Investigation of enhancing coal permeability with high-temperature treatment

•To reveal the mechanism of heat treatment enhance permeability of coal.•Thermal fracture behavior is categorized considering thermal fracture mechanism.•Relationship between temperatures and mechanical property of coal is determined.•The heat treatment temperature of well was suggested. Enhancing coal permeability with formation heat treatment (FHT) is a prospective method in coalbed methane (CBM) exploitation. Here, the mechanisms of coal permeability enhancement induce by heat treatment are investigated. Residual water coal samples were heated in a helium atmosphere at 50 °C, 100 °C, 200 °C, 400 °C, 600 °C, and 800 °C in sequence. The samples’ permeability and acoustic wave velocity were measured after each heat treatment, and Young’s modulus and shear modulus were calculated to determine the change in the mechanical properties. In addition, changes in the pore structure and surface properties were analyzed with low-pressure N2 adsorption and SEM. The results indicate that in the range of 50–200 °C, the bulk water and interlayer water were removed, which alleviated the aqueous phase trapping effect, enhanced gas flow channels and increased permeability by 4–14 times. Between 400 and 600 °C, the coal samples started to pyrolysis, weakening the mechanical properties of the coal, and fractures were initiated and extended, which caused permeability to increase by 79–1400 times. After 800 °C, the surface of the coal was completely pyrolyzed, samples were cracked, and the permeability increased by more than 20,000 times. Stimulation mechanisms of FHT in CBM reservoirs consist of aqueous phase trapping alleviation, coal fractures caused by mechanical properties weakening and combustion, and finally, the C–C bond and C–O bond of molecules are broken, which induced coal fracture to produce more gas flow channels. For field application of FHT, both advantages and disadvantages are analyzed.