Thermo-Hydro-Mechanical (THM) Coupled Model for Gas Extraction from Deep Coal Seams and Factors Influencing the Effective Extraction Radius

Gas extraction from deep coal seams is strongly affected by high geothermal temperature, high gas pressure, and high in situ stress, making accurate prediction of the effective gas extraction radius important for optimizing gas extraction parameters. In this study, a thermo-hydro-mechanical (THM) coupled model incorporating coal deformation, gas flow, heat transfer, temperature-dependent gas adsorption, and dynamic porosity–permeability evolution was developed and implemented in COMSOL Multiphysics. Comparison with four site-specific field measurements yielded relative errors of 5.28–9.27%. These discrepancies may be attributed to the idealized model assumptions and the inherent spatial heterogeneity of the actual coal seam. The effects of extraction duration, temperature, initial gas pressure, coal seam permeability, extraction negative pressure, and borehole diameter on the effective gas extraction radius were then investigated. The results showed that the effective radius increased with permeability, extraction duration, borehole diameter, and extraction negative pressure, but decreased with initial gas pressure and temperature. At a fixed extraction duration of 180 d and within the investigated parameter ranges and simulation conditions, Deng’s grey relational analysis showed that the grey relational grades followed the order: permeability > initial gas pressure > borehole diameter > temperature > extraction negative pressure. Permeability and initial gas pressure exhibited the strongest associations within the investigated ranges. These findings provide a quantitative basis for predicting the effective gas extraction radius and optimizing gas extraction parameters in deep coal seams.