Research on Torque of Based Hydraulic Rotary Hole Cleaning Tool on Halbach Permanent Magnet Array

Abstract: The hydraulic rotary hole cleaning tool does not require a drill string to provide torque, so it is compatible with downhole motors in the sliding phase of directional drilling. It facilitates efficient cutting cleaning, especially in the directional interval and horizontal interval. The magnet array structure must be optimized to enhance the magnetic torque transmission capacity and thus increase the output power of this tool. Magnetic field analysis was conducted in this study using Laplace and Poisson equations. A mathematical model was established to calculate the magnetic torque transmitted by the hydraulic rotary hole cleaning tool. The effects of various magnetic field and tool design parameters on the magnetic torque transmission capacity were compared among different permanent magnet array structures. The negative-positive Halbach magnet array structure shows enhanced magnetic torque transmission capacity compared to the traditional radial magnet array structure; it achieves a better excitation magnetic field direction of the permanent magnets on the magnetic shafts and an increased magnetic flux density within the tool housing. The suggested tool housing thickness under the field operation safety requirement is 9-10 mm. The optimum tool design features include a negative-positive Halbach magnet array structure, 100% magnet field coverage within the tool housing and a permanent magnet thickness range of 9.5-10.5 mm. The maximum magnet torque transmission capacity can be obtained within these parameters. The proposed magnetic torque transmission capacity model was compared against laboratory experiment data to prove that it is reliable and precise enough to optimize existing hydraulic rotary hole cleaning tools with the negative-positive Halbach magnet array structures.