Study on CH4 Characteristics of CO2 Displacement Coal under Thermomechanical Coupling

When coal resources enter deep mining, the coal body is exposed to a complex geological condition characterized by “high ground stress”, “high osmotic pressure” and “high temperature”. Under such geological conditions, the permeability evolution law of the coal body is determined by the coexistence of multi-phase and multi-field coupling. Moreover, the permeability of the coal body directly impacts the safety of coal mining operations. Thermodynamic coupling tests were conducted on coking coal samples. The scanning electron microscope and high-pressure adsorption instrument were employed to characterize the pore structure and adsorption capacity on the coal surface. Subsequently, the changes in the coal pore structure and gas displacement efficiency before and after the thermodynamic coupling treatment were compared and analyzed. The results demonstrated that, under thermodynamic coupling, the adsorption of CH4 in the coal rises with the increase in pressure. However, as the temperature increases, the adsorption of CH4 in the coal exhibits an opposite tendency. With the growth of injection pressure, the replacement ratio of CH4 decreases linearly. Likewise, with the elevation of temperature, the CH4 replacement ratio also diminishes. After the thermodynamic coupling, the pore structure of the coal sample is well-developed and the porosity is enhanced. The research findings can offer a theoretical foundation and technical guidance for the study of multi-field coupled gas migration and coal seam gas treatment.