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An oil well is a term for any perforation through the Earth's surface designed to find and release both petroleum oil and gas hydrocarbons.

Contents 1 History 2 Life of a well 2.1 Drilling 2.1.1 See also 2.2 Completion 2.3 Production 2.4 Abandonment 3 Types of wells 4 Cost 5 Reefs 6 See also 7 External links

From the 9th century, oil fields were exploited in the area around modern Baku, Azerbaijan, to produce naphtha for the petroleum industry of the medieval Islamic world. These fields were described by al-Masudi in the 10th century, and by Marco Polo in the 13th century, who described the output of those oil wells as hundreds of shiploads.^[1] 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, inasmuch as a hundred shiploads might be taken from it at one time."

Shallow pits were dug at the Baku seeps in ancient times to facilitate collecting oil, and hand-dug holes up to 35 meters (115 feet) deep were in use by 1594. These holes were essentially oil wells, which makes Baku the first true field. Apparently 116 of these wells in 1830 produced 3,840 metric tons (about 28000 barrels) of oil. Later, Russian engineer F.N. Semyenov used a cable tool to drill an oil well on the Apsheron Peninsula, ten years before Colonel Drake's famous well in Pennsylvania. Also, offshore drilling started up at Baku at Bibi-Eibat field near the end of the 19th century, about the same time that the "first" offshore oil well was drilled in 1896 at Summerland field on the California Coast.

The earliest oil wells were drilled percussively by hammering a 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 Borehole used non-rotary mud motor drilling to achieve a depth of over 12 000 meters (38,000 feet). Until the 1970s, most oil wells were vertical (although different lithology 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 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.

The creation and life of a well can be divided up into five segments:

• Planning
• Drilling
• Completion
• Production
• Abandonment

The well is created by drilling a hole 5 to 30 inches (13 – 76 cm) diameter into the earth with an oil rig which rotates a drill bit. After the hole is drilled, a steel pipe (casing) slightly smaller than the hole is placed in the hole, and secured with cement. 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 2-5 sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.

To drill the well,

• The drill bit, aided by the weight of drill collars above it, breaks up the earth.
• Drilling fluid (aka "mud") is pumped down the inside of the drill pipe and exits at the drill bit and aids to break up the rock, keeping pressure on top of the bit, as well as cleaning, cooling and lubricating the bit.
• The generated rock "cuttings" are swept up by the drilling fluid as it circulates back to surface outside the drill pipe. The fluid then goes through "shakers" which strain the cuttings from the good fluid which is returned to the bit. Watching for abnormalities in the returning cuttings and volume of returning fluid are imperative to catch "kicks" (when the pressure below the bit is more than that above, causing gas and mud to come up uncontrollably) early.
• The pipe or drill string to which the bit is attached is gradually lengthened as the well gets deeper by screwing in several 30-foot (10 m) joints of pipe at surface. Usually joints are combined into 3 joints equaling 1 stand. Some smaller rigs only use 2 joints and newer rigs can handle stands of 4 joints.

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 pressures, remove cuttings from the drilling fluid, and generate onsite power for these operations.

• Drilling Fluid
• Directional Drilling
• Well logging
• Mud logging
• MWD (Measurement While Drilling)
• LWD (Logging While Drilling)
• Geosteering
• Formation evaluation
• Wildcat Drilling
• Expandable Tubular Technology
• Subsea

Main article: Completion (oil well)

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 in offshore fields.

After a flow path is made, acids and fracturing fluids are 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 diameter of the tubing produces hydrocarbons at an increased velocity in order to overcome the hydrostatic effects of heavy fluids such as water.

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 (e.g., the "nodding donkey" pumps dotting the countryside in old oil fields in Texas and Oklahoma). The use of artificial lift technology in a field is often termed as "secondary recovery" in the industry. Many new systems in the last ten years have been introduced into the well completion field. Multiple packer systems with frac ports or port collars in an all in one system installation have cut completion costs and improved production, especially in the case of the horizontal well. 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". These valves regulate pressures, control flows, and allow access to the wellbore in case further completion work needs to be performed. From the outlet valve of the Christmas 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, this Christmas 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 parrafin removal, repeated acid matrix jobs, or even completing new zones of interest in a shallower reservoir. Such remedial work can be performed using workover rigs – also known as pulling units – to pull and replace tubing, or by the use of a well intervention technique called coiled tubing.

