Special Types of Collar
• Anti-wall stick
When drilling through certain formations the large diameter drillcollars can become stuck against the borehole (differential sticking). This is likely to happen when the formation is highly porous, a large overbalance of mud pressure is being used and the well is highly deviated. One method of preventing this problem is to reduce the contact area of the collar against the wellbore. Spiral grooves can be cut into the surface of the collar to reduce its surface area. (Figure 7)
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Figure 7 Spiral drillcollar Institute of Petroleum Engineering, Heriot-Watt University 15
• Square collars
These collars are usually 1/16” less than bit size, and are run to provide maximum stabilisation of the bottom hole assembly.
• Monel collars
These collars are made of a special non-magnetic steel alloy. Their purpose is to isolate directional survey instruments from magnetic distortion due to the steel drillstring.