A composition-based model for methane adsorption of overmature shales in Wufeng and Longmaxi Formation, Sichuan Basin

The Wufeng-Longmaxi Formation shale in Sichuan Basin is the main strata of shale gas exploration and devel-opment in China. Adsorbed methane is an important part of the total shale gas content. The influence of shale composition on adsorption characteristics is complex, and it is difficult to obtain a comprehensive understanding without a sufficient amount of data. In this work, the total organic carbon (TOC), X-ray diffraction (XRD) mineral analysis, and the excess methane adsorption isotherms with pressure up to 50 MPa at 40 degrees C of 106 overmature shale samples with TOC of 0.8%-9.0% and clay content of 2%-58% that belong to Wufeng-Longmaxi formation from 10 shale gas wells in Sichuan Basin, China are experimentally investigated and analyzed. New correlations found between compositions and methane adsorption characteristic parameters guide the establishment of a composition-based methane adsorption model that the excess adsorption isotherm of shale is the content-weighted sum of equivalent unit-content excess isotherms for organic matter and clay. With this model, the differences of excess isotherms of these two compositions and the differences of clay-contribution to methane adsorption for different sublayers are disclosed. It is beneficial to improve the accuracy of shale reservoir evaluation of studied blocks under the condition of limited coring and deepening the understanding of the underlying mechanism of methane adsorption characteristics.