Numerical simulation on enhanced coalbed methane extraction by hot flue gas based on the ECM-DFM model

Abstract

The multi-scale characteristics of the storage and transportation structure of coal seams will lead to abnormal heat and mass transport phenomena. It is necessary to explore the thermo-hydro-mechanical (THM) coupling mechanism of coalbed methane displacement by hot flue gas (CBMDHFG) and the mechanical response behaviour of macro fractures (MFs). Therefore, this paper develops a THM-coupled dual-medium model, which includes both the equivalent continuous medium (ECM) representing the coal matrix and micro fractures and the discrete fracture medium (DFM) representing the MFs. The fracture network randomly generated by the MATLAB code is imported into COMSOL to realize the solution of the model. Based on this, the evolution of multiple fields and the response law of gas flow rate during the CBMDHFG process are studied. The results show that the hot flue gas first migrates in the high-permeability MFs and then evolves towards the surrounding low-permeability ECM system. The high temperature and high pressure of the hot flue gas effectively weaken the compressive stress applied to the MFs in the near field and significantly promote shear dilation and expansion. A high gas injection temperature may induce the compression and closure of MFs outside the high-temperature excitation zone. CO₂ and N₂ mainly contribute to the shear dilation of MFs in the near field and far field respectively. Appropriately increasing the gas injection pressure, temperature, and the proportion of CO₂ in the early stage and gradually increasing the proportion of N₂ in the later stage can achieve the long-term development of gas. Loading high anisotropic stress induces complex THM coupling effects, leading to the inhomogeneity of the aperture of MFs. The side closer to the maximum principal stress is more likely to undergo shear dilation, and the peak gas production is higher. The research results are expected to provide new insights for exploring the enhanced recovery mechanism of CBMDHFG and optimizing the production scheme.