Pressure-dependent cross-property relationships between elastic and electrical properties of partially saturated porous sandstones

Abstract

Successful joint inversion of seismic and electromagnetic survey data to estimate parameters and improve the characterization of the earth’s subsurface relies on the understanding of the cross-property relations between the elastic and electrical properties. Pressure-dependent cross-property relationships of rocks with varying degrees of fluid saturation remain poorly understood even though such conditions are encountered frequently. We investigate this problem by designing and undertaking dedicated laboratory experiments to simultaneously measure the elastic velocity and electrical resistivity in two Berea sandstone samples subject to varying confining pressure and water saturation, using a state-of-the-art joint elastic-electrical measurement system. P- and S-wave velocities are found to increase exponentially with increasing confining pressure at each water saturation, whereas electrical conductivity can either increase or decrease with confining pressure depending on the degree of water saturation. More interestingly, the elastic and electrical properties of the two samples are shown to be systematically correlated as functions of water saturation and the applied confining pressure. The correlations reveal the natural link between the elastic and electrical properties in the two porous sandstones with partial water saturation, and they help to better indicate the fluid flow within compliant pores in addition to the known flow between stiff and compliant pores. The results will form the theoretical basis for the successful joint interpretation of seismic and electromagnetic survey data acquired from partially saturated porous rocks in the subsurface of the earth.