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Oilfield Glossary

Short course on petrophysics

Petroleum (from Latin petra – rock and oleum – oil) is a black, dark brown or greenish liquid found in formations in the earth.Commonly defined as "a substance, generally liquid, occurring naturally in the earth and composed mainly of mixtures of chemical compounds of carbon and hydrogen with or without other nonmetallic elements such as sulfur, oxygen, and nitrogen."
Known reserves of petroleum are estimated at around 1000 gigabarrels,and consumption is currently around 31 gigabarrels per year. At current consumption levels, world oil supply would be gone in about 33 years. However, this ignores any additions to known reserves, changes in demand, etc. As the supply of petroleum becomes more scarce, consumers may look to alternative fuel sources such as ethanol, fuel cells, electric vehicles, or clean-burning hydrogen. Petroleum forms naturally within the earth too slowly to be sustainable for human use.


Petroleum, in some form or other, is not a substance new in the world's history. More than four thousand years ago, according to Herodotus and confirmed by Diodorus Siculus, asphalt was employed in the construction of the walls and towers of Babylon; there were oil pits near Ardericca (near Babylon), and a pitch spring on Zacynthus.Great quantities of it were found on the banks of the river Issus, one of the tributaries of the Euphrates. Ancient Persian tablets indicate the medicinal and lighting uses of petroleum in the upper levels of their society.
The first oil wells were drilled in China in the 4th century or earlier. They had depths of up to 243 meters and were drilled using bits attached to bamboo poles. The oil was burned to evaporate brine and produce salt.
By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The ancient records of China and Japan are said to contain many allusions to the use of natural gas for lighting and heating. Petroleum was known as burning water in Japan in the 7th century.
In the 8th century, the streets of the newly constructed Baghdad were paved with tar, derived from easily accessible petroleum from natural fields in the region.
In the 9th century, oil fields were exploited in Baku, Azerbaijan, to produce naphtha. These fields were described by the geographer Masudi in the 10th century, and by Marco Polo in the 13th century, who described the output of those wells as hundreds of shiploads.
The modern history of petroleum began in 1846, with the discovery of the process of refining kerosene from coal by Atlantic Canada's Abraham Pineo Gesner. Poland's Ignacy Łukasiewicz discovered a means of refining kerosene from the more readily available "rock oil" ("petr-oleum") in 1852 and the first rock oil mine was built in Bobrka, near Krosno in southern Poland in the following year. These discoveries rapidly spread around the world, and Meerzoeff built the first Russian refinery in the mature oil fields at Baku in 1861. At that time Baku produced about 90% of the world's oil. The battle of Stalingrad was fought over Baku Oil field in California, 1938. The first modern oil well was drilled in 1848 by Russian engineer F.N. Semyonov, on the Aspheron Peninsula north-east of Baku.The first commercial oil well drilled in North America was in Oil Springs, Ontario, Canada in 1858, dug by James Miller Williams. The American petroleum industry began with Edwin Drake's drilling of a 69-foot-deep oil well in 1859, on Oil Creek near Titusville, Pennsylvania, for the Seneca Oil Company.The industry grew slowly in the 1800s, driven by the demand for kerosene and oil lamps. It became a major national concern in the early part of the 20th century; the introduction of the internal combustion engine provided a demand that has largely sustained the industry to this day. Early "local" finds like those in Pennsylvania and Ontario were quickly exhausted, leading to "oil booms" in Texas, Oklahoma, and California. By 1910, significant oil fields had been discovered in Canada (specifically, in the province of Alberta), the Dutch East Indies (1885, in Sumatra), Persia (1908, in Masjed Soleiman), Peru, Venezuela, and Mexico, and were being developed at an industrial level. Even until the mid-1950s, coal was still the world's foremost fuel, but oil quickly took over. Following the 1973 energy crisis and the 1979 energy crisis, there was significant media coverage of oil supply levels. This brought to light the concern that oil is a limited resource that will eventually run out, at least as an economically viable energy source. At the time, the most common and popular predictions were always quite dire, and when they did not come true, many dismissed all such discussion. The future of petroleum as a fuel remains somewhat controversial. Today, about 90% of vehicular fuel needs are met by oil. Petroleum also makes up 40% of total energy consumption in the United States, but is responsible for only 2% of electricity generation.The top three oil producing countries are Saudi Arabia, Russia, and the United States. About 80% of the world's readily accessible reserves are located in the Middle East, with 62.5% coming from the Arab 5: Saudi Arabia (12.5%), UAE, Iraq, Qatar and Kuwait. However, with today's oil prices, Venezuela has larger reserves than Saudi Arabia due to nonconventional crude reserves derived from bitumen.
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The chemical structure of petroleum is composed of hydrocarbon chains of different lengths. These different hydrocarbon chemicals are separated by distillation at an oil refinery to produce gasoline, jet fuel, kerosene, and other hydrocarbons. Carbon and hydrogen can be grouped in many ways. The ones important for petroleum are paraffins, naphthenes, aromatics and asphaltenes.

