A multi-field coupled model for gas migration in high-temperature coal seams considering fracture zones and fracture roughness.

Abstract

Gas migration in high-temperature coal seams is governed by fracture-zone connectivity, fracture-surface roughness, and thermo-mechanical deformation. However, fracture roughness is often treated qualitatively or absorbed into empirical permeability, which limits its use in field-scale gas-hazard assessment. This study develops a thermo-hydro-mechanical coupled model for gas-bearing coal seams with fracture zones. A dimensionless roughness parameter is introduced to quantify the relative asperity height of fracture surfaces and link fracture morphology with equivalent permeability, gas desorption, heat transfer, and coal-rock deformation. The model was validated using thermal-response data from the 16035 working face in the Liupanshui mining area and gas-production data from a coalbed methane well. It was then applied to analyze gas migration and coal-rock response in the 16035 working face. Results show that connected fracture zones act as preferential pathways for gas and heat transfer and control the spatial distributions of pressure, temperature, stress, and displacement near the roadway. Increasing the initial coal-seam temperature from 40°C to 50°C increases the maximum displacement by 15.8% and the maximum gas pressure by 4.3%. A lower roughness index also enhances gas transport. When the roughness index decreases from 0.4 to 0.1, the maximum permeability increases by 6.18%, and the gas desorption intensity at 3000s increases by 75.7%. These findings indicate that gas hazard in high-temperature coal seams should be evaluated by jointly considering fracture-zone geometry, quantifiable fracture roughness, and thermo-mechanical coupling effects.