Study of Radial Vibration Impact on Friction and Torque of Rotary Drill String

Abstract

During extended-reach well drilling, the torque of a rotary drill string is too large in the horizontal section, which results in the failure of the drill string. The effect of radial vibration on the friction and torque of the rotary drill string is unclear. In this work, first, the principle of the impact of radial vibration on the friction and torque of the rotary drill string in the horizontal section is analysed. Then, joints with an elliptical outer edge of the section are designed, which can generate low-amplitude vibration as the drill string rotates owing to the interaction with the wellbore wall. The radial vibration characteristics can be tuned by changing the major to minor radius ratio of the ellipse. A self-developed experimental device was designed to test the performance of tools with reduced friction and torque. The drill string torque with different penetration rates, rotation speeds, and ratios of the major radius to minor radius of the ellipse were systematically studied. The experimental results show that the average and the maximum amplitudes of the torque fluctuation first decrease to the lowest value and then increase with the increase of the ratio. When the ratio reaches 1.065, the average and the maximum amplitudes of the torque fluctuation evidently reduce; therefore, a joint with a ratio of 1.065 can effectively reduce frictional torque. The average torque reduces when the penetration rate and rotation speed decrease. At rotation speeds of 45 and 60 r/min, the maximum amplitude of torque fluctuation increases first and then decreases with an increase in penetration rate, and it reaches a maximum value at a penetration rate of 22.5 m/h. The fundamental frequency of torque fluctuation is almost linearly related to rotation speed, which is irrelevant to the penetration rate and the ratio of major to minor radius of the ellipse.