Wellbore Surveying Procedures [PDF]

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Drilling & Measurements Procedures

Wellbore Surveying

Revision 2.00

Proprietary Notice This information is confidential and is the trade secret property of Schlumberger. Do not use, disclose, or reproduce without prior written permission of Schlumberger. Schlumberger makes no warranties; express, implied, or statutory, with respect to the product described herein, including without limitation, any warranties of merchantability or fitness for a particular purpose.

Unpublished work © 2004 Schlumberger All rights reserved under copyright law

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Wellbore Surveying Procedures

Drilling & Measurements Procedures

Wellbore Surveying

Document Information Document Type

D&M Surveying Procedures

Software Version

Microsoft Word 2000 for Windows 2000 Wellbore Surveying_200.doc

Source File

Author Author Information

Revision History

29-Apr-2002 20-Oct-2004

Original Draft Version 1.00 Final Technical Review 2.00

Technical Review

Wayne Phillips, Surveying Project Manager, SPC Chandra Singam, Area Survey Specialist, MEA Benny Poedjono, Survey and Data Manager, NSA Mike Terpening, Survey Specialist, NGC Julian Fletcher, MWD InTouch Engineer Norman Kamanga, MWD InTouch Engineer

Approval

Paul Wand, D&M Business Development Manager Drilling Technologies

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Chris Chia D&M Surveying Manager Sugarland Product Center 110 Schlumberger Drive, MD11 Sugar Land, Texas 77478, USA Tel: (281) 285 7350 email [email protected]

Drilling & Measurements Procedures

Surveying

Table of Contents 1 1.1 1.2 1.3 1.4 1.5 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 3

4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.7 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2 6.3 6.4 7 7.1 7.2 8 8.1 8.2 8.3

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3.1 3.2 3.3 3.4

GENERAL ...........................................................................................................................................................1 Scope .............................................................................................................................................................1 Application ....................................................................................................................................................1 Purpose ..........................................................................................................................................................1 Responsibility ................................................................................................................................................2 Process Management .....................................................................................................................................2 SURVEYING PRINCIPLES ....................................................................................................................................3 The Purpose of Surveying .............................................................................................................................3 Well Positioning Objectives ..........................................................................................................................3 Definition of a Survey ...................................................................................................................................4 The Survey Program......................................................................................................................................4 Survey Trajectory Calculations .....................................................................................................................5 Manual Survey Trajectory Calculation..........................................................................................................6 Reference Corrections – Grid Convergence and Declination........................................................................7 SURVEYING APPLICATIONS AND METHODS .....................................................................................................12 Surveying Operations Overview .................................................................................................................12 Magnetic Survey Tools................................................................................................................................13 Gyro Survey Tools ......................................................................................................................................15 Surveying Methods......................................................................................................................................16 SURVEY QUALITY CONTROL ...........................................................................................................................24 The Wellbore Surveying Quality Control Loop ..........................................................................................24 The Support Organization - Survey Specialist Support...............................................................................25 Quality Control Measures for Surveys ........................................................................................................25 MWD Survey Quality Control ....................................................................................................................25 Gyro and EMS Testing and Calibration Procedures....................................................................................28 Responsibility for Contractor Survey Quality Control ................................................................................29 Contractor Survey Execution Procedure Flowchart ....................................................................................29 Contractor Survey Execution Procedure Flowchart ....................................................................................30 MAGNETIC SURVEYING AT THE RIGSITE ..........................................................................................................31 Rigsite Survey Acceptance of Magnetic Surveys........................................................................................31 MWD Running Procedures .........................................................................................................................32 MWD Benchmarks and Checkshots............................................................................................................37 Planning for External Magnetic Interference ..............................................................................................38 Drillstring Interference Correction Algorithms - Assumptions for Use ......................................................49 Drillstring Interference Correction Algorithms - Invalidating Factors........................................................49 DMAG Correction for Drillstring Magnetic Interference ...........................................................................49 Single Station Drillstring Interference Correction (MagCor / SUCOP)......................................................52 Correcting MWD Surveys for NMR Offset Effects ....................................................................................52 GYRO SURVEYING AT THE RIGSITE..................................................................................................................54 Rigsite Survey Acceptance of Gyro Surveys...............................................................................................54 Gyro Survey Running Procedures ...............................................................................................................55 Gyro Orientation Operations .......................................................................................................................56 Surface Hole Change Over From Gyro to MWD Surveying ......................................................................59 SURVEY REPORTING ........................................................................................................................................60 Survey Reporting.........................................................................................................................................60 Reporting Frequency ...................................................................................................................................60 ENHANCED SURVEYING TECHNIQUES..............................................................................................................61 SAG Correction ...........................................................................................................................................61 The Estimated Drillstring Interference (EDI) Calculator ............................................................................65 Magnetic Storms..........................................................................................................................................68

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9

DEPTH MEASUREMENT ....................................................................................................................................70 Depth Measurement Accuracy ....................................................................................................................70 Drillpipe Depth Measurement .....................................................................................................................70 Wireline Depth Measurement......................................................................................................................71 10 ACTIONS FOR FAILED SURVEY CONDITIONS ....................................................................................................73 10.1 Main Cause of Failed or Out of Tolerance MWD Surveys .................................................................73 10.2 Actions if Surveys Fail Validation Check ...........................................................................................76 10.3 MWD Survey Sensor Dependencies ...................................................................................................80 9.1 9.2 9.3

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General In this section; •

These procedures are part of the D&M Well Surveying and Anticollision Standard.

•

They cover all D&M surveying planning and operations, and apply to all D&M personnel.

•

The OSC organization is the owner of these procedures.

1.1

Scope

The Schlumberger Drilling and Measurements (D&M) Wellbore Surveying Procedures are written in support of the D&M Well Surveying and Anticollision Standard (D&M-SQ-S002). They cover the necessary elements associated with surveying and survey quality control, during the planning and execution phases of a directional well. These procedures are confidential as they describe methodologies that are internal to D&M and also refer to specific Schlumberger software. The target users are Drilling Engineering personnel, Operations Supports Center (OSC) personnel, D&M Line Management, Engineers in the Integrated Project Management (IPM) Segment of Schlumberger, who are involved in well construction activities, may also find this document useful. 1.2

Application

The Surveying Procedures apply to all Schlumberger Drilling and Measurements directional well survey planning and execution activities. These Surveying Procedures supersede the “1997 Anadrill MWD Surveying Procedures Manual”. The Surveying Procedures are designed to encompass the standard treatment of wellbore surveying embodied by the applicable D&M Standard. It is also a condition of the Standard Anticollision Procedures that these Surveying Procedures are adhered to, particularly with regard to survey quality control.

1.3

Purpose

The purpose of this document is to provide a set of guiding procedures with which to manage the attainment of the well positioning objectives. They are intended to provide an essential reference to all D&M personnel who need to know about surveying, survey quality control and surveying related procedural issues. Well positioning is mainly concerned with the management of risk, in this case the risk of failing to place the well in the intended position. Failure to meet this risk appropriately can of course lead to a collision with another well, penetration of an undesirable subsurface feature, or failure to achieve the intended positional target; but in all cases increased cost.

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Directional Drilling Coordinators, Directional Drillers, MWD Engineers and Survey Engineers. Drilling

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1.4

Wellbore Surveying

Responsibility

It is the responsibility of the sign-off authority to ensure that any planned survey program is constructed with the expectation of satisfying these procedures. It is the responsibility of D&M Line Management to ensure that the planned MWD surveys are executed in conformance with this procedure. Where D&M are also the Survey Management services provider, it is also the responsibility of the Survey Specialist to ensure that all third party surveys, which are intended to be included in the final well trajectory calculation, conform to this procedure. It is the responsibility of the Directional Driller and the Drilling Engineer to ensure that all third party drilling surveys, such as Gyro singleshots, orientations and Gyro While Drilling (GWD) surveys, conform to the execution monitoring standards of this procedure. 1.5

Process Management

The technical integrity of the survey program is the responsibility of the OSC Manager, who may delegate this responsibility to the Survey Specialist. In this respect the OSC organization must own the entire process, and are responsible for the supervision of the application of all aspects of these procedures. The responsibility for the management of the operational execution of the Survey Procedures lies with the The correct observance of the survey program and provision of sufficient resources to ensure that this is done is also key to the success of the operation. The effectiveness of these procedures is entirely reliant upon this commitment and ownership.

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Line Manager. In this respect the Line Manager is also the owner of the execution monitoring process.

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Wellbore Surveying

Surveying Principles

In this section; •

The well positioning objectives and the purpose of wellbore surveying.

•

Survey calculations and acceptable manual estimation methods.

•

Grid Convergence and Declination, and the reference data sign-off procedure.

2.1

The Purpose of Surveying

In satisfying these procedures, the survey program should be designed such that the position of the wellbore is known with sufficient accuracy at all stages of construction to; Meet local and governmental regulations,

•

Minimize the risk of well collision,

•

Penetrate geological targets at an appropriate level of dimensionality,

•

Drill a relief well.

Key to achieving this are the three basic tenets of surveying; that the survey instruments used to survey the well have been qualified to do so, and have been run in accordance with known running procedures; that the performance of each instrument run has been tested by validation under controlled conditions, with the use of known instrument calibration techniques; and that an instrument performance model [or error model] exists for each instrument which adequately describes these performance specifications and provides a degree of statistical position confidence for any surveyed position on a given wellpath. 2.2

Well Positioning Objectives

Drilling target analysis and anticollision clearance objectives are the main drivers to define how accurately the position of a well must be known while it is being drilled. Whole field planning and reservoir drainage plans also help define how the required level of accuracy of the final well position. These objectives are achieved using a fit-for-purpose survey program, with accuracy, efficiency and economy borne in mind. The principle technical considerations are; •

Communication with Geoscientists; Ultimately the data produced by wellbore surveying is of greatest value to the geoscientists and their needs must be respected and not traded solely for operational expediency. During the planning phase, well positioning accuracy requirements need to be defined in line with seismic or other logging data positional accuracy.

•

All client instructions concerning well position must be received in writing; Receipted and retained in the Well Design File. Any changes to these instructions must be entered in the revision pages of the file. Vertical depth accuracy is of particular value in the calculation of field reserves and can impact the economics of field development.

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•

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•

Wellbore Surveying

Geological Target Sizing; Proper geological targets should be set, based on where the Geoscientists require the well to be positioned. A minimum standard is an absolute radius; but polygon target shapes defined by fault boundaries and depth requirements are preferred. Horizontal targets can be defined as three-dimensional polygons where possible.

•

Drilling Target Sizing; Geological targets are re-sized (reduced) according to the relevant survey errors inherent in the survey program specification, to effectively guarantee that if the drilling target has been penetrated then indeed the geological target objectives have been achieved. This target sizing should to be based on the part of the survey program active from approximately 1000ft above the target horizon extended to the target horizon. If a more accurate survey instrument is programmed immediately above a target, this can sometimes be too late to correct the wellbore position.

2.3

Definition of a Survey

The last known position of the path of the drilling bit is when it visibly passes through the drill floor rotary also be known from engineering drawings, although it could still be argued that some depth error for the surveyed position exists already at this point. A survey is an instrument measurement taken at a point in the well so that it can be joined to the last known position or previous survey point, to provide a progressive description of the wellpath or trajectory. Most survey instruments in use today provide a survey point that is referenced to measured [or along-hole] depth that is obtained from the driller’s pipe tally, or from a wireline spooling measurement. The survey instrument provides inclination (hole angle) and azimuth (direction) measurements. When these parameters are combined to create a survey point defined by measured depth, inclination and azimuth, the horizontal displacement from the origin, and the vertical depth from the elevation reference are derived. 2.4

The Survey Program

The survey program is the planned series of survey instruments to be used, and surveying requirements to be met during the execution of the well design, in order to satisfy these procedures. The fundamental purpose of the survey program is to ensure that sufficient and quality surveying is carried out in order to achieve the target at the minimum of both cost and risk of unplanned collision. In doing so, it is also highly desirable to provide sufficient redundancy of data to ensure that each dataset included in the final well trajectory has been independently verified (redundant surveying). This procedure will deal with the aspects of survey program design specifically related to survey quality, with anticollision requirements having been defined elsewhere in the Standard Anticollision Procedures.

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table to begin drilling the well. On some installations the horizontal position at seabed or ground level may

Drilling & Measurements Procedures

2.5

Wellbore Surveying

Survey Trajectory Calculations

The wellbore trajectory is defined within the chosen coordinate system as a series of surveyed points in three-dimensional space. These are joined together to form a continuous trajectory using an accepted geometrical calculation method, such as Minimum Radius of Curvature. This ‘Minimum Curvature’ method assumes a smooth spherical arc between successive survey stations. Alternative methods such as radius of curvature, average angle and balanced tangential are still in use, especially for historical data, but Minimum Curvature has now become the industry standard because of the processing power of modern computers. Continued use of legacy trajectory calculation methods, though sometimes useful for validating legacy data, is not best practice and these are not recommended for operational use.

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2.6

Wellbore Surveying

Manual Survey Trajectory Calculation

In the absence of a computer calculation method for establishing a surveyed position, it is often found that Directional Drillers resort to the tangential method of calculation. This is a poor choice, which is prone to large errors especially since, with very little extra effort, the average-angle method will produce a much better result. Where inclination i1 and azimuth A1 are measured at point S1, and (i2 , A2) is measured at point S2, which is at a distance ∆L from point S1.

⎛i +i ⎞ ∆Z = ∆L cos⎜ 1 2 ⎟ ⎝ 2 ⎠

⎛i +i ⎞ ∆D = ∆L sin ⎜ 1 2 ⎟ ⎝ 2 ⎠

⎛ A + A2 ⎞ ∆Y = ∆D cos⎜ 1 ⎟ ⎝ 2 ⎠

⎛ A + A2 ⎞ ∆X = ∆D sin ⎜ 1 ⎟ ⎝ 2 ⎠

These formulae will then give vertical depth ∆Z, horizontal displacement ∆D, relative north or south Schlumberger Confidential

component ∆Y and relative east or west component ∆X.

S1

Average Angle

Ta ng

Mi

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n im um

Cu rva

en

tur e

tia l

S2

6

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2.7

Reference Corrections – Grid Convergence and Declination

2.71

Magnetic Declination

Magnetic declination correction is used to convert MWD azimuth values from a Magnetic North reference into a True North reference. The magnetic declination correction, which is applied for a particular location at a particular date, is the angle subtended between the direction of Magnetic North at that location and the direction of True North. The convention is that when Magnetic North lies to the West of True North, this gives a WEST DECLINATION CORRECTION while if Magnetic North lies to the East of True North this gives an EAST DECLINATION CORRECTION. For a West declination correction the observed magnetic azimuth is greater than the true azimuth, therefore the declination correction is subtracted from the observed magnetic azimuth. For an East declination correction the observed magnetic azimuth is less than the true azimuth, therefore the declination correction is added to the observed magnetic azimuth. Magnetic North varies widely at different locations around the world and with time.

b) East Declination

True North

True North

Magnetic North

Magnetic North

D Az im uth

D

Bo re

AMN

ho le

ATN

ATN = AMN - |D|

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a) West Declination

AMN ATN r Bo

e ol eh

A

zim

h ut

ATN = AMN + |D|

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2.72

Wellbore Surveying

Grid Convergence

The grid convergence correction is used to convert azimuth values from being True North referenced into a Grid North reference. The convergence correction, which should be applied for a particular location in a specific coordinate system, is the angle between the direction to Grid North at that location and the direction of True North. The diagrams below show the relationships between true azimuth, convergence and grid azimuth for various positions of a location with respect to the central meridian of the coordinate system grid zone of the observer and the equator. The grid convergence can also be estimated by hand with reasonable practical accuracy by: Grid Convergence = Sin Latitude x (Longitude – CM) Where CM = the central meridian longitude of the observers grid zone Northern Hemisphere

West of Central Meridian

East of Central Meridian

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Grid North

Grid North

ATN AGN r Bo

e ol eh

im Az

AGN = ATN + |C|

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C AGN ATN

Az

h ut

Central Meridian (CM)

C

im uth

True North

True North

Bo reh ole

2.721

AGN = ATN - |C|

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2.722

Wellbore Surveying

Southern Hemisphere West of Central Meridian

East of Central Meridian

Grid North

Grid North

True North

ATN

Central Meridian (CM)

AGN

Bo reh ole

C

Az im uth

True North

C ATN AGN

AGN = ATN - |C|

le ho

h ut

AGN = ATN + |C|

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True South

re Bo

im Az

True South

2.73

Using Drilling Office to Check Declination and Grid Convergence

2.731

Check Declination

Drilling Office uses the BGGM (British Geological Survey Global Geomagnetic Model) to compute the main field reference criteria for a given location for a given date. This is licensed software, which is resident in both Drilling Office, IDEAL (known as GeoMag in IDEAL), and in the Survey Toolbox. In order to check the value of magnetic declination for a given location in Drilling Office, with the precondition that you have set up a field, slot and well with the correct latitude, longitude and coordinate system, launch either Well Design or Survey Editor and create a new worksheet. Then, tie the worksheet onto your new location using select tie-on, or by clicking the tie-on icon button. Click on the Options/Compute Magnetic Declination menu item to bring up an interface giving the coordinates entered for the location and the Magnetic Field reference data. The Declination value is given in this case with a sign convention for negative west / positive east, so using the diagrams in section 2.71 (above) where we are applying the magnitude of the correction either positive or negative, this is already done for us.

