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Petroleum - نفت



Petroleum, or crude oil, naturally occurring oily, bituminous liquid composed of various organic chemicals. It is found in large quantities below the surface of Earth and is used as a fuel and as a raw material in the chemical industry. Modern industrial societies use it primarily to achieve a degree of mobility—on land, at sea, and in the air—that was barely imaginable less than 100 years ago. In addition, petroleum and its derivatives are used in the manufacture of medicines and fertilizers, foodstuffs, plastics, building materials, paints, and cloth and to generate electricity.

In fact, modern industrial civilization depends on petroleum and its products; the physical structure and way of life of the suburban communities that surround the great cities are the result of an ample and inexpensive supply of petroleum. In addition, the goals of developing countries—to exploit their natural resources and to supply foodstuffs for the burgeoning populations—are based on the assumption of petroleum availability. In recent years, however, the worldwide availability of petroleum has steadily declined and its relative cost has increased. Many experts forecast that petroleum will no longer be a common commercial material by the mid-21st century. World Energy Supply.



The chemical composition of all petroleum is principally hydrocarbons, although a few sulfur-containing and oxygen-containing compounds are usually present; the sulfur content varies from about 0.1 to 5 percent. Petroleum contains gaseous, liquid, and solid elements. The consistency of petroleum varies from liquid as thin as gasoline to liquid so thick that it will barely pour. Small quantities of gaseous compounds are usually dissolved in the liquid; when larger quantities of these compounds are present, the petroleum deposit is associated with a deposit of natural gas (see Gases, Fuel).

Three broad classes of crude petroleum exist: the paraffin types, the asphaltic types, and the mixed-base types. The paraffin types are composed of molecules in which the number of hydrogen atoms is always two more than twice the number of carbon atoms. The characteristic molecules in the asphaltic types are naphthenes, composed of twice as many hydrogen atoms as carbon atoms. In the mixed-base group are both paraffin hydrocarbons and naphthenes.



Petroleum is formed under Earth’s surface by the decomposition of marine organisms. The remains of tiny organisms that live in the sea—and, to a lesser extent, those of land organisms that are carried down to the sea in rivers and of plants that grow on the ocean bottoms—are enmeshed with the fine sands and silts that settle to the bottom in quiet sea basins. Such deposits, which are rich in organic materials, become the source rocks for the generation of crude oil. The process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the seafloor under their own weight. As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees. The mud and sand harden into shale and sandstone; carbonate precipitates and skeletal shells harden into limestone; and the remains of the dead organisms are transformed into crude oil and natural gas.

Once the petroleum forms, it flows upward in Earth’s crust because it has a lower density than the brines that saturate the interstices of the shales, sands, and carbonate rocks that constitute the crust of Earth. The crude oil and natural gas rise into the microscopic pores of the coarser sediments lying above. Frequently, the rising material encounters an impermeable shale or dense layer of rock that prevents further migration; the oil has become trapped, and a reservoir of petroleum is formed. A significant amount of the upward-migrating oil, however, does not encounter impermeable rock but instead flows out at the surface of Earth or onto the ocean floor. Surface deposits also include bituminous lakes and escaping natural gas.



These surface deposits of crude oil have been known to humans for thousands of years. In the areas where they occurred, they were long used for limited purposes, such as caulking boats, waterproofing cloth, and fueling torches. By the time the Renaissance began in the 14th century, some surface deposits were being distilled to obtain lubricants and medicinal products, but the real exploitation of crude oil did not begin until the 19th century. The Industrial Revolution had by then brought about a search for new fuels, and the social changes it effected had produced a need for good, cheap oil for lamps; people wished to be able to work and read after dark. Whale oil, however, was available only to the rich, tallow candles had an unpleasant odor, and gas jets were available only in then-modern houses and apartments in metropolitan areas.

The search for a better lamp fuel led to a great demand for “rock oil”—that is, crude oil—and various scientists in the mid-19th century were developing processes to make commercial use of it. Thus British entrepreneur James Young, with others, began to manufacture various products from crude oil, but he later turned to coal distillation and the exploitation of oil shales. In 1852 Canadian physician and geologist Abraham Gessner obtained a patent for producing from crude oil a relatively clean-burning, affordable lamp fuel called kerosene; and in 1855 an American chemist, Benjamin Silliman, published a report indicating the wide range of useful products that could be derived through the distillation of petroleum.

