Influence of Rough Fracture on Nonlinear Flow under the Coupling of Temperature and Effective Stress

Exploring the evolutionary traits of gas flow within rough fracture surfaces under combined temperature–effective stress effects can deliver references and a theoretical basis for gas resource exploitation in fractured complex strata of abandoned mines. Combining physical experiments and numerical simulations, this research investigates the intricate gas migration behavior inside fractures under geo-stress (5–20 MPa) and formation temperature (25–45 °C) conditions that replicate actual coal seam burial depths. Experimental findings reveal that fracture permeability rises steadily as surface roughness declines. Both elevated effective stress and ambient temperature weaken flow conductivity, with thermal variation exerting a stronger regulatory effect on highly rough fracture surfaces. As stress loading intensifies, the nonlinear flow factor of fracture surfaces climbs steadily, yet such growing tendency slows down notably once the stress exceeds 10 MPa. In the numerical simulation section, this work adopts the fracture contact ratio and rock thermal expansion coefficient to establish a three-dimensional thermo-hydro-mechanical (THM) coupling model with explicitly characterized fractures. It analyzes the streamline distribution patterns of fractures with distinct roughness grades amid variable temperature and pressure evolution. The outcomes demonstrate that minor-scale backflow widely emerges along rough fractures under low effective stress, whereas smooth fracture surfaces generate faint vortex responses. Gas velocity maintains a balanced distribution inside fractures of two different roughness levels, and thermal elevation increases the flow velocity within local flow channels of fracture surfaces. Under high effective stress, high-vorticity zones concentrate around contact regions, and temperature increment mainly alters local zones with relatively fast flow speed. This study clarifies the internal THM interaction mechanisms of fracture zones in abandoned mine settings, offering innovative insights for characterizing and predicting fracture flow behaviors.