Characterization of methane adsorption on overmature Lower Silurian-Upper Ordovician shales in Sichuan Basin, southwest China: Experimental results and geological implications

A series of methane adsorption isotherms were measured at 35.4 °C, 50.6 °C, and 65.4 °C at pressures up to 15.0 MPa for eight dried, overmature Lower Silurian-Upper Ordovician shale samples collected from the Sichuan Basin with TOC values in the range of 1.87-5.74%. The measured maxima of excess adsorption capacity of methane range from 1.25 to 2.50 cm3/g rock at 65.4 °C; the maxima are slightly enhanced at 35.4 °C, but all are positively correlated with total organic carbon (TOC). Both the supercritical Dubinin-Radushkevich (SDR)- and Langmuir-based excess adsorption models were found to represent the experimental excess adsorption isotherms equally well within the experimental range. The temperature-dependent densities of adsorbed methane resulting from the parameter fit of the SDR-based excess adsorption model are in the range of 297-415 mg/cm3; for the Langmuir-based excess adsorption model, the adjusted densities range from 386 mg/cm3 to 1027 mg/cm3 and most of them are much larger than the liquid density of methane at its boiling point (424 mg/cm3). Nevertheless, the maxima of absolute methane adsorption capacity fitted by both models are not significantly different and are linearly correlated. One of the contributors to the uncertainty of the gas-in-place estimation in geological conditions is the inconsistent utilization of experimental excess sorption data as “absolute sorption” values, particularly at high pressures. However, the choice of adsorption model itself (Langmuir- or SDR-based) and the fitting procedure, assuming either constant or temperature-dependent adsorbed phase density and maximum sorption capacity, do not significantly affect the estimated GIPs for the geological system studied here with depths of less than 4000 m.