Dynamic fracture toughness and fractal characteristics of crack in pressurized acidified Qinshui coal under impact loading

To investigate the effect of acid fracturing fluid on the fracture toughness and fractal properties of crack propagation in Qinshui coal under impact load, the 50 mm diameter split Hopkinson pressure bar device was employed to carry out mode I dynamic fracture toughness tests on Qinshui anthracite samples treated with acid fracturing fluid and water-based fracturing fluid under different impact pressures. Coal samples were subjected to force saturation and acidity treatment using an innovative apparatus. The fracture propagation phase of the specimen was acquired by a high-speed camera sensor. Combined with Image J analysis software and PCAS image recognition system, the macroscopic crack propagation trajectory and probability entropy of micro pores in coal samples were quantitatively analyzed. These findings revealed that dynamic fracture toughness endowed a strong rate-response relationship. When the impact pressure is 0.3, 0.4, and 0.5 MPa, the average fracture toughness of the water-based fracturing fluid group coal specimens was respectively 0.64, 1.20, and 1.31 times better that of the acidic fracturing fluid group. The rate of crack propagation and the dynamic fracture toughness of coal were reduced after acidification of the specimens. The crack growth rate initially surged, then decreased rapidly until it reached a stable state under impact load, while the variation in crack growth length and opening breadth showed a time-dependent increase. Crack propagation resistance and dynamic fracture toughness of coal are reduced by the acidification of the specimens. The fractal dimension of cracks in specimen increased under the impact of pressure growth. The fractal dimensions of crack in coal samples under the action of acidic fracturing fluid at 0.30, 0.40, and 0.50 MPa are 1.066, 1.078, and 1.087 times that of water-based fracturing fluid, as well as 1.119, 1.136, and 1.157 times that of natural state coal samples. With the increase in impact pressure, the entropy magnitude of the pore probability on the fracture surface of the coal sample also increased. The fracture surface morphology of coal sample transformed from compact and neat to loose and porous with the action of acidification. The dual mechanism of weakening and enhancing the fracture behavior of anthracite coal by fracturing fluid under different loading rates was explored, and a microscopic fracture mechanics model incorporating loading rate was developed based on the dual nature of the fracturing fluid and linear elastic fracture mechanics theory. The study results offer empirical evidence to investigate the process of fracture initiation and propagation in acid fracturing in Qinshui coal, and provide theoretical direction for designing acid fracturing in coal seams and controlling complicated fracture network.