Thus the quest for greater supplies of crude oil began. For several years people had known that wells drilled for water and salt were occasionally infiltrated by petroleum, so the concept of drilling for crude oil itself soon followed. The first such wells were dug in Germany from 1857 to 1859, but the event that gained world fame was the drilling of an oil well near Oil Creek, Pennsylvania, by “Colonel” Edwin L. Drake in 1859. Drake, contracted by the American industrialist George H. Bissell—who had also supplied Silliman with rock-oil samples for producing his report—drilled to find the supposed “mother pool” from which the oil seeps of western Pennsylvania were assumed to be emanating. The reservoir Drake tapped was shallow—only 21.2 m (69.5 ft) deep—and the petroleum was a paraffin type that flowed readily and was easy to distill.

Drake’s success marked the beginning of the rapid growth of the modern petroleum industry. Soon petroleum received the attention of the scientific community, and coherent hypotheses were developed for its formation, migration upward through the earth, and entrapment. With the invention of the automobile and the energy needs brought on by World War I (1914-1918), the petroleum industry became one of the foundations of industrial society.



In order to find crude oil underground, geologists must search for a sedimentary basin in which shales rich in organic material have been buried for a sufficiently long time for petroleum to have formed. The petroleum must also have had an opportunity to migrate into porous traps that are capable of holding large amounts of fluid. The occurrence of crude oil in Earth’s crust is limited both by these conditions, which must be met simultaneously, and by the time span of tens of millions to a hundred million years required for the oil’s formation.

Petroleum geologists and geophysicists have many tools at their disposal to assist in identifying potential areas for drilling. Thus, surface mapping of outcrops of sedimentary beds makes possible the interpretation of subsurface features, which can then be supplemented with information obtained by drilling into the crust and retrieving cores or samples of the rock layers encountered. In addition, increasingly sophisticated seismic techniques—the reflection and refraction of sound waves propagated through Earth—reveal details of the structure and interrelationship of various layers in the subsurface. Ultimately, however, the only way to prove that oil is present in the subsurface is to drill a well. In fact, most of the oil provinces in the world have initially been identified by the presence of surface seeps, and most of the actual reservoirs have been discovered by so-called wildcatters who relied perhaps as much on intuition as on science. (The term wildcatter comes from West Texas, where in the early 1920s drilling crews encountered many wildcats as they cleared locations for exploratory wells. Shot wildcats were hung on the oil derricks, and the wells became known as wildcat wells.)

An oil field, once found, may comprise more than one reservoir—that is, more than one single, continuous, bounded accumulation of oil. Several reservoirs may be stacked one above the other, isolated by intervening shales and impervious rock strata. Such reservoirs may vary in size from a few tens of hectares to tens of square kilometers, and from a few meters in thickness to several hundred or more. Most of the oil that has been discovered and exploited in the world has been found in a relatively few large reservoirs. In the United States, for example, 60 of approximately 10,000 oil fields have accounted for half of the productive capacity and reserves.




Most oil wells in the United States are drilled by the rotary method that was first described in a British patent in 1844 assigned to R. Beart. In rotary drilling, the drill string, a series of connected pipes, is supported by a derrick. The string is rotated by being coupled to the rotating table on the derrick floor. The drill bit at the end of the string is generally designed with three cone-shaped wheels tipped with hardened teeth. Drill cuttings are lifted continually to the surface by a circulating-fluid system driven by a pump.

Trapped crude oil is under pressure; were it not trapped by impermeable rock it would have continued to migrate upward, because of the pressure differential caused by its buoyancy, until it escaped at the surface of Earth. When a well bore is drilled into this pressured accumulation of oil, the oil expands into the low-pressure sink created by the well bore in communication with Earth’s surface. As the well fills up with fluid, however, a back pressure is exerted on the reservoir, and the flow of additional fluid into the well bore would soon stop, were no other conditions involved. Most crude oils, however, contain a significant amount of natural gas in solution, and this gas is kept in solution by the high pressure in the reservoir. The gas comes out of solution when the low pressure in the well bore is encountered, and the gas, once liberated, immediately begins to expand. This expansion, together with the dilution of the column of oil by the less dense gas, results in the propulsion of oil up to Earth’s surface.

