Correlation of magnetic pore fabrics with traditional pore fabric characterization and permeability anisotropy in typical sedimentary rocks and hot isostatically pressed samples

Pore-scale experiments are crucial to obtain pore geometry and distribution, i.e., pore fabrics, controlling preferred fluid flow directions in rocks. Pore fabrics are characterized to derive models, which are important for hydrocarbon exploration and geothermal applications. X-ray computed tomography (XRCT) is one typical method to obtain three-dimensional pore fabrics, but limited by its micron-scale resolution in 1-inch cores. Magnetic pore fabrics (MPFs) were proposed as a fast and efficient way to indirectly measure the pore fabrics, and target micropores down to 10 nm. Empirical relationships exist between MPFs and pore space properties, and between MPFs and permeability anisotropy. Previous studies investigated a limited number of rock types or plastic synthetic samples with simplified pores to compare MPFs, pore fabrics and permeability anisotropy. Permeability is commonly estimated from measurements parallel and perpendicular to the macroscopic fabric, and thus the measurement needs a priori information on the fabric orientation. This study integrates complementary measurements to characterize pore fabrics on various scales: pycnometer porosity, MPF, XRCT, and permeability anisotropy measurements. The specimens include various kinds of sandstones and carbonates to cover the main sedimentary lithologies, and hot isostatically pressed (HIP) samples of simple and controlled compositions to bridge the gap between the synthetic samples of previous studies and complex natural rocks. HIP samples were made by mixing calcite and muscovite powders in different proportions and grain sizes, and were cold pressed at 20 MPa and then hot pressed at 160 MPa and 670 °C. Full permeability tensors including confidence angles were determined, and each tensor was calculated from 7 directional permeability measurements for natural rocks. Considering the uniaxial symmetry of the HIP samples, 3 directional measurements are sufficient to calculate a tensor with confidence angles. One additional core from each block was scanned by XRCT with ~5.5 µm pixel size for 3D pore fabric analysis, prior to being impregnated with ferrofluid to measure MPFs. A total shape ellipsoid, representing the average XRCT-derived pore fabric, is compared with other second-order tensors, permeability anisotropy and MPFs. Initial data suggest that the maximum principal directions of permeability anisotropy, total shape ellipsoids and MPFs are coaxial in homogeneous samples with consistent pore space anisotropy. These confirmed quantitative correlations help to apply MPFs as an efficient method to determine pore fabrics and predict preferred flow direction.