Stress-sensitive porosity and permeability in carbonate rocks for underground hydrogen storage: A digital rock simulation study

Abstract Hydrogen, a genuinely clean energy, is a promising alternative to fossil fuels. Inspired by underground gas storage of methane, establishing underground hydrogen storage (UHS) in depleted oil and gas reservoirs has emerged as a significant research focus. Carbonate reservoirs, where widely-presented fractures can facilitate the high-speed injection and production of gases, are hence ideal candidates for building underground hydrogen storage facilities. During the cyclic injection and extraction processes of UHS, the formation is subjected to stress disturbances, leading to stress sensitivity. Understanding the stress sensitivity patterns of carbonate rocks is crucial for optimizing injection and production strategies. This study reconstructed three-dimensional digital models of fractured carbonate rocks from the L gas field using micro-CT scanning technology. Utilizing the finite element method, we investigated the microscopic permeability characteristics of carbonate rocks and analyzed the impact of stress loading direction and confining stress on stress sensitivity. The findings reveal that the stress loading direction significantly influences the stress sensitivity of fractured carbonate rocks. When a stress of 60 MPa is applied perpendicular to the fracture direction, the permeability reduction ratio can reach 17.32%. In contrast, when the same stress is applied parallel to the fracture direction, the permeability reduction ratio is only 4.82%. Furthermore, a simulation of UHS with cyclic injection and production of H 2 in the target block was conducted. When both permeability and porosity stress sensitivity were considered, the working gas volume for UHS decreased by only 3.4%, demonstrating that fractured carbonate reservoirs are feasible candidates for constructing underground hydrogen storage.