Mitigating Torsional Stick/Slip Vibrations in Oilwell Drilling Through PDC-Bit Design: Putting Theories to the Test

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 146561, ’Mitigation of Torsional Stick/Slip Vibrations in Oilwell Drilling Through PDC-Bit Design: Putting Theories to the Test,’ by Jayesh R. Jain, SPE, L.W. Ledgerwood III, SPE, Olivier J. Hoffmann, SPE, Thorsten Schwefe, SPE, and Danielle M. Fuselier, SPE, Baker Hughes, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. Stick/slip vibration of drillstrings is gaining renewed interest as operating parameters for polycrystalline-diamond-compact (PDC) bits move into the stick/slip regime of higher bit weight and lower rotary speed to enhance drilling performance. The main objective of this investigation was to determine the influence of bit design on stick/slip behavior of the drilling system. Pairs of PDC bits were designed and manufactured. Each pair had a bit with a standard design and one that embodied a stick/slip theory. Full-scale tests were performed under controlled conditions with a research drilling rig. Introduction Drilling vibrations can result in failure of bits and bottomhole-assembly (BHA) components. Axial and torsional vibrations are prominent with roller-cone bits. With PDC bits suffering catastrophic failure caused by backward whirl, the focus has been on lateral vibrations. Much attention has been given to combating backward whirl by use of antiwhirl bit designs. Meanwhile, cutter technology progressed dramatically with thermally stable PDC cutters that are more impact and abrasion resistant. Consequently, operating parameters for PDC bits have shifted to higher weight on bit (WOB) and lower rotary speed to enhance drilling performance. This parameter combination also avoids the low-WOB and high-rotary-speed regime (Fig. 1) in which backward whirl can destroy bits after drilling only a few feet. Unfortunately, higher WOB and lower rotary speed instigate torsional instability that matures into stick/slip vibrations. It has been shown in the field that for a surface rotary speed of 60 rev/min, the measured downhole rotary speed reached 400 rev/min during the slip phase. Lateral vibrations during the slip phase correlated well with observed damage. More-severe cases of measured stick/slip have been reported in the literature when the bit experienced reverse rotation at the end of the slip phase before coming to a standstill. A desirable solution for mitigating stick/slip is an inherently stable sys-tem that expands the stable-operating zone through improved bit and BHA/drillstring design to allow drilling at more-efficient operating parameters of high WOB and low rotary speed. The BHA design and the geometry (size and number) of drillpipe and drill collars can have a substantial effect on the stick/slip behavior of the system. Although larger drillpipe can mitigate stick/slip and enable more-efficient drilling, its availability and cost may limit its effectiveness as a solution. Prevailing industry perceptions were reviewed, and five leading theories regarding the root cause of stick/slip were identified.