Hydrogen diffusion into water and cushion gases – Relevance for hydrogen geo-storage

Hydrogen (H 2 ) has been recognized as a promising solution to reduce carbon dioxide (CO 2 ) emissions. H 2 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, H 2 is a highly diffusive compound compared to other natural gases, raising concerns about the possibility of H 2 loss in geo-storage (e.g. in underground geological formations such as depleted oil/gas reservoirs, aquifers or shale formations) or H 2 leak via pipelines when blending H 2 with natural gas in existing pipeline systems. Thus, understanding H 2 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 H 2 in water and cushion gases (CH 4 and N 2 ) at relevant geothermal conditions (i.e. 300 K–373 K and pressures up to 50 MPa). The findings showed that H 2 self-diffusion in methane and nitrogen increases with increasing temperature but decreases with increasing pressure. However, H 2 self-diffusion in water increases with increasing temperature but is not impacted by increasing or decreasing pressure. The results also indicated that the rate of H 2 self-diffusion in cushion gas is faster than in water, about exceeding two-digit times. Furthermore, the outcomes reported extended or new data on H 2 self-diffusion for the binary system of H 2 –H 2 O, H 2 –CH 4 , and H 2 –N 2 . This study is beneficial and contributes to assessing efficiency and safety for executing H 2 transportation and underground hydrogen storage (UHS) schemes. • Predict the self-diffusion coefficients of H 2 in H 2 O, CH 4 and N 2 from 300 K to 373 K and pressures up to 50 MPa. • H 2 self-diffusion in CH 4 and N 2 increased with increasing temperature but decreased with increasing pressure. • H 2 self-diffusion in water increases with increasing temperature but is weakly impacted by pressure. • Provide extending or new H 2 self-diffusion data to assess efficiency and safety for H 2 transportation and UHS schemes.