Microscopic Mechanism of Hot Flue Gas Displacement of Coalbed Methane under Thermo-Mechanical Coupling: A Molecular Simulation Perspective

The High-Temperature Flue Gas Enhanced Permeability and Displacement (HFG-ECBM) technology combines the advantages of low gas source costs and thermal-induced permeability enhancement, making it a key technical approach to overcoming bottlenecks in deep coalbed methane (CBM) extraction. Based on multispectral characterization results, this study constructed a dynamic displacement model of coal nanopores under different stress levels. Using Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods, the study investigated the microscopic mechanisms of HFG displacement of CBM under thermo-mechanical coupling. The results indicate that as the triaxial stress gradient increases, the coal matrix undergoes significant anisotropic compressive deformation, with porosity decreasing from 10.92% under stress-free conditions to 8.91% under high stress; medium- and large-sized pores close preferentially, and the methane saturation adsorption capacity exhibits a monotonically decreasing trend. The displacement of CBM by HFG exhibits a coupled regulation pattern characterized by “stress-dominated, temperature-modulated” behavior. High ground stress compresses pores and impedes gas migration, causing the diffusion coefficient of HFG to decrease by an average of 62.55% and that of CBM to decrease by 22.3%–24.3%. Increasing temperature weakens the adsorption binding force of CBM and lowers the desorption energy barrier while simultaneously enhancing the diffusion capacity of HFG; this mitigating effect is more pronounced under high ground-stress conditions. The research findings provide a microscopic theoretical basis and a foundation for optimizing parameters in the field application of hot flue gas displacement technology for coalbed methane extraction in deep, high-stress coal seams.