Investigation of methane sorption of overmature Wufeng-Longmaxi shale in the Jiaoshiba area, Eastern Sichuan Basin, China

In China, shale gas of production capacity 203, 000 m(3) per day has been identified in Well JY1 in the upper Ordovician Wufeng formation shale (O(3)w) and the lower Silurian Longmaxi formation shale (S(1)l), Jiaoshiba area, Sichuan Basin. However, the adsorbed gas capacity and the relative ratio between free gas and adsorbed gas are unknown in the Jiaoshiba area, and this is important for further shale gas exploration and development. And different adsorption models need to be calibrated very well to praise the sorption capacity. The samples were obtained from the well JY1. Sorption isotherm experiments were performed at a range of pressures (up to 17 MPa) and temperatures (up to 90 degrees C) to determine the effects of TOC, moisture, and clay minerals on methane adsorption capacity. The methane maxima adsorbed capacity values have a clear positive relationship with TOC. Clay content displays a weak correlation with methane adsorbed amounts normalized to TOC, indicating that clay minerals are a minor contributor to shale adsorption capacity. Moisture reduces methane sorption capacity by around 20%. Both the supercritical Dubinin-Radushkevich (SDR)-based and the Langmuir-based excess adsorption models are used to fit measured adsorption isotherms. Most of the methane adsorption phase density values fitted by both models are greater than the density of liquid phase methane at its boiling points, 424 mg/cm(3). For the Langmuir-based model, a fixed adsorption density of 424, or 373 mg/cm(3) was used in fitting the experimental data, which will result in higher maximum adsorbed capacity values. For the SDR-based model, maxima adsorbed capacity values remain stable and have nothing to do with sorption phase density.