Hydrogen Diffusion into Water and Cushion Gases - Relevance for Hydrogen Geo-storage

Hydrogen (H2) has been recognized as a promising solution to reduce carbon dioxide (CO2) emissions. H2 is considered a green energy carrier for energy storage, transport, and usage, and it can be produced from renewable energy resources (such as solar, hydropower, and wind energy). However, H2 is a highly diffusive compound compared to other natural gases, raising concerns about the possibility of H2 loss in geo-storage (e.g. in underground geological formations such as depleted oil/gas reservoirs, aquifers or shale formations) or H2 leak via pipelines when blending H2 with natural gas in existing pipeline systems. Thus, understanding H2 diffusion in subsurface formations and pipeline systems is vital. However, despite its importance, only limited data is available to assess the above situations. Therefore, in this study, molecular dynamics simulations were used to predict the self-diffusion coefficients of H2 in water and cushion gases (CH4 and N2) at relevant geothermal conditions (i.e. 300 K–373 K and pressures up to 50 MPa). The findings showed that H2 self-diffusion in methane and nitrogen increases with increasing temperature but decreases with increasing pressure. However, H2 selfdiffusion in water increases with increasing temperature but is not impacted by increasing or decreasing pres sure. The results also indicated that the rate of H2 self-diffusion in cushion gas is faster than in water, about exceeding two-digit times. Furthermore, the outcomes reported extended or new data on H2 self-diffusion for the binary system of H2–H2O, H2–CH4, and H2–N2. This study is beneficial and contributes to assessing efficiency and safety for executing H2 transportation and underground hydrogen storage (UHS) schemes.