Study on the influence law of ultrasound on coal structure and seepage characteristics under water pressure regulation

Ultrasonic permeability enhancement technology has great application potential in coal seam permeability improvement, and water environment serves as an important medium and energy transfer carrier for ultrasonic excitation. To investigate the degree of coal structural damage and degradation and the evolution law of coal permeability induced by ultrasonic excitation under the influence of ambient water pressure, this study targets the difficult gas extraction problem in Hujiahe Mine of Binchang Mining Area. Typical coal samples from the 402101 working face of Hujiahe Mine were selected. The permeability of coal under different water pressures and ultrasonic actions was measured using a BRS-Ⅲ gas pore-permeability combined tester. ImageJ image processing software was employed to quantitatively characterize the damage degree of coal surfaces before and after ultrasonic excitation, and the effects of water pressure and ultrasonic action on coal structural damage and seepage characteristics of the 402101 working face were explored. The results show that with the increase of ambient water pressure, multi-directional fractures are generated on the coal surface under ultrasonic excitation, and the surface roughness increases. After pressurized ultrasonic excitation, both the aperture and length of coal fractures increase, and the proportion of damaged area rises significantly (from 1.271% at 0 MPa to 21.513% at 0.3 MPa). Ultrasonic excitation promotes the development and connection of coal pore-fracture structures, forming a complex fracture network and improving coal permeability. The permeability change rate before and after excitation increases significantly with the rise of water pressure, with the variation range expanding from 6.8%-48.7% to 415.9%-2251.8%. The permeability characteristics of coal show a significant positive correlation with the degree of damage and degradation. Within a certain water pressure range, pressurized ultrasound can promote the development of coal pores and fractures to varying degrees and enhance the permeability of coal reservoirs.