These are the most abundant form of hydrocarbons having the general formula CnH2n+2.The name is derived from the Latin parum (= barely) + affinis with the meaning here of "lacking affinity", or "lacking reactivity".An example of paraffins is methane (CH4)
Nearly pure methane gas is referred to as dry gas. All paraffins having the value n<6, are gaseous and are commonly referred to as wet gas.Values of n, greater than 5 but less than 15, are normally crude oil and paraffins with n>15 become waxy and viscous.

Cycloalkanes (also called naphthenes) are chemical compounds with one or more carbon rings to which hydrogen atoms are attached according to the formula CnH2n. Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins.
Cycloalkanes are classified into small, normal and bigger cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the normal ones, and the rest are the bigger ones. As an example, following is the structure of cyclobutane:
                             H H
                             | |
                             | |
                             | |
                             H H
An aromatic hydrocarbon (abbreviated as AH) is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible aromatic hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic or polycyclic.The following is the structure of benzene:
                              // \
                           H-C    C-H
                             |   ||
                           H-C    C-H
                              \\ /

They are presumed to be very large, for example, with molecular weights that can be in the millions. But even this is not certain, as different methods used to derive the weights often yield wildly different results. The chemical structure is, too, difficult to determine and can vary from source to source. Usually, they are composed of oxygen, nitrogen, and sulfur, combined with the metals nickel, vanadium, and/or iron. But their classification depends largely on their behavior in solvents.
The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties.
They are of particular interest to the petroleum industry because of their depositional effect on drilling equipment. In some early wells, holes had to be re-drilled because the deposits were so disruptive. They continue to be a problem for the industry, as their removal is a time-consuming and expensive process.
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Most geologists view crude oil and natural gas, as the product of compression and heating of ancient organic materials over geological time. According to this theory, oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea bottom in large quantities under anoxic conditions.Over geological time this organic matter, mixed with mud, is buried under heavy layers of sediment. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis.
Kerogens are of various types:
  1. Algal: When matures mostly yields crude oil.
  2. Mixed: Also oil pro but yields more natural gas than type 1.
  3. Coaly: Yields mostly natural gas.Low capacity to form oil.
  4. Inert: Highly oxidized.Very rare and has no ability to generate oil or gas
Kerogen type is one way of determining whether the reservoir will contain oil or natural gas.Burial and high temperature is another factor.At 60oC is biogenic methane (burial depth of 1-2 km). This is where petroleum formation starts. Crude oil is generated here. This oil generation peaks at 100oC. Then at about 175oC (burial depth of 3-4 km) oil generation ceases and gas formation becomes dominant.When temperature exceeds 225oC kerogen has used up its petroleum generating capacity.Methane can still be produced.
Because most hydrocarbons are lighter than rock or water, these sometimes migrate upward through adjacent rock layers until they become trapped beneath impermeable rocks, within porous rocks called reservoirs. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.
Geologists often refer to an "oil window" which is the temperature range that oil forms in—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Though this happens at different depths in different locations around the world, a 'typical' depth for the oil window might be 4–6 km. Note that even if oil is formed at extreme depths, it may be trapped at much shallower depths, even if it is not formed there.Three conditions must be present for oil reservoirs to form:
  • a source rock rich in organic material buried deep enough for subterranean heat to cook it into oil;
  • a porous and permeable reservoir rock for it to accumulate in;
  • a cap rock (seal) that prevents it from escaping to the surface.
If an oil well were to run dry and be capped, it would likely fill back to its original supply eventually. There is considerable question about how long this would take. Some formations appear to have a regeneration time of decades. Majority opinion is that oil is being formed at less than 1% of the current consumption rate.
