- Casing (borehole)
Casing is large diameter pipe that is assembled and inserted into a recently drilled section of a
boreholeand typically cemented into place.
Casing that is cemented in place aids the drilling process in several ways:
* Prevent contamination of fresh water well zones.
* Prevent unstable upper formations from caving-in and sticking the drill string or forming large caverns.
* Provides a strong upper foundation to use high-density
drilling fluidto continue drilling deeper.
* Isolates different zones (that may have different pressures or fluids - known as zonal isolation) in the drilled formations from one another.
* Seals off high pressure zones from the surface, avoiding potential for a blowout
* Prevents fluid loss into or contamination of production zones.
* Provides a smooth internal bore for installing production equipment
A slightly different metal string, called
production tubing, is often used without cement in the smallest casing of a well completion 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, 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 wellheador the christmas tree.
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
* Production 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. It can normally vary from sizes such as 18" to 30".
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. A typical size of surface casing is 13⅜".
"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". As with the casing intervals described above, the production casing string extends to the surface where it is hung off. As the smallest casing, it will former the outer boundary of the 'A' annulus, which may involve it being used for
gas liftand well kills. A typical size is 9⅝".
In order to reduce cost, a liner may be used which extends just above the shoe (bottom) of the previous casing interval and hung off downhole rather than at the surface. It may typically be 7", although many liners match the diameter of the
Few wells actually produce through casing, since producing fluids can corrode steel or form deposits such as
asphaltenes or paraffins and the larger diameter can make flow unstable. Production tubingis 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 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.
Expandable Tubular Technology
* [http://www.glossary.oilfield.slb.com/Display.cfm?Term=casing Schlumberger Oilfield Glossary: Casing]
Wikimedia Foundation. 2010.
Look at other dictionaries:
Casing — may refer to:* Casing (ammunition), a shell enclosing the explosive propellant in ammunition * Casing (sausage), a thin covering holding the food contents of sausage * Casing (borehole), a metal tube used during the drilling of a well * Casing… … Wikipedia
Borehole mining — (BHM) is a remote operated method of extracting (mining) mineral resources through boreholes by means of high pressure water jets. This process can be carried out from land surface, open pit floor, underground mine or floating platform or vessel… … Wikipedia
Casing string — is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe joints are typically about 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double female threaded pipe… … Wikipedia
casing — [kās′iŋ] n. 1. the act of encasing 2. a protective covering; specif., ☆ a) a cleaned intestine as of cattle, a plastic membrane, etc., used to encase processed meats ☆ b) the supporting framework of a pneumatic rubber tire exclusive of an inner… … English World dictionary
Borehole — A borehole is the generalised term for any narrow shaft drilled in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes including the extraction of water or other liquid (such as petroleum) or… … Wikipedia
Casing shoe — In oil drilling and borehole mining, a casing shoe or guide shoe is a bull nose shaped device which is attached to the bottom of the casing string. A casing hanger, which allows the casing to be suspended from the wellhead, is attached to the top … Wikipedia
casing shoe — /ˈkeɪsɪŋ ʃu/ (say kaysing shooh) noun a steel sleeve fitted to the bottom end of a casing (def. 7) put into a borehole … Australian English dictionary
Offshore oil spill prevention and response — Inspector on offshore oil drilling rig Offshore oil spill prevention and response is the study and practice of reducing the number of offshore incidents that release oil or hazardous substances into the environment and limiting the amount… … Wikipedia
List of oil field acronyms — Contents 1 # 2 A 3 B 4 C … Wikipedia
petroleum production — Introduction recovery of crude oil and, often, associated natural gas from the Earth. petroleum is a naturally occurring hydrocarbon material that is believed to have formed in deep sedimentary (sedimentary rock) beds from animal and … Universalium