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

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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.