Coal breakage features and fragment size distribution in water jet drilling for coalbed methane development

Water jet drilling (WJD) technology has been widely used to extract coalbed methane. Coal breakage features and fragment size in WJD determine the drilling efficiency. The smaller the coal fragments, the easier the fragments are discharged from hydraulic boreholes. This study discloses the coal breakage features and size distributions of fragments, field experiments under the original in-situ stress condition were conducted to obtain the coal fragments generated in WJD. The effects of the jet pressure and coal strength on fragment size distributions were investigated based on the Weibull distribution and fractal model. The results indicate that fine particles dominate the coal fragments and its proportion increases with the increase of jet pressure and the decrease of coal strength. The fragment size distributions accord with the Weibull distribution. The breakage degree index of fragments decreases in the logarithmic form as the jet pressure increases. The coal fragments have the fractal characteristic, and the fractal dimension increases with the increasing jet pressure and decreases with the increasing coal strength. Additionally, the fractal dimension increases with the decreases in breakage degree index. This work could provide a guide for research on the coal fragments transportation and the selections of hydraulic parameters in WJD.