Fundamentals of Coalbed Methane Reservoir Engineering

415 Pages·2011·43.37 MB·English

by  John Seidle

Fundamentals of
Coalbed
Methane
Reservoir Engineering
John Seidle
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Disclaimer. The recommendations, advice, descriptions, and the methods in this book are presented solely for
educational purposes. The author and publisher assume no liability whatsoever for any loss or damage that results
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Library of Congress Cataloging-in-Publication Data
Seidle, John.
Fundamentals of coalbed methane reservoir engineering / John Seidle.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-59370-001-0
1. Gas reservoirs. 2. Coalbed methane. 3. Gas engineering. I. Title.
TN880.S45 2011
622’.3385--dc23
2011019056
All rights reserved. No part of this book may be reproduced, stored in a retrieval system,
or transcribed in any form or by any means, electronic or mechanical, including photocopying
and recording, without the prior written permission of the publisher.
Printed in the United States of America
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To Dana and our grandchildren
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Contents
Preface.....................................................................................xi
Acknowledgments .........................................................................xiii
1. Introduction ............................................................................... 1
What Is Coal Gas? ..........................................................................1
Estimated Worldwide Coal Gas Resources......................................................3
Reservoir Properties of Selected Coals .........................................................5
Construction of Coal Gas Analogs ...........................................................11
Coal Gas Pilots ............................................................................13
Statistical Nature of Coal Gas Exploitation.....................................................15
Current Challenges to Coal Gas Exploitation ..................................................16
References ................................................................................18
2. Coal Fundamentals ........................................................................ 19
Introduction ..............................................................................19
Coal Rank ................................................................................19
Proximate and Ultimate Analyses ............................................................22
Example 2.1. Comparison of daf and dmmf fractions........................................24
Number of Samples and Confidence Limits....................................................25
Example 2.2. Arkoma Basin—Hartshorne coal density.......................................27
Sample Collection and Preservation ..........................................................28
Macerals..................................................................................29
Cleats ....................................................................................30
Coal Porosity..............................................................................32
Coal Density ..............................................................................36
Example 2.3. Organic fraction and ash density of San Juan Basin Fruitland coal .................38
Coal Gas Composition and Gas Properties ....................................................40
Example 2.4. Average coal gas Z factors....................................................45
Gas Desorption............................................................................47
Example 2.5. Desorption times...........................................................50
Coal Rock Properties.......................................................................51
Nomenclature .............................................................................54
References ................................................................................56
3. Geologic Aspects of Coal Gas Reservoir Engineering .......................................... 59
Introduction ..............................................................................59
Coal Ages, Distributions, and Depositional Environments.......................................60
Coal Rank ................................................................................69
Coal Cleats................................................................................72
Coal Gas Origin and Composition—Thermogenic and Biogenic Gas..............................73
Emerging Geoscience Concepts for Coal Gas Reservoir Engineering ..............................80
Wireline Logs .............................................................................81
Nomenclature .............................................................................87
References ................................................................................88
4. Measurement of Coalbed Gas Content ....................................................... 93
Introduction ..............................................................................93
Direct Methods............................................................................93
vii
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viii Fundamentals of Coalbed Methane Reservoir Engineering
Lost Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
Residual Gas ............................................................................ 104
USBM Direct Method .................................................................... 105
Smith and Williams Method............................................................... 106
Curve Fit Methods ....................................................................... 109
Mass Normalized Gas Contents............................................................ 110
Sources of Error in Gas Content Measurements .............................................. 112
Typical Coalbed Gas Contents ............................................................. 112
Mudlog Gas Content ..................................................................... 113
Number of Samples ...................................................................... 113
Acknowledgements ...................................................................... 121
Nomenclature ........................................................................... 122
References .............................................................................. 123
5. Sorption of Gas on Coals .................................................................. 125
