Use of isotope techniques to trace the origin of acidic fluids in geothermal systems

204 Pages·2005·4.24 MB·English

IAEA-TECDOC-1448
Use of isotope techniques to
trace the origin of acidic fluids in
geothermal systems
April 2005
IAEA-TECDOC-1448
Use of isotope techniques to
trace the origin of acidic fluids in
geothermal systems
April 2005
The originating Section of this publication in the IAEA was:
Isotope Hydrology Section
International Atomic Energy Agency
Wagramer Strasse 5
P.O. Box 100
A-1400 Vienna, Austria

USE OF ISOTOPE TECHNIQUES TO TRACE THE ORIGIN OF
ACIDIC FLUIDS IN GEOTHERMAL SYSTEMS
IAEA, VIENNA, 2005
IAEA-TECDOC-1448
ISBN 92–0–102805–9
ISSN 1011–4289
© IAEA, 2005
Printed by the IAEA in Austria
April 2005
FOREWORD

Geothermal energy is an indigenous source of energy and, if used properly, also a renewable one.
Many IAEA Member States are currently using these resources either for electric power generation or
for direct use of the heat. However, geothermal development involves high risks due to its geological
uncertainty, intensive drilling investment and other technical difficulties.

Natural waters with pH values lower than 5 found in geothermal areas are called ‘acidic fluids’. The
presence of these fluids constrains the development of geothermal resources in many geothermal
systems around the globe. Acid wells are either plugged or abandoned. Limited understanding of their
origin prevents effective management of many high temperature geothermal reservoirs.

This issue was identified as a major problem encountered in geothermal development and
management during an advisory group meeting organized by the IAEA and held in Vienna from
29 May to 1 June 1995. At that meeting, the state of the art on this subject was reviewed and research
needs identified by a group of 19 experts from 13 Member States. It was suggested that a research
project be supported by the IAEA to promote the application of isotope techniques, in particular,
isotopes of sulphur compounds, to study the origin of acidic fluids in geothermal reservoirs.

Based on the recommendations of the meeting, the IAEA implemented a Coordinated Research
Project (CRP) on The Use of Isotope Techniques in Problems Associated with Geothermal
Exploitation (1997–2000). Research groups from China, Indonesia, Italy, Japan, Mexico, the
Philippines, the Russian Federation, Turkey and the United States of America participated in and
contributed to the project. The current publication is a compilation of final reports on ten individual
studies carried out under the CRP.

The IAEA would like to thank A. Truesdell and H. Ferrer for reviewing the manuscripts. The IAEA
officer responsible for this publication was Zhonghe Pang of the Division of Physical and Chemical
Sciences.
EDITORIAL NOTE
The papers in these proceedings are reproduced as submitted by the authors and have not undergone
rigorous editorial review by the IAEA.
The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating
Member States or the nominating organizations.
The use of particular designations of countries or territories does not imply any judgement by the
publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and
institutions or of the delimitation of their boundaries.
The mention of names of specific companies or products (whether or not indicated as registered) does
not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement
or recommendation on the part of the IAEA.
The authors are responsible for having obtained the necessary permission for the IAEA to reproduce,
translate or use material from sources already protected by copyrights.
CONTENTS

SUMMARY OF THE COORDINATED RESEARCH PROJECT........................................................ 1
Origin of sulphur compounds and application of isotope geothermometry in
selected geothermal systems of China .............................................................................................. 5
Zhonghe Pang
Experimental study and modelling of water-rock interaction in
active geothermal fields: Los Azufres ............................................................................................. 21
Wenbin Zhou, Zahanshi Zhang
Environmental isotopes of geothermal fluids in Sibayak geothermal field .......................................... 37
Z. Abidin, D. Alip, L. Nenneng, P.I. Ristin, A. Fauzi
Preliminary notes on the acid fluids of the Miravalles geothermal field
(Guanacaste, Costa Rica) ................................................................................................................. 61
F. Gherardi, C. Paninchi, A. Yock-Fung, J. Gerardo-Abaya
Isotope techniques for clarifying origin of SO4 type acid geothermal-fluid —
Case studies of geothermal areas in Kyushu, Japan......................................................................... 83
K. Matsuda, K. Shimada, Y. Kiyota
Chemical and isotopic study to define the origin of acidity in
the Los Humeros geothermal reservoir ........................................................................................... 97
E.H. Tello, R.A. Tovar, M.P. Verma
Sulphur isotope ratios in Philippine geothermal systems ................................................................... 111
F.E.B. Bayon, H. Ferrer
Isotope geochemistry of thermal springs in the Karymsky geothermal areas,
Kamchatka, Russian Federation..................................................................................................... 133
G.A. Karpov, A.D. Esikov
Research on isotope techniques for exploitation of geothermal reservoirs
in western Turkey........................................................................................................................... 155
S. Simsek
Chemistry of neutral and acid production fluids from the Onikobe geothermal field,
Miyagi Prefecture, Honshu, Japan ................................................................................................. 169
A.H. Truesdell, S. Nakanishi