Nevertheless, as fluid withdrawal continues from the reservoir, the pressure within the reservoir gradually decreases, and the amount of gas in solution decreases. As a result, the flow rate of fluid into the well bore decreases, and less gas is liberated. The fluid may not reach the surface, so a pump (artificial lift) must be installed in the well bore to continue producing the crude oil.

Eventually, the flow rate of the crude oil becomes so small, and the cost of lifting the oil to the surface becomes so great, that the well costs more to operate than the revenues that can be gained from selling the crude oil (after discounting the price for operating costs, taxes, insurance, and return on capital). The well’s economic limit has then been reached and it is abandoned.



In primary production, no extraneous energy is added to the reservoir other than that required for lifting fluids from the producing wells. Most reservoirs are developed by numerous wells; and as primary production approaches its economic limit, perhaps only a few percent and no more than about 25 percent of the crude oil has been withdrawn from a given reservoir.

The oil industry has developed methods for supplementing the production of crude oil that can be obtained mostly by taking advantage of the natural reservoir energy. These supplementary methods, collectively known as enhanced oil recovery technology, can increase the recovery of crude oil, but only at the additional cost of supplying extraneous energy to the reservoir. In this way, the recovery of crude oil has been increased to an overall average of 33 percent of the original oil. Two successful supplementary methods are in use at this time: water injection and steam injection.


Water Injection

In a completely developed oil field, the wells may be drilled anywhere from 60 to 600 m (200 to 2,000 ft) from one another, depending on the nature of the reservoir. If water is pumped into alternate wells in such a field, the pressure in the reservoir as a whole can be maintained or even increased. In this way the rate of production of the crude oil also can be increased; in addition, the water physically displaces the oil, thus increasing the recovery efficiency. In some reservoirs with a high degree of uniformity and little clay content, water flooding may increase the recovery efficiency to as much as 60 percent or more of the original oil in place. Water flooding was first introduced in the Pennsylvania oil fields, more or less accidentally, in the late 19th century, and it has since spread throughout the world.


Steam Injection

Steam injection is used in reservoirs that contain very viscous oils, those that are thick and flow slowly. The steam not only provides a source of energy to displace the oil, it also causes a marked reduction in viscosity (by raising the temperature of the reservoir), so that the crude oil flows faster under any given pressure differential. This scheme has been used extensively in the states of California, in the United States, and of Zulia, in Venezuela, where large reservoirs exist that contain viscous oil. Experiments are also under way to attempt to prove the usefulness of this technology in recovering the vast accumulations of viscous crude oil (bitumens) along the Athabasca River in north central Alberta, Canada, and along the Orinoco River in eastern Venezuela.



Another method to increase oil-field production has been the construction and operation of offshore drilling rigs. The drilling rigs are installed, operated, and serviced on an offshore platform in water up to a depth of several hundred meters; the platform may either float or sit on legs planted on the ocean floor, where it is capable of resisting waves, wind, and—in Arctic regions—ice floes.

As in traditional rigs, the derrick is basically a device for suspending and rotating the drill pipe, to the end of which is attached the drill bit. Additional lengths of drill pipe are added to the drill string as the bit penetrates farther and farther into Earth’s crust. The force required for cutting into the earth comes from the weight of the drill pipe itself. To facilitate the removal of the cuttings, mud is constantly circulated down through the drill pipe, out through nozzles in the drill bit, and then up to the surface through the space between the drill pipe and the bore through the earth (the diameter of the bit is somewhat greater than that of the pipe). Successful bore holes have been drilled right on target, in this way, to depths of more than 6.4 km (more than 4 mi) from the surface of the ocean. Offshore drilling has resulted in the development of a significant additional reserve of petroleum—in the United States, about 5 percent of the total reserves.











Once oil has been produced from an oil field, it is treated with chemicals and heat to remove water and solids, and the natural gas is separated. The oil is then stored in a tank, or battery of tanks, and later transported to a refinery by truck, railroad tank car, barge, or pipeline. Large oil fields all have direct outlets to major, common-carrier pipelines.



























