Borehole Casing is large diameter pipe that is inserted into a recently drilled section of a borehole, to protect and support the borehole stream. The lower portion is held in place with cement. Deeper strings usually are not cemented all the way to the surface therefore, the weight of the pipe must be partly supported by a Borehole Casing hanger in the borehole head.
The Purpose of Casing
Casing that is cemented in place aids the drilling process in several ways:
- Prevents pollution of water from the borehole.
- Prevents unbalanced upper formations from caving in and sticking the drill string.
- Provides a strong upper foundation to allow use of high-density drilling fluid to continue drilling deeper.
- Isolates various zones, which may have different pressures or fluids, in the drilled formations from one another.
- Seals off high pressure zones from the surface.
- Prevents fluid loss into or contamination of production zones.
- Provides a smooth internal bore for installing production equipment.
Optimum design of the Borehole Casing program decreases the borehole construction costs and improves the productivity of operations and reduces the environmental impacts.
A string known as production tubing, is often used without cement inside the final Borehole Casing string of a borehole to hold production fluids and transport them to the surface from an underground reservoir.
Typically, a borehole holds multiple intervals of Borehole Casing continuously placed within the previous casing run. The following casing intervals are typically used in an oil or gas borehole:
- Conductor casing
- Surface casing
- Intermediate casing (optional)
- Production casing
- Production liner
The conductor Borehole 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
Borehole Casing arranged on a rack at a drilling rig in preparation for installation. Cementing is done by mixing a cement slurry from inside of the casing and out into the annulus over 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 dislocation fluid behind the cement slurry column, that bumps in the casing shoe and stops further flow of fluid through the shoe.
This bump is visible on the surface as a pressure spike at the cement pump. To prevent the cement from flowing back 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.