Coal–Water Interfacial Controls on Methane Adsorption–Desorption and Pore-Scale Transport in Representative Coal Samples from the Ordos Basin

Methane production from water-bearing coal reservoirs is governed not only by methane adsorption capacity but also by the accessibility of adsorption domains and the efficiency of pore-scale transport during pressure depletion. However, the interfacial mechanism by which coal wettability and water occurrence regulate methane adsorption–desorption reversibility remains insufficiently understood. In this study, three representative Ordos Basin coal samples with different pore structures and surface polarities, denoted as OBC-L, OBC-M, and OBC-H, were investigated to explore the pore-scale mechanisms governing water-mediated methane storage and release rather than to establish basin-wide statistical relationships. A combined experimental workflow involving N2 adsorption–desorption, FTIR and XPS analyses, contact angle and Zeta potential measurements, low-field NMR, high-pressure methane adsorption–desorption tests, kinetic modeling, hysteresis evaluation, and Pearson correlation analysis was used to clarify the coupling among pore structure, coal–water interfacial properties, water occurrence, methane storage, and methane release. The results show that OBC-H possesses the strongest dry-state methane storage potential, with the BET surface area increasing from 5.82 m2/g for OBC-L to 12.94 m2/g for OBC-H and the fitted Langmuir volume (VL) reaching 22.3 cm3/g. Nevertheless, OBC-H also shows stronger water affinity, as reflected by an increase in the XPS-derived O/C atomic ratio from 0.118 to 0.186, a decrease in contact angle from 82.6° to 51.8°, and an increase in bound water fraction from 46.3% to 69.4%. With the transition from dry to saturated conditions, the fitted VL of OBC-H decreases from 22.3 to 15.2 cm3/g, while the Langmuir pressure (PL) increases from 1.38 to 3.00 MPa, indicating a simultaneous reduction in the model-estimated maximum methane adsorption capacity and apparent methane affinity. More importantly, the desorption results demonstrate that high adsorption capacity does not necessarily correspond to high methane deliverability. For OBC-H, the final desorption efficiency decreases from 79.6% to 54.2%, the effective diffusion coefficient decreases from 2.74 × 10−11 to 0.86 × 10−11 m2/s, and the hysteresis index increases from 12.8% to 36.4% under saturated water conditions. Correlation analysis further confirms that bound water fraction is positively associated with adsorption–desorption hysteresis but negatively associated with desorption efficiency, desorption rate constant, and effective diffusion coefficient. These findings are consistent with two distinct water-mediated constraints: adsorbed/bound interfacial water contributes to surface-site shielding, whereas capillary and saturated water occupation contributes to pore-throat transport restriction; together, these effects reduce methane release efficiency and enhancing desorption irreversibility. This study provides an interfacial interpretation of methane deliverability based on representative water-bearing coal samples and offers a mechanistic basis for understanding wettability- and water-retention-related transport constraints; broader applicability across the Ordos Basin requires validation using a larger number of samples from different coal seams and reservoir settings.