CASING AND TUBING

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. Casing is large diameter pipe which is held in place in the borehole using cement.
The diameters of 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.
The oil well casing pipe is further divided into:

• Conductor Casing – Conductor casing is the first boundary between the surface and the subsurface. It helps prevent well collapse, keeps away the groundwater, and most importantly provides structural support. Conductor casing is installed in the ground by either drilling it or hammering it down. It is available in sizes ranging from 18″ to 36″.
• Surface Casing – The purpose of surface casing is primarily environmental and to ensure safety. It helps in separating the freshwater zones, offers protection from blowout, and supports the wellhead and blow out prevention (BOP) equipment. Therefore surface casing is required to maintain very high safety standards. It is responsible for supporting the next casing in line. It may vary in size depending on the purpose, but the most common size of surface casing is 13 3/8”.
• Intermediate Casing – Intermediate casing is not always required in all types of oil and gas production. If used, it is placed between the surface and the production casing. It is available in sizes ranging from 13 3/8” to 16”.
• Production Casing – Production casing helps provide structural integrity and pressure control of the hydrocarbon-bearing sections, mainly during the production of oil.
• Production Liner – Production liners can be suspended from the production casing or one of the previous casings. They help in reducing the cost significantly as the liner is not required to run through the full depth of the well. Liners can be pre-perforated to help save time and cost.

The oil well casing is subject to high axial tension because of the external pressure received from the rocks surrounding it and the internal pressure during the oil extraction. A well-cemented casing can be very useful in the drilling process in several ways:

• Holds the drill string, preventing the unstable upper formation from caving in
• Helps the prevention of contamination of water well zones
• Provides an even, internal bore for easy installation of the production equipment
• Helps prevention of the contamination of production zones
• Helps prevention of fluid (oil) loss
• Helps seal off the high-pressure zones from the surface of the earth

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.