Pressure effects on the anisotropic electrical conductivity of artificial porous rocks with aligned fractures

Abstract

Understanding the electrical properties of reservoir rocks has important applications in oil and gas exploration. Small-scale fractures that are widely existing in reservoir rocks are usually aligned along a certain preferential direction due to reasons such as tectonic movement, making the electrical conductivity of the rocks anisotropic. Since all rocks are experiencing geological pressure, it is theoretically and practically important to study the influence of pressure on the fractures and its control on the anisotropic conductivity of reservoir rocks. We first prepared artificial porous sandstone samples with and without penny-shaped fractures and experimentally measured the anisotropic conductivity of the samples under differential pressure (the difference between confining pressure and pore fluid pressure). Then, we inverted from the experimental data for the fracture porosity and fracture aspect ratio at each differential pressure. The variation of the fracture parameters caused by the differential pressure and their influence on the anisotropic electrical conductivity of the rocks were further analysed theoretically. The results show that as the differential pressure increases, the measured anisotropic conductivity of the artificial samples with and without fractures all decreases exponentially. We also show that both the inverted fracture porosity and fracture aspect ratio decrease exponentially with the increase of differential pressure, and the fracture porosity and fracture aspect ratio are linearly correlated as an implicit function of differential pressure. The modelling results show that the differential pressure-induced decrease in the fracture porosity reduces the electrical conductivity in all directions, and the conductivity reduction in the rock parallel to the direction of the fractures is most significant. Comparing with the influence of the fracture porosity resulting from the applied differential pressure, the pressure-caused variation in the fracture aspect ratio was shown to play a secondary role on affecting the pressuredependent anisotropic electrical properties.

Experimental Study of Complex Resistivity Characteristics of a Sandstone Reservoir under Different Measurement Conditions

“…Liu et al [15] systematically summarized and analyzed the influence of pressure and temperature on the conductivity and wave velocity of basalt. Li et al [16] by measuring the anisotropic conductivity of artificial porous sandstone samples under pressure difference, the change of fracture parameters caused by the differential pressure and their influence on the anisotropic electrical conductivity of the rocks were further analyzed theoretically. The complex resistivity of argillaceous sandstone was tested and analyzed by Sun et al [17] under high-temperature and high-pressure conditions, and the influences of the salinity and depth of the rock reservoir on the complex resistivity were obtained.…”

A rapid 3D magnetotelluric forward approach for arbitrary anisotropic conductivities in the Fourier domain

“…It is well known that the conductivity of subsurface media often exhibits anisotropy. Laboratory observations and studies have shown that gneiss and other types of rocks have significant electrical conductivity anisotropy (Parkhomenko, 1967;Keller, 1982;Andréa and Li, 2022;Li et al, 2022;Yin et al, 2022). During the development of magnetotellurics, the research on anisotropy included all aspects of data analysis, forward modeling, and inversion.…”