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Keith Nicholson
Geothermal Fluids Chemistry and Exploration Techniques
With 30 Figures
Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest
Dr. Keith Nicholson School of Applied Sciences The Robert Gordon University Aberdeen AB 1 IHG, Scotland United Kingdom
ISBN-13: 978-3-642-77846-9 001: 10.1007/978-3-642-77844-5
e-ISBN-13 :978-3-642-77844-5
Library of Congress Cataloging-in-Publication Data. Nicholson, Keith, 1955- , Geothennal fluids: chemistry and exploration techniques/Keith Nicholson. p. cm. . Includes bibliographical references and index ISBN-13: 978-3-642-77846-9 1. Geothennal resources. 2. Geochemistry. 3. Water chemistry. I. Title. GBll99.5.N53 1993 551.2'3--dc20 93-29590
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© Springer-Verlag Berlin Heidelberg 1993 Softcover reprint of the hardcover 1st edition 1993
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Preface
This book introduces aqueous geochemistry applied to geothermal systems. It
is specifically designed for readers first entering into the world of geothermal energy
from a variety of scientific and engineering backgrounds, and consequently is not
intended to be the last word on geothermal chemistry. Instead it is intended to
provide readers with sufficient background knowledge to permit them to subsequently
understand more complex texts and scientific papers on geothermal energy. The book
is structured into two parts. The first explains how geothermal fluids and their
associated chemistry evolve, and shows how the chemistry of these fluids can be used
to, deduce information about the resource. The second part concentrates on survey
techniques explaining how these should be performed and the procedures which need
to be adopted to ensure reliable sampling and analytical data are obtained.
A geothermal system requires a heat source and a fluid which transfers the heat
towards the surface. The fluid could be molten rock (magma) or water. This book
concentrates on the chemistry of the water, or hydrothermal, systems. Consequently,
magma-energy systems are not considered. Hot-dry rock (HDR) systems are
similarly outside the scope of this text, principally because they contain no indigenous
fluid for study. Both magma-energy and HDR systems have potential as energy
sources but await technological developments before they can be exploited
commercially. Geothermal systems based on water, however, are proven energy
resources which have been successfully developed throughout the world. The energy
within such systems has been directly employed for space heating and other direct-use
applications, and for the generation of electricity.
The chemistry of the terrestrial geothermal systems considered in this work are
(or should be) also of interest to economic geologists, since they represent the modem
equivalents of certain epithermal-gold and porphyry-copper deposits. Terrestrial
systems, being more accessible, also provide insights into the processes active in
submarine geothermal systems which are themselves modem analogues of ancient
stratiform iron, manganese and massive sulphide mineralisation.
Being able to touch waters which have circulated through the Earth's crust,
waters which may have deposited gold just a few hundred metres below and which
can provide clues to geochemical processes active in the Earth, is one of the most
exciting aspects of geothermal chemistry. All this and energy too.
Aberdeen, Scotland.
August 1993
DEDICATION
For
KEITH NICHOLSON
Acknowledgements
When a work such as this is produced over a long period of time, it is difficult
to limit acknowledgements as many people have contributed in different, often subtle
ways. However, I should first like to thank Dick Glover, John Hulston and Mike
Stewart of DSIR, New Zealand, for finding time in busy schedules to provide helpful,
detailed reviews of earlier chapters. . I am also grateful to the many graduate students,
from over 25 countries, who I taught during my time in New Zealand. Discussions
about geothermal developments in their countries, made me appreciate the difficulties
scientists and engineers from different technical backgrounds have in interpreting
geothermal fluid chemistry. These discussions convinced me of the need for an
introductory text on geothermal chemistry, and formed the basis for the contents of
this work. I should also like to thank Pat Browne (Auckland University) who
provided moral support and valuable discussions over the contents of the book,
particularly in the early months of writing. In the later months, John Harper (RGU)
generously provided me with the time and facilities to complete this work.
Throughout all this time however, my greatest supporter and source of encouragement
has been Angela, my wife, who not only drew all the figures in this volume, but also
patiently got on with the more serious business of building a home, while I got deeper
into hot water.