The vast majority of oil that has been produced by the earth has long ago escaped to the surface and been biodegraded by oil-eating bacteria. What oil companies are looking for is the small fraction that has been trapped by this rare combination of circumstances. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping, but contain so much migrating oil that, although most of it has escaped, vast amounts are still present - more than can be found in conventional oil reservoirs. On the other hand, oil shales are source rocks that have never been buried deep enough to convert their trapped kerogen into oil.
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Locating an oil field is the first obstacle to be overcome. Today, geologists use seismic surveys to search for geological structures that may form oil reservoirs. The "classic" method includes making underground explosion nearby and observing the seismic response that provides information about the geological structures under the ground. However the "passive" methods that extract information from naturally-occurring seismic waves are also known.
Other instruments such as gravimeters and magnetometers are also sometimes used in the search for petroleum. Generally, the first stage in the extraction of crude oil is to drill a well into the underground reservoir. When an oil bearing structure has been tapped, the wellsite geologist (known on the rig as the "mudlogger") will note its presence. Historically, some oil fields existed where the oil rose naturally to the surface, but most of these fields have long since been depleted, except for certain remote locations. Often many wells (called multilateral wells) are drilled into the same reservoir, to ensure that the extraction rate will be economically viable. Also, some wells (secondary wells) may be used to pump water, steam, acids or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and so maintain an economic extraction rate.
If the underground pressure in the oil reservoir is sufficient, then the oil will be forced to the surface under this pressure. Gaseous fuels, natural gas or water are usually present, which also supply needed underground pressure. In this situation it is sufficient to place a complex arrangement of valves (the Christmas tree) on the well head to connect the well to a pipeline network for storage and processing. This is called primary oil recovery. Usually, only about 20% of the oil in a reservoir can be extracted this way.
The amount of oil that is recoverable is determined by a number of factors including the permeability of the rocks, the strength of natural drives (the gas present, pressure from adjacent water or gravity), and the viscosity of the oil. When the reservoir rocks are "tight" such as shale, oil generally cannot flow through but when they are permeable such as in sandstone, oil flows freely. The flow of oil is often helped by natural pressures surrounding the reservoir rocks including natural gas that may be dissolved in the oil, natural gas present above the oil, water below the oil and the strength of gravity. Oils tend to span a large range of viscosity from liquids as light as gasoline to heavy as tar. The lightest forms tend to result in higher production rates.
Over the lifetime of the well the pressure will fall, and at some point there will be insufficient underground pressure to force the oil to the surface. If economical, as often is, the remaining oil in the well is extracted using secondary oil recovery methods. Secondary oil recovery uses various techniques to aid in recovering oil from depleted or low-pressure reservoirs. Sometimes pumps, such as beam pumps and electrical submersible pumps (ESPs), are used to bring the oil to the surface. Other secondary recovery techniques increase the reservoir's pressure by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the reservoir. Together, primary and secondary recovery allow 25% to 35% of the reservoir's oil to be recovered.
Tertiary oil recovery reduces the oil's viscosity to increase oil production. Tertiary recovery is started when secondary oil recovery techniques are no longer enough to sustain production, but only when the oil can still be extracted profitably. This depends on the cost of the extraction method and the current price of crude oil. When prices are high, previously unprofitable wells are brought back into production and when they are low, production is curtailed. Thermally enhanced oil recovery methods (TEOR) are tertiary recovery techniques that heat the oil and make it easier to extract. Steam injection is the most common form of TEOR, and is often done with a cogeneration plant. In this type of cogeneration plant, a gas turbine is used to generate electricity and the waste heat is used to produce steam, which is then injected into the reservoir. In-situ burning is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil. Occasionally, detergents are also used to decrease oil viscosity. Tertiary recovery allows another 5% to 15% of the reservoir's oil to be recovered.
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Life of a well

