Study On Rock-Breaking Experiment By High-Pressure Water Jet Under Confining Pressure And Its Application In Coal Seam Permeability

China has abundant coal resources,but the gas disasters associated with coal mining are also very serious.Gas drainage is the most fundamental measure for gas control,however,the permeability of most coal seams in China is generally low,which makes gas drainage extremely difficult.It has been proved that the high-pressure water jet slotting in boreholes is an effective way to promote gas drainage.However,due to the complex geological conditions and the geostress environment tremendous differences in slotting effect are found in practical application.In order to improve the water jet slotting technology in boreholes,reasonable parameters of water jet must be selected.Based on the application of high-pressure water jet slotting technology on coal seam permeability enhancement in Pingshu Coal Mine,Shanxi Province,this paper studies the characteristics of rock-breaking by high-pressure water jet under confining pressure and optimizes the technical parameters of water jet slotting technology.The main research results are as follows:(1)The key parameters affecting the coal breaking performance by water jet are:jet pressure,impact target distance and nozzle diameter.The coal breaking process by water jet can be divided into three stages:surface impact,impact expansion and impact retardation;the initial impact pit and internal crack are formed in the surface impact stage.In the impact expansion stage,the specimen is mainly affected by the impact dynamic load of water jet.With the impact pit depth increasing,the impact effect of water jet decreases greatly in depth,and the impact force of water jet changes into stagnation pressure.In this stage,the failure of specimens is mainly caused by crack propagation under stagnation pressure.(2)The water jet experimental system was improved and the water jet impact test was carried out under different confining pressure.The specimen under free impact conditions was easier to break than those under different confining pressure.The break time of the specimen under confining pressure decreased significantly with the increase of jet pressure,and the break time of specimen under the same jet pressure decreased when the lateral pressure coefficient decrease.The threshold pressure of specimen failure under confining pressure should be about 15MPa.When the jet pressure is20MPa,the efficiency of rock-breaking by water jet is the highest.The surface strain of specimens under free impact condition has piecewise characteristics of rapid response,continuous accumulation,reverse catastrophe and linear change.Under uniaxial loading,the strain characteristics of different stages are as follows:initial steady stage,rapid accumulation stage and step response stage.The instantaneous catastrophe is significantly greater than that under free loading.(3)The parameters of water jet slotting were optimized through orthogonal test.The influence of jet pressure P,nozzle diameter D and impact time T on the failure of specimens were studied by range analysis and variance analysis.The significance ranking of influence factors associate with impact depth and mass loss was P>D>T.The optimal combination of parameters in the test range was P=25MPa,D=3mm,T=60s.(4)Field test shows that the coal output is the largest when the jet parameters are P=25MPa and D=3mm,and there is little difference between them when the jet parameters are P=20MPa and D=3mm.From sake of safety and efficiency,the suitable slotting parameters are P=20MPa,D=3mm,and the water consumption for slotting should be about 6m~3 per borehole.(5)The cumulative gas emission of single borehole after slotting is 11 times more than that of the conventional borehole.The speed of gas emission in slotting borehole is 9 times more than of conventional borehole when the gas emission is stable.The direct disturbance range of surrounding area caused by water jet slotting is about 2~3m,and the effective extraction range of single slotting drilling hole can reach 4~5m in 120days.The effect of coal seam permeability enhancement is significant.This paper contains 57 figures,27 tables and 113 references.