Introduction ............................................................................ 125
Langmuir’s Equation ..................................................................... 126
Effect of Ash and Moisture, Daf and Dmmf Isotherms......................................... 130
Effect of Coal Rank on Sorption............................................................ 133
Effect of Temperature on Sorption.......................................................... 134
Example 5.1. Temperature effects on sorption isotherms—Dietz 3 coal, Powder River Basin..... 136
Number of Isotherms Required ............................................................ 139
Number of isotherms to characterize a single seam........................................ 139
Number of isotherms to characterize multiple seams ...................................... 141
Number of isotherms to characterize a project............................................ 143
Number of isotherms to characterize a basin ............................................. 143
Undersaturation ......................................................................... 146
Sorption Isotherms for CO , Nitrogen, and Other Gases....................................... 147
2
Multicomponent Langmuir Isotherms ...................................................... 148
Apparent Oversaturation.................................................................. 149
Nomenclature ........................................................................... 151
References .............................................................................. 152
6. Coal Permeability......................................................................... 155
Introduction ............................................................................ 155
Theoretical Absolute Coal Permeability ..................................................... 156
Example 6.1. Hartshorne coal, Arkoma Basin—cleat permeability and porosity ............... 159
Stress Dependence of Coal Permeability..................................................... 159
Gas-Water Relative Permeabilities in Coal................................................... 166
Matrix Deformation Due to Sorption ....................................................... 174
Combined Stress and Matrix Shrinkage Influences on Coal Permeability......................... 175
Nomenclature ........................................................................... 181
References .............................................................................. 182
7. Coal Well Pressure Transient Tests.......................................................... 185
Introduction ............................................................................ 185
Injection/Falloff Tests..................................................................... 185
Example 7.1. Coal well prefrac falloff test ................................................ 187
Drillstem Tests .......................................................................... 192
Tank Tests .............................................................................. 193
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Contents ix
Slug Tests ............................................................................... 194
Diagnostic Fracture Injection Tests......................................................... 195
Drawdown and Buildup Tests.............................................................. 196
Example 7.2. Coal well pressure buildup test ............................................. 200
Sorption Compressibility.................................................................. 205
Two-Phase Pseudopressures............................................................... 207
Why Dual Porosity Behavior Is Not Apparent in Coal Well Tests................................ 208
Interference Tests ........................................................................ 210
Micropilot Injectivity Tests ................................................................ 211
Nomenclature ........................................................................... 212
References .............................................................................. 214
8. Gas and Water Mass Balances in Coals ...................................................... 217
Coal Gas Mass Balance Equation........................................................... 217
Solution of the Coal Gas Mass Balance Equation ............................................. 224
Example 8.1. Coal gas reserves—King method—GRI well.................................. 224
Example 8.2. Coal gas reserves—King method—Canyon coal, Powder River Basin ............ 227
Modified King Method ................................................................... 228
Example 8.3. Coal gas reserves—modified King method—GRI coal well ..................... 229
Example 8.4. Coal gas reserves—modified King method—Canyon coal well .................. 230
Jensen and Smith Modified Material Balance................................................. 231
Example 8.5. Coal gas reserves—Jensen and Smith method—GRI coal well................... 231
Example 8.6. Coal gas reserves—Jensen and Smith method—Canyon coal well................ 232
Coal Gas Recovery Factor................................................................. 233
Example 8.7. Gas recovery factor—unidentified GRI well .................................. 235
Example 8.8. Gas recovery factor—Canyon coal well ...................................... 236
Gas Mass Balance Equation for Undersaturated Coals......................................... 238
Gas Mass Balance Equation for Multicomponent Gases ....................................... 239
Example 8.9. Multicomponent gas recovery .............................................. 242
Nomenclature ........................................................................... 245
References .............................................................................. 246
9. Gas and Water Flow in Coals ............................................................... 247
Introduction ............................................................................ 247
Gas Flow in Coal Seams .................................................................. 247
Example 9.1. Time to pseudosteady-state flow............................................ 253
Characteristic Times for Sorption and Darcy Flow............................................ 255
Bottomhole Pressure—Constant Rate Gas Production—Infinite Coal ........................... 256
Example 9.2. Bottomhole flowing pressure for a Warrior Basin coal well ..................... 257
Bottomhole Pressure—Constant Rate Gas Production—Bounded Drainage...................... 260
