Experimental study on pore expansion and permeability enhancement in coal induced by pulsed ultrasonic wave

This study attempts to investigate the effects of pulsed ultrasonic wave on the pore structure of coal matrix and the patterns of gas seepage. We developed an acoustic-liquid coupling ultrasonic excitation experimental platform for coal samples to analyze the pore expansion and permeability enhancement effects in coal under pulsed ultrasonic excitation by using gas permeability measurement of coal cores and nuclear magnetic resonance (NMR) testing. Results show that increasing pulsed ultrasonic frequency excitation would lead to an increase in T₂ spectrum area, effective porosity and permeability of the coal matrix, while a decrease in residual porosity, T₂ cutoff value, and fractal dimension of seepage pores. The variation rates of these parameters exhibited an exponential relationship with the pulsed frequency. When the pulsed frequency increased from 1 to 16 times, permeability growth rate increased from 19% to 172%. Pulsed ultrasonic excitation promoted the transformation of closed pores into open pores in coal samples, forming more optimal seepage channels and effectively improving permeability. Under the synergistic effect of mechanical and cavitation effects, pulsed ultrasonic wave efficiently induce cumulative fatigue damage in coal, promoting the development and expansion of pores, and enhancing coal porosity and permeability. Increasing pulsed frequency under the same energy consumption could enhance permeability, providing a basis for the optimized application of pulsed ultrasonic technology in permeability enhancement treatment of low-permeability coal seams.