The Initial State Geochemical Model and Reservoir Response of 2 Years Production at Silangkitang, a Fault-Controlled Geothermal System Along the Great Sumatera Fault

Abstract

The Silangkitang (SIL) geothermal field which located along the Great Sumatera Fault (GSF) is currently producing 110 MW with the support from 4 production wells and 9 injection wells. Silangkitang is liquid-dominated system with high temperature (higher than 280°C) and relatively moderate gas (below 1 wt.% in reservoir liquid). Most of the production and injection wells are drilled perpendicular to the Great Sumatera Fault and some of the injection wells are drilled away from the fault. The production wells are drilled toward the upflow zone in the central of the prospect while the injection wells are drilled in the outflow zone which located in northwest and southeast of the field. The produced NCG at the wellhead is within 3.0 – 4.2 wt.% in steam with the highest one is shown by the southernmost production well. The fluid chemistry shows neutral pH with Na, K, Cl and SiO2 concentrations below 1000 ppm and very low Mg concentration. In addition, the oxygen-18 and deuterium show meteoric origin and the Na-K-Mg Giggenbach demonstrates full equilibrium state of reservoir fluid. The homogeneous chemical characteristic at Silangkitang strongly supports the good connection within the reservoir as the response to the fault-controlled system. The 2 years operation starts showing the chemical response which indicates an injection breakthrough without significant enthalpy impact. This result signifies the uniqueness of a geothermal reservoir within a mega fault zone in Sumatera Island.