Drilling Engineering
Directional Drilling
above situation. Initially, low weight-on-bit should be used, again to avoid bending the pendulum towards the low side of the hole. Once the dropping trend has been established, moderate weight can be used to achieve a respectable penetration rate.
Tangency point
W Effective length of pendulum
WOB
Figure 5-41: Reduction of pendulum force due to wall contact.
Some elementary texts on directional drilling depict the pendulum effect as shown in Figure 5-42. The implication being that part of the dropping tendency is produced by a downward tilt of the bit axis. It is interesting to note that if this picture were true then the dropping tendency would be increased by increasing WOB and reducing rotary speed, the precise opposite of what is recommended. Figure 5-42 is possible for certain lengths of pendulum when there is no near-bit stabilizer and only one string stabilizer. The collars above the upper stabilizer will sag towards the low side of the hole causing a fulcrum effect about the string stabilizer and tilting the upper portion of the pendulum towards the high side of the hole.Experienced has shown instances of pendulum assemblies dropping faster with high WOB and low rotary speed.
Workbook 80270H Rev. B / December 1995
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