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Therefore, the sign convention rule for whatever correction comes from the Drilling Office output interface is; - Declination must be ADDED to the magnetic azimuth to produce a true north azimuth. This means that when we add a negative number (if the declination is westerly) to our azimuths then our azimuths become smaller. 2.732

Check Grid Convergence

The grid convergence is given as a function of the latitude and the distance from the central meridian, of the specific coordinate system type and zone that has been chosen. It varies with distance from the central meridian and distance from the equator as a result of trying to produce a scaled grid map of a curved surface, so that the relative positions of objects (such as platforms, fields and wells) may be plotted in a relevant and usable manner. In Databrowser, the grid convergence is displayed at each of the Field, Structure and Well levels, and is calculated from the coordinate system, latitude and longitude entered by the user. The result is displayed declination, but in this case indicates our longitudinal position relative to the central meridian. Once again, using the diagrams in sections 2.721 and 2.722 (above) where we are applying the magnitude of the correction either positive or negative, this is already done for us. Therefore, whatever correction comes from the Drilling Office output interface; - must be SUBTRACTED from the true north azimuth to produce a grid north azimuth. This means that when we subtract a negative number (if the grid convergence is westerly) to our azimuths then our azimuths become greater. In the southern hemisphere the sign convention is reversed, but the rule of subtraction from the true north azimuth continues to apply because Drilling Office will take the southerly latitude into account. Using Drilling Office generated outputs and sign conventions for declination and grid convergence we get;

D&M Azimuth Reference Correction Rule: Grid Azimuth = Magnetic Azimuth + Declination – Grid Convergence

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in the same convention negative west / positive east in the Northern Hemisphere as given for

Drilling & Measurements Procedures

2.74

Wellbore Surveying

Reference Data Sign-off

During planning, it is the responsibility of the Drilling Engineer, the Survey Specialist and the signoff authority to ensure that all reference data are current and correct. At the wellsite, it is the responsibility of the MWD Engineer or Surveyor to cross check and obtain a signoff from the Company Man and the Directional Driller on grid convergence, magnetic declination, well reference location and all relevant reference data on the standard form. For the MWD acquisition system this is done using the D&IInits signoff sheet, and with other surveying systems using similarly formatted confirmation reports. This procedure is mandatory, and must be strictly followed, as reference error corrections are one of the most common sources of gross errors in surveying. A well collision can be the direct and immediate result of the misapplication of a reference correction.

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Wellbore Surveying

Surveying Applications and Methods

In this section; •

Surveying operations overview.

•

Magnetic surveying tools, methods and applications.

•

Gyro surveying tools, methods and applications.

•

Specialist surveying applications.

3.1

Surveying Operations Overview

3.11

Surface Hole

Surveying the wellbore can generally be split for discussion into three main areas. At the beginning of the well there are one or more large diameter hole sections to be cased with conductors. In this case the presence of external magnetic interference and other environmental effects such as weather. Depending on the surveying systems in use any one or more of these error sources may hamper the effectiveness and quality of the surveys. In the case where external magnetic interference is a problem, the use of magnetic systems, such as Measurement While Drilling (MWD), is limited to gravity-based toolface readings once sufficient hole angle is achieved to resolve a reliable measurement. For Schlumberger MWD systems this is achieved when a minimum hole angle of 3° has been reached. Where high-accuracy azimuths are required, and where external magnetic interference is also a problem [which results in the MWD surveys falling outside of their field acceptance criteria], then gyroscopes must be used for collision avoidance or to achieve the desired well trajectory. Traditionally, the most feasible options for this are the Northseeking Gyro (NSG) or the Surface Readout Gyro (SRG) systems. The NSG is a more accurate and the preferred instrument for this application, but it is also more sensitive to environmental movement, and the presence of a lot of “noise” at or near surface may degrade the survey quality beyond the point of usefulness. The SRG however, is more suited to noisy conditions because it is a much less sensitive instrument. Its disadvantage is lower accuracy [though still perfectly acceptable in many cases], and azimuth drift over time if the run length becomes extended. More recently, the introduction of GyroMWD systems to the industry has reduced the requirement for wireline borne gyro singleshots and orientations for the sake of saving rig time. In surveying terms these systems can be considered to be at a similar level of accuracy as the SRG system.

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surveying challenge is usually the avoidance of other nearby wells in high-well-density areas, and in the

Drilling & Measurements Procedures

3.12

Wellbore Surveying

Intermediate Hole

Once the first conductor strings are in place, the next sections of the well to be surveyed can generally be classed as intermediate. In this area it is likely that Gyro or GyroMWD survey tools will be used if required until clear of external magnetic interference, at which time the MWD instrument can be used to provide real-time survey data. Depending on the drilling target size available or any other positional constraints, it may often be necessary to run an additional ‘gyro multishot’ either in drillpipe for high-angle applications, or in cased hole for lower-angle sections, upon completion of the hole section. The choice of instrument and its conveyance method should be made to minimize operational difficulty, risk and cost, while maximizing accuracy and the reduction in position uncertainty gained. A good test of this principle is to examine the survey program for the purpose of setting a minimum survey depth to reach, in order to achieve the survey objective, rather than simply attempting to reach the maximum depth possible, which may be significantly more difficult and time consuming. One highly successful alternative to the intermediate gyro survey approach is in the use higher accuracy magnetic surveys using the Geomagnetic Referencing Service, which does not require an additional gyro tool run, but relies on higher accuracy MWD survey processing techniques and the use of a local crustal

3.13

Final Hole Section

Once the intermediate casing has been set and [if run,] the intermediate gyro multishot survey completed, the final section of the well [in surveying terms] is reached. The surveying objectives may again become more demanding in this area as we are now concerned with achieving the desired geological target. Often it is preferred not to run gyro surveys in this section, since they must be run through drillpipe in open hole at or near the end of the section. This can create dangers of hole stability and of the Bottom Hole Assembly (BHA) becoming stuck, as a result of the excessive non-drilling time that a long gyro survey might effect. In this case, it is preferred to enhance MWD surveying methods in some way, typically by use of SAG Correction, Inhole Referencing or Geomagnetic Referencing. 3.2

Magnetic Survey Tools

For MWD survey applications, the survey is sent uphole for surface signal reception using mud-pulse telemetry. The pressure changes in the standpipe are read and decoded into survey measurements at surface. The less common alternative to mud-pulse telemetry is electromagnetic telemetry which is used today in applications where a mud column is not employed or is only partially employed such as with air drilling, foam drilling and multi-phase underbalanced drilling. In this case a varying voltage is created across the MWD tool, which creates an electromagnetic wave that can be detected by a surface receiver mounted in the ground (or at seabed).

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field reference model in place of the BGGM.

Drilling & Measurements Procedures

Wellbore Surveying

All magnetic survey tools use accelerometers and magnetometers to measure the surveyed inclination and azimuth. Each sensor package contains three accelerometers, and three magnetometers with each group configured into an orthogonal set with one principal axis aligned in the direction of the major axis of the tool [and therefore the BHA]. The accelerometers each measure a component of the Earth’s gravity vector, and the magnetometers each measure a component of the apparent Earth’s magnetic field. In practice, about 95% or so of what the magnetometer set measures is the Earth’s main magnetic field, with the remainder being made up of the following sources of interference or offset; •

Crustal magnetism that is not modeled, and is trapped in the solid rock formations,

•

External magnetic interference from nearby cultural artifacts, or other nearby wells,

•

Disturbance field effects from diurnal and solar magnetospheric activity (solar storms),

•

The MWD sensors own biases and scale factors (that we try to calibrate for).

Electronic Magnetic Multishot (EMS or EMMS) tools are solid-state magnetic survey tools with a sensor package very similar to that of an MWD tool. This type of survey tool is dropped or launched through the steering tool or core orientation tool. Where MWD has previously been run, an EMS will not greatly improve positional accuracy, but its principal purpose will be in survey validation through redundant surveying at little or no rig-time cost. The EMS may also be chosen as the definitive survey for the section, although this is a convention based on the tool often being better magnetically spaced than the MWD sensor [in a non-magnetic collar located behind above the MWD], and being run in tandem (two tools simultaneously) giving “same-time” independent verification. Running an EMS survey relies upon having sufficient non-magnetic drill collars in the drillstring within which the EMS tool can be dropped, thereby minimizing drillstring interference. In addition, this requires a Totco ring or a universal-bottomhole-orienting (UBHO) sub to be present in the drillstring in the correct position. There are also a large number of film system magnetic singleshot kits still in use in the field today. These are often held at the rigsite as a backup check against an MWD, or to provide checkshots against other surveying systems. A magnetic singleshot tool consists of an angle unit containing the magnetic compass with a graduated inclinometer scale for different inclination bands, and a camera unit which houses the film disk and takes a single exposure photograph which is developed by the surveyor and provides inclination, direction and toolface relative to the rig heading. Due to the potential inaccuracy of this type of survey instrument, it is not recommended for uses as a survey tool, but will serve as a redundant check, especially on a Totco or other mechanical inclination-only tool, and both of these would only be used to record the wellpath where no other higher accuracy data exists.

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drillstring in freefall mode at or near section total depth (TD), or run on wireline as a (type of MWD)

Drilling & Measurements Procedures

3.3

Wellbore Surveying

Gyro Survey Tools

Any gyro system that sends real-time survey data to surface via an electric wireline can be referred to as a surface-readout gyro, but the SRG description most commonly refers to a specific type of optically referenced free gyro. Unlike the NSG, the SRG has no independent direction-finding capability. The surveyor will line up the SRG on a known bearing, sometimes called a “foresight,” and run the tool in the hole to take a survey. On completion of the survey, the tool is placed back into its alignment position and an estimate of the gyro drift is recorded by rechecking the foresight. The total drift is then applied using time curves to correct the survey with an estimate of the drift that had occurred at survey time. Northseeking gyroscopes (NSG) align themselves with the direction of true north by sensing the rotation of the earth. When an NSG is held stationary in a wellbore, the tool senses earth rotation as torque. The magnitude of the torque is a function of latitude (greatest at the equator) and orientation (greatest when the tool axis is aligned with the earth’s axis). This ‘earthrate’ can be approximated by; •

Earthrate = 15.041*Cos(Latitude), in degrees per hour.

north. These NSG tools are very accurate except where the latitude exceeds 70° north or south, and within a few degrees of high angle east-west azimuth orientation; or where the drill string is not held perfectly stationary in the well. NSG tools can be run in cased hole on wireline if the maximum wellbore inclination does not exceed about 60°, or on drillpipe where it may be possible to “drop’ a batterypowered memory NSG, much like an EMS. For higher inclination applications [above 70° inclination], the NSG system also becomes less accurate than a standard MWD system, and must be run in a second operating mode known as continuous northseeking gyro (CNSG). In CNSG mode the tool is switched into a local-rate measurement mode. Instead of measuring the earthrate, the tool measures displacement in two planes from a known local initialization point. This allows the CNSG to be run at higher inclinations, where it can be pumped-down through drillpipe even in horizontal hole. In addition, the CNSG is designed to be able to collect data dynamically at speeds of up to 200 ft/min (1m/s). In both surveying modes, i.e. NSG and CNSG, the rate gyro requires an independent depth reference input, which comes from the wireline depth counter when run in wireline mode or the drillpipe tally when run in drillpipe-conveyed mode. This tends to be a limiting factor with the rate gyro application because it often results in a significant vertical uncertainty problem in the very area in which this is a concern e.g. when landing the well or at the reservoir. Inertial gyro systems do not suffer with this problem, but those currently available are limited by their size in being able to access these high angle areas of the well. Each of the available northseeking gyro systems can be run concurrently with gamma ray, casing collar locators and many of the other wireline conveyed logging tools.

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Only the horizontal component of the earth’s spin vector is used to determine the direction relative to true

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3.4

Wellbore Surveying

Surveying Methods

This section broadly describes the range of surveying methods available on the market today, along with the basic operating modes for which the instruments or service being described should be considered. It is not intended that this section take the place of practical surveying operations experience, or the role of the Survey Specialist, but instead is intended to provide sufficient guidance such that the users of these procedures can determine when more expert and experienced advice might need to be called upon. 3.4.1

Measurement While Drilling (MWD) Surveys

MWD surveys are run during drilling operations using a survey instrument package housed in a nonmagnetic drill collar that is placed in the BHA. It is usually spaced within the BHA along with other nonmagnetic BHA elements so as to minimize any potential effects of drillstring magnetic interference. This is the primary drilling survey tool in use today because the tools are robust, relatively easy to set up, and they can be operated in the drilling environment thereby using up very little ‘non-drilling’ rig time. The major disadvantage of using MWD, or any other magnetic survey tool is that the reference system [the Earth’s magnetic field] is not constant, is not aligned to a geographical or mapping reference, and it can

MWD is not normally used in nominally vertical single wells until the reservoir section is reached, and it is common to find exploration well survey data containing only a few survey points between surface and the beginning of the reservoir section as a result. MWD is also of limited use in high well density areas where external magnetic interference from other nearby wells is likely to affect survey quality. Where the section to be drilled is relatively short or shallow, and external magnetic interference is expected over the entire interval, it is often advisable not to run the MWD tool in the BHA at all, in order to be able to survey using the gyro tool landed in the UBHO sub at a position as close to the drill bit as possible. MWD surveys are taken when the tool is stationary, and the pumps are off. The survey data is then pumped back to surface when the pumps are turned back on. In some applications a ‘continuous’ MWD survey can also be sent to surface whilst drilling is actually taking place. In D&M, the standard MWD survey includes the full six-axis sensor measurements and the computed survey parameters. This sixaxis data should normally be sent to surface to allow quality assurance checks and magnetic interference checks and corrections to be made. It is possible, in some configurations, for the MWD tool to send only ‘short surveys’ up which do not contain the six-axis data, and in cases where the full standard survey is not retained or cannot be downloaded later from the tool memory, this will prevent full quality assurance or the application of any further specialist corrections (such as DMAG) to be applied to the data. For this reason it is not recommended that short surveys are programmed for use where the recorded mode long surveys will not be available for later analysis, unless absolutely operationally necessary.

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be adversely affected by a number of error inducing interfering sources.

