- Crude oil
- Hydrocarbon exploration
- Natural gas
Saturday, September 5, 2009
Oil and gas
Oil and gas is a collective term that refers to liquid and gas hydrocarbons extracted from the subsurface. The term oil and gas may refer to:
Future of petroleum production
The future of petroleum as a fuel remains somewhat controversial. USA Today news reported in 2004 that there were 40 years of petroleum left in the ground. Some argue that because the total amount of petroleum is finite, the dire predictions of the 1970s have merely been postponed. Others claim that technology will continue to allow for the production of cheap hydrocarbons and that the earth has vast sources of unconventional petroleum reserves in the form of tar sands, bitumen fields and oil shale that will allow for petroleum use to continue in the future. They argue that both the Canadian tar sands and United States oil shale deposits represent potential reserves with just as much oil as current liquid petroleum deposits.
Price of petroleum
After the collapse of the OPEC-administered pricing system in 1985, and a short lived experiment with netback pricing, oil-exporting countries adopted a market-linked pricing mechanism.[25] First adopted by PEMEX in 1986, market-linked pricing received wide acceptance and by 1988 became and still is the main method for pricing crude oil in international trade.[25] The current reference, or pricing markers, are Brent, WTI .
Petroleum industry
The petroleum industry is involved in the global processes of exploration, extraction, refining, transporting (often with oil tankers and pipelines), and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline (petrol). Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics. The industry is usually divided into three major components: upstream, midstream and downstream. Midstream operations are usually included in the downstream category.
Petroleum is vital to many industries, and is of importance to the maintenance of industrialized civilization itself, and thus is critical concern to many nations. Oil accounts for a large percentage of the world’s energy consumption, ranging from a low of 32% for Europe and Asia, up to a high of 53% for the Middle East. Other geographic regions’ consumption patterns are as follows: South and Central America (44%), Africa (41%), and North America (40%). The world at large consumes 30 billion barrels (4.8 km³) of oil per year, and the top oil consumers largely consist of developed nations. In fact, 24% of the oil consumed in 2004 went to the United States alone [20], though by 2007 this had dropped to 21% of world oil consumed.
In the US, in the states of Arizona, California, Hawaii, Nevada, Oregon and Washington, the Western States Petroleum Association (WSPA) is responsible for producing, distributing, refining, transporting and marketing petroleum. This non-profit trade association was founded in 1907, and is the oldest petroleum trade association in the United States.
Petroleum is vital to many industries, and is of importance to the maintenance of industrialized civilization itself, and thus is critical concern to many nations. Oil accounts for a large percentage of the world’s energy consumption, ranging from a low of 32% for Europe and Asia, up to a high of 53% for the Middle East. Other geographic regions’ consumption patterns are as follows: South and Central America (44%), Africa (41%), and North America (40%). The world at large consumes 30 billion barrels (4.8 km³) of oil per year, and the top oil consumers largely consist of developed nations. In fact, 24% of the oil consumed in 2004 went to the United States alone [20], though by 2007 this had dropped to 21% of world oil consumed.
In the US, in the states of Arizona, California, Hawaii, Nevada, Oregon and Washington, the Western States Petroleum Association (WSPA) is responsible for producing, distributing, refining, transporting and marketing petroleum. This non-profit trade association was founded in 1907, and is the oldest petroleum trade association in the United States.
Sunday, August 30, 2009
Oil reserves
Oil reserves are the estimated quantities of crude oil that are claimed to be recoverable under existing economic and operating conditions.[1]
The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place. However, because of reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of producible oil reserves to total oil in place for a given field is often referred to as the recovery factor. Recovery factors vary greatly among oil fields. The recovery factor of any particular field may change over time based on operating history and in response to changes in technology and economics. The recovery factor may also rise over time if additional investment is made in enhanced oil recovery techniques such as gas injection, water-flooding[2], or microbial enhanced oil recovery.
Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time. This phenomenon is called reserves growth.[3]
Many oil producing nations do not reveal their reservoir engineering field data, and instead provide unaudited claims for their oil reserves. The numbers disclosed by some national governments are suspected of being manipulated for political reasons.
The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place. However, because of reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of producible oil reserves to total oil in place for a given field is often referred to as the recovery factor. Recovery factors vary greatly among oil fields. The recovery factor of any particular field may change over time based on operating history and in response to changes in technology and economics. The recovery factor may also rise over time if additional investment is made in enhanced oil recovery techniques such as gas injection, water-flooding[2], or microbial enhanced oil recovery.
Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time. This phenomenon is called reserves growth.[3]
Many oil producing nations do not reveal their reservoir engineering field data, and instead provide unaudited claims for their oil reserves. The numbers disclosed by some national governments are suspected of being manipulated for political reasons.
