Casing (oil well)

From Wikipedia, the free encyclopedia

Casing is a metal tube used during drilling an oil well in combination with cement to sequentially stabilize recently drilled formation.

1 Purpose
2 Design
3 Intervals
4 Cementing
5 External links

[edit] Purpose

Casing that is cemented in place aids the drilling process in several ways:

Isolates different zones (that may have different pressures or fluids - known as zonal isolation) in the drilled formation from one another
Seals off high pressure zones from the surface, avoiding potential for a blowout
Provides a strong upper foundation to use high-density drilling fluid to continue drilling deeper
Provides a smooth internal bore for installing production equipment
Isolates contaminating or corrosive fluids

Smaller casing is often used without cement in a well completion to contain production fluids and convey them to the surface from an underground reservoir oil reservoir. Used for this purpose, it is often called tubing.

[edit] Design

In the planning stages of a well a drilling engineer, usually with input from geologists and others, will pick strategic depths at which the hole will need to be cased in order for drilling to reach the desired total depth. This decision is often based on subsurface data such as formation pressures, strengths, and makeup, and is balanced against the cost objectives and desired drilling strategy.

With the casing set depths determined, hole sizes and casing sizes must follow. The hole drilled for each casing string must be large enough to easily fit the casing inside it, allowing room for cement between the outside of the casing and the hole. Also,the inside diameter of the first casing string must be large enough to fit the second bit that will continue drilling. Thus, each casing string will have a subsequently smaller diameter.

Casing strings are supported by casing hangers that are set in the wellhead or the christmas tree.

[edit] 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.

[edit] 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.