Shape Optimization Of The Initial Slot Of Directional Hydraulic Fracturing Based On Particle Flow Simulation

The thick medium sandstone lays upon 9# coal seam is the main factor of rockburst in-1000m-West-No.3-dip mining district of Zhangshuanglou coal mine.Directional hydraulic fracturing can weaken the rockburst danger by reducing the thickness and overhang length of the hard thick roof through the carack propagation.However,rules of crack propagation can be hardly investigated in field as affected by the shape of initial slot,water injection pressure,situ stress,and other conditions.Therefore,the use of particle flow simulation to study the crack propagation law under different slot shapes and stress conditions is of great significance to reveal the mechanism of directional hydraulic fracturing to prevent rockburst.In this paper,based on laboratory experiments and particle flow numerical simulation,the crack propagation law under different crack morphology was studied.Using crack initiation pressure,several cracks,crack turning direction,crack propagation distance,etc.as indicators,the reasonable kerf formation under different stress field conditions was evaluated.It provides a basis for optimizing the parameters of the initial slotting tool.The main research results are as follows:(1)According to the national standard "Measurement,Monitoring and Prevention of Rock Burst Pressure",the impact propensity test and analysis of the 9 coal roof of Zhangshuanglou Coal Mine were carried out,and the physical and mechanical parameters and impact propensity of the roof rock was obtained,which provided simulation Basic parameters.Using PFC2 D to simulate the single-axis pressure strength experiment and Brazilian split experiment,a detailed parameter describing the physical and mechanical properties of the top plate sample was obtained.Based on the minimum initiation strength of conventional hydraulic fracturing,a conventional hydraulic fracturing model was established,and the meso-permeability coefficient of the directional hydraulic fracturing model without confining pressure was obtained.(2)The mathematical expressions of the ellipse and rhombic initial slot boundaries were determined,and the simulation of the ellipse and rhombus hole of different forms is realized in PFC2 D.The evaluation index of the shape of the cut the initial slot is put forward,that is," the initial slot shape","the number of crackles","water injection pressure","cracking pressure" and "crack expansion direction control ability".(3)Based on the above indexes,the shape of the directional hydraulic fracture was optimized.The results show that compared with the conventional hydraulic fracture,the elliptical and rhombus initial slot can reduce the injection pressure required for cracking and enhance the control ability of the direction of crack expansion,rhombus initial slot with the reduction of the initial slot shaft ratio,the effect is worse.Under the same conditions,the rhombic initial slot is superior to the elliptical initial slot in terms of crack quantity,water injection pressure,cracking pressure,and crack expansion direction control ability.It is finally determined that the rhombus initial slot with an axis ratio of 10:2 is the optimal shape of the initial slot under the condition of no boundary stress.(4)When the long axis of the initial slot is parallel to the minimum principal stress,the cracks in the elliptical crack model deflect and extend towards the direction of greater stress.In the rhombicfracture model,when the initial slot axis is relatively large,the fracture mainly extends in the vertical direction,the axial ratio decreases,the vertical direction extends the distance decreases,and the horizontal direction increases.The rhomboid initial slot with an axial ratio of 10:2 has a better cracking effect in the vertical direction.The rhomboid initial slots with an axial ratio of 10:5 have a more balanced cracking effect in vertical and horizontal directions.(5)When the long axis of the initial slot is parallel to the maximum principal stress,the cracks in the elliptical crack model mainly extend in the horizontal direction.The fracture axial ratio is negatively correlated with the number of fractures and the deflection Angle,and positively correlated with the degree of stress concentration,the maximum fracture extension distance and the ability to control the direction of fracture expansion.In the rhombicfracture model,when the initial slot axis is relatively large,the fracture mainly extends in the vertical direction,decreases the axial ratio,and expands in the horizontal and vertical directions.Elliptical joints with an axial ratio of10:2 have a better cracking effect in the horizontal direction,while rhombicjoints with an axial ratio of 10:2 have a better cracking effect in the vertical direction,and rhombicjoints with an axial ratio of 10:5 have a better cracking effect in the vertical and horizontal directions.(6)When the long axis of the crack is parallel to the vertical stress,the fractures in the elliptical crack model mainly extend in the vertical direction,and the axial ratio of the crack is negatively correlated with the number of fractures and the deflection Angle,and positively correlated with the degree of stress concentration,the maximum extension distance of the crack and the control ability of the direction of the crack expansion.In the rhombicfracture model,the fracture axial ratio decreases,the fracture propagation distance in the horizontal direction decreases,and the fracture propagation distance in the vertical direction increases.The elliptical initial slot with an axial ratio of 10:2 has a better cracking effect in the vertical direction,while the rhombicinitial slot with an axial ratio of 10:5 has a better cracking effect in the vertical direction and horizontal direction.The research results of the thesis provide a basis for determining the directional hydraulic fracturing to rockburst prevention and control parameters of the sandstone roof in the 9th coal-thick layer of Zhang Shuanglou Coal Mine,and at the same time provide technical support for the optimization of the slotting tool.The paper has 113 figures,9 tables,and 84 references.