Numerical study on the effect of injection pressure on hot flue gas induced coal deformation and enhanced coalbed methane recovery

The use of hot flue gas emitted from gas power plants as injection gas source for enhanced coalbed methane recovery (ECBM) can achieve the dual goals of carbon storage and ECBM. In this paper, a THM-coupled model for the process of coalbed methane (CBM) displacement by hot flue gas was established. The effects of injection pressure on reservoir temperature, permeability, and displacement efficiency were investigated. The study also focused on the extent of the contribution of strain due to adsorption/desorption of a single-component gas to the total adsorption/desorption strain. The results indicate that increasing injection pressure augments both CH4 production and hot flue gas storage while having a negligible impact on reservoir temperature. As injection pressure increases, the reduction in CH4 desorption strain becomes more substantial, and the increment in the adsorption strains of CO2 and N2 becomes more significant. When the coal body is influenced solely by N2, the total adsorption/desorption strain, which is predominantly governed by CH4 desorption, exhibits a decreasing trend. The increase in total adsorption/desorption strain is dominated by adsorption of CO2, and the coal matrix shrinkage due to CH4 desorption exhibits a time-sharing dominance phenomenon in the vicinity of the injection well. Injection of hot flue gas can increase the permeability of reservoir in the vicinity of the injection wells, thereby increasing the injection capacity of the injection wells. The change in reservoir permeability is dominated by gas adsorption and desorption, and the magnitude of the change is proportional to the injection pressure.