What are the types of drill collars?

Date:2024-07-18Tags:drill collar, drill string, drill pipe

The drill collar is an important part of the drill string. Its main function is to apply drilling pressure to the drill bit and pump drilling fluid, so that the lower drilling tool assembly has greater rigidity, ensure the stability of the drill bit, and reduce the problem of well deviation. Since the drilling pressure is formed by the weight of the drill string, the compressive stress of the lower part of the drill string is relatively large. Usually, the drilling pressure value is greater than the critical value of the pressure rod stability of the drill string, which will cause the drill string in the lower compressed part to bend under the action of its own weight. During the rotation process, the bent drill string will cause alternating stress, resulting in fatigue damage of the drill string. In order to reduce the degree of fatigue damage and reduce bending stress, drill collars are usually used instead of drill rods in the compressed part of the lower end of the drill string. Drill collars are generally made of thick-walled seamless steel pipes, and the types include cylindrical drill collars, spiral drill collars, non-magnetic drill collars, square drill collars, etc. The following will introduce the characteristics of various types of drill collars and their manufacturing processes in detail.

drill collar

1. Cylindrical drill collar

Cylindrical drill collar is made of ordinary alloy steel, and the inner and outer diameters of the cross section of the tube body are both circular. Its main function is to bear the compressive stress of the lower part of the drill string, prevent the drill string from bending when rotating, and thus reduce the influence of alternating stress. Cylindrical drill collar is made of AISI 4145H chrome-molybdenum steel, which has good mechanical properties and durability. During the manufacturing process, the solid rod is quenched and tempered to obtain the required mechanical properties, and then the hole is opened, punched and threaded.


This drill collar can be up to 12 meters in length and is suitable for deep wells and high-pressure jet drilling. The new double-shoulder threaded oil drill collar currently developed by the company is also based on this type, with stronger fatigue resistance and higher rigidity, and can better adapt to the working environment under composite loads. Its advantages are high strength and durability, which can effectively reduce the fatigue damage of the drill string.


2. Spiral Drill Collars

The outer surface of the spiral drill collar is machined with spiral grooves, which help the drilling fluid flow evenly around the diameter of the drill collar, thereby balancing pressure and reducing the occurrence of differential drill sticking. This design effectively reduces the contact area with the well wall, thereby reducing the risk of sticking drill. In directional drilling, spiral drill collars are particularly important because they can reduce the friction between the drill collar and the well wall caused by large deviated wells.


The spiral drill collar is made of ordinary alloy steel and is processed by opening and punching to meet the requirements of API Spec. 7-1. Full-length heat treatment ensures that its mechanical properties meet or exceed API standards, with a hardness range of 285-341 Brinell hardness 1 inch below the surface and a Charpy "V" notch minimum impact strength of 40 foot pounds (70 °F). This drill collar weighs 4% less than a cylindrical drill collar of the same size, making it more suitable for long-term and high-intensity drilling operations.


3. Non-magnetic drill collars

Non-magnetic drill collars are made of stainless alloy steel with very low magnetic permeability and are suitable for scenarios where magnetic field interference needs to be avoided in directional well engineering. Its main function is to avoid the compass being affected by the magnetic field of the drill string when using a magnetic inclinometer to measure the borehole inclination angle and azimuth, thereby ensuring the accuracy of the measurement results. The materials of non-magnetic drill collars have passed strict chemical composition analysis and mechanical property tests to ensure that the hardness, toughness, impact value and corrosion resistance meet the standards.


The maximum value of magnetic permeability (magnetic field strength = 1x105/4πNm) is 1.010, the average value does not exceed 1.005, and the maximum value of magnetic field gradient (hot spot) is B≤0.05. These characteristics make non-magnetic drill collars perform well in complex drilling environments and can effectively avoid measurement errors caused by magnetic field interference. The design of non-magnetic drill collars is the same as that of general drill collars, but their special material composition and processing technology give them unique advantages in specific application scenarios.