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. The creation and life of a well can be divided up into five segments:
  • Planning
  • Drilling
  • Completion
  • Production
  • Abandonment


The first part of planning is acquiring a license for developing a well.After that the exploration phase starts. This phase starts with the outcrop studies and analysis by the geologists to locate probable well sites which is followed by 2D or 3D seismic. Seismic is the process in which sound waves are generated and sent out and then their reflections tell us whether the area contains conditions suitable for the presence of hydrocarbons or not. In 2D seismic, the sound waves are generated using a vibrator, dynamite or an air gun for onshore and for offshore only air guns are used. Out of the three techniques mentioned here, the vibrators are preferred as frequency can be controlled. However, dynamites are used where the test is run for greater depths as its penetration is more than the vibrators due to a lower frequency.


The well is created by drilling a hole 5 to 30 inches (13–76 cm) wide 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 size is placed the hole, and is secured in the hole 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 rotary torque and the compressive 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 clean, cool and lubricate the bit.
  • The generated rock "cuttings" are swept up by the drilling fluid as it circulates back to surface outside the drill pipe. Then go over "shakers" which shakes out the cuttings over screens allowing the good fluid to return back into the pits. 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 so than above causing gas and mud to come back 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.
A drilling rig is a structure housing equipment used to drill for water, oil, natural gas from underground reservoirs or to obtain mineral core samples. The term can refer to a land-based rig, a marine-based structure commonly called an 'offshore rig', or a structure that drills oil wells called an 'oil rig'. The term correctly refers to the equipment that drills oil wells or extracts mineral samples, including the rig derrick (which looks like a metal frame tower).
Sometimes a drilling rig is also used to complete an oil well, preparing it for production. However, the rig is not involved with the extraction of the oil; its primary function is to make a hole in the ground so the oil can be produced.
Laymen may refer to the structure which sits on top of offshore wells as a 'rig', but this is not correct. The correct name for the structure in a marine environment is platform. A structure upon which wells produce is a production platform. A floating vessel upon which a drilling rig sits is a floating rig or semi-submersible rig because the whole purpose of the structure is for drilling.
Drilling rigs can be:-
  • Small and portable, such as those used in mineral exploration drilling.
  • Huge, capable of drilling through thousands of meters of the Earth's crust. Large "mud pumps" are used to circulate drilling mud (slurry) through the drill bit and the casing, for cooling and removing the "cuttings" while a well is drilled. Hoists in the rig can lift thousands of tons of pipe. Other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or mineral sample; and permanent living accommodation and catering for crews which may be more than a hundred. Marine rigs may operate many hundreds of miles or kilometres offshore with infrequent crew rotation.
Drilling rig classification
There are many types and designs here when one shows up on time of drilling rigs, depending on their purpose and improvements; many drilling rigs are capable of switching or combining different drilling technologies.
  • by power used
    • electric - rig is connected to a power grid usually produced by its own generators
    • mechanic - rig produces power with its own (diesel) engines
    • hydraulic - most movements are done with hydraulic power
    • pneumatic - pressured air is used to generate small scale movements
  • by pipe used
    • cable - a cable is used to slam the bit on the rock (used for small geotechnical wells)
    • conventional - uses drill pipes
    • coil tubing - uses a giant coil of tube and a downhole drilling motor
  • by height
    • single - can drill only single drill pipes, has no vertical pipe racks (most small drilling rigs)
    • double - can store double pipe stands in the pipe rack
    • triple - can store stands composed of three pipes in the pipe rack (most large drilling rigs)
    • quad - can store stands composed of four pipes in the pipe rack
  • by method of rotation
    • no rotation (most service rigs)
    • rotary table - rotation is achieved by turning a hexagonal pipe (the kelly) at drill floor level.
    • top-drive - rotation and circulation is done at the top of the drillstring, on a motor that moves along the derrick.
  • by position of derrick
    • conventional - derrick is vertical
    • slant - derrick is at an angle (this is used to achieve deviation without an expensive downhole motor)
Once a well is drilled. a VSP(Vertical Seismic Profile) is sent down the hole. The results of this tests are then placed on the already obtained siesmic and the seismic is callibrated accordingly. Different tools are then run there giving us the different logs that are processed and interpreted by the petrophysicists to determine where the hydrocarbons exist and in what quantity. The geologists run their tests to determine the exact geographical structure and the reservoir engineers test the probable zones using equipment such as MDT.


