Oil well, Oil field and reservoir, Chemistry tutorial


An oil well is a common word for any boring via the surface of earth which is designed to determine and produce petroleum oil. Generally some of the natural gas is produced all along with the oil. A well designed to produce primarily or only gas might be known as a gas well.

Oil Field:

Oil field is an area having a plenty of oil wells extracting petroleum (that is, crude oil) from below ground.  As the oil reservoirs generally extend over a big area, possibly some hundred kilometers across, full exploitation entails multiple wells scattered across the area. Moreover, there might be exploratory wells probing the edges, pipelines to transport the oil elsewhere, and support facilities. As oil field might be remote from civilization, establishing a field is often a very complex exercise in logistics. For example, workers have to work there for months or years and need housing. In turn, housing and equipment need water and electricity. Pipelines in cold areas might require to be heated. Surplus natural gas requires to be burned off if there is no way to make use of it, needing a furnace and stacks, and pipes to carry it from well to the furnace.

Therefore, the typical oil field looks like a small self-contained city in the midst of a landscape dotted by drilling rigs and/or the pump jacks termed as 'nodding donkeys due to their bobbing arm. Some companies, like BJ Services, Esso, Bechtel, Schlumberger Limited, Baker Hughes and Halliburton, have organizations which specialize in the large-scale construction of the infrastructure and giving specialized services needed to operate a field profitably.

Above 40,000 oil fields are scattered around the globe, on land and offshore. The largest are the Ghawar Field in Saudi Arabia and the Burgan Field in Kuwait, having more than 60 billion barrels (9.5×109 m3) estimated in each. Most of the oil fields are much smaller. According to the US Department of Energy (Energy Information Administration), as of the year 2003 the US alone had over 30,000 oil fields.

In the modern period, the position of oil fields having proven oil reserves is a key underlying factor in numerous geopolitical conflicts. The word oil field is as well employed as shorthand to refer to the whole petroleum industry.

Oil Well:

The oil well is a common word for any boring via the earth's surface which is designed to find out and produce petroleum oil hydrocarbons. Generally some of the natural gas is produced all along by the oil. A well designed to generate mainly or only gas might be known as a gas well.

The well is made up by drilling a hole of around 5 to 36 inches (127.0 mm to 914.4 mm) diameter into the earth by a drilling rig which rotates a drill string by a bit attached. After the hole is drilled, sections of steel pipe (that is, casing), slightly smaller in diameter than the borehole, are positioned in the hole. Cement might be put between the outside of the casing and the borehole. The casing gives structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from one other and from the surface.

By such zones safely isolated and the formation protected via the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) having a smaller bit, and as well cased by a smaller size casing. Modern wells often encompass 2 to 5 sets of subsequently smaller hole sizes drilled inside one other, each cemented by casing.

=> To drill the well: 

The drill bit, aided via the weight of thick walled pipes termed as 'drill collars' above it, cuts into the rock. There are various kinds of drill bit; some causes the rock to fail via compressive failure. Others shear slices off the rock as the bit turns.

Drilling fluid (that is, 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 should be carefully tailored to give the correct chemical and physical features needed to safely drill the well. Specific functions of the drilling mud comprise cooling the bit, lifting rock cuttings to the surface, preventing destabilization of the rock in the wellbore walls and overcoming the pressure of fluids within the rock in such a way that these fluids don't enter the wellbore.

The made rock 'cuttings' are swept up via the drilling fluid as it circulates back to the surface outside the drill pipe. The fluid then goes via 'shakers' that strain the cuttings from the good fluid that 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' (whenever the formation pressure at the depth of the bit is greater than the hydrostatic head of the mud above, which if not controlled temporarily via closing the blowout preventers and ultimately by raising the density of the drilling fluid would let formation fluids and mud to come up nonstop) early.

