Non-Magnetic Drill Collars
Requirements relating to NMDC use when surveying with magnetic instruments..
Every magnetic compass placed in a steel BHA will exhibit an azimuth error. This is caused by the interference between the magnetic field of the Earth and any BHA components which cannot be demagnetised (e.g. mudmotors).
To reduce this error to an acceptable level, the correct length of non-magnetic drill collars (NMDC) must be run in every BHA in which magnetic surveys are to be taken.
The selection of the correct length of NMDC and . . .