Molecular Understanding of Hydrogen Behavior in Moist Kerogen Matrix with Residual Methane Presence: Implications for Underground Hydrogen Storage

Summary The global shift toward low-carbon energy systems has spotlighted hydrogen as a pivotal energy carrier, yet its low volumetric density poses significant challenges for large-scale storage. Underground hydrogen storage (UHS) in shale reservoirs offers a promising solution, leveraging adsorption in a nanoporous kerogen matrix. However, residual methane and connate water introduce complexities, forming a ternary mixture that alters gas behavior under nano-confinement. For this work, Grand Canonical Monte Carlo (GCMC) simulations were conducted to comprehensively understand the molecular mechanisms governing hydrogen adsorption behaviors within a moist kerogen matrix. Our results reveal that methane exhibits preferential adsorption due to its stronger affinity for kerogen, reaching saturation at low pressures, while hydrogen displays a nearly linear adsorption increase with pressure, dominating post-methane saturation with a 41~72% contribution to adsorption growth. In the case of hydrogen, its intrinsic permanent quadrupole moment plays a key role in nanoconfined kerogen, weakly interacting with moisture-induced clusters, thereby influencing its adsorption. Low moisture content slightly enhances hydrogen adsorption by creating additional electrostatic interaction sites, whereas higher moisture levels significantly reduce methane/hydrogen adsorption due to water cluster formation, which occupies pore volume. A higher molar fraction of methane in the mixture, combined with increased moisture content, results in a significant increase in selectivity with a steeper rise, although it fails to challenge methane’s dominant role in the system. Additionally, we proposed a simplified injection-production process using a flexible kerogen matrix across a moisture range of 0~2 wt%, revealing that high moisture conditions induce significant kerogen matrix deformation, resulting in desorption hysteresis during extraction. Furthermore, hydrogen recovery improves with increasing moisture content, benefiting from a strengthened trapping effect on CH4 due to water-induced pore expansion and reduced confinement for hydrogen, with the recovery rate potentially reaching up to 65%. These results could provide fundamental insights into gas behaviors, with implications for optimizing storage and production processes during UHS.