Theory and experiment of the dual-bit torque self-balancing drilling system for rescue drilling

The rapid and safe construction of rescue channel is an effective method to save the trapped person underground after mine accidents. The conventional rescue hole drilling technologies mainly include the downhole hammer percussion drilling technology and the composite drilling technology, which have the disadvantages of large rig structure, complex drilling process, great disturbance to the formation and low target hit rate. In particular, it is easy to have secondary accidents in the fractured strata, and the rescue requirements could not be effectively met. In order to ensure the formation adaptability and wall stability of rescue boreholes, the dual-bit torque self-balancing drilling method was proposed and the dual-bit reverse drive mathematical model was established. Specifically, the rock was broken by synchronous reverse rotation of the dual bits through the independent driving of the inner and outer drill bits in reverse. The upper drilling tool was mainly subjected to the simple axial tensile pressure, with weak disturbance to the well wall. Meanwhile, the dual bits were fed alternately to support each other, which could effectively prevent the drilling deflection and achieve the precise target hitting. The dual-bit torque self-balancing drilling system mainly consists of three major parts, namely, the local torque closed self-balancing for near-bit drive, the drilling pressure adjustment of inner and outer bits, and the torque isolation for drilling tool transmission. The overall structure of the drilling tool system was designed based on the functional requirements of the drilling system, and the strength of the key components was calibrated and optimized with the finite element analysis software, including the torque transmission shaft of inner bit, the pressure regulating screw rod and the pressure transmission spacer bearing seat. Besides, the functional prototype of torque self-balancing drilling system was developed. In addition, a series of tests, regulation and detection were performed on the synchronous rotation of the dual bits, alternate feed, system sealing and automatic control of the complete system, and on this basis, laboratory indoor drilling tests were carried out for the soft, medium-hard and hard rocks of three different lithologies. The test results show that the system has little disturbance to the borehole wall and core, high target hit rate, and favorable drilling speed in different lithological strata, which verifies the feasibility of the dual-bit torque self-balancing drilling under the near-bit drive, thus providing a new method for the rapid and safe construction of life-supporting and rescue channels for mine disasters.