Simulation Of Crack Propagation In Directional Hydraulic Fracture

China is the country most seriously affected by rock burst disasters.Field statistics and studies show that hard and thick roof is still the main inducing factor of coal mine disasters in China,and the controllable crack of thick and hard roof is a hot scientific issue in current research,which is of great significance to the safety production of rock burst mines in China.The principle of directional hydraulic fracturing is to use special "cutting tools" to cut directional fractures in the rock mass in advance,and then inject high pressure water to make the cracks continue to expand along the directional prefracture.Directional hydraulic fracturing can conduct directional stratification and block fracturing on the roof,which can control the damage to the integrity of hard roof and release the elastic energy of the roof,so as to reduce or eliminate the impact risk.Based on the crack growth mechanism of directional fracturing,this paper uses the discrete element numerical simulation software PFC(Particle Flow Code)to simulate and analyze the crack growth rule of directional fracturing.Considering the key factors such as different in-situ stress states,water injection pressure,slot Angle,rock mass structure,etc.,the main conclusions of the study on the pattern,direction,path and length of crack propagation are as follows:(1)according to the two indexes of "whether the crack bifurcates" and "crack growth length",the advantages and disadvantages of the directional pre-fracture geometry were evaluated.Rhombic fractures with a aspect ratio of 1:4 perform better than fractures with a ratio of 1:3,1:5,and 1:6.Finally,the rhomboid fracture with the aspect ratio of 1:4 was determined as the optimal directional pre-fracture.(2)the initiation pressure of directional hydraulic fracturing is greater than the sum of the tensile strength and the minimum principal stress of the rock mass,and less than the sum of the tensile strength of the rock mass and the ground stress perpendicular to the orienting direction of the slot.Under the condition of constant water pressure,the crack grows symmetrically from the center to both sides,and the growth rate changes from fast to slow.When the crack begins to crack,the nature of the crack is tension crack.Fracturing in parallel to the direction of crack propagation in the process of basic without accumulation phenomenon,contact force in the contact force is perpendicular to the crack propagation direction accumulation phenomenon significantly and pointing in the direction of the lateral force,namely the mesoscopic mutual acting force between particles showed to perpendicular to the direction of crack propagation on both sides of the expansion of the trend,in conformity with the tension crack of mesoscopic performance;In the process of fracturing,the model can be roughly divided into three zones from the inside to the outside: the fracturing zone,the disturbed zone and the undisturbed zone.The velocity and displacement of the particles in the fracturing zone and the disturbed zone are both perpendicular to the direction of crack propagation and pointing outward.(3)whether the orienting crack is parallel to the maximum principal stress or perpendicular to the maximum principal stress,the crack diameter increases with the increase of water pressure,and the number of bond failure between rock particles also increases with the increase of water pressure;The crack propagation diameter decreases with the increase of confining pressure ratio,and the number of failure of intergranular bond decreases with the increase of confining pressure ratio.The propagation diameter of directional precrack perpendicular to the maximum principal stress is slightly smaller than that parallel to the maximum principal stress.When the directional pre-crack is inclined to the maximum principal stress,the crack expands along the orientation direction of the directional pre-crack,and the crack length decreases with the increase of the intersection angle.In other words,directional pre-fracture has an effective guiding effect on the propagation of hydraulic fracturing crack under different stress fields.(4)the soft rock layer has no obvious influence on the direction of crack growth,but has a significant influence on the crack growth rate.The growth rate of the crack in the soft rock mass is higher than that in the hard rock mass,and the crack is prone to bifurcation when the strength of the soft rock mass is very low.The bond strength and inclination Angle of the weak thin interlayer have an effect on crack growth.The larger the dip Angle,the smaller the influence on crack growth,the easier the crack to pass through the thin interlayer,and the more symmetrical and flat the crack growth.