Comparative analysis of 3-phase and 4-phase surface phenomena in hydrogen, CH₄/CO₂/N₂-rich cushion gas, n-octane and rock systems

Hydrogen storage in depleted reservoirs is governed by multiphase interfacial phenomena that influence wettability, fluid displacement, containment, and recovery. This study experimentally evaluated three-phase interfacial tension (IFT; brine-n-octane-gas) and four-phase contact angle (CA; brine-n-octane-gas-rock) for CH₄- and CO₂-rich cushion-gas formulations at pressures of 500-3000 psi, temperatures of 30-70 ^oC, and NaCl salinities of 2 and 10 wt%, with previously published N₂-rich data used as a benchmark. Pendant-drop and captive-bubble measurements showed that CH₄-rich systems generally maintained the strongest water-wet conditions and highest IFTs, resulting in the largest estimated hydrogen column heights, although their wettability was highly sensitive to salinity. CO₂-rich systems exhibited the greatest pressure-induced increases in CA and reductions in IFT, indicating comparatively weaker capillary containment but potentially improved gas withdrawal. N₂-rich systems generally showed intermediate behaviour. Increasing temperature reduced both CA and IFT, while the stronger water-wetness associated with lower CA contributed to increased calculated column heights. Increasing salinity raised CA and IFT but generally reduced column height because the reduction in water affinity outweighed the IFT increase. These findings demonstrate that cushion-gas composition and reservoir conditions jointly control multiphase interfacial behaviour and should be considered when balancing hydrogen containment and recovery in depleted reservoirs.