comes out of an oil field?
Crude oil. (Texas tea, Black gold)
Gas (Natural gas, Methane).
Condensate and natural gas liquids (only sometimes found in an
Crude oil comes in a variety of colours and thicknesses.
be black, it may be brown, it may be greenish. It may be
thin, it may be relatively thick.
The 'thickness', or density, of crude is measured against a
developed by the American Petroleum Institute. The denser and
the oil, the lower on the 'API' scale it is.
Regardless of what it looks like, the raw crude oil as it is
of the ground is not much
use. To be able to
use crude oil, the different components that make it up need to
seperated out. Crude oil run through various processes in oil
refineries will seperate out various proportions of the
that particular crude. But when crude is refined, it yields
of the various component parts ('fractions'), depending on how
crude was formed, and what kind of organic material it
Crude from one field may have a different 'yeild profile' to
a field somewhere else.
Several thousand different chemicals can be identified in the
The huge pressure and high temperature in the deep underground
reservoir means that some chemical compounds which would be
normal temperatures and pressures above the ground remain liquid
the ground, and form a part of the liquid crude. When the crude
pumped up from the depths, the pressure is released and
drops. The liquified gases in the crude are then released.
normally methane, and is not usually associated with any other
compounds which liquify at normal above-ground temperature and
pressure. The gas usually associated with oil wells is therefore
'dry gas'. Historically, the gas that bubbles out as the crude
surface has simply been diverted and burnt ('flared off') from
of a tall pipe at the well head.
Depending on the field, some crude oils are naturally 'runny'
light. They are easy to refine, and are highly sought after.
lower density oils.
Crude oil with very little sulfur in it. Excess sulfur has to be
removed from crude at the refinery, a process that costs extra
Others are very thick, viscous, and heavy. Heavier oils are
relatively close to the surface. Any lighter more volatile
that might have been formed have vaporised and found their way
surface and disappeared. They are made up of large molecules
hexadecane (16 carbon & 34 hydrogen atoms, or C16
and octadecane (18 carbon & 38 hydrogen atoms or C18
They have to be refined in
specialised refineries especially built to handle them. These
molecules are split or 'cracked' into smaller molecules.
in effect fuel oil - can be cracked down to a mix of octane,
and a little ethylene. Octane and hexane are components of
This is done by heating.
Some crudes are naturally high in sulphur. If there is more than
sulfur present, they are called 'sour'
Natural gas liquids
What comes out of a
Gas (Natural gas, Methane).
Condensate and natural gas liquids (frequently associated with a
The pressure and high temperatures in the deep underground gas
reservoir means that some low boiling point hydrocarbon
compounds (which would be liquids at normal temperatures and
pressures above the ground) become gases under the ground.
These 'gasified liquids' form a part of the flow of gas when it
is piped up from the reservoir. When the gas is flows up from
the depths, the pressure is released and temperature drops. The
'gasified liquids' in the cooling gas stream then condense (just
as steam condenses back to water as it cools). These liquid
condensates and natural gas liquids are quite usual in gas
fields. The gas from gas fields is therefore usually 'wet gas'.
Natural gas liquids
Natural gas, as sold to the consumer, is methane. Gas wells (and
the gas on top of oil wells) contain 5% to 20% of gases and
'gasified liquids' that are not
These 'other' hydrocarbons in the natural gas (methane) stream
that can be relatively easily turned
a liquid with application of moderate pressure or
temperatures and pressures (known as natural gas condensate, or
The 'normally liquid' portion of the 'natural gas liquids' can
be separated from the gas stream either at the oil or gas fields
adjacent facilities, or elsewhere at a specialist natural gas
The term 'Natural gas liquids' includes both
'condensed' gaseous liquids captured at specialised natural gas
the 'normally liquid' lease condensate, often removed right at
are usually made up of both:
(i) lighter hydrocarbons, predominatly ethane, and propane, and
(ii) heavier hydrocarbons, such as pentane and heavier.
