Interfacial thermodynamics of underground hydrogen storage: Linking mineralogy, cushion gas, and capillary sealing

Secure underground hydrogen storage (UHS) depends on interfacial properties governing wettability and capillarity sealing. While liquid-gas interfacial tension ( ɤ LG ) data are widely available, the roles of solid-gas ( ɤ SG ) and solid-liquid ( ɤ SL ) tensions under reservoir conditions, particularly in UHS systems, employing CO 2 as a cushion gas, remain poorly understood. This study presents an integrated framework to quantify ɤ SG and ɤ SL under reservoir-relevant conditions and to evaluate how mineralogy, pressure, temperature, salinity, and gas composition influence wettability in UHS. Experimentally measured contact angles and ɤ LG data were combined with Young's relation and the linear Neumann equation-of-state formulation using a fixed β = 1.057 × 10 −4 (m 2 /mJ) 2 for every mineral/rock, aqueous phase, pressure, temperature, and gas composition. Seven representative substrates (calcite, dolomite, quartz, basalt, granite, shale, and gypsum) were evaluated over 10–100 bar, 20–80 °C, and gas compositions from pure H 2 to pure CO 2 . The fixed-β results show that EOS-derived effective ɤ SG decreases with pressure for every investigated combination, whereas ɤ SL also exhibits condition-dependent pressure variation. At 70 bar and 40 °C, mean ɤ SG decreases from 51.99 to 18.35 mN/m in distilled water and from 46.70 to 16.17 mN/m in brine as CO 2 increases from 0 to 100%, although individual substrate responses are not strictly monotonic. The derived quantities are interpreted as fixed-β, model-dependent effective interfacial energies rather than direct measurements of intrinsic solid surface energies or adsorption. These findings advance understanding of interfacial thermodynamics in hydrogen-bearing subsurface systems and provide pore-scale insight for UHS site screening and reservoir modelling.