Comprehensive Overview of Type of Oil Well Casing and Functions in Drilling and Production Operations

Oil well casing, a fundamental component in the realm of energy extraction, plays a pivotal role in ensuring the safety, efficiency, and environmental responsibility of oil and gas operations. This exploration unveils the diverse types of casing, their critical functions, and the integral part they play in stabilizing wellbores, preventing contamination, and enabling selective production. Join us as we navigate the crucial technology that supports the extraction of valuable resources from beneath the Earth’s surface.

Current Types of Casing in Use in Oil and Gas Industry:



Conductor Pipe:

Extending from the surface to a shallow depth, the conductor pipe is used to protect near-surface unconsolidated formations, seals off shallow-water zones, and protects against shallow gas flows. It also acts as a conduit for drilling mud and supports the platform’s foundation in offshore operations. Conductor pipes are cemented to the surface, with sizes varying (e.g., 20 in. in the Middle East or 26 or 30 in. in North Sea exploration wells). They support subsequent casing strings and wellhead equipment, with installation methods including driving, drilling, or a combination of both.

Surface Casing:

Deployed to prevent collapse in weak formations at shallow depths, surface casing is set in competent rocks like hard limestone. This casing, typically 13 3/8 in. in the Middle East or 18 5/8 in. to 20 in. in the North Sea, protects against shallow blowouts. It is strategically placed to shield troublesome formations, thief zones, water sands, shallow hydrocarbon zones, and build-up sections in deviated wells. BOPs are connected to the top of this casing for added protection.

Intermediate Casing:

Positioned in the transition zone below or above an over-pressured zone, intermediate casing seals off severe-loss zones or guards against problematic formations like mobile salt zones or caving shales. Ensuring good cementation is vital to prevent communication behind the casing. Commonly sized at 9 5/8 or 10 ¾ in., this casing may undergo multistage cementing to prevent high hydrostatic pressure on weak formations from a continuous, long column of cement.

Production Casing:

As the final casing string, production casing isolates producing zones, controls reservoir fluid, and allows selective production in multizone operations. Sized at 4 1/2, 5, and 7 in., it serves as the completion string for the well.


Acting as a string of casing that doesn’t reach the surface, liners are hung on the intermediate casing using a liner hanger. In liner completions, both the liner and the intermediate casing function as the production string. The primary design consideration for liners is their ability to withstand the maximum expected collapse pressure due to their positioning at the bottom and hanging from the intermediate casing.

Production Tubing:

The production tubing is installed in a well after the production casing is run and the completion equipment is run with this string. Its key functions include serving as a conduit for extracting oil, gas, and water from the formations, as well as safeguarding the production casing against corrosion, wear, and deposition caused by reservoir fluids. The tubing must possess sufficient strength to endure production loads and allow for future workover operations.


Jonathan Bellarby, 2009. Well Completion Design, Volume 56 (Developments in Petroleum Science). 1 Edition. Elsevier Science.

Wan Renpu, 2011. Advanced Well Completion Engineering, Third Edition. 3 Edition. Gulf Professional Publishing.

Ted G. Byrom, 2014. Casing and Liners for Drilling and Completion, Second Edition: Design and Application (Gulf Drilling Guides). 2 Edition. Gulf Professional Publishing.

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