August 1993
Contents
Preface .......•.•.................•.............•.•••.......•...................•....•................................... v
Acknowledgements .............................................................................................. vii
Part I Geothermal Fluid Chemistry
1 GEOTHERMAL SYSTEMS
1.1 SYSTEM TYPES AND CHARACfERISTICS ....................................... 1 Classification ............................................................................................... 3
1.2 GENESIS OF GEOTHERMAL FLUIDS ................................................ 6 Origin of water and solutes ............................................................................ 6 Evolution of geothermal fluids ................................................................... 7 Evolution of steam: boiling point-depth relations ....................................... 8 Age of geothermal fluids and geothermal systems ..................................... 12
1.3 THERMAL, HYDROLOGICAL AND CHEMICAL STRUCfURE ........ 12 Liquid-dominated systems .......................................................................... 14 Vapour-dominated systems .......................................................................... 16
2 WATER CHEMISTRY
2.1 WATER TYPES .............................................................................................. 19 Chloride Sulphate
....................................................................................................... 19 20
Bicarbonate .............................................................................................. 22 Sulphate-chloride ..................................................................................... 22 Dilute chloride-(bicarbonate) ........................................................................... 23 A diagnostic plot of water chemistry ........................................................ 27
X Geothennal Fluids
2.2 PROCESSES AFFECTING WATER COMPOSmON ............................ 28 Mineral-fluid equilibria .....•.•..........................................................•........ 28 Boiling (adiabatic) cooling .......................•.................•................................. 34 Conductive cooling .......................................•............................................. 34 Mixing (dilution) with other waters ............................................................... 35
2.3 INTERPRETATION OF WATER CHEMISTRy ..................................... 35 pH ................................................................................................................. 36 Common solutes .............................................................................................. 39 Chemical behaviour of common species ........................................................ 39 Chemical indicators of physico-chemical processes ..................................... 49 Statistical analysis of water chemistry ........................................................ 50
2.4 MIXING MODELS ..................................................................................... 52 Enthalpy - chloride diagrams ......................................................................... 52 Enthalpy - silica diagrams ..........•................................................................ 62 Carbonate - silica/chloride ...................................•....................................... 65
2.5 SOLUTE GEOTHERMOMETERS .................................................................. 67 Silica ................................................................................................................. 69 NaIK geothermometer ..................................................................................... 72 Na-K-Ca geothermometer ........................................................................... 73 Na/Li geothermometer ..................................................................................... 76 K/Mg and Li/Mg geothermometers ...................................................... ~............ 77 Na-K-Mg geothermometer ........................................................................... 78 Ca/Mg & S04fF geothermometers for carbonate reservoirs ......................... 80
2.6 CALCULATING RESERVOIR FLUID CHEMISTRY ............................ 80 Heat and mass balance equation ...................................................... ............ 80 Steam fraction calculation ........................................................................... 81 Weirbox composition ..................................................................................... 82 Reservoir fluid composition ........................................................................... 82 Total discharge (fD) composition .................................................................. 83 Reservoir fluid pH ..................................................................................... 83
3 GAS CHEMISTRY
3.1 INTRODUCflON ........................................................................................... 87
3.2 DISCHARGE FEATURES ........................................................................... 88
Contents XI
3.3 PROCESSES AFFECTING STEAM COMPOSmON ............................ 89 Geothermal system ..... .............. ...... ............ ........ ........ ........ ........ .... ........ .... 90 Solubility ....................................................................................................... 90 Steam formation .............................................................................................. 96 Condensation .............................................................................................. 102 Oxidation ....................................................................................................... 102 Rock-steam reactions ..................................................................................... 102
Mineral equilibria and buffering .................................................................. 102 Seismicity ....................................................................................................... 103 Contamination .............................................................................................. ,103
3.4 INTERPRETATION OF GAS CHEMISTRY ............................ ", ................. 103 Behaviour of common geothermal gases ........................................................ 104
Indicators of physico-chemical processes ........................................................ 108
3.5 GAS GEOTIIERMOMETERS .................................................................. 112 COz-HzS-Hz-CH4 geothermometer (D' Amore & Panichi) ............................ 112
CO-based geothermometer ........................................................................... 113 COz-geothermometer ..................................................................................... 114 Hz-Ar geothermometer ..................................................................................... 114 gas~water - based geothermometers ........................................................ 114
4 ISOTOPE CHEMISTRY
4.1 INTRODUCTION .............................................................................................. 117
Notation ....................................................................................................... 118
Standards ....................................................................................................... 120
4.2 GEOTHERMAL APPLICATIONS OF ISOTOPES ..................................... 120
Stable isotope .............................................................................................. 120
Radioactive isotopes ..................................................................................... 125
4.3 ISOTOPE GEOTHERMOMETERS .................................................................. 126
Isotope geothermometry equations .................................................................. 127
Oxygen isotope geothermometers .................................................................. 128 Hydrogen isotope geothermometers ........................................................ 134 Carbon isotope geothermometers .................................................................. 135 Sulphur isotope geothermometer .................................................................. 137
XII Geothermal Fluids
Part II Exploration Techniques & Surveys
5 EXPLORATION TECHNIQUES
5.1 INTRODUCfION .............................................................................................. 141
5.2 EXPLORATION SURVEYS ........................................................................... 142 Active systems .............................................................................................. 142 Fossil/epithermal systems ........................................................................... 145
6 WATER SURVEYS
6.1 PRE-FIELD LABORATORY PREPARATION ............................................... 151 Collection bottles .............................................................................................. 151 Bottle cleaning .............................................................................................. 152 Sampling equipment ..................................................................................... 154
