A fully coupled thermo-hydro-mechanical model for heat and gas transfer in thermal stimulation enhanced coal seam gas recovery

Thermal stimulation is an important artificial technique in enhanced coal seam gas (CSG) recovery, where temperature change is often coupled with coal deformation and gas flow. The fully coupled thermo-hydro-mechanical condition in thermal stimulation enhanced coal seam gas recovery triggers a series of thermal coal-gas interactions for the heat or gas transfer. However, prior studies normally ignored the thermo-mechanical interaction of thermal fracturing, the combination of heat and gas transfer, and have not validated their models with actual data. In this study, a fully coupled thermo-hydro-mechanical (THM) model of coal deformation, gas flow and heat transport was developed considering the competitive effects of thermal expansion, non-isothermal gas sorption and thermal fracturing. Subsequently, the model was well validated by historic experimental data, in-situ production data and numerical simulation data, respectively. Finally, the coupled THM model was implemented into a numerical simulation of thermal stimulation enhanced coal seam gas recovery by using the finite element approach. The heat and gas transfer properties, as well as the efficiency of gas production were evaluated. The modeling results show that: thermal stimulation has promoting effects on gas recovery, however higher temperature does not mean higher efficiency of thermal stimulation to gas recovery; larger thermal sorption coefficient and lower fracturing coefficient correspond to higher gas production; thermal stimulation enhances gas recovery by promoting gas desorption. Our fully coupled THM model can improve current understandings of heat and mass transfer in thermal stimulation enhanced coal seam gas recovery.