Causes of Downhole Vibrations
|Types of Vibration||Causes|
|Bit Bounce||Bit/Formation Interaction
Multilobe Pattern of roller cone bits
Mud Pressure Fluctuation
|Slip Stick||Bit/Formation Interaction
High WOB/Low RPM
Friction between the Drillstring/BHA and Hole Wall
High Wellbore Tortuosity
Whirl and Bending
Lack of Axial Symmetry
Uneven Vertical Loading on Bit Face
Imbalanced Cutter Forces
High RPM/Low WOB
Rotation Off Bottom Reaming
Formation Hardness/Formation Change
|Types of Elimination||Causes|
|Bit Whirl||Increase WOB
Increase mud lubricity
Bit Whirl – the motion a bit makes when it does not rotate around its center, but in a spiral motion. This phenomenon usually occurs in softer formations and results in an oversize hole and excessive bit wear.
Slip Stick – A destructive vibration or oscillation caused by one of two things: bit or BHA. Slip Stick occurs when the bit bites into formation and, momentarily, sufficient torque is lacking to shear the formation. Thus, the bit “stops” while the torsional energy in the drill string builds up. Once enough energy is built up, the bit breaks free at rotational speeds much higher than normal resulting in bit failure/damage, BHA failure and lost time.
Coupling – Bending of the drill string coupled with WOB. Linear coupling is easy to visualize by taking a piece of paper, giving it a slight curve, and then pressing axially on the ends. The paper responds by additional bending in the plane of the initial curvature.
Bit Bounce – a downhole vibration caused mostly by bit/formation interaction. Bit bounce can occur when a bit cannot get a sufficient “bite” into formation to produce a smooth drilling action but actually bounces off the bottom of the hole.
|Slip Stick||BHA Whirl||Bit Whirl||Bounce|
|Tool Joint Wear|
|Increase Weight on bit||to||Decrease Bit-Whirl|
|Decrease Weight on Bit||to||Decrease Slip-Stick|
|Increase Rotary Speed||to||Decrease Slip-Stick|
|Decrease Rotary Speed||to||Decrease Bit Whirl|
|Increase Mud Lubricity||to||Decrease Slip-Stick|