Introduction to Geophysical Formation Evaluation 9781566702638, 9780367400750, 1566702631

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Introduction to

GEOPHYSICAL FORMATION EVALUATION James K. Hallenburg

0

CRC Press Taylor & Francis Group Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 1998 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylo r & Francis Group, an In forma business

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ISBN: 9781566702638 (hbk) ISBN: 9780367400750 (pbk)

Contents Preface ................................................................................................ vii 1.

Introduction ................................................................................. 1 1.1 Formation Evaluation .....................................................................1 1.2 Operation ..........................................................................................2 1.3 Methods of Gathering lnformation ...............................................4 1.4 The Borehole Environment ..........................................................12 1.5 Data Reduction ..............................................................................12 1.6 Reserve Calculations .....................................................................14

2.

History ........................................................................................ 17 2.1 General ............................................................................................ 17

3.

Formation Materials Characteristics ........................................ 25 3.1 Petrophysics ...................................................................•...............25 3.2 Rock Types -Sandstones ............................................................28 3.2.1 Carbonates ........................................................................29 3.2.2 Clay Minerals ....................................................................31 3.2.2.1 Clays ..................................................................31 3.2.2.2 Shales .................................................................31 3.2.3 Other Sedimentary Materials .........................................32 3.2.4 Hard-Rock Environments ...............................................33 3.3 General Considerations ................................................................33 3.3.1 Compaction and Overpressure ......................................34 3.4 Fluids ...............................................................................................35 3.4.1 Water Characteristics .......................................................35 3.4.2 Ionic Conduction in Water ..............................................39 3.4.3 Hydrocarbons and Gases ................................................42

4.

Porosity, Permeability, Tortuosity, and Saturation ................. 59 4.1 General ............................................................................................59 4.2 Porosity .............................................................................. ,............59 4.3 Tortuosity ........................................................................................61 4.4 Permeability ...................................................................................65 4.5 Saturation........................................................................................71 4.6 Overpressure ..................................................................................?3

4.7

Saturation Evaluation ...................................................................74

4.8

Demonstration ...............................................................................77

4.9

5.

Water Cut ........................................................................................80 4.9.1 Water-Cut Determinations .............................................. 80

Borehole, Mud, and Formation Effects .................................... 83

5.1 5.2

5.3 5.4 5.5 5.6 5.7 5.8

Borehole Effects ..............................................................................83 Mud Logging..................................................................................86 5.2.1 I.a.g .....................................................................•.•.•..•.•.......ffl Hydrocarbon Logging ..................................................................88 5.3.1 Sources of Gas ...................................................................89 5.3.2 Gas Deflection................................................................... 90 Analysis of the Drilling Data .......................................................90 5.4.1 Shale Factor .......................................................................92 Invasion ...........................................................................................93 Hole Considerations......................................................................% Logging Tool Position ...................................................................98 Cased Holes ....................................................................................99

6.

Cores, Core Analysis, and Cuttings ....................................... 103 6.1 Introduction .................................................................................. 103 6.2 Uses of Coring ..............................................................................103 6.3 Core Quality .................................................................................109 6.4 Core Analysis ............................................................................... 111 6.5 Core Information ......................................................................... 111 6.6 Laboratory Measurements on Cores ......................................... 112 6.6.1 Extractors......................................................................... 112 6.6.2 Retorting .......................................................................... 114 6.6.3 Bulk Volume .................................................................... 115 6.6.4 Pore Volume- Boyle's Law Method ......................... 115 6.6.5 Pore Volume- Washburn-Bunting Method ............. 115 6.6.6 Grain I>ensity .................................................................. 115 6.6.7 Usual Practice, Pore Volume......................................... 116 6.7 Core-Derived Permeability ........................................................ 116 6.7.1 Laboratory Measurements ............................................ 117 6.8 Problems Associated with Core-Derived Data........................ 119 6.9 Cuttings Samples ......................................................................... 119 6.10 Sidewall Coring ...........................................................................120 6.11 Fluid Sampling.............................................................................120

7.

