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| {{mergefrom|Oil Well Drilling Problems|discuss=Talk:Oil well#Merger proposal|date=March 2013}}
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| {{Refimprove|date=February 2008}}
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| [[Image:West Texas Pumpjack.JPG|thumb|300px|right|The [[pumpjack]], such as this one located south of [[Midland, Texas]], is a common sight in [[West Texas]]]]
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| 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'''.
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| Usually attached to the oil [[derrick]] is the [[oil well dog house]] where the decisions are made for actions taken on the [[rig floor]]
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| Newer oil wells have hydraulic feet that are called ''walking rigs'' that allow for the rig to move several meters on its own without taking the derrick apart. This saves time as dual laterals can be drilled.
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| ==History==
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| [[File:Bottom Part of an Oil Well in Brazoria County.jpg|thumb|left|Bottom Part of an Oil Well in Brazoria County, Texas (Harry Walker Photograph, circa 1940)]]
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| The earliest known oil wells were drilled in [[History of Science and Technology in China|China]] in 347 [[Common Era|CE]]. These wells had depths of up to about {{convert|240|m}} and were drilled using [[drill bit|bits]] attached to [[bamboo]] poles.<ref name=ASTM>[http://www.astm.org/COMMIT/D02/to1899_index.html ASTM timeline of oil]</ref> The oil was burned to evaporate [[brine]] and produce [[sodium chloride|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.<ref name=EB1911/>
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| According to Kasem Ajram, petroleum was [[Distillation|distilled]] by the [[Persian people|Persian]] alchemist [[Muhammad ibn Zakarīya Rāzi]] (Rhazes) in the 9th century, producing chemicals such as [[kerosene]] in the [[alembic]] (''al-ambiq''),<ref name=Ajram>{{Cite book|author=Dr. Kasem Ajram|title=The Miracle of Islam Science|edition=2nd Edition|publisher=Knowledge House Publishers|year=1992|isbn=0-911119-43-4}}</ref>{{Verify source|date=September 2010}} and which was mainly used for [[kerosene lamp]]s.<ref>Zayn Bilkadi ([[University of California, Berkeley]]), "The Oil Weapons", ''[[Saudi Aramco World]]'', January–February 1995, pp. 20–7</ref> [[Alchemy and chemistry in Islam|Arab and Persian chemists]] also distilled crude oil in order to produce [[Flammability|flammable]] products for military purposes. Through [[Al-Andalus|Islamic Spain]], distillation became available in [[Western Europe]] by the 12th century.<ref name=EB1911>{{Cite web|title=petroleum|publisher=''[[Encyclopædia Britannica]]''|author=Joseph P. Riva Jr. and Gordon I. Atwater|url=http://www.britannica.com/EBchecked/topic/454269/petroleum|accessdate=2008-06-30}}</ref>
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| Some sources claim that from the 9th century, [[oil field]]s were exploited in the area around modern [[Baku]], [[Azerbaijan]], to produce [[Petroleum naphtha|naphtha]] for the [[petroleum industry]]. These fields 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 the Azerbaijani city of 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."{{citation needed|date=June 2013}}
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| [[Image:Oil well burning in Bibi Eibat.JPG|thumb|1904 [[oil well fire]] at Bibi-Eibat]]
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| Shallow pits were dug at the Baku seeps in ancient times to facilitate collecting oil, and hand-dug holes up to {{convert|35|m|ft}} deep were in use by 1594. These holes were essentially oil wells. Apparently 116 of these wells in 1830 produced 3,840 metric tons (about 28,000 barrels) of oil. Also, offshore drilling started up at Baku (then [[Russian Empire]]) at Bibi-Eibat field in 1846. In the New World, 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.
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| The earliest oil wells in modern times were drilled percussively, by hammering a [[Drilling rig#Cable tool drilling|cable tool]] into the earth. Soon after, cable tools were replaced with [[rotary drilling]], which could drill boreholes to much greater depths and in less time. The record-depth [[Kola Superdeep Borehole|Kola Borehole]] used non-rotary mud motor drilling to achieve a depth of over {{convert|12000|m|ft}}. Until the 1970s, most oil wells were vertical, although [[Lithology|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 [[petroleum geology|reservoir]] rocks which contain hydrocarbons are usually horizontal, or sub-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.
