Dispersion of hydrogen in different potential cushion gases

Underground hydrogen storage in porous media (UHSP), typically employing depleted natural gas reservoirs, is being increasingly contemplated as a potentially viable energy storage option to compensate for intermittent renewable energy sources. A layer of cushion gas (typically N₂, CH₄ or CO₂) is proposed to be accommodated in such UHSP systems to allow for pressure and volume control during hydrogen (the so-called working gas) injection and withdrawal, while also serving as a barrier to prevent excess water contamination. One significant challenge in such UHSP systems is the potential mixing between this working gas (hydrogen) and the cushion gas within the relevant reservoir. Such mixing increases the risk of unacceptable extracted hydrogen contamination (with the cushion gas), often requiring expensive surface hydrogen purification processes. Therefore, the use of a cushion gas has potential economic implications. Accurate determination of the extent of this mixing requires reservoir simulations, which depend on dispersion coefficient (K_L) data as input. Such data are however not available in the literature for hydrogen under relevant reservoir conditions. Here we present novel measurements of K_L for the dispersion of hydrogen in the three prospective cushion gases as a function of system pressure (50–100 bar) and for displacement velocities ranging from 0.2 to 8.4 cm/min.