Study on Damage Evolution Law of Coal Pore Structure under Multipass Pulsating Hydraulic Loads

Summary Coal seam water injection prefracturing to reduce dust is one of the important measures to control coal mine dust. At present, the hydraulic load used for coal seam water injection can be divided into pulsating hydraulic load and conventional hydraulic load. Pulsating hydraulic load can produce a more complex pore-fracture network under the same equivalent pressure as conventional hydraulic load, which is of remarkable significance for improving the effect of coal mine dust control. This study is based on the self-built pulsating hydraulic fracturing experimental device and combined with the nondestructive 3D reconstruction technology of computed tomography (CT) scanning, aiming to investigate the evolution law of coal fracture propagation under various pulsating peak pressures and loading times at the microscopic level. The results show that the primary pore structure governs the trend of coal damage. The arrangement directions of pores and small fractures can be made to slope in the direction of water injection by the pulsating hydraulic load, and the higher the pulsating peak pressure, the better the fracture penetration effect. The propagation of pores with a diameter of 100–500 μm is the main cause of the fracture section formation, and macropores that have a diameter larger than 500 μm constitute the principal part of the pore structure, indicating the propagation and penetration of fractures; the average surface porosity increment of coal increases with the increase of peak pressure. Under different pulsating peak pressures and loading times, the pore fractal dimension of coal samples all have a bimodal distribution of small and large peaks, but varied pulsating peak pressures also have an impact on the distribution range, number, and occurrence stage of this distribution.