Why Are the Only Volcano-Hosted Vapor-Dominated Geothermal Systems in West Java, Indonesia?

Abstract

Although volcano-hosted vapor-dominated geothermal systems are rare, West Java, Indonesia, has five such systems: Kamojang, Darajat, Patuha, Telaga Bodas, and Wayang Windu. This paper seeks an explanation for such vapor-dominated geothermal systems through heat and mass transfer numerical modeling based on the characteristics of these five systems. Using a basal heat input of 12 MW/km2 into a cube-shaped reservoir, vapordominated systems are produced when the host rock permeability is less than 3 x 10-16 m2 and the caprock permeability varies between 10-17 and 10-15 m2. The caprock permeability assures that the escaping steam exceeds the incoming liquids; the host rock permeability prevents flooding until the heat source is turned off. The maximum duration of the vapor reservoirs is 2000 years, at which point the reservoir either dries out or floods. The timescale for vapor reservoir stability is an order of magnitude less than that for the formation or cooling of liquid-dominated geothermal reservoirs. The factors contributing to the occurrence of the five vapor-dominated reservoirs in West Java are intense heating due to prolonged active volcanism, an absence of shear faulting, and the restrictive permeability range of the host and caprocks surrounding a relatively permeable reservoir.