The three main types of Oil Country Tubular Goods are drill pipe, casing, and tubing.

Drill pipe:
Drill pipe is heavy, seamless pipe which provides rotation for circulating the drilling fluid. The pipe helps the drilling fluid to be pumped back up the annulus. The annulus of an oil well is the space between the casing and the piping or any immediate space between any casing and tubing surrounding it. The annulus helps in the easy circulation of the fluid in the well. The pipe is subject to axial tension, as high torque is required for rotating the down-hole assembly and drilling the fluid and circulating it. All this creates a large amount of stress on the drill pipe, hence the OCTG pipe should be extremely strong.
This drill pipe is used only at the drilling rigs and they are available in different sizes and different qualities. The quality differs based on the grade of strength, weight, and length of the pipe. The OCTG Drill Pipe is sturdy steel pipe which is required to meet the standards of API 5DP and API SPEC 7-1.

Casing:
Casing is used for lining the complete borehole which has been dug into the ground to procure oil. Similar to the drill pipe, the oil well casing pipe is also subjected to axial tension, therefore they are required to be made from high-quality strong steel material. OCTG casing is large diameter pipe which is held in place in the borehole using cement.
The diameters of OCTG casing pipe range from 4.5″ to 36″. The well-cemented pipe acts as the structural component of the well and offers hole integrity. It prevents the well from collapsing when the drilling process is on. Good quality, well cemented casing can stay in place for the whole life of the well.

Tubing:
Oil well tubing pipe is inserted inside the wellbore through the oil well casing pipe, which helps the transportation of the hydrocarbons to the surface. It is pipe either used for transporting natural gas or crude oil to the facilities for further processing after the drilling is complete. Similar to the drill pipe and casing, tubing is subjected to high pressure during the extraction process, and needs to be thick, strong, and sturdy. Production tubing is available in various sizes ranging from ¾” to 4 ½”.

The oil well tubing protects the oil well casing from wear and tear due to corrosive fluids, sand, paraffin etc. Tubing can be replaced in case of damage, which is not possible with oil well casing.