A SYNCHROTRON X-RAY NANOTOMOGRAPHY STUDY OF THE PORE MORPHOLOGY OF SHALE, IMPLICATIONS FOR CAPROCK INTEGRITY IN GEOLOGIC HYDROGEN STORAGE

Geological storage of H₂ has emerged as a critical strategy in lowering anthropogenic greenhouse gas emission. Depleted oil and gas reservoirs, such as those in California's Central Valley, present promising opportunities for long-term storage due to their proven capacity for geological fluids and potentially favorable geomechanical conditions post-production. However, the feasibility of large-scale storage projects in depleted reservoirs depends on the ability of caprock to permanently seal the reservoir. In this study, we evaluate the integrity of caprock for subsurface hydrogen storage, using core samples from Miocene Monterey Shale at the Elk Hills Oil Field. This field, located in the Southern San Joaquin Basin, is renowned for its substantial hydrocarbon deposits derived from rich organic materials, and has been extensively produced for over a century. Advanced synchrotron X-ray tomography, performed at the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS), enabled three-dimensional and non-destructive characterization of pore morphology in caprock samples. We quantify key petrophysical properties for the caprock including porosity, pore size distribution, and pore throat size using the nanotomography data. Image processing and segmentation techniques highlight intricate pore structures within the rock samples. Complementary analyses utilizing scanning electron microscopy energy dispersive X-ray spectroscopy (SEM EDS), x-ray diffraction (XRD), and induced polarization measurement provide further insights into the geochemical and mineralogical characteristics of the caprock samples. The results highlight the complex pore structures and pore connectivity due to diagenetic alternation, and the potential of using a combined approach of synchrotron X-ray tomography, SEM EDS, and XRD to study petrophysical properties of caprocks. Through the comprehensive characterization, we aim to evaluate the feasibility of underground gas storage in California, and to advance the understanding and application of geological storage technologies crucial for sustainable energy transitions.