An oil well is a boring in the Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released along with the oil. A well that is designed to produce only gas may be termed a gas well.
The earliest known oil wells were drilled in China in 347 CE. These wells had depths of up to about 240 metres (790 ft) and were drilled using bits attached to bamboo poles. The oil was burned to evaporate brine and produce salt. By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The ancient records of China and Japan are said to contain many allusions to the use of natural gas for lighting and heating. Petroleum was known as Burning water in Japan in the 7th century.
According to Kasem Ajram, petroleum was distilled by the Persian alchemist Muhammad ibn Zakarīya Rāzi (Rhazes) in the 9th century, producing chemicals such as kerosene in the alembic (al-ambiq), and which was mainly used for kerosene lamps. Arab and Persian chemists also distilled crude oil in order to produce flammable products for military purposes. Through Islamic Spain, distillation became available in Western Europe by the 12th century.
Some sources claim that from the 9th century, oil fields were exploited in the area around modern Baku, Azerbaijan, to produce naphtha for the petroleum industry. These places were described by Marco Polo in the 13th century, who described the output of those oil wells as hundreds of shiploads. When Marco Polo in 1264 visited Baku, on the shores of the Caspian Sea, he saw oil being collected from seeps. He wrote that "on the confines toward Geirgine there is a fountain from which oil springs in great abundance, in as much as a hundred shiploads might be taken from it at one time."
In 1846, Baku (settlement Bibi-Heybat) the first ever well was drilled with percussion tools to a depth of 21 meters for oil exploration. In 1848, the first modern oil well was drilled on the Absheron Peninsula north-east of Baku, by Russian engineer F.N. Semyenov.
Ignacy Łukasiewicz, a Polish pharmacist and petroleum industry pioneer built one of the world's first modern oil wells in 1854 in Polish village Bóbrka, Krosno County who in 1856 built one of the world's first oil refineries.
In North America, the first commercial oil well entered operation in Oil Springs, Ontario in 1858, while the first offshore oil well was drilled in 1896 at the Summerland Oil Field on the California Coast.
The earliest oil wells in modern times were drilled percussively, by repeatedly raising and dropping a cable tool into the earth. In the 20th century, cable tools were largely replaced with rotary drilling, which could drill boreholes to much greater depths and in less time. The record-depth Kola Borehole used non-rotary mud motor drilling to achieve a depth of over 12,000 metres (39,000 ft).
Until the 1970s, most oil wells were vertical, although lithological and mechanical imperfections cause most wells to deviate at least slightly from true vertical. However, modern directional drilling technologies allow for strongly deviated wells which can, given sufficient depth and with the proper tools, actually become horizontal. This is of great value as the reservoir rocks which contain hydrocarbons are usually horizontal or nearly horizontal; a horizontal wellbore placed in a production zone has more surface area in the production zone than a vertical well, resulting in a higher production rate. The use of deviated and horizontal drilling has also made it possible to reach reservoirs several kilometers or miles away from the drilling location (extended reach drilling), allowing for the production of hydrocarbons located below locations that are either difficult to place a drilling rig on, environmentally sensitive, or populated.
Before a well is drilled, a geologic target is identified by a geologist or geophysicist to meet the objectives of the well.
The target (the end point of the well) will be matched with a surface location (the starting point of the well), and a trajectory between the two will be designed. There are many considerations to take into account when designing the trajectory such as the clearance to any nearby wells (anti-collision) or if this well will get in the way of future wells, trying to avoid faults if possible and certain formations may be easier/more difficult to drill at certain inclinations or azimuths.
When the well path is identified, a team of geoscientists and engineers will develop a set of presumed properties of the subsurface that will be drilled through to reach the target. These properties include pore pressure, fracture gradient, wellbore stability, porosity, permeability, lithology, faults, and clay content. This set of assumptions is used by a well engineering team to perform the casing design and completion design for the well, and then detailed planning, where, for example, the drill bits are selected, a BHA is designed, the drilling fluid is selected, and step-by-step procedures are written to provide instruction for executing the well in a safe and cost-efficient manner.
