Table Of ContentWELLL OGGING
FOR
EARTSHC IENTISTS
DARWINV . ELLIS
ELSEVIER
NEWY ORK AMSTERDAMLO NDON
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PREFACE
A poll of casual users of the geophysical information derived from well
logging might indicate that the two words most frequently associated with
well logging are "black magic." An equally disheartening survey conducted
among students might produce a single associative word: "boring." It was in
this climate that I was invited to accept the challenge to produce a course on
well logging to combat these negative associations. This text is my attempt
to demystify well logging and to communicate the excitement of the research
which underlies these frequently derided geophysical measurements. The
reader will have to be the judge of the extent to which I have succeeded in
this effort. The text is directed at students heading toward a career in the
petroleum industry but should be useful to practitioners who want to know
more about the logging measurements than can be extracted from
conventional sources. It should also appeal to a growing number of earth
scientists who have discovered that sub-surface "field work can be performed
with logging measurements.
This book is the outgrowth of a course taught at Stanford University's
School of Earth Sciences during the winter quarters of 1985 and 1986. The
students enrolled were primarily graduate students from the Departments of
Petroleum Engineering, Geophysics, and Geology. A further critical review
of the course was provided by Dr. J. Jensen of Heriot-Watt University whose
students in the Master's of Engineering program used a draft copy of the
present text for the well logging sequence of their petroleum engineering
course.
Until recent years, relatively few books on well logging have appeared,
and none were particularly suited for classroom use. The material that was
available tended to provide only a superficial discussion of the measurements
with little discussion of the underlying physics. My bias in emphasizing the
importance of the physics of well logging is clearly reflected throughout the
text. However, I have attempted to balance this by including discussions and
problems which explain the mechanics of obtaining the usual petrophysical
parameters from well log data.
The approach taken is to begin with a survey of the major categories of
measurement: electrical, nuclear, and acoustic. The measured physical
parameters are related to desired petrophysical parameters. As a first step in
the interpretation process the measurements are examined in terms of
limitations imposed by practical engineering considerations and the borehole
environment in which the logging tools must operate. Later chapters provide
discussion and examples of combinations of measurements and the types of
interpretations which can be made from them. Throughout, large sections of
the text have been set off in italics which may be skipped by the casual
reader. These detailed sections are included for those who would like to
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xiv
know more about the particular measurement techniques exploited and may
be of more interest to researchers.
Logging can be considered as an experiment. One which is conducted,
not in a pristine laboratory environment, but in an uncontrolled and hostile
environment with unknown conditions. The goal is to infer petrophysical
information from rather indirect means. The extraction of this information
sometimes comes from rather unscientific rules of thumb, and other times
from straightforward application of physical principles. The elementary
extraction of the petrophysical parameters usually desired from logging
measurements, porosity, and water saturation, are treated from a historical
perspective to show the origin of the rules of thumb and indicate their
weaknesses. Directions of current research to refine the estimate of the
desired petrophysical properties from logging measurements are also
discussed. Problem sets at the end of the chapters are designed either to
familiarize the student with the mechanics of obtaining useful answers from
the logging measurements or to illustrate non-conventional utilization of
logging responses to questions which will hopefully appeal to earth scientists
outside the petroleum industry. The message I have tried to communicate is
that, if you are interested in the subsurface of the earth, well logging has
something to offer.
For the opportunity to develop this course I am indebted to Schlumberger
for their permission to devote preparation and teaching time away from the
laboratory, to the School of Earth Sciences at Stanford University for
welcoming me to their faculty for two consecutive years, and to Prof. G.
Stewart for arranging the use of the draft text and for inviting me to
participate in the lectures at Heriot-Watt.
For indispensable help in the refinement of this text I would like to thank
Jay Tittman for his more than careful reading and rereading of major portions
of the manuscript. Additional thanks for reviewing and comments go to J.
Robinson of Shell, and Schlumberger colleagues H. Scott, J. Singer, and J.
Tab anou.
For relief from the first year’s teaching load I would like to thank guest
lecturers, P. Day, W. Kenyon, M. Herron, T. Plona, and M. Orktaglio. For
efforts in suggesting, locating, and providing figures, references, and generally
helpful suggestions I am indebted to: L. Jacobsen, P. Day, J. Howard, M.
