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 highprecision 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 events/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 critical 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.