Investigation of enhancing coal permeability with high-temperature treatment

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 degrees C, 100 degrees C, 200 degrees C, 400 degrees C, 600 degrees C, and 800 degrees 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 N-2 adsorption and SEM. The results indicate that in the range of 50-200 degrees 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 degrees 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 degrees 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.

Automated summary

Enhancing coal permeability through formation heat treatment is an effective method for coalbed methane exploitation. The process involves removing residual water and inducing cracks in the coal structure, significantly increasing permeability. Additionally, the stimulation mechanisms consist of alleviating aqueous phase trapping, weakening coal mechanical properties, and inducing coal fracture to produce more gas flow channels.