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Open Hole Electrical LoggingLecture Presentation

October 18, 21, and 23, 2002

Carlos Torres-Verdín, Ph.D.Assistant Professor

PGE368Fall 2002 Semester

Objectives:

•To understand the physical principles behind the operation of laterolog and induction tools,

• To understand the principles behind the interpretation of apparent resistivity curves acquired with laterolog and induction tools,

• To understand the importance of environmental corrections, and

• To introduce the physical principles behind the operation of LWD resistivity tools.

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Complementary Reading Assignments:

1. Bassiouni, Z., 1994, Theory, Measurement, and Interpretation of Well Logs, Chapter 5: “Resistivity Logs.”

2. Schlumberger’s Computer Animated Presentations on Induction Principles and Laterolog Principlesavailable from our course web site.

Open HoleBorehole

Environment

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Open-Hole Logging Environment

Dynamic Mud Filtrate Invasion and Mud Cake Buildup

Source: Oilfield Review, Schlumberger

LABORATORY SAMPLEBrine-Water Saturation

+ -

++++

----

VI

R =VI

R

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ELECTRICAL LOGGING TOOLS

Induction Galvanic (Laterolog)

Low Frequency Excitation: 10 Hz – 500 KHz

ELECTRICAL LOGGING TOOLS

Induction Laterolog

Electrical Conductivity of Mud is an Important Issue

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250 cm 200 150 100 50 0 cm

80 cm

80 cm

40 cm30 cm 20 cm

60 cm

5 cm 2 cm 0 cm

INDUCTION LOG

LATEROLOG

NEUTRONGAMMA RAY

DENSITY

SONICMICRO RESISTIVITY

MICROLOGDIPMETER

DEPTH OF INVESTIGATION

RES

OLU

TIO

NRESISTIVITY

RADIOACTIVITY

RESISTIVITY

ACOUSTIC

Logging Tools

INDUCTION vs.

LATEROLOG, When?

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NORMAL MEASUREMENT IN A BOREHOLE

Resistivity in a Homogeneous Medium

−=

=

==

=−

∫∞

Ir

drdVR

IVrR

rRI

rdrRIV

drr

RIdV

r

2

2

2

4

4

44

4

π

π

ππ

π

Current lines

Equipotential spheres

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LATERAL MEASUREMENT

GUARDED ELECTRODE MEASUREMENT

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16” Short Normal - 1979• 1927: 1st wireline resistivity • 1st resistivity while drilling• Rapparent = G V/I• Requires conductive, water-

based drilling fluid– Large borehole effects limit

range Rb < R < 20 Rb– Low quality suitable for

correlation and resistivity trends– Insulation prone to failure– Obsolete technology

LATEROLOG 7

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DUAL LATEROLOG

LATEROLOG(GALVANIC)

TOOLS

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LATEROLOGTOOL CONFIGURATION

ACTUAL TOOL

ELECTRICAL CONDUCTION PHENOMENALaterolog Tools

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LATEROLOG TOOLDifferent Current Focusing Strategies

LLD CORRECTION CHART FOR BOREHOLE EFFECTS

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LLDCORRECTION CHART

FOR INVASION EFFECTS

LLD CORRECTION CHART FOR BED THICKNESS

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LLS CORRECTION CHART FOR BED THICKNESS

MICROLATEROLOG

DEVICE:a

Pad Tool

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MSFL TOOL:a

Micro-LaterologDevice

FORMATION MICRO-IMAGING TOOL

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PHYSICALPRINCIPLE OF

INDUCTIONTOOLS

PHYSICAL PRINCIPLE OF INDUCTION TOOLS

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INDUCTION SOUNDING

PHYSICAL PRINCIPLE OF INDUCTION TOOLS

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Dual Phasor Array Induction

Multi-Frequency Acquisition

PRINCIPLE OF INDUCTION FOCUSING

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INDUCTION TOOL SENSITIVITY

Ideal RadialGeometric Factors

Actual RadialGeometric Factors

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SENSITIVITIES FOR DIFFERENT COIL SUBARRAYS

LINEAR APPROXIMATION THEORY(Geometric Factor Approximation)

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FOCUSED SENSITIVITY FUNCTIONS

SKIN EFFECT CORRECTION(Correction for Frequency-Dependent

Propagation Effects)

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BOREHOLE CORRECTION(Correction for Mud Conductivity

And Borehole Size)

BOREHOLE CORRECTION(Correction for Mud Conductivity And Borehole Size)

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BED THICKNESS CORRECTION(Induction Log)

INVASIONCORRECTION(Induction Log)

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SUMMARYApproximate Interpretation Cycle

EXAMPLESkin and Borehole Corrected Induction Curves

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EXAMPLEFocused Induction Curves

MODERN INTERPRETATION TECHNIQUES

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COMPREHENSIVEINTERPRETATION PROCESS

(Numerical Simulation andInversion)

EXAMPLE2-D Inversion Results

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LWD RESISTIVITY TOOLS

16” Short Normal - 1979• 1927: 1st wireline resistivity • 1st resistivity while drilling• Rapparent = G V/I• Requires conductive, water-

based drilling fluid– Large borehole effects limit

range Rb < R < 20 Rb– Low quality suitable for

correlation and resistivity trends– Insulation prone to failure– Obsolete technology

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2 MHz Propagation - 1984• 1967 patent by M.

Gouilloud • Transverse E-field• Works in conductive or

insulating drilling fluids• Small borehole effects in

smooth boreholes• 1st quantitative LWD

resistivity measurement• ~0.1 – 200 ohm-m range

2 MHz Propagation - 1988• Symmetric array

- increased accuracy- reduced effects in rugose

holes• Two resistivities derived

from Phase Shift and Attenuation

• Dual radial depths-of-investigation

• Anisotropic formations

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Loop antennas located under slotted metal shields.

Close-Up of Tool

• Phase Shift provides a shallow resistivity with high axial resolution

• Attenuation provides a deep resistivity with lower axial resolution

Dual Depths

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Advances in Propagation Resistivity

• 1991 – Array with 4 depths-of-investigation• 1995 – Array with 10 depths-of-investigation• 1995 – Dual frequencies 400 kHz and 2 MHz• Different size drill collars (3” to 9” OD)

1993: Toroidal Resistivity• Electrodes held at collar potential

& currents measured• Improves S/N, dynamic range &

provides high spatial resolution• 1st azimuthal resistivity• Provides borehole images and dip• Multiple depths-of-investigation• Active focusing technique • 0.1-20,000 ohm-m range• Minimal borehole effects

Rb < R < 100,000 Rb

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6.75” Toroidal Resistivity

ButtonsToroidRing Toroid

Location of drill bit

Borehole Resistivity Imaging• Each button scans 360°

as collar rotates• Stacked scans provide

continuous image• Geological features:

– Beds– Dipping formations– Fractures– Faults

• Geosteering

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Acknowledgements

• Schlumberger

• Baker Atlas

• Repsol-YPF