Role of methane as a cushion gas for hydrogen storage in depleted gas reservoirs

Abstract

Depleted natural gas reservoirs play an important role as a viable option for large-scale hydrogen storage and production. However, its deployment depends on the accurate knowledge of the cushion gas (such as CH4, CO2, and N2) compositions, which are key components affecting the rock-fluid interfacial phenomenon. In addition, there are currently few reported studies on rock/brine/gas-mixture wettability and gas-mixture/brine surface tension representing this type of reservoir. Hence, we report the feasibility of using CH4 as a cushion gas (in the presence of CO2 and N2) for H2 storage at various pressures (500 up to 3000 psi), temperatures (30 up to 70) oC, and salinities (2 up to 20) wt.% using drop shape analyzer equipment. Contact angle (CA) and surface tension (ST) experiments were extensively conducted for the different gas mixtures (H2–CH4–CO2–N2) to establish relevant data for H2 storage in depleted gas reservoirs.Our result indicates that unless when the rock's initial wetting state is altered, the studied gas-mixture compositions (Test case 1: 80% H2 – 10% CH4 – 5% CO2 – 5% N2; case 2: 70% H2 – 20% CH4 – 5% CO2 – 5% N2; case 3: 60% H2 – 30% CH4 – 5% CO2 – 5% N2; case 4: 50% H2 – 40% CH4 – 5% CO2 – 5% N2; case 5: 40% H2 – 50% CH4 – 5% CO2 – 5% N2; case 6: 30% H2 – 60% CH4 – 5% CO2 – 5% N2; and case 7: 20% H2 – 70% CH4 – 5% CO2 – 5% N2) will exhibit comparable wettability behavior as the CAs ranged between [20 to 41°] irrespective of the reservoir pressure, temperature, and salinity. ST decreases with increasing temperature and linearly with increasing pressure. ST for each gas mixture increased with salinity. ST decreases systematically with increasing CH4 fraction (at any given salinity, temperature, and pressure) with the highest observed in Test case 1 and the lowest in Test case 7 compositions. Test cases 3 and 4 with H2 (50–60%) and CH4 (30–40%) fractions was selected as the optimal gas mixture based on CA and ST for H2 storage and withdrawal. The study's findings offer precise and useful input data for the reservoir-scale simulation used in geo-storage optimization in depleted natural gas reservoirs.