The role of fault zones on structure, operation and prospects of geothermal reservoirs - A case study in Lahendong, Indonesia

Abstract

Expiration of fossil fuels and climate irregularities directs the energy demands towards renewable energy sources for the energy supply in future. In this frame, geothermal energy gives a substantial contribution to the strategies of renewable-source based energy production. Efficiency of this component requires to develop new geothermal sites and to improve the performance of existing systems. The main contribution of geoscience is to optimize and characterize the potential of geothermal sites. One of the essential steps of reservoir characterisation is the understanding of fluid flow in the reservoir. Fluid flow in tectonic active areas is mainly controlled by fault zones. In this study, structural mapping and hydrogeological analysis is used to provide insights into the regional reservoir setup. Here, geohydrochemical analysis is performed to characterize fluid- and rock-composition and the interaction between fluids and rocks. On the other hand, numerical simulations are used to explain the role of fault hydraulic conductivity and fluid properties on temperature and pressure distribution in the study area. The study area is the high-enthalpy geothermal field Lahendong in Sulawesi-Indonesia. It hosts a producing geothermal power plant producing 80 MW of electricity. Geoscientific investigations in the Lahendong geothermal field have started early 1970s. However, the evolution and distribution of thermal fluids within the target area is still in debate. The present day conceptual model shows that the geothermal field consists of two sub-reservoirs separated by horizontally less permeable fault zones. Brine of low pH is predominantly seen in the north while moderate pH fluids exist in the south and east. Accordingly, production rates vary between the northern and southern parts by a factor of five. However, faults behave permeable sub-parallel to the strike. Therefore, hot springs arise mostly along or at junction of faults. Lahendong area is characterized by basaltic andesite, tuff and volcanic breccia. Detailed investigations on hydraulic conductivity of fault zones show that faults either act as fault-normal flow barriers due to sealing of the fault core, or as conductive pathways in the damage zone sub-parallel to the fault strike. The damage zone, especially in case of extensional faults, is characterized by fractures. The impermeable fault core is a barrier between one reservoir section, which is characterized by acidic water, considerable gas discharge, high productivity and strongly altered and fractured rocks and another section, which hosts pH-neutral waters, high temperatures and less altered rocks. Those reservoir conditions observed on-site have been converged in numerical hydrochemical models. The fault-controlled vertical and horizontal fluid flow is used to simulate different reservoir sections. Recharge and discharge in the model occurs along the faults. However, fluid flow is also influenced by fluid phase transition. Steam propagation at top of faults stimulates vertical fluid rise, because steam propagates faster due to lower density. Therefore, in case of 2-phase flow simulations, permeabilities have to be lower to satisfy same pressure and temperature conditions. The main contribution of this study is to show that systematically performed structural analysis helps to understand the fluid flow in geothermal reservoirs. It has been confirmed that the hydrotectonic concept combining the tectonic and hydrogeological information essentially improves the understanding of subsurface flow of thermal fluids, which is the basic source of geothermal power plants. This is crucial for site selection and smart drilling strategies, which supports a sustainable exploitation of the geothermal field avoiding risks, such as low-productive wells or the production of highly corroding waters. Results also guide reservoir management in case of a potential for field extension, as performed in Lahendong.