Well Log Interpretation [PDF]

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Basic Well Log Analysis Reading Rocks from Wireline Logs

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Wire Line Logging Tool strings used in wireline logging operations

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Core-log Integration

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Important Principles You Will Need To Know To Make Any Sense of the Wiggle Traces on Wireline Log Strip Charts

• Porosity = pore volume/total volume of a rock – Porosity can range from 0% to in excess of 40%

• Saturation = volume of the porosity occupied by some fluid. – The possible fluids are almost always water or hydrocarbons; either liquid or gas. – SW = water saturation in percent, – 1 - SW is hydrocarbon saturation in percent.

• Lithology = rock type, including fluid filled pores, with physical characteristics of: – – – – – –

Resistivity spontaneous potential; SP natural radioactivity; e.g. Gamma Ray emissions bulk density hydrogen content of rock and fluid filled pores interval transit time (sonic velocity) 4

Basic Well Log Analysis • Logs Help Define – – – – –

physical rock characteristics Lithology/mineralogy, porosity, pore geometry, and permeability.

• Logging data are used to: – – – –

identify productive zones, determine depth and thickness of zones, distinguish between oil, gas, or water in a reservoir, and to estimate hydrocarbon reserves 5

Log Properties of Interest • The most frequently used logs are open hole logs – Logs are recorded in the uncased portion of the wellbore.

• The two primary parameters determined from well log measurements are – Porosity, fluid composition and relative saturation

• Log interpretations are determined by one of three general types of logs: – Electrical – Nuclear – Acoustic or sonic logs

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Bore Hole Environment • Where a hole is drilled into a formation, the rock plus the fluids in it are altered in the vicinity of the borehole

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Borehole Environment • The formations encountered in the bore hole during drilling are invaded to some extent by drilling fluids ("mud") • Mud is used to – lubricate the bit, – circulate the broken rock fragments produced during drilling and most significantly to – maintain pressure in the hole to prevent blow out.

• The mud invades the formation to at least some degree – in order to make useful physical measurements of the insitu rock properties the measurement's must be made well into the rock (if possible) or – mud infiltration must be accounted for. 8

Cased Holes • Steel pipe "casing" is set in bore holes to prevent damage and caving • Only certain down hole tools can make useful measurements through pipe, ie. – gamma ray, – neutron porosity

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LITHOLOGY LOGS • Natural Gamma Ray (γ-ray) Logs – Decay of radioactive elements produces high energy gamma ray emissions – Radioactive elements (K, U, Th) are normally concentrated in shaley rocks while most sandstones are very weakly radioactive. – Because radioactive material is concentrated in shale, shale has high gamma ray log readings – Clay-free sandstone and carbonate rocks have low gamma ray log readings 10

Determination of Lithology from γ -Ray Logging Tools

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Neutron Logs • Neutron logs (NL or GRN) measure the hydrogen ion concentration in a formation. – In clay-free formations where porosity is filled with water or hydrocarbons the neutron log measures liquid filled pores (the only significant occurrence of hydrogen). – The neutron log measures energy loss when neutrons emitted from the tool collide with other particles in the formation. – The maximum energy loss during a neutron collision occurs when – A neutron collides with a particle of equal mass, that is a hydrogen atom. 12

Neutron Logs • A lower neutron log reading (fewer energetic back scattered neutrons) indicates abundant formation hydrogen. – Clay rich formations contain hydrogen in the crystal structure ofthe clay minerals and give anomalous values for liquid filled pore volume.

• Neutron log excursions (decreasing in value from right to left) indicate higher proportions of hydrogen in the Formation – either increased liquid filled porosity or – higher shale content.

• Neutron log excursions increasing from left to right indicate – less porosity and/or – less shale

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Gamma Ray – Neutron Log

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Compensated Neutron Logs • Newer “radiation” logs called CNL (for compensated neutron logs) are calibrated so that the scale is in porosity units, or neutron porosity units • The CNL (sometimes called the NPHI, for Neutron porosity {φ}) is almost always displayed with • The formation density log and these logs, in combination, can be used to infer lithology

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Bulk Density • Formation density (compensated; FDC) logs measure the density (grams/cm3) of the formation based on the density of electrons in the formation • Electron density is a function of the absolute amount of matter comprising the formation – measured by the back scatter of gamma rays emitted from a gamma ray source in the logging tool

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Bulk Density • The absolute amount of matter in the formation is – inversely proportional to the degree of gamma ray penetration into the formation without back scatter to the detector

• Since the tool averages the electron density – porous formations composed of dense minerals will appear similar to low porosity formations with lower density rock matrix

• Bulk density is read on a log increasing from left to right. 17

Mineral Densities

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FDC-CNL Log

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FDC-CNL Log (showing density φ, DPHI)

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Lithology interpretation from FDC-CNL logs • An industry standard "quick-look" overlay methodology can be used with CNL-FDC wire-line logs • When Neutron porosity (CNL dashed curve) and Bulk Density (FDC, solid curve) logs are overlain on a common, limestone equivalent porosity scale changes in lithology can be inferred with depth

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Hypothetical neutron-density overlay patterns for simple logbased lithofacies. The overlay uses a common calibration to an equivalent limestone

porosity scale.

(From Doveton, 1986).

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The Photoelectric Index (PE or PEF) • The photoelectric index (Pe or PEF) is a supplementary measurement by the latest generation of density logging tools • PEF records the absorption of low-energy gamma rays by the formation in units of barns m() per electron • The logged value is a direct function of the aggregate atomic number (Z) of the elements in the formation, and so is a sensitive indicator of mineralogy. • The common reservoir mineral reference values are : quartz 1.81 ; dolomite 3.14 ; calcite 5.08 barns/electron.

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Digitally Enhanced Log Displays

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E- LOGS • Electric logs, resistivity and spontaneous potential, were the first wireline logging tools. • Instruments were (and still are) lowered down bore holes and physical measurements were made regarding the electrical properties of the rocks encountered.

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Resistivity • Resistance of rock R = rA/L (ohm-meter2/meter, contracted to ohm-meter or ohm-m) – r is the resistance (ohms) – A is the cross-sectional area – L is the length of the resistor

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Resistivity • The resistivities of sedimentary rocks are determined by the rock component types and their geometry. – hydrocarbons, rock, and fresh water are all insulators (nonconductive, or at least very highly resistive) to electric current flow. – Salt water is a conductor and has a low resistivity

• The measurement of resistivity is a measurement of the amount (and salinity) of the formation (connate) water. 27

Spontaneous Potential • Electrical current generated across – the boundaries between formation fluids and drilling fluids (if these fluids are of different salinity) and – the boundary between interbedded shale and sandstone.

• The spontaneous potential associated with shale and sandstones is the result of higher permeability in sandstone relative to lower permeability in shale. 28

Typical e-log Response to Variable Lithology and Fluid Content

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Wireline Logging Traces and Geophysical Logging Tools

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Trends in γ -Ray Traces and Interpretation of Depositional Environment

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γ -Ray Log Cross Section

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The Oz Machine

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