What is a Drill Collar?

What is a Drill Collar?

The drill collar is located at the bottom of the drill string and is the main component of the lower drill bit assembly. It is a thick-walled, high-strength, heavy-weight seamless steel pipe located at the bottom of the drill string and above the drill bit. Its main features are large wall thickness (generally 45~75mm, equivalent to 4~8 times the wall thickness of the drill pipe), large gravity and rigidity. In order to facilitate the drilling work, the outer surface of the internal thread of the drill collar can be processed with hanging grooves and slip grooves.

Functions of Drill Collar:

(1) Apply drilling pressure to the drill bit.
This is the most basic function. The huge weight of the drill collar itself provides the pressure required for the drill bit to break the rock downward. Without the drill collar, the drill string is too light and cannot be drilled effectively.

(2) Ensure the necessary strength under compression conditions.

(3) Reduce the vibration, swing, and jump of the drill bit, reduce fatigue damage, and make the drill bit work smoothly. 

The drill collar concentrates the complex bending stress at the bottom, protecting the relatively weak drill pipe at the top and extending the service life of the entire drill string.

(4) Maintaining the wellbore trajectory and controlling the well inclination.
Due to the high rigidity and heavy weight of the drill collar, it can effectively overcome the natural deflection force of the formation, and like a "ballast", it helps the drill string to remain vertical or drill at the designed angle, preventing the wellbore from being excessively bent.

Classification of Drill Collars:

According to their function and structure, drill collars are generally divided into three types: integral drill collars (smooth drill collars), spiral drill collars, and non-magnetic drill collars.

(1) Integral drill collars are smooth, thick-walled round tubes with connecting threads machined at both ends. This is the most basic model.

(2) Spiral drill collars have three right-hand spiral grooves machined on the outer cylindrical surface of the round drill collar to reduce the contact area with the well wall, which can effectively prevent pressure differential jamming. Drilling fluid is also easier to pass through, and it is currently the most commonly used type.

(3) Non-magnetic drill collars are mainly used for monitoring during oil drilling. Their structure is the same as that of integral drill collars. Non-magnetic drill collars are made of special non-magnetic materials (such as monel alloy, stainless steel), which are forged after rigorous chemical composition analysis. The material has been tested for mechanical properties to ensure that the hardness, toughness, impact value and corrosion resistance meet the standards. It has good low magnetic permeability and good machinability.

Drill Collar Materials:

Drill collars need to withstand huge pressure, torque, bending stress and corrosion from well fluids, so the requirements for materials are extremely high. They are mainly divided into the following two categories:

1. Conventional steel drill collars

Material standards:
Usually, alloy steels specified in the API SPEC 7-1 standard of the American Petroleum Institute are used. This is the most common and widely used type.

Common steel grades:
4140 steel: This is the most mainstream material, a chromium-molybdenum alloy steel. It has high strength, good toughness and excellent fatigue resistance, and is very cost-effective.
4145H Steel: Based on 4140, it has increased carbon and manganese content for higher strength and is commonly used to manufacture larger drill collars.

Heat Treatment: These steels require a tempering heat treatment (quenching followed by high-temperature tempering) to achieve uniform mechanical properties and sufficient hardness to resist wear and corrosion downhole.

2. Non-Magnetic Drill Collars

Used to accommodate measurement-while-drilling instruments, non-magnetic materials must be used to avoid interfering with magnetometer readings.

Material Types:
Monel: A nickel-copper alloy, the earliest non-magnetic material used, offers excellent corrosion resistance but is very expensive.
Stainless Steel: Austenitic stainless steel, such as Cr-Ni-Mn-N, is currently the most popular non-magnetic drill collar material. It offers excellent non-magnetic properties, sufficient strength, and a lower cost than monel.
Non-Magnetic Composite Alloys: Some manufacturers have also developed specialized high-nickel-chromium non-magnetic composite alloys.

Common Drill Collar Sizes:

Drill collar size is primarily determined by its outer diameter and inner diameter (waterhole), and its selection must match the wellbore size.

1. Outer Diameter Range

Drill collar outer diameters range widely to accommodate various requirements, from small to large wellbores. 

Common outer diameters range from 3-1/8 inches to 12-1/4 inches.

Most commonly used sizes: 6-1/4" (158.8 mm), 6-1/2" (165.1 mm), 7" (177.8 mm), 8" (203.2 mm), 9" (228.6 mm), and 9-1/2" (241.3 mm).

Selection Principle: The outer diameter of the drill collar should be close to, but smaller than, the inner diameter of the preceding casing and the drill bit size being used. It is typically 1/4" to 1/2" smaller than the drill bit diameter to ensure smooth tripping and provide an adequate return path for the drilling fluid.

2. Inner Diameter
The inner diameter of a drill collar is much smaller than its outer diameter, which contributes to its high weight.
Features: The inner diameter is typically small and consistent to ensure structural strength.
Purpose: A smaller inner diameter increases the flow rate of drilling fluid within the drill collar, aiding in the transport of cuttings, but also increases pumping pressure.

3. Length
A single drill collar is typically around 9.1 meters (30 feet) or 9.5 meters (31 feet) in length. In practice, multiple drill collars are connected to form a "drill collar string" to meet the required total weight and rigidity.

4. Connecting Thread

Typically NC or FH series.

Drill Collar Working Positions and Combinations:

Drill collars are not used individually; rather, multiple drill collars are connected together to form a "drill collar string." It is located at the bottom of the drill string:

Drill bit → Drill collars (several to dozens) →While-drilling tools (such as jars, MWD) → Drill pipe

A typical drill collar set can weigh tens to hundreds of tons, providing sufficient weight on bit (WOB) for the drill bit.

Typical Applications of Drill Collars:

1. All drilling operations requiring mechanical rock breaking. - Providing WOB
2. Vertical well deflection prevention and directional/horizontal well control. - Controlling wellbore trajectory
3. Any well section with alternating stresses and bending. - Reducing drill pipe fatigue and damage
4. Wells using measurement while drilling (MWD) and logging while drilling (LWD) tools. - Ensuring a safe working environment for downhole tools (no magnetic interference)
5. Drilling in steep formations, faults, and other easily deflected formations; drilling in hard, abrasive formations. - Managing complex working conditions

Conclusion:

Drill collars are an indispensable and heavyweight tool in drilling operations. It uses its own weight to power the drill bit and its rigidity to ensure wellbore quality, making it a critical piece of equipment for efficient, safe, and designed drilling. Conventional 4140/4145H alloy steel offers a perfect balance of strength and cost-effectiveness, while non-magnetic stainless steel addresses the critical issue of downhole measurement. The size selection strictly adheres to the wellbore structural design, ensuring that the drill collar can fulfill its core role of providing weight on bit and stabilizing the wellbore within the specified wellbore.

Read more: Difference between Drill Pipe and Drill Collar or Drill Pipe Connection Method