Hydrogen mixing dynamics in depleted gas reservoirs

Depleted gas reservoirs are suggested as a suitable choice for the seasonal storage and utilization of hydrogen (H₂). However, hydrogen mixing with in-situ natural gas and cushion gas leads to contamination and subsequent loss of hydrogen. Accordingly, we conduct several numerical simulations of H₂ injection into a depleted gas reservoir to investigate the impacts of several geological and operational parameters on the recovery factor, purity of produced hydrogen, and unwanted fluid production. The results reveal that anticline geometry can utilize gravity segregation to facilitate the withdrawal of H₂, and the H₂ purity is strongly impaired in horizontal geometry, and up to 23 % reduction is found in horizontal geometry. As the cushion and in-situ gas amount increases, the H₂ purity and hydrogen recovery factor (HRF), decrease in each withdrawal. With the equivalent molar composition, N₂ provides a slightly better HRF than CH₄ in each cycle (<1 %). With equivalent molar composition, HRF using injected N₂ as cushion gas is slightly better than using in-situ gas as cushion gas (up to 0.22 % in each cycle). The gas mixing zone expands as dispersivity increases and larger longitudinal dispersivity values (e.g. 50 m) promote greater hydrogen retention within the reservoir. While prolonging the injection period reduces the mechanical dispersion, it only slightly increases HRF, which may not justify additional operational cost.