Experimental Study on Temperature–Pressure Coupling Sensitivity and Burial Depth Response of Coal Permeability

The coupled effect of in situ temperature and stress complicates the permeability evolution of coal reservoirs, which restricts the exploration and evaluation of deep coalbed methane (CBM). Two high-rank coal samples were collected from the Sihe (SH) and Zhaozhuang (ZZ) mining areas, and multi-gradient coupled temperature–stress seepage experiments (20–50 °C, 8–32 MPa) as well as supporting triaxial mechanical tests were carried out to investigate the temperature and stress sensitivity of coal permeability. Combined with coal mechanical deformation characteristics, the transition depth mechanism of permeability evolution with burial depth was revealed. Experimental results indicate that coal permeability follows a negative exponential decay trend with increasing effective stress, and the evolution process can be divided into three stages: rapid attenuation, slow decline and stabilization. Temperature rise can weaken the stress attenuation degree of coal permeability under continuous effective stress loading and effectively reduce the stress sensitivity of coal reservoirs. Under constant confining pressure, permeability decreases linearly with rising temperature; the temperature-induced damage effect is prominent at low effective stress, while the regulatory effect of temperature is greatly weakened when fractures are compacted under high effective stress. An exponential function between permeability and burial depth was established based on coupled temperature–stress experimental data, and the critical burial depth of permeability transition depth in the study area was determined to be 550–600 m. The abrupt change interval of elastic modulus against confining pressure is consistent with the burial depth of permeability transition depth, which acts as the key mechanical factor dominating the nonlinear transition of reservoir permeability. This study provides experimental and theoretical support for the development of deep CBM in the study area. The results represent non-adsorbing gas (nitrogen) permeability under the investigated temperature–stress window (20–50 °C, 8–32 MPa) and should not be extrapolated to methane-bearing CBM reservoirs without adsorption–swelling corrections. The transition depth of approximately 550–600 m is a laboratory-derived estimate rather than a field-verified reservoir threshold.