Non-Mag Drilling Tools: Ensuring Directional Drilling Accuracy and Safety

Meta Title: Non-Magnetic Drilling Collars & Tools – Enhancing Directional Drilling Precision
Meta Description: A technical deep-dive into non-magnetic drilling collars and tools. Learn how non-mag BHA components eliminate magnetic interference, their materials and specs, use cases in MWD/LWD, and tips for handling and QA/QC in oilfield operations.

Directional drilling and measurement-while-drilling (MWD) operations demand components that do not disturb the sensitive magnetic sensors used for navigation underground. Non-magnetic drilling tools, particularly non-mag drill collars, play a crucial role in these scenarios by providing the necessary weight and stiffness in the drill string without causing magnetic interferencepetrogears.com. In this article, we focus on non-magnetic drilling collars – their construction, material properties, practical uses in the bottom-hole assembly (BHA), and technical considerations. We also cover best practices for using and maintaining non-mag tools to ensure they deliver maximum benefit in terms of directional accuracy and drilling safety.

 Example of non-magnetic drill collars in a workshop. These collars are made of specialized stainless alloy steel, which appears shiny and is non-magnetic. They are machined with threaded connections on each end. Non-mag collars are typically placed in the BHA above MWD/LWD tools to provide weight on bit while not interfering with directional sensors.

What are Non-Magnetic Drilling Collars?

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 requirementsdrillingtools.com. 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 stressesdrillingtools.com. 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 availabledrillingtools.com.
• 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 toolsdrillingtools.competrogears.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 crackingdrillingtools.comdrillingtools.com. 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 MWDdrillingforgas.comdrillingforgas.com – the exact length needed can depend on the BHA design and how far the directional sensors need to be from any steel.

Role in Directional Drilling and MWD/LWD

The primary motivation for using non-mag drilling tools is to enable accurate directional surveying. MWD tools typically include a magnetometer (compass) and accelerometers. The magnetometer measures Earth’s magnetic field to determine azimuth (direction) of the wellbore. If there’s a big piece of magnetic steel nearby (like a drill collar), it will distort the field and introduce error in the measurement. This is known as BHA magnetic interference and can make the azimuth readings unreliable or even unusable.

By incorporating non-mag collars:

• The sensitive magnetometers in the MWD tool (or a separate single-shot survey tool) are surrounded by non-magnetic metal for a certain distance, shielding them from the influence of the rest of the steel drill stringpetrogears.com. Essentially, above and below the MWD tool, you would have non-mag sections. Above, you might have non-mag collars; below (between the tool and the bit) often the drill collars might still be steel, but the distance is usually sufficient that the bit and steel below doesn’t interfere significantly.
• The length of non-mag section needed is determined by modeling or empirical standards. For example, one guideline suggests that having the compass at least 15-30 feet away from any steel will keep the magnetic azimuth error below a certain threshold (like 0.25°)drillingforgas.com. Often companies have software that calculates the needed length given the expected magnetic properties of the BHA, the latitude (Earth’s field strength varies with location), etc.
• In practice, what happens is: you might run, say, 2–3 non-mag collars (each maybe 31 ft long) back-to-back. The MWD tool is typically housed in a non-mag collar itself (often the collar is designed to hold the MWD sensors inside). So the MWD tool might be placed in the bottommost non-mag collar. Additional non-mag collars above provide further buffer. Then above that, you transition to regular steel drill collars for weight. This arrangement ensures that when the MWD’s magnetometers measure the field, the nearest big steel masses are sufficiently far away that their influence is minor.
• Additionally, non-mag subs and other tools may be used: for example, a non-mag stabilizer or a non-mag crossover sub could be included, further extending the magnetically neutral area.
• Not only do they ensure accurate surveys, but non-mag collars maintain the required drilling performance. They act just like normal collars to provide weight on bit (WOB) and stiffness to the BHA. This is critical in directional drilling to control well trajectory – you often need a certain amount of weight and stiffness to maintain angle or build/drop as needed. Non-mag collars allow this without sacrificing survey quality.

In summary, non-magnetic drill collars are essential for housing MWD and LWD tools and allowing them to perform correctlypetrogears.com. Without them, the directional driller would not trust the azimuth readings, making it nearly impossible to steer the well accurately. Non-mag BHA components effectively isolate the instruments from magnetic noise.