Herron, J. Jensen, J. Banavar, R. Kleinberg, R. Plumb, S. Chang, T. Timur, J.
Kent, T. Obenchain, J. Schweitzer, and C. Straley.
For assistance with grueling details of the physical production, I would
like to thank Mary Savoca, C. Green and T. Killpack. And finally, for their
hard work and patience in providing the graphics for the text I wish to credit
and thank Allison Fazio, R. Giuliano and M. Dalton.
CONTENTS
PREFACE xiii
1 AN OVERVIEW OF WELL LOGGING 1
Introduction 1
What Is Logging? 2
Properties of Reservoir Rocks 5
Well Logging: The Narrow View 6
Measurement Techniques 8
How is Logging Viewed by Others? 9
References 13
2 INTRODUCTION TO WELL LOG
INTERPRETATION: FINDING THE HYDROCARBON 15
Introduction 15
Rudimentary Well Site Interpretation 15
The Borehole Environment 18
Qualitative Interpretation 20
Reading a Log 22
Examples of Curve Behavior and Log Display 27
A Sample Rapid Interpretation 32
References 34
Problems 34
3 BASIC RESISTIVITY AND
SPONTANEOUS POTENTIAL 37
Introduction 37
The Concept of Bulk Resistivity 38
Electrical Properties of Rocks and Brines 42
Spontaneous Potential 46
Log Example of the SP 51
References 52
Problems 52
4 EMPIRICISM: THE CORNERSTONE OF
RESISTIVITY INTERPRETATION 57
Introduction 57
Early Electric Log Interpretation 58
Empirical Approaches to Quantifying R, 61
Archie’s Formation Factor 61
Archie’s Synthesis 62
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A Review of Electrostatics 65
A Thought Experiment for a Logging Application 66
References 70
Problems 70
5 RESISTIVITY: ELECTRODE DEVICES AND
HOW THEY EVOLVED 73
Introduction 73
Unfocused Devices 73
The Short Normal 73
Estimating the Borehole Size Effect 75
Focused Devices 84
Laterolog Principle 84
Spherical Focusing 87
Pseudogeometric Factor 91
Dual Laterolog Example 94
References 97
Problems 97
6 INDUCTION DEVICES 99
Introduction 99
Review of Magnetostatics and Induction 100
The Two-Coil Induction Device 105
Geometric Factor for the Two-coil Sonde 107
Focusing the Two-coil Sonde 111
Skin Effect 114
Induction or Electrode? 118
Induction Log Example 119
References 121
Problems 122
7 ELECTRICAL DEVICES FOR
MEASUREMENTS OTHER THAN R, 123
Introduction 123
Microelectrode Devices 1 24
Uses for R,, 128
A Note on Reality 138
Characterizing Dielectrics 142
Tools to Measure Formation Dielectric Properties 146
Measurement Technique 147
Log Example 149
Appendix
Propagation of E-M Waves in Conductive Dielectric Materials 152
References 155
Problems 157
ix
8 BASIC NUCLEAR PHYSICS FOR
LOGGING APPLICATIONS: GAMMA RAYS 161
Introduction 161
Nuclear Radiation 162
Radioactive Decay and Statistics 163
Radiation Interactions 166
Fundamentals of Gamma Ray Interactions 167
Attenuation of Gamma Rays 172
Gamma Ray Detectors 173
References 179
Problems 179
9 GAMMA RAY DEVICES 181
Introduction 181
Sources of Natural Radioactivity 183
Gamma Ray Devices 186
Uses of the Gamma Ray Measurement 188
Spectral Gamma Ray Logging 190
Spectral Stripping 195
A Note on Depth of Investigation 197
References 198
Problems 199
10 GAMMA RAY SCATTERING AND
ABSORPTION MEASUREMENTS 20 1
Introduction 20 1
Density and Gamma Ray Attenuation 202
Density Measurement Technique 204
Lithology Logging 212
Estimating Porosity from Density Measurements 222
References 223
Problems 224
11 BASIC NEUTRON PHYSICS FOR
LOGGING APPLICATIONS 221
Introduction 227
Fundamentals of Neutron Interactions 228
Nuclear Reactions and Neutron Sources 233
Useful Bulk Parameters 234
Neutron Detectors 240
References 24 1
Problems 242
243
