Molecular dynamics insights into gas-water interfacial tension: Optimizing hydrogen storage in subsurface conditions

Abstract

This study presents a comprehensive analysis of interfacial tension (IFT) between pore water and hydrogen-cushion gas mixtures in subsurface porous media, a key variable in the distribution of gas mixtures for hydrogen storage. Employing molecular dynamics simulations, we developed two IFT models at 20 MPa and 373 K: one for an H2–CO2–H2O system and another for an H2–CH4–H2O system. The models reveal that increasing cushion gas concentration reduces gas-water IFT. Notably, for H2–CO2–H2O mixtures, the IFT substantially decreases with CO2 concentration up to 40%, after which the reduction rate diminishes. In H2–CH4–H2O mixtures, the IFT decreases linearly with CH4 concentration. The study attributes these variations to local density distributions and molecular orientation effects. Specifically, CO2 adsorption at the interface up to 40% concentration significantly lowers IFT, while CH4 adsorption proportionally decreases IFT. The influence of CO2 on water molecule orientation at the interface, in contrast to the non-effect of CH4, is also critical in enhancing IFT reduction. Our results also highlight the reduction of H2 self-diffusion at the interface caused by CO2 and CH4, an essential factor in optimizing subsurface hydrogen storage operations.