Improved Bit Stability Reduces Downhole Harmonics (Vibrations)
Authors: Ortiz Blas , Casallas Cesar , Parra Hector 
Published: December 2007

Abstract
There is growing demand for application of Polycrystalline Diamond Compact (PDC) bits in hard formations. This may be due in part to the fact that, in these formations, roller cone bits are considered "high risk" because they incorporate moving parts and are more likely to fail due to bearing fatigue which in turn causes roller cone loss. A lost cone means extra round trips and the costly downtime associated with fishing.
On the other hand, hard formation drilling presents significant challenges to PDC bits also: The brittle synthetic diamond table of the compact cutter fractures readily along crystalline cleavage planes, making them susceptible to severe impact damage. However, the major cause of reduced PDC bit life is lateral downhole vibration, or "bit whirl", which is especially troublesome in hard formation drilling. This lateral vibration is induced by an imbalance of forces, similar to the way an unbalanced car tire wobbles, then begins to vibrate the entire car.
In addition, harder formations require greater energy to fail, which means higher energy in the drillstring and greater likelihood of dynamics problems. This is especially true in surface hole because of the related proportions of drill pipe and drill collar to the bit. Very little BHA to dampen the vibratory effects created by downhole harmonics.
Over the years, several design methods have been introduced to reduce downhole vibration, including force balancing, blade spiraling, use of asymmetrical blade layouts, high back rake gauge cutters and impact arrestors, as well as introduction of new cutter technology to eliminate cutter overlap and induce a scalloped or ridged bottomhole pattern.
In addition, one "forgotten" element of bit design recently has been reintroduced to further reduce bit whirl. The improved stability afforded by this "overlooked" design feature has resulted in a more durable PDC bit that is less likely to fail due to lateral vibrations, as evidenced by a performance comparison with case histories in which vibration was the major cause of PDC bit failure.

Introduction and Background
Langeveld in 1992, revealed that poor performance of PDC bits in harder formations is caused primarily by downhole vibrations, but that proper operating conditions and new technology for vibration control can successfully extend PDC applications into harder formations. More recently, dynamics modelling has given way to monitoring surface and downhole measurements to identify harmful operating conditions. When sensors indicate vibration levels have exceeded some predetermined "safe" level, weight on bit and/or rotary speed are adjusted.
It is understood that downhole vibration can induce three components of motion in the drillstring and bit: axial motion along the drillstring axis; torsional motion, causing twist/torque; and lateral or side-to-side motion.
Axial vibration, also known as "bit bounce," results when large weight on bit fluctuations cause the bit to repeatedly lift off and impact the formation. Torsional vibration, known as "slip stick" or non-uniform bit rotation, occurs when the bit periodically stops rotating, causing the string to torque up and then spin free, accelerating the bit to high speeds. Lateral vibration is bit whirl, the eccentric rotation of the bit to high speeds. Bit whirl in the lateral direction with respect to the wellbore causes excessive side forces that result in wellbore gearing.
For the purposes of this paper, we focus on the effects of torsional and lateral vibrations on PDC bits; specifically, how torsional creates an imbalance of forces which causes the bit to engage the wellbore inducing off-center bit rotation and thus lateral vibration.

The Phenomenon of Bit Whirl
Simply stated, bit whirl, resulting from downhole harmonics, is the motion produced when the instantaneous center of rotation shifts from the bit's geometric center. Although this vibration motion can be described mathematically, it is very sensitive to formation properties, operating features, and bit design features, making any mathematical prediction of bit whirl virtually impossible.
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