Molecular dynamics insights into interfacial tension and solubility of hydrogen-cushion gas-water systems for underground hydrogen storage

Underground hydrogen storage (UHS) has been a promising option for large-scale hydrogen storage. Gas-water interfacial tension (IFT) and gas solubility are important parameters affecting the flow and distribution of hydrogen in underground porous media. The IFT and solubility of hydrogen-water systems at temperatures ranging from 298.15 K to 373.15 K, pressures ranging from 2.76 MPa to 46.88 MPa, and salinities up to 4.95 mol/kg were investigated using molecular simulation methods. The IFT of hydrogen-water systems exhibits a negative correlation with temperature and pressure but a positive correlation with salinity. Hydrogen solubility exhibits a positive correlation with pressure while showing a negative correlation with temperature and salinity. Hence, the high salinity caprock has a higher hydrogen-water IFT and lower hydrogen solubility, which is favorable for UHS projects. Cushion gas is used to maintain formation pressure and meanwhile mixed with hydrogen gas and diffused with each other. The effects of three different cushion gas types, including N₂, CO₂ and CH₄, and various cushion gas contents on the IFT and solubility of hydrogen-cushion gas-water systems were also investigated. Contrasting these three cushion gases, N₂ and CH₄ affect the IFT and solubility of hydrogen-cushion gas-water systems to a similar extent. In particular, CO₂ exhibits a propensity for interfacial accumulation, and can greatly reduce the gas-water IFT and has a high solubility. CO₂ is an excellent cushion gas, which is not only conducive to the solubility trapping of CO₂, but also forms a barrier at the gas-water interface and reduces the hydrogen loss by dissolution. This study focuses on revealing the influence of cushion gas on the two-phase system of UHS and clarifying the underlying mechanisms. Thus, it contributes to the selection of target formations and cushion gas types for UHS.