Fractal characteristics of pore damage evolution and permeability control mechanisms in coal during low-temperature freezing: A case study of medium- to high-rank coals in southwest guizhou mining area

Abstract

To investigate mechanisms controlling coal reservoir permeability during low-temperature freezing (-20 to 0 °C), we analyzed medium- to high-rank coals from Southwest Guizhou using nuclear magnetic resonance (NMR), computed tomography (CT), uniaxial compression, and acoustic emission (AE) monitoring. Results show that frost heave and thermal stress transform micropores (<10 nm) into mesopores (10–100 nm) and seepage pores (>100 nm). This conversion accelerated below -15 °C. At -20 °C, seepage pore area increased by 21.28 % in bituminous coal and 26.12 % in anthracite. CT scans showed fracture rates rose by 81.57 % and 87.7 %, respectively. This indicates a transition from stable expansion to unstable penetration. T₂ spectra revealed distinct pore structures: bituminous coal showed double/triple peaks, while anthracite displayed mainly triple peaks. Bituminous coal had higher fractal dimension in adsorption pores, suggesting greater microstructural adaptability. In contrast, anthracite developed more seepage pores, enhancing flow pathways. Freezing reduced mechanical properties. Uniaxial strength, elastic modulus, and AE activity all declined with temperature, confirming pre-damage. A dynamic permeability model based on porosity-temperature data showed a nonlinear positive correlation between permeability increase and porosity change rate. Permeability increased by 5.35–5.97 times in bituminous coal and 4.4–6.42 times in anthracite. These findings demonstrate that low-temperature freezing enhances permeability through "pore transformation–fracture oversaturation–mechanical degradation." This supports using cryogenic fracturing for coalbed methane extraction.