Laboratory determination of the full electrical resistivity tensor of heterogeneous carbonate rocks at elevated pressures

We describe a measurement system capable of determining the full resistivity tensor of core samples at elevated, geologically representative, pressures using a galvanic method. It is suitable for heterogeneous rocks where it is difficult to measure tensorial resistivity without bias from sample selection and heterogeneity. We demonstrate the efficacy of the system using both synthetic data and measurements on carbonate rock core samples. The apparatus employs a computer controlled array of 16 electrodes to inject current into, and measure boundary voltages on, a 5 cm diameter cylindrical sample. A computationally efficient FE algorithm is used to retrieve the full resistivity tensor from the measured voltages. The algorithm uses isotropic Finite Element code to calculate anisotropic solutions for samples of arbitrary geometry. Initial results from Jurassic limestone and Triassic dolomite samples, reveal cm-scale heterogeneity and significant bulk anisotropy consistent with rock fabric observed in X-ray Computed Tomography scan images.