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Shale gas stored in deep shale is in a supercritical state. Therefore, it is necessary to study the adsorption and desorption properties of supercritical shale gas. To accurately determine the state of methane (CH4) in the pores of deep shale, the fractal characteristics of several shale samples drilled at a depth of 2650 m are analyzed using scanning electron microscopy (SEM) and image analysis. We find nanopores with different fractal features in the shale. The effects of adsorption energy and substrate strain on adsorption capacity are clarified. The virial coefficients of CH4 are obtained by molecular dynamics (MD) simulations and are consistent with the experiment. The adsorption and desorption of CH4 in different fractal nanopores are modeled using grand canonical Monte Carlo (GCMC) simulations at different temperatures and pressures (from capillary condensation to supercritical state). Additionally, the gas-in-place (GIP), excess adsorption, and absolute adsorption isotherms are obtained. We find the crossover of excess adsorption isotherms, which was observed in the experiment, and the absolute adsorption amount increases with the increase in pressure in the case of ultrahigh pressure (>40 MPa). Moreover, we obtain an ultrahigh-pressure dual-site Langmuir equation, and it can accurately describe observed adsorption isotherms from low pressure to ultrahigh pressure. Our study visually reproduces the adsorption/desorption behaviors of CH4 under in situ conditions in deep shale and reveals their microscopic mechanism.
Lin et al. (Tue,) studied this question.