The mechanism of permeability enhancement of coal seams by hydraulic fracturing based on the characterization of coal physical properties.

To further understand the mechanism of pressure relief and permeability enhancement in coal seams through hydraulic punching, this study employs microscopic testing methods such as porosity testing, spectral analysis, and image scanning, combined with simulation techniques, to analyze the impact of hydraulic punching on both the macro- and microstructures of coal from a microscopic perspective. The results indicate that the particle size distribution of the punch samples is similar to that of the original coal and drilled samples, with differences only in the larger particle size range. The impact damage during drilling and punching is mechanical in nature and primarily affects the macro dimensions of the coal. The punching process significantly modifies the internal microporous and chemical structures of coal, greatly increasing the number of micropores larger than 1.1 nm. Additionally, the punching process introduces secondary structural defects in the macromolecular structure of coal, leading to an increase in disordered carbon structures within the coal samples. The hydraulic punching process primarily achieves pressure relief and permeability enhancement through both macrofractures and micropores. This study integrates both microscopic and macroscopic perspectives to analyze the effects of hydraulic punching on pressure relief and permeability enhancement in coal.