Some physical properties of Cerro Prieto cores

Abstract

A number of physical properties of core samples taken from the general area of the Cerro Prieto geothermal field have been measured at elevated pressures and temperatures. Results are reported for compressional (P) and shear (S) wave velocities, permeability and thermal conductivities. Additional tests being run include formation resistivity factor, bulk and pore compressibilities, and thermal expansion. Most of the core samples used in the tests have been sent from the University of California Riverside project and, thus, mineralogical data are available for most of the cores. For the most part, the wells from which core samples were obtained were outside of the limits of geothermal development and, thus, the data may not be representative of formations within the present producing area. Core samples are from the same geologic formations, continental deltaic sediments derived from the Colorado River, and are taken from approximately the same depth as the producing wells. Shear and compressional wave velocities were measured on one core taken from well M-3 at a depth of 2395 m. Tests were run at temperatures of 25, 100 and 175°C over a pressure range of 500 – 10,000 psi with the core dry and fully saturated with brine. A considerable temperature effect was noted, both velocities decreasing with increased temperatures. Increase of velocity with pressure was close to that predicted by theory. Elastic moduli were calculated from the velocity data. It was noted that the ratios of values of Poisson's ratio may well serve as a means of detecting liquid-vapour interfaces if values of both P- and S- wave velocities were measured in wells. Permeabilities measured at elevated temperatures showed substantial decrease with increased temperature — as much as 70% reduction when the temperature is increased only to 155°C. Much of this decrease is recovered upon returning the sample to room temperature, there being about 25% permanent reduction. The cause of this permeability reduction with temperature is not fully understood but much of it is believed to be due to quartz — clay mineral reactions which are accentuated at the higher temperatures and stresses. Thermal conductivities of a large range of core types are being measured and will be reported in another paper. Results of this work are being compared with earlier correlations and modifications of these correlations are being made so that thermal properties may be predicted for other cores apart from Cerro Prieto and at other environmental conditions.