Pore Evolution Law of a Coal Body Fractured by Dual-Frequency Ultrasound

To investigate the influence of dual-frequency ultrasound on coal fracturing performance, we employed a dual-frequency ultrasonic fracturing system to conduct experiments across various frequency combinations. Coupled with low-field nuclear magnetic resonance measurements, we examined the evolution of pore structure, the changes in permeability, and the multifractal characteristics of coal samples under dual-frequency conditions. The experimental results indicated that, following dual-frequency ultrasound treatment, the T2 spectrum showed a diminished small-pore peak, an expanded midpore distribution range, and a heightened, broadened large-pore peak. As the second frequency increased, the large-pore area growth rate rose exponentially, reaching up to 1,800%. Porosity evolution revealed that heterogeneous-frequency ultrasound more effectively enhances coal sample porosity than homogeneous-frequency ultrasound—by an additional 11.06%. Moreover, with increasing total ultrasonic frequency, the permeability growth rate of the coal samples exhibited an exponential increase, peaking at 475.6%. Multifractal dimension analysis confirmed that dual-frequency ultrasound can extend pore scales and markedly improve the pore structure of coal samples.