(1) The smaller molecules such as ethane, ethylene, propane,
butane, butylene, isobutane, and isobutylene can be retrieved
from the natural gas stream and converted into liquids at the
specialist natural gas processing plants by methods such as
freezing and pressurizing. For example, the second smallest
molecule, propane, can be turned into a liquid at -42o
These now liquified gases may be mixed together and taken to a
specialist plant for fractionation, where the individual
products are split out and sold separately. Ethane is an
important feedstock in the chemical industry, making, amongst
other things, ethylene. Propane is used in home heating and
cooking. Butane is used as a gasoline additive as an oxygenate
to reduce pollution.
If not sold separately, the fractionates can be mixed together
to form 'liquified
petroleum gases', or LPG. This blend has to remain
pressurized to be
liquid. This gas can be held in relatively thin walled bottles,
so is sold worldwide for both domestic cooking as 'bottled gas'
and as a transport fuel.
Some heavier hydrocarbons with more hydrogen atoms, such as
hexane and heptane, can also be recovered from the
fractionisation of the gas stream. They can be used to 'make'
gasoline, but have to be blended with other liquid hydrocarbons
from distillation of crude oil in order to be useful. These
heavier gaseous natural gas liquids cannot be counted as 'oil
equivalent' by themselves - they depend on crude oil to become
(2) The heavier hydrocarbons that
are liquid at normal temperatures
are often called
'lease condensate', 'natural gas condensate', 'natural
gasoline', or 'casinghead gasoline'.
A typical 'natural gas liquids' breakdown would be about 83%
gaseous liquids, and 17% natural gasoline (it varies, and can be
as much as 22%).
Condensate refers to a specific portion
of the 'Natural Gas
Liquids'. It is more usually referred to as 'lease condensate'.
This is a mixture of those heavier hydrocarbons that
condense out to a liquid at normal pressure and temperature
which is recovered from the natural gas in the lease operators
separation facilities near by the gas field.
lighter gaseous molecules in the gas stream, such as propane and
butane. As mentioned, these are recovered at specialist natural
gas processing plants (see below).
LNG - Liquified Natural Gas
Natural gas (methane) can be transported in its normal gaseous
state inside a pipeline. But transcontinental gas pipelines are
very expensive. As a result there is increasing demand for
natural gas that has been compressed and cooled (to minus 160
degrees celsius) until it is liquid - at which point it is
around 1/600th of its original volume - then transported by ship
to a facility that can turn the liquid back into a gas in a
controlled manner. The gas can then be distributed via pipelines
in the usual way.
What comes out
of a refinery?
the crude oil that went in.
Crude is heated through a defined temperature range that causes
liquids in the desired fraction to boil off. For 'straight run'
gasoline, for example, as the crude is heated at the bottom of a
'still' the low boiling point liquids that make up gasoline all
vaporise. These 'gasoline vapors' reach the top of the still
temperatures are not more than 200o
C, and the vapors
low boiling point fraction of the crude are drawn off and
It is hotter lower down the column, and higher boiling point
are first vaporised, then drawn off and condensed at this lower
These tall column stills are known as 'fractionating towers'.
The distilled liquid fractions may then be further refined by
impurities with various chemicals.
The less desirable higher boiling point liquids that make up
are usually further broken down using catalysts to more
suitable for gasoline and for making aviation gas. The gasoline
from this process are blended with 'straight run' gasoline. A
of other chemical techniques, such as isomerisation and
dehydrogenation, are used to improve some refinery liquids for
The light gas oils, or fuel oils, can either be further split
manufacture of more gasoline, or retained and refined for when
for furnace oil is high (winter), or when extra quantities of
fuel are needed. Refineries have some flexibility in changing
product mix between more or less proportions of gasoline versus
oils, but it can't be done instantly.
The type of irreducible residues that remain from distillation
on the makeup of the crude. It may be tars, or 'asphalt'
Some crudes - heavy crudes - have no appreciable gasoline
They have to be subjected to heating cycles to crack down into
hydrocarbon liquids to be able to make gasoline. This requires
heating than light crudes. Heavy crudes therefore cost more to
As a result, heavy crudes are less expensive to buy as the cost
dealing with them is significantly higher and there is less
them. In addition, they require specially configured refinery
processes requiring more capital to build. Once built,
handling heavy crudes make more gross profit (at least) than
handling light crude - but only so long as heavu crudes sell
As more refineries are converted to handle heavy crudes, demand
up, and the price advatage reduce.
