Field-scale dispersion between hydrogen and cushion gas in underground hydrogen storage and co-relation with gravity segregation

Underground hydrogen storage (UHS) is an emerging technology aimed at addressing the intermittency of renewable energy by storing the surplus electricity underground in porous reservoirs as gaseous hydrogen. To ensure deliverability a UHS reservoir is pressurised by a cushion gas, either hydrogen or some secondary gas which can mix with hydrogen in the subsurface. Alternative gases have been considered in the literature due to cost or availability, but investigations into the velocity driven mixing (dispersion) of the gases are limited compared to studies modelling concentration driven mixing (diffusion). This study examines the processes in an idealised porous reservoir under a series of parameters chosen to guide future UHS site selection. Dispersion was shown to induce hydrogen losses of up to 18% in the first year of operation, and up to 12% at ten years. High pressure and low temperatures were shown to cause greater gravity segregation of hydrogen from cushion gas, but did not affect mixing. High porosity caused more mixing, and permeability was shown to increase segregation, especially when the vertical permeability rose above 1 × 10 −14 m 2 . Dispersion was shown to alleviate the effects of gravity segregation by lessening the density contrasts that drive it. • Macrodispersion is evaluated in a model of an underground hydrogen storage reservoir. • Losses from dispersion can be upwards of 18% of the total injected hydrogen. • Dispersion can reduce the effects of gravity segregation by up to 8%. • Vertical permeability above 1 × 10 −14 m 2 results in large hydrogen losses. • Hot, lower pressure, lower porosity and permeability reservoirs minimised losses.