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.

Fractured reservoirs, as one kind of unconventional reservoirs, have great potential for oil and gas development, and their accurate characterization requires the development of rock-physics models that better simulate real fractured rocks. However, current models focus mainly on the elastic properties of rocks with aligned cracks, while the effects of randomly orienting cracks in transversely isotropic (TI) rocks are poorly studied even though such conditions are frequently encountered in the earth. To address this problem, we have derived models for the elastic properties of rocks with a TI background permeated by 3D inclined cracks and randomly orienting cracks. Then, based on the developed models, we comprehensively study the effects of the two inclination angles (i.e., the dip angle between the cracks and the isotropic plane and the rotation angle between the cracks and the plane normal to the isotropic plane, respectively) of 3D inclined cracks on the elastic properties of TI rocks. We determine that the two angles have significant influences on the elastic coefficients and hence the elastic velocities, and that their influences on the elastic properties are varying in different directions. We further investigate the effects of crack density and aspect ratio of randomly orienting cracks on the elastic properties of the fractured rocks with a TI background. The results show that the increasing crack density and crack aspect ratio reduce the elastic coefficients and velocities for rocks with randomly orienting cracks, in which the relations between compressional-wave velocities and the crack properties (i.e., crack density and crack aspect ratio) are obtained to aid the interpretation of the acquired acoustic exploration data. The proposed new models can greatly improve the modeling capability for the elastic properties of rocks with a TI background permeated by inclined and randomly orienting cracks.

Geophysical methods detecting electromagnetic properties (e.g., conductivity, relative permittivity, and dielectric loss factor) have been developed into an important tool to accurately assess the hydrate-bearing reservoirs. The key to the satisfying inversion of the electromagnetic collected data is the precise understanding of the electromagnetic responses in hydrate-bearing reservoirs. However, the frequency-dependent anisotropic electromagnetic responses in fracture-filling hydrate reservoirs remain poorly understood. To acquire the above understanding, we use a numerical simulation method to investigate the frequency-dependent anisotropic conductivities, relative permittivities, and dielectric loss factors of the constructed fracture-filling hydrate digital cores. The fractures in digital cores are aligned along a certain direction and contain the hydrate with various distributions and saturations. The simulated electromagnetic behaviors are comprehensively analyzed and explained by the effects of the electrical polarization, the aligned fractures, as well as the occurrence of hydrate with various distributions and saturation in fractures. The results show that the conductivities enhance while the relative permittivities reduce with increasing frequency, respectively, and their dispersion amplitudes at the frequency scope roughly between 100 MHz and 3000 MHz are more dramatic than that at other frequency ranges. The obtained dielectric loss factors increase first and then decrease with frequency and display peak value at the frequency of approximately 1000 MHz. The conductivities, relative permittivities, and loss factor peak values vertical to the fractures are lower than that in the other direction under the same conditions, respectively. It is found that these three parameters for all hydrate distributions and frequencies reduce with the enhancement of hydrate saturation, whereas the decreasing trends in conductivities and relative permittivities are distinct among the various hydrate distributions. The research results would be possible to pave a way for better identifying and evaluating hydrate resources of fractured reservoirs using the collected electromagnetic data.