Example 9.3. Bottomhole flowing pressure for an Arkoma Basin coal well .................... 260
Gas Production Rate—Pseudosteady State................................................... 262
Gas Production Rate—Constant Bottomhole Pressure—Bounded Drainage...................... 262
Water Flow in Coal Seams................................................................. 263
Example 9.4. Bottomhole flowing pressure of a well in an undersaturated coal ................ 264
Nomenclature ........................................................................... 267
References .............................................................................. 268

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x Fundamentals of Coalbed Methane Reservoir Engineering
10. Depletion of Gas and Water in Coals........................................................ 271
Introduction ............................................................................ 271
Tank-Type Model of Coal Depletion........................................................ 272
Example 10.1. Depletion of Utah #25-7-6, Drunkard’s Wash Field, Uinta Basin................ 275
Gas Production from Dry Coals............................................................ 287
Example 10.2. Depletion of two Arkoma Basin coal wells .................................. 287
Depletion of Undersaturated Coals......................................................... 291
Example 10.3. Depletion of a Marylee coal well, Rock Creek Project, Warrior Basin............ 294
Coal Well Decline Curves ................................................................. 296
Example 10.4. Decline curve analysis of Utah #25-7-6, Drunkard’s Wash Field, Utah........... 298
Example 10.5. Decline curve analysis of two Arkoma Basin coal wells........................ 299
Gas Composition during Depletion......................................................... 301
Example 10.6. Gas composition during laboratory depletion................................ 305
Example 10.7. Gas composition during depletion of a San Juan coal well ..................... 310
Depletion of Variable Permeability Coal..................................................... 311
Nomenclature ........................................................................... 314
References .............................................................................. 316
11. Simulation of Coal Well Performance ....................................................... 319
Introduction ............................................................................ 319
Development of Coalbed Methane Simulators ............................................... 319
Simulation Insights into Coal Reservoir Physics.............................................. 322
Probabilistic Coal Well Simulations......................................................... 323
Simulation of Enhanced Coalbed Methane Recovery and CO Sequestration ..................... 325
2
Required Inputs ......................................................................... 326
History Matching ........................................................................ 327
Example 11.1. Simulation of undersaturated coal with tank and gridded models............... 331
Example 11.2. Cleat compressibility effects in undersaturated coal........................... 335
Example 11.3. Effect of sorption time on coal well performance............................. 338
Nomenclature ........................................................................... 343
References .............................................................................. 344
12. Enhanced Coalbed Methane Recovery and CO Sequestration ................................. 347
2
Introduction ............................................................................ 347
Binary Langmuir Sorption ................................................................ 347
Example 12.1. Coalbed gas contents of a two-component gas mixture........................ 348
Early History of ECBM and CO Sequestration in Coals....................................... 350
2
Linear Flooding of a Coal Deposit.......................................................... 356
Example 12.2. Nitrogen breakthrough in the Tiffany ECBM Pilot, San Juan Basin ............. 362
Example 12.3. Effect of Langmuir pressure on CO breakthrough in ECBM................... 366
2
Coal Absolute Permeability Variation during ECBM or CO Sequestration....................... 372
2
Tank Model for CO Sequestration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
2
Example 12.4. CO sequestration in San Juan coal......................................... 376
2
Nomenclature ........................................................................... 382
References .............................................................................. 384
Index .................................................................................... 387
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Preface
Coalbed methane developed from a safety hazard in coal mines to an unconventional gas reservoir in the last
quarter of the 20th century. Many articles dealing with coal geoscience and reservoir engineering have been
written over the last 30 or 40 years, and the field is now sufficiently mature that several good textbooks and
monographs on these subjects have appeared. Why add another? Three reasons. First, coalbed methane can now
be differentiated into separate specialties such as geology, geochemistry, geophysics, drilling and completions
engineering, and operations and facilities engineering in addition to reservoir engineering. Secondly, the coalbed
methane industry is increasingly global, allowing regional perspectives of previous work to be fused into a larger
vision. Lastly, coalbed methane reservoir engineering has evolved many specialized methods in recent years.
Entry into the field could perhaps be eased by a book stressing the fundamental aspects of coal gas reservoir
engineering.
This book is not meant to be an exhaustive compilation of all coalbed methane reservoir engineering,
a rapidly evolving field. It is meant to be a useful introduction to a unique unconventional gas resource for
students and practicing engineers as well as a basic handbook for those involved in coalbed methane on a daily
basis and require straightforward, practical answers in the fast-paced business world. I am grateful to all the
people who have assisted me in bringing this book to completion, but all errors remain mine and mine alone.
I will consider this book successful to the extent that it promotes additional developments in coalbed methane
reservoir engineering that render it obsolete.
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