Volcano-hosted vapor-dominated geothermal systems in permeability space

Although volcano-hosted vapor-dominated geothermal systems are rare, West Java, Indonesia, has five such systems: Kamojang, Darajat, Patuha, Telagabodas, 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. Specifically, the 3D model TOUGH2 is used to explore heat input and permeability conditions that lead to vapor-dominated systems. Our generic model consists of a reservoir, a host rock, and a caprock. The size of the reservoir is 16 km2 and 2.8 km thick, with 10% porosity and a permeability of 10−13 m2. The initial condition of the model is fully liquid saturated at a normal geotherm with hydrostatic pressure. An intrusive heatpulse of 8 MW/km2 for 9 kyr is applied to the base to the reservoir and the resulting geothermal system characteristics are mapped out in permeability space. A vapor-dominated system is produced when the hostrock permeability is 5 × 10−17 m2 or less accompanied by the caprock permeability varying from 10−17 to 4 × 10−16 m2. The caprock permeability assures that the escaping steam exceeds the incoming liquids; the hostrock permeability prevents flooding until the heat source is turned off. Increasing the basal heat input to 12 MW/km2 shifts the maximum host-rock permeability for vapor systems to 10−16 m2, but there is minimal change in the maximum caprock permeability. In all models the maximum duration of the vapor reservoirs is 1.5 kyr, 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 range of low permeability in the host and caprocks surrounding a relatively permeable reservoir.