Cracking mechanism of coal under high-pressure water jet and its applications for enhanced coalbed methane drainage
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This paper is concerned with the mechanism of coal breakage under high-pressure water jet (HPWJ) and its applications. A model of HPWJ impinging on coal target was established to study the cracking mechanism of coal under impact load. The characteristic and pressure distribution of HPWJ, the propagation characteristics of stress wave in coal, the mechanical properties of different coal particles, and the fracture characteristics of coal under HPWJ erosion were investigated theoretically and numerically. The results show that the shock wave and water wedge pressure are the main factors that cause coal breakage and crack propagation. The damage to the far-field coal particles affected by HPWJ is primarily caused by tensile stress, and the damage to the near-field coal particles affected by HPWJ is caused by the coupled effects of tensile stress and compressive stress. An erosion cavity is formed in the coal model with diameters of 1.25 to 2.5 times that of the jet at different depths. Meanwhile, the strong quasi-static pressure at the crack discontinuities further promotes the propagation of radial cracks around the erosion cavity to form a fracture zone, and the diameter of the fracture zone at different depths is 3.5 to 4.0 times that of the jet. In addition, the results of field application show that there is a significant difference between the methane parameters in the hydraulic flushing borehole and the conventional borehole; the average methane volume fraction and the average methane flow rate in hydraulic flushing boreholes are 3.85 and 3.67 times, respectively, that in conventional boreholes. Indicating hydraulic flushing can effectively promote the initiation and propagation of coal cracks. These results are of great significance to improve coalbed methane drainage technology and prevent gas disaster accidents in coal mines.
KeywordsHigh-pressure water jet erosion Coal breakage Crack propagation Coalbed methane
The authors thank the National Natural Science Foundation of China (No. 41072118 and No. 41430640). Additionally, the authors would like to acknowledge Dr. Pengfei Lv, China University of Mining and Technology Beijing, for his helpful discussion and suggestions. In addition, the authors appreciate the staff of Yi’an Mining Co., Ltd., for their long-term support and help during the field work. The authors also thank the anonymous reviewer for the valuable comments and helpful suggestions.
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