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.


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 the wellbore is filled with cement so that the flowpath from the reservoir to the surface is plugged.
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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. 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 Southwestern United States, and is 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.

Well Logs

A well log is a graphical presentation of physio-chemical characteristic of the geologic formations measured in a borehole as a function of depth. Before the invention of well logs, the methods used for this purpose were:
  1. inspection and analysis of drill cuttings and cores
  2. formation tests
An ever increasing variety of logs are now available depending upon the nature of the rock characteristics being measured. The two main groups are:
  1. those measuring properties which exist naturally in rocks (SP and gamma ray belong to this group)
  2. those having as a common denominator the transmission of a certain physical signal through the formation.Examples of this group are resistivity, acoustic and radioactivity logs
The following are some of the characteristics studied in these logs:
  • Rock characteristics
    The rocks serving as seals which trap hydrocarbons are fine grained, compacted formations with zero or negligible permeability such as shales,salts and anhydirtes.
    • Quartz grains are the principal constituents of sandstones.The most common cementing material is silica or calcium carbonate
    • Carobnate rocks are made up of the remains of marine animals and plants such as corals and algae.The texture of carbonate rocks may vary from unconsolidated bodies of shells to compacted crystalline rocks
    • Shales are the noncommercial part of an oilfield.They normally act as seal rocks and prevent the natural migration of hydrocarbons
  • Porosity
    Normally, the porosity ranges between 5 and 50 in sandstones and 1 and 25 percent in carbonate rocks.Porosity can be classified as:
    • Effective porosity is the ratio of volume Vip associated with the interconnected pores to bulk volume Vt of rock.
      porosity = Vip
    • Absolute or total porosity is the ratio of total pore volume to bulk volume of rock
    Another classification of porosity can be:
    • Primary porosity: based on the pores resulting from original deposits and further cementation of original sediments
    • Secondary porosity: the result of diagenic effects which give rise to solution chaneels.The fractures are also common form of secondary porosity
  • Water & hydrocarbon saturation
    Water saturation is defined as the ratio of the pore volume occupied by water to the total pore volume

    Sw = Vw
    Hydrocarbon saturation is the ratio of the pore volume occupied by hydrocarbons to total pore volume.

    So = Vhc = 1-Sw
  • Temperature and resistivity relationship
    At a constant temperature, water resistivity decreases with increaing salinity whereas at a constant salinity, resistivity decreases with increasing temperature.In log interpretation, the temperature and resistivity relationship is given by,

    R2 = R1 * T1+7
    where T is in degrees Farenheit.
  • Permeability
    Permeability is measured in darcies.A rock is defined to have one darcy permeability when it allows a fluid of one centipoise viscosity to flow at a rate of one cubic centimeter per second through a rock of one centimeter length and one square centimeter cross-section area under one atmoshpere of differential pressure.

