Drill Collar: Definition, Weight on Bit, and BHA Design

What Is a Drill Collar?

A drill collar provides the weight on bit (WOB) and compressive stiffness needed to keep the bottom hole assembly on trajectory by placing the heavy, thick-walled pipe section directly above the bit in compression, preventing the lighter drill pipe above from buckling, and supplying the axial force that drives the cutting structure into the formation, manufactured to API Specification 7-2 with outside diameters from 79 mm (3-1/8 in) to 279 mm (11 in).

Key Takeaways

Drill collars function in compression rather than tension like drill pipe, using their weight to apply force to the bit while the drill pipe above remains in tension to prevent buckling and string failures.
Standard drill collar unit weights range from 29 kg/m (19.5 lb/ft) for a 79 mm (3-1/8 in) OD collar to 296 kg/m (199 lb/ft) for a 279 mm (11 in) OD collar; typical WOB applied through drill collars runs 44-222 kN (10,000-50,000 lb).
Drilling engineers, directional drillers, and BHA designers specify drill collar dimensions; drilling contractors supply the collars; company representatives and regulators verify BHA design compliance with approved well programs.
API Specification 7-2 governs drill collar manufacturing in North America and internationally; NORSOK D-001 in Norway and AER Directive 059 in Canada incorporate API 7-2 requirements by reference for drilling program review.
Nonmagnetic (monel) drill collars placed around MWD and LWD tools prevent the magnetic steel collar body from corrupting the magnetometer readings used for directional surveying.

How Drill Collars Work

The drill collar's fundamental mechanical function exploits a basic structural principle: a column of pipe running in tension does not buckle, while a column running in compression beyond its critical load will buckle, contact the wellbore wall, and cause deviation from the intended trajectory. Drill pipe is relatively thin-walled and will buckle sinusoidally, then helically, at compressive loads well below what is needed to drive a bit into rock. Drill collars, with their thick walls and heavy cross-sectional areas, have a much higher critical buckling load and can safely carry the WOB in compression. The neutral point, where the string transitions from compression below to tension above, should always fall within the drill collar section, never in the drill pipe, to protect the lighter pipe from cyclic bending fatigue.

Wall thickness is the defining characteristic of a drill collar compared to drill pipe. A 6-5/8 in (168 mm) OD drill collar has a typical ID (inner diameter) of 2-13/16 in (71 mm), giving a wall thickness of approximately 48 mm (1.9 in). A 6-5/8 in OD drill pipe joint has a wall thickness of only 9-11 mm (0.35-0.43 in). This massive wall thickness gives the drill collar its weight per foot, its stiffness, and its resistance to the compressive fatigue forces generated by WOB cycles, vibration, and tool rotation. API Specification 7-2 sets out the dimensional tolerances, minimum wall thickness requirements, and straightness standards for drill collars, requiring a maximum bow of 0.5 mm/m (0.006 in/ft) over the entire collar length for standard service.

Drill collars are manufactured in standard lengths of 9.14 m (30 ft) for slick (non-spiral) collars and 9.5 m (31.2 ft) for spiral collars. Spiral grooves machined into the collar OD reduce the contact area between collar and wellbore wall, lowering differential sticking risk in permeable formations. The spiral reduces OD contact by approximately 35 percent compared to a slick collar of the same OD, significantly reducing the force required to free a differentially stuck string. API 7-2 specifies allowable spiral profiles, groove depth, and width to ensure the structural integrity of the collar body is not compromised by the grooving.

Drill Collar Across International Jurisdictions

In Canada's Deep Basin and Montney plays of Alberta, BHA designs for 4,500-6,000 m (14,764-19,685 ft) wells typically use five to eight 8 in (203 mm) OD drill collars providing 90,000-140,000 lb (400-623 kN) of available WOB, plus two nonmagnetic monel collars surrounding the MWD/LWD tools. Precision Drilling and Ensign Energy Services maintain drill collar inventories at their major Canadian rig yards. AER Directive 059 requires that the BHA string description including collar dimensions appear in the approved well program before spud, and that any changes to the BHA during drilling be documented in the morning report and the AER-format daily drilling log.

