Experimental study on the influence of ultrasonic excitation duration on the pore and fracture structure and permeability of coal body

The study aims to explore the effect of ultrasonic duration on the pore structure and seepage characteristics of coal. Herein, we adopted a self-developed ultrasonic excitation experiment system using low-field nuclear magnetic resonance (NMR) technology and NM-4B non-metallic ultrasonic wave velocity detector. The study investigated NMR relaxation time T2 patterns and the number of pores of each size, internal structural damage, and seepage characteristics of the coal body before and after ultrasonic fracturing. Based on the Warren-Root model, the permeability model of coal under ultrasonic fracturing was established, revealing the changing law of ultrasonic fracturing at different times on the change of pore scale, connectivity, and nuclear magnetic permeability of the coal body. The mechanism of ultrasonic fractured coal body evolution at different times was clarified. The results showed that the ultrasonic fracturing at different times improved the number and volume of all size pores in the coal body, expanded primary fractures, and developed newborn fractures. The rate of change in pore size, porosity, wave velocity, and coal permeability had a linear relationship with the fracturing time. The growth rate of seepage pore volume proportion was positively correlated to the decreased rate of adsorption pore volume proportion. The fracturing duration leads to the interconnection of micropores within the coal body, forming medium and large pore fractures. This transformation between pores and fractures enhances the pore structure and seepage characteristics of the coal body. The coal rock permeability model can be used to calculate the permeability of a coal body, which follows the same change law as nuclear magnetic permeability. When evaluating the index using absolute and relative errors, the range of relative error is 0.97%-7.91%. The error variation range is small, indicating that the established model can effectively reflect the change law of porosity and permeability of the coal body responding to ultrasonic fracturing. The ultrasonic fracturing rendered the ultrasonic cavitation takes micropores as fracturing units, developed pores of all sizes, connected closed pores, increased free fluid space proportion, and improved the coal permeability.