Surface Phenomena of Hydrogen Storage in Unconventional Reservoirs

Underground hydrogen storage in unconventional reservoirs has emerged as a promising solution for large-scale, low-carbon energy buffering, yet its feasibility remains governed by complex rock–fluid interactions that are not fully understood. This review critically examines the wettability, interfacial properties, and pore-scale transport mechanisms controlling hydrogen behaviour in shale, coal, and other low-permeability formations. Key influencing factors such as pressure, temperature, salinity, mineralogy, total organic carbon, organic acids, cushion-gas composition, and brine chemistry are systematically analyzed to clarify their roles in modifying contact angle, interfacial tension, and capillary sealing. Results from multi-phase experiments show that hydrogen exposure can drive wettability transitions that alter injectivity, storage security, and withdrawal efficiency, while cushion gases such as methane and carbon dioxide substantially modify contact angle and interfacial tension trends relative to pure hydrogen systems. Adsorption-diffusion dynamics further complicate storage behavior: hydrogen exhibits weak adsorption but rapid diffusivity, particularly within inorganic nanopores, increasing its mobility and potential leakage risk in poorly sealed formations. Mineralogical heterogeneity, kerogen maturity, and the presence of organic acids significantly affect both trapping and wettability stability. A bibliometric analysis highlights a young but rapidly expanding research domain, with major contributions concentrated in key hydrogen-focused countries. Overall, the review identifies critical knowledge gaps - including the need for standardized high-pressure, high-temperature wettability protocols, integrated adsorption-diffusion-wettability models, and mechanistic simulations-underscoring the importance of advancing experimental and theoretical frameworks to ensure safe and efficient underground hydrogen storage deployment.