Hydrogen adsorption and diffusion in caprock mineral slits: A molecular dynamics study for underground storage

Underground hydrogen storage (UHS) is vital for large-scale, long-term energy storage. Safe implementation requires understanding hydrogen adsorption and transport in realistic porous media containing water and brine. We employ a hybrid framework that combines grand canonical Monte Carlo and molecular dynamics simulations to predict hydrogen behavior in representative mineral pores and to evaluate factors such as mineral type, surface termination, pore size, temperature, and pressure. Our analysis extends to partially water-saturated and saline conditions (water saturation Sw ranging from 0.1 to 0.9, NaCl molalities m ranging from 1 to 5) to quantify the effects of confinement, hydration, and ionic strength. In dry pores, hydrogen exhibits weak physisorption and exists predominantly as a homogeneous free gas, with diffusion coefficients of ∼10−6 m2/s that are influenced by temperature, pore size, and pressure. The diffusion rate is fastest in kaolinite, followed by chlorite, calcite, and slowest in montmorillonite; surface variations further modulate this behavior. The introduction of water diminishes accessible surface area and gas connectivity, thereby reducing adsorption and effective diffusivity. Hydrogen primarily partitions into the gas phase at gas–water–mineral interfaces. Increasing water saturation (Sw) monotonically suppresses adsorption and diffusion, with these effects more pronounced in smaller pores and on hydrophilic surfaces. Elevated salinity (m = 1–5) modestly enhances interfacial structuring and salting-out phenomena, further decreasing diffusivity at a given Sw, while mineral-dependent rankings are preserved. These insights support the prediction of hydrogen storage and transport in brine formations and help select and operate UHS sites.