LIST OF PRINCIPAL SCIENTIFIC INVESTIGATORS.................................................................. 195

LIST OF RELATED IAEA PUBLICATIONS................................................................................... 197
SUMMARY OF THE COORDINATED RESEARCH PROJECT
Background and objectives

Natural waters with pH values lower than 5 found in geothermal areas are called ‘acidic fluids’. These
fluids occur in geothermal well discharges, especially in geothermal areas associated with recent
volcanism such as the geothermal areas in Central America, Japan and the Philippines. Acidic fluids
cause serious damage to production wells. Their origin needs to be understood in order to design
appropriate preventive or treatment measures.

Previous research has mainly aimed at determination of geological occurrence and chemical
characterization of acidic fluids. Two major types of acidity had been identified, sulphate acidity and
HCl acidity. The former was believed to be the result of oxidation of H2S or SO2 from a deep source to
SO4. The latter was hypothesed as resulting from the reaction of feldspar with superheated steam.

Realizing the potential contribution of stable isotopes of the water molecule and those of sulphur
compounds in tracing the sources of acidic fluids, especially the sulphate type of acidity in geothermal
well discharges, the IAEA organized a Coordinated Research Project (CRP) with the objectives of
integrating isotope techniques, in particular the isotopes of sulphur compounds, into:

(1) identification of the origin of the water component in acidic fluids;
(2) identification of the origin of sulphur compounds in acidic fluids and;
(3) study on mixing of waters from different sources to form acidic fluids.

In addition to these objectives, the CRP was also to test isotope geothermometry based on sulphur
compounds in geothermal fluids.

Ten research groups from China, Indonesia, Italy, Japan, Mexico, the Philippines, the Russian
Federation, Turkey and the United States of America participated in the CRP and carried out field and
laboratory investigations on twenty geothermal fields. Samples of geothermal waters, gases and
18 2 3 34
minerals were collected and analyzed for the following isotopes: δ O, δ H, H of water, δ S in H2S
34 18 34
gas, δ S in aqueous sulphate (SO4), δ O in aqueous sulphate (SO4), δ S in anhydrite and pyrite and
18
δ O in anhydrite as required by the individual research projects.

Table I provides an overview of geothermal fields investigated, main technical issues involved and
isotope methodologies used and major findings of the project. Results of the research can be
summarized as follows.

Origin of water in acidic fluids
In this CRP, isotopic composition of oxygen-18 and deuterium in waters sampled from acidic wells
indicates that these waters are mixtures of meteoric and magmatic waters. Examples of this type of
geothermal systems are Miravalles in Costa Rica, Onikobe in Japan, Los Humeros in Mexico and
Several fields in the Philippines. It has further shown that acidic fluids are isotopically heavier in some
systems, implying a larger fraction of magmatic inputs. This isotopic evidence confirms the hypothesis
that some acidic fluids are un-neutralized magmatic water.

Origin of sulphur compounds in acidic fluids
There are three possible origins for the aqueous sulphate in geothermal well discharges of a volcanic
geothermal system:
a. oxidation of H2S gas similar to that producing acidic hot springs and ponds at the surface,
b. hydrolysis of magmatic SO2 gas,
c. hydrolysis of native sulphur at shallow depth.
1
34
δ S in aqueous sulphate (SO4) formed by the first process is close to 0 ‰ VCDT, similar to those
34
found in surface thermal manifestations. However, δ S (SO4) is enriched (up to 30‰ VCDT) in acidic
well discharges. Isotopically enriched fluids are found to be also higher in SO4 concentration, such as
34
those in Kyushu, Japan. This δ S (SO4) enrichment has been interpreted by different authors as either
from hydrolysis of native sulphur at a shallow depth or as hydrolysis of deep magmatic SO2. If there
were additional evidence on the origin of the water component in the respective well discharges, it
would have been possible to distinguish the two origins.

