Study On The Influence Of High Pressure Water Jet On Slot Formation Of Roof

The occurrence of the hard and hard roof above the coal seam is the main factor that causes the frequent occurrence of coal mine rockburst accidents.The preliminary exploration on the technology of pre splitting and increasing permeability in high gas coal seam shows that the radius of pre splitting slot is larger and the fracture radius can be expanded by using water jet to cut seam,but there are few reports on the application of high-pressure water jet to pre splitting slot in hard roof.In order to improve the rock breaking effect of high-pressure water jet,this paper studies the slot forming law of roof high-pressure water jet by means of theoretical analysis,numerical simulation,laboratory experiment and field application,determines six key influencing factors of high-pressure water jet,analyzes the rock breaking process and rock breaking characteristics of high-pressure water jet.The effects of nozzle structure,jet pressure,nozzle diameter,nozzle moving speed,cutting target distance and repeated cutting times on the slot formation of high pressure water jet are studied.Finally,the field application is carried out,and the main results are as follows:(1)According to the theory,the key factors affecting the slot formation of highpressure water jet are nozzle structure,jet pressure,nozzle diameter,nozzle moving speed,cutting target distance and repeated cutting times.(2)Numerical simulation studies the rock breaking process and rock breaking characteristics of high pressure water jet,and the jet velocity field and pressure field under three different nozzle structures are simulated.The nozzle structure with better performance is obtained.It is considered that the cone-shaped straight nozzle is better than the cone-shaped nozzle,and the cone-shaped nozzle is better than the straight nozzle.(3)The results show that the characteristic index depth,width and erosion volume of rock slot are positively correlated with jet pressure,nozzle diameter and repeated cutting times,and negatively correlated with nozzle transverse velocity.With the increase of target distance,the characteristic index of rock slot increases first and then decreases,and the optimal target distance of cutting efficiency is 5D.(4)The experimental results show that the primary and secondary relations of the five key factors affecting the slot depth are nozzle moving speed,repeated cutting times,jet pressure,nozzle diameter and target distance.The primary and secondary relations of the five key factors affecting the slot width are nozzle diameter,repeated cutting times,nozzle moving speed and jet pressure.The five key factors that affect the erosion volume per unit are nozzle moving speed,repeated cutting times,jet pressure,nozzle diameter and target distance.(5)Based on the research results,combined with hydraulic fracturing,the roof high-pressure water jet slotting is applied in the field,which can be manually slotted without withdrawing the drill pipe.For the roof rock with uniaxial compressive strength of 60 MPa,it can form a 4-5mm wide artificial slot,reduce the fracture initiation pressure during fracturing,and expand the fracture radius.Through the field test,under the guidance of the accumulation energy of the water jet prefabricated slot in the roof strata,the maximum single fracturing radius can reach 15 m.The single hole back fracturing is used for many times to improve the construction efficiency.The field measurement shows that the application effect is good.