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 δ 18O 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