Determination of Interfacial Tension for H2-H2s-Water and Ch4-H2s-Water Mixtures Under Different Geological Conditions for Underground Hydrogen Storage

Abstract

Hydrogen, a renewable energy source, is expected to see steadily increasing production to meet global net-zero targets. Depleted hydrocarbon reservoirs, with their large storage capacity, offer a promising solution for large-scale underground hydrogen storage (UHS). Stored hydrogen (H₂) and cushion gases (e.g., CH₄) may coexist with residual hydrogen sulfide (H₂S) or H₂S generated locally through geochemical reactions or microbial activity. As gas composition significantly influences the interfacial tension (IFT) between gas and pore water, the distribution and dynamics of H₂ and CH₄ in depleted reservoirs are affected by H₂S under reservoir conditions. This study presents newly determined IFTs of H₂-H₂S-water and CH₄-H₂S-water mixtures at 298 K and 343 K with pressures ranging from 10 to 30 MPa using molecular dynamics (MD) simulations. Results reveal that IFT decreases with increasing pressure, temperature, and H2S concentration, with distinct mechanisms driving these variations. Notably, even small amounts of H₂S (e.g., 10 mol%) significantly reduce IFT, underscoring its critical role in gas-water interactions. With simulated IFT data, two regression models are developed to predict the IFTs of  H₂-H₂S-water and CH₄-H₂S-water mixtures under geological conditions. This study provides comprehensive IFT expressions for CH₄-H₂S-water and H₂-H₂S-water mixtures and offers insights into the mechanisms underlying IFT variations for UHS applications.

Keywords: underground hydrogen storage, gas-water interface, molecular dynamics simulation, interfacial tension, hydrogen sulfide, regression model