Mechanism of coal pore modification induced by broadband ultrasonic excitation and its regulation on gas desorption behavior

Ultrasonic excitation can effectively transform the pore structure of coal and increase coalbed methane production capacity and recoverable reserves. In order to explore the influence of broadband ultrasound on the cross−scale pore evolution and gas desorption characteristics of coal, the cross−scale pore structure evolution and gas desorption and diffusion laws of two high−rank coals under five frequencies of ultrasonic excitation in the range of 25.0−82.5 kHz were studied using a water environment broadband ultrasonic excitation test system, and the frequency response difference mechanism of the pore structure was clarified. The results show that the effect of ultrasonic frequency on micropores and mesopores is coal−type dependent. The micropore structure of CS coal did not change significantly after ultrasonic excitation, but the pore expansion effect increased with the increase of frequency; XT coal underwent selective reconstruction of micropores (some 0.48 nm pores merged and expanded to 0.56 nm pores), resulting in a decrease in total micropore volume and specific surface area, weakening the CO₂ adsorption capacity, while the average pore size of the pores increased and the pore structure merged and reorganized. The transformation effect of micropores and mesopores in XT coal is positively correlated with the ultrasonic frequency. Ultrasonic excitation also induces significant development of macropores, and the pore expansion amplitude shows a “U−shaped” change with increasing frequency, which first decreases and then increases. With the increase of equilibrium gas pressure, the gas adsorption of coal gradually increases, and the negative correlation between adsorption and frequency becomes more and more significant. The adsorption constant a value of the coal sample decays linearly with increasing frequency, and the largest decrease is at 82.5 kHz compared with the original coal (CS coal 18.27%, XT coal 18.72%); the adsorption constant b value of CS coal shows a “U−shaped relationship” with increasing frequency, which first decreases and then increases, while the b value of XT coal has no obvious correlation with frequency, which is due to the difference in the primary pore structure of the two coals, but the b values of both appear at 82.5 kHz. The diffusion coefficient also shows a trend of first decreasing and then increasing with increasing frequency, with an increase of 10.66%−35.01%. The effect of broadband ultrasound on coal body couples mechanical vibration and cavitation effect, realizing the precise regulation of cross−scale pore structure: micropore and mesopore transformation is dominated by mechanical effect, while macropore transformation is dominated by cavitation effect. The research results provide theoretical support for ultrasonic high−efficiency permeability enhancement technologies such as ultrasonic precise control of the “bottleneck” hole section of low−permeability coal seams and multi−frequency synergistic linkage high−efficiency modification.