Technology And Engineering Practice Of High Gas And Low Permeability Coal Seam Drainage With Hydraulic Slit

China’s coal development has gradually entered the deep stage,the characteristics of "three high and one low" are more and more prominent,especially the low permeability,the efficiency of conventional gas extraction means is low,it is necessary to carry out anti-reflection coal seam gas extraction.Hydraulic slotting technology is widely used as an efficient anti-reflection technology.However,how to set reasonable seam parameters and arrangement mode is particularly important for different coal quality and different layer coal seams in hydraulic seam cutting.In order to solve this problem,the paper analyzed the coal breaking depth with different water jet parameters through numerical simulation,studied the effect of pressure relief and reflection improvement with different slit parameters,carried out hydraulic slit engineering practice,and studied the reasonable slit parameters and process flow suitable for the target coal seam.Aiming at the coal breaking depth of water jet with different slit parameters,a prediction model for coal breaking depth of water jet with different driving pressure and nozzle diameter was proposed.The gas-liquid two-phase flow model was established by Fluent numerical simulation software,and the velocity field of water jet with different driving water pressure and nozzle diameter was simulated.The coal breaking depth of water jet with different parameters was ob,tained by quasi-static rock breaking model,and the driving water pressure and nozzle diameter used for hydraulic cutting were further determined.Through FLAC3D numerical simulation,single control variable method and response surface test method,the influence law of different slot diameters and drilling inclination Angle on coal pressure relief was studied,and the mechanical mechanism of slot pressure relief was analyzed.The interaction effects of slot diameter,width and drilling Angle on coal pressure relief range are analyzed,and the optimum slot parameters and drilling spacing are determined.Determine the hydraulic slotted anti-reflection coal field testing scheme and equipment type selection,test process in the coal quantity data collection period,different nozzle diameter,water pressure,slotted time slot on the influence of coal,identified the different slot depth of water pressure,nozzle diameter under the condition of the coal,analyses the hydraulic cutting seam extraction gas effect and the outburst prevention effect.The test results show that the pure gas extraction amount of hydraulic slit drilling increases by 3.5～9.5 times,and the maximum gas concentration of driving face decreases by more than 40%after hydraulic slit is used.The gas extraction efficiency is obviously improved,the effect of on-site outburst elimination is good,and the pre-extraction time of coal seam gas is shortened,which provides a certain theoretical basis and engineering practice reference for gas prevention and control in working face.