With the interplay with many of the elements in a well design and making a change to one will have a knock on effect on many other things, often trajectories and designs go through several iterations before a plan is finalised.
The well is created by drilling a hole 12 cm to 1 meter (5 in to 40 in) in diameter into the earth with a drilling rig that rotates a drill string with a bit attached. After the hole is drilled, sections of steel pipe (casing), slightly smaller in diameter than the borehole, are placed in the hole. Cement may be placed between the outside of the casing and the borehole known as the annulus. The casing provides structural integrity to the newly drilled wellbore, in addition to isolating potentially dangerous high pressure zones from each other and from the surface.
With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) with a smaller bit, and also cased with a smaller size casing. Modern wells often have two to five sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.
This process is all facilitated by a drilling rig which contains all necessary equipment to circulate the drilling fluid, hoist and turn the pipe, control downhole, remove cuttings from the drilling fluid, and generate on-site power for these operations.
After drilling and casing the well, it must be 'completed'. Completion is the process in which the well is enabled to produce oil or gas.
In a cased-hole completion, small holes called perforations are made in the portion of the casing which passed through the production zone, to provide a path for the oil to flow from the surrounding rock into the production tubing. In open hole completion, often 'sand screens' or a 'gravel pack' is installed in the last drilled, uncased reservoir section. These maintain structural integrity of the wellbore in the absence of casing, while still allowing flow from the reservoir into the wellbore. Screens also control the migration of formation sands into production tubulars and surface equipment, which can cause washouts and other problems, particularly from unconsolidated sand formations of offshore fields.
After a flow path is made, acids and fracturing fluids may be pumped into the well to fracture, clean, or otherwise prepare and stimulate the reservoir rock to optimally produce hydrocarbons into the wellbore. Finally, the area above the reservoir section of the well is packed off inside the casing, and connected to the surface via a smaller diameter pipe called tubing. This arrangement provides a redundant barrier to leaks of hydrocarbons as well as allowing damaged sections to be replaced. Also, the smaller cross-sectional area of the tubing produces reservoir fluids at an increased velocity in order to minimize liquid fallback that would create additional back pressure, and shields the casing from corrosive well fluids.
In many wells, the natural pressure of the subsurface reservoir is high enough for the oil or gas to flow to the surface. However, this is not always the case, especially in depleted fields where the pressures have been lowered by other producing wells, or in low permeability oil reservoirs. Installing a smaller diameter tubing may be enough to help the production, but artificial lift methods may also be needed. Common solutions include downhole pumps, gas lift, or surface pump jacks. Many new systems in the last ten years have been introduced for well completion. Multiple packer systems with frac ports or port collars in an all in one system have cut completion costs and improved production, especially in the case of horizontal wells. These new systems allow casings to run into the lateral zone with proper packer/frac port placement for optimal hydrocarbon recovery.
The production stage is the most important stage of a well's life; when the oil and gas are produced. By this time, the oil rigs and workover rigs used to drill and complete the well have moved off the wellbore, and the top is usually outfitted with a collection of valves called a Christmas tree or production tree. These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work is needed. From the outlet valve of the production tree, the flow can be connected to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations, or oil export terminals.
As long as the pressure in the reservoir remains high enough, the production tree is all that is required to produce the well. If the pressure depletes and it is considered economically viable, an artificial lift method mentioned in the completions section can be employed.
Workovers are often necessary in older wells, which may need smaller diameter tubing, scale or paraffin removal, acid matrix jobs, or completing new zones of interest in a shallower reservoir. Such remedial work can be performed using workover rigs – also known as pulling units, completion rigs or "service rigs" – to pull and replace tubing, or by the use of well intervention techniques utilizing coiled tubing. Depending on the type of lift system and wellhead a rod rig or flushby can be used to change a pump without pulling the tubing.
