Experimental Study on the Methane Adsorption of Massive Shale Considering the Effective Stress and the Participation of Nanopores of Varying Sizes

To explore the shale gas occurrence mechanism in shale with an intact pore structure under actual reservoir conditions, an adsorption experiment on massive shale was performed. Considering the change in the pore volume of massive shale under effective stress, the adsorption mechanism and free gas storage space of massive shale were investigated. Based on the adsorption mechanism assumptions of micropore filling and mesopore multilayer adsorption, the adsorbed phase densities of pores of varying pore sizes were calculated and applied to the conversion of the absolute adsorption amount of massive shale. The results show the existence of isolated pores in the massive shale, resulting in a lower adsorption capacity in comparison to granular samples. When subjected to the combined effects of in situ stress and pore pressure, the pore volume of massive shale gradually decreases with the increase in effective stress. Shale gas is mainly adsorbed in micropores, but with increasing pressure, the adsorption amount of micropores approaches saturation, and the contribution of mesopores to the total adsorption amount gradually increases. The main adsorption mechanism of shale gas is based on micropore filling, and the multilayer surface adsorption of mesopores should also be considered. By combining the simplified local density model and the Ono-Kondo lattice model, the adsorption behavior of shale gas can be accurately described. To accurately estimate shale gas reserves, it is necessary to take into account the actual pore size distribution, pore volume compressibility, and connected porosity of the shale samples.