Heat–fluid–solid coupling model for gas-bearing coal seam and numerical modeling on gas drainage promotion by heat injection

Abstract

Improving the absorbed gas to active desorption and seepage and delaying gas drainage attenuation are considered as key methods for increasing drainage efficiency and gas output. According to the solid mechanics theory, the nonlinear Darcy seepage theory and thermodynamics, the heat–fluid–solid coupling model for gassy coal has been improved. The numerical model was founded from the improved multi-field coupling model by COMSOL Multiphysics and gas drainage by borehole down the coal seam enhanced by heat injection was modelled. The results show that the heat–fluid–solid model with adsorption effects for gassy coal was well simulated by the improved multi-field model. The mechanism of coal seam gas desorption seepage under the combined action of temperature, stress and adsorption can be well described. Gas desorption and seepage can be enhanced by heat injection into coal seams. The gas drainage rate was directly proportional to the temperature of injected heat in the scope of 30–150 °C and increasing in the whole modelled drainage process (0–1000 d). The increased level was maximum in the initial drainage time and decreasing gradually along with drainage time. The increasing ratio of drainage rate was maximum when the temperature raised from 30 to 60 °C. Although the drainage rate would increase along with increasing temperature, when exceeding 60 °C, the increasing ratio of drainage rate with rising temperature would decrease. Gas drainage promotion was more effective in coal seams with lower permeability than with higher permeability. The coal seam temperature in a 5 m distance surrounding the heat injection borehole would rise to around 60 °C in 3 months. That was much less than the time of gas drainage in the coal mines in sites with low permeability coal seams. Therefore, it is valuable and feasible to inject heat into coal seams to promote gas drainage, and this has strong feasibility for coal seams with low permeability which are widespread in China.