Numerical Analysis of Heat and Gas Transfer Characteristics during Heat Injection Processes Based on a Thermo-Hydro-Mechanical Model

Heat injection is an important artificial technique, which can significantly enhance the extraction efficiency of coal seam gas (CSG) and reduce the outburst risk caused by CSG. Although heat injection has been comprehensively investigated, the effect of temperature on the coal-gas interactions in CSG extraction is still not clear. In this study, a thermo-hydro-mechanical model was developed considering the expansion of coal mass and the change of adsorption capacity induced by heat injection. Subsequently, the reliability of the model was verified through a comparison with other theoretical models and field data. Finally, a numerical simulation and parameter analysis of the heat injection process were performed and compared with the traditional gas extraction method. The simulation results show that heat injection can significantly increase the gas production rate and cumulative gas production through the gas desorption and the permeability increase. The gas content in the coal seam dramatically decreases in the vicinity of the production and heat injection wells under the condition of heat injection, which greatly accelerates the gas drainage. The coal deformation caused by thermal-induced gas desorption has a more dominant effect on the porosity than other factors, i.e., pore pressure, thermal strain and compressive strain.