Classifications
Reserves are those quantities of petroleum claimed to be commercially recoverable by application of development projects to known accumulations under defined conditions.[6] Reserves must satisfy four criteria: They must be:
discovered through one or more exploratory wells[6]
recoverable using existing technology[6]
commercially viable[6]
remaining in the ground[6]
All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications - proved and unproved.[6] Unproved reserves can further be divided into two subcategories - probable and possible to indicate the relative degree of uncertainty about their existence.[6] The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997
discovered through one or more exploratory wells[6]
recoverable using existing technology[6]
commercially viable[6]
remaining in the ground[6]
All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications - proved and unproved.[6] Unproved reserves can further be divided into two subcategories - probable and possible to indicate the relative degree of uncertainty about their existence.[6] The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997
Proved reserves
Proved reserves are those reserves claimed to have a reasonable certainty (normally at least 90% confidence) of being recoverable under existing economic and political conditions, with existing technology. Industry specialists refer to this as P90 (i.e. having a 90% certainty of being produced). Proved reserves are also known in the industry as 1P.[8][9]
Proved reserves are further subdivided into Proved Developed (PD) and Proved Undeveloped (PUD).[9][10] PD reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required.[10] PUD reserves require additional capital investment (e.g. drilling new wells) to bring the oil to the surface.[8][10]
Proved reserves are the only type the U.S. Securities and Exchange Commission allows oil companies to report to investors. Companies listed on U.S. stock exchanges must substantiate their claims, but many governments and national oil companies do not disclose verifying data to support their claims.
Proved reserves are further subdivided into Proved Developed (PD) and Proved Undeveloped (PUD).[9][10] PD reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required.[10] PUD reserves require additional capital investment (e.g. drilling new wells) to bring the oil to the surface.[8][10]
Proved reserves are the only type the U.S. Securities and Exchange Commission allows oil companies to report to investors. Companies listed on U.S. stock exchanges must substantiate their claims, but many governments and national oil companies do not disclose verifying data to support their claims.
Unproved reserves
Unproved reserves are based on geological and/or engineering data similar to that used in estimates of proved reserves, but technical, contractual, or regulatory uncertainties preclude such reserves being classified as proved. [11] Unproved reserves may be used internally by oil companies and government agencies for future planning purposes, but are not routinely compiled. They are sub classified as probable and possible [11]
Probable reserves are are attributed to known accumulations, and claim a 50% confidence level of recovery. Industry specialists refer to this as P50 (i.e. having a 50% certainty of being produced). Referred to in the industry as 2P (proved plus probable).[8]
Possible reserves are attributed to known accumulations which have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced (P10). Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas), projected reserves based on future recovery methods. Referred to in the industry as 3P (proved plus probable plus possible).
Probable reserves are are attributed to known accumulations, and claim a 50% confidence level of recovery. Industry specialists refer to this as P50 (i.e. having a 50% certainty of being produced). Referred to in the industry as 2P (proved plus probable).[8]
Possible reserves are attributed to known accumulations which have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced (P10). Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas), projected reserves based on future recovery methods. Referred to in the industry as 3P (proved plus probable plus possible).
Oil, gas reserves discovered
ISLAMABAD (APP) - The Oil and Gas Development Company Limited (OGDCL) as operator along with its joint venture partners Government Holdings (Pvt) Limited (GHPL) and Orient Petroleum International Inc. (OPII) has discovered oil and gas reserves in Sanghar City, Sindh Province.OGDCL sources told APP that the Company as operator along with its joint venture partners has discovered oil and gas reserves from its exploratory Well Baloch #01, in Sinjhoro Exploration License which is located at a distance of 22.5 km North West of Sanghar City, Sindh Province.The structure of Baloch Well # 01 was delineated drilled and tested by OGDCL’s in house expertise.The well was drilled down to the depth of 3463 meters, targeting to test the potential of sand body (Massive Sands) of Lower Goru formation of Cretaceous age.The significant reserves of hydrocarbons have been found at Baloch Well#01. The hydrocarbon bearing Zone comprising of 32.5 meters has tested 780 BBL of oil per day and about 3.5 MMCFD of gas through 32/64" choke, at well head flowing pressure of 880 Psi.
Oil refinery
An oil refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas. Oil refineries are typically large sprawling industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units.
Exploration methods of Oil
Visible surface features such as oil seeps, natural gas seeps, pockmarks (underwater craters caused by escaping gas) provide basic evidence of hydrocarbon generation (be it shallow or deep in the Earth). However, most exploration depends on highly sophisticated technology to detect and determine the extent of these deposits using exploration geophysics. Areas thought to contain hydrocarbons are initially subjected to a gravity survey, magneic survey, passi seismicor regional seismic reflection surveys to detect large scale features of the sub-surface geology. Features of interest (known as leads) are subjected to more detailed seismic surveys which work on the principle of the time it takes for reflected sound waves to travel through matter (rock) of varying densities and using the process of depth conversion to create a profile of the substructure. Finally, when a prospect has been identified and evaluated and passes the oil company's selection criteria, an exploration well is drilled in an attempt to conclusively determine the presence or absence of oil or gas.Oil exploration is an expensive, high-risk operation. Offshore and remote area exploration is generally only undertaken by very large corporations or national governments. Typical Shallow shelf oil wells (e.g. North sea) cost USD$10 - 30 Million, while deep water wells can cost up to USD$100 million plus. Hundreds of smaller companies search for onshore hydrocarbon deposits worldwide, with some wells costing as little as USD$100,000.
