A non-magnetic drilling collar (NMDC) is a thick-walled tubular piece of the drill string, similar in dimensions to standard drill collars, but made from a non-magnetic alloy. Standard drill collars are usually made from high-strength alloy steel (like AISI 4145H) which is ferromagnetic. In contrast, non-mag collars use special alloys (often a type of stainless steel with high chromium, nickel, and sometimes manganese content) that have very low magnetic permeability.
Key characteristics of non-mag drilling collars:
Material: Common materials for non-mag collars include high-chrome stainless steels such as Monel or proprietary alloys like P530 or P550. These are essentially austenitic steel alloys that are engineered to have mechanical strength while being essentially non-magnetic. The manufacturing process often involves a strict heat treatment and maybe even a quench in a way that preserves a non-magnetic austenitic grain structure. Non-mag alloys are often slightly lower in yield strength compared to conventional steel drill collars, but they still meet the needs of providing weight on bit and stiffness.
API Specification Compliance: Non-mag collars are generally manufactured to API Spec 7-1 (which covers drill collar specifications) just like regular collars, to ensure they meet standard dimensions and mechanical property requirements. They have the same threading (like NC50, 6⅝ REG, etc.) as their steel counterparts, so they can be interchanged in the drill string assembly.
Physical Properties: Typically, a non-mag collar will have yield strength on the order of 90,000–120,000 psi (depending on grade), tensile strength slightly above yield (maybe 120–135 ksi), and adequate toughness (Charpy V-notch values) to handle drilling stresses. Modern non-mag materials are designed to achieve these strengths so that using a non-mag doesn't mean sacrificing drill string integrity. Hardness is usually controlled to avoid magnetization (very high hardness could lead to some ferromagnetic phases forming).
Geometry: They mirror normal drill collars in outer diameter (commonly ranging from 3.5 inches OD for small collars up to 8+ inches for large collars) and inner bore (often around 1.25–2.5 inches depending on OD). Non-mag collars can be slick (plain surface), spiral grooved (to reduce differential sticking risk, just like steel spiral collars), or flex (with a thinner mid-section for more flexibility) – all the usual variants are available.
Magnetic Permeability: The defining feature is their magnetic permeability is very close to 1 (same as the surrounding drilling mud and rock), meaning they do not significantly distort the Earth's magnetic field. In practice, using non-mag material neutralizes magnetic interference caused by the drill string, enabling accurate readings from magnetic sensors in MWD toolspetrogears.com. The collars are tested for this property; a common requirement is a max relative permeability of <1.05 or even closer to 1.0. Often, each non-mag collar may be checked with a magnetometer to ensure no "hot spots" (localized areas of higher magnetism).
Corrosion Resistance: Non-mag alloys, being stainless, have good resistance to corrosion (including H₂S environments). However, one issue can be stress corrosion cracking in certain environments. Manufacturers mitigate this by processes like shot peening the ID of the collar, which induces compressive stresses and reduces susceptibility to stress-corrosion cracking. This is particularly important if the collars will be exposed to corrosive drilling fluids or sour gas.
In essence, a non-mag drilling collar is a chunk of heavy, non-magnetic metal that is placed in the drill string usually directly above the MWD and LWD (logging-while-drilling) tools. By doing so, it provides a "magnetically transparent" zone around those instruments. Typically, a series of non-mag collars (often a certain length calculated based on the expected interference radius of steel above and below) are used. For instance, a common guidance might be to use 30 ft or more of non-mag collar in the BHA when running MWD–the exact length needed can depend on the BHA design and how far the directional sensors need to be from any steel.
Vigor delivers specialized Non-Magnetic Drilling Collars (NMDCs) engineered to eliminate magnetic interference in critical measurement-while-drilling (MWD) and logging-while-drilling (LWD) operations. Our collars-manufactured from ultra-low permeability alloys like Monel K500, Inconel 725, and custom austenitic steels-are sourced through client partnerships to meet exacting specifications: magnetic permeability <1.005 Gauss/Oersted, dimensional tolerances within ±0.05mm, and API 7-1/ISO 11961 compliance.
Beyond procurement, Vigor's drilling engineers provide integrated technical collaboration: analyzing formation magnetism, wellbore trajectory, and bottom-hole assembly (BHA) dynamics to optimize collar placement and mitigate survey errors. Each NMDC undergoes rigorous multi-stage validation, including:
- Real-time eddy current scanning for permeability uniformity,
- Charpy V-notch impact testing at operating temperatures,
- Full material traceability and ultrasonic inspection (UT).
Leveraging strategic supplier alliances, Vigor reduces lead times by 30%+ and costs by 15–25%, while our proactive engineering ensures accelerated deployment of field-proven solutions. The result: high-fidelity directional data, elimination of costly sidetracks, and enhanced drilling efficiency in complex subsurface environments.
For more information, you can write to our mailbox info@vigorpetroleum.com & marketing@vigordrilling.com