Drilling & Measurements Procedures

3.4.2

Wellbore Surveying

Continuous Measurement While Drilling (cDNI) Surveys

Continuous MWD measurements (cDNI) consist of real-time survey measurements made during drilling and which are sent to surface in real time at frequent intervals. Unlike the traditional or ‘static’ MWD surveys which are usually taken during drilling connections [every thirty or ninety feet], the cDNI surveys provide additional information to the Driller about the well trajectory and dogleg severity in the interval between the static surveys. 3.4.3

Electronic Multishot Surveys (EMS)

An Electronic Multishot Survey (EMS), or ‘drop multishot’ instrument is usually run as one of a pair or tandem stack [or in some cases a triple-stack] of solid-state magnetic survey instruments. These instruments are housed in a small diameter pressure barrel, so that they can be dropped or released in free-fall mode inside the drillstring before tripping out of hole. Survey data is recorded internally within the survey instrument against elapsed time, and the Surveyor observes the trip out of hole whilst recording elapsed time versus sensor depth based on the drillpipe tally. The survey instruments are spaced using of the MWD tool, the survey sensors are equally spaced within the nonmagnetic collar above the MWD. A typical EMS tool when made up can vary in length from thirty feet up to forty-five feet in length, and will contain a tandem sets of probes, batteries, rubber finger-pin stabilizers and spacer bars depending on the BHA configuration, depth and hole angle to which they are to be run. 3.4.4

Gyro Survey Modes

The two most common conditions for running gyro surveys are well proximity issues in surface hole where external magnetic interference makes the use of MWD impractical, and in longer ‘multishot’ survey applications which require the lateral position accuracy of the gyro survey to ensure that the drilling target can be achieved, or that some positional feature such as a fault can be avoided. Gyro surveys are usually run in one of three modes; memory mode, where the survey tool is initialized or surface referenced and then placed or dropped within in the drillstring to record survey data to a memory module at regular intervals; real time mode, sometimes generically referred to as surface readout mode, where the survey tool is run on an electric wireline and the survey data is transmitted directly to surface for immediate interpretation; and MWD mode where the gyro mounted in a drill collar transmits the survey to a standard MWD telemetry package, which then sends the survey to surface while drilling.

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nonmagnetic spacer bars so that when the survey barrel lands on the ‘Totco ring’, ‘baffle plate’ or on top

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The main benefits of the memory mode gyro is that high accuracy wellbore surveys can be obtained with relatively little ‘rig time’ overhead, as the survey tool is tripped into or out of the hole whilst sitting in the BHA, much like the EMS. It is then recovered or fished to surface using a sand line, or wireline retrieval system, or it is ‘tripped’ to surface and recovered directly from the BHA at the rig floor. The main disadvantages of the memory gyro system is that there is no guarantee that a satisfactory survey has been completed until the tool is recovered to surface where, despite some technological improvements, these gyro tools continue to be prone to shock failure. There is also some trade-off in terms of accuracy with a drillpipe conveyed gyro survey in that the depth measurement system [manually strapping the drillpipe] can be prone to gross error, and the systematic misalignment and SAG errors need to be carefully handled since the gyro lies in a static position within the BHA. Generally, wireline conveyed gyro survey tools can be split into two major instrument groups; surface referenced lower accuracy free gyro’s which are often ideal for the surface hole application but which suffer from excessive drift and other time-based errors when run at depth and/or temperature; and high accuracy northseeking and inertial grade gyros which are designed primarily to satisfy the extended survey or target penetration application.

running procedures. The high accuracy gyro is often run unnecessarily in the surface hole because the economics or availability of the other systems makes it operationally expedient to do so. For mobile rigs in bad weather areas this can often create more problems than it solves, with regard to survey data quality. Similarly, the use of lower accuracy gyro systems for extended multishot applications is to be discouraged, both because of the capability of the instruments themselves, and also as a result of the limitations of drift corrections and where manual interpretation of data needs to be made from a film disk or spool. In many of these circumstances the standard MWD system is both a more accurate and a less time consuming surveying service. More recently there has been a number of Gyro-MWD (GMWD) tools introduced to the industry, and these are primarily aimed at the low angle surface ‘kick off’ market. The main advantage of running GMWD is that there is a rig time saving to be gained by not having to run successive wireline trips into and out of the drillstring for gyro singleshots, whilst drilling is suspended. This can be particularly attractive for deepwater applications, where the extended wireline run times are very costly in terms of the daily rig cost for the rig time. The main disadvantage of these systems is that they are generally less accurate than the full high accuracy NSG system as a result of having been ruggedized for the drilling environment, and they also continue to be prone to surface noise and vibration problems when trying to sense the earthrate in a dynamic and noisy environment. For this reason they are not recommended for applications that require gyro surveys at inclinations in excess of 20°, or for surface kick-off operations on mobile rigs in areas where moderate to heavy sea conditions are commonly expected.

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The operational strength of the lower accuracy gyros is in their relative robustness, using well established

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3.4.5

Wellbore Surveying

Film System Gyro

Film system or photomechanical gyro systems are still in limited use today, and many old film system gyro surveys are also found in legacy survey databases. A film system gyro consists of a free gyro that is surface referenced to a foresight bearing or to the rig heading. Once referenced and running at full operating temperature, the gyro would be loaded into a survey barrel whilst running on battery power. At this time the gyro tool would be conveyed through the drillpipe to land in the BHA where a photograph would be taken once a preset time delay had elapsed. The camera unit would then take a lookdown photograph of the gyro compass card that also contains a graduated overlaid angular scale for inclination. A marker or lubber line indicates the relative (rig heading) azimuth, the toolface and the inclination. These photographs would be taken singly on a circular film disk, or sequentially on a photographic roll of film depending on the instrument type, both of which had to be manually developed and interpreted at surface. The most common film disk gyro system still in use today is probably the ‘Humphrey’ gyro tool. 3.4.6

Surface Readout Gyro

referenced or foresight bearing referenced free gyros that are run on electric wireline. Most systems are usually provided with a remote rig floor readout for gyro toolface, which the driller can use to orient the steerable BHA whilst by rotating the rotary table when the gyro is in place sitting in the UBHO sub. Once the correct toolface has been set, the gyro is retrieved to surface, the connection made up and drilling resumed. Generally these tools are robust, and work well in the relatively noisy surveying conditions found at surface. SRG tools do suffer from time-based drift and whilst they are perfectly adequate for surface hole operations, they are less adequate when run at depth, or in deep water where the run times are extended. As a result of the accuracy restrictions of the SRG tool, it is not recommended to run the tool above 20 degrees inclination or below about 1500ft measured depth.

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Surface Readout Gyro (SRG) tools are still in common use in many parts of the world. They are surface

Drilling & Measurements Procedures

3.4.7

Wellbore Surveying

Gyro Multishot Surveys

The term multishot traditionally referred to the ‘stringing’ together of a series of ‘singleshots’ or snapshot surveys of the wellpath, to form a trajectory or surveyed position of an interval of the wellpath. Photomechanical gyro multishot tools which used rolls of film, and later Northseeking gyros which sent data directly to a computer at surface, were used to ‘map’ sections of the well. Generally multishot surveys were run in cased hole, and because of the relative accuracy of the tools, particularly the Northseeking tools, they were considered to be the most accurate well survey available. Disadvantages of traditional gyro multishots were that the tool had to be held stationary at a given measured depth to obtain an accurate reading of direction and inclination, and this took a lot of rig time for long surveys. The tools were not very accurate at high angles of inclination over 70 degrees [in fact, they are less accurate than MWD surveys at this orientation]. The advent of the continuous northseeking gyro (see below) solved these two main problems. 3.4.8

Gyro Singleshot and Orientation Surveys

gyro in the drillstring in order to take a ‘snapshot’ survey at a single depth once the gyro has come to rest in the BHA (singleshot survey), or to ascertain the toolface, and /or set the toolface of the steerable BHA once the gyro had landed in the UBHO sub. This operation is usually done in areas of high well density, where external magnetic interference negates the use of MWD orientations. In the past, this was one of the earlier applications for free gyros, or SRG tools, which were adequate to the task, reasonably rugged, and did not suffer too much from surface ‘noise’ or vibration and movement. More recently NSG tools have been put to use for this purpose, and this has brought an improvement in accuracy at the cost of greater sensitivity to the ‘noisy’ surface hole environment. This operational tradeoff in performance versus surveying time taken must be carefully considered when planning the survey program, since many operations mistake or misinterpret the value of this trade-off. Using a northseeking gyro tool for orientations during a large diameter hole section kick-off at or near surface where tool movement and noise is a problem that can be overcome or reduced in several ways. The most obvious answer is to replace the use of the northseeking gyro tool with the use of a Surface Readout Gyro such as the SRO (Sperry-Sun) or SRG (Scientific Drilling). As an alternative, Gyrodata created an IMT tool (intelligent modified tool). This tool is used in noisy (tool movement rate) environments and is a modification to their standard NSG tool whereby the sample period has been lengthened (to 2 minutes or 4 minutes) so that the noisier survey data can be averaged out. The downside of this is a fall off in accuracy, but probably no more than would be the replacement with an SRG tool. Alternatively, the SDI Keeper tool can be run in sightline mode (a psuedo surface referenced mode), and this can also alleviate some of the noise problems. The Gyrodata tool cannot be run in 'sightline' mode.

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Gyro singleshots and gyro orientations are generic terms used to describe the operations of running a

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3.4.9

Wellbore Surveying

Continuous Gyro Surveys

The continuous northseeking gyro (CNSG) is currently the standard gyro multishot survey application for all deviated wells. This is an engineering variation on the standard NSG whereby all axis outputs of the tool are combined to measure local rate, or displacement in the horizontal and vertical from an initialization point, with an independent electronic depth feed to provide the complete survey. The result is that the survey tool can be run dynamically without the need to stop to take discrete survey measurements [except for initializations and bias updates], which saves rig time and provides a more accurate survey. The disadvantage of this method of gyro surveying is that the quality of the survey rests largely on the quality of the initialization, rather than a series of somewhat independent discrete measurements as in the traditional NSG multishot. This survey type therefore requires a higher degree of internal quality assurance by the survey vendor. 3.4.10 Inertial Gyro Surveys Inertial gyros form the most accurate survey systems available for surveying the well. There is only one normally restricted to cased hole surveying only, as the tool has an approximate diameter of about 5½ inches. The survey runs are limited to inclinations up to about seventy degrees, beyond which a wireline logging tool would ordinarily be unable to free-fall under it’s own tool weight [depending on the tool configuration and the type of wellbore fluid present]. A wireline tractor device can be used to run the tool at higher inclinations, but this is a very high cost survey solution that is only warranted for the highest survey accuracy requirements.

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commercially available tool [Baker Hughes Inteq – RIGS Tool] on the market today, and these tools are

Drilling & Measurements Procedures

Wellbore Surveying

3.4.11 Inclination Only Tools As suggested by the name, inclination only tools are designed to measure the hole angle without anything being known about the direction. This type of instrument has a limited application as a surveying tool because of this, and even their use in isolated exploration or appraisal wells should be considered carefully because of the resulting size of the position uncertainty that will be associated with having no knowledge of direction. The most common use for inclination only surveys is as a surface hole drilling survey for single well applications where the well is going to be resurveyed later in the operation with a more accurate survey tool. There is a secondary inclination-only application that must be considered on occasion, and this is where an MWD survey has failed it’s magnetic field acceptance criteria (FAC), but the gravity measurement is still good. In this case the resulting survey can be considered to be an inclination only survey. Care must be taken when implementing this application for MWD surveys, since one of the most common causes of out of specification magnetics for an MWD tool is external interference from the presence of other nearby wells. In this situation the application requires the highest survey accuracy to avoid a well

3.4.12 Other Specialist Survey Services Other specialist survey services currently under introduction in the industry include Gyro While Drilling (GyroMWD) of which there are several commercially available tools on the market, and magnetic ranging tools both passive and active. The GyroMWD tools are generally higher cost than standard MWD, and their primary application is in the drilling of surface hole in high well density areas where standard MWD cannot be used due to external magnetic interference. These tools are made attractive by the rig time savings available when not having to trip a wireline gyro into and out of the hole, particularly in deep water or high rig cost operations. Generally, the improvements in magnetic surveying accuracy with techniques such as geomagnetic referencing have largely removed the need for GyroMWD surveying further down the hole where the improvements in accuracy can be offset by performance issues and the running costs of the tools themselves. Even though these tools consist of an NSG co-located in the MWD collar (Scientific Drilling), or located in an additional nonmagnetic collar below the MWD (GyroPulse), it must be understood that these tools have been significantly modified and dampened in order survive the drilling environment. This has resulted in a fall off in overall system accuracy, and the current generations of these tools should not be considered to be any more accurate than a traditional SRG tool.

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collision, and gyro surveying is the only viable option to achieve this.

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Wellbore Surveying

Magnetic ranging is a technique that is used for relief well drilling or to allow one well to be drilled a fixed and controlled distance away from another well in tandem, such as for a steam assisted gravity drainage (SAGD) project. In the SAGD application a steam injector is drilled above a tandem producer so that heavy oil or tar sands can be exploited by heating using steam injection, and where the oil then drains down into the producer to be pumped to surface. Magnetic ranging can be done either passively or actively. In the passive application a magnetic ranging tool is run in the well being drilled and attempts are made to recognize the magnetic signature of the offset cased well or fish. A variation of this method can also include the magnetization of the offset well casing prior to it’s being run in the first of the tandem wells. The passive method is less robust and much harder to achieve effectively than the active method. Active magnetic ranging relies upon concurrent access to the tandem well so that an electromagnetic source can be run in one well and the range logged from the other. Variations on this include running the electromagnetic source in either the drilling well or the tandem well. Two companies, Vector Magnetics and Scientific Drilling, are the main service providers in active and passive magnetic ranging services respectively. Schlumberger Confidential

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4

Wellbore Surveying

Survey Quality Control

In this section; •

Principles of survey quality control.

•

MWD surveying conformance to the quality control loop.

•

Survey subcontractor quality control.

•

D&M monitoring of survey subcontractor quality.

4.1

The Wellbore Surveying Quality Control Loop

The premise of a quality surveying process is that, within the bounds of practicality, the survey instruments are correctly calibrated to a validated standard of accuracy; they are shown to perform to within the calibration specification prior to the job; the survey at the rigsite is conducted using accepted quality running procedures, and the data obtained meets the required field acceptance criteria. The return to the test or maintenance facility.

Pre-job Calibration

QC Review & Idependent Verification

Validated Running Procedures

Post-job Calibration Checks

Quality Checks During Survey Whilst there is some variation between survey service operators regarding what constitutes a calibration check as against master recalibrations, and what the specific methods and calibration tolerance thresholds are for the various tools used across the industry, the process stated above holds true in all cases, and this represents the underlying basis for all position error modeling, anticollision scanning, drilling target sizing and many other operational and economic decisions that are made as a result of determining the well’s position.

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survey instruments should then be required to perform to within the calibration specification again on

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4.2

Wellbore Surveying

The Support Organization - Survey Specialist Support

Each of the D&M areas, and many of the major locations now have Survey Specialists operating within the OSC community. The Drilling, Planning and Execution (DPE) InTouch helpdesk, MWD InTouch helpdesk, Acquisition Systems (AS) InTouch helpdesk, and the Well Engineering community within Eureka can also provide invaluable support and access to surveying expertise. 4.3

Quality Control Measures for Surveys

Survey quality control can be a difficult process to document. Often, for a given survey instrument, an apparently satisfied set of field acceptance criteria can still produce an invalid survey, whilst one or more violated field acceptance specifications (short of a tool failure) do not always constitute a survey failure. The reasons for this apparent paradox are that no one single set of field acceptance criteria is infallible for every type of surveying service, and often what is required is an examination of the assumptions underlying the apparently bad survey. Most common of these are gross errors, which are often the hardest to detect (e.g. applying the grid convergence in the wrong direction), or some failure to apply the technology appropriately. Generally, the basic quality control criteria and field acceptance specifications surveyor will be given an indication of when a survey specialist should be called, or when additional technical support is required. 4.4

MWD Survey Quality Control

4.4.1

MWD Testing and Calibration Procedures

MWD service companies generally follow a standard of pre-job and post-job testing which incorporates calibration checks, but which is also aimed at ensuring the ruggedness of the tool and the testing and preparation that is required to ensure that the tool will survive for an extended period in the drilling environment. Generally, MWD tools must also remain at the rigsite for extended periods of time, and so the most weight in the ‘QC loop’ for an MWD service must be given to the pre-job calibration check; the comparative benchmarks between successively run MWD tools; and the onsite field acceptance criteria (FAC) for each tool run. This is because the post-job calibration check often takes a considerable time to complete as a result of extended run times, shipping and logistics. Often the end of well report, or the MWD survey report is completed and has been delivered to the client before the final qualifying post-job checks are complete. In principle this represents an incomplete QC loop as far as a quality system of surveying is concerned, but the general robustness of magnetic survey tools and the usually immediate indication of ‘out-of-tolerance’ behavior of the tools onsite is considered a practical replacement for this aspect of the quality control loop, until such time as the tools are returned to the base.

