Molecular Insights into the Effects of Cushion Gas on Underground Hydrogen Storage

In underground hydrogen storage (UHS), a cushion gas is commonly used to provide pressure support to assist the withdrawal of hydrogen. Molecular dynamics (MD) simulations of the H 2 + cushion gas + water ternary system were conducted at 323–448 K and 20–150 MPa. We considered N 2, CH 4, and CO 2 as a cushion gas candidate, with H 2 mole fractions in the gas-rich phase x H 2 = 0.7 and 0.3. The MD results are reasonably consistent with those obtained by density gradient theory based on the CPA equation of state. Our computed interfacial tensions (IFTs) are also reasonably consistent with experiments. Our previous studies showed that the IFTs of the H 2 + water system were not strongly influenced by pressure, and they decreased with increasing temperature. The IFTs of the H 2 + cushion gas + water system decreased with decreasing x H 2 . A key observation is that the effect of gas type on the IFTs of the H 2 + cushion gas + water system followed the order: CO 2 > CH 4 ≈ N 2 . This can be attributed to the relatively high surface excess of CO 2 . We also performed MD simulations of the H 2 + cushion gas + water + silica (hydrophilic) and H 2 + cushion gas + water + silica (hydrophobic) systems. Our previous studies showed that the water contact angles (CAs) of the H 2 + water + silica systems were not strongly influenced by the pressure or temperature. The water CAs of the H 2 + cushion gas + water + silica systems increased with decreasing x H 2 . Importantly, the effect of gas type on the water CAs of the H 2 + cushion gas + water + silica systems followed the order: CO 2 > CH 4 > N 2 . The capillary pressure of the H 2 + cushion gas + water + silica (hydrophilic) system decreased with decreasing x H 2, suggesting that the studied gases might enhance the withdrawal process of H 2 in UHS.