Hydrogen Mixing Dynamics in Depleted Gas Reservoirs

Abstract Depleted gas reservoirs are suggested as a suitable choice for the seasonal storage and utilization of hydrogen (H2) with the presence of surface infrastructure, large storage capacity, and available history data. However, hydrogen mixing with in-situ natural gas and cushion gas leads to contamination and subsequent loss of hydrogen. Hydrodynamic dispersion is an important driving mechanism for gas mixing during cyclic hydrogen injection/withdrawal. Accordingly, we investigate the mixing dynamics of hydrogen, cushion gas, in-situ gas, hydrodynamic dispersion, and formation geometry and their impacts on the recovery factor and purity of back-produced hydrogen. We construct a numerical model based on the finite-element method considering hydrodynamic dispersion. The model is then utilized to examine the mixing dynamics of injected hydrogen under various geological and operational parameters. The results reveal that the amount of injected cushion gas and in-situ significantly influences the purity of produced hydrogen. As the cushion and in-situ gas amount increases, the H2 purity, as well as the H2 recovery factor, decreases in each withdrawal. With the equivalent molar composition, the hydrogen recovery factor using in-situ gas as cushion gas is slightly better than using injected N2 as cushion gas in each cycle. The hydrodynamic dispersion negatively impacts the produced H2 purity due to the expansion of the mixing region, leading to H2 contamination and a reduced recovery factor. The hydrogen recovery factor will drop 1-2% when longitudinal dispersivity is at 10−1 – 10° orders, however, it will drop 3-6% when longitudinal dispersivity is at 101 orders. However, compared with cushion and in-situ gas amount, the hydrodynamic plays a minor role in the performance of underground hydrogen storage. The anticline geometry can utilize gravity segregation to facilitate the withdrawal of H2. Compared with anticline geometry, the H2 recovery factor of a horizontal geometry will drop 17% - 23% in each cycle.