Estimation of shale gas adsorption capacity of the Longmaxi Formation in the Upper Yangtze Platform, China

The methane isothermal adsorption experiment was conducted on the Lower Silurian marine shale of the Longmaxi Formation from the Upper Yangtze Platform, China, at pressures up to 10.94 MPa and temperatures of 30 °C and 39 °C to investigate the key factors affecting its methane adsorption capacity. Basic shale reservoir characteristics were obtained using total organic carbon measurements, thermal maturity assessment, X-ray diffraction, and nitrogen adsorption. An abundance of organic matter in the reservoir with total organic carbon (TOC) ranging between 0.49 and 4.8 wt% was overmature. The matrix minerals mainly consisted of clay (16%-33%) and quartz (37%-53.9%). The Brunauer-Emmett-Teller (BET) surface areas of the shale samples varied from 6.34 to 21.3 m2/g. The Langmuir-based absolute adsorption model was used to correct the measured adsorption isotherms into absolute adsorption isotherms, which represented the actual adsorption. Furthermore, the Langmuir volume was in the range of 1.04-5.36 m3/t. Shale gas adsorption capacity presented a significantly positive correlation with an abundance of organic matter that provided a larger BET surface area. The absence of apparent correlation with the clay indicated that the clay did not significantly contribute to the gas adsorption capacity in the shale samples. Variations in temperature and pressure were also key factors affecting shale gas adsorption capacity. Therefore, TOC, temperature, and pressure were optimized to establish the forecast model, combined with multiple regression methods and a set of cited data. The results of the isothermal adsorption experiment in this study were used to verify the model's feasibility for predicting the shale gas adsorption capacity of the Longmaxi Formation. The predicted results were highly correlated with the actual measured results, with a correlation coefficient as high as 0.9887. The function of shale gas adsorption capacity derived and analyses employed in this study can be used to illustrate the impact of pressure and temperature regimes on shale gas adsorption capacity, given varying abundance of organic matter and compositions of matrix minerals.