Hydrogen-methane transport in clay nanopores: Insights from molecular dynamics simulations

Underground hydrogen storage (UHS) is recognized as one of the most promising ways to achieve large-scale long-term storage of hydrogen, and bridge the gap between demand and supply of the renewable energy sources. Recent studies show that depleted shale gas reservoirs might also be a favorable candidate for hydrogen storage because of its unique adsorption and sealing capacities. However, the interaction of hydrogen-cushion gas-water in shale is still unclear, and there is a lack of research revealing the transport mechanisms of their mixtures in shale nanopores. Here, we used the molecular dynamics simulations to investigate the hydrogenmethane transport through Na-montmorillonite clay (i.e. a common clay mineral in shale) in the presence of water. The effect of pore size, water content and driving pressure gradient on fluid behavior was discussed. The results show that: (i) The pores prefer to adsorb water molecules and form water films near the pore walls. If the water content is below 10%, it can be considered to increase the proportion of cushion gas to reduce the adsorption loss of hydrogen. (ii) When the water content increases from 10% to 50%, the thickness of water film in the pores increases by 2-3 times, and the self-diffusion coefficient of hydrogen decreases more than that of methane. (iii) The increased pressure gradient promotes the desorption of water molecules, and results in an increased amount of hydrogen accumulating near the pore walls. The results in the study provide deep insights into how hydrogen-methane mixtures transport through clay nanopores, which is important for enhancing experimental and modelling design aimed at improving hydrogen injection and production efficiency in shale.