Reservoir Porosity Analysis at the Ddarajat Geothermal Field

Abstract

Lithology at the Darajat Geothermal Field consists of microdiorite intrusions, andesite lavas and sequences of pyroclastic rocks (bottom to top), with the majority of the Darajat reservoir being composed of andesite lavas. Volcanic rocks typically give a wide range of primary matrix porosity, which can be enhanced by fracturing and leaching, or reduced by mineralization. Therefore, evaluation of the porosity in a geothermal reservoir can be very difficult. The integration of Formation Micro Scanner (FMS), Accelerator Porosity Sonde (APS) logs, drilling data and core plug data were applied to assess matrix and fracture porosity. FMS and drilling data were used to identify fracture zones and lithology type, while APS and core plugs were used to identify porosity and mineralization. The porosity distribution in the reservoir appears to be related to rock type. The average porosity of andesite lava from wells located on the margins of the field, which represents a non fractured zone, is much lower compared to the field center. The average porosity of this zone is approximately 2 %. Fracture related porosity was observed from wells penetrating major fault zones in the reservoir. The steam entry or loss zone related fractures are characterized by an average porosity greater than 13 %. Outside these steam entry or loss zones, the background porosity in the reservoir is 7 %. This appears to indicate that fracturing significantly enhances the primary matrix porosity of the host rock. It has also been observed that there is no apparent trend of decreasing porosity with depth at Darajat.