Research Of Directional Hydraulic Fracturing Between Supports Applied To Fully Mechanized Caving Face Of Difficult Caving Top Coal

When thick and hard coal utilized fully mechanized top coal caving,top coal was difficult to break.Even if top coal broke,cracked blocks were too large and not easy to release,which directly affected the recovery rate of top coal.Through extensive investigation and field observation,the paper used directional hydraulic fracturing between supports to weaken difficult caving top coal,so as to improve the recovery rate of top coal in fully mechanized caving face.The "short cantilever beam" structure was monitored in the difficult caving thick coal such as Luzigou mine and Tiebei mine utilized fully mechanized top coal caving,using deep hole non-contact displacement detection method and intelligent drilling television imaging technology.The short cantilever beam structure had a certain self-stabilizing ability,and its falling lagged behind the working face,which was the key factor affecting the top coal to release.Based on field observation,the mechanical model of difficult caving coal was established.The short cantilever beam’s limit thickness analytical formulas of tensile failure and shear failure were obtained,in order to establish the theoretical basis for determining the location of hydraulic fracturing piecewise slotting.Based on the maximum tensile stress criterion of elastic mechanics,hydraulic fracturing initiation mechanical models of non-slotted borehole and slotted borehole were analyzed,which respectively obtained the initiation pressure and the initiation position.At the same time,the ABAQUS numerical simulation method was used to compare and analyze the stress distribution around the non-slotted borehole and slotted borehole at different angles.Hydraulic fracturing propagation mechanical model at slotted borehole’s tip was established,which was divided into three stages,namely the initiation stage,the stage of slot and borehole’s combined effects and the stage of Penny-shaped fracture model.According to the type I fracture propagation criteria of fracture mechanics,Criterion for fracture propagation of each stage was expressed by discharge,and also type I fracture propagation termination radius was obtained.The distribution of hydraulic fracturing propagation areas was detected by electromagnetic computed tomography,which provided a reference for the determination of borehole spacing.Finally,according to Tiebei mine’s coal seam conditions,based on the parameters of theoretical analysis and numerical simulation,directional hydraulic fracturing between supports was used to weaken difficult caving top coal.The industrial practice showed that top coal of the mine could fall along with mining,cracked blocks were moderate,and top coal was released smoothly.The top coal recovery rate of the working face has been greatly improved.The directional hydraulic fracturing is an effective way to weaken the difficult caving coal,and has wide application value.