Isotherms, thermodynamics and kinetics of methane-shale adsorption pair under supercritical condition: Implications for understanding the nature of shale gas adsorption process

This research article selects supercritical methane and organic-rich shale as adsorbate-adsorbent pair to investigate methane adsorption behavior and enrich our understanding of the nature of shale gas adsorption process. The isotherms and kinetics of methane-shale adsorption pair are measured at temperatures of 303 K, 323 K, 343 K and 363 K by using a volumetric experimental setup. Then, the Langmuir-based (Langmuir, Langmuir + k, Langmuir + Henry), BET-based (BET, BET + k, BET + Henry) and DA-based (DA, DA + k and DA + Henry) excess models are used to interpret measured excess isotherms, and the Unipore Diffusion (UD), Bidisperse Diffusion (BD) and Two Combined First-Order Rate (TCFOR) models are used to interpret the adsorption kinetics data. Instead of using the coefficient of determination (R-2), this work used the corrected Akaike's Information Criterion (AIC(c)) for model selection. It is found that the DA + Henry model is more suitable for excess adsorption isotherms, and the TCFOR model is more appropriate for adsorption kinetics study. Additionally, for methane-shale adsorption under supercritical condition, the fugacity is of great significance in evaluating thermodynamic properties including isosteric heat of adsorption (q(st)), enthalpy change (Delta H), entropy change (Delta S) and Gibbs free energy change (Delta G). These properties show strong dependence on adsorption amount and temperature, and suggest that supercritical methane adsorption on organic-rich shale is a process of physisorption, exothermic and spontaneous. Further, the kinetics parameters extracted from kinetics curves suggest that the methane adsorption at each pressure step is a two-stage process, with a fast macropore diffusion process at early time, followed by a slow micropore diffusion process at later time. Additionally, the fast macropore diffusion dominates the two-stage adsorption process at lower pressures, while at higher pressures slow micropore diffusion dominates.