Molecular simulations of hydrogen diffusion in underground porous media: Implications for storage under varying pressure, confinement, and surface chemistry conditions

Assessing the feasibility of porous formations for hydrogen geo-storage demands a comprehensive understanding of hydrogen (H 2 ) transport behavior in porous media at subsurface conditions. In this study, we employ molecular dynamics simulations to evaluate the effects of pressure (20 - 500 atm), pore size (2 - 20 nm), and surface composition on H 2 diffusion and interactions within organic and inorganic slit pores. Our analysis of H 2 density profiles and interaction energies reveals a distinct preference for H 2 molecules to adsorb more readily onto graphene surfaces than kaolinite surfaces. However, self -diffusion results demonstrate that H 2 molecules interact relatively weakly with both substrate types. Further insights are provided by velocity autocorrelation functions, emphasizing the occurrence of wall -mediated collisions as H 2 molecules diffuse along the pore surfaces, particularly within kaolinite. This highlights the significance of surface roughness in mitigating H 2 loss via diffusion in subsurface nanopores, given the small molecular size of H 2 and its limited interactions with both organic and inorganic materials. Furthermore, the results demonstrate that self -diffusion coefficients in both pore types increase with pore size and decrease with pressure. Notably, surface composition plays a critical role in low-pressure environments, with self -diffusion coefficients beginning to converge beyond a pressure of 100 atm. Self -diffusion coefficients also become less sensitive to slit aperture beyond a pressure of 50 atm. In high-pressure environments, hydrogen transport is governed by thermal collisions between gas molecules, leading to negligible property variations between pore types. These findings hold significance in the investigation of efficient H 2 storage within porous media and caprock formations.