Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 219

4.4.2 Stick-slip oscillations.
Here we will describe in detail the complicated dynamical process that is
known as “stick-slip” oscillation. When rotation commences, the drillstring
twists in place and stores torsional strain (or, “spring potential”) energy until the
applied torque exceeds the total static frictional torque acting on the bottomhole
assembly. The BHA now starts to rotate in a steady state mode. This steady
motion may not be permanent. If the static friction exceeds the dynamic friction
acting at the bit, the previously stored potential energy may be transferred into
kinetic energy, so that acceleration to a speed faster than steady state rotation is
possible. Sometimes this process can result in self-sustaining oscillations that
cause drillpipe fatigue and damage to the bit. Equally dangerous “drillstring
unwinding” can occur, as described, through axial-torsional mode coupling.
When longitudinal vibrations induce momentary bit bounce or bottom bit
slippage, severe pendulum-like unwinding and winding is possible. These
effects have been experimentally observed in numerous field tests.
Brett (1991) notes that relatively moderate and continuous reductions in
dynamic rotating torque (obtained as PDC bit rotary speeds increase, while held
at constant weight-on-bit) may create similar vibrations. That is, it is not
necessary for pure static friction to exist before self-excited torsional oscillations
occur. Brett (1992) presents lab and field torsional measurements, comparing
these with modeling results, to show that the observed vibrations can be
explained by a reduction in PDC bit torque as rotary speed increases.
Apparently, the reduction in torque during rotational acceleration releases static
strain energy for transfer into dynamic component modes. This so-called
“negative damping” is a manifestation of the interaction between static and
dynamic torsional states. Brett' s laboratory results for PDC bits show that the
reduction in torque with rotary speed is greatest for a dull bit in hard rock, and
that as weight-on-bit increases, the absolute value of the reduction in torque also
increases. From Brett (1991), we might tentatively conclude that torsional
vibrations that arise from bit torque characteristics are more common with PDC
bits than with tricones; also, they appear to be more severe at higher applied
weight-on-bits, at lower rotary speeds, and with duller bits.
Dufeyte and Henneuse (1991) and Henneuse (1992) describe a number of
Elf Aquitaine experiments in which downhole vibrations were continuously
monitored at the surface. Their results, obtained over numerous operating hours
and a variety of well types, showed that stick-slip phenomena represent very
frequent occurrences. In particular, stick-slip oscillation appeared as a result of
a natural tendency for PDC bits to develop substantial torsional waves. The
exact severity of any damaging backward spin depended on the bits' cutting or
abrasion operating mode.