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Detailed Explanation of Non-Magnetic Drill Collar Technology: Materials, Magnetic Properties, and MWD Applications
Author:
WHH
Source:
LONGWAY
Published:
2026-09-23
In directional drilling, MWD (Measurement While Drilling) and LWD (Logging While Drilling) tools rely on magnetometers to measure wellbore azimuth and toolface angle. If magnetic interference is present in the drill string assembly, the measurement data can become distorted, leading to deviations of the wellbore trajectory from the design. Non‑magnetic drill collars are specially engineered drill collars designed to address this issue.
This paper systematically reviews the material requirements, magnetic performance specifications, structural features, MWD application selection criteria, and key procurement and acceptance considerations for non-magnetic drill collars, providing technical guidance for directional drilling operations.
I. Why is it necessary? Non-magnetic drill collar ？
The magnetometer in MWD tools measures the direction of the Earth’s magnetic field. If the drill collars in the drill string are magnetic, they generate an additional magnetic field that can interfere with the magnetometer readings, resulting in:
Azimuth measurement error
The tool face angle is inaccurate.
Wellbore trajectory loss of control
Increased costs for targeted correction and adjustment
The function of a non-magnetic drill collar is to provide a non-magnetic environment around the MWD tools, ensuring that the magnetometer measures only the Earth’s magnetic field and is not affected by the magnetic properties of the drilling equipment.
Key principle: Non-magnetic drill collars must be positioned above and below the MWD tool, with a length sufficient to isolate the magnetometer from interference caused by other magnetic components in the drill string, such as standard drill collars and drill pipe joints.
II. Material Requirements: N1310 Low-Carbon Alloy Steel
Non-magnetic drill collars are manufactured from N1310 low-carbon alloy steel, an austenitic stainless steel specifically developed for non-magnetic applications.
| Performance Metrics | Requirements |
| Material Type | N1310 Low-Carbon Alloy Steel (Austenitic) |
| Permeability (μr) | ≤1.005 |
| Tensile strength | ≥970 MPa |
| Yield strength | ≥830 MPa |
| Elongation | ≥13% |
| Brinell hardness | 285-341 HB |
| Impact energy (Akv) | ≥40 ft-lbs |
The chemical composition of N1310 is characterized by high chromium (Cr), high nickel (Ni), and high manganese (Mn), with low carbon (C), and it incorporates nitrogen (N) for solid-solution strengthening. This compositional design ensures that the material maintains non-magnetic properties while exhibiting adequate strength and toughness.
III. Magnetic Property Requirements and Testing
The key performance indicator of a non-magnetic drill collar is its magnetic permeability. The lower the magnetic permeability, the less it is susceptible to external magnetic field interference.
| Test Items | Technical Requirements | Detection method |
| Permeability (μr) | ≤1.005 | Permeability tester |
| Magnetic flux density | Complies with API 7-1 requirements | Fluxgate magnetometer |
| Remanence | As low as possible | Gaussmeter |
Inspection Location: Magnetic property testing shall be conducted along the entire length of the drill collar, with particular attention to the threaded sections at both ends and the intermediate section. Any localized deviation of magnetic permeability from the specified limit may result in measurement errors.
API standard requirements: API Spec 7-1 stipulates that the average magnetic permeability μr of non-magnetic drill collars shall be ≤ 1.005. In addition, the material must undergo solution treatment to ensure a stable austenitic microstructure.
IV. Structural Features and Dimensions
The structure of a non-magnetic drill collar is identical to that of a conventional drill collar, consisting of a thick-walled round tube with API‑standard threads machined at both ends. The primary differences lie in the material and magnetic properties.
| Parameter | Typical range |
| Outer Diameter (OD) | 79.4mm - 279.4mm (3-1/8″ - 11″) |
| Inner diameter (ID) | Determined according to the outer diameter and wall thickness. |
| Length | Typically available in lengths of 3 m, 6 m, or 9 m, or customized to customer specifications. |
| Thread type | NC, REG, IF, FH series |
| Thread processing | Phosphating or cold rolling |
The wall thickness of a non-magnetic drill collar is typically slightly greater than that of a standard drill collar with the same outer diameter, to compensate for the relatively lower yield strength of N1310 material and ensure that the overall load-carrying capacity meets the required specifications.
V. Key Considerations for Selecting MWD Applications
1. Length Selection
The length of a non-magnetic drill collar shall meet two requirements:
Isolation of magnetic interference: Ensuring that the MWD magnetometer is unaffected by the magnetic fields generated by the drill string above and below it.
