Summary

Inherent drilling characteristics of polycrystalline diamond compact (PDC) bits alone can be sufficient to excite severe torsional drillstring vibrations. The work presented here 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.

Introduction

Torsional drill string vibrations have been studied for some time. However, most previous literature attributed these vibrations to drill string static friction effects. Briefly (and at the risk of oversimplification), this previous work shows that if the static friction coefficient is sufficiently higher than the dynamic friction coefficient then torsional drillstring vibrations can occur. The basic cause of this vibration is a "slip-stick" phenomenon. When drillstring rotation begins, the drillpipe stores torsional energy (in the form of a spring) until the applied torque exceeds the total static frictional torque on the bottomhole assembly (BHA). The BHA then begins to rotate and, because the static friction is higher than the dynamic friction, the stored energy in the drillpipe spring is transferred to inertial energy in the BHA. It then can accelerate to a speed faster than the steady-state rotational speed. This earlier work shows that, under some circumstances, the transfer of energy from the drillpipe spring to the BHA mass can be self-sustaining and will result in torsional vibrations. The facts that torsional vibrations sometimes can occur while off bottom and that these previous models can match field results closely indicate that drill string static friction effects can cause torsional drillstring vibrations.

This paper's purpose is to present data and a model to show that bit characteristics can initiate torsional drill string vibrations. This work shows how an inherent characteristic of PDC bits, the reduction in torque with increased rotary speed, can cause torsional vibrations during drilling ahead, even though the drillstring may rotate smoothly when the bit is off bottom. The work presented here also explains why torsional vibrations (1) can occur in shallow vertical holes where drillstring friction is low; (2) are more common with PDC bits than with three-cone bits; and (3) are more severe with PDC bits at higher applied weight-on-bit (WOB), with dull bits, and at lower rotary speeds.

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