The dynamic evolution of geothermal system in Lahendong, North Sulawesi, Indonesia

Abstract

This study discusses integrated approaches to characterizing dynamical evolution of the exploited high-enthalpy geothermal system of Lahendong, North-Sulawesi, Indonesia. Lahendong has two primary reservoirs, the southern and the northern. The main focus is characterisation of detailed fluid dynamics in the system with respect to geological structures and permeability distribution. The approaches combine integrated analysis of well data and detailed hydrochemical analysis to understand how the geothermal system evolves and the distribution of changes due to structural influence and prolonged exploitation of the reservoirs.Two different methods are implemented into two different chapters. Chapter I focuses on integrating well behaviour data, detailed surface fault mapping, analysis of well development, tracer tests, and logs. Chapter II emphasizes the hydrochemistry dynamics in subsurface and surface using physicochemical and hydrochemical data, spatial analysis, relationship analysis, ternary chart evolutions, geothermometer, XRD (X-Ray Diffraction) of cores and surface alteration.Result of Chapter I confirm the general trend of important faults (NE-SW, NW-SE, E-W, N-S) in the study area. The faults compartmentalize the reservoir. A comparison of the southern and the northern reservoir gives a broad insight of structures, permeability and hydrogeology, derived from the integrated analysis of well behavior data. All of this relates to changes in reservoir fluid character with exploitation.Observation of hydrochemistry perspective in Chapter II reveals the reservoir fluids have generally become more saline and equilibrated with boiling as possible mechanisms according to ternary chart evolutions and geothermometer analysis. Over time, spring waters have generally become more acidic followed by significant changes in physical features (ebullition, etc) and spatial changes. These changes demonstrate permeability evolution and also the role of structural permeability in responding to changing reservoir conditions.The combined and detailed analysis of Chapter I and Chapter II bring important and broad knowledge of evolution on fluid dynamics in Lahendong geothermal system. The exploitation of the system has triggered the changes. Observing these changes with continuous and careful reservoir monitoring is highly important to ensure the long-term sustainability of the system.Keywords: structural controls, permeability controls, fluid dynamics, high enthalpy system, exploitation, integrated analysis, hydrochemistry, well behavior.