Research on the physical properties of geothermal reservoir rock. Quarterly report, March 1978

Abstract

A laboratory study of the P-wave velocity and electric resistivity was undertaken on Cenozoic volcanic rocks collected from the Columbia Plateau volcanic basin (C) and the Jemez volcanic field (NM). Electric resistivities of cylindrical samples saturated with 0.1 N NaCl solution were measured using a four electrode system and a 1.0 KHz frequency source. Seismic P-wave velocities were calculated from measured transit time of mechanical pulses generated and received by piezoelectric transducers. The electric resistivity of water saturated samples decreased as temperature increased to the boiling point of water. Above boiling point, resistivity increased rapidly as water changed to vapor. Resistivity is most sensitive to temperature changes between 35/sup 0/C to 65/sup 0/C. Resistivities of samples increased with decrease in saturation. The effect is more pronounced at lower temperatures. No dependence of seismic P-wave velocities on temperature was observed. Both resistivity and P-wave velocity depend on porosity. The increase in porosity results in a decrease in the resistivity formation factor. Assuming a relationship FF = a phi/sup -m/ (Archie's Law), where FF and phi represent the formation factor and porosity respectively, least squares indicate a variation of a between .5 and 2.0. The value of m varied between 1.2 to 1.7. Seismic velocities (v) decrease as porosity increases. Porosity appears to be linearly related to log v. Several samples show anomalous relationship between porosity and resistivity. Most of these samples also show anomalous seismic velocities. The majority of these samples have coarse grains or large pores. The effect of saturation on P-wave velocity is small and can be observed in few samples. In these samples, seismic velocities decrease with increase in saturation at high saturation (100% to 85%), and show a reverse relationship at low saturation. Between 15% and 85% saturation in velocity is constant.