The pipe or drill string to which the bit is joined is steadily lengthened as the well gets deeper via screwing in additional 30-foot (10 m) joints (that is, sections) of pipe under the Kelly or top drive at the surface. This method is termed making a connection. Generally joints are joined into 3 joints equaling 1 stand. Some of the smaller rigs only utilize 2 joints and a few rigs can handle stands of 4 joints.

This method is all facilitated via a drilling rig that includes all the essential equipment to circulate the drilling fluid, hoist and turn the pipe, control down hole pressures, take away cuttings from the drilling fluid and produce onsite power for such operations.


After drilling and casing the well, it should be 'completed'. Completion is the method in which the well is allowed to produce oil or gas.

In a cased-hole completion, small holes termed as perforations are made in the part of casing that passed via the production zone, to give 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 part. These maintain structural integrity of the wellbore in the lack of casing, whereas still allowing flow from the reservoir into the wellbore. Screens as well control the migration of formation sands into production tubular and surface equipment that can cause washouts and other problems, specifically from unconsolidated sand formations in the offshore fields.

After a flow path is formed, acids and fracturing fluids are pumped to the well to fracture, clean or else prepare and stimulate the reservoir rock to optimally produce hydrocarbons to the wellbore. Ultimately, the area above the reservoir part of the well is packed off within the casing, and joined to the surface through a smaller diameter pipe termed as tubing. This arrangement gives a redundant barrier to leaks of hydrocarbons and also allowing damaged parts to be replaced. As well, the smaller diameter of the tubing generates hydrocarbons at an increased velocity in order to overcome the hydrostatic effects of heavy fluids like water.

In most of the wells, the natural pressure of the subsurface reservoir is high adequate for the oil or gas to flow to the surface. Though, this is not for all time the case, particularly in depleted fields where the pressures have been lowered via other producing wells or in low permeability oil reservoirs. Installing smaller diameter tubing might be adequate to assist the production; however artificial lift processes might as well be required. Common solutions comprise down hole pumps, gas lift or surface pump jacks. 

Most of the new systems in the last 10 years have been introduced for well completion. Multiple packer systems having frac ports or port collars in an all in one system have cut completion costs and enhanced production, particularly in the case of horizontal wells. These new systems let casings to run to the lateral zone having proper packer/frac port placement for the optimal hydrocarbon recovery.


The production phase is the most significant phase of a well's life, whenever the oil and gas are produced. By this time, the oil rigs and work over rigs employed to drill and complete the well have moved off the wellbore, and the top is generally outfitted by a collection of valves termed as a production tree. These valves control pressures, control flows and allow access to the wellbore in case further completion work is required. From the outlet valve of the production tree, the flow can be joined to a distribution network of pipelines and tanks to supply the product to refineries, natural gas compressor stations and oil export terminals.

As long as the pressure in the reservoir remains high adequate, the production tree is all that is needed to produce the well. If the pressure depletes and it is considered economically feasible, an artificial lift process illustrated in the completions part can be used.

Workovers are often essential in older wells, which might require smaller diameter tubing, scale or paraffin removal, acid matrix jobs and completing new zones of interest in a shallower reservoir. This remedial work can be functioned by using workover rigs - as well termed as pulling units or completion rigs - to pull and substitute tubing, or via the use of well intervention methods utilizing coiled tubing. Based on the kind of lift system and wellhead a rod rig or flushby can be employed to change a pump without pulling the tubing.

Improved recovery methods like water flooding, steam flooding or CO2 flooding might be employed to increase the reservoir pressure and give a 'sweep' effect to push hydrocarbons out of the reservoir. These methods need the use of injection wells (often selected from old production wells in a carefully determined pattern), and are employed whenever facing problems with reservoir pressure depletion, high oil viscosity, or can even be used early in a field's life. In some cases, based on the reservoir's geo-mechanics, reservoir engineers might find out that ultimate recoverable oil might be raised by applying a water flooding strategy early in the field's growth instead of later. These enhanced recovery methods are often termed as 'tertiary recovery'.


