Measurement and Analysis of Stick-Slip and High-Frequency Torsional Coupled Oscillations of Drill String
The self-excited torsional oscillation of the drilling system can reduce the contact force between the bit and the formation or the relative velocity of the cutting force.To mitigate the impact of this mechanism,the tor-sional oscillation of the drill string was investigated through analysis of triaxial oscillation in time and frequency do-mains.It is found that low-frequency torsional oscillation(LFTO)causes stick-slip oscillation.The frequency of stick-slip oscillation is 0.128 Hz,and the triaxial oscillation and rotational speed fluctuate periodically.When the drill string is subjected to high-frequency torsional oscillation(HFTO),the main frequency of 177.2 Hz is present in all triaxial accelerations.Time-domain analysis reveals that the tangential acceleration is much higher than the peaks of axial and normal accelerations,and the root-mean-square(RMS)value is also higher,indicating that the tangential oscillation fluctuates greatly with high energy.This means that torsional oscillation is occurring down-hole.When stick-slip and HFTO are coupled,two main frequencies of normal acceleration appear:the main fre-quency of stick-slip and the main frequency of HFTO.High torsional frequency can increase the torque and me-chanical speed,leading to drilling tool fatigue.The study results provide guidance for describing the characteris-tics of torsional oscillation,judging whether stick-slip or HFTO occurs in drilling process,and taking proper meas-ures to eliminate stick-slip oscillation and relieve drilling tool fatigue.
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