Usages of drill string, casing and tubing in oil drilling

Oil drilling and production steel pipes can be generally classified into drill string (including kelly, drill pipe, weighted drill pipe, drill collar), casing (including surface casing, technical casing, oil layer casing liner) and tubing according to various structures, forms, uses and performance.

Usages of drill string, casing and tubing in oil drilling

  1. Drill string:
  • Kelly: The kelly is located at the top of the drill string, connected with the drill pipe below. The structure is characterized by an internal round external square or an internal round external hexagon.Its function is to transfer the rotary power of the surface rotary table to the downhole bit through the drill string, to break the bottom rock layer, transfer the well flushing fluid, cool the bit and clean the bottom rock header.
  • Drill pipe: The drill pipe is located in the middle of the drill string, under the kelly, and weighted above the drill pipe or drill chain.The main function is to transfer the ground rotating power to the drill bit through the kelly, which serves as the intermediate medium, and gradually lengthen the connection of the drill pipe to make the depth increase continuously.Start drilling and replace the drill bit.Transfer tools and drilling fluid into the well.The drill pipe is made of two parts of pipe body and joint by friction welding.Hot-rolled alloy steel seamless pipe is adopted to increase the strength of the welded part between the pipe and the joint.The two ends of the tube body shall be upset and thickened at the welding part. The thickening forms include: internal thickening and external thickening, and internal and external thickening, respectively represented by IU, EU and IEU symbols.Drill pipe steel grades are E-75, X-95,G-105 and S-135.Two or three digits after the letter indicate the minimum yield strength of the grade.The drill pipe joints are generally made of high-strength alloy steel by rolling, forging, heat treatment and mechanical processing into butt welding joints of different thread types. The thread types mainly include inner flat, full hole and normal, which are respectively represented by IF,FH and REG.Butt weld joints of different sizes and thread types are required for drill pipe with different steel grades and specifications.Since the outside diameter of the butt welding drill pipe joint is larger than the outside diameter of the pipe body, it is easy to wear during drilling, so the joint material is required to have high strength and wear resistance.In order to improve the wear resistance of the joint, in addition to strengthening treatment and increasing the hardness of the joint, it is generally possible to spray welding on the surface of the joint with higher hardness and wear-resistant materials, thus greatly increasing the service life of the joint.
  • Weighted drill pipe: it is a kind of drill pipe of medium weight similar to drill pipe, with wall thickness 2-3 times of drill pipe.At both ends of the thick-walled tube body, there are extra-long extra-thick pipe joints, and part of extra-thick pipe joints in the middle.The weighted drill pipe is generally added between the drill pipe and the drill collar when forming the drill string to prevent the sudden change of the drill string section and reduce the drill pipe fatigue.
  • Drill collar: located at the lower part of the drill pipe or weighted drill pipe, connected with the drill pipe or weighted drill pipe at the top, and connected with the drill bit at the bottom.These include alloy drill collars, non magnetic drill collars and spiral drill collars, square drill collars, etc.By virtue of its own weight and high rigidity, apply bit pressure and bending resistance to the well, so that the bit can work smoothly, prevent well deviation, and maintain the shaft strike.


  1. Casing:

In order for the underground oil and gas reservoir to be transported to the surface smoothly, it is necessary to run the oil “casing” from the bottom hole to the top of the well to construct a channel to prevent blowout and leakage and isolate the different oil, gas and water layers.Can be divided into surface casing, technical casing, oil layer casing, liner according to different uses .

1) Surface casing: used for drilling through the soft and prone to collapse of the ground to reinforce the shaft wall, prevent collapse, and make the drilling proceed smoothly.Common specifications are 13 3/8″ and 10 3/4.

2) Technical casing: In drilling, in order to prevent well collapse, leakage and blowout in complex formation and prevent high-pressure brine layer fluid flow into the well, the technical casing must be applied to isolate and reinforce the borehole wall.Common specifications are 9 5/8″ and 8 5/8″.

3) Reservoir casing: after drilling to the target layer, in order to prevent interference between reservoirs with different pressures and other fluids from immersing in the well, it is necessary to go into the reservoir casing to isolate the oil, gas and water layers, so as to realize layered exploitation and layered water injection.Common specifications are 4 1/2″, 5 1/2″, 6 5/8″, 7″.

Usages of drill string, casing and tubing in oil drilling

  1. Tubing:

It is mainly used for oil recovery and gas extraction, to export underground oil and gas to the surface through tubing.According to its end structure, the tubing can be divided into three types: flat tubing, external thickening tubing and integral joint tubing.

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