What is a drill pipe? What is its function?
A
drill pipe is a steel pipe with a threaded tail, which is used to connect the surface equipment of the drilling rig and the drilling and grinding equipment or bottom hole device at the bottom of the well. The drill pipe is the core component of the drill string in oil and gas drilling operations. It is mainly used to transmit power (torque and tension) and transport drilling fluid, and connect the surface drilling rig with downhole drilling tools (such as drill collars, drill bits, etc.). Drill pipes are divided into three categories: square drill pipes, drill pipes and heavy weight drill pipes.
The main functions of drill pipes:
1. Power transmission: The rotational torque and axial pressure of the surface drilling rig are transmitted to the drill bit to achieve formation crushing.
2. Drilling fluid channel: As a channel for mud circulation, it cools the drill bit and carries cuttings back to the surface.
3. Well depth extension: By lengthening the drill pipe combination, the depth of the wellbore is gradually increased.
The purpose of the drill pipe is to transport drilling mud to the drill bit and to raise, lower or rotate the bottom hole device together with the drill bit. Drill pipes must be able to withstand huge internal and external pressures, twisting, bending and vibration. Drill pipes can be used multiple times during the extraction and refining of oil and gas.
The common components of a drill string are: drill bit,
drill collar, drill pipe, stabilizer, special joint and square drill pipe.
Basic structure of drill pipe:
Pipe body:
Usually made of seamless steel pipe, with a wall thickness of 9~11mm, and the two ends are thickened (internal thickening, external thickening or internal and external thickening) to enhance strength.
Appearance: Hollow thick-walled steel pipe, with thickened tool joints (Tool Joint) at both ends, connected to the drill string by threads.
Joint:
It is divided into male and female threaded joints, which are combined with the pipe body by friction welding or threaded connection, and are responsible for transmitting torque, tensile load and sealing drilling fluid.
Male joint (Pin): With external threads, inserted into the female joint of the next drill pipe.
Female joint (Box): With internal threads, used to connect the male joint.
Steel grade and size of drill pipe:
Material: high-strength alloy steel, E-75, X-95, G-105, S-135 grades in API standards. The higher the steel grade, the greater the yield strength. It must have good tensile, torsional and fatigue resistance.
Size:
The size of the drill pipe is mainly determined by the outer diameter (OD), inner diameter (ID) and weight per foot, and follows the API 5DP standard.
Common sizes include 2⅜", 2⅞", 3½", 4½", 5", 5½", 6⅝", etc. (API standard).
Length: A single drill pipe is usually 9~12 meters (30~40 feet), and can reach several thousand meters after multiple pipes are connected.
Wall thickness: Thinner than drill collars, but thicker than ordinary steel pipes to withstand high pressure and complex loads.
Key parameters of drill pipe size:
(1) Outer diameter (OD)
Range: Usually 2⅜" ~ 6⅝" (60.3mm ~ 168.3mm), larger sizes need to be customized.
Selection basis: Wellbore size, drilling rig capacity, drilling fluid displacement requirements.
(2) Inner diameter (ID)
Influence: Determines drilling fluid flow rate and annular return velocity. The smaller the inner diameter, the higher the flow rate (but the pressure drop increases).
ID=OD−2×wall thickness
Dimensions of special types of drill pipe:
(1) Heavy-weight Drill Pipe
Outer diameter: the same as conventional drill pipe (such as 3½", 4½", 5").
Unit weight: 20%~50% higher than ordinary drill pipe.
Use: Replace part of the drill collar to reduce the risk of stuck drill, often used in the transition section of directional wells.
(2) Slim Hole Drill Pipe
Outer diameter: less than 2⅜" (such as 1⅞", 2⅛"), used for small wells or sidetracking.
Challenges: low torsional strength, high steel grade (such as S-135) is required.
Types of drill pipe:
Drill pipes are divided into three categories: square drill pipe, drill pipe and weighted drill pipe. The connection order is square drill pipe (1 piece) + drill pipe (n pieces, determined by the well depth) + weighted drill pipe (n pieces, determined by the design of the drilling tool assembly).
1. Square drill pipe
The square drill pipe is located at the top of the drill string and has two types: square and hexagonal.
Function:
① During drilling, the square drill pipe cooperates with the bushing and the turntable bushing to transmit the ground rotation torque to the drill pipe to drive the drill string and the drill bit to rotate;
② Bear the entire weight of the drill string;
③ The channel for the circulation of drilling fluid.
Structure:
The square drill rod consists of upper and lower joints and a pipe body. The pipe body has two structures: square or hexagonal (mostly square structures in oil drilling); the upper joint is a left-handed female thread (reverse thread), which is connected to the faucet. During the rotation process, the left-handed female thread prevents reverse threading; the lower joint is a right-handed male thread, which is connected to the drill rod. When working, the upper end of the square drill rod is always above the turntable surface, and the lower part is below the turntable surface.
