Mitigation of Multi-Frequency Stick/Slip

Abstract

Stick/slip-induced vibration has been recognized as a cause for bit wear, premature tool failure, and poor drilling performance, which represents a contribution of approximately 30% of drilling vibration dysfunction. Dynamic modeling of stick/slip phenomena in drillstrings shows that the vibrational waves travel back and forth along the drillstring between the bit and top drive, which typically leads to 15% fluctuation in surface torque. It is also found that stick/slip is much more likely to occur with certain drilling fluid types and in deviated holes with large dogleg severity. One method of stick/slip mitigation is through control of the top drive. In existing applications, the vibrational wave at a fundamental frequency is absorbed by tuning a proportional-integral (PI) controller. Stick/slip-induced vibrations do not exist at a single frequency, and the simple PI controller cannot mitigate stick/slip occurrence at all vibration frequencies. Vibrations at frequencies other than the frequency chosen for mitigation can be amplified using existing tuning methods. In tests in which the method was applied, there were cases in which the vibration shifts to the second mode when the first torsional mode is mitigated. Therefore, the challenge is to target more than one vibration frequency. A new control system has been designed to observe stick/slip frequencies and then to dampen the stick/slip across a wide frequency range, while regulating the rotational speed of the drillstring at the desired set point. All control algorithms are implemented on a standard programmable logic controller (PLC). To eliminate the need to modify the existing top variable-frequency drives (VFD), this paper also proposes several methods to seamlessly implement the proposed controller. Existing configurations and stick/slip mitigation tests based on PI controller-gained tuning have been achieved on a test rig. Field tests demonstrating the new control method have been performed, and the results are presented and analyzed in this paper.