Bit bounce is one of the forms of vibrations which can be faced during drilling operations. It happens when the bit loses contact with the formation. Bit bounce can cause severe damage to the drilling bit and the other BHA components like PDM motors and MWD tools.
Bit bounce is common when drilling with the tri-cone bits. This phenomena starts when the bit tends to generate a tri-lobed bottom hole pattern. The generated pattern is consisting of three ridges, it can be noticed at the recovered cores after using a tri-cone drilling bit. The combination of the formation hardness and the pattern forms force the bit to bounce three times per revolution. This frequency of 3 bounces per rotary speed will be amplified if can match the axial natural frequency of the drill string leading the drilling bit to lose contact with the formation. There are also other factors which contribute in the excitation of this axial vibration such as variation of the weight on bit, changes in mud pressure, formation change, BHA interaction with borehole walls and the vibration modes coupling.
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| Fig 1 - Formation pattern & Bit trajectory on it |
The bit bounce results in accelerated bearing and tooth wear, damage to down hole equipment, reduction in penetration rate, short run lengths.
One of the solutions to overcome the axial vibration is using shock subs which are also called vibration dampener. The drill string can be fitted with this type of tools and it can be placed above the bit to reduce the high level of vibration frequency due to bit bouncing. It absorbs the vertical vibration and reduce the impact on the down-hole tools and also the surface equipment. Using the shock sub can help to prolong the life of the drilling bit and the drill string. The use of the shock sub with the PDC bit is not recommended because can prematurely fail the PDC cutters.
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| Fig 2 - Shock Sub |
Destroying the tri-lobed pattern is necessary and thus removing the source of vibration excitation. This action can be performed by lifting the drill string off bottom and starting drilling with reduced parameters till getting steady situation. Changing the drilling parameters is also one the actions to eliminate or at least reducing the axial vibrations. Increasing the weight on bit when recovering drilling and after removing the pattern can help to start with steady vibration frequency leading to better drilling performance.
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