Study of the macromolecular structure of kerogen for CO2/H2 and CH4/H2 competitive adsorption capacity: 3D molecular reconstruction, spectroscopic experiments, molecular simulations

The nanopores in organic matter (OM) in shale are considered to be the main storage space for methane. However, there is still limited understanding of the role of OM in underground hydrogen storage(UHS) in retaining shale gas reservoirs. To investigate the influence of kerogen on hydrogen storage, this study employs multiple spectroscopic techniques (solid-state ¹³C nuclear magnetic resonance, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy) to establish macromolecular structure models of kerogens of high and over-mature stages. Using molecular simulation techniques (GCMC and MD methods), the adsorption characteristics of hydrogen on kerogen under conditions of 333.15 K-393.15 K and 0-30 MPa are studied. The result shows: Longmaxi kerogen is carbon-dominant (>80%), featuring an extensive aromatic framework in over-mature stages. The high content of protonated and branched aromatic carbon, alongside a well-developed graphite (002) crystal plane, confirms high graphite-like crystallinity in over-mature structures. CH₄/H₂ competitive adsorption is primarily governed by van der Waals forces. CH₄ molecule exhibits stronger surface affinity, preferentially occupying high-energy sites with densities exceeding twice the bulk phase. Conversely, H₂ interactions are extremely weak, primarily controlled by pore space confinement and thermodynamic conditions, leading to a bulk-phase distribution. CH₄/H₂ selectivity decreases with pressure. The limited impact of maturity on selectivity reflects the stability of the dispersion-dominated mechanism. CO₂ molecule exhibits strong electrostatic and inductive interactions with polar functional groups. This leads to markedly higher isosteric heats and selectivity compared to the CH₄ system. The CO₂ molecule exhibits strong electrostatic and inductive interactions with polar groups and aromatic structures, showing significantly higher isosteric heat and selectivity coefficients compared to the CH₄/H₂ system. The CO₂ molecule has the lowest diffusion coefficient due to stable adsorption configurations and long residence times. At high pressures, a pore confinement effect restricts the H₂ molecule mean free path, increasing collision resistance and reducing its effective diffusion rate. These findings provide critical theoretical support for assessing the safety and capacity of large-scale UHS in depleted shale gas reservoirs.