Study on Fracture Propagation Behavior of Composite Directional Fracturing with Prefabricated Crack and Control Holes in Coal Seams

In order to achieve the goal of changing the direction of cracks and actively relieving pressure to increase permeability in local areas, and to prevent excessive stress concentration and areas with poor anti reflection effect in coal seams, this paper proposes a method of coordinating prefabricated cracks and control holes to control the propagation of hydraulic cracks. A collaborative guided fracturing mechanics model with prefabricated cracks and control holes was established using linear elastic fracture mechanics and fluid mechanics theory. Based on this model, RFPA2D Flow software was used to numerically simulate the effects of geostress, prefabricated crack parameters, and control-hole parameters on crack propagation trajectory. The results show that the stress concentration effect on the weak surface of the prefabricated crack and the high pore water pressure field of the control hole work together to significantly reduce the crack-initiation pressure, and form a low resistance propagation channel in the connecting area between the prefabricated crack and the control hole, guiding the crack to deflect towards the direction of the control hole. When the lateral stress coefficient, prefabricated crack angle, and hole spacing increase, cracks are more likely to deflect towards the direction of maximum horizontal principal stress; when the length of the prefabricated crack and the water pressure of the control hole increase, the guiding effect of the crack expanding towards the control-hole direction is enhanced. The initiation angle of hydraulic fractures increases with the increase in the lateral stress coefficient. When the prefabricated cutting angle θ ≤ 45°, the initiation angle of hydraulic fractures increases with an increase in θ. When the prefabricated cutting angle θ > 45°, the initiation angle decreases with an increase in θ.