Hydrogen's Next Top Model: Exploring Processes and Practice in Hydrogen-Cushion Gas Mixing

In underground hydrogen storage, mixing between Hydrogen and cushion gas could present a problem to the recoverability of working gas and may be a controlling factor in subsurface reactions. The conventional modelling approach focuses mainly on diffusion as the primary mixing process, while little attention is paid to dispersive mixing. Using the finite element simulator COMSOL this work focuses on assessing the relative magnitude of transport between the two processes, including diffusive processes such as thermodiffusion and surface diffusion. Molecular diffusion is shown to be the dominant segregative process, but still transports an order of magnitude less mass than mechanical dispersion. Necessary adjustments should be made when considering implementation of mixing processes in numerical models, with  attention being given to the dispersion model and its reliance on a scale dependent dispersivity coupled with grid size.