Pore pressure effects on the anisotropic joint elastic-electrical properties of artificial porous sandstones with aligned fractures

Abstract

Fractures widely exist in rocks and joint interpretation of elastic and electrical survey data can improve the characterization of fractured rocks. However, the cross-property relationship between the elastic and electrical rock properties, which is the key to the successful joint elastic-electrical interpretation, remains poorly understood in fractured rocks with varying pore pressure. This knowledge gap was by developing a method for making calcite-cemented sandstones with penny-shaped fractures and by simultaneously measuring the anisotropic elastic and electrical properties of the artificial sandstones with and without fractures as a function of pore pressure through the development of an anisotropic joint elastic-electrical measurement system. We find that elastic velocities and electrical resistivities of the intact and fractured samples reduce with increasing pore pressure, and the reduction in the physical properties of the fractured rock is less significant than that of the intact sample. We also find that elastic and electrical anisotropic parameters decrease in the sample without fractures but decrease first and then increase with increasing pore pressure in the sample with fractures. Most interestingly, the anisotropic elastic and electrical properties are found to exhibit a strong linear correlation in the intact and fractured samples with varying pore pressure and a difference exists in the slopes between the samples with and without fractures. The experimental data are explained and interpreted through the contrary contributions of the dilation of the random microcracks and the aligned fractures with increasing pore pressure to the anisotropic elastic and electrical properties. The results reveal the pore pressure effects on the anisotropic joint elastic-electrical properties of sandstones with fractures and provide a potential for the detection of overpressure in fractured rocks through integrated elastic and electrical surveys.