Casing is the steel pipe that is run into a drilled hole and cemented in place to form a permanent, protected wellbore. A well is never drilled and cased in one go. Instead, it is built in sections, and each section is lined with a different casing string of a smaller diameter. In this article you will learn the five main types of casing, what each one does, and how they work together.
Why Do We Run Casing?
Without casing, the hole would collapse, drilling fluid would leak into weak formations, and fresh water zones could be contaminated. The main functions of casing are:
- To furnish a permanent and gauged wellbore of precise diameter through which later drilling, completion and production operations can take place.
- To allow selective production from different formations without flow between them (interformational flow).
- To provide a means of attaching the wellhead system and Xmas tree, which control and handle the produced fluid.
- To isolate unstable, weak or high-pressure zones so that drilling can continue safely.
The Five Main Types of Casing
Casing strings are named for the job they do and the depth at which they are set. From the top of the well to the bottom, there are five main types:
- The conductor casing (or stove pipe)
- The surface casing
- The intermediate casing
- The production casing
- The production liner system, a liner set through the productive interval and landed inside the previous casing on a hanger, or tied back to the surface
1. Conductor Casing
The conductor is the first and largest string, usually between 16 and 30 inches in diameter. It is driven or drilled and cemented a short distance below the surface, often 30 to 150 feet on land and deeper offshore. It stops the loose, shallow soil from caving in and washing out, and it gives the drilling fluid a path to flow back to the pits.
2. Surface Casing
The surface casing is set below the shallow fresh water sands, commonly at a few hundred to a couple of thousand feet. It is always cemented back to surface. Its main jobs are to protect fresh water from contamination by drilling fluid, oil, gas or formation water, to hold the weak shallow formations, and to carry the first blowout preventer (BOP) stack. For these reasons it must be strong enough to hold pressure if the well kicks.
The conductor and surface casing are used to:
- Control caving and washing out of poorly consolidated surface formations
- Furnish the means of handling the return flow of drilling fluid
- Protect fresh water sands from possible contamination by drilling fluid, oil, gas or formation water
- Allow the attachment of BOP systems
3. Intermediate Casing
Intermediate casing, also called protection casing, is set between the surface and production casing. A well may need none, one, or several intermediate strings. Their purpose is to seal off troublesome zones that contaminate the drilling fluid or cause hole problems, for example unstable shales, lost-circulation zones, salt sections or abnormally pressured formations. Setting depths are normally chosen using the pore pressure and fracture gradient, which is checked with a leak-off test. The properties of the drilling mud are also matched to each hole section.
4. Production Casing
The production casing is run through or just above the pay zone. It isolates the productive interval from all other zones, houses the production tubing and completion equipment, and withstands the pressures of production and well-stimulation work. It is usually the string that stays in service for the life of the well, so its grade and thickness are chosen carefully.
5. Production Liner
A liner is a shorter string that does not reach the surface. It hangs from the bottom of the previous casing on a liner hanger and covers only the open hole below it. Because less steel is used, liners can cut cost in deep wells. If the liner is later connected back to the surface, it is called a tieback. The production string or liner provides the means of segregating the pay section from all other zones and allows for selective production.
Casing Types at a Glance
| Casing | Typical purpose | Cemented to surface? |
|---|---|---|
| Conductor | Stops shallow caving, guides drilling fluid returns | Yes |
| Surface | Protects fresh water, supports BOP | Yes |
| Intermediate | Isolates problem zones | Often partly |
| Production | Isolates the pay zone, houses completion | Depends on design |
| Liner | Covers the lower open hole, saves cost | No (hangs from previous casing) |
How Casing Is Designed
Engineers work from the bottom up. They first decide where the production string must go, then choose setting depths for the strings above using pore pressure and fracture gradient. Each string must fit inside the last one, so hole and casing sizes step down as the well gets deeper. Casing is then selected for burst, collapse and tension loads. For a broader overview of casing programs, see the introduction to casing and cementing.
Frequently Asked Questions
How many casing strings does a well have?
It depends on depth and geology. A shallow well may have only a conductor, surface and production string, while a deep or complex well may have several intermediate strings and a liner.
What is the difference between casing and a liner?
Casing runs from its setting depth all the way to the surface. A liner hangs from inside the previous casing and does not reach the surface.
Why is casing cemented?
Cement holds the pipe in place, seals the space between the pipe and the rock, and prevents fluids from moving between formations.
Key Takeaways
- Casing is built in strings, from the large conductor down to the production casing or liner.
- Surface casing protects fresh water and carries the BOP.
- Intermediate casing isolates problem zones such as unstable shales or high pressures.
- Production casing and liners isolate the pay zone and house the completion.
Casing programs are an important part of well planning.
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