Effect of acid fracturing fluid modifying coal microstructure stimulated by ultrasonic

• Ultrasonic-assisted acid fracturing fluid significantly enhances coal microstructure modification, improving acid fracturing efficiency. • Acetic acid treatment reduces aromatic and branching parameters in coal, indicating chemical alteration of organic matter. • Acid dissolution of carbonates and clay minerals creates new pores, increasing porosity, pore volume, and fractal dimension. • Optimal concentration of 7% acetic acid increases pore volume by 5.7 times, improving pore connectivity and surface morphology.