Research on the promotion of ultrasonic vibration on the development of coal pore fractures and permeability

Coal seam water injection can effectively reduce the amount of dust suspended in the working face during coal mining. However, the permeability of the coal seam gradually deteriorates with the increasing depth of mining. Ultrasound excitation can improve the water injection conditions of coal seams. To explore the mechanism of ultrasound excitation in improving the water permeability of the coal seam, the evolution of the coal mass fracture after ultrasound excitation was investigated through low-temperature nitrogen adsorption experiments and microscopic observation. Additionally, the changes in the pore volume and specific surface area of the coal mass were quantitatively analyzed. The changes in the effective permeability of water-bearing coal samples stimulated by ultrasonic waves were also investigated through seepage experiments. The research results indicated that ultrasound excitation produced pores in the size scale range of 2–10 nm and concentrated the distribution of the mesopores, thereby increasing the pore volume and the specific surface area of the coal mass. The confining pressure gradually closed the fractures and decreased the permeability of the coal body. Injection pressure can improve the effect of ultrasound excitation on fracturing. There is a critical value for ultrasound excitation, below which more small pores will be produced, and above which small pores will be rapidly converted into large pores, increasing the permeability of the coal body.