Effect of adsorption phase and matrix deformation on methane adsorption isotherm of Fuling shale

The controlling factors and mechanisms of gas adsorption are useful in gas content evaluation and production forecast for shale gas. To determine the adsorption characteristics of the Fuling shale at high pressures, and investigate the root cause of isotherms with abnormal shapes, a series of methane adsorption isotherms on the Fuling shale samples were measured utilizing gravimetric and volumetric methods at the temperature of 88 °C and several pressures up to 40 MPa. The measurement data was fitted by the original Langmuir equation and the Langmuir equation involving the adsorbed phase. Then, the factors that influence the shape of the isotherm under high pressure were investigated, and three main factors including adsorption phase, shale matrix and matrix compression were later incorporated to construct further modified models for sample volume, void volume and adsorption amount. It is applied to investigate the difference in isotherms obtained by gravimetric and volumetric methods, and the difference in isotherms for clay minerals and organic matter. The results indicate that the range of methane adsorption capacities on Fuling shale is 1.6–4.1 × 10−3 m3/kg. The excess adsorption isotherms obtained by the gravimetric method can be fitted with good accuracy by the Langmuir-AP model accounting for the adsorbed phase, and the calculated absolute adsorption isotherms for shale samples are all classified to Type I. From the volumetric method and the gravimetric method respectively, the shape of excess adsorption isotherms are similar at low pressure, but different at high pressure. The effects of the adsorption phase and matrix swelling on adsorption amounts are less from the volumetric method than that from the gravimetric method. While the role of matrix compression played on the results is more sensitive from the volumetric method than that from the gravimetric method, especially under high pressure. The effect of matrix compression on adsorption for organic matter differs from that for clay minerals, as the deformation of organic matter under high pressure is larger than that of clay.