Casing (oil well)
Encyclopedia
Casing is large diameter pipe
that is assembled and inserted into a recently drilled section of a borehole
and typically held into place with cement
.
A slightly different metal
string, called production tubing
, is often used without cement in the smallest casing of a well completion to contain production fluid
s and convey them to the surface from an underground reservoir
.
s 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.
The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing
and associated hardware such as packers, gas lift mandrels and subsurface safety valves.
Casing design for each size is done by calculating the worst condition that may be faced during drilling and production. Mechanical properties of designed pipes such as collapse resistance, burst pressure, and tensile strength must be sufficient for the worst conditions.
Casing strings are supported by casing hanger
s that are set in the wellhead
, which later will be topped with the Christmas tree
. The wellhead usually is installed on top of the first casing string after it has been cemented in place.
or gas well:
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 inches.
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 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 form the outer boundary of the 'A' annulus
, which may involve it being used for gas lift
and well kill
s. A typical size is 9⅝ inches.
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 production tubing
.
Few wells actually produce through casing, since producing fluids can corrode steel
or form deposits such as asphaltenes or paraffin
s and the larger diameter can make flow unstable. 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. It is significantly lighter than casing and does not require a drilling rig
to run in and out of hole; smaller "service rigs" are used for this purpose.
ing 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.
Pipe
Pipe may refer to:* Pipe , a hollow cylinder following certain dimension rules* Smoking pipe* Pipe or butt, a cask measurement* Pipe , a type of metal casting defect...
that is assembled and inserted into a recently drilled section of a borehole
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
and typically held into place with cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
.
Purpose
Casing that is cemented in place aids the drilling process in several ways:- Prevent contamination of fresh waterFresh WaterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
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 fluidDrilling fluidIn geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...
to continue drilling deeper. - Isolates different zones, that may have different pressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
s 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 blowoutBlowout (well drilling)A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....
- Prevents fluid loss into or contamination of production zones.
- Provides a smooth internal bore for installing production equipment.
A slightly different metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
string, called production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
, is often used without cement in the smallest casing of a well completion to contain production fluid
Production fluid
Production fluid is the fluid mixture of oil, gas and water in formation fluid that flows to the surface of an oil well from a reservoir. Its consistency and composition varies.Fluids may be described by a multitude of characteristics including:...
s and convey them to the surface from an underground reservoir
Oil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...
.
Design
In the planning stages of a well a drilling engineer, usually with input from geologistGeologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s 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
Casing string
Casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe segments are typically about in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings...
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.
The inside diameter of the final casing string (or penultimate one in some instances of a liner completion) must accommodate the production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
and associated hardware such as packers, gas lift mandrels and subsurface safety valves.
Casing design for each size is done by calculating the worst condition that may be faced during drilling and production. Mechanical properties of designed pipes such as collapse resistance, burst pressure, and tensile strength must be sufficient for the worst conditions.
Casing strings are supported by casing hanger
Casing hanger
The casing hanger is that portion of a wellhead assembly which provides support for the casing string when it is lowered into the wellbore. It serves to ensure that the casing is properly located. When the casing string has been run into the wellbore it is hung off, or suspended, by a casing...
s that are set in the wellhead
Wellhead
A wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....
, which later will be topped with the Christmas tree
Christmas tree (oil well)
In petroleum and natural gas extraction, a Christmas tree, or "tree", is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells...
. The wellhead usually is installed on top of the first casing string after it has been cemented in place.
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 oilOil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
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
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
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 inches.
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 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 form the outer boundary of the 'A' annulus
Annulus (oil well)
The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
, which may involve it being used for gas lift
Gas lift
Gas lift is one of a number of processes used to artificially lift oil or water from wells where there is insufficient reservoir pressure to produce the well. The process involves injecting gas through the tubing-casing annulus. Injected gas aerates the fluid to reduce its density; the formation...
and well kill
Well kill
A well kill is the operation of placing a column of heavy fluid into a well bore in order to prevent the flow of reservoir fluids without the need for pressure control equipment at the surface. It works on the principle that the weight of the "kill fluid" or "kill mud" will be enough to suppress...
s. A typical size is 9⅝ inches.
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 production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
.
Few wells actually produce through casing, since producing fluids can corrode steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
or form deposits such as asphaltenes or paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
s and the larger diameter can make flow unstable. Production tubing
Production tubing
Production tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...
is therefore installed inside the last casing string and the tubing annulus is usually sealed at the bottom of the tubing by a packer
Production packer
A production packer is a standard component of the completion hardware of oil or gas wells used to provide a seal between the outside of the production tubing and the inside of the casing, liner, or wellbore wall....
. Tubing is easier to remove for maintenance, replacement, or for various types of workover operations. It is significantly lighter than casing and does not require a drilling rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...
to run in and out of hole; smaller "service rigs" are used for this purpose.
Cementing
CementCement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
ing is performed by circulating a cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
slurry through the inside of the casing and out into the annulus through the casing shoe
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 of the casing....
at the bottom of the casing string
Casing string
Casing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe segments are typically about in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings...
. 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
Check valve
A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction....
and prevents fluid from flowing up through the shoe from the annulus.
See also
- AnnulusAnnulus (oil well)The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
- Casing stringCasing stringCasing string is a long section of connected oilfield pipe that is lowered into a wellbore and cemented. The pipe segments are typically about in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings...
- Conductor tensionerConductor tensionerA conductor tensioner unit is installed on an offshore jackup rig to apply upward tension on the conductor. The conductor tensioner is installed into the conductor deck and is usually removable in order to protect the unit during transit....
- Expandable tubular technologyExpandable Tubular TechnologyExpandable tubular technology is a system for increasing the diameter of the casing or liner of an oil well by up to 30% after it has been run down-hole.-Challenges:...
- Oil wellOil wellAn oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
- Production packerProduction packerA production packer is a standard component of the completion hardware of oil or gas wells used to provide a seal between the outside of the production tubing and the inside of the casing, liner, or wellbore wall....
- Production tubingProduction tubingProduction tubing is a tube used in a wellbore through which production fluids are produced .Production tubing is run into the drilled well after the casing is run and cemented in place. Production tubing protects wellbore casing from wear, tear, corrosion, and deposition of by-products, such as...