Effect of drilling fluid hydraulics on drill stem vibrations

- The effect of dynamic fluid pressure on drill stem vibration is investigated.
- Dynamic fluid pressures have significant impact on lateral frequencies.
- Significant change in lateral frequency occur when including the fluid model.
- Vibration free RPM road maps are highly dependent on model selection.

Drill stem vibration may cause drilling inefficiencies and often leads to non-productive time. Drill stem vibration analysis is performed to prevent and identify RPM values that may cause damaging standing waves in the drill stem. However, different models and assumptions give different vibration results. One simplification commonly done is excluding drilling fluids. This paper investigates the effect of including drilling hydraulics and the dynamic pressure on drill stem vibrations. The model assumes the drill stem is acting as a Euler Bernoulli beam and was discretized using a finite element formulation. Two fluid rheological models, Herschel Bulkley and Power Law, were used to determine the uncoupled dynamic pressure drop across the drill stem. The results show that including the dynamic pressure in the drill stem vibration model causes the critical rotational speeds to change as much as 32% (i.e. 59 RPM). As expected, axial and torsional vibrations are not affected, while lateral vibration changes significantly when the ratio of the wellbore diameter to the drill stem outside diameter is below 1.25, with a narrow total drill bit flow area, or when operating at high flow rates. Therefore, when creating RPM road maps to avoid vibrations while drilling, the contribution of the dynamic pressure should be included.