In the US Permian Basin, WOB requirements for drilling through hard Permian carbonates and cemented sandstones run toward the high end of the typical range: 30,000-50,000 lb (133-222 kN) for PDC bits in the Bone Spring and Wolfcamp formations. Pioneer Natural Resources (now ExxonMobil), Devon Energy, and ConocoPhillips design BHAs with 8-12 drill collars per run. BSEE requires that offshore BHA descriptions including collar sizes and weights appear in the well permit application and that deviations from the permitted BHA be reported within 24 hours.

On Norway's Continental Shelf, Equinor and Aker BP operate under NORSOK D-001, which requires that BHA design documents be prepared by a certified drilling engineer and approved before each well phase. Drill collar procurement for North Sea operations follows both API 7-2 and NORSOK M-120 material standards for high-strength drill string components. The Johan Sverdrup platform wells use BHAs with 7 in (178 mm) OD drill collars in the 8-1/2 in (216 mm) hole section and 4-3/4 in (121 mm) OD collars in the 6 in (152 mm) reservoir section, all nonmagnetic to protect MWD accuracy in the deviated wellbore trajectories required by the platform slot pattern.

Saudi Aramco's high-volume drilling program for Ghawar and Shaybah fields uses standardized BHA designs approved through the company's Drilling Engineering department. Monel drill collars for MWD tools are provided by Saudi Aramco's in-house drill string inventory, the largest single operator inventory in the world. ADNOC Drilling in Abu Dhabi specifies collar sizes in its contractor rig packages, with 6-3/4 in (171 mm) collars standard for 8-1/2 in (216 mm) hole sections in the Arab Formation carbonates of the Zakum and Asab fields.

Fast Facts

A single 9.14 m (30 ft) long, 8 in (203 mm) OD drill collar weighs approximately 1,600 kg (3,527 lb) in air and is machined from a single billet of 4145H modified chromium-molybdenum alloy steel heat-treated to yield strength of 120,000 PSI (827 MPa), making it one of the most metallurgically demanding and precisely toleranced components in the entire drill string.

Drill Collar Types, Grades, and Nonmagnetic Specifications

Slick drill collars are the standard type: a plain cylindrical body with no external features except the threaded connection boxes at each end. They are used in the majority of BHA configurations below the MWD/LWD tool section. Spiral drill collars, as described above, add machined grooves to reduce differential sticking risk. Monel (nonmagnetic) drill collars are manufactured from Monel K-500, a nickel-copper alloy with very low magnetic permeability (typically less than 1.01 relative permeability versus 100+ for carbon steel). MWD directional sensors are placed inside monel collars to prevent the magnetic field of the steel collar body from biasing the magnetometer readings used to compute azimuth.

The minimum nonmagnetic spacing required around an MWD magnetometer depends on the local magnetic inclination and the accuracy requirement for the survey. In high-latitude regions of Canada and Norway where the magnetic dip angle is steep, longer nonmagnetic spacing is required: typically 24 m (79 ft) or more of monel collars surrounding the sensor. At lower latitudes in the Middle East, 12 m (39 ft) of nonmagnetic spacing may be sufficient. Directional drilling service companies including SLB, Halliburton, and Baker Hughes provide nonmagnetic spacing calculations for each well based on the local magnetic field parameters, well profile, and required survey accuracy.