4. Square drill collar

The square drill collar has a square cross section and is mainly used in drilling operations that require higher torsional strength. This drill collar is usually made of high-strength alloy steel and heat-treated to improve its mechanical properties. The unique shape of the square drill collar makes it excellent in bending and torsion resistance, and is suitable for drilling operations in complex formations and high-pressure environments.


During the manufacturing process, the square drill collar also needs to undergo strict quality control and mechanical property testing to ensure that it meets API standards. Its durability and high rigidity make it an ideal choice for deep well drilling and drilling in complex formations.


5. Ordinary drill collar (integral drill collar)

Ordinary drill collar, also known as integral drill collar, refers to a drill collar made of thick-walled seamless steel pipe. It is designed to apply weight to the drill bit and pump drilling fluid. It is an integral thick-walled joint made of a solid bar of modified alloy steel. The solid bar is quenched and tempered to obtain the required mechanical properties, and then holed, punched and threaded.


The advantages of ordinary drill collars are their high strength and durability, which can effectively reduce fatigue damage of drill strings. The company has currently developed a series of drill collars of various specifications within 12 meters in length, and has developed a new type of double-shoulder threaded oil drill collar to be suitable for deep well and high-pressure jet drilling under composite loads. These products are made of AISI 4145H chromium-molybdenum steel, and the content of trace elements is effectively controlled to ensure that their mechanical properties meet or exceed API standards.


6. Non-magnetic spiral drill collar

Non-magnetic spiral drill collar is a type of drill collar that combines non-magnetic and spiral designs. Its outer surface is processed with spiral grooves to reduce the contact area with the well wall and reduce the risk of sticking and drilling. It also has low magnetic permeability and is suitable for directional well projects that need to avoid magnetic field interference. Non-magnetic spiral drill collars are made of low-chromium-manganese alloy steel and undergo rigorous chemical composition analysis and mechanical property testing to ensure that their hardness, toughness, impact value and corrosion resistance meet the standards.

The maximum value of magnetic permeability (magnetic field strength = 1x105/4πNm) is 1.010, the average value does not exceed 1.005, and the maximum value of magnetic field gradient (hot spot) is B≤0.05. The design of non-magnetic spiral drill collars can effectively avoid magnetic field interference in complex drilling environments and ensure the accuracy of measurement results.


7. Spiral drill collar

The main function of the spiral drill collar is to reduce the occurrence of sticking and drilling. There are three right-hand spiral grooves on its outer cylinder, and a 305-560mm section of uncut round drill collar is left in the external thread joint part and a 475-610mm section of uncut round drill collar is left in the internal thread end joint part. This design makes the weight of the spiral drill collar 4% less than that of the round drill collar of the same size.


Spiral drill collars are not included in the API standard, but many manufacturers in various countries have produced them. Chinese domestic drill collars have also been officially put into production, and their specifications and dimensions are the same as those of foreign products. During the manufacturing process, spiral drill collars need to undergo strict quality control and mechanical performance testing to try to make them meet the API standard.


8. Non-magnetic drill collar

The structure of non-magnetic drill collar is the same as that of general drill collar, but its chemical composition and mechanical properties are different from those of ordinary drill collar. Non-magnetic drill collar is a drilling tool with special functions in directional well engineering. When a magnetic inclinometer is used in directional wells to measure the inclination angle and azimuth of the wellbore, in order to prevent the compass from being affected by the magnetic field of the drill string, a non-magnetic drill collar of a certain length must be equipped in the drill string so that the magnetic inclinometer is located at a suitable position in the non-magnetic drill collar for inclination measurement, otherwise the correct measurement result cannot be obtained.


In summary, the application of drill collars in oil development is still very common. Its different types and process characteristics enable it to adapt to various complex drilling environments and needs. Whether it is a cylindrical drill collar, a spiral drill collar, a non-magnetic drill collar or a square drill collar, the design and manufacturing process of each type has undergone strict quality control and performance testing, which is also the reason why drill collars can be derived into various types.

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