Experimental study on hydraulic fracturing crack propagation of coal-rock complex with different hole-interface distance

To reveal the influence law of the distance between the bottom of fracturing borehole and the coal-rock interface on the crack propagation during the hydraulic fracturing of the coal seam roof, hydraulic fracturing characteristic experiments with varying hole-interface distances were conducted.Similar composite specimens with hole-interface distances of 2 mm, 5 mm, 7 mm, 10 mm, 12 mm, and 15 mm were prepared.The hydraulic fracturing process was simulated using an RTR-4600 high-precision measurement testing machine for coal and rock mechanical parameters combined with a DS5-8B full-information acoustic emission signal analyzer to capture acoustic emission characteristics.Experimental results show that the fracture initiation pressure decreases with increasing hole-interface distance, but it is not completely linear.The optimal hole-interface distance is 5 mm.The crack propagation rate and range are balanced.The distribution density of acoustic emission points and the damage coverage rate are 5.2 PCS/mm³ and 83% respectively.Under this condition, the complexity and permeability of the fracture network increase most significantly.The hole-interface distance dominates the damage evolution path by regulating the multi-field coupling relationship of fluid - stress - damage.Under the condition of larger hole-interface distance, the crack propagation dominated by the interface is limited by the diffusion effect of the pressure field.The optimal hole-interface distance can achieve the best match between stress concentration and the distribution of weak surfaces in the structure.The ultra-small hole-interface distance leads to the instability and degradation of the crack network due to the over-limit near-field stress.