Solubility and interfacial tension of hydrogen and hydrogen-methane mixtures in water: Effect of pH, salinity, and gas composition

Underground hydrogen storage (UHS) in deep aquifers is a promising strategy for enabling a large-scale hydrogen economy, primarily due to its secure and high storage capacity. However, hydrogen solubility in brine and the interfacial interactions with formation water under realistic reservoir conditions remain poorly understood, particularly in the presence of cushion gases like methane (CH₄). Thus, this study provides comprehensive experimental data on high-pressure solubility and IFT behavior of H₂, CH₄ and their mixtures in deionized water and 10 % NaCl brine across varying pH levels (4, 7, and 10) at 298 K and pressure up to 300 bar. The results demonstrate that gas solubility is strongly dependent on pressure and gas composition, with CH₄-rich mixtures showing significantly higher solubility and sharper IFT reductions than H₂-rich systems. Salinity was shown to reduce gas solubility due to the salting-out effect, while pH had a negligible influence on solubility and IFT under the tested conditions. Importantly, the solubility of H₂ was found to be largely reversible during depressurization, supporting the feasibility of cyclic gas withdrawal in UHS operations. Interfacial tension data revealed that CH₄ content and pressure were the dominant factors influencing gas–liquid interface behavior. In addition, a thermodynamic solubility model -incorporating gas-specific fugacity coefficients and Setchenov salinity corrections-demonstrated excellent agreement across all tested systems, with correlation coefficients (R²) exceeding 96 %. The model advances predictive capabilities for subsurface hydrogen behavior and can support more reliable reservoir simulation. This work fills a critical experimental data gap for UHS modeling, providing quantitative insights into how gas composition, pressure, and salinity affect H₂ solubility and interfacial behavior in brine systems. Together, these findings offer critical insights for optimizing gas injection strategies, cushion gas selection, and operational design in saline aquifers and depleted gas reservoirs.