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for all surveys form a ‘road map’ which need to provide sufficient checks and balances so that the

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When the MWD tools are returned to the base and prepared for their next job, they must receive the full quality control process to be made ready to ship. This will include an ‘outgoing systems test’ (OST) to confirm the calibration status of the tool. Not only does this OST now represent the pre-job calibration check for the next job, it also represents the post-job calibration check for the previous job (however asynchronous this may have become in relation to the previous job) for the purposes of completing the QC loop. Therefore the MWD repair and maintenance system must incorporate both a procedure for dealing with MWD tools found to be out of specification from failing their OST, so that the tool can be repaired and accepted for shipping to it’s next job, but also a system for notifying the OSC organization of the failure of a tool to pass it’s OST with respect to the last job it was on. The data quality and benchmarks checks can also be reviewed at this time and reconfirmed by the Survey Specialist who will examine whether any service quality failure has occurred. 4.4.2

Pre-Job Data and Data Signoff Sheet

Prior to the commencement of any MWD survey service at the rigsite, a number of checks need to be made on the tools themselves, usually consisting of function tests and setting up checks. In addition, a reference data to identify the survey location and field acceptance criteria under which the survey will be performed. This will include some or all of; surface location, latitude, longitude, grid convergence, declination, Geomag (BGGM) field acceptance criteria, toolface offset, elevation reference and other relevant data depending on the type of survey service to be provided. For gyro and EMS surveys, a single sheet should be prepared and signed off by the surveyor, directional driller and client representative prior to commencement of the job. For MWD services a sign-off sheet should be completed for each MWD tool run and/or reinitialization of the surface acquisition system. The MWD reference sign-off sheet should be completed, checked and signed off by the MWD surveyor, Directional Driller and client representative in each case. In cases where D&M are providing survey management services, or the well is at a position critical stage [such as a high well density anticollision monitoring area] then it may also be necessary to transmit each completed reference sign-off sheet in to the OSC Manager or the Survey Specialist. 4.4.3

Nonmag Collar Inspections for Magnetic Hotspots

All MWD and nonmagnetic drill collars (NMDC) and components must be inspected regularly for magnetic hotspots. Inspection certificates for all nonmagnetic equipment at the rigsite must be on file in the repair and maintenance office. Collars must be cleaned thoroughly and maintained as required by the appropriate maintenance procedures. Any collars that are repaired via a welding operation, or have threads re-cut will be re-inspected and recertified for magnetic hotspots. All rental NMDC components must be furnished with a contractor certification for magnetic hotspot inspection.

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pre-job (DNIInits) data sign-off sheet is required to be completed, which contains all of the necessary

Drilling & Measurements Procedures

4.4.4

Wellbore Surveying

MWD Field Acceptance Criteria

The field acceptance criteria (FAC) reference values provided by the Geomag program are the primary wellsite survey quality control metric for MWD surveys. Where magnetic surveys are out of tolerance, these reference values must not be arbitrarily adjusted. There are very few occasions where the Geomag outputs are at odds with the actual field location values, and this is typically at higher latitudes and where the presence of unmodeled local magnetic anomalies can affect tool readings. In most cases the surveys concerned are found to be out of tolerance for another reason; this is usually found to be either the presence of external magnetic interference, which can result in a well collision, or drillstring interference, which can cause the drilling target to be missed if not corrected for, and in either case a failure to meet the well positioning objectives. In the past, the field calibration procedure did not involve the use of a total field proton magnetometer with the calibration stand assembly. A practice was used whereby the FAC could be manually adjusted at the wellsite based on the first five to ten good MWD surveys. This practice is no longer approved or necessary, and since the calibration technique has been improved with the mandatory use of the proton tolerance, this is the first warning that the engineer must make further efforts to find the root of the problem. In most cases, assistance in doing this may also come from the OSC or the Survey Specialist, who must be consulted by line management, and to whom all relevant data should be sent for analysis. Further details for specific wellsite practice on the use of FAC are contained in section 5.

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magnetometer. Generally, if the tool produces apparently ‘bad’ surveys because they are out of

Drilling & Measurements Procedures

4.4.5

Wellbore Surveying

MWD Benchmarks and Checkshots

The purpose of benchmarking is to ensure that successive MWD runs can be positively shown to be providing comparison surveys within expected specification. This is a fundamental quality control measure to ensure continuous service quality, as well as a means to satisfy the redundant surveying principle. On conducting the shallow hole test to ensure tool functionality, nothing useful is usually known about the survey measurement at that point other than the fact that the sensors are providing a reading which is affected by external magnetic interference. It is desirable therefore to make a quality check of the survey sensors at the earliest opportunity when clear of external interference, or on each occasion that the BHA is tripped out of hole and the BHA is changed (and therefore the drillstring magnetic signature of the BHA has changed) and/or where the MWD tool is changed out. Further details on benchmark and checkshot procedures can be found in section 5. 4.5

Gyro and EMS Testing and Calibration Procedures

follow rigorous and complex calibration and calibration-check procedures on their survey instruments. In most cases this requires the use of high accuracy test stands, and dedicated high cost facilities in order to provide and meet the demands of these services. Gyro sensors will receive a pre-job calibration check immediately prior to being sent to the field, and again immediately after their return from the job. This is made possible by the relatively short term in which the tools are actually at the rigsite (usually as a result of high day-rate charges for tool hire), and their size and mobility allowing the instruments to be transported fairly easily. At the rigsite, the gyro surveyor will complete a wellsite information sign-off sheet (much like a DNIInits sign-off sheet) containing all of the reference details for the well to be used to calculate the final survey. The surveyor will conduct a readiness check on the survey equipment, run the job, and subsequently complete a wellsite QA/QC report that must also be signed off. An ‘end of job’ report is usually produced by the gyro company which provides a statement of the completed quality control loop; that of calibration check, approved survey obtained, and calibration recheck successfully completed. If, at any time, some aspect of this QC loop is not confirmed or is found to be out of specification, then this must call into question the veracity of the entire survey, and is therefore a violation of the survey program until resolved.

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The gyro companies [who also provide EMS surveying services in broadly the same manner] generally

Drilling & Measurements Procedures

4.6

Wellbore Surveying

Responsibility for Contractor Survey Quality Control

The impact of subcontractor surveys can be critical to the success of Schlumberger’s provision of directional services. The OSC Manager is responsible for ensuring that all subcontractor surveys conducted in any well design being executed, whether contracted directly by Schlumberger or not, conform to the requirements of these procedures. The OSC Manager or the Survey Specialist shall determine if a contractor survey is in violation of any aspect the accepted running procedures for that service, or the survey program itself, and shall recommend any corrective action required to the client and to D&M line management. The OSC Manager is also responsible for investigation into any instances of reduced quality surveys and shall assess their impact upon the survey program, invoking any contingency that may be required to maintain the integrity of the survey program. Where the integrity of the survey program has been breached or is in question, drilling must cease until the impact on the positional objectives of the well have been established and a solution or contingency invoked which will retrieve the situation.

must each be faxed to the FSM or OSC in town immediately upon completion, so that they can be checked and reconfirmed in good time for correction or to re-run a failed survey.

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For all D&M wells, the survey subcontractor wellsite information sheet and the wellsite QA/QC sheet

Drilling & Measurements Procedures

4.7

Wellbore Surveying

Contractor Survey Execution Procedure Flowchart Survey Contractor

Survey Equipment Called Out

D&M OSC Manager

Calibration Check OK

Tools Ready Calibration Checked

Calibration Facility Audit

Written Procedure for Job

Review Procedure

Brief Personnel

Specify Special Requirements

No Yes

Repair and Maintenance

Yes

Ship Equipment To Rigsite

No

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Recalibration Possible No

Rigsite Equipment Check

Replace Survey Tools

Wellsite Checks OK

No

Onsite Repair Feasible

Yes No Execute Job

Yes

Yes Assess Impact to Results

No

Onsite QC Checks OK

Complete Survey Acceptance

Yes Correction Acceptable

Yes

Fax Data/QC to Survey Specialist

Check Post Job Calibration

Issue Temporary Tie-on Point

Survey History Sheet Entry

Tools RTB Post Job Calibration

Database Update

No Backup Instrument Available

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5

Wellbore Surveying

Magnetic Surveying at the Rigsite

In this section; •

MWD field acceptance criteria (FAC).

•

Benchmarking and checkshots.

•

Planning for external magnetic interference.

•

Dealing with drillstring magnetic interference.

5.1

Rigsite Survey Acceptance of Magnetic Surveys

For magnetic surveys, the rigsite survey acceptance is usually based on the magnetic survey data having met the field acceptance criteria when compared with the expected reference values from the BGGM model and the gravity calculation model contained in the surface acquisition system. Once the surveyor produces the pre-job reference sign-off sheet and it’s correctness is confirmed, sufficient information is surveys. For EMS tools, on completion of the survey the tool memory is downloaded and the surveyor then matches survey depths from his tripping worksheet to survey times given by the tool memory clock which assigned times to each survey station recorded. The surveyor chooses the best quality survey point for each depth surveyed and can then output a survey quality control report which gives the ‘spread values’ in Total G (gravity field), Total B (magnetic field strength) and Dip Angle (magnetic field vector) measurement summary for all accepted surveys. Provided that the observed spread values fall within the allowed quality control spread values then this validates the use of the EMS survey to satisfy the requirements for that part of the survey program. The typical spread values allowed for an entire dataset from a ‘Champ’ type EMS tool are; 7mG in gravity field, 700nT in magnetic field strength and 0.7° in Dip Angle. For MWD tools a similar validation is conducted in real time during drilling against these field acceptance criteria (FAC) for each MWD survey station as it is obtained. In the case of Schlumberger MWD tools the tolerance limits for FAC for each individual survey station are; 2.5mG in gravity field, 300nT in magnetic field strength and 0.45° in Dip Angle. MWD surveys that do not meet their field acceptance criteria must not be accepted as standard MWD surveys as required by the survey program (see section 4).

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now available from this sheet to provide rigsite quality control checks on the subsequent magnetic

Drilling & Measurements Procedures

5.2

MWD Running Procedures

5.2.1

Software Initialization

Wellbore Surveying

In order to complete the MWD survey initialization the surveyor must enter the required direction and inclination (D&I) initialization data in the surface system computer. The version of the BGGM used must be the most recent available or it’s immediate predecessor if within six calendar months of reissue of the model (see section 2.8). These values of |G|, |B|, Dip and Magnetic Declination must be used as reference values and for survey acceptance purposes unless geomagnetic referencing services are being provided and additional written instructions to update the FAC for the local crustal magnetic field have been received from the DEC or the survey specialist. Evidence of local variations to the BGGM reference values and confirmation of the correct values to be used must be sought from the DEC manager or the survey specialist where the standard model values are not used, or are found to be apparently invalid for any reason. The most common cause of gross error in magnetic surveying is in the incorrect use of reference values or their incorrect application. The grid convergence angle may be provided by the Directional Driller, but in all cases must be confirmed grid convergence calculation software. Further information on the derivation and use of grid convergence and declination can be found in section 2.7. The initialization process is complete when the sign-off sheet has been checked and signed off by all responsible parties at the rigsite. 5.2.2

DNI Initialization At the wellsite, the MWD engineer must run Geomag from surface system software IDEAL to obtain well data for the D&I initialization. The MWD engineer clicks on “D&I Init” from “TOP CP” and the following window appears.

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by the DEC manager or the survey specialist and by checking against the definitive survey database or

Drilling & Measurements Procedures

Wellbore Surveying

5.2.2.1 Geomag Inputs To compute these values, Geomag needs the following input data for the wellbore location; •

Elevation - This is the elevation above mean sea level.

•

Latitude - This is the latitude in decimal degrees. Positive degrees for the northern hemisphere and negative degrees for the southern hemisphere.

•

Longitude - This is the longitude in decimal degrees. Positive degrees for the eastern hemisphere and negative degrees for the western hemisphere.

•

Date - When you open the Geomag panel, every field is blank except the date field, which contains, by default, the current date.

•

Check that the date is correct. The correct date is important because the earth's magnetic field changes over time. The wrong date can cause Geomag to calculate incorrect values.

After clicking on “Run Geomag” as highlighted above, the following Geomag Control Panel window will appear;

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Compute: Click “Compute” in order to get output. Geomag calculates well data from these inputs when the MWD engineer clicks Compute. The calculated well data is used as a quality check for downhole survey data.

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Wellbore Surveying

5.2.2.2 Geomag Outputs •

Location G (the gravitational force)

•

Location H (total magnetic field strength)

•

Magnetic declination

•

Magnetic dip angle

•

Deviations/year for the magnetic field outputs

•

Deviations/year for the magnetic field outputs

These outputs are date sensitive.

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After running Geomag, double-check the following: •

The magnetic declination - It should equal what is listed on the well plan. If it does not, verify the actual magnetic declination of the location with the client and directional driller.

•

The location - Verify that you entered the correct sign for latitude and longitude.

•

Click “apply” and “OK” to save the Geomag output to DNI Inits and return to DNI Init Panel.

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Wellbore Surveying

5.2.2.3 Grid Convergence & Tool Face Correction angles After running Geomag, the outputs are displayed in their respective fields on the D&I Init panel as shown below.

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If the client has chosen to reference the survey data to grid north, then they will supply the correct grid convergence angle. If the reference is to true north, then grid convergence needs to be set to zero. After the MWD engineer enters the angle, the software computes total correction as follows;

Total Correction = Magnetic declination - Grid Convergence. Lastly, the MWD engineer enters the Tool Face Correction angle for the BHA to be used in the run. The angle should be entered before data acquisition is started because it is used for tool face computations. There is no default value. If a wrong value is entered, the wellbore will be steered in the wrong direction. Click on “save” and then “exit”. A file named “DNIInits_1.TXT” is created in the well folder. The field engineer will need to print this page and get it signed off before faxing it to town (OSC and FSM) for crosschecking and record keeping.

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Drilling & Measurements Procedures

5.2.3

Wellbore Surveying

Tool Face Correction Measurement

The field engineer measures the Tool Face Correction arc and the circumference of the MWD collar. The arc is measured from the MWD scribe-line to the Mud motor bend, clockwise looking downhole. When running Rotary Steerable directional tools, a toolface of 0 may be entered since no toolface measurement is taken or needed. The Tool Face Correction angle = (arc / circumference) * 360°. 5.2.4

Shallow Hole Test

When conducting the shallow hole test (SHT) to check the MWD tool functionality, make sure the reference |G| satisfies the acceptance criteria established in section 3.5.5. Unless this test is conducted in open hole a sufficient distance below the rig floor then the reference |B| and Dip values must be disregarded due to the presence of external interference. 5.2.5

Recommended Roll Test Once Clear of Casing

an unexpectedly large dependence on toolface. Since the test will take twenty minutes to complete, not all clients will agree to provide the required rig time. For this reason this test is not a mandatory requirement, but is a quality control check that can be conducted as an additional check. The roll test consists of a set of four rotation surveys taken at the same depth (+/- 1 meter), one in each quadrant as shown in the table below. If the formation is soft and washouts are a concern, the surveys may be taken at different depths but try to keep them as close together as practical. This test must be conducted at least 50 meters away from the last casing shoe or any other potential sources of external interference such as adjacent wells. Verify that the all the surveys satisfy the field acceptance criteria in section 3.6.6 below. 5.2.6

MWD Field Acceptance Criteria

The acceptance criteria for all Schlumberger MWD surveys are as follows: |G| = Reference +/- 2.5 mg |B| = Reference +/- 300nT (6 Counts) Dip = Reference +/- 0.45° The reference value for |G| is computed from the latitude of the rigsite. The computation is available on the D&I calibration system computer or D&M surface acquisition systems.