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| ==Life of a well==
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| [[File:Pump Jack labelled.png|thumb|left|A schematic of a typical oil well being produced by a [[pumpjack]], which is used to produce the remaining recoverable oil after natural pressure is no longer sufficient to raise oil to the surface]] | |
| {{see also|Oil well control}}
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| The creation and life of a well can be divided up into five segments:
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| * Planning
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| * Drilling
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| * Completion
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| * Production
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| * Abandonment
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| ===Drilling===
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| {{See also|Boring (earth)}}
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| 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 (borehole)|casing]]), slightly smaller in diameter than the borehole, are placed in the hole. Concrete may be placed between the outside of the casing and the borehole. 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.
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| 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.
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| ;To drill the well
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| [[File:Casing & Temporary Home.JPG|thumb|Oil Rig Casing]]
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| * The drill bit, aided by the weight of thick walled pipes called "drill collars" above it, cuts into the rock. There are different types of drill bit; some cause the rock to disintegrate by compressive failure, while others shear slices off the rock as the bit turns.
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| * [[Drilling fluid]], a.k.a. "mud", is pumped down the inside of the drill pipe and exits at the drill bit. Drilling mud is a complex mixture of fluids, solids and chemicals that must be carefully tailored to provide the correct physical and chemical characteristics required to safely drill the well. Particular functions of the drilling mud include cooling the bit, lifting rock cuttings to the surface, preventing destabilisation of the rock in the wellbore walls and overcoming the pressure of fluids inside the rock so that these fluids do not enter the wellbore.
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| [[Image:Mudlogging.JPG|thumb|[[Mud log]] in process, a common way to study the lithology when drilling oil wells]]
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| * The generated rock "[[drill cuttings|cuttings]]" are swept up by the drilling fluid as it circulates back to surface outside the drill pipe. The fluid then goes through "[[shale shakers|shakers]]" which strain the cuttings from the good fluid which is returned to the pit. Watching for abnormalities in the returning cuttings and monitoring pit volume or rate of returning fluid are imperative to catch "kicks" early. A "kick" is when the formation pressure at the depth of the bit is more than the hydrostatic head of the mud above, which if not controlled temporarily by closing the [[blowout preventer]]s and ultimately by increasing the density of the drilling fluid would allow formation fluids and mud to come up through the annulus uncontrollably.
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| * The pipe or [[drill string]] to which the bit is attached is gradually lengthened as the well gets deeper by screwing in additional 30-foot (9 m) sections or "joints" of pipe under the [[Kelly drive|kelly]] or topdrive at the surface. This process is called making a connection, or "tripping". Joints can be combined for more efficient tripping when pulling out of the whole by creating stands of multiple joints. A conventional triple, for example, would pull pipe out of the hole three joints at a time and stack them in the derrick. Many modern rigs, called "super singles," trip pipe one at a time, laying it out on racks as they go.
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| 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.
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| ===Completion===
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| {{Main|Completion (oil and gas wells)}}
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| [[Image:H104.jpg|thumb|left|Modern driller Argentina]]
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| After drilling and casing the well, it must be 'completed'. Completion is the process in which the well is enabled to produce [[Petroleum|oil]] or gas.
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| In a cased-hole completion, small holes called [[Perforation (oil well)|perforations]] are made in the portion of the [[Casing (oil well)|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.
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| After a flow path is made, acids and fracturing fluids may be pumped into the well to [[Hydraulic fracturing|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.
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| [[File:Labeled oil derrick.JPG|thumb|Labeled oil derrick]]
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| 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 [[pumpjack|pump jacks]]. Many new systems in the last ten years have been introduced for well completion. Multiple [[Production packer|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.
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| ===Production===
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| 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|workover rig]]s 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 (oil well)|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.
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| 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.
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| [[Workover]]s 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.
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| Enhanced recovery methods such as water flooding, steam flooding, or CO<sub>2</sub> 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 "[[Extraction of petroleum#Tertiary recovery|tertiary recovery]]".
