Research on drag reduction technology for long boreholes drilling in coal mine

In view of problem in long boreholes drilling in coal mine that frictional resistance between the drill string and the borehole wall seriously affects the transmission of weight-on-bit and hinders the further increase of the borehole depth, drag reduction technology for long borehole drilling were studied. Technical bottlenecks of the current composite directional drilling technology are analyzed, that is, it ca't be oriented in the composite drilling, and the efficiency is low in the deep hole sliding direction, as a result, it is impossible to develop branches and correct deviations when the drilling is deep. The solutions of hydraulic oscillator drag reduction technology，roller centralizer drag reduction technology，impact vibration drag reduction technology and rotary steering technology were proposed, and the principles, technical advantages and limitations of each scheme were introduced, and the stratum adaptability of related technologies and the requirements for drilling equipment were pointed out. The actual application results of the related technology were put forward:after application of hydraulic oscillator in Huainan Mining Group Zhangji Mine,average feed gauge pressure was about 4 MPa, which was 33% lower than before, and the average drilling efficiency was increased by 126%, the pressure drop consumption was about 2 MPa, although the mud pump pressure increases to 1.7-2.5 MPa, it could still meet the needs of mud pumps in current coal mine underground drilling system and deep hole drilling with directional drilling tools;Two directional test boreholes were completed on the site of Tangjiahui Coal Mine with single-bend screw motor and hydraulic impactor compound drilling tool,and the drilling depths were 546 m and 564 m,the construction footage using the drilling tool combination were 411 m and 429 m,the time were 8 230 min and 8 570 min respectively, and the ROP was about 3 m/h,while the ROP of conventional directional drilling in the same formation was 0.83 m/h,the ROP of composite drilling tools increased by about 260%;Impact screw motor was used in Sihe Mine of Jinmei Group for drilling footage of 1 743 m, the cumulative time was 200 h, and the average mechanical speed could reach 8.34 m/h, which was 20%-30% higher than the conventional screw motor.It was proposed that the development of small-diameter rotary steering technology is a new break through direction for underground drilling tools in coal mines.