Basic Distillation

The basic refining tool is the distillation unit. In the United States after the Civil War (1861-1865), more than 100 still refineries were already in operation. Crude oil begins to vaporize at a temperature somewhat less than that required to boil water. Hydrocarbons with the lowest molecular weight vaporize at the lowest temperatures, whereas successively higher temperatures are required to distill larger molecules. The first material to be distilled from crude oil is the gasoline fraction, followed in turn by naphtha and then by kerosene. The residue in the kettle, in the old still refineries, was then treated with caustic and sulfuric acid, and finally steam distilled thereafter. Lubricants and distillate fuel oils were obtained from the upper regions and waxes and asphalt from the lower regions of the distillation apparatus.

In the later 19th century the gasoline and naphtha fractions were actually considered a nuisance because little need for them existed, and the demand for kerosene also began to decline because of the growing production of electricity and the use of electric lights. With the introduction of the automobile, however, the demand for gasoline suddenly burgeoned, and the need for greater supplies of crude oil increased accordingly.


Thermal Cracking

In an effort to increase the yield from distillation, the thermal cracking process was developed. In this process, the heavier portions of the crude oil were heated under pressure and at higher temperatures. This resulted in the large hydrocarbon molecules being split into smaller ones, so that the yield of gasoline from a barrel of crude oil was increased. The efficiency of the process was limited, however, because at the high temperatures and pressures that were used, a large amount of coke was deposited in the reactors. This in turn required the use of still higher temperatures and pressures to crack the crude oil. A coking process was then invented in which fluids were recirculated; the process ran for a much longer time, with far less buildup of coke. Many refiners quickly adopted the process of thermal cracking.


Alkylation and Catalytic Cracking

Two additional basic processes, alkylation and catalytic cracking, were introduced in the 1930s and further increased the gasoline yield from a barrel of crude oil. In alkylation small molecules produced by thermal cracking are recombined in the presence of a catalyst. This produces branched molecules in the gasoline boiling range that have superior properties—for example, higher antiknock ratings—as a fuel for high-powered engines such as those used in today’s commercial planes.

In the catalytic-cracking process, the crude oil is cracked in the presence of a finely divided catalyst. This permits the refiner to produce many diverse hydrocarbons that can then be recombined by alkylation, isomerization, and catalytic reforming to produce high antiknock engine fuels and specialty chemicals. The production of these chemicals has given birth to the gigantic petrochemical industry, which turns out alcohols, detergents, synthetic rubber, glycerin, fertilizers, sulfur, solvents, and the feedstocks for the manufacture of drugs, nylon, plastics, paints, polyesters, food additives and supplements, explosives, dyes, and insulating materials. The petrochemical industry uses about 5 percent of the total supply of oil and gas in the United States.


Product Percentages

In 1920 a U.S. barrel of crude oil, containing 42 gallons, yielded 11 gallons of gasoline, 5.3 gallons of kerosene, 20.4 gallons of gas oil and distillates, and 5.3 gallons of heavier distillates. In recent years, by contrast, the yield of crude oil has increased to almost 21 gallons of gasoline, 3 gallons of jet fuel, 9 gallons of gas oil and distillates, and somewhat less than 4 gallons of lubricants and 3 gallons of heavier residues.



The disciplines employed by exploration and petroleum engineers are drawn from virtually every field of science and engineering. Thus the exploration staffs include geologists who specialize in surface mapping in order to try to reconstruct the subsurface configuration of the various sedimentary strata that will afford clues to the presence of petroleum traps. Subsurface specialists then study drill cuttings and interpret data on the subsurface formations that is relayed to surface recorders from electrical, sonic, and nuclear logging devices lowered into the bore hole on a wire line. Seismologists interpret sophisticated signals returning to the surface from sound waves that are propagated through Earth’s crust. Geochemists study the transformation of organic matter and the means for detecting and predicting the occurrence of such matter in subsurface strata. In addition, physicists, chemists, biologists, and mathematicians all support the basic research and development of sophisticated exploration techniques.

Petroleum engineers are responsible for the development of discovered oil accumulations. They usually specialize in one of the important categories of production operation: drilling and surface facilities, petrophysical and geological analysis of the reservoir, reserve estimation and specification of optimal development practices, or production control and surveillance. Although many of these specialists have formal training as petroleum engineers, many others are drawn from the ranks of chemical, mechanical, electrical, and civil engineers; physicists, chemists, and mathematicians; and geologists.

