Annulus (oil well)

Annulus (oil well)

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The annulus of an oil well refers to any void between any piping, 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...

or casing
Casing (oil well)
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.-Purpose:Casing that is cemented in place aids the drilling process in several ways:...

and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept
Annulus (mathematics)
In mathematics, an annulus is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. Or, it is the area between two concentric circles...

. The presence of an annulus gives the ability to circulate fluid in the well, provided that excess drill cuttings
Drill cuttings
Drill cuttings are the broken bits of solid material removed from a borehole drilled by rotary, percussion, or auger methods. Boreholes drilled in this way include oil or gas wells, water wells, and holes drilled for geotechnical investigations or mineral exploration.The drill cuttings are commonly...

have not accumulated in the annulus, preventing fluid movement and possibly sticking the pipe in the 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...

.

For a new well in the process of being drilled, this would be the void between the drill string
Drill string
A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...

and the formation being drilled. An easy way to visualise this would be to stand a straw (purple in diagram) straight up in the center of a glass of water. All of the water in between the straw and the sides of the glass would be the annulus (yellow area in diagram), with the straw itself representing the drill string
Drill string
A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...

and the sides of the glass representing the formation
Formation
Formation may refer to:* Formation flying, aerobatics performed with several aircraft* Formation , a high-level military organization* Tactical formation, the physical deployment of military forces-Sports:...

. While drilling, drilling fluid
Drilling fluid
In 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...

is pumped down the inside of the drill string and pushes the drill cuttings
Drill cuttings
Drill cuttings are the broken bits of solid material removed from a borehole drilled by rotary, percussion, or auger methods. Boreholes drilled in this way include oil or gas wells, water wells, and holes drilled for geotechnical investigations or mineral exploration.The drill cuttings are commonly...

up the annulus to the surface, where the cuttings are removed from the drilling fluid
Drilling fluid
In 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...

(drilling mud) by the shale shakers
Shale shakers
Shale shakers are devices that remove drill cuttings from the drilling fluid that is used for boring holes into the earth. Controlling the solids in drilling fluid is an important component of the cost of drilling, so research into improved shaker design is ongoing.-Shale shaker design:Shale...

.

In a completed well, there may be many annuli. The 'A' annulus is the void between 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 the smallest casing string
Casing (oil well)
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.-Purpose:Casing that is cemented in place aids the drilling process in several ways:...

. The A annulus can serve a number of crucial tasks, including 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 normal well will also have a 'B' and frequently a 'C' annulus, between the different casing strings. These annuli do not normally have any connection to well bore fluids, but maintaining pressure in them is important in order to ensure integrity of the casing strings.

Though all annuli in a completed well are expected to be isolated from 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 each other, connections allowing the flow of fluids between them may sometimes occur, due to either intervention
Well intervention
A well intervention, or 'well work', is any operation carried out on an oil or gas well during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well....

or wear and tear. In these situations, it is said that there is "communication" between them.

During coiled tubing
Coiled tubing
In the oil and gas industries, coiled tubing refers to metal piping, normally 1" to 3.25" in diameter, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing is often used to carry out operations...

interventions, the void between the coil and the production tubing can also be considered an annulus and be used for circulation.

• Oil well
Oil 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...

• 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...

• Casing
Casing (oil well)
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.-Purpose:Casing that is cemented in place aids the drilling process in several ways:...

• Coiled tubing
Coiled tubing
In the oil and gas industries, coiled tubing refers to metal piping, normally 1" to 3.25" in diameter, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing is often used to carry out operations...

• 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...

• Mud Gas Separator
Mud Gas Separator
Mud Gas Separator is commonly called a gas-buster or poor boy degasser. It captures and separates large volume of free gas within the drilling fluid. If there is a "KICK" situation, this vessel separates the mud and the gas by allowing it to flow over baffle plates. The gas then is forced to flow...