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A roll test is recommended at the beginning of each bit run to check that the survey results do not exhibit

Drilling & Measurements Procedures

5.3

Wellbore Surveying

MWD Benchmarks and Checkshots

The purpose of benchmarking is to ensure that successive MWD runs can be positively shown to be providing comparison surveys within expected specification. This is a quality control measure to ensure continuous service quality, as well as a means to satisfy the redundant surveying principle. On conducting the shallow hole test to ensure tool functionality, nothing useful is usually known about the survey measurement at that point other than the fact that the sensors are providing a reading which is affected by external magnetic interference. It is desirable therefore to make a quality check of the survey sensors at the earliest opportunity when clear of interference. Should a sensor problem be detected, it is less desirable to have made a long trip to bottom before discovering this than it would have been by conducting this simple check at a shallower depth. In addition, if the section of open hole traversed is extensive, it is also desirable to take an ‘off bottom’ checkshot against the previous run prior to drilling ahead, to reconfirm the hole direction and continue to satisfy the redundant surveying principle. A benchmark is established when MWD surveys at the same depth from two independent tools are compared, and some estimate is made to weight the tolerance criteria between those two surveys based on geomagnetic location, orientation, BHA configuration and sensor errors. This can be done using the hole condition allows. The depths chosen to carry out checkshots should also be selected so as to minimize potential hole problems. Secondarily, it is preferable to obtain benchmarks at depths or areas of lower dogleg where possible, so as to obtain the best comparison survey. On the first trip in a new hole section, a checkshot should be taken as soon as the MWD tool is clear of external magnetic interference from the previous casing shoe (or adjacent wells or fish). Then, for a repeated trip in the same section with a new MWD tool or drilling assembly, a checkshot should be taken against the first run, and a benchmark calculated which will weight the two surveys accordingly. In either case this is unlikely to be nearer than 50m to the last shoe. On future trips in the hole repeated checkshots at this depth should fall within the tolerance values given from the original benchmark result. Once the open hole section exceeds 1000m (3000ft), a second checkshot (and repeated benchmark calculation) should be carried out, off bottom, before drilling ahead. This second benchmark should be used for successive checkshots with the benchmark position being extended with each 1000m of progress accordingly, and a new benchmark being calculated each time. In cases where a repeated checkshot at the first depth clear of the shoe is ‘drifts’ out of tolerance but all of the other tool and FAC parameters are acceptable, it may be that the shape of the hole at the upper checkshot depth has changed since the first run (e.g. as a result of washing out, having been reamed or from extended circulating). In this case, the upper benchmark may be recalculated by moving to a deeper position in the well, provided that the second lower benchmark remains within tolerance. In any case the principle of ensuring that the FAC for successive MWD runs as soon as clear of external magnetic interference should always be maintained.

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Survey ToolBox software and the prerequisite for any checkshot or benchmarking survey is of course that

Drilling & Measurements Procedures

Wellbore Surveying

An alternative to this procedure of moving the upper benchmark may be to take a clustershot or rotation survey of at least four surveys equally spaced in toolface, and resolve out a more accurate checkshot survey at the first benchmark point. This is technically the preferred procedure because it also satisfies the roll test recommended in section 3.6.5, but may not be practical if the client is reticent about spending additional rig time for the rotation survey even though it may in the end be more effective than having to make a full trip out of hole. In any case where the engineer suspects a tool problem from carrying out this procedure, he should proceed as directed by the UOP or ORM in force. This should be done before tripping to bottom, where the second benchmark should be used only to confirm that correct operation of the tool has been established. Where a second trip in hole with a new MWD tool replaces a contingency EMS survey in the survey program for that hole section, a third benchmark at an appropriate depth would be expected to justify the saving made. This will only be required where three separate benchmarks have not been calculated already as a result of following the above procedure. Planning for External Magnetic Interference

MWD surveys that fail to meet their FAC quality control metrics cannot be accepted or used for the purpose of well collision risk management. At best, if the gravity reference values are within specification, these surveys could be used as inclination only surveys. At worst, and since we are probably dealing with interference from other nearby wells, this could result in a well collision, and a potentially catastrophic failure. MWD sensors use the earth’s magnetic field as a reference and surveys are calculated from these measurements based on the underlying assumption that the effects of other sources of potential magnetic interference are minimized to the point where we can reasonably estimate the accuracy of a standard MWD survey. The FAC metrics are used as a basic ‘first pass’ quality filter that allows the engineer to quickly recognize a problem (or the potential development of one).

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5.4

Drilling & Measurements Procedures

5.4.1

Wellbore Surveying

MWD Surveys that Fail Field Acceptance

When an MWD survey fails the magnetic field quality requirements of the FAC metrics, then the cause of this can be as a result of; 1) The tool has made an erroneous measurement. •

There is a tool sensor problem, or a sensor has failed,

•

There is a failure condition in the tool circuitry that is causing a local magnetic disturbance field near the sensor cartridge (e.g. a ground loop).

•

There is a tool calibration problem; the tool has been loaded with the wrong calibration, or the calibration is invalid.

2) The measurement environment is not what we expect. There is external magnetic interference from one or more nearby wells or objects (e.g. the rig, anchor chains, sunken wrecks or junk in the well),

•

The is crustal magnetism present in the surrounding rock formations that is not accounted for in the reference field model,

•

There is extreme magnetospheric activity taking place as a result of a solar magnetic storm, and the subject well is at high latitude,

•

The surface system has been incorrectly initialized, or the rig location, date or surface elevation entered are incorrect,

•

The reference field model is wrong.

Clearly, the field engineer can quickly check some of these failure conditions and this should be taken care of as a matter of course. Some of these conditions are also easily ruled out where consistent checkshots have been made across MWD tool run boundaries, or where redundant surveying has been used as an additional quality control metric. In high well density locations it is also easy to check whether MWD surveys from previous runs (even those from offset wells), have met their FAC, and whether those FAC correlate to the values in use for the current run. The most likely problem in our scenario is the presence of external magnetic interference and we can assume, as is most often the case, that the MWD surveys are out of specification and that gyros are being run until clear of interference.

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•

Drilling & Measurements Procedures

5.4.2

Wellbore Surveying

The Effect of Magnetic Interference from Offset Wells

The effect of external magnetic interference on the MWD surveys follows a fairly straightforward inverse square law. For a given field strength of interfering magnetic pole the effect on the survey sensor is a function of one divided by the square of the separation distance.

Interference effect = Pole strength x (1/Distance2) What this means is that if there are two nearby offset wells, that are say 2 feet and 10 feet away from our subject well respectively, and they share the same magnetic pole strength (of say, one unit of interference), then they will affect our MWD surveys at this point by; •

1/22 = 1/4 = 0.25, and

•

1/102 = 1/100 = 0.001

So, the well that is only 2 feet away will affect our magnetic surveys by a factor of 250 times more than combination as an overall interfering field, this can be estimated as the root sum of these two numbers;

Combined effect of external interference = Pole strength x √(1/W12 + 1/W22 … +1//Wn2) Again, using our example here, this would result in; •

√(0.252 + 0.0012) = √0.062501 = 0.250002

In other words, the external interference from the well which is only 2 feet away is driving the overall external interference effect, since using the inverse square law makes the effect of the proximity of the 10 feet away well significantly less. However, this example was given for the sake of clarity. In reality when we have significant interference from multiple wells, one of which is say, 6 feet away, and with say, five other wells that are 8 feet away, then this might mistakenly make us think that we should only care about the single closest well. •

√(1/36)2 = 1/36 = 0.027778

•

√((1/36)2 + (1/64) 2 + (1/64) 2+ (1/64) 2+ (1/64) 2+ (1/64) 2) = √0.0019923 = 0.044635

This demonstrates the idea that whilst the nearest source of external interference definitely contributes the greatest effect on the MWD sensor, the presence of multiple nearby wells can significantly increase this effect (in this simple example, by more than 60%). Of course, we made an underlying assumption with this approach that each of the offset wells contributes an interfering field of equal magnetic field strength, and this will be discussed further below.

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the well that is 10 feet away. Not only that, since they do not affect our MWD sensors in turn, but in

Drilling & Measurements Procedures

5.4.3

Wellbore Surveying

Magnetic Interference Scan report

When an anticollision scan is run against offset wells, one of the calculations that are done is a ‘behindthe-scenes’ root sum of the center-to-center squared distance. In other words, the exact calculation shown in the example of the previous section, for each anticollision scan interval. This information is tabulated in the “Magnetic Interference Scan Report” or ‘MagScan report’ in the Close Approach module of Drilling Office. The report gives standard header information on the subject well with a list of each of the offset wells that have been included in the combined scan, and finally the tabulated results of the magnetic scan calculation as shown in the example below: Magnetic Scan Results CC-1 Survey TVD (ft) 0.00

DISTANCE (ft) 2.08

100.00

100.00

2.08

200.00

200.00

2.08

300.00

300.00

2.08

400.00

400.00

2.08

500.00

500.00

2.08

600.00

600.00

2.08

700.00

700.00

2.08

800.00

800.00

2.09

This example output shows the measured depth and TVD of the subject well versus the equivalent combined magnetic clearance scan distance of all of the scanned nearby wells (i.e. as if all of the magnetic interference were going to come from a single well at this equivalent distance). In theory, we would want to ensure that all possible nearby wells are included in this scan, but in practice for this application our magnetic clearance scan report containing at least all of the wells present from the platform, rig or installation that the subject well is being drilled from will probably be sufficient for these purposes.

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MD (ft) 0.00

Drilling & Measurements Procedures

5.4.4

Wellbore Surveying

DMAG –Drillstring Magnetic Interference Multistation Correction

The DMAG software application is an MWD survey correction platform that is designed to correct for drillstring magnetic interference. In this case (where the source of the interference is external), DMAG is unable to provide a useful solution to the problem, and gyro surveys must be used until clear of magnetic interference. However, DMAG can be effectively used to calibrate the magnetic interference scan report for a specific location. This is simply done by gathering the standard raw MWD surveys for the current run in the normal fashion until clear of external magnetic interference, and the MWD surveys are now within specification (or where the only remaining problem with them is drillstring magnetic interference, which DMAG will take care of). Having reached this point in the well, DMAG will be unlikely to provide good corrected surveys as a result of just ‘dumping’ in all of the raw surveys for the current run. This is because there are too few good surveys, and DMAG will not work unless the input dataset contains a predominance (i.e. more than half) of clean ‘drillstring interference only’ surveys. In practice the engineer should begin processing using DMAG from the current point in the well, and good corrected survey outputs. In our first case study, where external interference from offset wells is the problem, it can be clearly seen from DMAG when this point is reached. 50000

External Magnetic Interference Case Study 1 External magnetic interference decreasing as effective magnetic clearance scan distance increases

49000

Total Field Strength (nT)

49500

48500

Reference Field Strength 48000

+/- 300 nT FAC

Measured Depth (ft)

47500 2300

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3300

3800

4300

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working backwards up the well (shallower) adding surveys for as long as DMAG can continue to produce

Drilling & Measurements Procedures

Wellbore Surveying

In the example shown above, external magnetic interference can clearly be seen at 2300ft measured depth as the total field measured by the MWD sensor approaches the FAC reference value with increasing well separation as drilling continues. In this case, the presence of some drillstring magnetic interference is also indicated beyond about 3300ft MD, where we can estimate ‘by eye’ that we are probably clear of the external interference effects. In this case, DMAG is used to correct the surveys for drillstring magnetic interference beginning from the deepest survey, and working back up the hole, by adding successive surveys, until a point is reached where the correction software cannot successfully reprocess them. From the plot shown below, it can be seen that this point is reached somewhere around 3000ft MD. What this means is that shallower than this depth the MWD surveys are too badly interfered with by external magnetic interference that they cannot be successfully corrected. 50000

External Magnetic Interference Case Study 1 External magnetic interference decreasing as effective magnetic clearance scan distance increases

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49000

Total Field Strength (nT)

49500

DMAG corrected surveys - processed in batches from the deepest survey by adding more survey data working back up the hole, until as many good surveys as possible are obtained.

48500

Reference Field Strength 48000

+/- 300 nT FAC

Measured Depth (ft)

47500 2300

2800

3300

3800

4300

It should also be noted at this point that this same technique could be used in single well applications to estimate the clearance distance required from exiting the last casing shoe before the MWD surveys will be clear of external interference from the casing. (This is not discussed further here.)

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Wellbore Surveying

It is important to understand some aspects of how DMAG works in order to be able to make an assessment about the validity of the results. In many cases the entire group of surveys shown in the plot above can be input into DMAG and numerous apparently ‘good’ surveys obtained from the results, from positions through the run. However, this is unlikely to include any (or many) of the very last (deepest) surveys in the group, in which we have the greatest confidence of being good, and this is a good indicator that DMAG has found the ‘wrong’ answer. The only way to successfully ensure that the best, and most correct surveys have been obtained in this example is to work from the deepest surveys first, and experience has shown that the results will be excellent whilst working back up (shallower) the hole until a point is reached where the surveys just can’t be effectively corrected and the output from DMAG will appear to ’flip’. What this means is that up until now we will have obtained almost all good corrected surveys until we reach this turnover point. At this point the next batch of DMAG processed data, with even one more ‘bad’ survey included for processing will cause the software to fail to reach the correct solution. At this stage it may even report some previously ‘good’ surveys as ‘bad’ and vice-versa. Calibrating the Magnetic Interference Scan

Now that we have the largest possible group of DMAG corrected ‘good’ surveys, working back up the hole from a point where we were confident that we were clear of any external magnetic interference effects, then we can look at the measured depth of the last good corrected survey to calibrate the magnetic interference scan report. 50000

External Magnetic Interference Case Study 1 External magnetic interference decreasing as effective magnetic clearance scan distance increases

49000

Total Field Strength (nT)

49500

DMAG correction back calculated until surveys can no longer be brought into specification, indicates minimum magnetic clearance scan point (3080ft MD).

48500

Reference Field Strength 48000

+/- 300 nT FAC

Measured Depth (ft)

47500 2300

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3300

3800

4300

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5.4.5

Drilling & Measurements Procedures

Wellbore Surveying

At this stage it is best to re-run the anticollision scan using all of the good actual survey data obtained for the well to date. In our multi-well installation example, this will include the gyro singleshots, and the corrected ‘good’ DMAG results. On running the anticollision scan on this data, the magnetic interference scan report can be generated. In order to provide a more useful graphical (visual) display of the magnetic scan data an excel spreadsheet can be used to plot the results of the magnetic scan report.

This was done using the results for the actual surveys from case study 2, comprising the gyro surveys plus the good DMAG results as follows; 80

Magnetic Scan Plot - Case Study 2 70

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50

Effective Magnetic Clearance (ft)

40

30

20

10

Measured Depth (ft) 0 0

500

1000

1500

2000

2500

3000

3500

In this case it can be seen that the effective magnetic clearance distance is less than 10ft until we are at about 1500ft MD in our well. In a sense this plot shows a graphical summary of the anticollision scenario, although not in a useful format for that application.

