Hydrogen Diffusion in Organic-Rich Porous Media: Implications for Hydrogen Geo-storage

Subsurface hydrogen storage for large-scale energy supply to decarbonize a variety of greenhouse gas-emitting activities is gaining momentum worldwide to combat climate change. Hydrogen's extensive range of flammable concentrations and low minimum ignition energy in air combined with its highly diffusive nature mandate implementation of comprehensive risk management protocols to maintain storage safety. In particular, the role of hydrogen diffusion process (self-diffusion) needs further investigation to understand H₂transport behavior and elude the associated risk of leakage for the long-term safety of the underground hydrogen storage process. Recent molecular simulation studies suggest that, compared to other subsurface storage options, depleted shale reservoirs may offer preferential storage characteristics due to excellent sealing and adsorption capabilities of shale. Data on hydrogen diffusion in organic (kerogen)-rich shale is scarce. Here, we applied molecular dynamics to compute hydrogen self-diffusivity in kerogen systems with different structures at various pressures, ranging from 3 to 41 MPa, and under an isothermal condition of 360 K. Two kerogen types of varying maturity (II-A and II-C) were utilized to create slit nanopores of 0.5 and 2 nm in size. The Knudsen number was calculated based on the mean free path and determined by the density values obtained from simulation to improve diffusion coefficient estimates. Results obtained indicate the K_ndata in the range from 0.64 to 9.72, demonstrating that the main transport mechanism was transitional in nature. With increasing pressure, diffusivity declined regardless of the slit pore size or kerogen type. H₂diffusion was greater for the 2 nm pore system compared to both the 0.5 nm in II-A (0.004-0.02 cm²/s) and II-C (0.0037 to 0.019 cm²/s). In addition, for a fixed nanopore size, thermal maturity does not seem to impact diffusivity. Finally, simulation results were regressed to delineate a continuous description for diffusivity as a function of pressure. For curve fitting, R²values were found to be 98 and 92% for 2 nm pore size of II-A and II-C, respectively, while 97 and 79% for 0.5 nm pore size of II-A and II-C, respectively. The results obtained are not only of interest for storage of hydrogen in shale but also for any subsurface hydrogen storage approach where a shale layer acts as a seal to form a trap.