Introduction to Electric Resistance and Resistivity ............. 123 7.1 Resistance and Resistivity ..........................................................123 7.2 Definitions .................................................................................... 123 7.2.1 Resistance ........................................................................124 7.2.2 Resistivity ........................................................................ 125 7.2.3 Units ................................................................................. 128 7.3 Formation Resistivity ..................................................................128

7.4 7.5 7.6 7.7 7.8 7.9 8.

Cementation Exponent .........................................•.......•..••........•131 Rock Texture, Sandstone............................................................. 135 Rock Texture, Carbonates ........................................................... 135 Salinity...........................................................................................136 RT vs ell Crossplot- The Pickett Plot ....................................... 138 The Non-Linear- (Hingle) Crossplot.....................................139

Introduction to Radioactivity ................................................. 143 8.1 Definitions and Terms .................................................................143 8.2 Particle Description .....................................................................147 8.2.1 Electrons, Beta Particles, and Positrons ......................147 8.2.2 Neutrons .......................................................................... 149 8.2.3 Protons and Similar Particles ....................................... 152 8.2.4 Alpha Particles ............................................................... 152 8.2.5 Atoms ............................................................................... 153 8.2.6 Photons ............................................................................ 154 8.2.6.1 Gamma-Photon Scattering Interactions ..... 155 8.2.6.2 Sources and the Scattering Process ............. 159 8.2.6.3 Compton Interactions ................................... 162 8.3 Geophysical Logging Considerations....................................... 166 8.4 Health Physics ..............................................................................167

Glossary ............................................................................................ 169 Bibliography ................................................................................... 173 Index ................................................................................................. 177

Preface

This volume, Introduction to Geophysical Formation Evaluation, is intended as a practical volume for the engineer, environmentalist, geologist, miner, exploration manager, and student. Although it contains some relatively detailed analyses, it is not intended to be used to design geophysical instruments, but rather, how to use them. Its focus is on the petroleum usage of geophysical methods: petroleum-funded research and engineering is where about 90% of present-day geophysical methods originate. Non-hydrocarbon methods and practice, as well as other-than-borehole methods are planned for subsequent books. It is important to know where these methods originate. Some of the newer instruments and/ or techniques have purposely been left out; others have been included, even though they are not yet available in the field. Still others have also been included, although they have fallen into apparent disuse. The author has attemptted to judge the future value of all of the instruments and techniques; obviously, this attempt is not going to be 100% successful, and revisions will no doubt have to be made to correct these mistakes. This is a rapidly changing field; even the approaches and character of the discipline are changing, making it difficult to keep up them. There are many books currently available on how to interpret logs, how to design instruments, and how to conduct exploration. Introduction to Geophysical Formation Evaluation attempts to answer why a certain procedure is used and why a particular instrument is chosen over another. A chapter on the history of logging and formation evaluation has been included. It appears to be important, as well as interesting. Perhaps some of the myths and errors of word-of-mouth stories can be corrected. For example, the author was the Project Engineer for the MicroLog and is continually amazed at how many "were present for the first test of the MicroLog". There are several versions of "the first Schlumberger log." Frankly, I do not know which is the first one. The one shown in this text is certainly one of the early ones. The references combine both the sources of some of the information included in the text and additional books and papers for further investigation by the reader.

A note of warning!! Numerous tables have been included in this volume. Bear in mind that the values listed are being upgraded almost daily and

are therefore subject to change. Use the values for estimation only. If you have a critical calculation or determination to make, the value listed in this text may be correct. But, be sure to find the best known value and verify

the one from this text.