Well Log Types

The following is a detailed description of the different well log types:
  • SP log
    The spontaneous potential of the formations in a well is defined as the potential difference between a moveable electrode in the borehole and a fixed electrode at the surface.
    The SP readings are made from the shale baseline to the left.These require that the hole be filled with a relatively conductive fluid of salinity lower than the formation water salinity.
    The SP curve measures electromotive force of electrokinetic and electrochemical origins.The electrokinetic potential is generally regarded as negligible in practice.Electrochemical potential can be sub-divided into:
    • Diffusion potentials: the potential difference generated when two saline solutions come in contact through a permeable membrane.This is the case of mud filtrate and the water in a permeable formation in a well.It is originated by movement of ions from the more concentrated (formation water)to the less concentrated(mud filtrate)
    • Membrane potentials: generated when two saline soutions are seperated by cationic membrane which allows the Na+ to move from the more concentrated to the dilute one.
    The main component of electrochemical potential is membrane potential. The general expression for SP is:
                               SP = -KlogRmf
    K -> a function of temperature
    Rmf -> mud filtrate resistivity
    Rw -> formation water resistivity
  • Gamma ray log
    The natural gamma ray logs measure the natural radioactivity of rocks.Rocks are constantly emitting gamma rays as a result of natural disintegration of small amounts of radioactive elements.Shales normally contain more radioactive element than sands and carbonate rocks.Thus, it can be considered a lithology log similar to the SP log.
    Radioactivity can be defined as a spontaneous disintegration of atoms accompanied by emission of radiation.The simpler atoms have stable nuclei.There are some heavier and more complex atoms which are only potentially stable, and they spontaneously transmute themselves to other more stable isotopes with a change of mass.
    Gamma ray logs can be obtained in open or cased holes, whether filled with mud or empty.These are normally scaled in API units.An API unit is defined as 1/200 of the difference in log deflections between two formations of known radioactivity in an artificial caliberation pit existing at the university of Housten.The gamma ray signal from the formation is mainly affected by thickness, casing, cement behind casing and mud within the borehole.Service companies provide correction charts for these effects.
  • Resistivity log
    1. Conventional resistivity log
      As a general criterion, high resistivity indicates low permeability, low porosity, high hydrocarbon saturation, fresh water or a combination of these conditions.
      Three resistivity curves are obtained i.e. short normal, long normal and the lateral. These curves are obtained by sending an alternating current through an electrode contained in the sonde. The difference betwen the current electrode and several reference electrodes is then measured and the corresponding resistivity is computed. The borehole must be filled with a conductive mud to establish the necessary electrical contact with the formation.
      The short normal is influenced by the mud filtrate invaded zone, whereas the long normal measures a value close to the true formation resistivity of the formation.Similar to the normal resistivity, the lateral device also measures the resistivity of the formation material.However, it penetrates deeper into the formation than the long normal, and hence provides more accurate values of the formation resistivity.
    2. Conventional microresistivity log
      The current and measurement electrodes are short spaced in this log. Two microresistivity curves are micronormal and microinverse.A microcaliper curve is also obtained, which records the hole diameter variations throughout the hole.
      The difference between the two microcurves, called separation, is commonly used to interpret these logs qualitatively.When micronormal resistivity is higher than microinverse curve, the separation is called positive. The permeable beds are characterized by a positive separation and presence of mud cake. Nonpermeable hard formations show no mud cake, high resistivity and no separation.
    3. Induction log
      These can be used in wells drilled with normal water-based muds. They have also replaced the conventional resistivity devices where the formation is less than 200 ohm-meters. These logs are interpreted similar to the conventional resistivity logs. The variations in resistivity represent changes in rock characteristics and fluid content. Since there is no flow of current through the mud, induction logs are less affected by borehole conditions than conventional logs.
    4. Focused current log
      These are useful in highly resistive formations and in the wells drilled with salt muds.For these the logarithmic scales are used.
    5. Focused microresistivity log
      This log determines more accurately the resistivity of the mud filtrate flushed zone as compared to the conventional microresistivity logs.
  • Neutron log
    Some elements contain in their nuclei more neutrons than others, and they are also loosely bound. These elements give up their neutrons when bombarded with alpha rays.
    Upon leaving its source, afast neutron suffers a series of collisions.If the neutron hits a heavy nucleus, it bounce with energy almost identical to the energy of emission. But if the collision is centered is against a nucleus of about the same weight, the energy of the fast moving neutron is practically transferred to the other nucleus. The effect is to slow down the fast neutron.
    Hydrogen is the major source for slowing down neutrons. Each time a neutron collides, it becomes slower until it finally reaches its minimum velocity at formation temperature and diffuses. It is at this stage in the reaction chain that the neutron is abosrbed by an atom, which then emits one or several gamma rays. The concept of neutron logging is based on the bombardment of formation by meutrons and their detection in the formation.
    In interpretation of this log, it is assumed that capture gamma rays are proportional to the amount of hydrocarbon present in the formation. Since most of the hydrogen is present in the water and hydrocarbons filling the rock pores, deflections of the neutron curve recorded should be proportional to formation porosity.
  • Density log
    Gamma rays emitted by the source collide with the electrons of the atoms of the formation material which they traverse. In this atomic reaction some gamma rays lose energy before reaching the detectors, while others are absorbed through different atomic absorption processes.
    The heavier the material, the fewer gamma rays that reach the detectors. Therefore, the counting rates of the detectors should be proportional to formation density.
  • Acoustic log
    These are based on the transmission of sound waves through the formation; they record the differential time which an acoustic compressional wave takes to travel a given distance in the formation. One application of acoustic logs is to determine formation porosity.

