Analysis of mechanical mechanism and influencing factors of directional fracturing of multi-hole sleeve

The huge impact of kinetic energy released by the breaking of hard roof is one of the important factors that induces dynamic disasters in coal mines. The key to preventing and controlling coal and rock dynamic disasters is the directional fracture of strata at a specific position and reducing the length of hard roof suspension. The multi-hole sleeve fracturing technology has the advantages of simple operation and wide application conditions and has a wide research prospect in the field of hard roof weakening. In order to understand the mechanism of multi-hole sleeve fracturing, the mechanical mechanism of multi-hole sleeve fracturing was studied using theoretical analysis and numerical simulation. The variation law of inter-hole stress under different influencing factors was revealed, and the crack propagation law and force chain distribution characteristics in the fracturing process were obtained. The pre-cracking effect of sleeve fracturing can be improved by changing the drilling structure through grooving. In order to determine the reasonable hole arrangement parameters, a mechanical model of the multi-hole sleeve fracturing with prefabricated cracks was established based on linear elastic fracture mechanics. The calculation equations of stress intensity factor, critical expansion pressure, and critical crack initiation angle of cracks were given, and the variation rules of stress intensity factor, critical expansion pressure, and critical crack initiation angle of cracks under different influencing factors were obtained. The results show that: ① The lateral pressure coefficient k has a significant effect on the minimum crack initiation stress of the borehole. With the lateral pressure coefficient k > 1, the minimum crack initiation stress decreases with the increase of the hole angle. With the lateral pressure coefficient k < 1, the minimum crack initiation stress decreases with the increase of the hole angle. ② The numerical simulation results show that there is a stress superposition effect between the sleeves, and the contact force chain is a 'radial' distribution. The fracturing process of the sleeve is mainly a tensile failure, and the deformation and failure at the connection of the hole center is the most severe, forming a 'banded' fracture surface along the direction of the hole. ③ The stress distribution near the slot end is changed by the slot, and the circumferential tensile stress is larger than that of the seamless slot model. When the slot length is 0.5 times the radius of the hole, the critical expansion pressure is the smallest, and the crack is most likely to expand. ④ The critical initiation angle is determined by K_I and K_II, which is less than 70.53°. Under the condition that the pump pressure and in-situ stress conditions cannot be changed, the directional fracturing of the rock can be realized by adjusting the hole angle and the length of the prefabricated slot.