High Temperature Behavior of Rocks Associated with Geothermal Type Reservoirs

Abstract

ABSTRACT Measurements of sonic velocities, bulk and matrix compressibilities and thermal conductivities have been made on a group of outcrop sandstones and a group of siltstone cores obtained from wells drilled in the Imperial Valley, California. Results of these measurements, which were made at temperatures as high as 200 °C and pressures to 16,000 psi, are presented and discussed relative to their application in high temperature underground reservoir studies. Sonic velocities were found to decrease with increased temperature and increase with increased pressure and with liquid saturation. If the effect of temperature on sonic velocities is ignored, overestimation of porosity from transit-time logs could result. Both bulk and matrix compressibilities were found to increase with increased temperature. Application of these data to surface subsidence calculations showed that although subsidence would be small due to fluid pore pressure reduction in these particular rocks, the effects of temperature on compressibility could increase subsidence substantially. Thermal conductivities were of the expected order of magnitude for sandstones and siltstones of the type tested. Vaporization-condensation reactions caused a more than two-fold increase in apparent thermal conductivity for the higher permeability sandstones but had essentially no effect on the conductivities of the low permeability siltstones. From results of the present work it was concluded that the temperature effects observed were of sufficient importance to warrant additional work at higher temperatures and on a wider variety of samples typical of those which might be encountered in high temperature subsurface reservoirs. INTRODUCTION In the development of geothermal power, an understanding of the properties and behavior of rocks and rock-fluid systems at high temperatures and pressures may be needed for a variety of applications. These include, among others, the interpretation of well logs, prediction of subsidence upon pore pressure reduction, calculation of reservoir storage capacity, and fluid flow and heat flow calculations. Properties needed for these applications include sonic velocities, bulk and pore compressibilities, thermal conductivity, permeability and electrical resistivity factor. Capillary pressure and relative permeability data measured at subsurface temperature and pressure conditions may also be required in multi-phase fluid and heat flow calculations.