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

XY Yao and AKN Nair and MFAC Ruslan and SY Sun and BC Yan, LANGMUIR, 41, 19953-19964 (2025).

DOI: 10.1021/acs.langmuir.5c01958

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 xH2 = 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 xH2. 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 approximate to 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 xH2. 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 xH2, suggesting that the studied gases might enhance the withdrawal process of H2 in UHS.

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