Reservoir Characterization and Performance Prediction for a Volcanic-Hosted Geothermal Reservoir: Darajat, Indonesia

Harrison, Chris¹, Julfi Hadi², Sri Rejeki³, Joao Keller¹, Viet Hoang¹, Onto Alamsyah² (1) ChevronTexaco Energy Technology Co, San Ramon, CA (2) Star Energy, Jakarta, Indonesia (3) ChevronTexaco Energy Indonesia, Jakarta, Indonesia

Darajat is a large geothermal field located southeast of Jakarta in Java, Indonesia. The field is a large asset with energy-equivalent reserves in excess of 200MMBOE, and includes the world’s highest-performing geothermal well. A combination of geothermal techniques and geostatistical modeling tools were used to produce a suite of fine scale models with dis­continuous layers of volcanic rocks and a distributed fracture system to characterise the reservoirs in the Field. These models were then scaled up and carried through reservoir sim­ulation to provide a probabilistic assessment of anticipated performance of a planned major expansion of the Darajat production facilities.

The facies model for vapor dominated fields is especially critical since in these systems the reserves are stored primarily within the rock matrix, with fractures controlling produc­tivity. Comparison of an idealised andesite stratovolcano model with cores and cuttings from 31 wells and surface samples led to the development of a conceptual volcanic lithofacies model for the Field which guided the distributions of matrix porosities and fracture densities in the models.

A suite of 3D models of the subsurface were generated in goCad™ modeling using Experimental Design methods to capture the effects of uncertainties in key factors. The series of dynamic models developed integrated the fine scale static models with other reser­voir uncertainties such as initial liquid saturation, recharge, reservoir depth, and relative per­meabilities. The probabilistic performance of the proposed Darajat expansion was evaluated using a response surface model and Monte Carlo methods as a proxy for the simulation models. Finally, on the technical basis of this study a major expansion of the Darajat plant was sanctioned during late 2004.