Material and Manufacturing Insights

The metallurgical aspect of non-mag tools is quite interesting and is worth highlighting for a technical audience:

• Austenitic Steels: The alloys used (like the aforementioned P530, which is a trade name for a non-mag alloy) are typically austenitic. Austenitic steels are generally non-magnetic (think of common 300-series stainless steels, e.g., 304, 316 – they’re non-magnetic in solution-annealed condition). However, most austenitic stainless by itself is too low in yield strength for drill collar use. So these special alloys add elements like manganese and molybdenum to boost strength. For instance, a typical chemistry might include ~18-22% Cr, 10-20% Ni, 3-5% Mo, plus small amounts of Nb or N to stabilize and strengthen, with controlled C. The result is an alloy that can be hardened via solid solution strengthening and perhaps a bit of cold work.
• Avoiding Magnetic Phases: During manufacturing, it’s crucial to avoid forming any martensite or ferromagnetic phases. Some stainless steels can become slightly magnetic if cold worked (due to strain-induced martensite). Manufacturers control this by heat treatments. Often, non-mag collars are hot-forged or rolled and then solution annealed to remove any residual stress/martensite. Final machining is done carefully to not induce magnetism (e.g., using carbide tools with proper feed/speed to minimize work hardening).
• Mechanical Properties Trade-off: Historically, an issue was that non-mag collars had lower strength than steel, meaning you might need to use more of them (adding weight and length to BHA). But modern non-mag grades have narrowed this gap. As seen in one technical summary: non-mag collars offer strength and hardness comparable to standard steel collars while neutralizing magnetic interferencedrillingtools.com. They meet API spec for mechanical properties, meaning they can handle the high tensile loads and bending stresses of deep drilling.
• Quality Control: Non-mag collars undergo rigorous inspections. This includes ultrasonic testing (to ensure no internal flaws, especially since the alloys are often slightly more expensive and sometimes trickier to heat treat), and magnetic permeability testing – each piece might be scanned with a sensitive gaussmeter. Also, their chemistry and heat treat are tightly controlled because any variance can introduce magnetism. Some manufacturers even have proprietary processes to de-magnetize or verify zero magnetic field after final machining.
• Coatings or Surface Treatments: Generally, non-mag collars are left bare (shiny steel) as in the photo, but sometimes a phosphate or anti-galling coating might be applied on threads, etc. The non-mag material tends to be more galling-prone than 4145 steel, because austenitic stainless can gall. Thus, thread surfaces might be cold rolled or coated to prevent galling during make-up. Molybdenum disulfide based lubricants or special thread compounds (besides standard dope) could be recommended to avoid galling non-mag threads.

One particular note: because the modulus of elasticity of these stainless alloys is slightly lower than steel, non-mag collars may have a bit more stretch or deflection under load. However, in practice the difference is minor in drilling use.

Practical Applications and Use Cases

Beyond just drill collars, “non-magnetic drilling tools” can encompass various BHA components. Let’s go through where and how non-magnetic materials are used in the drilling assembly:

• Non-Mag Drill Collars in BHA: As discussed, any BHA with an MWD tool will include a series of non-mag drill collars. Even in some cases without an MWD, if using certain magnetic survey tools, a non-mag might be used to reduce interference. They are also used in certain fishing or casing alignment applications where a magnet might be used (so non-mag prevents interference with magnetic fishing tools or casing alignment tools).
• Non-Magnetic Subs: There are non-mag crossover subs or pony collars. These are shorter pieces (maybe 1-5 ft) that can connect tools. For example, the MWD tool itself is often contained in a non-mag collar housingwhich is essentially a sub with the MWD inside. Also, when running a mud motor (which typically has a steel housing that can be magnetized from its power section), often a non-mag flex sub is placed between the motor and the MWD to ensure the motor’s magnetism doesn’t affect the tool.
• Measurement While Drilling (MWD) and Logging While Drilling (LWD): All these tools rely on a non-magnetic environment. The non-mag collars ensure accurate measurements of azimuth, as well as some LWD sensors that might use electromagnetic signals. In LWD, for example, nuclear tools or formation evaluation sensors are less affected by magnetics, but they often are run in the same string as MWD so they benefit from the placement of non-mag above.
• Steering Tools (e.g., Magnetic Mud Motors): If a steering tool uses magnetics (some older systems might have magnetostriction sensors or such), non-mag would be essential. In modern rotary steerable systems (RSS), usually the RSS is placed below the MWD, which is in a non-mag. The RSS itself might be largely non-mag as well, depending on its design, to avoid interference.
• Wireline Logging in Drill Pipe: Sometimes, wireline tools are pumped through drill pipe for logging (when open-hole logging is required in high-angle wells). Non-mag drill collars can provide a low-interference zone if any logging tools have magnetometers or if one is doing something like oriented coring, etc.
• Survey & Collision Avoidance: In complex wells (like multi-well pads or offshore with many wellbores), knowing exact wellbore position is critical to avoid collisions. The high accuracy surveys enabled by non-mag BHA components help in collision avoidance planning (since you reduce the uncertainty in well position by having good surveys). The better the magnetic environment (using non-mag), the smaller the positional uncertainty of the well path.

