What is a Drill String?

What is a Drill String?

The drill string is the collective name for the entire string of drilling tools used in oil and gas drilling, extending from the surface derrick to the drill bit at the bottom of the well. Its main components include kelly pipes, drill pipes, drill collars, various connectors, stabilizers, and other downhole tools. It is a crucial tool for fast, high-quality drilling, serving as the hub connecting the surface and underground.

Components of a Drill String:

A drill string is a complex system composed of components with different functions. Its typical components, from top to bottom, are as follows:

1. Kelly
Location: Topmost.

Function: With a square (or hexagonal) cross-section, it is held in place by the square bushing of the rotary table. The kelly pipe transfers power from the surface rotary table to the drill pipe, driving the drill bit.

2. Drill pipe
Location: It constitutes the main body of the drill string, accounting for over 90% of the total length.

Function:
a. Torque transmission: Transmits torque from top to bottom to the drill bit.

b. Drilling fluid transportation: The hollow tubular cavity within the drill pipe is the conduit for drilling fluid to flow to the bottom of the well. 

c. Drill String Extension: Continuously increasing the length of drill pipe deepens the wellbore.

3. Downhole Tools

Location: Between the drill pipe and the drill collar.

Function: Includes drill tool stabilizers (centralizers, used to stabilize the drill string), jars, and measurement-while-drilling/logging-while-drilling tools (used to collect real-time downhole data).

4. Drill Collar
Location: The drill collar is a thick-walled, heavy-duty pipe located below the drill pipe, directly connected to the drill bit (or bottomhole motor).

Function:
a. Provides weight-on-bit: The substantial weight provided by its thick walls applies the pressure required to break rock to the drill bit.
b. Stabilizes the Wellbore: It controls wellbore inclination through various combinations of the drill collar and stabilizers. Its high rigidity helps maintain wellbore straightness or the planned drilling trajectory.

5. Drill Bit
Location: The cutting tool located at the very bottom of the drill string, used to drill through rock.

Function: The drill collar is the tool that directly breaks rock and serves as the "terminal station" and working end of the drill string. The type of drill bit depends on the formation being drilled.

6. Bottom Hole Assembly (BHA)
A group of components located at the bottom of the drill string, including stabilizers, motors, and other tools, used to control the direction and performance of the drill bit.

All components are connected by threaded joints (drill pipe joints or mating joints). The lower end of the drill string is connected to the drill bit or a bottomhole motor. Power and torque from the surface are transmitted to the drill bit through the drill string. Drilling fluid can also be transported to the bottomhole through the drill string for well cleaning and drilling.

Certain specialized operations (such as fishing, cementing, and formation testing) can also be completed by connecting the drill string to related tools and lowering them into the well. Drill string threaded connections are divided into two types: positive threaded connections and negative threaded connections.

How Does the Drill String Work in Drilling?

1. Rotation: The drill string is rotated by the kelly or top drive at the surface, transmitting the rotational force to the drill bit, thereby grinding the rock. 

2. Penetration: Gravity forces the drill string downward into the rock, cutting through the formation.

3. Fluid Circulation: As the drill string rotates and penetrates, drilling fluid is pumped downward through the internal cavity of the pipe. The drilling fluid is then discharged through nozzles on the drill bit, flushing away rock cuttings from the space between the drill string and the wellbore (the annulus) and cooling the drill bit.

Drill String Forces:

During rotary table drilling, the forces acting on the drill string are quite complex. However, these loads can be categorized into two main types: constant and alternating. Constant stresses include tensile, compressive, and shear stresses; alternating stresses include bending stress, shear stress caused by torsional vibration, and tensile and compressive stresses caused by longitudinal vibration. Throughout the length of the drill string, the loads are characterized by the predominance of constant loads at the wellhead and alternating loads near the bottom. This alternating load is the primary cause of drill string fatigue failure.

The parts of the drill string that are most seriously stressed are:

(1) The lower part of the drill string is most seriously stressed during drilling. The drill string is subjected to axial pressure, torque and bending moment at the same time. What is more serious is that there is a violent alternating stress cycle during rotation, and the drill bit suddenly encounters resistance or jamming, which greatly increases the torque on the drill string.

(2) The forces on the drill string at the wellhead are complex during drilling and drilling. The drill string at the wellhead is subjected to the maximum tension during drilling. If the drill bit is lifted or braked suddenly during drilling, the axial tension on the drill string at the wellhead will be greatly increased. During drilling, the drill string at the wellhead is subjected to the maximum tension and torque, and the stress situation is also relatively serious.

(3) Due to factors such as changes in stratum lithology, impact of the drill bit and longitudinal vibration, the drilling pressure is uneven, causing the neutral point to move up and down. In this way, the drill string near the neutral point is subjected to alternating loads.

The Drill String's Function:

The drill string is a crucial tool for fast, high-quality drilling. It serves as the connecting link between the surface and the underground. In rotary table drilling, it transmits the energy needed to break rock, applies weight on bit to the bottomhole, and delivers flushing fluid. In downhole power drilling, the bottomhole power unit is delivered to the bottomhole via the drill string and withstands the reaction torque. The fluid energy required by turbodrills and screw drills is also delivered to the bottomhole via the drill string.

The drill string plays an irreplaceable role in the drilling process, and its functions can be summarized in the following six areas:

1. Torque Transmission: It transmits the rotational torque of the surface power to the drill bit, driving it to rotate and break rock.
2. Weight on Bit: The weight of the drill collar provides vertical downward pressure on the drill bit, enabling it to effectively penetrate the formation.

3. Drilling Fluid Delivery: It acts as a closed conduit, pumping high-pressure drilling fluid from the surface to the drill bit, completing the circulation. 

4. Tripping and Lowering the Drill Bit: When the drill bit wears, the entire drill string must be pulled out of the ground to be replaced and then lowered back into the well.

5. Controlling the Wellbore Trajectory: Through a suitable drill string assembly (such as with a stabilizer), the wellbore can be extended along a pre-designed trajectory and direction.
6. Obtaining Downhole Information: Measurement while drilling (MWD) tools transmit real-time data such as well inclination, azimuth, and formation characteristics to the surface to guide drilling operations.

During the drilling process, drill bit performance, wellbore conditions, and even various changes in the downhole formation are often only reflected to the surface through the drill string and various instruments. Optimal drilling parameters and other technical measures can only be achieved through the proper use of the drill string. In addition to normal drilling, other drilling operations, such as coring, handling complex downhole conditions, formation testing, cementing, and salvaging, rely on the drill string.

Read more: What Types of Oil Drill Pipes are there? or Drill Pipe vs. Drill Collar