12 NEUTRON POROSITY DEVICES
243
Introduction
X
Basis of Measurement 244
Measurement Technique 249
The Neutron Porosity Device: Response Characteristics 254
Shale Effect 254
Mamx Effect 256
Gas Effect 259
Depth of Investigation 264
Log Examples 266
Appendix
Depth of Investigation of Nuclear Logging Tools 269
References 27 8
Problems 279
13 PULSED NEUTRON DEVICES 28 1
Introduction 28 1
Thermal Neutron Die-away Logging 282
Thermal Neutron Capture 282
Measurement Technique 285
Instrumentation and Interpretation 288
Pulsed Neutron Spectroscopy 293
Appendix
A Solution of the Time-dependent Diffusion Equation 299
References 30 1
Problems 302
14 NUCLEAR MAGNETIC LOGGING 305
Introduction 305
Nuclear Resonance Magnetometers 306
Why Nuclear Magnetic Logging? 307
A Look at Magnetic Gyroscopes 308
The Precession of Atomic Magnets 309
Paramagnetism of Bulk Materials 310
Some Details of Nuclear Induction 313
Operation of a Conventional Nuclear Magnetic Logging Tool 32 1
Interpretation 326
Factors Affecting TI and T, 326
Surface Interactions 328
Applications 33 1
Free Fluid Index 33 1
Permeability from SurfacdVolume Information 33 1
Measurement of Residual Oil 333
A New Approach 334
References 336
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15 INTRODUCTION TO ACOUSTIC LOGGING 339
Introduction 339
A Short history of Acoustic Logging in Boreholes 340
Application of Borehole Acoustic Logging 342
Review of Elastic Properties 342
Wave Propagation 349
Rudimentary Acoustic Logging 353
Rudimentary Interpretation 355
References 357
Problems 358
16 ACOUSTIC WAVES IN ROCKS 359
Introduction 359
A Review of Laboratory Measurements 360
Acoustic Models of Rocks 366
Acoustic Waves in Boreholes 37 1
References 377
Problems 378
17 ACOUSTIC LOGGING METHODS AND
APPLICATIONS 381
Introduction 381
Transducers 382
Conventional Sonic Logging 382
Some Typical Problems 390
Newer Devices 391
Acoustic Logging Applications 397
Lithology and Pore Fluid Identification 400
Formation Fluid Pressure 402
Mechanical Properties and Fractures 403
Permeability 406
Ultrasonic Devices 408
References 412
Problems 413
18 LITHOLOGY IDENTIFICATION FROM
POROSITY LOGS 417
Introduction 417
Graphical Approach for Binary Mixtures 418
Combining Three Porosity Logs 425
Lithology Logging: Incorporating P, 430
Numerical Approaches to Lithology Determination 432
References 436
Problems 436
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19 CLAY TYPING AND
QUANTIFICATION FROM LOGS 439
Introduction 439
What is Clay/Shale? 440
Distribution of Clay 444
Influence on Logging Measurements 44 6
Some Traditional Indicators 448
Some New Methods of Clay Quantification 451
Interpretation of P, in Shaly Sands 45 1
Clay Mineral Parameters and Neutron Porosity Response 454
Response of C to Clay Minerals 457
Aluminum Activation 458
Clay Typing (Geochemical Logging) 459
References 467
Problems 468
20 SATURATION ESTIMATION 47 1
Introduction 47 1
Clean Formations 472
Shaly Formations 478
V, Models 480
Effect of Clay Minerals on Resistivity 48 1
Double Layer Models 483
Saturation Equations 484
References 486
Problems 487
21 EXTENDING MEASUREMENTS
AWAY FROM THE BOREHOLE 489
Introduction 489
Depth of Investigation and Resolution of Logging Measurements 490
Nuclear 490
Electrical 49 1
Acoustic 492
Surface Seismics 494
Borehole Seismics 498
The VSP 500
Rudimentary VSP Processing 504
Information from the VSP 508
Increasing the Lateral Investigation: Offset VSP 510
Appendix
Acoustic Impedance and Reflection Coefficients 517
References 519
INDEX 521
