Drill Collars Information

Figure 1: Drill collars are used in the drilling industry, serving as a component of the bottom hole assembly (BHA). Source: Public domain

Drill collars are used in the drilling industry, serving as a component of the bottom hole assembly (BHA). These elements are distinguishable from drill pipes due to their thicker walls, heavier weight, and increased rigidity. The primary function of drill collars is to provide weight on the drill bit, aiding in the drilling process and mitigating the effects of vibration and impact forces encountered during drilling operations.

Manufactured predominantly from solid carbon steel bar stock, drill collars are designed for durability and efficiency. Their structure includes a hollow center, running from one end to the other, and features a tool joint at each end — one with male threads and the other with female threads. The outer surface of drill collars varies; some have a smooth, slightly rounded finish known as slick drill collars, while others feature helical grooves, known as spiral grooves, to reduce sticking in the well bore.

In addition to traditional steel drill collars, non-magnetic varieties are also available. These are typically made from nickel-copper alloys or other nonmagnetic premium materials. Their primary use is in environments where it is essential to isolate measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools from magnetic interference.

Drill collars are significantly heavier than drill pipes. In simpler BHA configurations, their primary purpose is to exert force on the drill bit. However, in more complex setups, drill collars work in conjunction with heavy-weight drill pipes (HWDP), not only providing force to the drill bit but also aiding in directional control of the drilling operation.

The production and use of drill collars are guided by various standards, including GOST 22634, which outlines the basic dimensions and tolerances for connections of drill pipes with stabilizing collars and tool joints.

Configurations

Figure 2: Drill collars. Source: Joshua Doubek/CC BY-SA 3.0 DEED

Drill collars come in various configurations, each designed to meet specific needs in the drilling process. The choice of a particular configuration depends on factors such as the drilling environment, desired drilling direction, depth of the drilling operation, and the geological conditions. Each configuration offers distinct advantages and disadvantages and is suited for different operational scenarios. Understanding these configurations is crucial for optimal drilling efficiency and safety.

Slick Drill Collars

Slick drill collars are characterized by their smooth exterior. They are primarily used in straightforward drilling operations where the minimal deviation is required. The significance of this configuration lies in its simplicity and effectiveness in standard drilling conditions. Its smooth surface minimizes the risk of differential sticking, a common challenge in drilling. While highly effective in stable, less complex geological formations, slick drill collars are less suited for directional drilling or in highly deviated wells, where more specialized configurations are necessary.

Spiral Groove Drill Collars

Spiral groove drill collars feature helical grooves on their exterior, a design that serves multiple purposes. These grooves reduce the surface area in contact with the wellbore, thereby lowering the risk of differential sticking, a critical consideration in more complex or unstable geological formations. This configuration is especially beneficial in directional drilling operations, where the risk of sticking is heightened. While they offer improved operational efficiency in challenging environments, spiral groove drill collars can be more prone to wear and may require more careful handling and maintenance.

Non-Magnetic Drill Collars

Non-magnetic drill collars, made from materials like nickel-copper alloys, are essential in operations where magnetic interference can compromise the accuracy of downhole measurements. These collars are critical in environments where precise MWD and LWD are necessary. The non-magnetic property ensures that the data collected by sensitive instruments is accurate and reliable. However, the specialized material and manufacturing process make these collars more expensive and they are typically reserved for operations where their unique properties are a necessity.

HWDP

Although not traditional drill collars, HWDPs often complement drill collars in complex bottom-hole assemblies. HWDPs are intermediate-weight pipes that provide a gradual transition between the heavy drill collars and the lighter drill pipe. This transition is important in managing the stresses and strains in the drill string, especially in directional drilling. They offer better control and reduce the likelihood of fatigue failures in the drill string. However, integrating HWDP into the drill string adds complexity and requires careful planning and design.

Materials

The materials used in the manufacture of drill collars are varied, with each material bringing its unique properties and suitability to different drilling environments. The selection of material significantly influences the performance, durability, and efficiency of drill collars in various drilling conditions.

Carbon Steel

Figure 3: Normalized carbon steel microscope view. Source: DoITPoMS, University of Cambridge/CC BY-NC-SA 2.0 DEED

Carbon steel is the most commonly used material in drill collar manufacturing mostly due to its robustness and cost-effectiveness. Its high strength and durability make it ideal for general drilling operations. Carbon steel drill collars are particularly effective in standard drilling environments where high stress and wear are anticipated. However, their susceptibility to magnetic interference limits their use in operations where precise magnetic measurements are crucial. Additionally, while they offer good resistance to corrosion and wear, in extremely corrosive environments, other materials might be more suitable.