Enhanced recovery methods such as waterflooding, 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 picked 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. The application of such enhanced recovery techniques is often termed as "tertiary recovery" in the industry.

Finally, when the well no longer produces or produces so poorly that it is a liability to its owner, it is abandoned. In this simple process, tubing is removed from the well and sections of well-bore are filled with cement as 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 unnecessary and cost prohibitive.

Oil 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 it undergoes pressure reduction from the reservoir to the surface (similar to uncapping a bottle of pop where the carbon dioxide effervesces out.) Unwanted natural gas can actually be quite 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 and 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. Of course, 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.

Another obvious way to classify oil wells is by land or offshore wells. There really is very little difference in the well itself; an offshore well simply targets a reservoir that also happens to be underneath an ocean. Also, due to logistics, drilling an offshore well is far more costly than an onshore well. By far the most common type of well is of the onshore variety. These wells dot the Southern and Central Great Plains, Southwestern United States, and are also the most common type of well 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:

• production wells when they are drilled primarily for producing oil or gas, once the producing structure and characteristics are established
• appraisal wells when they are used to assess characteristics (such as flowrate) of a proven hydrocarbon accumulation
• exploration wells when they are drilled purely for exploratory (information gathering) purposes in a new area
• wildcat wells when a well is drilled, based on a large element of hope, in a frontier area where very little is known about the subsurface. In the early days of oil exploration in Texas, wildcats were common as productive areas were not yet established. In modern times, oil exploration in many areas has reached a very mature phase and the chances of finding oil simply by drilling at random are very low. Therefore, a lot more effort is placed in exploration and appraisal wells.

At a producing well site, active wells may be further categorised as:

• oil producers producing predominantly liquid hydrocarbons, but mostly with some associated gas.
• gas producers producing virtually entirely gaseous hydrocarbons.
• water injectors injecting water into the formation either to maintain 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 let alone into a fresh water source, in the case of onshore wells. Frequently, water injection has an element of reservoir management and produced water disposal.
• aquifer producers intentionally producing reservoir water for re-injection to manage pressure. This is in effect moving reservoir water from where it is not as useful, to where it is more useful. These wells will generally only be used if produced water from the oil or gas producers is insufficient for reservoir management purposes. Using aquifer produced water rather than sea water is due to the chemistry.
• gas injectors injecting gas into the reservoir often as a means of disposal or sequestering for later production, but also to maintain reservoir pressure.

Lahee classification [1]

• New Field Wildcat (NFW) – far from other producing fields and on a structure that has not previously produced.
• New Pool Wildcat (NPW) – new pools on already producing structure.
• Deeper Pool Test (DPT) – on already producing structure and pool, but on a deeper pay zone.
• Shallower Pool Test (SPT) – on already producing structure and pool, but on a shallower pay zone.
• Outpost (OUT) – usually two or more locations from nearest productive area.
• Development Well (DEV) – can be on the extension of a pay zone, or between existing wells (Infill).

The following is a quick comparison of average well costs for the UK Continental Shelf (UKCS). These costs exclude testing (e.g., flow rate testing), and are based on values from March 1998. Prices have doubled since then^{[citation needed]}:

Typical well costs for UKCS wells in 1998

Well location	Typical cost (in millions of £)
Northern North Sea	8 – 12
West of Shetland	5 – 15
Southern North Sea	7 – 12
Irish Sea	2 – 3

The cost of an offshore well depends strongly on the remoteness of the location being drilled. Hence the Irish Sea (shallow water, close to the coast) is cheap in comparison to the West of Shetland (deep water, far from the coast and other facilities). The 2006 cost of a Central North Sea high pressure, high temperature well is about $35-50 million. Deep water wells in the Gulf of Mexico can cost over $100 million.^{[citation needed]}

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]}

Offshore platforms, the well's supporting structure, produce artificial reefs.

• Directional drilling
• Drilling rig
• Hydraulic fracturing
• Oil gusher
• Oil platform
• Oil supplies
• Oil well fire
• Pumpjack
• Spindletop
• Subsea
• Well intervention

• Schlumberger Oilfield Glossary
• The History of the Oil Industry
• Satellite Photo of burning Oil wells, Iraq
• Images of an abandoned well site in the Houston area
• Interactive Map of US Oil Wells oilwellmaps.com
• Energy Resources in Kansas Kansas Geological Survey