Techniques of double-pipe and double-acting directional drilling with air in broken and soft coal seams in underground coal mines

In the broken and soft coal seam drilling, the outburst and collapse of the hole occur easily, resulting in shallow drilling depth and low rate of hole forming, which seriously restrict the efficient gas control in the broken and soft coal seam. With the rise of gas control models, such as replacing roadway with holes and progressive drainage of the broken and soft coal seam, higher requests have been put forward to the gas drainage hole. The further improvement of the drilling depth, drilling accuracy and hole forming rate of the broken and soft coal seam can meet the needs of coal mine safety, high efficiency and precise gas control. On this basis, the double-pipe and double-acting air directional drilling technology for the broken and soft coal seam is proposed. In view of the key drilling tools and drilling techniques involved in this drilling method, the grading of drilling tools such as outer casing, inner drilling tools, collar bits and reamers is analyzed and determined according to the specification and the passability test of underground air motor drilling tools. The parameters such as pitch and head number of casing blade are optimized by analyzing the motion speed of coal particles in auger drilling. According to the demand of cuttings discharge and the air motor drilling tool, the air volume and pressure are calculated. By using the numerical simulation method, the law of cuttings discharge and the optimal drilling parameters are studied. Through the field drilling test, the rationality of the method and the drilling tool grading are verified. The results show that increasing the air volume is more effective than increasing the rotation speed in cuttings discharge. When the casing speed is 40 r/min, the corresponding air volume should reach at least 500 m³/h. After the sliding directional drilling, drilling and cuttings discharge should be done as soon as possible. When the air volume is lower than 400 m³/h, cuttings should be discharged by sweeping the hole. The inner pipe drilling tool with ø90 mm collar bit, ø120 mm reamer, ø73 mm air motor, ø73/35 mm wide blade drilling pipe and the outer pipe drilling tool with ø140 mm casing bit, ø120/96 mm anti-spiral casing are combined with air flow of 1 000 m³/h and air pressure of 1.25 MPa, and the rotating speed between 40 r/min and 120 r/min. The hole trajectory can be controlled within the coal seam, when the drilling depth reaches 350 m and the casing depth reaches 168 m, which effectively improves the borehole depth and precision, and provides a new technological method for precise drilling in the broken and soft coal seams.