Wellhead
Encyclopedia
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.
The primary purpose of a wellhead is to provide the suspension point and pressure seals for the casing string
s that run from the bottom of the hole sections to the surface pressure control equipment.
While drilling
the oil well, surface pressure control is provided by a blowout preventer
(BOP). If the pressure is not contained during drilling operations by the column of drilling fluid, casings, wellhead, and BOP, a well blowout
could occur.
Once the well has been drilled, it is completed
to provide an interface with the reservoir rock and a tubular conduit for the well fluids. The surface pressure control is provided by a christmas tree
, which is installed on top of the wellhead, with isolation valves and choke equipment to control the flow of well fluids during production.
Wellheads are typically welded onto the first string of casing, which has been cemented in place during drilling operations, to form an integral structure of the well. In exploration wells that are later abandoned, the wellhead may be recovered for refurbishment and re-use.
Offshore, where a wellhead is located on the production platform
it is called a surface wellhead, and if located beneath the water then it is referred to as a subsea
wellhead or mudline wellhead.
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...
that provides the structural and pressure-containing interface for the drilling and production equipment.
The primary purpose of a wellhead is to provide the suspension point and pressure seals for 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...
s that run from the bottom of the hole sections to the surface pressure control equipment.
While drilling
Drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...
the oil well, surface pressure control is provided by a blowout preventer
Blowout preventer
A blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic...
(BOP). If the pressure is not contained during drilling operations by the column of drilling fluid, casings, wellhead, and BOP, a well blowout
Blowout (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....
could occur.
Once the well has been drilled, it is completed
Completion (oil and gas wells)
In petroleum production, completion is the process of making a well ready for production . 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...
to provide an interface with the reservoir rock and a tubular conduit for the well fluids. The surface pressure control is provided by a 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...
, which is installed on top of the wellhead, with isolation valves and choke equipment to control the flow of well fluids during production.
Wellheads are typically welded onto the first string of casing, which has been cemented in place during drilling operations, to form an integral structure of the well. In exploration wells that are later abandoned, the wellhead may be recovered for refurbishment and re-use.
Offshore, where a wellhead is located on the production platform
Oil platform
An oil platform, also referred to as an offshore platform or, somewhat incorrectly, oil rig, is a lаrge structure with facilities to drill wells, to extract and process oil and natural gas, and to temporarily store product until it can be brought to shore for refining and marketing...
it is called a surface wellhead, and if located beneath the water then it is referred to as a subsea
Subsea
Subsea is a general term frequently used to refer to equipment, technology, and methods employed in marine biology, undersea geology, offshore oil and gas developments, underwater mining, and offshore wind power industries.- Oil and gas :...
wellhead or mudline wellhead.
Functions
A wellhead serves numerous functions, some of which are:- Provide a means of casing suspension. (Casing is the permanently installed pipe used to line the well hole for pressure containment and collapse prevention during the drilling phase).
- Provides a means of tubing suspension. (Tubing is removable pipe installed in the well through which well fluids pass).
- Provides a means of pressure sealing and isolation between casing at surface when many casing strings are used.
- Provides pressure monitoring and pumping access to annuliAnnulus (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...
between the different casing/tubing strings. - Provides a means of attaching a blowout preventerBlowout preventerA blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic...
during drilling. - Provides a means of attaching a christmas treeChristmas 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...
for production operations. - Provides a reliable means of well access.
- Provides a means of attaching a well pump,
Components
The primary components of a wellhead system are:- casing headCasing headIn oil drilling, a casing head is a simple metal flange welded or screwed on to the top of the conductor pipe or the casing and forms part of the wellhead system for the well....
- casing spools
- casing hangerCasing hangerThe 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 - packoffs (isolation) seals
- bowl protectors / wear bushings
- test plugs
- mudline suspension systems
- tubing heads
- tubing hangers
- tubing head adapters
Design specification
The oil industry specifications for wellhead systems (materials, dimensions, test procedures and pressure ratings etc.) are :- APIAmerican Petroleum InstituteThe American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
6A Specification for Wellhead and Christmas Tree Equipment - ISO 10423 Wellhead and Christmas Tree Equipment