Drill collars are heavy tubes below the drill pipe, but above the drill bit, that add weight to the bit.
There is much more to a drill collar than the single sentence above implies, though. They make up most of the bottom hole assembly (BHA) and are just as important in a drill string as drill pipe. The thin walls of drill pipe make it more susceptible to buckling and failure, but drill collars can help reduce the risk of failure. Adding weight to the bottom of the BHA keeps the drill pipe in tension.
DRILL COLLAR BASICS
The word "collar" can be misleading. Instead of encasing a portion of the drill string, it's a long, thick-walled steel tubular which is actually part of the BHA. Drill collars continue pumping drilling fluid from the drill pipe down to the bit.
There are a few similarities between drill pipe and drill collars. Both have pin and box threaded ends. These shouldered connections allow the BHA to be joined to the rest of the drill string (usually with HWDP as a transitional pipe in-between to reduce fatigue failures and add additional weight to the drill bit). The average length of both a drill pipe and a drill collar are both around 31 feet.
Drill collars also have a larger outer diameter and smaller inner diameter than drill pipe. This means that the threaded ends can be machined directly onto the drill collar, and not applied after production, as with drill pipe.
Schlumberger, a large oilfield company, explains that drill collars are manufactured from "bars of steel (…) drilled from end to end to provide a passage to pumping drilling fluids through the collars. The outside diameter of the steel bars may be machined slightly to ensure roundness, and in some cases may be machined with helical grooves ("spiral collars")."
TYPES OF DRILL COLLARS
There are two main types of drill collars: slick and spiral. Both types are thicker-walled, more rigid, and heavier than drill pipe.
Slick drill collars are, as the name implies, slick. It is a smooth, rounded, thick-walled section. These are typically used in scenarios that require more contact with the wall of the wellbore.
The newer and more common type of drill collar is called spiral. These have grooves cut into the tubular which reduce the surface area and allow lubricant and cuttings to pass around the drill collar. Spiral drill collars are more likely to be used in situations where there is a chance the drill string can become stuck in the wellbore by minimizing wall contact. Spiral drill collars are especially useful for drilling through porous formations.
Both types of drill collars typically are sold in two formats: cold-rolled steel, or non-magnetic (plain carbon steel or premium non-magnetic alloys). Non-magnetic drill collars are often used during the measurement process, to protect magnetic measuring instruments, which calculate the borehole's geomagnetic field, but can be used in non-magnetic environments as well.
FUNCTIONS OF DRILL COLLARS
The primary purpose of a drill collar is to apply weight and force on the drill bit in order to help control direction and wellbore stability. However, that's not the only benefit it brings to a drill string.
Drill collars work to keep drill strings from buckling under pressure. They absorb much of the vibration, which reduces pressure on the drill pipe and maintains tension, reducing bending and fatigue failures. They also help control the direction and inclination of boreholes, which includes keeping the walls of the hole straight. This results in a reduction in sticking or jumping drill strings.
Drill collars are able to provide these benefits to a drill string because of their inclusion in the BHA. Adding weight behind the drill bit helps push the bit through rock it may not otherwise be able to cut through on its own. Schlumberger explains further: "To accurately control the amount of force applied to the bit, the driller carefully monitors the surface weight measured while the bit is just off the bottom of the wellbore. Next, the drill string (and the drill bit), is slowly and carefully lowered until it touches bottom. After that point, as the driller continues to lower the top of the drill string, more and more weight is applied to the bit, and correspondingly less weight is measured as hanging at the surface."
China Vigor as an in-depth practitioner in the oil and gas downhole drilling tool industry, we always insist on creating unique value for customers with high-quality products and services, so our team has reached a very close cooperative relationship with world-class customers. We will always maintain a learning mentality, constantly explore the latest developments in the oil and gas industry and apply them to product development. It is precisely because the Vigor team is always updating itself, so that we can always provide customers with the best quality products and top services, if you are interested in Vigor's downhole drilling and completion logging tools, please do not hesitate to contact us to get the most professional products and services.
For more information, you can write to our mailbox info@vigorpetroleum.com & mail@vigorpetroleum.com
