Acoustic and Electrical Properties of Mexican Geothermal Rock Samples

Abstract

The interpretation of geophysical well logs requires established relationships between the petrophysical properties of typical reservoir rocks. Laboratory and borehole log studies [Archie, 1947; Wgllie et al., 1956; Geertsma, 1961; King, 1966; Carothers, 19681 indicate a strong dependence of acoustic velocities and electrical resistivity on the porosity and state of fluid saturation of porous rocks. Effects of temperature, state of stress and pore texture have also been demonstrated to be significant. shear-wave velocities have been measured on a suite of ten sandstone samples obtained from wells in the Cerro Prieto geothermal field and on two rock samples from other Mexican geothermal fields. The samples were tested in both their dry and fully brine-saturated states at uniaxial stresses to 15 MPa. Elect rica l resistivities and associated phase angles have been measured on the same core samples as a function of frequency in the range 10 Hz to lo5 Ez under drained conditions at hydrostatic confining stresses to 10 MPa. The electrical properties were measured on samples tested in their fully saturated state, using brines of two different concentrations. A comprehensive summary of the existing information on laborato ry-measured petrophysical properties of sandstones from the Cerro Prieto geothermal field has been presented by Contreras et al. (1984). The summary includes sections on bulk density, porosity, fluid permeability, bulk and pore compressibility, thermal expansion and conductivity, acoustic velocities, and electrical resistivity.