Enhanced recovery methods such as water flooding, steam flooding, or CO2 flooding may be used to increase reservoir pressure and provide a "sweep" effect to push hydrocarbons out of the reservoir. Such methods require the use of injection wells (often chosen from old production wells in a carefully determined pattern), and are used when facing problems with reservoir pressure depletion, high oil viscosity, or can even be employed early in a field's life. In certain cases – depending on the reservoir's geomechanics – reservoir engineers may determine that ultimate recoverable oil may be increased by applying a waterflooding strategy early in the field's development rather than later. Such enhanced recovery techniques are often called "tertiary recovery".
A well is said to reach an "economic limit" when its most efficient production rate does not cover the operating expenses, including taxes.
The economic limit for oil and gas wells can be expressed using these formulae:
is an oil well's economic limit in oil barrels per month (bbls/month).
is a gas well's economic limit in thousand standard cubic feet per month (MSCF/month).
are the current prices of oil and gas in dollars per barrels and dollars per MSCF respectively.
is the lease operating expenses in dollars per well per month.
working interest, as a fraction.
net revenue interest, as a fraction.
gas/oil ratio as SCF/bbl.
condensate yield as barrel/million standard cubic feet.
production and severance taxes, as a fraction.
When the economic limit is raised, the life of the well is shortened and proven oil reserves are lost. Conversely, when the economic limit is lowered, the life of the well is lengthened.
When the economic limit is reached, the well becomes a liability and is abandoned. In this process, tubing is removed from the well and sections of well bore are filled with concrete to isolate the flow path between gas and water zones from each other, as well as the surface. Completely filling the well bore with concrete is costly and unnecessary. The surface around the wellhead is then excavated, and the wellhead and casing are cut off, a cap is welded in place and then buried.
At the economic limit there often is still a significant amount of unrecoverable oil left in the reservoir. It might be tempting to defer physical abandonment for an extended period of time, hoping that the oil price will go up or that new supplemental recovery techniques will be perfected. In these cases, temporary plugs will be placed downhole and locks attached to the wellhead to prevent tampering. There are thousands of "abandoned" wells throughout North America, waiting to see what the market will do before permanent abandonment. Often, lease provisions and governmental regulations usually require quick abandonment; liability and tax concerns also may favor abandonment.
In theory an abandoned well can be reentered and restored to production (or converted to injection service for supplemental recovery or for downhole hydrocarbons storage), but reentry often proves to be difficult mechanically and expensive. Traditionally elastomer and cement plugs have been used with varying degrees of success and reliability. Over time, they may deteriorate, particularly in corrosive environments, due to the materials from which they are manufactured. Conventional bridge plugs also have very small expansion ratios, limiting them for use in wells with restrictions. Alternatively, high expansion plugs, such as inflatable packers, do not have the differential pressure capabilities required for many well abandonments, nor do they provide a gas-tight seal. New tools have been developed that make re-entry easier, these tools offer higher expansion rations than conventional bridge plugs and higher differential pressure ratings than inflatable packers, all while providing a V0 rated, gas tight seal that cement cannot provide. 
Natural gas is almost always a byproduct of producing oil, since the small, light gas carbon chains come out of solution as they undergo pressure reduction from the reservoir to the surface, similar to uncapping a bottle of soda where the carbon dioxide effervesces. Unwanted natural gas can be a disposal problem at the well site. If it escapes into the atmosphere it becomes known as fugitive gas. If there is not a market for natural gas near the wellhead it is virtually valueless since it must be piped to the end user. Until recently, such unwanted gas was burned off at the wellsite, but due to environmental concerns this practice is becoming less common. Often, unwanted (or 'stranded' gas without a market) gas is pumped back into the reservoir with an 'injection' well for disposal or repressurizing the producing formation. Another solution is to export the natural gas as a liquid. Gas to liquid (GTL) is a developing technology that converts stranded natural gas into synthetic gasoline, diesel or jet fuel through the Fischer-Tropsch process developed in World War II Germany. Such fuels can be transported through conventional pipelines and tankers to users. Proponents claim GTL fuels burn cleaner than comparable petroleum fuels. Most major international oil companies are in advanced development stages of GTL production, e.g. the 140,000 bbl/d (22,000 m3/d) Pearl GTL plant in Qatar, scheduled to come online in 2011. In locations such as the United States with a high natural gas demand, pipelines are constructed to take the gas from the wellsite to the end consumer.
