Theoretical model of annular slotting depth for high pressure water jet and its application

The permeability coefficient of coal in China is generally low, especially in deep mining, the coal shows the characteristics of high in-situ stress, high gas pressure and low permeability.Using high pressure water jet slotting to reduce the in-situ stress of coal seam, increase the permeability of coal seam and improve the gas extraction efficiency of coal seam is one of the technical development directions.According to the different geological conditions of coal seams, the design of technical parameters related to water jet slotting is mainly based on experience, which may lead to an uneven control of water jet slotting boreholes in coal seams and the existence of blank zones or two adjacent boreholes, thus affecting gas extraction and coal seam outburst prevention.Based on the analysis of the impact of high pressure water jet on coal slotting, considering the impact characteristics of high pressure water jet and the zonal structure of high pressure water jet, the velocity distribution function of high pressure water jet was obtained through the boundary layer theory, and the attenuation law of average jet velocity in the process of high pressure water jet slotting was also obtained.The wave equation was established according to the seeding characteristics, and then the theoretical model of annular slot depth of high-pressure water jet was synthetically established based on the attenuation law of high-pressure water jet varying with distance and the maximum elongation line strain criterion.By tri-al-manufactured high-pressure water jet force acquisition device, under the conditions of nozzle diameter, jet pressure and jet distance, the change law of impact force of high-pressure water jet under different parameters was tested, and the attenuation law of average velocity of high-pressure water jet was theoretically verified.Based on the theoretical model of annular slotting depth of high-pressure water jet and the mechanical parameters of coal in Xinji No.2 Coal Mine, the slotting depth of coal under different jet pressures was calculated theoretically.The comparison error between the actual slotting depth and the predicted results of theoretical model is less than 7%.The research shows that the annular slotting model of high pressure water jet can predict coal slotting depth well, and can be used to guide the design of coal slotting parameters under high pressure water jet.