Drill Collar

DCs are heavy and stiff steel tubular, and they are used at the bottom of a BHA to provide weight on bit. There are different kinds of DCs available such as flush or spiral DC. In directional drilling, spiral DCs are preferable because the spiral grooves reduce the wall contact area by 40% for a reduction of weight of only 4%. Hence, chances of differential sticking can be greatly minimized.

There are other classifications of DCs as mentioned in the following.

Nonmagnetic drill collars: NMDCs are generally flush (nonspiral). They are manufactured from high-quality, corrosion-resistant, stainless steel. They are used while running the magnetic survey instruments to be located in a NMDC of sufficient length to allow the measurement of the earth's magnetic field without magnetic interference. Survey instruments are isolated from magnetic disturbance caused by other steel components in the BHA.

Short nonmagnetic drill collars (SNMDCs): SNMDCs are used between a mud motor and a mud collar to neutralize magnetic interference from the following