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Casing is a metal tube used in a well completion to contain production fluids and convey them to the surface from an underground reservoir oil reservoir. Casing installation follows the running of conductor pipe during the drilling of a well.
Usually multiple sizes of casing are used, the largest size being run first to the shallowest depth. After it is cemented into place, the next smallest size is run deeper and cemented. Typically, two or three casing sizes are run.
Cementing secures the casing in place, and also forms a solid continuous sheath around the casing to isolate different regions (known as zonal isolation). For example, if the casing is not cemented in place, water from a shallow underground aquifer could drain down the annulus (space between the casing and the drill bore) resulting in environmental issues and degrading of the production zone.
Casing strings are supported by casing hangers that are set in the wellhead or the christmas tree.
Casing intervals
Typically, a well contains multiple intervals of casing successively placed within the previous casing run. The following casing interval is typically used in an oil or gas well:
- Conductor casing
- Surface casing
- Intermediate casing (optional)
- Production casing or liner
The conductor casing serves as a support during drilling operations, to flowback returns during drilling and cementing of the surface casing, and to prevent collapse of the loose soil near the surface.
The purpose of surface casing is to isolate freshwater zones so that they are not contaminated during drilling and completion. Surface casing is the most strictly regulated due to these environmental concerns, which can include regulation of casing depth and cement quality.
Intermediate casing may be necessary on longer drilling intervals where necessary drilling mud weight to prevent blowouts may cause a hydrostatic pressure that can fracture deeper formations. Casing placement is selected so that the hydrostatic pressure of the drilling fluid remains between formation pore and fracture pressures.
The final interval is either production casing or production liner. As with the casing intervals described above, the production casing string extends to the surface where it is hung off. In order to reduce cost, a liner may be used instead which extends just above the shoe (bottom) of the previous casing interval and hung off downhole rather than at the surface.
Few wells actually produce through casing, since producing fluids can corrode steel or form deposits such as asphaltines or parrafins. Production tubing is therefore installed inside the last casing string and the tubing annulus is usually sealed at the bottom of the tubing by a packer. Tubing is easier to remove for maintenance, replacement, or for various types of workover operations. Tubing is significantly lighter than casing and does not require a drilling rig to run in and out of hole; smaller pulling units are used for this purpose.
Cementing
Cementing is performed by circulating a cement slurry through the inside of the casing and out into the annulus through the casing shoe at the bottom of the casing string. In order to precisely place the cement slurry at a required interval on the outside of the casing, a plug is pumped with a displacement fluid behind the cement slurry column, which "bumps" in the casing shoe and prevents further flow of fluid through the shoe. This bump can be seen at surface as a pressure spike at the cement pump. To prevent the cement from flowing back into the inside of the casing, a float collar above the casing shoe acts as a check valve and prevents fluid from flowing up through the shoe from the annulus.
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