Definition
Non-Magnetic Drill Collar — A non-magnetic drill collar is a drill collar machined from non-magnetic alloy that houses MWD and directional survey instruments, isolating them from the drill string's magnetic interference so accurate azimuth measurements can be taken.
Also known as: non-magnetic drill collar, monel collar, NMDC, non-mag collar
Why non-magnetic
Directional surveys rely on magnetometers that read the Earth’s magnetic field to determine azimuth. Ordinary steel drill collars distort that field, so survey instruments must be run inside a non-magnetic drill collar (NMDC) made from austenitic stainless or specialized copper- nickel and chrome-manganese alloys. Placing enough non-magnetic length above and below the sensors keeps steel far enough away that the magnetic reading stays accurate.
Design and placement
NMDCs carry the same rotary-shouldered connections and load capacity as steel collars but are selected for low magnetic permeability and, often, resistance to galling and corrosion. The number and spacing of non-magnetic collars around the MWD package is calculated from the well’s inclination, azimuth, and magnetic latitude to keep survey error within tolerance.
Why it matters for rentals
Non-magnetic alloys are costly and NMDCs are only needed while surveying directionally, making them a natural fit for rental on directional and horizontal wells.
See our drill collars for rental non-magnetic collars, or contact us for guidance on non-magnetic spacing for your survey program.