James K. Hallenburg Tulsa, 01clahoma

The Author

James K. Hallenburg is currently a retired geophysicist/petrophysicist living with his wife, Jaquelyn, in Tulsa, Oklahoma. Jim was born in Chicago in 1921, and has traveled to and worked in many comers of the world. During World War II, Jim was a bomber pilot and weather observer in the U.S. Army Air Corps. and still enjoys flying. Following his stint in the military, Jim returned to his studies, graduating with a B.A. in physics from Northwestern University. Throughout his career, he has attended a number of schools to further his education and has taught courses at several of them. Jim spent 18 years as an engineer, designing geophysical systems for Schlumberger Well Services He was a Senior Engineer for the Mohole Project and Chief Engineer for the Western Company of North America. He also operated and owned Data Line Logging Company in Casper, Wyoming. Later, he was Manager of Applications Engineering for Century Geophysical in Tulsa. Jim has also been a consultant for the International Atomic Energy Commission. Subsequently, Jim spent several years giving seminars in geophysics and formation evaluation. He is the author, co-author, and editor of several books and computer programs and has held office in numerous technical societies.

1 Introduction 1.1

Formation Evaluation

Formation evaluation is the process of using borehole and surface information in concert to evaluate the characteristics of subsurface formations. In doing so, the composition, character, and quality of the zone in which we are working, can be determined. Formation evaluation is the process of: 1. Estimating recoverable reserves of hydrocarbon, gas, minerals, water, or any other formation material

2. Estimating the target material in place (which, of course, is different from the recoverable material) 3. Determining the lithology and geology of the target environment 4. Assessing the general geological environment (correlation, mapping, depths, thicknesses, identifying depositional features) 5. Detection of abnormal pressures (especially in petroleum exploration) 6. Evaluation of rock stress and other pre-mining parameters 7. Locating and quantifying fluid contacts 8. Fracture detection, porosity type, and amount determination 9. Learning the history of the formation Formation evaluation for petroleum follows some predictable steps. The process for petroleum is aimed at determining, primarily, the water saturation of the target formation zone. We will learn the other information, such as rock type, as a byproduct of the saturation determination or as a deliberate separate effort. But, the determination of the saturations is the first concern. To do this, specific information will be obtained and examined, and the saturations then calculated. There are many ways to do this. The process is shown in Figure 1.1.

1

Introduction to Geophysical Formation Evaluation

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1.2 Operation In order to do these things, we must first see how the systems work and determine what their limitations and advantages are. In order to effec-

tively predict what a system will do - what it will and will not see - we must first understand how it works.

Introduction

3

FIGURE 1.2

The borehole resistivity measurement situation.

We will examine the methods of obtaining this information, We will determine what, where, and to what degree the target is being measured. We will examine common methods for correcting data to eliminate unwanted information for each system. We will see why a system is preferred and what the shortcomings are. We will investigate the methods for reducing and interpreting the data, determine a general model, and determine specific things. We will see how we can adapt methods to new and different uses. Once the systems are understood, the raw data can be processed. It must be reduced -it is seldom in a form that can be used unprocessed. That is, the raw signal values must be corrected for various unwanted and distorting factors, such as the presence of the borehole and the departure of the measurement geometry from ideal. This is because each measured volume is finite and will include more than the sample we wish to examine. See Figure 1.2. The corrected signal must then be converted to a useable form. For example, it is difficult to determine much about geological parameters with millivolts. If millivolts are converted to salinity, however, then the information can be used. Finally, one must decide what the particular value from the reduced data means, especially when used in correlation with other measured values. This last is formation evaluation. This, then will be the process we will study.