Non-Magnetic Nickel-Copper Alloys

Non-magnetic nickel-copper alloys are essential in scenarios where magnetic interference affects downhole measurements. These alloys are used to manufacture non-magnetic drill collars, which are indispensable in wells where MWD and LWD are critical. The non-magnetic properties ensure that the magnetic field of the Earth is not distorted, allowing for accurate readings. The downside is the higher cost associated with these materials, coupled with their slightly lower strength compared to carbon steel. These drill collars are primarily used in complex, high-technology drilling operations.

Chromium-Molybdenum Steel (Chromoly)

Chromium-molybdenum steel, or chromoly, is another material often used for drill collars, offering a balance between strength and non-magnetic properties. Chromoly is known for its excellent strength-to-weight ratio and its ability to withstand high stress and temperatures, making it suitable for deep well drilling. It also has a degree of resistance to corrosion, which is beneficial in environments with corrosive elements. However, chromoly drill collars can be more expensive than standard carbon steel and are usually reserved for more demanding drilling conditions where their properties are fully utilized.

Specialty Alloys

In certain highly specialized drilling situations, drill collars are made from unique alloy compositions designed to meet specific requirements. These specialty alloys can include elements like vanadium, titanium, or aluminum, each contributing to enhanced performance characteristics such as increased strength, reduced weight, or improved corrosion resistance. These materials are typically used in extreme drilling conditions, such as ultra-deep wells or highly corrosive environments. The primary drawback is their cost and the complexity of their manufacturing process, making them less common and more targeted in their application.

Industrial Applications

Figure 4: Drill collar diagram. Source: National Oilwell Varco/CC BY-SA 4.0 DEED

Drill collars are instrumental in various industrial applications beyond oil and gas exploration. Each sector presents unique challenges and requirements, influencing the choice of drill collar types and materials. This section highlights several key industrial applications of drill collars, underscoring their versatility and importance in different fields.

Oil and Gas Exploration

Figure 5: Offshore oil rig, WilPhoenix, in Cromarty Firth, near Invergordon, Scotland. Source: Mustang Joe/Public domain

In oil and gas exploration, drill collars are fundamental in ensuring efficient and accurate drilling. They provide the necessary weight and stability to the drill string, facilitating the penetration of tough geological formations. Non-magnetic drill collars are particularly vital in environments where precise MWD and LWD are required. This application demands high-strength materials capable of withstanding extreme pressures and temperatures.

Geothermal Drilling

Geothermal drilling utilizes drill collars to access heat resources deep within the Earth. The high temperatures and corrosive environments encountered in geothermal wells necessitate the use of drill collars made from materials with high thermal resistance and corrosion tolerance. These operations often require specialized drill collars, such as those made from chromium-molybdenum steel, to ensure operational integrity and longevity.

Construction

In the construction industry, especially in large-scale infrastructure projects, drill collars are used in the process of creating foundations for buildings, bridges, and other structures. They help in boring through various soil types and rock formations. The choice of drill collars in construction is typically guided by the soil conditions and the depth of drilling required. Carbon steel drill collars are commonly used due to their strength and durability.

Mining

Mining operations use drill collars for exploratory drilling and extraction processes. In these applications, the drill collars must withstand abrasive materials and potentially corrosive environments. The strength and wear resistance of carbon steel make it a suitable choice, though, in more demanding conditions, specialty alloys may be employed for their enhanced properties.

Water Well Drilling

Drill collars are also used in water well drilling, where they aid in reaching aquifers for water extraction. The requirements in water well drilling may not be as stringent as in oil and gas exploration, but the durability and efficiency of the drill collars remain essential. Carbon steel drill collars are often used, offering a balance between cost-effectiveness and performance.

History

The history of drill collars traces back to the early days of oil and gas exploration, evolving in response to the increasing complexities and demands of drilling technology. Initially, the drilling process relied on basic tools with limited capabilities, often leading to inefficiencies and operational challenges. As drilling depths increased and operations ventured into more challenging geological formations, the need for more specialized equipment became apparent. 

The introduction of drill collars allowed for deeper and more accurate drilling. Over the years, advancements in materials science and engineering have further refined drill collar designs, integrating features like non-magnetic properties and specialized coatings to meet the diverse needs of modern drilling operations. Today, drill collars are an important component of the drilling process, embodying a rich history of innovation and adaptation in the face of evolving industrial challenges.

Related Information

GlobalSpec—Video: Geothermal drillers dig deeper with new anchor system

 


Product Announcements
Hyperion Materials & Technologies
Staubli Electrical Connectors

UNLIMITED FREE
ACCESS
TO THE WORLD'S BEST IDEAS

SUBMIT
Already a GlobalSpec user? Log in.

This is embarrasing...

An error occurred while processing the form. Please try again in a few minutes.

Customize Your GlobalSpec Experience

Category: Drill Collars
Finish!
Privacy Policy

This is embarrasing...

An error occurred while processing the form. Please try again in a few minutes.