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Drilling & Measurements Procedures

Wellbore Surveying

In order to calibrate the magnetic interference scan, the survey data was reverse processed as described above using DMAG and the following results were obtained;

Therefore, in this case we were unable to back process any good DMAG surveys shallower than 2762.95ft MD. By using this depth, and our magnetic interference scan plot, we can now calibrate our magnetic interference scan results for this location. The plot below shows the magnetic interference scan report with a MD marker placed at 2672.95ft MD; Schlumberger Confidential

80

Magnetic Scan Plot - Case Study 2 70

60

Effective Clearance Requirement for this Location = 50ft

50

Effective Magnetic Clearance (ft)

40

30

20

10

Measured Depth (ft) 0 0

500

1000

1500

2000

2500

3000

3500

By subtending a horizontal marker from the intersection point on our interference scan at 2672.95ft MD, we get an effective magnetic clearance of 50ft. This is now a locally calibrated value for this location that can be applied (and improved by being updated with successive well data) for practical purposes.

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Drilling & Measurements Procedures

Wellbore Surveying

In the planning sense this value can be applied to future anticollision scans of plans versus offset wells to indicate the expected MD to which gyro surveys will be required. This information can be applied to the survey program, and also used to plan service costs and operational requirements for these services. At execution time, this information can be used as the well is drilled to estimate actual clearance versus planned clearance and to update the expected gyro surveying service requirements against the plan. Provided the same technique is applied consistently, the effective clearance requirement for this location can also be updated and modified as time and circumstances perhaps, affect the magnetic characteristics of the well casings and local external interference levels.

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Drilling & Measurements Procedures

5.4.6

Wellbore Surveying

A Note About the Underlying Assumptions

There are a number of underlying assumptions that must be used in order to make this technique of practical use; 1) The magnetic field strength of the affecting offset well conductors is the same for all offset wells (but will be different from location to location). •

This may not be true if different sized conductors are used in different slots on the installation.

•

Experience suggests that this is generally true where the conductor size is the same for a given installation.

2) The effective clearance required is different from installation to installation in different locations. •

Experience has shown that this is also generally true. The effective clearance requirement for one smaller platform in a mid-latitude northern hemisphere field was 33ft with 18-inch conductors.

•

Our example case study 2 shown here, is for a large platform in an equatorial region, using larger conductors, where the effective clearance requirement is 50ft.

•

This is true, and attempts should not be made to keep reverse processing random groups of surveys going back up the hole where DMAG could derive an incorrect solution and falsely report some bad surveys as ‘good’.

•

Care should be taken even when external interference is present and the standard MWD surveys are now drifting into specification. This does not necessarily indicate that the survey is clear of external interference, since this could be the combined effect with drillstring interference, or that some of the allowances made for the FAC for the GeoMag field uncertainty are being incorrectly applied as an allowance for external magnetic interference. This is in fact one of the ‘use-and-practice’ assumptions made when using the FAC as a quality metric when clearing external interference.

DMAG is the only consistent method to manage clearing external magnetic interference.

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3) DMAG can provide a useful measurement as to when the threshold for good MWD surveys has been reached.

Drilling & Measurements Procedures

5.5

Wellbore Surveying

Drillstring Interference Correction Algorithms - Assumptions for Use

The use of any drillstring interference correction algorithm comes with a number of qualifying assumptions; •

These algorithms are unable to distinguish the presence of external magnetic interference from nearby wells, casings, fish in the hole etc. Unless there is a predominance of data in the set that does not contain external interference, the algorithm will incorrectly attempt to resolve this interference into a drillstring interference problem.

•

The main field model (Geomag)) parameters for the location are accurate to within the residual uncertainty allowed for by the positional error model used to depict the uncertainty in the corrected surveys, and that local crustal affects are not invalidating the main field model.

5.6

Drillstring Interference Correction Algorithms - Invalidating Factors

A number of conditions may exist either individually, or concurrently, which may invalidate or adversely skew the results; Local crustal magnetic effects in the formations

•

Disturbance field effects (magnetic storms)

•

Inclusion of 4/5 axis or Hx bias offset corrected data

•

A tool failure condition

•

Tool out of calibration (SUCOP/MagCor only)

•

Insufficient 6 axis data in dataset (DMAG only)

•

Data from different BHA runs included for processing in the same dataset (DMAG only)

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5.7

•

DMAG Correction for Drillstring Magnetic Interference

The DMAG drillstring magnetic interference correction algorithm is a multistation analysis technique that uses the main field BGGM model as a local reference, and performs an optimization analysis on a group of MWD survey points. The DMAG algorithm is trying to solve a multiple optimization problem to find the best fit for the six scale factors, six biases, total B and Dip angle (in other words 14 parameters) for the entire dataset submitted for correction. It will provide drillstring interference corrected surveys and a quality indicator, and the engineer can use these results to calculate a drilling azimuth correction. All azimuth comparisons are based on standard MWD surveys corrected for declination (and grid convergence if applicable) only. Detailed running procedures for the use of DMAG are contained within the help files and documentation for that application. In ANY given scenario, DMAG provides a better answer than a single station algorithm (such as MagCor/SUCOP) or a simple Hx offset correction. Single station correction methods cannot be used at high angle or within fifty degrees of East/West because they result in a larger residual uncertainty after correction than before.

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DMAG does not suffer from this restriction directly, but this benefit does not come without a price. The orientation, change in orientation throughout the dataset, number of surveys and the quality of the reference data will all drive the final residual uncertainty in the results from the algorithm. With wider release and more direct use of DMAG by the field, this means that we must raise our awareness and understanding of the strengths and weaknesses of DMAG, and at least know when to call the Survey Specialist. As suggested by many field reports, the algorithm is best placed to come up with a good correction when there is a significant amount of data, and when there is also a large orientation change through the dataset. The reason for this is that the algorithm is able to establish the cross-axial elements of the drillstring effects, and modeling of the axial effects is fairly straightforward. Having sufficient quality data is often not the case in reality, especially in situations when we care most about the correction, and therefore we have to have some other way (such as detailed procedures or experience) to deal with these problems. The things that most commonly drive the algorithm to produce poor results are insufficient data and/or data with an insufficient amount of information in it (as a result of lack of orientation change) to effectively quality data from external interference will result in an invalid result and cannot be modeled effectively. Insufficient, but good data (in this case good means good total |G|, but maybe out of FAC as a result of drillstring interference only), will also result in a poor result, but in this case this will be represented in the magnitude of the uncertainty output numbers. 5.7.1

Practical Application of DMAG

Studies have shown (SPE 87977) that using DMAG provides more accurate results than standard MWD (based on using the ISCWSA standard error model), and in some cases DMAG can also be used to good effect to compensate for a reduction in nonmagnetic spacing material usually required in the BHA to isolate the MWD sensors from the effects of excessive drillstring magnetic interference. 5.7.2

Filtering out Bad Surveys From DMAG Input Datasets

DMAG is a multistation analysis optimization algorithm that is trying to resolve out the drillstring magnetic interference based on the observed behavior of a group of surveys. In principle, the larger the group of input surveys available, the better the results will be.

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model the BHA, or the presence of external magnetic interference in the data. A predominance of bad

Drilling & Measurements Procedures

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However, the precondition is that there must be a predominance of good surveys in the input data set for DMAG to find the answer. In this case a 'good' survey means a survey that may be bad, but that the only thing 'wrong' with it is drillstring interference. Where this is the case, then DMAG is capable of automatically filtering out the obviously ‘bad’ surveys without any intervention. However, in borderline case, or where there is clearly a large proportion of bad surveys in the dataset, then the engineer must use some of the batch mode filtering tools to manually edit out some of the clearly unusable data. In most cases, it will be sufficient to filter out surveys that are clearly out of specification on total |G| because of tool movement, and surveys that clearly had external interference present (see above) and showed impossible values for total |B| and Dip Angle. 5.7.3

Number of Surveys Required to Obtain Good Results From DMAG

To ensure the best results DMAG needs a sufficiently large enough dataset to perform a robust multistation analysis. A good rule-of-thumb starting point is 8 'good' (as in good for DMAG input) surveys in non-challenging orientations, and up to 20 'good' surveys for the most challenging orientations.

inclination, azimuth or toolface change through the run. DMAG will work effectively even at the most challenging orientations with more survey data for input, and with increasing change in orientation over the run. For example, if the run starts at vertical and goes to horizontal due east within several hundred feet, and then remains at a tangent, then DMAG will find the answer to this very quickly, even if most of the data is horizontal because it has a large change in orientation over the run length to work with. This rule of thumb is only valid however, provided again that there is no external interference present, and there is a predominance of good data present in the input dataset. In other words if you have a large number of bad surveys in the data set and/or you also have external interference, then DMAG will require a much larger set of surveys to provide the best results. Often, this will be reflected in the residual uncertainty given in the output results for the processed surveys. It is also possible for DMAG to process even a single survey and provide a good result. In this case DMAG will apply a single station solution to the problem until a series of internal checks are satisfied and the software automatically changes over to the full multistation mode. A guideline formula that could be used to calculate the minimum number of surveys required is; •

8+ sinI*sinAm*12 (and round to the nearest number)

•

Where sinI = the sine of the Inclination and sinAm = the absolute sine of the Magnetic Azimuth

•

The inclination and azimuth chosen for this calculation should be the one that produces the greatest value for the orientation weighting (sinI*sinAm).

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In this case, the most challenging orientation is due magnetic east/west at horizontal with very little

Drilling & Measurements Procedures

5.8

Wellbore Surveying

Single Station Drillstring Interference Correction (MagCor / SUCOP)

Current versions of IDEAL are fitted with an integrated magnetic correction algorithm that is licensed by Shell, called Shell Survey Platform (SSP). Prior to this, previous releases of this same Shell algorithm, (called SUCOP) were run in standalone mode, after manually transferring data files to the Shell application from the surface system. The application was eventually integrated with IDEAL so it could be run online in real time. The Schlumberger name for the SUCOP/SSP algorithm in IDEAL is MAGCOR, and this is used today for all Shell jobs that specify a requirement for it. This magnetic correction algorithm is what is referred to today as a single-station correction method. The [SUCOP] magnetic interference correction method attempts to correct magnetic survey data for crossaxial and axial magnetic drillstring interference, bias of the cross-axial sensors and toolface dependent alignment errors. This is broadly achieved by applying the processed results from a rotation shot, or series of rotation shots, to each individual survey station measurement in addition to the application of the main field model data from the BGGM reference model. The major advantage of this technique is that prior to its introduction there was not any other practical rotation shots are used, they have the effect of weighting all of the single surveys by the results from a very small dataset obtained somewhere else in the well. In addition, the residual uncertainty as a result of applying this type of correction degrades very quickly with hole orientation approaching east/west and high angle. This means that in practice there is a very large no-go zone (stipulated by Shell at greater than 50˚ inclination and within 40˚ of east/west) where the uncertainty left over after correction is larger than not applying the correction and it therefore cannot be used when drilling in these orientations. For detailed information regarding the use and application of SSP for Shell jobs, refer to the documentation found in the surface system software SSP application. For all other non-Shell jobs, DMAG must be used as the primary correction method except where SSP is specifically requested by the client. In this case this must be arranged with D&M line management beforehand, who will deal with the issue of royalty payments to Shell that may be required for it to be used by the client concerned. 5.9

Correcting MWD Surveys for NMR Offset Effects

The Nuclear Magnetic Resonance (NMR) tool contains a powerful permanent magnet that is used to induce a magnetic field in the formations being drilled. When this tool is run with the MWD, it also induces a large drillstring magnetic interference component that must be corrected for. The surface acquisition system contains an NMR offset correction calculation that can be used for this purpose, and the EDI calculator in the Survey ToolBox contains a similar calculation module so that the engineer can estimate what magnitude to expect the effects of the NMR tool to have on the MWD azimuth error.

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method available to deal with this problem. The major disadvantages of this technique are that when

Drilling & Measurements Procedures

Wellbore Surveying

The most correct option for dealing with the NMR offset interference to the MWD surveys is by using DMAG to correct the surveys. DMAG will treat the NMR interference in a similar manner to the other drillstring interfering elements, and will produce the best answer overall. However, there is a problem in practice with knowing when enough surveys have been obtained for DMAG to produce a robust answer. Since DMAG requires a large enough body of data to make a good estimate of the interference effects, and the size of this body of data varies with both the general orientation, and the change in orientation through the dataset, it is difficult to place an absolute value on the required number of surveys. Experience with real data from jobs run in the field have shown that the EDI calculator and the NMR offset correction calculation in the acquisition system will produce a similar magnitude expected correction. When too few surveys are submitted to DMAG during the early part of the run the results can be seen to be apparently ‘unstable’ and the size of the correction ‘jumps’ around. It should be noted that this problem can also be alleviated by increasing the size of the DMAG input dataset by taking rotation shots, without necessarily having to have drilled ahead in the well. The best procedure to follow for running NMR with MWD and correcting for NMR magnetic interference is

•

Check the planned BHA during planning for the worst expected magnitude of the NMR correction using the EDI calculator. Plan to run DMAG and apply an early manual offset correction

•

When the actual BHA is made up at the wellsite, recheck the EDI estimate, and observe that the NMR correction being applied by the surface system is of an equivalent magnitude.

•

Using the minimum number of surveys required estimate formula in section 5.7.3 above, estimate the minimum number of surveys required by DMAG to get good results.

•

Once that minimum has been reached, export the raw surveys into DMAG and process the corrections in batch mode.

•

When the corrected survey results are stable and the DMAG results are returning good results, upload the batch mode surveys to the surface system and accept them, and the start running DMAG in ‘autorun mode’.

•

An alternative procedure that would achieve the absolute best results is to proceed with the automatic NMR offset correction until the end of the BHA run, and then submit the entire run for batch mode processing to get the best results from DMAG.

•

Replace the NMR offset corrected results with the DMAG corrected results. These are now the definitive survey results for that run.

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as follows;

Drilling & Measurements Procedures

6

Wellbore Surveying

Gyro Surveying at the Rigsite

In this section; •

Gyro survey field acceptance criteria.

•

Gyro survey running procedures.

•

Benchmarks and checkshots.

6.1

Rigsite Survey Acceptance of Gyro Surveys

In order to satisfy survey acceptance at the rigsite, each gyro survey service provider stipulates service specific procedures as to what constitutes a valid survey at the jobsite. This includes quality control listings of drift checks and/or bias updates to be completed within specification; a statement of repeatability of surveys at multi-survey sampling points in the well; and/or earthrate listings depending on the type of tool run.

checkshots against the main outrun survey, this gives both a gross check on the survey comparison as well as the depth accuracy in areas of build or turn. For continuous gyro surveys the most useful first check is a comparison plot between the inrun survey and the outrun survey to ensure that they track the same well trajectory, in addition to a statement of the closure between the inrun and outrun. This is a calculated distance between the inrun survey and the outrun surveys at the deepest survey depth. To some extent these are each calculated ‘independently’ of each other from the initialization point. A second level required check is that the quality statement about the initialization point has been completed within specification, as this is a critical quality control measure affecting the entire survey. Provided that the surveyor satisfactorily completes and signs-off a post survey quality control report containing sufficient evidence (as defined by that service provider) that the survey has met the required standard, then this is sufficient for the rigsite database to be updated; the survey program to have been confirmed, and for drilling to continue.

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The most robust check for northseeking gyros run in gyrocompassing mode is a comparison of inrun

Drilling & Measurements Procedures

6.2

Wellbore Surveying

Gyro Survey Running Procedures

All gyro surveys must be run in accordance with their accepted running procedures as provided by the gyro service contractor. General operating and reporting procedures and documentation should be made available by the gyro contractor to assist Drilling Engineers during the planning phases of the survey program. Job specific running procedures may also be provided by the survey contractor for more demanding or special projects. Where this is the case, these job specific procedures must be approved by the Survey Specialist or the OSC Manager in order for the survey to be accepted for use in the survey program. In all cases the accepted and documented standard running procedures for the survey instrument to be run will be required to be met for the survey data to be accepted for use in the definitive survey. 6.2.1

Pre-Job Checks

On arrival at the rigsite, the surveyor should check all survey equipment and make up the survey tool to be run in the well to ensure that all equipment is complete and functioning. If the survey is to be run on tool run up at surface through the wireline cable-head with a full test conducted of all surface equipment in the wireline unit. In addition, the wireline unit depth system should be checked and a valid and current calibration certificate for the depth measurement unit should be sighted. If the survey tool is to be run in battery or mode, then a full check of all surface processing equipment should be carried out as well as a check on battery charge status and memory settings. A brief survey memory test should also be carried out using backup batteries if possible. 6.2.2

Running Gear Assembly

The running gear should be made up and checked at the pre-job check stage, and then again in good time for the survey run. Careful attention should be paid to the correct tightening of all connectors, insulation of conductors and particularly the configuration and placement of all centralizers, decentralizers or other alignment devices to ensure that no systematic misalignment errors are introduced as a result of poor running gear configuration.