4

1.3

Introduction to Geophysical Formation Evaluation

Methods of Gathering Information

In the process of formation evaluation, we will use drilling operation logs,

geophysical wireline logs, core analysis, and production type tests. Surface, airborne, and remote sensing methods may also be used, especially as preliminary indicators. Drill cuttings are a good source of information. Because the samples are small and incomplete, and because the depths are uncertain, cuttings are a supplemental method. Depths from which the cuttings came are sometimes difficult to determine accurately because of the time lag between the cutting and their arrival at the surface. Also, larger pieces may travel to the surface at a different rate than small pieces. Clays are mostly dispersed in the liquid. Cuttings are the bits and pieces which have been ground and cut out of the formation material by the drill bit. They have been mechanically damaged and washed severely with the borehole fluid. Descriptions of the washed drill cuttings are usually logged (tabulated) as a function of the drilling depth and then correlated with the core samples and wireline logs. They can be quite valuable if they are properly handled. Among the things we can learn from the cuttings are some of the details of the formation lithology, grain size, redox condition, and hydrocarbon presence. Many other things may be determined with varying degrees of uncertainty. Other peripheral logs are mud logs (Figure 1.3), drilling time logs (Figure 1.4), cuttings analysis, mud analysis, core gamma logs (Figure 1.5) and core analysis logs (Figure 1.6). These are not wireline logs, but are less expensive ways of gathering additional formation information since they seldom require additional rig time. They are also usually made on the surface, where the environment is less hostile than down hole and they can usually be made while other operations are in progress. The amount of information which can be obtained is sometimes rather limited. This does not mean, however, that the information is valueless. It should be gathered and used and become part of the record. This group of logs is often neglected in the descriptions and explanations of the sources of information for a project. They are valuable and must be considered. Some, such as the core analysis and the wireline logs, supply basic and necessary information. Many give information not available from any other source (drilling time and mud analysis). Other are used for correlation and verification (cuttings analysis and core gamma logs). Some serve dual purposes (drilling time logs and mud analysis). More details about the uses of some of these peripheral logs and sources are covered in later chapters. The equipment to gather these items of information are usually sophisticated and designed to operate in the field. A good example of this type of instrument is the wireline logging truck (Figure 1.7). This is a complete,

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compact, mobile laboratory. It is designed to collect, process, deliver, transmit, and (often) analyze the desired information. The instrumentation for the other types of data-gathering devices are of a similar level of usefulness and sophistication. Driller's logs are sometimes confused with mud logs, but they are not the same thing. The driller's log is a time log of the happenings on the rig: the trips, bit changes, mud changes, etc. This includes the drilling rate, which can be a valuable source of information. The drilling rate can alert you to possible trouble, it will correlate well with the electric logs, and it will often allow you to estimate missing data. The driller's log is often included on the mud log. Mud logs are designed primarily to determine the mud character. Mud control is vital, especially in deep oil wells. Since the drilling mud carries the cuttings and the formation solids and fluids from the zone through which the drill is passing, it is an ongoing record of the subsurface events and some of the surface events. Much information may be gained about formation fluids and hydrocarbon presence. Mud logs are seldom used on mineral, water, and scientific projects. Mud logs are time and depth records of the borehole fluid, the drilling mud. lYJ>ically, salinity, density, viscosity, additives, gas content, oil content,

Introduction

7

FIGURE 1.5

A surface core gamma log from Grayburg-San Andres, Andrews County, Texas. (From LeRoy, L. W. and LeRoy, D. 0 ., Core AMlysis, Core Laboratories, 1982. With permission.)

resistivity, and water loss can be logged, usually on the surface. These logs can furnish the analyst valuable information about the mud resistivity, temperature, depth of invasion, and general problems. Wireline geophysical logs are the lowest cost per unit of information and the most dependable sources of high quality information for formation evaluation. Whether they are wireline logs, surface or airborne measurements, cross-hole (hole-to-hole) logs, or MWD logs, the modem trend is to offer more variety of physical, mechanical and chemical measurements and methods of interpreting them. At the same time, accuracy and reliability have increased and continue to increase markedly. These are used, among other things, to determine stratigraphy, lithology, porosity, and many other useful physical properties. Therefore, we will spend most of our effort learning the use of the wireline geophysical logging methods. Wireline logs cost approximately ten times more than the drilling operation logs. They are, however, the least expensive source of detailed and precise data that we will be able to gather on a location. Thus, they are the

Introduction to Geophysical Fonnation Evaluation

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