As a well ages and the reservoir is depleted, the oil generated starts to drop. The production rate at which the well's revenue is so low that it no longer makes a gain is termed as the 'Economic limit'. The equation to find out the economic limit comprises taxes, operating cost, oil price, and royalty. Whenever oil taxes are raised, the economic limit is raised. If oil price is increased, the economic limit is lowered. If the economic limit is increased, the life of the well is shortened and proven oil reserves are lost. On the contrary, if the economic limit is lowered, the life of well is lengthened.

If the economic limit is reached, the well becomes a liability and it is abandoned. In this method, tubing is eliminated from the well and parts of well bore are filled by cement to isolate the flow path between gas and water zones from one other, and also the surface. Entirely filling the well bore with cement is expensive 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 financial limit, there often is still an important amount of unrecoverable oil left in the reservoir. It might be tempting to defer the physical abandonment for an extended time period; hoping that the oil price will go up or those new supplemental recovery methods will be perfected.  Though, lease provisions and governmental regulations generally need quick abandonment, liability and tax concerns as well might favor abandonment.

In theory, an abandoned well can be reentered and restored to production (or transformed to injection service for the supplemental recovery or for down hole hydrocarbons storage), however reentry often proves to be hard mechanically and not cost efficient.

Types of Wells:

Oil wells come in numerous varieties. There can be wells that produce oil, wells which produce oil and natural gas, or wells which only produce natural gas. Natural gas is almost for all time a by-product of producing oil, as the small, light gas carbon chains come out of solution as it undergoes pressure reduction from the reservoir to the surface, identical to uncapping a bottle of soda  pop where the carbon-dioxide effervesces. The unwanted natural gas can be a disposal problem at the well site. If there is not a market for natural gas close to the wellhead it is virtually valueless as it should be piped to the end user. Till recently, such unwanted gas was burned  off  at  the  well  site,  however  due  to  ecological  concerns  this practice is becoming  less common. Often, unwanted (or stranded gas devoid of a market) gas is pumped back to the reservoir having an 'injection' well for the disposal or depressurizing the producing formation. The other solution is to export the natural gas as a liquid. Gas-to-Liquid, (GTL) is a developing technology which transforms stranded natural gas to the synthetic gasoline, diesel or jet fuel via the Fischer-Tropsch method. These fuels can be transported via conventional pipelines and tankers to users. Proponents claim GTL fuels burn cleaner than comparable petroleum fuels. The other obvious way to categorize oil wells is via land or offshore wells. There is extremely little difference in the well itself. The offshore well targets a reservoir that occurs to be underneath an ocean. Because of logistics, drilling an offshore well is far more costly than an onshore well. By far the most general kind is the onshore well.

The other manner to categorize oil wells is by their purpose in contributing to the growth of a resource. They can be characterized as:

a) Production wells are drilled mainly for producing oil or gas, once the producing structure and features are found out.

b) Appraisal wells are employed to assess the features (like flow rate) of a proven hydrocarbon accumulation.

c) Exploration wells are drilled purely for the exploratory (information gathering) reason in a new area.

d) Wildcat wells are those drilled outside of and not in the vicinity of the known oil or gas fields.

At a producing well site, active wells might be further categorized as:

a) Oil producers, generating predominantly liquid hydrocarbons, however mostly by some related gas.

b) Gas producers, generating nearly wholly gaseous hydrocarbons. Water injectors, injecting water to the formation to maintain the reservoir pressure or simply to dispose of water produced by the hydrocarbons as even after treatment, it would be too oily and too saline to be considered clean for dumping the overboard, let alone to a fresh water source, in case of the onshore wells. Often water injection consists of an element of reservoir management and produced water disposal.

c) Aquifer producers, intentionally generating reservoir water for re-injection to manage pressure. This is in effect moving reservoir water from where it is not as helpful to where it is more helpful. Such wells will in general only be employed if produced water from the oil or gas producers is inadequate for the reservoir management purposes. By using aquifer produced water instead of sea water is because of the chemistry.

d) Gas injectors, injecting gas to the reservoir often as a means of disposal or sequestering for later production, however as well to maintain the reservoir pressure.

