Application of ultra-high pressure hydraulic slotting technology in medium hardness and low permeability coal seam

In order to improve the gas pre-drainage efficiency of medium hard coal seam with high gas and low permeability, the applicable conditions, advantages and disadvantages of hydraulic piercing, hydraulic fracturing and hydraulic cutting seam anti-permeability technology were discussed. Based on the principle of ultra-high pressure hydraulic slotting technology, a kind of ultra-high pressure hydraulic slotting device for through-layer drilling is developed, it is mainly composed of diamond hydraulic slotting bit, shallow spiral drill pipe, ultra-high pressure rotary joint, ultra-high pressure clean water pump, high-low pressure converter, ultra-high pressure rubber pipe, etc. The water pressure reaches 60−100 MPa, which can realize the integration of drilling and cutting, and is simple and convenient to use. The device was used to carry out field tests in the pre-drainage boreholes of 11-2 coal seam through the floor roadway of 1361(1) haulage gateway in Dingji coal mine. The coal seam gas pressure was 1.43 MPa, the gas content was 8.05 m³/t, and the gas permeability coefficient was 0.013 m²/ (MPa²·d), the coal seam firmness coefficient is 0.79; 1361(1) transportation roadway floor No.11−No.15  drilling area unit length 227 meters, using high-pressure hydraulic slotting anti-permeability measures, 1361(1) transportation roadway floor No.6—No.10 drilling area unit length 213 meters, anti-permeability measures of low-pressure water punching in coal mine. The results show that the average single-knife slitting time of ultra-high pressure hydraulic slitting drilling is 10.7 min, the single-knife coal output is 0.31 t, the equivalent slitting radius is 1.38 m, the slitting density of the coal hole section is 1 knife/m, and the average cutting rate per hole is The average gas drainage concentration of ultra-high pressure hydraulic slotted holes is 56.97%, which is 2.37 times that of low-pressure punching; The time to reach the standard is about 23 days, which is 74.4% and 54.9% shorter than that of ordinary drilling and hydraulic punching technology respectively. Compared with ordinary drilling and hydraulic punching technology, ultra-high pressure hydraulic slotting technology is more ideal for gas drainage in low permeability medium-hard coal seams.