Provides sufficient weight: As part of the drill collar section, it helps to apply drilling pressure.
Rule of thumb: The total length of a non-magnetic drill collar is typically 6–9 meters (2–3 sections), depending on the location of magnetic components in the drill string and the sensitivity of the MWD tools.
2. Placement Location
Non-magnetic drill collars should be positioned directly above and below the MWD tool, forming a symmetrical non-magnetic section. The MWD tool itself should be located at the midpoint of the non-magnetic drill collar section.
Typical BHA configuration (from top to bottom):
Drill pipe → Standard drill collar → Non-magnetic drill collar (upper) → MWD tool → Non-magnetic drill collar (lower) → Drill bit
3. Compatibility with standard drill collars
Non-magnetic drill collars cannot fully replace conventional drill collars—their primary function is to provide a non-magnetic environment, not to generate the main drilling weight. In BHA design, conventional drill collars supply the primary drilling weight, while non-magnetic drill collars furnish the localized non-magnetic section required for MWD.
VI. Key Points for Procurement and Acceptance
When procuring non-magnetic drill collars, the following requirements should be clearly specified in the contract:
| Acceptance Items | Technical Requirements | Verification method |
| Materials | N1310 low-carbon alloy steel | MTC Chemical Composition Report |
| Permeability | ≤1.005 | Permeability Test Report |
| Mechanical properties | Tensile strength ≥ 970 MPa, yield strength ≥ 830 MPa | MTC Mechanical Properties Report |
| Hardness | 285-341 HB | Hardness Test Report |
| Thread | API standard, phosphating/cold rolling | Thread gauge inspection |
| Nondestructive testing | UT+MT, free of internal defects and surface cracks | Test Report |
| Traceability | The heat number is consistent with the MTC. | Verify the furnace number. |
| Third-party inspection | Supports on-site inspections by SGS/BV. | As per project requirements |
Special reminder: Magnetic property testing of non-magnetic drill collars shall be conducted on the finished product, not merely on the raw material. The manufacturing and heat‑treatment processes may affect the final magnetic properties.
VII. Frequently Asked Questions (FAQ)
Q1: What is the difference between a non-magnetic drill collar and a standard drill collar?
The primary difference lies in the material. Standard drill collars are made from AISI 4145H magnetic alloy steel, while non-magnetic drill collars are manufactured from N1310 non-magnetic alloy steel. Their structure, dimensions, and threaded connections are essentially identical.
Q2: Can a non-magnetic drill collar be used on its own?
No. It must be used in conjunction with MWD/LWD tools, positioned on the upper and lower sides of the tool to create a non-magnetic section.
Q3: What is the standard permeability of a non-magnetic drill collar?
API 7-1 specifies that the average permeability μr shall be ≤ 1.005.
Q4: What testing items are required for non-magnetic drill collars?
Material chemical composition, magnetic permeability, mechanical properties (tensile strength, yield strength, elongation, impact energy), hardness, thread geometry, and nondestructive testing.
Q5: Can a non-magnetic drill collar be repaired?
The threads can be re-cut or refaced, but the magnetic properties must be re‑tested afterward. When the tube body is severely damaged, it is typically scrapped.
VIII. Summary
| Key points | Core content |
| Core Features | Provide a non-magnetic environment for MWD/LWD to prevent magnetic interference. |
| Materials | N1310 low-carbon alloy steel, permeability ≤ 1.005 |
| Mechanical properties | Tensile strength ≥ 970 MPa, yield strength ≥ 830 MPa, hardness 285–341 HB |
| Structure | Same as a standard drill collar: thick-walled round tube with API threads. |
| Application | Placed on the upper and lower sides of the MWD tool, with a total length of 6–9 meters. |
| Procurement and Acceptance | Permeability test report, mechanical properties, nondestructive testing, traceability |
Non-magnetic drill collars are indispensable key components in directional drilling. Proper selection, rigorous inspection, and standardized operation are essential to ensuring the accuracy of MWD measurements and effective wellbore trajectory control.
Do you need to procure non-magnetic drill collars for your directional drilling project?
Contact Hebei Longway Petroleum Equipment Co., Ltd.
Obtain professional technical consulting
Hebei Longway supplies non-magnetic drill collars that comply with API 7‑1 standards, manufactured from N1310 low‑carbon alloy steel with a magnetic permeability of ≤1.005, and supported by third‑party inspection.
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