Non-Darcy thermal-hydraulic-mechanical damage model for enhancing coalbed methane extraction

Under the thermal-hydraulic-mechanical coupling conditions, structural change of the damaged coal and rock mass will affect its thermodynamic properties, permeability characteristics, and even the flow state. This study proposes a nonlinear (Non-Darcy) thermal-hydraulic-mechanical damage model based on the equivalent matrix scale and dynamic diffusion (EDN-THMD), which is solved via the finite element method combined with programming. The model is validated with field data and compared with different coupled models. The influence of different factors on gas extraction promoted by fracturing is analyzed. The results show that ad/desorption thermal effect dramatically influences the temperature field. The gas production is underestimated if ignoring equivalent matrix scale, especially after fracturing, but overestimated without considering water effect and dynamic diffusion. Consistency of direction between the fracture hole centerline and the maximum principal stress contributes to damage propagation, and the key to fracture-enhanced extraction is whether damage connection across the hole. The Non-Darcy factor evolution is tortuous, and the Non-Darcy effect is the most significant in the dehydration period which can reduce the extraction volume by 0.04-1.7%, and is directly proportional to Non-Darcy flow coefficient.