The effect of temperature and ultrasonic power on the microstructure evolution of coal modified by clean fracturing fluid: An experimental study

Ultrasonic-assisted fracturing permeability enhancement technology offers several advantages, including a broad range of hydraulic fracturing permeability enhancement, microstructure modified by fracturing fluid, and ultrasonic promotion of gas desorption and extraction. This technology can significantly improve the efficiency of coalbed methane extraction and has broad application prospects. To investigate the effect of ultrasonic action power and temperature on the microstructure evolution of coal by using clean fracturing fluid, experiments were conducted using coal samples from a mine in Shenyang. The pore structure of these samples was analyzed by scanning electron microscopy (SEM) and mercury intrusion method (MIP), while the chemical microstructure was evaluated with energy dispersive spectroscopy (EDS), X-ray diffraction experiment (XRD) and Fourier transform infrared spectroscopy (FTIR). The results showed that ultrasonic treatment improved the pore structure characteristics and connectivity by facilitating chemical reactions between fracturing fluid and mineral impurities. Specifically, when the ultrasonic parameter was set at 600W-60 °C, the pore volume of coal samples increased by 26.09 %, tortuosity decreased by 31.81 %, pore fractal dimension increased by 3.93 %, permeability increased by 100.45 %, aromaticity increased by 16.69 %, and the degree of branching aliphatic side-chains increased by 108.70 %. Under the same ultrasonic power, the effective temperature for ultrasonic-assisted fracturing fluid treatment on coal rock was found to be 60 °C. Additionally, under the same temperature conditions, an increase in ultrasonic power resulted in significant enhancements in both pore structure parameters and chemical structure parameters in coal, showing a certain linear relationship. The research results enrich the theoretical system of coalbed methane mining and provide theoretical guidance for the field practice of this technology. •The pore structure and chemical microstructure of coal under different conditions were analyzed.•There exists an optimal temperature for ultrasonic-assisted fracturing fluid when modifying coal.•The permeability increment was 100.45 % at 600 W-60 °C.