Cushion gas effects on clay-hydrogen-brine wettability at conditions relevant to underground gas storage

Geological storage of hydrogen, and its retrieval as needed, could play a vital role in the transition from fossilfuel based energy to clean renewable energy production. Cushion gases, such as carbon dioxide and methane, can be used to maintain the reservoir pressure required to increase the efficiency of injection and extraction processes. Because water is ubiquitous in the subsurface, it can provide additional sealing mechanisms and affect the ability of gases to penetrate porous rocks. Because the interactions among the various gases and the wetting properties in the subsurface affect the sealing capacity of the caprock, they can provide important considerations for the proper design of geological storage and retrieval processes. Molecular dynamics simulations were used to evaluate the effects of varying compositions of cushion gases (CO2 and CH4) on brine-hydrogen-kaolinite clay wettability. Contact angles and liquid-gas interfacial tension were computed for 10% NaCl brines at 323 K and pressures in the range 5-40 MPa. These conditions are representative of underground gas storage. The results showed that, in pure H2, the kaolinite siloxane surface is 'intermediate wet', with contact angles ranging from 91 to 106. At constant temperature and pressure, CO2 and CH4 cause the surface to become less water-wet, yielding larger contact angles. We observed that CO2 led to a more significant increase in contact angles. This suggests that CO2 or CH4 lead to easier recovery of hydrogen. These cushion gases also reduce gas-brine interfacial tensions, with CH4 yielding a less pronounced effect than CO2. Reductions in interfacial tension translate to reduced capillary sealing pressure, which implies that hydrogen can be retrieved at lower pressures. The results presented suggest that the efficiency of a gas used as cushion gas is related to the density difference between the resultant gas mixture and water. At the conditions tested here, CO2 and CH4 are found to reduce the sealing capacity of kaolinite towards hydrogen storage, while they are likely to improve hydrogen recovery. This should be taken into consideration when intermittent hydrogen storage is attempted in geological repositories.