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| ===Abandonment===
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| A well is said to reach an "economic limit" when its most efficient production rate does not cover the operating expenses, including taxes.<ref name="econ-limit">Mohammed A. Mian, [http://books.google.com/books?id=sDYtmIcJ1ycC&pg=PT445&lpg=PT445&dq=%22oil+well%22+%22economic+limit%22&source=bl&ots=GLsc8_w24h&sig=foDF6GvmPeL42r6QEWsNrUT5uQA&hl=en&ei=QEKySsDAKaqltgf5kcCmDg&sa=X&oi=book_result&ct=result&resnum=10#v=onepage&q=%22oil%20well%22%20%22economic%20limit%22&f=false ''Petroleum Engineering Handbook for the Practicing Engineer''], Tulsa, Okla.: PennWell, 1992, p.447.</ref>
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| The economic limit for oil and gas wells can be expressed using these formula: | |
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| Oil fields:
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| <br />
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| <math>
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| {EL}_{oil}=\frac{{WI}\times{LOE}}{{NRI}[{P_o}+({P_g}\times{GOR})/1,000]\times(1-{T})}
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| </math>
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| <br /> | |
| Gas fields:
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| <br /> | |
| <math>
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| {EL}_{gas}=\frac{{WI}\times{LOE}}{{NRI}[({P_o}\times{Y})+{P_g}]\times(1-{T})}
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| </math>
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| <br />
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| Where:
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| <br />
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| <math>{EL}_{oil}</math> is an oil well's economic limit in [[bbls|oil barrels]] per month (bbls/month).<br />
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| <math>{EL}_{gas}</math> is a gas well's economic limit in [[Standard cubic foot|thousand standard cubic feet]] per month (MSCF/month).<br />
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| <math>{P}_{o}, {P}_{g}</math> are the current prices of oil and gas in dollars per barrels and dollars per MSCF respectively.<br />
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| <math>{LOE}</math> is the lease operating expenses in dollars per well per month.<br />
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| <math>{WI}</math> working interest, as a fraction.<ref name=work-inst>[http://www.glossary.oilfield.slb.com/Display.cfm?Term=working%20interest Oilfield Glossary - "working interest"]</ref><br />
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| <math>{NRI}</math> net revenue interest, as a fraction.<br />
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| <math>{GOR}</math> gas/oil ratio as bbls/MSCF.<br />
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| <math>{Y}</math> condensate yield as barrel/million standard cubic feet.<br />
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| <math>{T}</math> production and [[severance tax]]es, as a fraction.<br />
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| <ref name="econ-limit" />
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| 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.
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| 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.
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| 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.
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| 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 not cost effective.
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| ==Types of wells==
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| [[File:NaturalGasWell.jpg|thumb|A natural gas well in the southeast [[Lost Hills Oil Field|Lost Hills Field]], California, US.]]
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| Fossil-fuel wells come in many varieties. By produced fluid, there can be wells that produce oil, wells that produce [[oil]] ''and'' [[natural gas]], or wells that ''only'' produce natural gas. 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 [[Petroleum reservoir|reservoir]] to the surface, similar to uncapping a bottle of soda pop where the carbon dioxide [[effervesce]]s. Unwanted natural gas can be a disposal problem at the well site. 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.{{Citation needed|reason=Oct 2009|date=October 2009}} 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 [[LNG|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 {{convert|140000|oilbbl/d|abbr=on}} [[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]].
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| [[File:Raising the stern.JPG|thumb|Raising the derrick]]
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| Another obvious way to classify oil wells is by land or offshore wells. There is very 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.
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| Another way to classify oil wells is by their purpose in contributing to the development of a resource. They can be characterized as:
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| * ''wildcat wells'' are those drilled outside of and not in the vicinity of known oil or gas fields.
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| * ''exploration wells'' are drilled purely for exploratory (information gathering) purposes in a new area.
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| * ''appraisal wells'' are used to assess characteristics (such as flow rate) of a proven hydrocarbon accumulation.
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| * ''production wells'' are drilled primarily for producing oil or gas, once the producing structure and characteristics are determined.
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| [[Image:Bundesarchiv Bild 183-R00740, Boryslaw, Erdölgewinnung.jpg|thumb|Oil extraction in [[Boryslav]] in 1909]]
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| At a producing well site, active wells may be further categorised as:
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| * ''oil producers'' producing predominantly liquid hydrocarbons, but mostly with some associated gas.
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| * ''gas producers'' producing almost entirely gaseous hydrocarbons.
