ELECTRICAL RESISTIVITY MEASUREMENTS OF ANDESITE AND HYDROTHERMAL BRECCIA FROM THE AWIBENGKOK GEOTHERMAL FIELD, INDONESIA

Abstract

Laboratory measurements of the electrical resistivity of rocks and synthetic rocks with confining pressures up to 100 bars and temperatures between 20 and 211?C were performed to further investigate how the pore-size distribution and associated capillarity affects boiling in porous media. In all cases, resistivity gradually increased when pore pressure was decreased below the phase-boundary pressure of free water, indicating that boiling is controlled not only by temperature and pressure, but also by pore size distribution. This effect was first reported by us for metashale from The Geysers. Total changes in resistivity with boiling are comparable to Geysers rock for andesite from Awi 1-2 and much larger for a hydrothermal breccia. Large resistivity changes associated with boiling for the highly permeable breccia suggest that a resistivity increase would correlate with high-permeability zones if steam is present. If confirmed in further experiments, these results may lead to a new geophysical diagnostic for locating boiling in high-permeability areas of geothermal reservoirs, detecting steam in situ, and determining the pressure-temperature conditions of steam-cap formation.