Methane adsorption experiments over wide ranges of pressure (up to 30 MPa) and temperature (30–120 °C) were performed using a gravimetric method on the Longmaxi shale collected from the northeast boundary of Sichuan Basin, China. Organic geochemical analyses, shale composition determination, and porosity tests were also conducted. The experimental supercritical methane excess adsorption isotherms at different temperatures initially increase and then decrease with increasing pressure, giving a maximum excess adsorption capacity (G ex m = 1.86–2.87 cm3/g) at a certain pressure P m (6.71–12.90 MPa). The excess adsorption capacity decreases with increasing temperature below 28 MPa, while this effect reversed above 28 MPa. However, the absolute adsorption capacity decreases as the temperature increases over the full pressure range. Supercritical methane adsorption on shale is of temperature dependence because it is a physical exothermic process supported by calculated thermodynamic parameters. P m is positively correlated with the temperature, while the decline rates (0.021–0.058 cm3 g–1 MPa–1) in excess adsorption negatively correlate with the temperature. Meanwhile, Langmuir volume G L (3.07–4.04 cm3/g) decreases while Langmuir pressure P L (1.44–4.31 MPa) increases with temperature elevation. In comparison to the actual adsorbed gas (absolute adsorption), an underestimation exists in the excess adsorption calculation, which increases with increasing depth. The conventional method, without subtracting the volume occupied by adsorbed gas, overestimates the actual free gas content, especially for the deep shale reservoirs. In situ adsorbed gas is simultaneously controlled by the positive effect of the reservoir pressure and the adverse effect of the reservoir temperature. Nevertheless, in situ free gas is dominated by the positive effect of the reservoir pressure. Low-temperature overpressure reservoirs are favorable for shale gas enrichment. Geological application of gas-in-place estimation shows that, with increasing depth, the adsorbed gas content increases rapidly and then declines slowly, whereas the free gas content increases continuously. There was an equivalence point at which the contents of adsorbed and free gas are equal, and the equivalence point moved to the deep areas with increasing water saturation. Moreover, the adsorbed gas and free gas distribution are characterized by the dominant depth zones, providing the reference for shale gas exploration and development.

To investigate supercritical methane adsorption on shale and its controlling factors, high-pressure (up to 20 MPa) methane adsorption experiments were performed on overmature Niutitang shales from the Upper Yangtze area in China. Combining field emission scanning electron microscopy, low-pressure N2 adsorption (LP-N2-GA), and CO2 adsorption (LP-CO2-GA), the pore structure and fractal characteristics were studied. According to the LP-N2-GA and Frenkel–Halsey–Hill (FHH) model, pore surface and spatial structure are characterized by the fractal heterogeneity with corresponding fractal dimensions D 1 (2.32–2.69) and D 2 (2.49–2.82). The measured supercritical methane excess adsorption isotherms show three stages: (i) a sharp increase under 6 MPa, (ii) a slow increase until reaching the maximum (V ex m = 1.06–4.60 cm3/g) at the pressure of P m (7.53–10.41 MPa), and (iii) a decline at various rates over the P m. The rates of decline in excess adsorption at high pressures vary (0.031–0.074 cm3/g/MPa) and positively correlate with the total organic carbon (TOC) content, pore volume, and specific surface area (SSA) of micropores and fractal dimension D 1, whereas the P m possesses weakly negative relationships with these factors. The excess adsorption data can be accurately fitted by the supercritical Langmuir-based adsorption model with the maximum absolute adsorption capacities (V L) ranging from 2.88 to 6.57 cm3/g. Misinterpreting the low-pressure (0–10 MPa) experimental excess adsorption data as the absolute adsorption values to fit the adsorption isotherms, the actual adsorption capacity will be underestimated with the errors ranging from 18.44 to 45.34% for the calculated V L, and an underestimation will exist in extrapolated in situ adsorbed gas content. TOC still plays an important role in promoting methane adsorption capacity even for the overmature shales. Meanwhile, the methane adsorption capacity is positively correlated with the SSA, micropore volume, and fractal dimension D 1, and microporosity is the governing factor on adsorbed gas occurrence.

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