Analysis of enhanced permeability and gas displacement in coal reservoirs by hot flue gas

Injection of CO₂-containing hot flue gas as a heat-carrying medium into deep coal seams can improve coal permeability and gas displacement effect, while achieving geological sequestration of CO₂. Elucidating the multi-field coupling mechanism of enhanced coal permeability and gas displacement by hot flue gas is a crucial point for the scientific regulation and control of gas recovery enhancement efficiency. A geological model of a heterogeneous coal reservoir was constructed based on the heterogeneous distribution characteristics of mechanical parameters obtained from Nano indentation tests. Numerical simulations were conducted to analyze the spatiotemporal evolution characteristics of multi-fields during the coal permeability enhancement and gas displacement by hot flue gas. On this basis, an evaluation index for gas recovery enhancement efficiency considering gas breakthrough effect was utilized for exploring the intrinsic response relationships between main engineering control parameters (temperature, CO₂ volume fraction, and pressure of flue gas) and the gas displacement efficiency by hot flue gas. The results show that: Weibull probability distribution function is more suitable for describing the spatially heterogeneous distribution of the microscopic elastic modulus of coal. The input of the released elastic strain energy from coal damage and the heat from mineral dissolution leads to the development of localized abnormally high-temperature zones. The gas displacement modes in the near field and far field are thermal displacement and CO₂ displacement, respectively. Although N₂ can rapidly break through the reservoir, it plays a role in increasing the seepage pressure gradient and reducing the flow resistance of the mixed gas during displacement. Within the heating range of coal adjacent to the hot flue gas injection well, the stronger mineral dissolution effect is attributed to coal damage under the high temperature and pressure effect of hot flue gas, which facilitates CO₂ enrichment. In insufficient heating zones, coal temperature is not high enough to induce coal damage, and excessive thermal expansion of the matrix leads to a significant reduction in permeability. In the early displacement stage, the range of insufficient heating zones surrounding the hot flue gas injection well remains unchanged, but extensive CO₂ enrichment causes the low-permeability front to expand outward. As displacement time increases, the range and degree of coal damage near the hot flue gas injection well rapidly increase and gradually stabilize, while the soluble mineral content in fractures decreases significantly, resulting in a significant permeability enhancement effect. During the gas displacement by hot flue gas, the area of the low-permeability front first increases and then decreases. To optimize gas recovery enhancement efficiency through multi-field regulation, the main engineering control parameters should be determined according to the actual duration of the recovery enhancement cycle. For short production cycles, flue gas pressure has a greater influence than CO₂ volume fraction or flue gas temperature, and low-pressure flue gas contributes to higher recovery enhancement efficiency of gas injection. For long production cycles, high flue gas pressure, low CO₂ volume fraction, and high flue gas temperature can lead to higher recovery enhancement efficiency of gas injection.