What Is a Drill String?

Drill string is a general term for steel pipe columns above the drill bit and below the faucet. Its main body includes square drill pipe, drill pipe, drill collar, various connection joints and stabilizers and other downhole tools. It is an important tool for fast and high-quality drilling. It is a hub that connects the ground with the ground. ^[1]

In rotary drilling, the drill string includes the entire system's drilling tools connected below the faucet. It consists of square drill pipe, drill pipe, drill collar, joint, stabilizer and other components. The role of the square drill pipe is to transfer the power of the ground turntable to the drill pipe to drive the drill bit to rotate. The role of the drill pipe is to transmit the power from the ground to the drill bit, and to gradually deepen the wellbore by the gradual extension of the drill pipe. The drill collar is located below the drill pipe and is directly connected to the drill bit (or bottom hole power machine), relying on itself The weight of the pressure is increased, and the inclination of the wellbore is controlled by various combinations of it and the stabilizer, and the different components of the drill string are interconnected) with the help of a drill pipe joint or a joint.

When drilling on a rotary table, the forces on the drill string are more complicated. However, all these loads can be divided into two categories: constant and alternating. The tensile stress, compressive stress, and shear stress belong to the invariant stress; the bending stress, the shear stress caused by torsional vibration, and the tensile and compressive stress caused by longitudinal vibration are the alternating stress. Throughout the length of the drill string, the effect of the load is mainly the effect of constant load at the wellhead and the effect of alternating load near the bottom of the well. This alternating load is the main reason for fatigue failure of drill string.

The severely stressed parts of the drill string are:
(1) The lower part of the drill string is most stressed when drilling. The drill string is subject to the effects of axial pressure, torque and bending moment at the same time. What's more serious is that there is a violent alternating stress cycle during rotation, and the drill bit suddenly encounters a jam, which will greatly increase the torque experienced by the drill string. .
(2) When drilling and when drilling, the force on the drill string at the wellhead is complicated. The drill string at the wellhead is subjected to the maximum tensile force when lifting and lowering. If it is lifted and braked suddenly when lifting, the axial tensile force at the wellhead at the wellhead is greatly increased. When drilling, the drill string at the wellhead is subjected to the highest tensile and torsional forces, and the stress is also severe.
(3) Due to the changes in formation lithology, the impact of the drill bit and the longitudinal vibration, etc., the drilling pressure is not uniform, so the neutral point position is moved up and down. In this way, the drill string near the neutral point is subjected to an alternating load. ^[2]

Drill string is an important tool for fast and high-quality drilling. It is a hub that connects the ground with the ground. It is used to transfer the energy required to break the rock when drilling on the rotary table, to apply drilling pressure to the bottom of the well, and to send well washing fluid to the well. In downhole power drilling, the downhole power machine uses the drill string to send to the bottom of the well and bears the counter torque, and the liquid energy required by the turbine and screw drilling tools is also transmitted to the bottom of the well through the drill string.

During the drilling process, the work of the drill bit, the condition of the wellbore, and even various changes in the formation downhole can often be reflected on the surface through the drill string and various instruments. Reasonable drilling technical parameters and other technical measures can only be achieved under the condition of proper use of the drill string. In addition to normal drilling, various other operations in the drilling process, such as coring, handling downhole complex conditions, formation testing, cement extruding, and falling objects, etc., all rely on drill strings.