Oil and Gas Companies All Over The World
Abu Dhabi National Oil Company (ADNOC).
AGIP (Azienda Generale Italiana Petroli) since 1998 ENI-Agip Exploration and Production.
Anglo Siberian Oil Company plc.
Anonima Petroli Italiana (API)
AnzoilARCO (Atlantic Richfield Company) = BP-Amoco.
Australian Oil & Gas Corporation Ltd.
Australian Worldwide Exploration LimitedBay State Gas.
Bentec Drilling (Preussag DE).Benton Oil & Gas Company.
BHP Billiton.BJ Services Company.
Black Hills Exploration and Production.
(US).BP Amoco ARCO.
Chieftain International (now: Huntoil).
Chinese Petroleum Corporation (Taiwan TW).
Compagnie Générale de Géophysique (CGG).CONOCO (US) (incl. Gulf Canada).
ConocoPhillips.CSIRO Exploration and Mining.
Dana Petroleum plc.Deutag Drilling (Preussag DE) (now: KCA DEUTAG Drilling Limited)Devon Energy Corporation (US) (incl. Santa Fe Snyder Corporation, PennzEnergy).
Dominion Exploration & Production.Elf Aquitaine (TOTAL FINA ELF).
Empresa Colombiana de Petróleos (ECOPETROL).
Empresa Nacional del Petróleo - Chile (ENAP).
Encana.Energy Africa EA.Eni.Enterprise Oil plc. (UK).
Equity Oil Company.ESSO DeutschlandEvergreen Resources (US)ExxonMobilFINA (now:
Oil well
An oil well is a general term for any boring through the Earth's surface designed to find and produce petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil, and a well designed to produce mainly or only gas may be termed a gas well.
Life of a well
The creation and life of a well can be divided up into five segments:
- Planning
- Drilling
- Completion
- Production
- Abandonment
Drilling
The well is created by drilling a hole 5 to 36 inches (127.0 mm to 914.4 mm) diameter into the earth with a drilling rig which rotates a drill string with a bit attached. After the hole is drilled, sections of steel tubing (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. 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 thick walled pipes called "drill collars" above it, cuts into the rock. There are different types of drillbit, some cause the rock to fail by compressive failure. Others shear slices off the rock as the bit turns.
Drilling fluid (aka "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 which 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 don't enter the wellbore.
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 pit. Watching for abnormalities in the returning cuttings and monitoring pit volume or rate of returning fluid are imperative to catch "kicks" (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 preventers and ultimately by increasing the density of the drilling fluid would allow formation fluids 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 additional 30-foot (10 m) joints (i.e., sections) of pipe under the kelly or topdrive at the surface. This process is called making a connection. Usually joints are combined into 3 joints equaling 1 stand. Some smaller rigs only use 2 joints and some 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.
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 drillbit, some cause the rock to fail by compressive failure. Others shear slices off the rock as the bit turns.
Drilling fluid (aka "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 which 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 don't enter the wellbore.
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 pit. Watching for abnormalities in the returning cuttings and monitoring pit volume or rate of returning fluid are imperative to catch "kicks" (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 preventers and ultimately by increasing the density of the drilling fluid would allow formation fluids 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 additional 30-foot (10 m) joints (i.e., sections) of pipe under the kelly or topdrive at the surface. This process is called making a connection. Usually joints are combined into 3 joints equaling 1 stand. Some smaller rigs only use 2 joints and some 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.
Completion
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. 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.Production
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 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 or completion 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".
Abandonment
When the well no longer produces or produces so poorly that it is a liability, it is abandoned. In this process, tubing is removed from the well and sections of well bore are filled with cement to isolate the flow path between gas and water zones from each other, as well as the surface. Completely filling the well bore with cement 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.The point at which the well no longer makes a profit and is plugged and abandoned is called the “economic limit.” The equation to determine the economic limit contains four factors, namely: (1) taxes, (2) operating cost, (3) oil price, and (4) royalty. When oil taxes are raised, the economic limit is raised. When oil price is increased, the economic limit is lowered.When the economic limit is raised, the life of the well is decreased. Proven oil reserves are lost when the life of an oil well is decreased. Inversely, when the economic limit is lowered, the life of the well is increased. Proven oil reserves are increased when the life of the well is increased.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. However, 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 not cost effective.
Types of wells
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 soda pop where the carbon dioxide effervesces. 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. 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, with a world-scale (140,000 bbl/day) GTL plant in Qatar scheduled to come online before 2010. 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 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.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 are drilled primarily for producing oil or gas, once the producing structure and characteristics are determined
- appraisal wells are used to assess characteristics (such as flow rate) of a proven hydrocarbon accumulation
- exploration wells are drilled purely for exploratory (information gathering) purposes in a new area
- wildcat wells are those drilled outside of and not in the vicinity of known oil or gas fields.
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