Technology of cavity completion in surface cluster wells for assisting rock cross-cut coal uncovering in high outburst coal seam

In view of the problems of high risk in rock cross-cut coal uncovering of deep outburst coal seam, and borehole blowout and low efficiency during underground drilling, based on numerical simulation and field experiments, the mechanism of reducing outburst by cavity completion in surface boreholes for assisting rock cross-cut coal uncovering in high outburst coal seam was studied, and the key process parameters were optimized. Firstly, the multi-stage cavity formation technology and supporting system for surface cluster wells were developed. Then, the evolution of physics of stress-relief coal seam was revealed, and the optimization method of key process parameters for cavity creation was proposed. Finally, the technology of cavity completion in surface boreholes for assisting rock cross-cut coal uncovering was successfully implemented in the field, and it proves that this technique is effective. The research results show that： A multi-field collaborative outburst prevention method of “well network pressure relief-borehole group energy dissipation-curtain solidification” was proposed. The mechanism of outburst prevention by cavity completion in surface cluster well was revealed. A multi-stage caving technology of “borehole mechanical reaming-hydraulic jetting-water drainage pressure relief” was developed, and an integrated system of “cavity creation-slag discharge-water slag separation” was also developed. The range of plastic zone around the cavity is linearly related to the volume of the cavity, with a ratio of 81.90. The gas pressure in the low permeability area near the cavity decreases first and then increases with the increase of cavity diameter, and the corresponding optimal cavity diameter is 2.0 m. When the roadway is arranged along the direction perpendicular to the maximum principal stress, the outburst risk of coal on both sides of the roadway are easier to be eliminated after cavity completion. The technology of cavity completion in surface boreholes for assisting rock cross-cut coal uncovering was successfully implemented in the field, and the results show that the permeability coefficient of the coal seam has increased by about 10 times. The number of boreholes for in the test site has decreased by 24%, and the drilling length has decreased by 21%. The technical method described here can be further expanded to form the “coal-gas co-extraction method with borehole”, which is expected to achieve synergistic and efficient co-extraction of coal and gas in difficult to extract coal seams.