Hydrogen Diffusion in Clay Slit: Implications for the Geological Storage

We determined the self-diffusion coefficients of hydrogen in clay (montmorillonite) nanopores using molecular dynamics under subsurface conditions. We explored the effects of temperature, pressure, pore size, moisture content, and salinity. Our results show that the self-diffusion coefficient of hydrogen is on the order of magnitude of 10(-8) m(2)/s. The diffusivity of confined hydrogen increases moderately with temperature and slit aperture but declines with pressure. The estimated density profile suggests that only one dense layer of hydrogen molecules is adsorbed near the slit surface. The distinct diffusion coefficients in the parallel and perpendicular directions to the basal surfaces confirm the confinement effect of the substrates. As the volume ratio of hydrogen increases, the existing pattern of hydrogen changes from a droplet to a layer sandwiched by the aqueous solution. The water bridge will act as a piston for the hydrogen droplet and impede hydrogen diffusion. However, when the hydrogen and brine form a stratified structure, the self-diffusion coefficient of hydrogen sandwiched by two brine films is similar to that of confined pure gas at the same pressure and temperature conditions. If the brine salinity reaches some extent, part of brine and hydrogen molecules will mix as a new phase, which slightly inhibits the hydrogen diffusion. This work provides a better insight into hydrogen diffusion through the clay nanopores, which is critical for reliably assessing the risk of hydrogen leakage through caprocks.