Data Gathering

Data can be collected using direct as well as indirect methods. The following is some detail of these methods:
  • Direct methods
    • Coring
      Core samples are used
      • to gain understanding of the composition of the reservoir rock, inter-reservoir seals and the reservoir pore system
      • to establish physical rock properties by direct measurement in a laboratory
      • to describe the depositional environment, sedimentary features and the diagenetic history of the sequence
      • in pre-development stage, to test the compatibility of injection fluids with the formation and to establish the probability of formation failure and sand production.
      Coring is performed in between drilling operations. Once the formation for which a core is required is identified on the mudlog, the drilling assembly is pulled out of the hole.
      For coring operations a special assembly is run comprising of a core bit and a core barrel.
      A core bit can be visualized as a hollow cylinder with an arrangement of cutters on the outside.These cut a circular groove into the formation.Inside, the groove remains an intact cylinder of rock which moves into the inner core barrel as the coring process progresses.Evetually, the core is cut free (broken)and prevented from falling out of the barrel while being brought to the surface by an arrangement of steel fingers or slips.Typical core diameters are three to seven inches and are usually 90 feet long.
      If a conventional core has been cut, it will be retrieved from the barrel on the rig floor and crated.It is common to do a lithologic description at this stage.To avoid drying out of core samples and the escape of light hydrocarbons some sections will be immediately sealed in a coating of hot wax and foil.
      If the formation selected is very friable or unconsolidated, a fibre glass inner barrel instead of a steel inner barrel is used. The fibre glass lining containing the core is cut upon retrieval with a handsaw into 10 foot sections and the ends are sealed for transport.
      Upon arrival in the laboratory the core will be sectioned (one third:two thirds) along its entire length and photographed under normal and UV light.After that, plugs (small cylinders 2cm diameter and 5cm long) are cut from the slabbed core, usually at about 30cm intervals.
      Standard analysis pf plugs will include determination of:
      • porosoty
      • horizontal air permeability
      • grain density
      Special core analysis include
      • vertical air permeability
      • relative permeability
      • capillary pressure
      • cementation and saturations exponents
      Core analysis is complex and may involve different laboratories.It may therefore, take months before final results arrive.As a result of the relatively high costs and a long lead time of core evaluations the technique is only used in selected intervals in a number of wells drilled.
    • Mudlogging
      A Wellsite Geologist (usually called a "Mudlogger" or "Mudlogging Field Geologist") works to describe cuttings, monitor formation gas and general drilling operations while drilling is in progress. The geologist/mudlogger analyzes the rock samples (cuttings) coming out of the circulating mud/fluids off of the 'flow line' from the drill string/pipe. Cuttings are sampled as they move across the shale shakers at drilled depth intervals (10', 20', 30', 50') described by the chief geologist. The 'Mud Log' is prepared by a Mud-logging company that has been hired by an operating company. A Mud Log displays formation gas (gas units and ppm), rate of penetration (ROP in min/ft); lithological sample descriptions; interpretive geology based upon ROP, formation gas/oil cut-stain-fluorescence, and gas curves including a total gas (gas units = ppm/1000) curve and Methane through Pentane (ppm). A mud log also displays bit information, drilling parameters, deviation surveys, and formation tops.