From a drilling performance perspective, non-mag collars also contribute to directional control. They behave like normal drill collars in terms of stiffening the BHA. This is important: for instance, in a directional BHA, you might have a bent motor with some flexible collars to build angle, and then stiff collars to lock in the angle. Non-mag collars can be part of that design (some companies might choose to place one non-mag collar near the bit even if not strictly needed for MWD, just to have some weight and also reduce magnetic interference for some near-bit measurement devices).

Non-mag tools are also critical in special applications like relief well drilling – you might run additional non-mag in a relief well if using magnetic ranging tools (which deliberately detect magnetic fields from another well). In such scenarios, one might actually temporarily magnetize a section of the target well’s casing and use a magnetic sensor in the relief well to find it. The relief well’s BHA might include non-mag sections to not obscure the target’s signal.

Benefits of Non-Magnetic Drill Collars

To summarize the advantages that non-mag drilling collars provide:

• Accurate Directional Measurements: By far the biggest benefit, as repeated, is the accuracy of MWD surveys. Non-mag collars ensure reliable and precise data from downhole survey instruments, which allows the drilling team to hit target reservoirs with confidencepetrogears.com. This precision can lead to more efficient drilling (less time spent orienting or re-drilling due to uncertainty) and better wellbore placement (crucial in reservoir navigation).
• Safety and Risk Reduction: Using non-mag tools reduces the risk of directional drilling problems. If one were to drill without them, the uncertainty in well path could cause unplanned collisions or the need for extra sidetracks. Also, proper directional control prevents issues like wellbore collision or exiting the desired drilling window, which can be safety hazards. Moreover, maintaining well integrity in risky zones (like in high-pressure wells or in relief well scenarios) often depends on accurate drilling – non-mag tools indirectly contribute to safety by enabling that accuracygalipequipment.com.
• Maintaining BHA Function: Unlike other solutions to reduce interference (e.g., spacing out tools or using small drill pipe), non-mag collars do not compromise the mechanical function of the BHA. They still provide the required weight on bit and stiffness, so drilling performance (rate of penetration, ability to transmit weight and rotary power to the bit) is maintained. In other words, they solve the magnetic problem without creating a drilling problem.
• Durability in Harsh Conditions: Non-mag materials are inherently corrosion-resistant. In high-pressure, high-temperature (HPHT) wells, or wells with sour gas (H₂S), non-mag collars can sometimes outperform normal steel collars which might suffer hydrogen embrittlement or corrosion. The non-mag won’t prevent all such issues, but their alloys are generally less prone to sulfide stress cracking (especially with the shot-peened ID as mentioned)drillingtools.com. This means in extremely harsh downhole environments, non-mag collars may have a longer service life, providing an added reliability factor.
• Magnetic Ranging and Specialized Ops: In cases where active magnetic ranging is used (e.g., intersecting a blown-out well, or pinpointing a fish with a magnet), having a non-mag assembly can make those operations feasible. The isolation of magnetic components means any intentional magnetic signal or detection is clearer.

Operational Tips and Best Practices for Non-Mag Tools

Oilfield professionals (drilling engineers, tool pushers, directional drillers, and QC inspectors) should consider the following when working with non-magnetic drilling tools:

