Wetting behaviors of water on kerogen surfaces from molecular level: Implication for gas extraction and hydrogen storage in shale

Shale formations serve as primary reservoirs for natural gas and emerging candidates for hydrogen storage, where the wetting behaviors of organic matter (i.e., kerogen) play a critical role in fluid retention and transport. This study employed molecular dynamics simulations to investigate the pressure-dependent wettability of kerogen surfaces in H₂ and CH₄ environments under geological conditions (333 K, 10-100 MPa). Results reveal distinct gas-specific mechanisms governing wettability evolution. For CH₄-H₂O systems, increasing pressure induces a wettability transition from weakly water-wet to gas-wet due to the strong interaction between CH₄ and the kerogen surfaces, which results in a smaller gas-solid interfacial tension compared with liquid-solid interfacial tension. Meanwhile, both the reduced gas-liquid and gas-solid interfacial tension contributes to a linear rise in contact angles (88° to 119°). In contrast, H₂ exhibits weaker interactions with the kerogen surfaces and experiences a minimal decrease in gas-liquid interfacial tension, thus presenting persistently water-wet characteristics (53° to 69.5°) even at 100 MPa. Crucially, the Young-Laplace equation remains valid at the nanoscale, as evidenced by direct capillary pressure measurements aligning with theoretical predictions, confirming classical interfacial thermodynamics govern nanoconfined fluid behavior. These mechanistic insights elucidate how gas-specific molecular interactions dictate shale wettability, providing a physicochemical basis for optimizing CH₄ recovery through pressure-managed wettability alteration and ensuring H₂ storage security in hydrophobic kerogen network.