    • Sidewall sampling
      The sidewall sampling tool can be used to obtain small plugs directly from the borehole wall.The tool is run on wireline after the hole has been drilled.Some 20 to 30 individual bullets are fired from each gun at different depths.The hollow bullet will penetrate the formation and a rock sample will be trapped inside the steel cylinder.By pulling the tool upwards, wires connected to the gun pull the bullet and sample from the borehole well.
      These are useful to obtain direct indications of hydrocarbons and to differentiate between oil and gas.The technique is applied extensively to sample microfossils and pollen for stratigraphic analysis.
      Upto 20 samples can be individually cut and are stored in a container inside the tool.
  • Indirect methods
    • Seismic
      Seismology (from the Greek seismos = earthquake and logos = word) is the scientific study of earthquakes and the movement of waves through the Earth.
      Earthquakes, and other earth movements, produce different types of seismic waves. These waves travel through rock, and provide an effective way to "see" events and structures deep in the Earth. There are three basic types of seismic waves: P-waves, S-waves, and surface waves.
      Pressure waves, also called Primary waves or P-waves, travel the fastest and are therefore the first waves to appear on a seismogram. P-waves are pressure or compressional waves that move (propagate) through a material by alternately compressing and expanding (dialating) materials. For a visual example of this movement, try laying a coil (like a Slinky) on a flat surface. Tap lightly on one end, and you will see the coil compress and then expand along the whole length of the coil. This is a P-wave.
      S-waves, also called Shear waves or secondary waves, travel slower than P-waves and appear second on a seismogram.
      One of the earliest important discoveries was that the outer core of the Earth is liquid. Pressure waves (P-waves) pass through the core. Transverse or shear waves (S-waves) that shake side-to-side require rigid material so they do not pass through the core. Thus, the liquid core causes a "shadow" on the side of the planet opposite of the earthquake where no direct S-waves are observed.
      Seismic waves produced by explosions have been used to map salt domes, faults, anticlines and other geologic traps in petroleum-bearing rocks, geological faults, rock types, and long-buried giant meteor craters. For example, the Chicxulub impactor, which is believed to have killed the dinosaurs, was localized to Central America by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration.
    • Logging while drilling
      Logging While Drilling (LWD), along with Measurement While Drilling (MWD) systems provide wellbore directional surveys, petrophysical well logs, and drilling information in real-time while drilling. MWD refers to measurements acquired down hole while drilling that specifically describe directional surveying and drilling-related measurements. LWD refers to petrophysical measurements, similar to open hole wireline logs, acquired while drilling. These systems are based on mud telemetry (mud pulse), where variations in pressure exercised by the tool can be sensed on the surface via a computer, and thus communication is established.