Study on characteristics of methane adsorption and thermodynamic in water-bearing shale

Shale reservoirs generally contain water that has a significant impact on shale gas adsorption behavior and geological reserve evaluation. In order to explore the influence of water content on gas adsorption behavior in shale, an adsorption model considering the influence of water content was established, the rationality of the new model was verified by combining several sets of experimental data, and gas adsorption behavior in water-bearing shale was further analyzed. On this basis, the contribution of different adsorption mechanisms to the isothermal adsorption of methane in shale was discussed, and the thermodynamic characteristics of adsorption in dry and wet shale were compared by thermodynamic parameters. The results shown that, the methane adsorption amount shown a typical “three-stage” change in the process of gas pressure change. Among them, water has an inhibitory effect on methane adsorption capacity, gas adsorption capacity in water-bearing shale was significantly lower than that in dry shale, and gas adsorption capacity decreased with the increasing water content. In addition, temperature also inhibited methane adsorption, and gas adsorption capacity in shale decreased with the increasing temperature when the water content was constant. At the same time, the contribution of different adsorption mechanisms to adsorption capacity was affected by the combination of water and pressure. The higher the pressure and water content, the lower the contribution of microporous filled adsorption to the total adsorption capacity. Thermodynamic parameters indicated that methane adsorption in both dry and water-bearing shale was physical adsorption, and the isosteric adsorption heat in dry shale was always higher than that in water-bearing shale under the same adsorption capacity. Water occupied high-energy adsorption sites and affected intermolecular forces by altering shale heterogeneity, which combined to reduce isosteric adsorption heat and hence gas adsorption capacity in water-bearing shale. The research results contributed to the accurate calculation of gas adsorption capacity in shale, and provided a theoretical basis for the development programme and compilation of shale gas reservoirs.