Hydraulic fracturing technology for permeability improvement through underground long borehole along coal seam

Aiming at the general poor gas drainage effect of coal seams with low permeability in China, the paper put forward the technical measures of hydraulic fracturing for permeability improvement through large-diameter long boreholes, the mechanism of permeability improvement by hydraulic fracturing in long boreholes was discussed, and gas drainage test using hydraulic fracturing in coal seams in underground coal mines was carried out. On the basis of successful drilling of long boreholes along coal seams, a rapid sealing tool assembly suitable for underground hydraulic fracturing operation was developed. The change law of parameters during fracturing was analyzed, the method for evaluation of the influence range of hydraulic fracturing, the effect of hydraulic fracturing and gas drainage was put forward, investigated and assessed. The research indicated that the technology had overcame the problems existing generally in traditional underground hydraulic fracturing, such as poor hole-sealing quality and small influence range of fracturing. After fracturing, the permeability coefficient of seam increased by 2.67 times, the maximum influence range of fracturing reached 58 m, during 130 d totally 31.39×10⁴ m³ pure gas was drained with the maximum daily extraction of 2 668 m³ and the mean gas volume fraction of 70.05%, the drained pure gas quantity of 100 m of borehole was up to 0.55 m³/min.