Oil Reservoir:

A petroleum reservoir or an oil and gas reservoir is the subsurface pool of hydrocarbons contained in the porous or fractured rock formations. The naturally occurring hydrocarbons are trapped via overlying rock formations by lower permeability.

Once a source rock produces and expels petroleum, the petroleum migrates from the source rock to a rock which can store the petroleum. A rock capable of storing petroleum in its pore spaces, the void spaces between the grains of sediment in a rock, is termed as a reservoir rock. Rocks which have adequate pore space via which petroleum can move comprise sandstone, limestone, and rocks that have numerous fractures. A good reservoir rock might encompass pore space which surpasses 30% of the rock volume. Poor quality reservoir rocks encompass less than 10% void space capable of storing petroleum. Rocks which lack pore space tend to lack permeability, the property of rock which lets fluid to pass via the pore spaces of the rock. Having very few pores, it is not probable that the pores are joined and less likely that fluid will flow via the rock than in a rock having larger or richer pore spaces. Highly porous rocks tend to encompass better permeability due to the greater number of pores and bigger pore sizes which tend to let fluids to move via the reservoir more easily. The property of permeability is vital to producing petroleum: If fluids can't migrate via a reservoir rock to a petroleum production well, the well will not produce much petroleum and the money used up to drill the well has been wasted.

In order for a reservoir to have petroleum, the reservoir should be shaped and sealed similar to a container. Good petroleum reservoirs are sealed through a less porous and permeable rock termed as a seal or cap rock. The seal prevents the petroleum from further migration. Rocks similar to shale and salt give excellent seals for the reservoir rocks as they don't allow fluids to pass via them easily. Seal-forming rocks tend to be made up of small particles of sediment which fit closely altogether in such a way that the pore spaces are small and poorly joined. The permeability of a seal should be virtually zero in order to retain petroleum in the reservoir rock for millions to hundreds of millions of years, the time span between the formation of petroleum to the discovery and production of numerous petroleum fields. Similarly, the seal should not be subject to forces in the earth that might cause fractures or other breaks in the seal to form.

Reservoir rocks and seals work altogether to make a trap for petroleum. Typical traps for petroleum comprise hills shaped such as upside-down bowls beneath the surface of earth, termed as anticlines, or traps made by faults. Abrupt changes in rock type can make good traps, like sandstone deposits subsequent to the shale deposits, particularly if a sand deposit is encased in a rock which is adequately rich in organic matter to act as a petroleum source and endowed by the properties of a good seal. The significant feature of the formation of petroleum accumulations is timing. The reservoir should have been deposited prior to the petroleum migration from the source rock to the reservoir rock. The seal and trap should have been developed previous to petroleum accumulating in the reservoir, or else the petroleum would have migrated farther. The source rock should have been exposed to the suitable temperature and pressure conditions over long periods of time to modify the organic matter to petroleum. The required coincidence of some conditions is difficult to accomplish in nature.

Petroleum is generally found below the earth's surface in accumulations termed as fields. Fields can have oil, tar, gas, water and other substances; however oil, gas and water are the most frequent. In order for a field to form, there should be some sort of structure to trap the petroleum, a seal on the trap which prohibit leakage of the petroleum, and a reservoir rock which consists of sufficient pore space, or void space, to hold the petroleum.

To find these characteristics altogether in an area in which petroleum has been produced by chemical reactions influencing the organic remain needs lots of coincidences of timing of natural processes. Rocks occur in numerous environments, comprising lakes, deep areas of the seas and oceans and swamps. The source rocks should be buried deep enough beneath the earth's surface of the earth to heat up the organic material, however not so deep that the rocks metamorphose or that the organic material changes to graphite or materials other than the hydrocarbons. Temperatures less than 302° F (150°C) are usual for the petroleum generation.


