Updated conceptual model of the Patuha geothermal field, Indonesia

Abstract

The 55 MW Patuha geothermal power plant is tapping steam from a vapourdominated reservoir located in West Java Province of Indonesia. Systematic analysis of downhole data from the 31 deep and shallow wells drilled to date has produced initial pressure and temperature models for the extended resource area. The reservoir is split into four zones by their pressure potential. At top is a cold groundwater reservoir that extends down to about 1750 m a.s.l. Below is a clay cap, observed as a linear and conductive type thermal gradient down to elevations of 1050-1300 m a.s.l. The top of the productive high-temperature reservoir is located at this depth. The reservoir is mostly hosted within andesite, breccia and microdiorite intrusive rock. Matrix permeability may play a significant role in this lithological unit. A steam cap is encountered in its shallower part, underlain by a single-phase water reservoir of hydrostatic pressure. The steam cap has a thickness of 220-830 m. Its pressure ranges from as high as 50 bars at the Putih Crater, down to about 20 bars in the eastern wellfield. Such a lateral pressure gradient may relate to reservoir compartmentalization along faults that shift the reservoir host rock unit.

The waterdominated reservoir has its top at 400-500 m a.s.l.; its details remain to be seen as most wells do not reach this depth. The updated conceptual reservoir model presented here is similar to previous ones with the exception of having the reservoir largely hosted by the same lithological unit. A single hot and magmatic upflow zone under the Putih Crater is proposed and compartmentalization is used to explain the lateral pressure gradient within the steam cap. Several maps of initial temperature and pressure distribution allow for estimating essential parameters for a volumetric generating capacity model. The extended reservoir area is between 6 and 20 km2 , its thickness is between 900 and 2000 m and the temperature ranges from 220 to 260°C. These model parameters yield a generating capacity in excess of the currently installed 55 MW. Due to the vapour-dominated nature of the resource, a more detailed 3D model should be applied, in particular, to study the behaviour of the liquid dominated section during long term production. Modelling studies of other vapour-dominated fields on Java suggest that steam caps like that of Patuha are supported by their underlying liquid water zone. Long term resource management should consider this.