Microscopic insights into geochemical reactions impacts on confined fluid phase behavior in underground hydrogen storage

In underground hydrogen storage (UHS), geochemical reactions and nanopore confinement critically regulate hydrogen loss and fluid behavior. To elucidate the coupled effects of reaction-induced brine evolution and nanopore confinement on molecular/ionic behavior in UHS, this study integrates geochemical modeling with molecular dynamics to examine water/ion distributions and diffusion in calcite–brine–hydrogen systems at 393 K and 90 bar. Results show georeactions enrich 95 % Na + enrichment at calcite interfaces and ∼60 % Ca 2+ adsorption within 0.25 nm of calcite surfaces, reducing interfacial water by 40–60 %. Nanopore size critically controls diffusion: 2 nm pores suppress H 2 diffusion below 10 −13 m 2 /s, while 8 nm pores restore near-bulk behavior (∼1.44 × 10 −9 m 2 /s). Hydrogen bond lifetimes decrease with larger pores and ion disruption, though water-calcite bonds stabilize due to ion-induced immobilization. These findings reveal how geochemistry and confinement jointly regulate pore-scale fluid dynamics in UHS. • Confined fluid behaviors in underground hydrogen storage condition were explored. • Ions occupy calcite surface through competitive adsorption. • Cations prefer to adsorb on the calcite surface via electrostatic interactions. • Increasing pore size significantly enhances molecular and ionic mobility. • Geochemical reaction and large pore size reduce the lifetimes of hydrogen bonds.