Bulk-like H2/CH4 mixture density in micro- and nanopores for underground hydrogen storage: a wettability-dependent modified Peng-Robinson equation

Underground hydrogen storage in depleted gas reservoirs involves nanopores where wall-molecule interactions rival intermolecular forces; how H 2 /CH 4 core density departs from bulk behavior as composition changes remains unclear. A wettability-dependent modified Peng-Robinson (PR) equation is developed for the bulk-like core density of methane-bearing pores, not near-wall or whole-pore densities. One CH 4 molecular diameter defines the wall-affected thickness, pore corrections act at component level, and the mixing rule retains H 2 -H 2 , H 2 -CH 4 , and CH 4 -CH 4 contributions. Validation against 45 wall-resolved molecular-simulation points gives 7.75% average absolute relative deviation. At 343 K and 50 MPa (extrapolated) in a 5 nm cylindrical pore, the deviation from conventional PR, Δ ρ , changes from −5.63% at x H 2 = 0.25 to −1.18% at x H 2 = 0.90, and enlarging the pore from 3.5 to 20 nm at equimolar composition reduces | Δ ρ | from 4.97% to 0.83%. CH 4 -related attraction, not mixture molar mass, controls the response.