Numerical study on injection of flue gas as a heat carrier into coal reservoir to enhance CBM recovery

A hydraulic-mechanical-thermal coupled numerical model for enhanced CBM recovery by injecting the flue gas (flue gas ECBM) is established, which fully couples the gas-water two-phase flow, competitive adsorption and temperature change as well as coal deformation. The model is first validated, then used to analyze the effects of different injectant components and temperature on CH4 production, CO2 storage and the evolution of permeability. The mechanism that enhanced CBM recovery and CO2 sequestration by injecting flue gas is discussed from the perspectives of competitive adsorption and selective diffusion in the molecule scale as well as the permeability evolution. CO2 can be competitively adsorbed on the surface of coal pores to desorb adsorbed CH4, while N-2 desorbs adsorbed CH4 by effectively reducing the CH4 partial pressure in the pores, enhanced CBM recovery by injection of flue gas is the result of the combination of these two effects. During the flue gas ECBM process, the permeability first decreases under the coaction of coal matrix expansion and effective stress increase; subsequently, the desorption of adsorbed CH4 induced by the N-2 component is dominant, which leads to a significant increase in permeability. Appropriately increasing the injection temperature of flue gas is conducive to the desorption of adsorbed CH4, and thus beneficial to the permeability and CO2 sequestration. Before the arrival of CO2, some adsorbed CH4 can be desorbed first under the action of N-2, which is not only beneficial to permeability, but also provides more adsorption sites for CO2 adsorption. However, the strategies for managing N-2 breakthroughs are needed to achieve an optimal balance between CH4 production, CH4 purity and CO2 sequestration over the entire project period.