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 H₂ + cushion gas + water ternary system were conducted at 323-448 K and 20-150 MPa. We considered N₂, CH₄, and CO₂ as a cushion gas candidate, with H₂ mole fractions in the gas-rich phase x_H2 = 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 H₂ + water system were not strongly influenced by pressure, and they decreased with increasing temperature. The IFTs of the H₂ + cushion gas + water system decreased with decreasing x_H2. A key observation is that the effect of gas type on the IFTs of the H₂ + cushion gas + water system followed the order: CO₂ > CH₄ ≈ N₂. This can be attributed to the relatively high surface excess of CO₂. We also performed MD simulations of the H₂ + cushion gas + water + silica (hydrophilic) and H₂ + cushion gas + water + silica (hydrophobic) systems. Our previous studies showed that the water contact angles (CAs) of the H₂ + water + silica systems were not strongly influenced by the pressure or temperature. The water CAs of the H₂ + cushion gas + water + silica systems increased with decreasing x_H2. Importantly, the effect of gas type on the water CAs of the H₂ + cushion gas + water + silica systems followed the order: CO₂ > CH₄ > N₂. The capillary pressure of the H₂ + cushion gas + water + silica (hydrophilic) system decreased with decreasing x_H2, suggesting that the studied gases might enhance the withdrawal process of H₂ in UHS.