Since the mechanisms of methane-mudstone interactions are important for estimating shale gas reserves, methane adsorption under supercritical conditions of 30 MPa pressure and 303.15, 333.15, 363.15 K temperatures was studied to measure the excess methane adsorption in two mudstone samples from Yanchang Formation, Ordos Basin. Excess adsorption features inflection points where the amount of adsorbed gas changes from increasing to decreasing concentrations. Three methods (fixed, slope, and freely fitted density) were applied to calculate the adsorbed-phase density (ρ_ad), which was then used to fit the measured excess adsorption. Two criteria, the goodness-of-fit and whether the fitting can obtain reasonable absolute adsorption, were applied to determine the most accurate model. Results indicated that the supercritical Dubinin-Radushkevich (SDR) model with freely fitted ρ_ad was the most reasonable model. The volume of adsorbed methane at 363.15 K is close to the micropore (d < 2 nm) volume of the corresponding mudstone. Considering the actual geological conditions, the adsorbed gas should be predominantly stored in micropores. Thermodynamic parameters reveal that the methane adsorption on mudstone is a physisorption process that is jointly controlled by the heterogeneity of, and interaction forces between the methane molecule and, the rock surface.

Shale matrix permeability is an important indicator for evaluating gas transport and production. However, the effects of pore connectivity and water saturation on the matrix permeability in deep gas shales have not been adequately studied. In this study, the permeability of deep shales in the Yichang area of the Middle Yangtze was characterized using three methods. These included the determination of apparent permeability in different directions via pulse-decay, also matrix permeability obtained via the Gas Research Institute method, and the connected pore network permeability via the mercury injection capillary pressure technique. The results revealed a significant difference between the horizontal and vertical permeability of deep shales. The smaller the size of the multiple connected pore network, the larger was the effective tortuosity and the lower the permeability. Comparison of the three permeabilities and combined microscopic observations revealed that microfractures and laminae were the dominant gas transport channels. Importantly, the matrix permeability decreased exponentially with increasing water saturation, with water vapor adsorption experiments revealing that water occupation of pores and pore-throat spaces smaller than 10 nm in diameter was the main reason for this decrease in matrix permeability. Collectively, proposed method of evaluating effective permeability with an index for shale gas reservoirs is significant for sweet spot selection and production prediction of shale gas reservoirs around the globe.