2. Drill pipe
Function:
The drill rod is the basic component of the drill string. Its main function is to transmit torque and transport drilling fluid, and rely on the gradual rotation of the drill rod. Lengthening makes the wellbore deeper and deeper. Therefore, the drill pipe occupies a very important position in oil drilling.
Structure:
The drill pipe structure is made of seamless steel pipe, and the wall thickness is generally 9-11mm; it consists of two parts: the drill pipe body and the drill pipe joint. The pipe body and the joint are welded together by friction welding.
3. Heavy weight drill pipe
The
heavy weight drill pipe is similar to the oil drill pipe, and is also a hollow steel column with a length of about 10 meters. However, the weight of a single rod is heavier than that of an oil drill pipe, and the wall thickness is 2-3 times that of a drill pipe. The heavy weight drill pipe is connected between the drill pipe and the drill collar to prevent fatigue damage due to changes in the cross section of the drill string. It can also replace part of the drill collar, but its suspension The lifting is simple, the drilling operation is convenient, and the drilling time can be saved.
Classification by special purpose:
1. Heavy weight drill pipe
The wall thickness is thicker than ordinary drill pipe, and the weight is close to the drill collar. It is used to replace part of the drill collar to reduce the risk of stuck drill.
Commonly used in the transition section of directional wells and horizontal wells.
2. Anti-sulfur drill pipe
Use special materials (such as SS-75, SS-95) to prevent hydrogen sulfide (H₂S) corrosion.
Used in sulfur-containing oil and gas fields.
3. High-toughness drill pipe
Optimize the heat treatment process to improve the impact resistance, suitable for extremely cold or deep sea environments.
Typical applications of drill pipe:
1. Rotary drilling: transmit the drilling rig torque and drive the drill bit to rotate Rock breaking.
2. Directional wells/horizontal wells: Flexibly adapt to changes in wellbore trajectory, with downhole motors (PDM) or rotary steerable systems (RSS).
3. Deep wells/ultra-deep wells: Use high-strength steel grades (such as S-135) to withstand huge tension and torque.
4. Underbalanced drilling: High-pressure drill pipes prevent blowouts.
Classification of drill pipe joints:
The drill pipe joint is a component of the drill pipe, divided into male and female joints, connected to both ends of the drill pipe body. In order to enhance the connection strength of the joint, the wall thickness of the pipe body needs to be increased at the joint. According to the thickening method, it can be divided into three forms: internal thickening, external thickening, and internal and external thickening. The joint has threads on the car (rough buckle), used to connect each single drill pipe. The thread of the drill pipe joint is a tapered pipe thread with a sealing shoulder. The shoulder surface is tightened to seal, and the thread only serves as a connection. The thread buckle type of the corresponding joint is different for different thickening methods.
Three conditions that must be met for buckle connection:
① Equal size;
② Same thread type;
③ Matching of male and female buckles.
According to the thread buckle type, drill pipe joints are divided into four connection types:
1. Internal flush connection (IF): Mainly used for external thickening drill pipe. Its characteristics are that the drill pipe has the same internal diameter throughout the body and small drilling fluid flow resistance; but the outer diameter is large and easy to wear.
2. Full hole connection (FH): Mainly used for internal and external thickening Thick drill pipe. Its characteristic is that the drill pipe has two inner diameters, the drilling fluid flow resistance is greater than the inner flush type, but its outer diameter is smaller than the regular type.
3. Regular connection (REG): Mainly used for internal thickened drill pipe, drill bit, and salvage tools. Its characteristic is that the inner diameter of the thickened inner diameter of the joint is smaller than the inner diameter of the pipe body, the drilling fluid flow resistance is large, and the relative flow resistance is the largest among the three buckle types, but the outer diameter is the smallest and the strength is greater. The above three types of joints all use V-type threads, but the buckle type, buckle distance, taper and size are very different.
4. Numbered connection (NC): The digital series joint is the American national standard coarse thread series. It has now been adopted as an international standard by API. NC thread is also a V-thread. Some NC joints have the same pitch diameter, taper, pitch and thread length as the old API standard joints and can be used interchangeably.
Most oil drill pipe joints are in the form of external thickening or internal and external thickening. The outer diameter of the drill pipe joint is larger than the outer diameter of the pipe body. During the drilling process, it is constantly in contact and friction with the well wall or
casing, causing wear. In order to avoid drilling accidents such as broken drill pipe and tripping caused by joint wear, a drill pipe joint anti-wear belt must be provided on the drill pipe female joint.
Material Innovation:
In addition to traditional steel, aluminum alloy, titanium alloy and composite drill pipes are gradually used in special working conditions due to their lightweight, high-strength and corrosion-resistant properties.
Read more: API Tubing and Casing Size Chart