The traps needed in the last step of the reservoir formation method have been categorized by petroleum geologists into two kinds: Structural and Stratigraphic. A reservoir can be made up by one type of trap or a combination of both.

Structural Traps:

Structural traps are made up by a deformation in the rock layer that includes the hydrocarbons. Domes, anticlines and folds are general structures. Fault-related features as well might be categorized as structural traps if closure is present. Structural traps are the simplest to locate via surface and subsurface geological and geophysical studies. They are the most frequent among traps and have received a greater amount of attention in the search for oil than all other kinds of traps. An illustration of this type of trap begins if salt is deposited via shallow seas. Later, a sinking seafloor deposits organic-rich shale over the salt, that is in turn covered by layers of sandstone and shale. Deeply buried salt tends to increase unevenly in swells or salt domes and any oil generated in the sediments is trapped where the sandstones are pushed up over or nearby to the salt dome.

Stratigraphic Traps:

Stratigraphic traps are made when other beds seal a reservoir bed or whenever the permeability changes (facies change) in the reservoir bed itself. Stratigraphic traps can form against either the younger or older time surfaces.

Estimating Reserves:

After the discovery of a reservoir, a program of appraisal will search for to build an enhanced picture of the accumulation.  In simple text book illustration of a uniform reservoir, the first phase is to make use of seismic to find out the possible size of the trap. Appraisal wells can be employed to find out the location of oil-water contact and with it, the height of oil-bearing sands. Often coupled by seismic data, it is possible to approximate the volume of oil bearing reservoir. The subsequent step is to make use of information from appraisal wells to approximate the porosity of the rock. This is generally 20 to 35% or less (that is, the percentage of the net volume which includes fluids instead of solid rock). This can provide a picture of the real capacity. Laboratory testing can find out the features of the reservoir fluids, specifically the expansion factor of the oil (how much the oil will expand if brought from the high pressure, high temperature atmosphere of the reservoir to 'stock tank' conditions at the surface.

By this knowledge, it is then possible to estimate how many 'stock tank' barrels of oil are situated in the reservoir. This is known as the Stock Tank Oil Initially In Place (STOIIP). As an outcome of studying properties like the permeability of the rock (how simply fluids can flow via the rock) and possible drive methods, it is possible to then approximate the recovery factor (what proportion of the oil in place can be reasonably expected to be generated). This is generally between 30 to 35%. This finally provides a value for the recoverable reserves. The complexity in practice is that reservoirs are not uniform. They encompass a variable porosities and permeabilities and might be compartmentalized; having fractures and faults breaking them up and making complex fluid flow.


To get the contents of the oil reservoir, it is generally essential to drill to the crust of Earth; however surface oil seeps exist in several parts of the world. A virgin reservoir might be under adequate pressure to initially push the hydrocarbons to surface. Though, as the fluids are generated, the pressure will often decline, and production will falter with it. Though, the reservoir might respond to fluid withdrawal in a manner that will tend to maintain the pressure. Artificial drive processes might be essential. This method (as well termed as depletion drive) based on the related gas of the oil. The virgin reservoir might be completely liquid, however will be expected to encompass gaseous hydrocarbons in solution because of the pressure. As the reservoir depletes, the pressure falls beneath the bubble point and the gas comes out of solution to make a gas cap at the top. This gas cap pushes down on the liquid assisting to maintain the pressure.

In reservoirs already having a gas cap (that is, the virgin pressure is already beneath bubble point), the gas cap expands by the depletion of the reservoir, pushing down on the liquid parts applying additional pressure.

Beneath the hydrocarbons might be a ground water aquifer. Water, as by all liquids, is compressible to a small degree. As the hydrocarbons are depleted, the reduction in pressure in the reservoir causes the water to enlarge slightly. However this expansion is minute, if the aquifer is large adequate, this will translate to a large increase in volume, that will push up on the hydrocarbons, maintaining the pressure. If the natural drives are inadequate, as they very often are, then the pressure can be artificially maintained via injecting water to the aquifer or gas to the gas cap.

