Factors influencing hydrogen migration in cap rocks: Establishing new screening criteria for the selection of underground hydrogen storage locations

Safe and efficient underground storage of hydrogen requires cap rocks with effective sealing capabilities. This study investigates the interplay between various factors and their influence on hydrogen leakage from cap rocks. Compared to other commonly stored gases, hydrogen exhibits a significantly faster leakage rate. This study reveals that pore size and tortuosity of cap rocks play a crucial role. Larger pore sizes and lower tortuosity factors lead to increased leakage. Conversely, higher tortuosity (more twisted pores) enhances the sealing capability. Cap rocks with a tortuosity greater than 3 or an effective pore size below 5 nm demonstrated exceptional sealing capabilities. Beyond pore geometry, the study underscores the dependence of hydrogen diffusivity on prevailing underground conditions. Higher reservoir temperatures translate to a faster leakage rate, whereas increased pressure reduces the mean free path of hydrogen molecules, hindering diffusion and promoting retention within the storage formation. Reservoirs of temperature below 350 K and under a pressure exceeding 13.80MPa (2000 psi) exhibited minimal leakage. These findings underscore the crucial role of meticulous cap rock selection for safe and efficient underground hydrogen storage. This research paves the way for future investigations exploring the impact of additional factors such as the presence of other gases, the chemical composition of the cap rock itself, and the long-term effects of hydrogen exposure on cap rock integrity. A comprehensive understanding of these factors is essential for optimizing underground hydrogen storage strategies and ensuring long-term environmental sustainability.