The drilling engineer specifies and supervises the actual program by which a well will be bored into the Earth, the kind of drilling mud to be used, the way in which the steel casing that isolates the productive strata from all other subsurface strata will be cemented, and how the productive strata will be exposed to the well bore. The facilities-engineering specialists specify and design the surface equipment that must be installed to support the production operation, the well-head pumps, the field measurement and collection of produced fluids and gas separation systems, the storage tankage, the dehydration system for removing water from the produced oil, and the facilities for enhanced recovery programs.

The petrophysical and geological engineer, after interpreting the data supplied by analysis of cores and by various logging devices, develops a description of the reservoir rock and its permeability, porosity, and continuity. The reservoir engineer then develops the plan for the number and location of the wells to be drilled into the reservoir, the rates of production that can be sustained for optimum recovery, and the need for supplementary recovery technology. The reservoir engineer also estimates the productivity and ultimate recovery (reserves) that can be achieved from the reservoir, in terms of time, operating costs, and value of the crude oil produced.

Finally, the production engineer monitors the performance of the wells. The engineer recommends and implements remedial tasks such as fracturing, acidizing, deepening, adjusting gas to oil and water to oil ratios, and any other measures that will improve the economic performance of the reservoir.



Crude oil is perhaps the most useful and versatile raw material that has become available for exploitation. By 2003, the United States was using 7 billion barrels of petroleum per year, and worldwide consumption of petroleum was 29.3 billion barrels per year.



The world’s technically recoverable reserves of crude oil—the amount of oil that experts are certain of being able to extract without regard to cost from Earth—add up to about 1,000 billion barrels, of which some 73 billion barrels are in North America. However, only a small fraction of this can be extracted at current prices. Of the known oil reserves that can be profitably extracted at current prices, more than half are in the Middle East; only a small fraction are in North America.



It is likely that some additional discoveries will be made of new reserves in coming years, and new technologies will be developed that permit the recovery efficiency from already known resources to be increased. The supply of crude oil will at any rate extend into the early decades of the 21st century. Virtually no expectation exists among experts, however, that discoveries and inventions will extend the availability of cheap crude oil much beyond that period. For example, the Prudhoe Bay field on the North Slope of Alaska is the largest field ever discovered in the Western Hemisphere. The ultimate recovery of crude oil from this field is anticipated to be about 10 billion barrels, which is sufficient to supply the current needs of the United States for less than two years, but only one such field was discovered in the West in more than a century of exploration. Furthermore, drilling activity has not halted the steady decline of North American crude oil reserves that began during the 1970s.



In light of the reserves available and the dismal projections, it is apparent that alternative energy sources will be required to sustain the civilized societies of the world in the future. The options are indeed few, however, when the massive energy requirements of the industrial world come to be appreciated. Commercial oil shale recovery and the production of a synthetic crude oil have yet to be demonstrated successfully, and serious questions exist as to the competitiveness of production costs and production volumes that can be achieved by these potential new sources.

Although alternative energy sources, such as geothermal energy, solar energy, and nuclear energy, hold much promise, none has proved an economically viable replacement for petroleum products.





Adding to the urgency of finding alternatives to petroleum and other fossil fuels is the problem of global warming. Petroleum combustion releases carbon dioxide, a greenhouse gas, into the atmosphere, and most atmospheric scientists believe that rising levels of greenhouse gases are driving climate change. These changes could cause numerous environmental problems, including disrupted weather patterns and polar ice cap melting. Disrupted weather patterns could lead to extensive drought and desertification. Polar ice cap melting could cause flooding and profound changes in ocean circulation. Many environmental organizations are urging governments and individuals to reduce greenhouse gas emissions by conserving energy with fuel-efficient technologies and other measures. In the United States most environmental groups have urged the U.S. government to ratify the Kyōto Protocol, a global treaty that sets a specific timetable for reducing greenhouse gas emissions. See also Ocean and Oceanography.

Drilling oil wells also creates environmental problems because the petroleum pumped up from deep reservoir rocks is often accompanied by large volumes of salt water. This brine contains numerous impurities, so it must either be injected back into the reservoir rocks or treated for safe surface disposal.

Petroleum usually must also be transported long distances by tanker or pipeline to reach a refinery. Transport of petroleum occasionally leads to accidental spills. Oil spills, especially in large volumes, can be detrimental to wildlife and habitat.

Contributed By:
Todd M. Doscher

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

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