Oil in Place:

Oil in place is the net hydrocarbon content of an oil reservoir and is often abbreviated STOOIP, that stands for Stock Tank Original Oil In Place, or STOIIP for Stock Tank Oil Initially In Place, referring to the oil in place prior to the commencement of production. In this case, stock tank refers to the storage vessel (often purely notional) having the oil after production.

Oil in place should not be confused having oil reserves which are the technically and economically recoverable part of oil volume in the reservoir. Present recovery factors for oil fields around the world usually range between 10 and 60%; some are over 80%. The broad variance is due to largely to the diversity of fluid and reservoir features for various deposits. Accurate computation of the value of STOOIP needs knowledge of:

  • Volume of rock having oil (bulk rock volume, in the USA this is generally in acre-feet)
  • Percentage porosity of the rock in the reservoir.
  • Percentage water content of that porosity
  • Amount of shrinkage which the oil undergoes whenever brought to the surface of earth and is achieved by using the formula

N = 7758 Vb Φ (1 - Sw)/Boi

Or N = Vb Φ (1 - Sw)/Boi [m3]


N = STOIIP (barrels)

Vb = Bulk (rock) volume (acre-feet or cubic meters)

Φ = Fluid-filled porosity of the rock (fraction)

Sw = Water saturation - water-filled part of this porosity (fraction)

Boi = Formation volume factor (that is, dimensionless factor for the change in volume between reservoir and standard conditions at surface)

Gas saturation Sg is traditionally absent from this equation.

The constant value 7758 transforms acre-feet to stock tank barrels. An acre of reservoir 1 foot thick would have 7758 barrels of oil in the limiting case of 100% porosity, zero water saturation and no oil shrinkage. Whenever the metric system is being employed, a conversion factor of 6.289808 can be employed to transform cubic meters to stock tank barrels. A one-cubic metre vessel would hold 6.289808 barrels of oil.

Formation Volume Factor:

Whenever oil is produced, the high reservoir temperature and pressure reduces to surface conditions and gas bubbles out of the oil. As the gas bubbles out of the oil, the volume of oil reduces. Stabilized oil under surface conditions (either 60o F and 14.7 psi or 15o C and 101.325 kPa) is termed as stock tank oil. Oil reserves are computed in terms of stock tank oil volumes instead of reservoir oil volumes. The ratio of stock tank volume to oil volume under reservoir conditions is termed as the formation volume factor (FVF). It generally differs from 1.0 to 1.7. A formation volume factor of 1.4 is the characteristic of high-shrinkage oil and 1.2 of low-shrinkage oil.

Reservoir Engineering:

Reservoir engineering is the branch of petroleum engineering that applies scientific principles to the drainage problems occurring throughout the growth and production of oil and gas reservoirs so as to get a high economic recovery. The working tools of the reservoir engineer are subsurface geology, applied mathematics, and the fundamental laws of physics and chemistry regulating the behavior of liquid and vapor stages of crude oil, natural gas and water in reservoir rock. 

Of specific interest to reservoir engineers is producing accurate reserves estimates for use in the financial reporting to SEC and other regulatory bodies. The other job responsibilities comprise numerical reservoir modeling, production forecasting, well testing, well drilling and workover planning, economic modeling and PVT analysis of the reservoir fluids.

Reservoir engineers as well play a central role in the field development planning, recommending suitable and cost efficient reservoir depletion schemes like water flooding or gas injection to maximize the hydrocarbon recovery.

Reservoir engineers often specialize in two main areas:

Surveillance (or production) engineering, that is, monitoring of existing fields and optimization of production and injection rates. Surveillance engineers generally make use of analytical and empirical methods to perform their work, comprising decline curve analysis, material balance modeling, and inflow or outflow analysis. Simulation modeling, that is, the conduct of reservoir simulation studies to find out the optimal growth plans for oil and gas reservoirs.

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