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| * ''water injectors'' [[Water injection (oil production)|injecting water]] into the formation to maintain [[Oil reservoir|reservoir]] pressure, or simply to dispose of water produced with the hydrocarbons because even after treatment, it would be too oily and too saline to be considered clean for dumping overboard offshore, let alone into a fresh water resource in the case of onshore wells. Water injection into the producing zone frequently has an element of reservoir management; however, often produced water disposal is into shallower zones safely beneath any fresh water zones.
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| * ''aquifer producers'' intentionally producing water for re-injection to manage pressure. If possible this water will come from the [[Oil reservoir|reservoir]] itself. Using aquifer produced water rather than water from other sources is to preclude chemical incompatibility that might lead to reservoir-plugging precipitates. These wells will generally be needed only if produced water from the oil or gas producers is insufficient for reservoir management purposes.
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| * ''gas injectors'' injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.
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| '''Lahee classification''' [http://www.pttc.org/tech_sum/ts_v91_10.htm ]
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| * ''New Field Wildcat'' (NFW) – far from other producing fields and on a structure that has not previously produced.
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| * ''New Pool Wildcat'' (NPW) – new pools on already producing structure.
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| * ''Deeper Pool Test'' (DPT) – on already producing structure and pool, but on a deeper pay zone.
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| * ''Shallower Pool Test'' (SPT) – on already producing structure and pool, but on a shallower pay zone.
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| * ''Outpost'' (OUT) – usually two or more locations from nearest productive area.
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| * ''Development Well'' (DEV) – can be on the extension of a pay zone, or between existing wells (''Infill'').
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| ==Cost==
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| 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).
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| The daily rates of offshore drilling rigs vary by their capability, and the market availability. Rig rates reported by industry web service<ref name="rigzone.com">Rigzone - Rig day rates : http://www.rigzone.com/data/dayrates/</ref> 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.
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| With deepwater drilling rig rates in 2010 of around $420,000/day,<ref name="rigzone.com"/> and similar additional spread costs, a deep water well of duration of 100 days can cost around US$100 million.
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| With high performance jackup rig rates in 2010 of around $150,000,<ref name="rigzone.com"/> and similar service costs, a high pressure, high temperature well of duration 100 days can cost about US$30 million.
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| 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. {{Citation needed|date=February 2007}}
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| 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.
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| ==Reefs==
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| {{Expand section|date=October 2009}}
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| [[Oil platform|Offshore platforms]] (the structure supporting the wells) often provide habitat for marine life. After the wells have been abandoned, sometimes the platforms can be toppled in place or moved elsewhere to be dropped to the ocean floor to produce [[artificial reef]]s.{{Citation needed|date=July 2011}}
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| ==See also==
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| * [[Hydraulic fracturing]]
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| * [[Offshore drilling]]
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| * [[Oil spill]]
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| ==References==
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| {{Reflist|2}}
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| ==External links==
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| * [http://www.glossary.oilfield.slb.com/ Schlumberger Oilfield Glossary]
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| * [http://www.sjgs.com/history.html The History of the Oil Industry]
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| * [http://books.google.com/books?id=deIDAAAAMBAJ&pg=PA401&dq=popular+mechanics+1930+aircraft&hl=en&ei=KaMmTZ-LCentnQeE6ZzjAQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDYQ6AEwBA#v=onepage&q=popular%20mechanics%201930%20aircraft&f=true "Black Gold"] ''Popular Mechanics'', January 1930 - large photo article on oil drilling in the 1920s and 1930s
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| * [http://books.google.com/books?id=eCYDAAAAMBAJ&pg=PA21&dq=Popular+Science+1932+plane&hl=en&ei=G-VPTarQJ8P58AbZ7_yCDw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CC4Q6AEwATgo#v=onepage&q=Popular%20Science%201932%20plane&f=true "World's Deepest Well"] ''Popular Science'', August 1938, article on the late 1930s technology of drilling oil wells
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| * [http://www.cseg.ca/publications/recorder/2004/06jun/06jun-ancient-chinese-drilling.pdf 'Ancient Chinese Drilling'] article from June 2004 CSEG Recorder
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| {{Petroleum industry}}
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| {{DEFAULTSORT:Oil Well}}
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| [[Category:Petroleum production]]
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| [[Category:Oil wells| ]]
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| [[Category:Drilling technology]]
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| [[Category:Chinese inventions]]
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| [[Category:Russian inventions]]
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| [[Category:American inventions]]
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