| Title | Structurally Controlled Fluid Flow in a High-Enthalpy Geothermal System – Case Study Lahendong, Sulawesi (Indonesia) |
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| Authors | Maren BREHME, Bettina WIEGAND, Fiorenza DEON, Guido BLÖCHER, Mauro CACACE, Warwick KISSLING, Christoph HAASE, Kemal ERBAS, Yustin KAMAH, Simona REGENSPURG, Inga MOECK, Günter ZIMMERMANN, Martin SAUTER |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | hydraulically conductive fault, hydrochemistry, structural controls, directional permeability, fault core, geochemistry, water-rock interaction, high-enthalpy system, Indonesia |
| Abstract | This study investigates a magmatic structurally controlled geothermal reservoir in North Sulawesi (Indonesia) combining structural geology, hydrochemistry including isotopes as well as geochemistry and permeability determination. The integration of all data into a thermal-hydraulic model allows prediction of reservoir behavior and supports the sustainable use of it. Methods used throughout the study are structural mapping, fault plane analysis, field based determination of physicochemical properties of well and spring waters, discharge measurements in rivers, analysis of major and minor elements as well as isotopes of waters and rocks in the laboratory completed by investigations on permeability of rocks. Modeling of hydrochemical water properties was done using PHREEQC and subsequently a thermal-hydraulic modeling was carried out with Feflow. Hydrogeological and geothermal relevant tectonic structures in this volcanically active area include two fault zone patterns, joints, and fractures at different scales. Discharge of several thermal springs is mostly connected by the fault zones while subsurface fluid flow is presumably controlled by fractured striking NW-SE. The study area is therefore highly differentiated with respect to hydraulic properties and chemical composition of the fluids. There are two types of fluids that can be roughly classified into acid and neutral waters. Isotopes suggest a meteoric origin of geothermal groundwater. Variable trace element concentrations are subject to differences in weathering regime and geochemical equilibrium conditions in the aquifer. The main rock types in the Lahendong area are andesite and breccia. The thin sections from the surface rock material of the springs along with the cores from well indicate different stages of alteration. Temperature calculations by geothermometers are in good agreement with in situ measured temperature data, ranging between 232°C and 341°C. A compartmentalisation of the reservoir was derived from stress field analysis of the tectonic elements in combination with hydrogeological interpretation. Temperature anomalies and loss of river water suggest surface water infiltration into fault zones. The Lahendong geothermal field is subdivided into two sub-reservoirs by horizontally less permeable fault zones and permeable fracture patterns parallel to the strike of the main fault. Our study shows that geological-structural analysis in combination with hydrothermal- and geochemical investigations are the essential tools for geothermal reservoir characterization and sustainability prediction of geothermal energy exploitation. |