Analysis of methane diffusion on permeability rebound and recovery in coal reservoirs: Implications for deep coalbed methane-enhanced extraction

A proper understanding of the effect of methane diffusion on coal reservoir permeability rebound and recovery is essential, as coal reservoir permeability is the key parameter influencing the efficiency of coalbed methane migration and computational research on it is lacking. In this paper, the multifield coupling model for methane migration was established. Then, two parameters, the influence coefficient of diffusion on permeability rebound (DPRB) and the influence coefficient of diffusion on permeability recovery (DPRC), were proposed to quantify the effect of methane diffusion on rebound and recovery of coal reservoir permeability. Subsequently, we used COMSOL software to study the variation rules of the coal reservoir permeability rebound time, permeability recovery time, and permeability rebound value, DPRB, and DPRC for different geologic parameters. The results shown that the permeability rebound time and recovery time are proportional to the coal seam initial pressure, but inversely proportional to the initial permeability and initial diffusion coefficient. The rebound value decreases with increasing coal seam initial pressure and initial permeability, but ascends with rising initial diffusion coefficient. DPRB declines with increasing coal seam initial pressure, initial permeability, and initial diffusion coefficient, but they are all greater than 0.7, indicating that methane diffusion has a significant effect on permeability rebound. The DPRC values for different coal seam initial pressures, initial permeabilities, and initial diffusion coefficients are above 0.98, which implies that methane diffusion dominates the permeability recovery process. Finally, a conceptual model was presented to research the mechanism of diffusion influence on rebound and recovery of coal reservoir permeability, and the implications for enhanced drainage of deep coalbed methane were discussed. Therefore, the results of this paper can provide a theoretical foundation for deep coalbed methane-enhanced extraction.