Wells can be located:
Offshore wells can further be subdivided into
While the location of the well will be a large factor in the type of equipment used to drill it, there is actually little difference in the well itself. An offshore well targets a reservoir that happens to be underneath an ocean. Due to logistics, drilling an offshore well is far more costly than an onshore well. By far the most common type is the onshore well. These wells dot the Southern and Central Great Plains, Southwestern United States, and are the most common wells in the Middle East.
Another way to classify oil wells is by their purpose in contributing to the development of a resource. They can be characterized as:
At a producing well site, active wells may be further categorised as:
Lahee classification 
The cost of a well depends mainly on the daily rate of the drilling rig, the extra services required to drill the well, the duration of the well program (including downtime and weather time), and the remoteness of the location (logistic supply costs).
The daily rates of offshore drilling rigs vary by their capability, and the market availability. Rig rates reported by industry web service show that the deepwater water floating drilling rigs are over twice that of the shallow water fleet, and rates for jackup fleet can vary by factor of 3 depending upon capability.
With deepwater drilling rig rates in 2015 of around $520,000/day, and similar additional spread costs, a deep water well of duration of 100 days can cost around US$100 million.
With high performance jackup rig rates in 2015 of around $177,000, and similar service costs, a high pressure, high temperature well of duration 100 days can cost about US$30 million.
Onshore wells can be considerably cheaper, particularly if the field is at a shallow depth, where costs range from less than $1 million to $15 million for deep and difficult wells.
The total cost of an oil well mentioned does not include the costs associated with the risk of explosion and leakage of oil. Those costs include the cost of protecting against such disasters, the cost of the cleanup effort, and the hard-to-calculate cost of damage to the company's image.
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. Modern wells have blowout preventers intended to prevent such an occurrence. An accidental spark during a blowout can lead to a catastrophic oil or gas fire.
Prior to the advent of pressure control equipment in the 1920s, the uncontrolled release of oil and gas from a well while drilling was common and was known as an oil gusher, gusher or wild well.Casing (borehole)
Casing is 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.Christmas tree (oil well)
In petroleum and natural gas extraction, a Christmas tree, or "tree", is an assembly of valves, spools, and fittings used to regulate the flow of pipes in an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells. It was named for its resemblance to the series of starting lights at a drag racing strip (Christmas tree (drag racing)).Completion (oil and gas wells)
Well completion is the process of making a well ready for production (or injection). 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. Casing ensures that this will not happen while also protecting the wellstream from outside incumbents, like water or sand.First Oil Well, Bahrain
As its name suggests, it is the first oil well in the Arabian side of the Persian Gulf and is located in Bahrain. The well is situated below Jebel Dukhan. It was discovered and operated by Bahrain Petroleum Company (BAPCO), established in 1929 in Canada by Standard Oil Company of California.Oil first spurted from this well on 16 October 1931, and the well finally began to blow heads of oil on the morning of 2 June 1932. The initial oil flow rate was 9,600 barrels per day (1,530 m3/d); by the 1970s the well produced 70,000 bbl/d (11,000 m3/d), and after that it stabilized at about 35,000 bbl/d (5,600 m3/d). In 1980, BAPCO was taken over by the Government of Bahrain. Close to the well, which has been reconstructed to its first appearance, is a stable.Bahrain was the first place on the Arabian side of the Persian Gulf where oil was discovered, and it coincided with the collapse of the world pearl market.First Oil Well in Western Canada
The First Oil Well in Western Canada National Historic Site of Canada commemorates the 1902 oil strike in what is now Waterton Lakes National Park, Alberta. Drilled in 1902, the well was the first productive oil well in the western Canadian provinces.
