Hydrogen storage and diffusion in shale nanopores: molecular dynamics effects of pressure, temperature, confinement, and wall chemistry

Abstract

Underground hydrogen storage in shale is governed by pore occupancy, interfacial interactions, and molecular diffusion. This study examines their combined effects on hydrogen retention and transport. Molecular dynamics simulations were performed for five mineral slit-pore models and type I–III kerogen pores. The effects of temperature, pressure-associated loading, pore width, kerogen type, and pore-wall type were assessed using adsorption heat, pore H₂ population (the mean number of H₂ molecules within the pore region), density profiles, and directionally averaged self-diffusion coefficients. Higher prescribed loadings corresponding to higher pressures increased pore H₂ population and were accompanied by reduced diffusion. Heating decreased adsorption heat and pore population while enhancing diffusion. Narrower pores accommodated fewer molecules and restricted molecular motion, with the largest change in diffusion between the sampled pore widths occurring from 2 to 3 nm in quartz. Differences among kerogen types were modest, although type III exhibited a tendency toward stronger H₂ retention. Pore-wall type yielded more pronounced differences: quartz exhibited the highest adsorption heat, calcite combined the largest pore population with the lowest diffusion coefficient, and graphite supported the fastest diffusion. The differing rankings of adsorption heat, pore population, and diffusion indicate that interaction strength alone cannot adequately characterize hydrogen storage and transport. Joint assessment of these descriptors provides molecular-scale insights into shale pore structures and the trade-offs between hydrogen retention and transport.