• Avoid Accidental Magnetization: Non-mag does not mean it can never become magnetic. Improper handling can induce magnetism. For example, dropping a non-mag collar or striking it with a hammer can introduce residual magnetism (mechanical shock can align domains in the metal). Likewise, welding on a non-mag component or exposing it to strong magnetic fields (like setting it next to a large electric motor) can magnetize it. Always handle non-mag collars with care – use non-sparking brass hammers or rubber mallets if needed, and avoid placing them near generators or other electromagnets. Many companies will routinely demagnetize (degauss) their non-mag collars after each job as a precaution.
• Inspection for Magnetism: Include a magnetism check in the BHA inspection procedure. A simple tool is a hand-held magnetic compass or a field gaussmeter. Run it along the collar – the needle should not deflect significantly. If any hot spots are found, the collar should be degaussed or replaced. It’s particularly wise to check the threaded connections; sometimes thread dressing or repair can induce slight magnetism. Some directional drillers carry a pocket compass and will personally check the non-mag collars on the rig floor before running in hole – a quick sanity check that could save a lot of headache later.
• Thread Galling Precautions: As mentioned, non-mag material can gall, especially when making up to steel connections (e.g., a non-mag collar pin into a steel collar box). Use a good thread lubricant (preferably the one recommended by the non-mag tool manufacturer). Ensure threads are clean and free of debris. It might be beneficial to use lower make-up speed when making up non-mag to steel to prevent heat/galling. Some operators apply a silver plating or coating on non-mag threads to minimize galling.
• Proper Placement in BHA: Work with directional drilling engineers to calculate the required number and placement of non-mag collars. More is not always better because too many non-mag collars (which are usually heavier and less flexible) can alter the drill string dynamics. The goal is to have enough length to protect the instruments. Use established formulas or software from directional tool providers to get the needed length. Ensure that if you have any magnetic tool below the non-mag section (like some mud motors can have residual magnetism), that distance is accounted for as well.
• Handling and Storage: Mark non-mag collars clearly (usually they are painted or engraved with “Non-Mag”). Store them separately from regular drill collars to avoid confusion. Because they are stainless, they might be a bit more susceptible to surface damage (they don’t have the same oxide layer as a phosphate-coated steel collar), so apply thread protectors and perhaps a light oil coating for storage to prevent any surface rust (some non-mag alloys can still get superficial rust if contaminated with carbon steel grinding dust, for example).
• Documentation and Traceability: Keep material certificates and service history for non-mag tools. Because the material is unique, you want to ensure that any repair welding or heat treating is done by qualified facilities that understand non-mag requirements. If a non-mag collar is machined or re-threaded by a shop, verify that the shop has procedures to maintain non-mag properties (some shops inadvertently can ruin a non-mag by improper heat treat). After any major repair, re-certify the collar’s magnetic permeability. Quality assurance is key – many failures of non-mag tools historically come not from the concept, but from improper maintenance or undocumented mix-ups (e.g., an incorrect steel sub mistakenly used in place of non-mag).
• Utilize Manufacturer Guidance: Different non-mag alloy grades might have specific guidelines (for example, one grade might require a different thread compound if it’s more prone to galling; another might have a torque correction factor because of different friction). Always refer to the manufacturer’s technical data. Some provide recommended maximum makeup torques specifically for non-mag connections, or limits on re-cutting threads.
• Monitor MWD Data for Clues: Interestingly, one can sometimes detect if a non-mag is insufficient via MWD data itself. If while drilling, the MWD’s magnetometer readings seem erratic or have high dip angle variations, it could be a sign of magnetic interference (meaning either the non-mag is magnetized or not enough non-mag length was used for the latitude). In such cases, consider pulling out to check the non-mag. It’s costly, but less costly than steering the well wrong. Modern MWD tools also often have quality indicators for their measurements – if those indicate magnetic distortions, take it seriously.
• Leverage the Advantage for Complex Wells: Non-mag tools are sometimes optional in vertical wells (with no MWD, or if using gyro surveys). But in any complex well (long horizontal, multiwell pad), always plan for non-mag usage. They enhance the quality of directional control, which in the end saves time and prevents collisions. It’s a best practice in such scenarios to not cut corners by omitting non-mag subs; the short-term saving is not worth the risk.

In conclusion, non-magnetic drilling collars and related tools are indispensable in modern directional drilling. They ensure that the technological marvels we send downhole (MWD/LWD tools) can do their job accurately by providing a magnetically “quiet” environmentpetrogears.com. The use of non-mag tools reflects a trade-off: slightly more expensive and specialized equipment in exchange for vastly improved drilling intelligence and safety. For oil and gas operators and drilling contractors, this trade-off is overwhelmingly worth it – almost every directional well today relies on non-mag components at its core. By understanding the technical nuances of these tools and handling them with proper care, drilling teams can significantly improve their well placement precision and avoid many pitfalls associated with magnetic interference. Non-mag drilling tools exemplify how material science and drilling engineering come together to solve practical problems, enabling the feats of precise directional drilling that have become routine in the industry.