Characterizing Reservoir Dynamics Using Hydrochemical and Structural-Geological Data in a High-Enthalpy Geothermal System in Indonesia

Abstract

Aim of this study is to provide a recent reservoir characterization with a focus on the dynamics of a structurally controlled system in a high-enthalpy geothermal field in Indonesia. Combination of hydrochemistry and structural geology allows an integrated view on historical changes in deep reservoir behaviour and surface thermal manifestations as result of 15 years exploitation. Implemented methods throughout the study include detailed fault surface mapping in the field, analysis of physicochemical properties and major and minor ions. The combination of recent fault mapping and previous studies confirms the existence of four general fault trends in the area including NE-SW, NW-SE, N-S and E-W. Along these faults and at intersections points surface thermal springs occur, which show physical and chemical evolution over the exploitation time. At present time, spring waters generally turn into more acidic followed by significant changes in physical features (e.g. size, steam fraction). The reservoir fluids generally become more saline nowadays with boiling as a possible reason. The physical and chemical changes in thermal springs and the deep reservoir indicate recent fluid dynamics in the geothermal system. The exploitation of the system triggers changes of fluid flow pathways, leading to mixing and changing fluid volumes in springs and wells. Observing these processes in a continuous and careful reservoir monitoring is highly important to ensure the long-term sustainability of the system.