Numerical simulations of depleted (CH 4 ) and cushion (CO 2 ) gases impacts on H 2 withdrawal and CO 2 storage efficiencies in a depleted gas reservoir

Hydrogen (H 2 ) storage in depleted gas (mainly CH 4 ) reservoirs associated with carbon dioxide (CO 2 ) injection as cushion gas is one promising strategy to promote the energy transition and decarbonization. However, it is still unknown about how depletion time, gas volume ratio of H 2, CH 4 and CO 2 ( VRH2:CH4/CO2 ), and injection/withdrawal rate will influence the performances of H 2 withdrawal and CO 2 storage in depleted gas reservoirs. Therefore herein, numerical simulations were performed to examine these influences systematically. The following results are demonstrated: 1) in the presence of H 2 and CH 4, as the VRH2:CH4 decreases from 100 %: 0 to 50 %: 50 %, H 2 withdrawal factor ( WF-H2 ) and purity ( WP-H2 ) firstly increase and then decrease, with the maximum WF-H2  = 42 % and the minimum WP-H2  = 51 %-91 % occurring at 60 %: 40 %; 2) in the presence of H 2 and CO 2, as VRH2:CO2 increases from 50 %: 50 % to 75 %: 25 %, WF-H2 and WP-H2 continuously decreases with the maximum WF-H2  = 38 % and the maximum WP-H2  = 29 %-96 % occurring at VRH2:CO2  = 75 %: 25 %, while CO 2 storage factor ( SF-CO2 ) continuously decreases from 59 % to 2 %; 3) in the simultaneous presence of H 2, CH 4 and CO 2, synergistic impacts of CH 4 and CO 2 result in 4 % WF-H2 and 60 % SF-CO2 enhancements, respectively, occurring at VRH2:CH4  = 90 %: 10 %, and VRH2:CO2  = 75 %: 25 %; 4) in the case of H 2 and CH 4, a smaller H 2 withdrawal rate ( WR-H2 ) results in a lower WF-H2, but a higher WP-H2. These insights provide crucial guidance for the implementation of temporary H 2 and permanent CO 2 geo-storage in depleted gas reservoirs.