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Concept
Casing (borehole)
Summary
Casing is a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. Similar to the bones of a spine protecting the spinal cord, casing is set inside the drilled borehole to protect and support the wellstream. The lower portion (and sometimes the entirety) is typically held in place with cement. Deeper strings usually are not cemented all the way to the surface, so the weight of the pipe must be partially supported by a casing hanger in the wellhead.
Casing that is cemented in place aids the drilling process in several ways:
Prevents contamination of fresh water well zones.
Prevents unstable upper formations from caving in and sticking the drill string or forming large caverns.
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, minimizing potential for a blowout.
Prevents fluid loss into or contamination of production zones.
Provides a smooth internal bore for installing production equipment.
Optimum design of the casing program decreases the well construction costs, enhances the efficiency of operations and also diminishes the environmental impacts.
A slightly different metal string, called production tubing, is often used without cement inside the final casing string of a well to contain production fluids and convey them to the surface from an underground reservoir.
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 and strengths, well integrity, and is balanced against the cost objectives and desired drilling strategy.
With the casing set depths determined, hole sizes and casing sizes must follow.
Official source
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Ontological neighbourhood
Related publications (10)
Related concepts (2)
Completion (oil and gas wells)
Well completion is the process of making a well ready for production (or injection) after drilling operations. This principally involves preparing the bottom of the hole to the required specifications, running in the production tubing and its associated down hole tools as well as perforating and stimulating as required. Sometimes, the process of running in and cementing the casing is also included. After a well has been drilled, should the drilling fluids be removed, the well would eventually close in upon itself.
Oil well
An oil well is a drillhole boring in Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released as associated petroleum gas along with the oil. A well that is designed to produce only gas may be termed a gas well. Wells are created by drilling down into an oil or gas reserve that is then mounted with an extraction device such as a pumpjack which allows extraction from the reserve.