The well was drilled by John Lineham, whose Rocky Mountain Development Company had a mineral claim on the land along Oil Creek (now Cameron Creek), a region of natural oil seeps. The area had been drilled unsuccessfully for oil in the early 1890s, without results. Lineham's well was drilled by a wood "Canadian Pole" rig powered by a 35 hp steam engine. The Lineham Discovery Well #1 struck oil at 311 metres (1,020 ft), producing saleable quantities of oil at the rate of 300 barrels per day (48 m3/d). However, the well casing quickly failed, and the bore became jammed with debris and drilling tools. It was cleared in 1904, and a pump was installed. Drill tools again jammed the well and the well was abandoned. The tools remain visible in the bore. Total production was about 8,000 barrels (1,300 m3) of oil.Production had dwindled before the well was blocked. Further explorations in the area yielded nothing useful, but general exploration in more northerly portions of Alberta yielded the Turner Valley field in 1914. The Oil Creek strike is believed to be the result of oil seepage along fault planes in the Lewis Overthrust, in which oil originating in younger Cretaceous rock has moved upwards through older Pre-Cambrian rock that has been forced over the oil-bearing layers. More oil in the Waterton area was eventually discovered at the Pincher Creek oil field in 1948.A small monument, depicting a stylized drill rig, was placed over the well in 1968. The site was designated a site of national significance in 1965.Halliburton
Halliburton Company is an American multinational corporation. One of the world's largest oil field service companies, it has operations in more than 70 countries. It owns hundreds of subsidiaries, affiliates, branches, brands, and divisions worldwide and employs approximately 55,000 people.The company has dual headquarters located in Houston and in Dubai, and it remains incorporated in the United States.Halliburton's major business segment is the Energy Services Group (ESG). It offers a broad array of products and services to upstream oil and gas customers worldwide through fourteen product service lines: Artificial Lift, Cementing, Completion Tools, Multi-Chem, Pipeline & Process Services, Production Enhancement, Production Solutions, Baroid, Drill Bits & Services, Landmark Software & Services, Sperry Drilling, Testing & Subsea, Wireline & Perforating, and Consulting & Project Management. Halliburton's former subsidiary, KBR, is a major construction company of refineries, oil fields, pipelines, and chemical plants. Halliburton announced on April 5, 2007 that it had sold the division and severed its corporate relationship with KBR, which had been its contracting, engineering and construction unit as a part of the company.The company has been involved in numerous controversies, including its involvement with Dick Cheney and the Iraq War, and the Deepwater Horizon, for which it agreed to settle outstanding legal claims against it by paying litigants $1.1 billion.
KBR, one of Halliburton's subsidiaries at the time, paid bribes to high-ranking Nigerian officials between 1994 and 2004. Under a deal reached with the U.S. Justice Department, Halliburton has agreed to pay $382 million to settle the bribery case.Jeff Miller was promoted to President of Halliburton on August 1, 2014, and CEO on June 1, 2017, replacing Dave Lesar.History of the oil industry in Saudi Arabia
Saudi Arabian oil was first discovered by the Americans in commercial quantities at Dammam oil well No. 7 in 1938 in what is now modern day Dhahran.Kuwaiti oil fires
The Kuwaiti oil fires were caused by Iraqi military forces setting fire to a reported 605 to 732 oil wells along with an unspecified number of oil filled low-lying areas, such as oil lakes and fire trenches, as part of a scorched earth policy while retreating from Kuwait in 1991 due to the advances of Coalition military forces in the Persian Gulf War. The fires were started in January and February 1991, and the first well fires were extinguished in early April 1991, with the last well capped on November 6, 1991.Lost circulation
In oil or gas well drilling, lost circulation occurs when drilling fluid, known commonly as "mud", flows into one or more geological formations instead of returning up the annulus.
