Study of the hydraulic cavitation mechanism and aperture optimization of drilling in high-gas coal seams

To enhance gas pre-extraction efficiency in low-permeability, high-gas coal seams and eliminate the danger of coal and gas outbursts at the Wulihou Coal Mine, this study analyzed the influence of the three stress zones surrounding cavitation holes (employed in alternating cavitation gas extraction technology) on gas extraction, based on the stress distribution characteristics around these holes. Two indices—the pressure relief zone area ratio and the plastic zone area ratio—were proposed to evaluate the pressure relief effect induced by the cavitation process. The effectiveness of cavitation holes for pressure relief and permeability enhancement was investigated using FLAC3D numerical simulation. Results indicate that as the cavitation hole size increases, the area of the associated stress relief zone expands, with its spatial extent best characterized by an exponential function. Similarly, the area of the plastic zone surrounding the cavitation hole also expands and is well-described by a cubic function. Both the pressure relief zone area ratio and the plastic zone area ratio exhibit an initial increase followed by a decrease with increasing cavitation hole radius, reaching their peak values within the radius range of [0.4, 0.5] m. Therefore, the optimal radius for cavitation holes is recommended to be between 0.4 m and 0.5 m. This research provides valuable reference for evaluating the pressure relief and permeability improvement effects, as well as for optimizing the aperture of hydraulic cavitation boreholes in high-gas coal seams.