Drill string vibration is an inevitable phenomena. Severe vibration can be harmful for the downhole equipment and causes many problems:
- Drill string failure
- Premature bit failure
- Damage to well bore
- Difficulties while directional drilling
- Reduced penetration rate
- Stabilizers wear
- Motor & MWD failure
Stick Slip
Stick slip or torsional stick slip is a result of the friction between the BHA and the bore hole walls which perturbs the normal or the steady rotation of the drill sting and the BHA.
Stick slip is an extremely severe torsional oscillation which can cause the drilling bit comes to complete stop and followed by an extra acceleration of the surface rotation which can exceed the normal chosen RPM for operations. Stick slip can be caused as mentioned above by high level of friction between the drill sting and the walls of the wellbore, but also the drill string buckling, stabilizers-borehole contact and drilling bit aggressivity can contribute in the overall level of vibration.
The group of the drill pipe and the BHA can be represented as a torsional spring and lumped mass. When the rotation is started from the surface, the mass and any frictional forces tend so resist to the rotation until a sufficient torque is generated to overcome the inertia then the whole system starts to rotate. The initial rotational speed tend to oscillate around the average surface rotary speed till getting to the moment when the drilling bit get the same rotary speed as surface speed.
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| Fig 1- RPM tendencies Surface-Drilling Bit |
In the case of the stick slip, the drill string can be stopped by the resisting forces. When sufficient torque develops in the string to overcome the resisting forces, then the string will rotate rapidly due the sudden release of the static energy. The sudden rotation level can be higher than the normal planned rotation. The instantaneous drilling bit speed can vary from zero to twice the surface speed and can be negative that means backward rotation.
The stick slip can be detected straight forward from surface parameters. The torque oscillates at more 20% of the average drilling torque. Rotation can vary severally between maximum RPM which can be higher than applied RPM and the minimum rotation which can be zero RPM (Rotary drive stalls). It can be featured by reduced penetration rate, over torqued connections, damaged drilling bits and in severe cases connections back-off.
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| Fig 2 - Drilling Bit oscillations (from zero RPM to Max ) |
There are many corrective actions which can be performed to overcome the stick slip:
- Increasing the rotary speed and decreasing the weight on bit can be one of the solutions.
- In severe cases, performing a wiper trips is a solution to minimize the tight spots which can cause the high level of friction between the BHA and the walls of the well bore.
- In future programs, the BHA design can be questioned.
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