The genesis of torsional drillstring vibrations

Inherent drilling characteristics of polycrystalline diamond compact (PDC) bits alone can be sufficient to excite severe torsional drillstring vibrations. The work presented in this paper augments previous findings that torsional vibrations can result from the drilling characteristics of the bit itself. Laboratory and field torsional measurements are compared with model results to show that the observed vibrations can be explained by a reduction in PDC bit torque as rotary speed increases. These torsional vibrations are important because they can cause drillpipe fatigue and may be severe enough to damage the bit. The model also is used to pinpoint possible solutions to the problem.