Lost circulation can be a serious problem during the drilling of an oil well or gas well.Matador, Texas
Matador is a town in and the county seat of Motley County, Texas, United States. The population was 740 at the 2000 census. In 1891, it was established by and named for the Matador Ranch. It is located 95 miles east of Lubbock, Texas.Oil well fire
Oil well fires are oil or gas wells that have caught on fire and burn. Oil well fires can be the result of human actions, such as accidents or arson, or natural events, such as lightning. They can exist on a small scale, such as an oil field spill catching fire, or on a huge scale, as in geyser-like jets of flames from ignited high pressure wells. A frequent cause of a well fire is a high-pressure blowout during drilling operations.Pumpjack
A pumpjack is the overground drive for a reciprocating piston pump in an oil well.It is used to mechanically lift liquid out of the well if not enough bottom hole pressure exists for the liquid to flow all the way to the surface. The arrangement is commonly used for onshore wells producing little oil. Pumpjacks are common in oil-rich areas.
Depending on the size of the pump, it generally produces 5 to 40 litres (1 to 9 imp gal; 1.5 to 10.5 US gal) of liquid at each stroke. Often this is an emulsion of crude oil and water. Pump size is also determined by the depth and weight of the oil to remove, with deeper extraction requiring more power to move the increased weight of the discharge column (discharge head).
A beam-type pumpjack converts the rotary motion of the motor to the vertical reciprocating motion necessary to drive the polished-rod and accompanying sucker rod and column (fluid) load. The engineering term for this type of mechanism is a walking beam. It was often employed in stationary and marine steam engine designs in the 18th and 19th centuries.Red Adair
Paul Neal "Red" Adair (June 18, 1915 – August 7, 2004) was an American oil well firefighter. He became notable as an innovator in the highly specialized and hazardous profession of extinguishing and capping oil well blowouts, both land-based and offshore.Singleton, West Sussex
Singleton is a village, Anglican parish and civil parish in the Chichester district of West Sussex, England. It lies in the Lavant valley, north of Chichester on the A286 road to Midhurst.
The village name is derived from the Anglo-Saxon 'sengel', which means "burnt clearing".
The civil parish has a land area of 1,602 hectares (3,960 acres). In the 2001 census there were 199 households containing 476 people, of whom 199 were economically active. The population marginally increased to 480 at the 2011 Census.Superman and the Mole Men
Superman and the Mole Men is an independently made 1951 American black-and-white superhero film released by Lippert Pictures Inc. Produced by Barney A. Sarecky and directed by Lee Sholem, it stars George Reeves as Superman and Phyllis Coates as Lois Lane. It is the first feature film based on any DC Comics character.
The storyline concerns reporters Clark Kent and Lois Lane arriving in the small town of Silsby to witness the drilling of the world's deepest oil well. The drill, however, has penetrated the underground home of a race of small, bald humanoids who, out of curiosity, climb to the surface at night. They glow in the dark, which scares the local townfolk, who form a mob intent on killing the strange visitors. Only Superman can intervene to prevent a tragedy.Washington oil field
The Washington oil field is an oil field and in Washington County, Pennsylvania. It also produced natural gas.
The oil field fueled a boom in Washington County, Pennsylvania from the 1880s to the early 1900s. For a time, the McGugin Gas Well was the largest flow of natural gas in the world and provided Pittsburgh with most of its natural gas needs.Well drilling
Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, brine, natural gas, or petroleum, for the injection of a fluid from surface to a subsurface reservoir or for subsurface formations evaluation or monitoring. Drilling for the exploration of the nature of the material underground (for instance in search of metallic ore) is best described as borehole drilling.
The earliest wells were water wells, shallow pits dug by hand in regions where the water table approached the surface, usually with masonry or wooden walls lining the interior to prevent collapse. Modern drilling techniques utilize long drill shafts, producing holes much narrower and deeper than could be produced by digging.
Well drilling can be done either manually or mechanically and the nature of required equipment varies from extremely simple and cheap to very sophisticated.
Managed Pressure Drilling (MPD) is defined by the International Association of Drilling Contractors (IADC) as “an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore." The objectives of MPD are “to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly."Wellhead
A wellhead is 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 strings 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.
When 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.