One of the more important considerations in drill string design is dealing with the vibrations and shock loads that are produced as the bit rotates on bottom. Such vibrations can cause "rough running" at the surface, and can result in damage to the bit, drill collars, drill pipe and other components.
There are a number of drilling parameters that determine whether vibration will occur, including:
• formation lithology (e.g., hard rock, broken formations, and formations with intermittent hard and soft streaks);
• depth;
• bottomhole assembly configuration; • weight on bit;
• rotary speed.
Engineers can use these and other parameters during the well planning process to develop computer models for optimizing drill string design. They can also monitor vibrations during the drilling of the well by means of surface or downhole (MWD) sensors.
We can often control or eliminate downhole vibrations by modifying the bottomhole assembly configuration, or simply by adjusting rotary speed and/or weight on bit. A vibration dampener, or shock sub ( Figure 1 ,
Figure 2 , and Figure 3 ), may be useful in some cases for absorbing vibrations and shock loads.
Figure 2
A vibration dampener is a type of shock absorber designed to prevent vibrations generated by the bit from traveling up the drill string to the surface. It is commonly run between the bit and the drill collars, or above the large-diameter short drill collar segment of a packed- hole drilling assembly. When properly used, vibration dampeners can result in faster drilling rates, longer bit life, less damage to the drill string and surface equipment, and reduced torsional impact.
Because of the complexity and magnitude of the forces acting on the bit and drill string, vibration dampeners cannot be considered a cure-all for vibration problems. They should not be routinely added to drill strings without first modeling their effects. Improperly applied, a shock sub may not only fail to absorb severe vibrations, but could even create additional vibrations, thereby accelerating, rather than preventing, drill string failure.
REFERENCES
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ADDITIONAL READING
Hill, T., Ellis, S., Lee, K., Reynolds, N., and Zheng, N. (2004). "An Innovative Design
Approach to Reduce Drill String Fatigue". SPE 87188, presented at the IADC/SPE Drilling Conference held in Dallas, Texas, USA.(2-4 March) . Richardson, TX, USA : Society of Petroleum Engineers.
Lou, A., and Souder, W. (1998). "Composite Drill Pipe for Oil and Gas E&P Operations".
SPE 48888, presented at the SPE International Oil & Gas Conference and Exhibition in China held in Beijing, China (2-6 November). Richardson, TX, USA : Society of Petroleum
Engineers.
Murch, C. (1996). "Application of Top Drive Drilling to Horizontal Wells". SPE 37100,
presented at the SPE Horizontal Well Technology Conference held in Calgary, Alberta, Canada (18-20 November) . Richardson , TX, USA : Society of Petroleum Engineers.
Shepard, S., Reiley, R., and Warren, T. (2001). "Casing Drilling: An Emerging
Technology". SPE 67731, presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands (27 February-1 March). Richardson, TX, USA: Society of Petroleum Engineers.
Summers, M., and Crabtree, S. (1998). "Drill String Management to Reduce Drilling
Risks". SPE 39325, presented at the IADC/SPE Drilling Conference held in Dallas, Texas (3- 6 March). Richardson, TX, USA: Society of Petroleum Engineers.