34
Some studies in this CRP report data of δ S in H2S and in SO4 samples from both acidic and neutral
34
wells of the same geothermal field. In neutral wells, they are identical, with δ S values in the range of
±5 ‰ deviation from 0 ‰ VCDT that was accepted for un-altered magmatic H2S. In acidic wells,
34
fractionation of S between H2S and SO4 has been recognized. Furthermore, there is a negative
34
correlation between pH of water and δ S (H2S) but positive correlation with that in SO4, implying an
isotope exchange between the two.

Application of geothermometers based on isotopes in sulphur compounds
34
Isotope geothermometry based on δ S (H2S) provides estimates of reservoir temperatures that are
compatible to measured temperature values for the acidic wells, but not for neutral pH wells. This
suggests that fractionation equilibrium is more easily attained in acidic wells as compared to neutral
pH wells.
18
Isotope geothermometer based on δ O in water and aqueous sulphate was found to be generally not
applicable to low temperature (<100ºC) formation waters in sedimentary basins, but gave agreeable
results for saline thermal waters of marine origin at reservoir temperature of 150ºC.

Future research needs
Results of the CRP have improved the understanding of acidic fluids in terms of the origin of water and
sulphur compounds in the fluids. Existing theories have been tested including geothermometry
methodologies. However, isotope data collected for sulphur compounds are still rather limited.
Extended sampling and measurements on isotopes will further verify the findings of this project.
Research on gaseous tracers, such as carbon gases and noble gases in geothermal systems, will increase
the knowledge on the origin of geothermal fluids in general.

2
3
Table I. Overview of research carried out in the geothermal fields under the coordinated research project on acidic fluids in geothermal reservoirs

Principal Name of geothermal Isotopes or other tools
investigator field Technical issues used Main findings and conclusions
18 2 34
Pang, Z. Thermal waters of Origin of thermal water and δ O, δ H, δ S (SO4), In Southern Fujian, thermal water is a mixture of
18
(China) Southern Fujian and aqueous sulphate, δ O (SO4) meteoric water and seawater and the sulphate is
Eastern Hebei applicability of aqueous marine origin. In Eastern Hebei, aqueous sulphate is
Provinces, China sulphate oxygen isotope non-marine in origin. Isotope geothermometry
geothermometry temperature estimates are compatible to reference
temperatures in the former case but not in the latter.
Zhou, W. Los Azufres, Mexico Chemistry of Water Rock Water and rock chemistry, Water-rock interactions in the geothermal field have
(China) interactions in an active reaction experiments, two stages producing different assemblages of
geothermal system numerical modelling secondary minerals. Acidic fluids may be produced by
oxidation of H2S gas.
18 2
Abidin, Z. Sibayak, Sumatra, Origin of geothermal water, δ O, δ H, The system doesn’t seem to gain magmatic inputs of
34 34
(Indonesia) Indonesia possible linkage to a δ S(H2S), δ S (H2S), δ the nearby Sibayak volcano.
34 18
nearby volcano S (SO
4), δ O (SO4)
18 2
Panichi, C. Miravalles, Costa Rica Origin of acidic fluids in δ O, δ H Acidic fluids found in production wells may originate
(Italy) production wells from inflow of immature volcanic waters or oxidation
of H2S near the surface.
34 34
Matsuda, K. Hatchobaru, Takigami, Origin of acidic fluids in δ S (H2S), δ S (SO4), Origin of acidic fluids in the production wells is from
18
(Japan) Shiramizugoe, and production wells δ O (SO4) shallow low temperature SO4-rich waters.
yamagawa, Kyushu,
Japan
18 2
Los Humeros, Mexico Origin of HCl acidity δ O, δ H, Tritium High HCl in the steam phase is produced by reactions
Tello, E.
between NaCl and Rock minerals.
(Mexico)
18 34 34
Bayon, B. Mt. Apo, Bacman, Origin of acidic fluids in δ O, δ S(H2S), δ S H2S in acid wells is magmatic origin. Isotopically light
18 34
(Philippines) Mahanagdong, production wells and (SO4), δ O (SO4), δ S acid sulphate waters are formed by oxidation of H2S
34
Palinpino, Philippines isotope geothermometry (Anhydrite), δ S(Pyrite), at shallower depth, heavier ones reflect equilibrium
δ18O (Anhydrite) with H2S in the reservoir or deeper hotter
environment. Aqueous sulphate isotope
geothermometry gives similar temperature estimates
to measured ones in lower pH environments but not in
neutral pH environments.