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