| Petroleum
(from Latin petra – rock and oleum – oil) or crude oil (also known as black gold)
is a black, dark brown or greenish liquid found in formations in the earth. The
American Petroleum Institute, in its Manual of Petroleum Measurement Standards
(MPMS), defines it 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."
Petroleum is found
in porous rock formations in the upper strata of some areas of the Earth's crust.
It consists of a complex mixture of hydrocarbons, mostly alkanes, but may vary
greatly in appearance and composition. Petroleum is used mostly, by volume, for
producing fuel oil and petrol (gasoline), both important "primary energy" sources
(IEA Key World Energy Statistics). Petroleum is also the raw material for many
chemical products, including solvents, fertilizers, pesticides, and plastics.
84% (37 of 42 gallons in a typical barrel) of all petroleum extracted is processed
as fuels, including gasoline, diesel, jet, heating, and other fuel oils, and liquefied
petroleum gas [1]; the other 16% is converted into other materials such as plastic.
Known reserves of
petroleum are typically estimated at around 1.2×10^12 barrels[2] with at least
one estimate as high as 3.74×10^12 barrels (3,740,000,000,000)[3]. Consumption
is currently around 84×106 barrels per day, or 31×10^9 barrels per year. Because
of pumping difficulties, usable oil reserves are only about 1/3 of total reserves.
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, photovoltaic, or clean-burning hydrogen. Petroleum
forms naturally within the earth too slowly to be sustainable for human use. Formation
Chemistry
Octane,
a hydrocarbon
found in petroleum, lines are single bonds, black spheres are carbon, white spheres
are hydrogen 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.
The general formula for these hydrocarbons is CnH2n+2.
For example 2,2,4-Trimethylpentane, widely used in gasoline, has a chemical formula
of C8H18 which reacts with oxygen exothermically. C8H18(aq)
+ 12.5O2(g) → 8CO2(g) + 9H2O(g)
+ heat Incomplete
combustion of petroleum or gasoline results in emission of poisionous gases such
as carbon
monoxide and/or nitric oxide. For example: C8H18(aq)
+ 12.5O2(g) + N2(g) → 6CO2(g)
+ 2CO(g) + 2NO(g) + 9H2O(g) + heat Formation
of petroleum occurs in a variety of mostly endothermic reactions in high temperature
and/or pressure. For example, a kerogen may break down into hydrocarbons of different
lengths:[5] CH1.45(s)
+ heat → .663CH1.6(aq) + .076CH2(aq) + .04CH2.6(g)
+ .006CH4(g) + .012CH2.6(s) + .018CH4.0(s)
+ .185CH.25(s) Biogenic
theory 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. (Terrestrial
plants tend to form coal) 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.
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. (In the case of the Athabasca Oil Sands, it is
found right at the surface.) Three conditions must be present for oil reservoirs
to form: first, a source rock rich in organic material buried deep enough for
subterranean heat to cook it into oil; second, a porous and permeable reservoir
rock for it to accumulate in; and last a cap rock (seal) that prevents it from
escaping to the surface. 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.
The reactions that
produce oil and natural gas are often modeled as first order breakdown reactions,
where kerogen is broken down to oil and natural gas by a set of parallel reactions,
and oil eventually breaks down to natural gas by another set of reactions. The
first set was originally patented in 1694 under British Crown Patent No. 330 covering
"a way to extract and make great quantityes of pitch, tarr, and oyle out of a
sort of stone." The latter set is regularly used in petrochemical plants and oil
refineries. Means
of production Extraction
The most
common method of obtaining petroleum is extracting it from oil wells found in
oil fields. After the well has been located, various methods are used to recover
the petroleum. Primary recovery methods are used to extract oil that is brought
to the surface by underground pressure, and can generally recover about 20% of
the oil present. After the oil pressure has depleated to the point that the oil
is no longer brought to the surface, secondary recovery methods draw another 5
to 10% of the oil in the well to the surface. Finally, when secondary oil recovery
methods are no longer viable, tertiary recovery methods reduce the viscosity of
the oil in order to bring more to the surface. Alternative
methods As oil
prices continue to escalate, other alternatives to producing oil have been gaining
importance. The best known such methods involve extracting oil from sources such
as oil shale or tar sands. These resources are known to exist in large quantities;
however, extracting the oil at low cost without negatively impacting the environment
remains a challenge. It is also possible to transform natural gas or coal into
oil (or, more precisely, the various hydrocarbons found in oil). The best-known
such method is the Fischer-Tropsch process. It was a concept pioneered in Nazi
Germany when imports of petroleum were restricted due to war and Germany found
a method to extract oil from coal. It was known as Ersatz ("substitute" in German),
and accounted for nearly half the total oil used in WWII by Germany. However,
the process was used only as a last resort as naturally occurring oil was much
cheaper. As crude oil prices increase, the cost of coal to oil conversion becomes
comparatively cheaper. The method involves converting high ash coal into synthetic
oil in a multistage process. Ideally, a ton of coal produces nearly 200 liters
(1.25 bbl, 52 US gallons) of crude, with by-products ranging from tar to rare
chemicals. |