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electric wireline, then the wireline resistance and insulation to the conductor should be checked, and the

Drilling & Measurements Procedures

6.3

Gyro Orientation Operations

6.3.1

Survey Tool Rotation

Wellbore Surveying

During periods of anticollision monitoring where there is a risk of a well proximity problem as defined in the Standard Anticollision Procedures, or where a risk assessment based exemption is in force, surveys should be taken every 10m (30ft) or more frequently with either surface readout gyro or northseeking gyro. In order to protect against gross errors, particularly unrecognized calibration problems arising, the survey tools provided at the rigsite should be cycled at least every four runs in hole, or two stands drilled (whichever occurs first). A quality control process should be maintained which confirms surveyed position across tool changeovers, and the positive seating of the gyro in the Universal-Bottom-Hole-Orienting (UBHO) sub. MWD surveys should not be used in any situation where external magnetic interference is likely to be a problem, and where the surveys cannot meet their field acceptance criteria. Drilling must cease when any confirmed, until the problem is resolved. 6.3.2

Gyro Kick-off Procedure

In a surface hole kick-off BHA, the gyro tool will land out and be roughly centralized in the UBHO sub. However, the survey sensors will be subject to some 'misalignment' based on the difference between the outside diameter of the gyro and the inside diameter of the collar above the UBHO. Whilst there is some allowance for this built into the error models, for inclinations below 15 degrees or so, it is also feasible to run a stabilizer on top of the gyro tool which is fitted with rubber fingers or spring steelbows in order to both centralize and help stabilize the gyro tool collar. With an assembly that has a steerable bend in it, the survey results and variation in successive surveys at the same depth may be highly sensitive to toolface at these lower angles of inclination. This can be tested by running a clustershot (a series of singleshots) over at least four equally spaced toolfaces, and calculating the vector sum result using the Survey Toolbox application. The resultant calculated survey will approximate the axis of the borehole as opposed to the axis of the drilling assembly in the larger diameter hole. In very tight tolerance kick-offs it is common practice to take clustershots at every survey until the spread in azimuth over the four rotation singleshots is less than 2 degrees. Once the hole inclination reaches about 12 degrees (depending on the degree of bend in the assembly), it will no longer be practical to obtain a singleshot in each of the four quadrants because the assembly will tend to 'fall' over at highside and at lowside. Usually by this time the variations in the survey results has reduced sufficiently to resort to singleshots-only in any case.

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inconsistency in any recorded survey is observed, or benchmark comparisons at any stage cannot be

Drilling & Measurements Procedures

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Another problem can be caused by the drilling assembly 'waving' around in oversize hole, causing unnecessary gyro noise. This is more common where the assembly has also been rotated during drilling and the hole angle is at or near vertical, or at or near seabed. Where a northseeking gyro is in use, a solution to this is to seat the gyro in the UBHO and then sit the bit on bottom to take the survey. This usually requires a stick up of at least a single, and then a roughneck is required to be lifted up on a riding belt to place the gyro in the drillstring after the top-drive has been raised clear. The gyro is run to bottom, and then the bales are used to pick up the assembly and rest it briefly on bottom to take the survey. The precondition for resting the bit on-bottom is of course that the possibility of getting stuck is minimal. This practice has fallen out of use over time though, and an alternative is to use a gyro steering tool or wet connect system, or a modified NSG tool, but these options involve more complex running procedures. 6.3.3

Clustershot Calculation

A clustershot is the vector sum of a series of rotation surveys (usually from 4 to 8 survey stations) evenly spaced in toolface. The purpose of a clustershot calculation is to find the true wellbore axis orientation as a result of taking multiple surveys to minimize the effect of toolface misalignments. These misalignments the survey package to describe a cone shaped path as it is rotated in the wellbore. In some respects this can be considered to be the toolface dependent SAG angle effects, and taking them into account can often be critical to the success of a surface hole kick-off in a high well density area where the proximity clearances are very small. Clustershots should be used (or toolface dependency at least checked) in low angle hole below 15° inclination, where toolface dependent misalignments are most likely to affect survey readings. 6.3.4

Gyro Orientation Survey Accuracy

The gyro singleshot in the UBHO will not be as accurate as a cased-hole gyro multishot, and this is reflected in the error models for this running configuration. There are two issues with a 'noisy' gyro survey in the UBHO; first, the misalignment term is much larger, because of the poorer quality centralization; and second, the toolface dependent misalignment is larger because of the steerable assembly and the SAG correction for the gyro in the BHA. Gyro singleshots may be SAG corrected using standard SAG calculation software, but allowance must be made in these corrections to model the centralized at the bottom and not at the top misalignment.

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are most commonly caused by the steerable bend in the BHA assembly, which has the effect of causing

Drilling & Measurements Procedures

Wellbore Surveying

The overall azimuth error is also larger because of a lesser quality measurement. This is particularly true if the extended sample periods or averaging is used, or where GyroMWD is used and it must be clear that this is not as good a quality of azimuth measurement as a standard NSG survey. At low angles of inclination the azimuth error will not drive the position uncertainty, but the misalignment will. Experience has also shown that to prevent gross errors, and particularly to trap instances where the gyro tool goes out of calibration as a result of too noisy an environment, multiple gyro tools should be repeatedly cycled (see section 6.3.1 above). The service vendors will have vendor specific operating requirements for doing this, and so this procedure should be seen as a minimum quality control requirement. 6.3.5

Survey Frequency Related Issues

Where proximity clearances are tight, gyro singleshots may be required more frequently than normally expected if a very aggressive steerable assembly is used, or if less accurate gyro systems are in use. The reason for this is that there is a danger that the capability of the BHA may exceed the propagation size and rate of the survey errors, particularly at or near surface where this may have a critical effect. Therefore, the engineer needs to be concerned with how far to proceed with drilling before a survey is should drive the minimum survey frequency and should be checked at the planning stage, and during initial execution. Some guidance on this can be obtained from the BHA Survey Frequency plots given in the Survey Toolbox application. 6.3.5

Gyro Benchmarks and Checkshots

When gyro singleshots and orientations are being run then benchmarks and checkshots will be required across tool run boundaries to confirm survey acceptance. At low angles of inclination clustershots or rotation surveys should be taken to account for any toolface dependent bias that may be present when a steerable drilling assembly is used. When this is the case, then the last clustershot survey taken with the previous gyro tool will become the benchmark for running a checkshot with the subsequent tool run and until the next tool change-out occurs. Where the last survey run with the previous gyro tool is a singleshot orientation survey only (and not a clustershot), then this will become the benchmark for the subsequent tool run but some care must be taken to account for any toolface dependent misalignments caused by the steerable section of the BHA.

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required to absolutely ensure that a well collision is prevented as a result of insufficient surveying. This

Drilling & Measurements Procedures

6.4

Wellbore Surveying

Surface Hole Change Over From Gyro to MWD Surveying

In situations where external magnetic interference is expected on the MWD tool from one or more nearby wells, the only option available to ensure the avoidance of an unintended well collision is to run gyro singleshot surveys. During drilling, prior to any decision being made to halt taking gyro surveys, it must be confirmed that the MWD tool is clear of external magnetic interference. The only practical way to ensure that this is the case is by ensuring that the MWD surveys meet their FAC, in addition to direct comparison between the gyro and MWD survey results. In determining what the degree of correlation should be, some allowance must be made for the orientation, geographic location and BHA configuration (or amount of drillstring interference present in the MWD surveys), as well as the respective tool accuracies. This calculation can be done using the Survey Toolbox Benchmark Calculator. The correlation should be confirmed over at least two successive surveys and should meet the calculated specification provided by the benchmark software. In addition, it should be further confirmed that the subject well is positively diverging from any offset well before halting the use of gyro surveys, and the gyro survey equipment Schlumberger Confidential

should remain on standby until there is no further possibility of external magnetic interference.

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7

Wellbore Surveying

Survey Reporting

In this section; •

Survey reporting requirement from wellsite to the OSC

•

Survey reporting frequency of communications.

7.1

Survey Reporting

All accepted MWD and Gyro surveys must be reported to the client and the directional driller in writing at least daily, and a hardcopy record of the days surveys retained in the well or job file. In addition, at the end of a hole section, or at the end of each multishot survey, a complete listing over the entire survey interval showing the tie-on point used and the calculated well position should be presented to the client representative, the directional driller, and sent in to the OSC Manager or Survey Specialist for inclusion into the definitive survey database. Supporting data such as quality control reports, checkshots, overlap onshore depending on the client or location. 7.2

Reporting Frequency

In addition to meeting the requirements of section 7.1 above, all MWD or Gyro surveys recorded on all D&M operations must be sent in to the line manager in town at least daily in time for review prior to morning calls, or otherwise as required. Constant communications with the onshore support organization must be maintained by the wellsite team. This includes frequent reports of surveying progress, survey listings or operational updates to ensure that when support and guidance is needed from the onshore team, that they are fully apprised of the situation as quickly as possible, and prepared to provide effective support. It is undesirable and unacceptable for the onshore support team to be briefed on the progress of D&M surveying operations by the client at any time, particularly at the morning call.

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or repeat sections, or inrun and outrun surveys may also be required to be presented both offshore and

Drilling & Measurements Procedures

8

Wellbore Surveying

Enhanced Surveying Techniques

In this section; •

SAG Correction

•

Use of the EDI calculator.

•

Effect of magnetic storms on MWD surveys.

8.1

SAG Correction

The SAG angle (i.e. the misalignment between the surveying package and the borehole axis in the vertical plane) must be computed for each BHA run. If the computed sag angle is greater than 0.25°, efforts should be made to reduce the computed angle by modifying the BHA. All calculated SAG angle corrections greater than 0.1° must be applied to provide SAG corrected surveys as required by the survey program. It follows therefore that if the survey program requires the use of SAG corrected surveys then they must fall into one of the following categories which all justify the use of a SAG corrected error model; slick steerable drilling assembly, therefore the SAG is zero and the SAG corrected error model also applies because TVD errors as a result of SAG will be minimal, •

stabilized drilling assembly, calculated SAG angle correction is 0.1° or less, and therefore the same case as the slick assembly applies,

•

stabilized drilling assembly where the calculated SAG angle correction has been applied to the surveys and therefore the correction has minimized the resulting TVD error.

Sag angle calculations that fall below the 0.1° threshold are considered too small to warrant the additional management required because of the overriding effect of the assumptions made in calculating the SAG angle. 8.1.1

Definition of SAG

Mechanical misalignment errors affecting directional survey measurements can be considered to be the difference between the orientation of the along-hole axis of the survey sensor and that of a line describing the geometric center of the wellpath. When considering the nature of a BHA lying on the ‘low side’ of the hole it is clear that there are a number of potential sources of misalignment of the survey sensor package present in a downhole assembly. These may include some or all of the following; •

mechanical misalignment of the sensor cartridge within the collar,

•

position, number and size relative to collar size, of any stabilizers,

•

position and degree of bend in steerable motor or assembly,

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•

Drilling & Measurements Procedures

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•

mud weight, and therefore flotation characteristics,

•

hole architecture effects caused by the BHA laying on the low side of the hole.

Estimation of the SAG angle correction is usually restricted to inclination effects, and therefore would affect the accuracy of vertical depth or TVD calculation. SAG misalignment may also be present in drillpipe conveyed gyro surveys, whether they be by drop gyro multishot or gyro steering tool where the gyro sits in one location within the drillstring; or by a wireline gyro pumped down or otherwise run in the string. This will generally cause the gyro survey to experience misalignment as a result of drillpipe misalignment in the hole (which can be observed in high density continuous surveys), and from riding over internal connection upsets, as well as any SAG error which occurs when gyro readings are obtained in the region of a stabilized BHA. Practical application of a SAG correction is considered below using manual estimation, which can be used to confirm the results from other software methods. In any estimation or software application method for determining SAG correction the overriding assumption is that the assembly is contained in a smooth parallel-sided uniform cylinder. This is clearly not the case in practice, and when determining the result, and therefore ignored. Generally there is also a data management issue when SAG correcting survey data in or near real-time, which requires greater resources not normally warranted for negligible corrections. This application threshold translates into 1.7ft per thousand feet of position uncertainty, and is (practically) close to the residual tolerance allowed by the SAG corrected error models in use (±0.08°).

8.1.2

Prerequisites for SAG Correction

The position of the sensor relative to the other elements of the BHA is crucial in estimating the SAG correction, so whilst estimates may be made from generic BHA designs during the planning phase, the actual BHA sheet detail is the primary prerequisite for making any useful calculations. In addition to standard BHA sheet information, additional information generally required by a software program is; •

the size and position of any sleeve stabilizers present in the motor assembly,

•

the position of, and degree of bend for any steerable elements in the BHA,

•

mud weight,

•

the expected survey angle range

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applicability of the SAG correction any calculated estimate of ±0.1° or less is considered negligible as a

Drilling & Measurements Procedures

8.1.3

Wellbore Surveying

Manual Estimation of Sag Correction

When using the manual estimation method, it is useful to think of the SAG angle to be the resultant of two overall sources of misalignment; direct mechanical misalignment from stabilization of the assembly; and sagging of the survey instrument package and/or the collar caused by the flexible bending attributes of the drillstring when acted upon by gravity. The purpose of carrying out manual estimation of the SAG angle is firstly to have some method which will always be available when it is known that SAG will exist, and where there may be no other methods available. Secondly, manual estimation will provide a common-sense check on software methods, which are not infallible and should always be handled cautiously. Manual estimation is done using the prerequisite information stated above and a standard scientific calculator. We will look at three typical scenarios; •

Slick steerable assembly

•

MWD stabilized at one end only

•

MWD stabilized at both ends Slick Steerable Assembly

Here we are dealing with a potential misalignment caused by the difference in collar diameter, which can generally be considered to be negligible. If we considered a 9” MWD collar with 9½” drill collars below and 9” collars above i.e. a ½” diameter differential, then the misalignment likely to be caused would be of the order of ±0.07°. Therefore, provided that the diameter differential is not significantly greater than this, it can be ignored as it falls below the threshold allowed by the SAG corrected error models. What is more likely to be a contributing factor to a survey misalignment error is the degree of bend of the steerable part of the assembly, and thereby the toolface upon which the survey is taken. Experience suggests that at less than 15° inclination this may be a problem that must be considered and which is generally dealt with using clustershot surveys and benchmarks. At angles in excess of 15° inclination toolface dependency is unlikely to be a problem as it would be unusual to find the drillstring oriented to either highside or lowside as the effects of gravity and torque will naturally prevent this from happening. Therefore, care should be taken to assess the effects of any low inclination misalignment, and to continue to be aware of possible survey toolface effects, even after exceeding these angles, in order to prevent unwanted and inconsistent errors being introduced. When obtaining surveys for overlap checkshots or repeats, attention should also be given to this problem when attempting to confirm the repeatability specification of succeeding survey tools.

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8.1.4

Drilling & Measurements Procedures

8.1.5

Wellbore Surveying

MWD Collar Stabilized at One End

With the MWD collar stabilized at or near one end only, we can treat the mechanical misalignment as a triangle of points of contact. The three sides of our triangle will be the outside of the collar, the upset caused by the stabilizer (which is equal to half the difference in outside diameter between collars and stabilizer), and the assumption of a smooth sided horizontal hole. In the diagram below, we can take a 12¼” stabilizer with 9” collars, giving an upset therefore of 12¼ - 9 = 3¼ then divided by 2 = 1.625 inches.

