What Is Casing in Drilling?
Casing is a large-diameter steel pipe that is run into a drilled wellbore and normally cemented in place. When discussing types of casing in drilling, it’s important to understand their main functions: maintaining wellbore integrity, isolating different formations and pressure zones, protecting freshwater formations, and providing structural support for well-control and completion equipment.
As drilling progresses deeper, different casing strings are installed according to the well design and formation conditions. In a typical oil or gas well, the main casing types are structural or drive pipe, conductor casing, surface casing, intermediate casing, production casing, and liner.
Types Of Casing in Oil and Gas Wells
The following table summarizes the main casing strings and their functions:
| Casing Type | Where It Is Used | Main Function |
|---|---|---|
| Structural / Drive Pipe | Very shallow section of the well | Provides initial structural support and stabilizes loose near-surface formations |
| Conductor Casing | Near-surface hole section | Supports shallow formations and provides a foundation for subsequent drilling operations |
| Surface Casing | Below conductor casing | Protects freshwater formations, isolates weak formations, and provides support for well-control equipment |
| Intermediate Casing | Between surface and production casing | Isolates troublesome formations, abnormal pressure zones, losses, or unstable intervals |
| Production Casing | Deepest full casing string, normally associated with the producing interval | Provides long-term well integrity and enables safe completion and production operations |
| Liner | Hung from inside the previous casing rather than extending to surface | Isolates deeper intervals while reducing casing cost, weight, and running requirements |
The exact casing program varies from well to well. Some wells may require several intermediate casing strings, while others may reach the reservoir with fewer strings. The final casing program is selected according to formation pressures, fracture gradients, hole stability, well-control requirements, drilling hazards, and the planned completion.
Structural Casing or Drive Pipe
This casing string provides initial hole support close to the surface and generally varies in length from about 30 to 300 feet. The casing may be driven or hole drilled. The casing may be cemented if it is necessary to drill a hole. Figure 1 represents a casing design for a deep, offshore well.
Conductor Casing
This string of casing in some areas may be the first casing string set and cemented. In others, particularly offshore rigs, it is the first string of casing that provides any protection against formation pore pressures.
The general practice is to cement around this string of casing back to the surface. If cement returns are not obtained at the surface, a so-called top job is typical, where cement is displaced into the annulus from the surface through a small string of pipe. We can summarize its purposes as follows:
- To Prevent erosion around the rig’s foundation where the surface sand is weak.
- To make the cellar more stable.
- To provide a good start for subsequent drilling
- To circulate drilling fluids through to the surface.
- To protect inner casings from corrosion.
The conductor casing type setting depth is usually based on the amount required to prevent washout of the shallow borehole when drilling to the depth of the surface casing and to support the weight of the surface casing.
A drilling diverter can be installed on the conductor to divert flow in case of a shallow kick. Usually, when the surface sand is stable, this casing string is unnecessary.
See also: Running procedures for 30″ conductor Pipes.
Surface Casing
This type of casing, which is used in drilling wells, serves several purposes:
- To protect shallow freshwater sands from possible contamination
- To prevent cave-in of unconsolidated, weaker, near-surface sediments and in the event of a kick.
- To allow the flow to be contained by the blowout preventers.
- Any subsequent casing strings run in the well to support and protect from corrosion.
See also: Casing Running and Cementing Preparation, Casing Running – installation patterns, Casing Running Procedures.
Intermediate Casing Type In Drilling
This string of casing, which is used on oil and gas wells, serves several purposes:
- Intermediate casing is similar to surface casing in that it permits the final depth objective to be reached safely.
- To isolate problem zones, i.e., abnormal pressure, lost circulation, sloughing, or caving zones between the surface and production casing depths.
More than one intermediate string can be set if necessary.
Liner Type Of Casing In Drilling
A liner can be used for drilling to isolate problem zones, production, or both.
A liner is a different type of casing profile in that it does not extend from the bottom of the drilled well to the surface but is suspended from the bottom of the next largest casing string. Usually, it only extends a few hundred feet above the last casing shoe depth.
One of the primary advantages of using liners is the savings in steel costs. Also, liners may be run quickly, compared to full casing strings, which improves the chances of getting the liner to the desired depth.
A disadvantage of a liner is that the previous casing string must withstand the pressures encountered after drilling below the liner. This means a heavier design with a larger diameter casing. The casing above the liner is also subject to drill string wear for extended periods.
Protective and production drilling casing liner types are set, and their purpose is the same as an entire casing string. Another problem with liners has been getting a good cement seal around the liner.
When running and cementing, liners are generally neither moved nor centralized ( Learn more about casing centralization). As a result, it is not easy to get cement on all sides of the liner. Liner hangers are available that permit liner movement during cementing; however, they are not commonly used. Other liner problems include standard clearances, which are sometimes small, particularly with deep production liners.
Production Casing
This string of casing is set and cemented to isolate the productive formations. The amount of cement used is generally related to company preference and would be based on the volume needed to bring the cement back to a pre-determined height.
The cement volume may be based on caliper log calculations or a given excess based on experience. Remember that cement will travel more readily up the large side of the hole. As a result, you may need to use more excess than cement calculations indicate to cover the casing adequately in the small part of the hole if it needs to be centralized.
Casing Type vs. Casing Grade and Connection
It is important to distinguish casing type from casing size, grade, weight, and connection. The casing type—such as surface, intermediate, or production casing—describes the string’s position and function in the well. Its size, weight, steel grade, and connection are engineering specifications selected during casing design.
For detailed engineering information, see our guides to casing sizes and specifications, casing design calculations, casing connections, and casing liners.