Abstract: To investigate the coupling effect among coal seam temperature, gas seepage field and stress field, a temperature field controlling equation was reestablished by employing binary-energy-state theory in coal gas flow. Then, the theoretical solution of desorption differential heat term in the equation was derived as well. Finally, a fully coupled thermal-hydrological-mechanical (THM) model was improved. Based on the improved THM model, this study discussed complex interactions among adsorption, desorption, stress field, temperature field and seepage field in coal gas flow. Moreover, the THM model was used to investigate the changes of temperature, gas pressure and permeability of coal seam during gas drainage. The model results showed good agreement with the results from previous experimental studies. The results indicated that the drop-down rate of coal seam temperature was controlled by the combined effect of the original gas content, pore pressure and permeability of coal seam during gas drainage. It means that the greater the permeability of the coal seam is, the faster the dropdown rate of the temperature is. The greater the coal seam gas content and gas pressure are, the faster the temperature drops. Meanwhile, the permeability of coal seam increased with the elapsed time of drainage and the amplitude of the increase declined with the increase of the distance from borehole axis along the radial direction.

Key words: temperature field, coal bed methane, gas seepage, thermal-hydrological-mechanical coupled model