OD and ID tolerances for API 7-2 drill collars are tightly controlled. OD tolerance is +0.79 mm / -0.79 mm (+1/32 in / -1/32 in) of nominal. ID tolerance is +1.59 mm / -0 mm (+1/16 in / -0) of nominal. These tolerances ensure compatibility with drill pipe connections and BOP ram sizing. The bore ID must be sufficient to pass the expected mud flow rates without excessive annular velocity losses; typical IDs of 2-13/16 in (71 mm) allow flow rates of 600-2,500 L/min (160-660 gpm) for 6-3/4 in (171 mm) to 9-1/2 in (241 mm) hole sizes. Connections are API NC38, NC46, NC50, 6-5/8 FH, or other standard API thread profiles matching the drill pipe connections used in the well program.

Drill collar condition monitoring follows API RP 7G guidelines. Visual inspection checks for corrosion pitting, fishing neck damage, and thread wear. Dimensional inspection measures OD wear (allowing up to 5 percent OD reduction) and ID enlargement. Magnetic particle inspection or dye penetrant testing checks for fatigue cracks at the stress concentration points: the pin base, box shoulder, and any spiral groove termination. Collars with cracks or OD wear exceeding 5 percent are downgraded or retired. A typical drill collar has a service life of 3,000-8,000 rotating hours depending on application severity.

Tip: When selecting drill collar OD for a new well program, size the collar to 75-85 percent of the bit OD to ensure adequate annular clearance for cuttings transport while maximizing available weight per foot. An 8-1/2 in (216 mm) hole section works optimally with 6-3/4 in (171 mm) collars (79 percent of bit OD), which provide 79 kg/m (53 lb/ft) unit weight. Investors reviewing drilling programs should note that operators using undersized collars in hard formations often struggle with low ROP and bit damage, directly inflating cost per foot metrics.

DC: Drill collar, the standard field abbreviation used in BHA diagrams, morning reports, and well cost tracking spreadsheets.
Monel collar: A nonmagnetic drill collar manufactured from Monel K-500 nickel-copper alloy; sometimes called a "non-mag collar" or simply "non-mag" in the field.
Slick collar: A plain-OD drill collar without spiral grooves; distinguishes it from spiral drill collars in BHA specification documents.
Heavy-wall drill pipe (HWDP): A drill pipe product with wall thickness intermediate between standard drill pipe and drill collars; used as a transition element between the drill collar section and standard drill pipe, particularly in directional wells where reducing WOB oscillations is important.

Related terms: BHA, MWD, LWD, rotary table, kelly, directional drilling, well control

Frequently Asked Questions

What is a drill collar and what does it do?

A drill collar is a thick-walled, heavy steel pipe placed in the bottom hole assembly directly above the drill bit. Its primary purpose is to provide weight on bit (WOB), the compressive downward force needed to push the bit into the formation at the desired penetration rate. Its secondary purpose is to keep the BHA in compression so the lighter drill pipe above stays in tension and does not buckle. Without drill collars, drilling through hard rock formations would be impractical because standard drill pipe would buckle long before generating sufficient WOB.

How much weight do drill collars provide?

Drill collar weight per unit length depends on the OD, ID, and steel density (approximately 7,850 kg/m3 or 490 lb/ft3). A 6-3/4 in (171 mm) OD collar weighs approximately 79 kg/m (53 lb/ft) in air and about 68 kg/m (46 lb/ft) in 1.5 SG (12.5 ppg) drilling fluid due to buoyancy. A typical BHA string of eight such collars in a 9.14 m (30 ft) length provides approximately 4,960 kg (10,936 lb) of available WOB at the bit, well within the 10,000-50,000 lb (44-222 kN) range typical for most PDC and roller-cone bit programs.

Why are nonmagnetic drill collars used near MWD tools?

MWD directional tools contain magnetometers that measure the Earth's magnetic field to calculate the drill string's azimuth (compass direction). If standard steel drill collars surround the magnetometers, the strong magnetic field of the steel corrupts the readings. Nonmagnetic collars made from Monel K-500 or other low-permeability alloys have negligible magnetic properties, allowing the magnetometer to measure only the Earth's true field. The required length of nonmagnetic spacing depends on the survey accuracy needed and the local magnetic field inclination angle.