Characterization of methane adsorption on overmature Lower Silurian Upper Ordovician shales in Sichuan Basin, southwest China: Experimental results and geological implications
Abstract A series of methane adsorption isotherms were measured at 35.4 degC, 50.6 degC, and 65.4 degC 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 cm 3 /g rock at 65.4 degC; the maxima are slightly enhanced at 35.4 degC, 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/cm 3 for the Langmuir based excess adsorption model, the adjusted densities range from 386 mg/cm 3 to 1027 mg/cm 3 and most of them are much larger than the liquid density of methane at its boiling point (424 mg/cm 3 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.