Porosity Study For Detail Reservoir Characterization in Darajat Geothermal Field, West Java, Indonesia

Abstract

Porosity is one of the critical factors in geothermal reserve estimation, as a majority of geothermal fluid reserves in a vapor system are stored in the reservoir rock matrix porosity. Volcanic rocks typically give a wide range of primary matrix porosity, which can be enhanced or reduced by alteration. Heterogeneity of rock distribution and alteration processes makes the evaluation of porosity in a geothermal reservoir difficult and can be misleading.
In order to realistically predict future field performance, porosity of Darajat reservoir rock has been studied in detail by combining core porosity measurements (total and effective porosity), petrography of blue-dye impregnated thin sections, X-Ray Diffraction (XRD) and Portable Infrared Mineral Analyzer (PIMA). Core data are mostly available from the wells located on the field's margin, and very limited from the field's center. A wireline porosity log estimate becomes critical where core data is not available. Integration of Schlumberger's Accelerator Porosity Sonde (APS) and Formation Micro Scanner (FMS) pseudo-resistivity were correlated with core data and provided porosity estimates for the field center. The core porosity data were used as primary data for reservoir simulation, while wireline log data were used to predict the range of porosity values.
The porosity distribution in the reservoir appears to be mainly related to rock type (texture, phenocryst size and abundance, etc.), fracture and hydrothermal alteration. Porosity of fresh lava is generally much lower compared to breccias, while porosity of breccias is lower than tuffs. A moderate to high alteration intensity generally leads to a wide range of porosity values. In lava, a higher intensity of alteration usually occurred along fracture zones and less altered in the rock matrix. In pyroclastics, alteration can occur anywhere.