Dynamic Response Mechanism of Low-Permeability Coal Pore-Fracture Structure and Methane Desorption Characteristics to Ultrasonic Cavitation

Water-based ultrasonic cavitation-enhanced coalbed methane recovery (WUC-ECBM) represents an innovative approach for enhancing coalbed permeability and demonstrates significant potential in practical applications. To investigate the impact of ultrasonic cavitation on the pore structure of coal and its influence on gas desorption from coal matrices, this study analyzed the changes in pore structure under varying ultrasonic powers using a self-developed experimental system utilizes water-based ultrasonic cavitation to fracture and improve the permeability of coal. Low-temperature nitrogen adsorption and scanning electron microscopy characterization techniques were employed to evaluate these changes. Combined with the numerical simulation of gas desorption at the microscopic scale within coal, the mechanisms of WUC-ECBM are revealed. The results indicate that as ultrasonic power increases, the increment in macropore volume of coal samples rises markedly, and the pore size distribution becomes more uniform. Additionally, the pore surfaces of coal exhibit varying degrees of damage, leading to rougher internal pore surfaces. Ultrasonic cavitation treatment effectively enlarges the low-pressure zone within coal matrices, thereby enhancing the gas desorption domain. This effect is attributed to the high-energy shock waves released during cavitation bubble collapse, which not only expand existing pore channels but also penetrate previously closed or semiclosed pores, potentially generating a new microfracture network. Consequently, gas desorption is promoted, with higher ultrasonic powers yielding more pronounced effects.