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.
See also Asphalt; Naphtha.
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.
| IV |
|
HISTORICAL
DEVELOPMENT |
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.
| VII |
|
ENHANCED OIL
RECOVERY |
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.
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 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.
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.
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.
| C |
|
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.
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.
| XI |
|
PRODUCTION VOLUMES AND
RESERVES |
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.
| XII |
|
ENVIRONMENTAL EFFECTS OF USING
PETROLEUM |
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.
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