Nanoconfinement Controls Hydrogen Transport in Shales

Underground hydrogen storage (UHS) in sedimentary formations requires understanding how hydrogen migrates, persists, and is released under nanoscale confinement in fine-grained rocks that may act as reservoirs or caprocks. Here, we combine in situ small-angle neutron scattering (SANS) with molecular dynamics simulations to examine how wetting state controls hydrogen reversibility and postcycling pore occupancy in shale nanopores. SANS-derived pore-size distributions reveal a clear hierarchy: dry shale exhibits near-complete reversibility, decane-saturated shale shows partial recovery, and water-saturated shale displays minimal recovery, with persistent postcycling occupancy concentrated in pores <2 nm. Molecular simulations support a mechanistic interpretation in which this hierarchy arises from coupled nanoscale effects, including cavity-formation penalties, confinement-induced stress anisotropy, and fluid-specific interfacial free-energy barriers at mineral and organic surfaces. Water-saturated nanopores are associated with strong thermodynamic and mechanical constraints consistent with reduced hydrogen reversibility, whereas these constraints are substantially weaker in dry pores and intermediate in decane-filled pores. Together, the experimental and molecular simulation results provide a framework linking nanoscale fluid–rock interactions to hydrogen hysteresis and retention, with implications for subsurface hydrogen recoverability and containment in underground storage systems.