Hole-bottom hydraulic pressurization technology for directional drilling in hard rock in underground coal mines

In order to solve the problem of large feeding pressure, low drilling efficiency and limited hole depth caused by the backing effect of drill string during directional drilling of deep hole in underground coal mines, a technical idea of using hydraulic tool to increase WOB was proposed. The structural design of the hydraulic pressurizer at the bottom of the hole was carried out by drawing on the structure of the hydraulic pressurizer in the field of oil drilling and combining the nearly horizontal drilling conditions in underground coal mines. The hydraulic parameters were designed by the theoretical calculation and the numerical simulation method. The indoor test results show that the axial force 2-10 kN is output in the range of flow 200-450 L/min with 12 mm, 13 mm, and 14 mm of the piston hole. Field test was carried out in Zhangji coal mine and a set of hole-bottom hydraulic pressurization and auxiliary pressurization were summed up. The test results show that the drilling depth was smoothly extended from 464 m to 578 m under the sliding directional drilling method. The maximum drilling efficiency of the deep hole drilling was increased from below 1 m/h to more than 3 m/h. The feeding pressure was reduced by 23.8% and the average drilling efficiency was increased by 137%.