Study on the Theory of High Pressure Water Jet Coal Breaking and Its Main Controlling Factors

In order to solve the problems of gas extraction and pressure relief and permeability enhancement under low permeability and high surrounding rock stress conditions in deep coal seams, the ultra-high pressure hydraulic cutting technology is proposed. The mechanical mechanism of coal breaking by high pressure water jet is analyzed. The process of coal breaking by jet is divided into stress wave effect and secondary crack propagation effect. The effects of physical and mechanical properties of coal and rock and jet parameters on jet cutting depth are analyzed theoretically. The results show that the shear strength, tensile strength, fracture and pore development degree of coal and rock correspond to different types of coal and rock failure. With the increase of jet pressure and flow rate (nozzle diameter), the jet cutting depth increases gradually. The influence of jet rotating speed on cutting depth is analyzed. It is concluded that reducing rotating speed should be adopted to improve cutting depth in field practice. The research results can effectively improve the stress state of coal body, play the role of pressure relief and permeability enhancement, and improve coal seam permeability and gas release capacity, which can well meet the control needs of coal mine gas complex dynamic disasters.