SAG ANGLE 1.625" (0.135417 ft)

x In this case we need to know the distance from the stabilizer to the point of contact between the collar and the sidewall of the hole (distance ‘x’). It is reasonable to assume that the collar will be bending and ‘sagging’ slightly as a result of the upset from the stabilizer and experience suggests that for 8” or smaller So following our example through and using ‘x’ = 30, we can calculate the sag angle as; •

Tan θ = (0.135417 ÷ 30) thus giving; θ = 0.26°

Therefore we can estimate that the SAG angle in this case is 0.26° at horizontal. If we want to then estimate what it would be at any other inclination, then simply multiply by the Sine of the inclination, for example; •

SAG angle at horizontal = 0.26°; therefore at 30° inclination = 0.26 x Sin 30° = 0.13°

To determine the sign of the SAG correction to the inclinations, it is clear that with only one stabilizer below the MWD the inclinations are artificially higher than they should be so the correction is negative, with the reverse also being true. If the survey package is farther than 25ft (30ft for large assemblies) away from the stabilizer, then the SAG misalignment can be considered negligible because the sensor is aligned along the side of the hole beyond the point of contact with the sidewall. 8.1.6

MWD Collar Stabilized at Both Ends

When the MWD collar is stabilized at or near both ends, where the separation between stabilizers is less than 40ft, we simply treat the problem as that of a rigid body, thereby only taking into account the differential in outside diameter of the two stabilizers as being the misaligning factor. For example if we have a 12¼” stabilizer, pony collar, MWD and then an 10¾” stabilizer, with a separation of 32ft between stabilizers then the misalignment would be calculated as follows; •

Offset between stabilizers is (12¼ -10¾ ) ÷ 2 = 0.75” (0.0625ft)

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collars an estimate of 25ft could be used, and for 9” or larger an estimate of 30ft would be more realistic.

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Tan θ = (0.0625 ÷ 32 ) thus giving; θ = 0.11°

This SAG correction would then be dealt with as in the previous section. Should the separation between the stabilizers be greater than 40ft (for small OD assemblies), or 60ft for larger assemblies, then the problem can be treated as if the survey package is offset at one end only; i.e. whichever end at which the survey package is nearest to a stabilizer. The special case here however is that if stabilized at both ends with a stabilizer separation of 40ft (or 60ft as above) or more, then if the survey package is within five feet of the mid point between stabilizers, the SAG misalignment can again be considered negligible. In this case the survey package would not be lying along the centerline of the hole but would be offset and parallel to it. Note: In the absence of any other available method the manual estimation of SAG correction is a useful guideline to alert the surveyor or driller to the possible presence of SAG. Some care must also be taken in estimating the position of the survey sensors and contact points because when a larger SAG correction exists, even small variations in these parameters can have a large impact on results. Manual estimation is a basic check only and relies largely on experience and common sense. Unusual or unexpected results consulted whenever this is the case. 8.2

The Estimated Drillstring Interference (EDI) Calculator

Due to the desire to place the survey instrument as close to the bit as is practicable, and the impracticability of drilling with very large intervals of nonmagnetic collars, drillstring magnetic interference of the survey sensor is impossible to avoid. The Survey ToolBox contains an EDI Calculator that is used to calculate the expected level of azimuth error generated by drillstring magnetic interference. It is essential that this tool be used correctly and as a part of a comprehensive procedure to deal with the problem of drillstring interference. This allows for the identification of possible problems with interference, which then have to be dealt with either by increasing the amount of nonmagnetic collars, improving the existing sensor spacing or correcting for the interference using the DMAG correction algorithm. It should always be borne in mind that it is possible to successfully drill a directional well with little or no regard to nonmagnetic spacing in the BHA. This may be because the steel elements in the BHA are not highly magnetized, the direction and inclination of the well to be drilled are magnetically benign, or that a positional uncertainty error model has been used to plan the anticollision and positional requirements for the well that allows a sufficiently large amount of magnetic interference so as to have compensated for the problem. However, none of any of these options is feasible or reasonable unless the amount of interference has been estimated, checked, corrected for (if necessary), or prevented by improving the magnetic spacing of the BHA.

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should always be checked by some other method, and the OSC manager or Survey Specialist should be

Drilling & Measurements Procedures

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The EDI Calculator is used to check the estimated azimuth error based on BHA size and configuration. If the EDI calculator indicates interference >0.5 degrees then the BHA can still be run, and the Field Acceptance Criteria used to determine whether DMAG is required to correct the MWD surveys. The Schlumberger Drilling and Measurements standard for expected drillstring interference at the 2sigma (95%) uncertainty level is not more than 0.5 degrees of azimuth error. The following procedure should be used in all cases and to check all planned BHA's to be run by Drilling and Measurements: 8.2.1 •

Estimated Azimuth Error 0.5° If the estimated azimuth error for the BHA configuration to be used exceeds 0.5 degrees, the first response is to use the EDI Calculator to estimate how much additional nonmag is required (and in which position) in the BHA to reduce it to an acceptable level.

•

The option of increasing this spacing must be explored first as it must always be our intention to not have to rely solely on a mathematical algorithm for quality surveyed directional control. If it is possible to increase the nonmag spacing accordingly to reduce the estimated interference to within acceptable limits, then proceed as per the previous section.

•

If increased spacing is not possible, then the running of the BHA in it’s current configuration must be subject to further conditions whereby we must either plan to correct for the actual interference (using DMAG) or use a less accurate error model (MWD-INC_ONLY) to define our position uncertainty. Therefore adequate checks to calculate the actual interference experienced need to be made in order to assess the next required action. On running the BHA and completing the shallow hole test and checks required by the MWD UOP or ORM, collect sufficient 6-axis surveys for the orientation in question, and process them using the DMAG application.

•

Check the actual calculated interference versus the estimate given from the EDI Calculator. If the actual interference encountered for the BHA in question results in 0.5 degrees or less, no further action need be taken at this time (subject to any update checks required as above), and the MWD+DMAG error model is validated and can be used.

•

If the actual interference calculated exceeds 0.5 degrees then the use of DMAG must continue for the remainder of the BHA run in regular cycles, or until such time as the actual interference reduces to less than 0.5 degrees (e.g. if dropping into a target and inclination is falling, or when turning away from east/west).

•

The most accurate and best quality position of the wellpath must be maintained at all stages of the drilling operation such that the plan, anticollision scan and directional objectives remain valid at all times. This includes ceasing all operations, and the replanning of the well using an appropriately worse positional error model if required.

•

The SLB_ISCWSA error models for MWD-STD (interference less than 0.5degrees), MWD+DMAG (where the correction is used), and MWD-INC_ONLY (where the interference exceeds 0.5 degrees and is not corrected for), have been constructed specifically for use with this procedure.

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Drilling & Measurements Procedures

8.3

Wellbore Surveying

Magnetic Storms

The purpose of this procedure is to enable MWD engineers and Directional Driller’s to be able to ascertain whether magnetic storm activity may be a factor when observing apparently poor or reduced quality magnetic surveys when in the field. Currently the only way to do this at the rigsite requires Internet access to geomagnetic websites as detailed below. The alternative is to contact your line manager, the OSC Manager or the Survey Specialist and ask them to make these checks. 8.3.1

Geomagnetic Monitoring

Live monitoring of geomagnetic activity for the North Sea area is carried out and reported by the British Geological Survey (BGS), by means of ‘k-values’. Eight ‘k-values’ are given for each day, which correspond to each four-hour period of the day (e.g. 0000-0400, 0400-0800 etc). Each value is assigned a number from 0 to 9 that indicates on a sliding scale, the level of magnetic disturbance for that period. Data is available from Lerwick (Shetland), Eskdalemuir (Scottish borders) or Hartland (southern England) and is available retrospectively for the previous days measurements. This data is extracted by the user inputting an e-mail address and the month and year of interest and is obtained from the BGS website at:

For northern North Sea areas, typical ‘k-values’ will range from very quiet (0-3) to moderate (4-6) to stormy (7-9). 8.3.2

Geomagnetic Storm Forecasting

Magnetic storm forecasting is a relatively difficult activity to do consistently and the average rate of success for forecasting medium size storms is about 50%. Considerable efforts are made with satellite measurements and solar observations to provide as much warning as possible or magnetic storm activity. Generally, the ability to provide notice of magnetic storms for say, within the next eight hours would probably be very much better than suggested above, but once again the scale of the effects are latitude and time dependent. The Geomagnetic Service of Canada (GSC) provides some of the best forecasting material available and this can be found at their main website under: http://www.geolab.nrcan.gc.ca/geomag/home_e.shtml If the suspected problem is for MWD data collected on previous day(s), check the ‘k-values’ on the BGS website using the observatory nearest in latitude to the rigsite. If the values are 5 or 6 then the MWD data collected during the period of the day for which the values apply needs to be checked. If the values are 7 or more, then any MWD data collected for that period probably needs to be corrected or repeated. If the problem is for data collected within the last few hours then check the forecasts on the GSC website. Generally, anything that is above the unsettled threshold needs to be checked further or repeated, and anything else is unlikely to be a magnetic storm problem.

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http://www.geomag.bgs.ac.uk/gifs/k_indices.html

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In either case the most effective way of dealing with the problem is to repeat the affected surveys once the magnetic storm activity has passed. This can be done either on a short trip or when finally tripping the assembly out of hole. In either case a return to expected values of |B| and Dip would indicate when this could be done. The presence of a magnetic storm does not invalidate MWD (or EMS) survey data. In most cases this activity lasts for less than a few hours only, and at most only a few MWD surveys from a dataset are usually affected. This is unlikely to significantly impact hole location, but may be a problem in a tight anticollision situation (where you might have a gyro available anyway). In addition, inclination measurements will be unaffected, and an interpolated azimuth for an apparently poor survey between two good surveys (which have passed their FAC) is probably the best description of the hole position if magnetic activity is suspected, until the raw data can be checked or the survey repeated. An EMS survey may be worse affected because of the shorter time spent surveying and this should be borne in mind. Finally, the likelihood of a magnetic storm adversely affecting magnetic survey data for mid and low latitude areas is unlikely. However, if suspected there are worldwide magnetic observatories in many Survey Specialist as required.

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locations that cover virtually all latitudes, and these produce daily data, which can be checked by the

Drilling & Measurements Procedures

9

Wellbore Surveying

Depth Measurement

In this section; •

Depth measurement accuracy.

•

Drillpipe depth measurement.

•

Wireline depth measurement.

9.1

Depth Measurement Accuracy

Depth measurement accuracy is of vital importance to the quality of the wellbore position overall, and it provides one third of the survey measurement triangle of depth, inclination and azimuth from which all other trajectory calculations spring. Generally, depth quality has received less attention than other aspects of survey quality, often because it is provided as a ‘third-party’ service or is not directly under the control of the survey contractor. Considering that depth error contributes to vertical or TVD uncertainty directly as a cosine function of inclination (i.e. everywhere except in horizontal hole sections), depth

9.2

Drillpipe Depth Measurement

Drillpipe depth, or the ‘drillers depth’, which is derived from the drillpipe tally, is used as the primary depth reference for all pipe conveyed surveying systems. These include MWD, EMS, drop northseeking gyro, SRG singleshots, and film based surveying systems. The drillpipe and other BHA components are usually measured horizontally on the pipe deck using a steel tape measure. These measurements are recorded in the drillers pipe tally book, which contain the individual and cumulative lengths of all drillstring elements placed in the well as they are picked up and laid out during normal drilling operations. When tripping in and out of the hole, a common practice is to reconfirm and check the existing drillers tally by ‘strapping’ the pipe in or out of hole using the steel tape measure. In this case the drillstring is racked back in the derrick in roughly ninety-six foot stands, and every time a stand is racked back or picked up it is measured by the roughneck who calls out the measurement to the driller as the derrick-man holds the end of the steel tape measure to the top edge of the joint collar of the stand being measured. The drillers’ depth measurement therefore contains errors associated with the use of the steel tape measure, but ignores any effects associated with pipe stretch and slump or thermal expansion, which can also be considerable.

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control probably requires far more attention to detail than it currently receives.

Drilling & Measurements Procedures

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As the drillstring is raised and lowered in the hole during drilling, the bit depth is tracked by a ‘geolograph’, which is part of the rig equipment connected electronically to various users of the depth measurement including the MWD unit. The ‘geolograph’ is set manually and then tracks the progress of the crown block as drilling progresses. Drillpipe depth measurements are therefore prone to error, and discrepancies often occur, usually when a stand, single or double are picked up or laid down without the surveyor being aware of it. The MWD engineer should therefore keep an independent check on the depth and the pipe tally and seek to resolve gross depth errors at the earliest opportunity and avoid reporting them into the definitive survey database. At this late stage they are often undetectable, particularly if the redundant survey principle has not been followed. It should always be borne in mind that all ‘logging whilst drilling’ (LWD) data obtained using drilling formation evaluation tools will also contain the same depth errors associated with drillpipe depth errors in the survey data, and this must not be overlooked when reconciling log data to positional data. 9.3

Wireline Depth Measurement

Wireline depth is derived from mechanical optical encoders or other mechanical wheel rolling counter wheel each time it completes a revolution. The measurement head of the wireline unit will house one or more measurement wheels for this purpose. Wireline depth measurement is uniquely different from drillpipe depth measurement in that apparently ‘more’ wire is spooled back onto the wireline drum of cable in most cases than was spooled out. This is usually a function of stretch of the wire cable and slippage of the measurement head assembly. Some wireline depth measurement systems also incorporate the use of ‘magnetic markers’, which are small magnetized pieces of magnetic foil that are embedded in the wireline cable usually at 100ft intervals. This provides an additional level of depth accuracy when used as the wireline operator can also monitor the rate of change between the 100ft markers as well as the overall stretch in the cable more effectively. Almost all wireline conveyed surveys are depth corrected for stretch in some way. Usually, this is done by the surveyor as a result of indications from the surface computer about tool movement, and the use of ‘experience’. A typical scenario is that for a gyro survey run in drillpipe the inrun survey depths will always be a little shallower than actually indicated on the depth counter because slack wire had to be ‘pumped’ down the hole to allow the frictional forces of the tool traverse to be overcome. On the outrun survey at depth the wireline is spooled in and fully stretched out, taking all of the slack out as well as ‘stretching’ the cable due to it’s inherent elasticity. This amount of ‘pick-up’ can usually be seen by the surveyor on the surface computer because the tool readings remain stable until all of the slack is taken up and the tool finally begins to move. For an example high angle 15000ft survey this may be as much as 70ft of slack and stretch to be removed from the surveyed depth at TD.

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devices which are calibrated to index the depth scale reading by the circumference of the measurement

Drilling & Measurements Procedures

Wellbore Surveying

On the resulting wireline trip out of hole the stretch will begin to come back into the cable, particularly as the hole angle and therefore frictional forces reduce. This means that the surveyor will usually apply an incremental stretch correction to the surveyed depth until having traversed the build section at which time the major portion of the stretch has been removed. On recovering the survey tool to surface it is normal to rezero the survey tool at the rig floor anywhere from ten to fifteen feet deeper than was zeroed going in the hole. This is usually the cumulative effect of stretch that stayed in the wireline cable (which varies as a function of age of the cable and the number of runs it has done), and the total slippage of the measurement system (which is calibrated and certified at frequent intervals). The primary quality control measure for wireline depth is the rezero figure when the tool returns to surface. Provided this indicates a rezero tolerance of less than 2ft in a thousand feet (0.2%) the depth accuracy is usually accepted. Some wireline contractors or survey providers quote less than 0.2% in accuracy, particularly when a Casing Collar Locater (CCL) is used in conjunction with the standard depth measurement. The CCL is a magnetic field sensor unit that is usually run on top of the survey tool, and which is tuned to detect the increased steel mass of the casing collars as they are traversed during the survey run. Each collar is identified by a spike on a logging trace, and can be assigned a depth from the

In all but the shortest runs (