6.2 FIELD SURVEY .............................................................................................. 154 Mapping springs .............................................................................................. 155 Field notes ....................................................................................................... 155 Field measurements ..................................................................................... 155 Selection and prioritizing springs for sampling ............................................... 157
6.3 SAMPLING METHODS ........................................................................... 158 Springs and pools ..................................................................................... 158 Wells ................................................................................................................. 160 Specialised sampling requirements .................................................................. 162 Sample storage .............................................................................................. 163
6.4 ANALYTICAL METHODS ........................................................................... 163 Introduction .............................................................................................. 163 Aluminium .............................................................................................. 164 Ammonia ....................................................................................................... 165 Arsenic ....................................................................................................... 167 Bicarbonate, Carbonate ........................................................................... 168 Boron ....................................................................................................... 169 Caesium ....................................................................................................... 172
Contents XIII
Calcium ....................................................................................................... 172 Chloride ....................................................................................................... 173 Fluoride ....................................................................................................... 174 Hydrogen sulphide ..................................................................................... 175 Iodide, Bromide .............................................................................................. 176 Iron ................................................................................................................. 177 Lithium ....................................................................................................... 178 Magnesium .............................................................................................. 179 Mercury ....................................................................................................... 179 pH ................................................................................................................ :180 Potassium ....................................................................................................... 181 Rubidium .................................................................................... ;-0 ................. 182 Silica ....................................................................................................... 182 Sodium ....................................................................................................... 184 Sulphate ....................................................................................................... 184 Total Dissolved Solids ..................................................................................... 186
6.5 DATA QUALITY AND PRESENTATION ............................................... 186 Quality control checks ..................................................................................... 186 Ionic balance .............................................................................................. 187 Mass balance .............................................................................................. 188 Units ................................................................................................................. 188 Tabulation of results ..................................................................................... 192
7 GAS SURVEYS
7.1 PRE-FIELD LABORATORY PREPARATION ............................................... 195 Sample flasks .............................................................................................. 195 Flask cleaning and preparation .................................................................. 195 Sampling equipment ..................................................................................... 196
7.2 FIELD SURVEY .............................................................................................. 197 Selection and prioritizing fumaroles for sampling ..................................... 197
7.3 SAMPLING METHODS ........................................................................... 197 Fumaroles ....................................................................................................... 197 Gas discharges from pools ........................................................................... 198 Wells ................................................................................................................. 198 General notes on sampling gases .................................................................. 199 Sample storage .............................................................................................. 199
XIV Geothermal fluids
7.4 ANALYTICAL METHODS ........................................................................... 200 Gas chromatographic determinations ........................................................ 201 Wet chemical determinations ........................................................................... 203 Calculation of gas in the total discharge (TOgas) ..................................... 205
7.5 GAS CONCENTRATION UNITS .................................................................. 205
8 SOIL AND SOIL-GAS SURVEYS
8.1 INTRODUCTION .............................................................................................. 209
8.2 SURVEY ORGANISATION ........................................................................... 210 Orientation survey ..................................................................................... 210 Sampling grid .............................................................................................. 211
8.3 SOIL SURVEYS .............................................................................................. 211 Soil sampling .............................................................................................. 212 Sample preparation ..................................................................................... 213 Ammonia ....................................................................................................... 214 Antimony, ....................................................................................................... 214 Arsenic ....................................................................................................... 214 Boron ....................................................................................................... 214 Mercury ....................................................................................................... 215
8.4 SOIL-GAS SURVEYS ..................................................................................... 216 Soil-gas sampling ..................................................................................... 216 Carbon dioxide .............................................................................................. 217 Helium ....................................................................................................... 217 Mercury vapour .............................................................................................. 218 Radon ....................................................................................................... 219
8.5 DATA QUALITY, PRESENTATION AND INTERPRETATION .................. 220 Presentation methods ..................................................................................... 220 Anomaly identification and interpretation ............................................... 220
APPENDICES
1. Steam tables ..................................................................................... 225 2. Atomic weights ..................................................................................... 233
Contents XV
REFERENCES .............................................................................................. 235
SUBJECT INDEX .............................................................................................. 255