Heavy crude takes more energy to process than light crude.
there is roughly 14%-18% more energy needed to refine a barrel
crude than to refine a barrel of light crude.
The gasoline-burning national car fleet of the USA means that
refineries try to maximise
gasoline production, whereas the increasingly diesel-powered
car fleet means European refineries try to maximise light gas
diesel. European refineries typically break a barrel of crude
into about 25% gasoline, 50%
light gas oil/diesel where USA refineries typically break a
crude down into around 50% gasoline and 25%
Crude oil (UK) or Petroleum (USA) is heated to extract the
with the lowest boiling point. The smaller molecule hydrocarbons
are usually a liquid at ambient temperatures are (smallest
largest) pentane, hexane, heptane,octane, decane, and dodecane.
is a liquid until the temperature hits 36o
C, when it
volatises into a gas. Dodecane is a liquid until temperatures
C. The other liquids turn into a gas at
temperatures between these ranges. When these six liquid
are put into a mixture together, the mixture is called
of the lighter liquids are chemically 'reformed' to make them
suitable as a car fuel.
The heptane component is a straight chain hydrocarbon molecule
to combust very quickly in high compression engines, causing
pre-ignition or 'knocking'. A chemically 're-formed'
form of octane (C8
'iso octane', is considered the ideal fuel, as it combusts at a
pace, giving better compression. Iso-octane is given a nominal
of 100, as the perfectly combusting fuel for modern high
car engines. Heptane, on the other hand, is given an 'octane
Blends of gasoline are measured against a standard comprised of
defined proportionate blend of iso-octane and heptane (the
Octane'). The closer to
100 the 'octane rating'* of the gasoline blend is, the better
explosion profile in the engine and the less tendency to
Many modern engines have tended to reduce the importance of the
rating as electronics automatically adjust the timing and
for optimal combustion regardless of the octane rating (to a
A barrel of crude ultimately yields about 45% gasoline product.
* Most of the world uses the 'Research
Octane Number' (RON) to measure octane. The USA uses an
average of RON
plus the 'Motor
Octane Number'. Thus a 91octane fuel is the equivalent of 87
The two heaviest and least volatile components of gasoline,
dodecane, when mixed together are known as 'kerosene'. A
modified blend of kerosine (avgas) is used in jet engines.
A barrel of crude ultimately yields about 4.5% kerosine.
Fuel oils or light gas oil
The liquids with the largest molecules in a crude are hexadecane
octadecane. Heavy crudes have little else. These molecules are
up and further split apart (with the aid of a catalyst) to make
usable. Hexadecane (C16
for example, can be split into various proportions of octane (C8
) and a small amount of ethylene (C2
octane and hexane liquids are used as components of gasoline.
Light gas oil is also further refined into grades for home
highly refined grades for diesel.
A barrel of crude ultimately yields about 36.5% fuel oil.
The fraction of the crude that has very many carbon atoms is
A barrel of crude ultimately yields about 2% lubricating oils.
The fraction of the crude that has even more carbon atoms is
semi-solid grease. High paraffin crudes are best for grease
The heaviest molecules in the crude are solids at normal
Paraffin wax is used for candles.
Averaged, a barrel of crude ultimately yields about 11.5%
tars, ethylene and other miscellaneous products.
The cultural divide
In America, the fuel we put in our cars tank is 'gasoline',
shortened to 'gas'. The tank is called a 'gas tank'.
In the UK and former colonies, the fuel we put in our cars tank
'petroleum', always shortened to 'petrol'. The tank is called a
American term 'gas; has infiltrated the language and is now used
interchangably for 'petrol', and almost as frequently).
In USA, the oil that comes out of the ground is called 'crude',
might be called 'petroleum'. (Petroleum is literally correct, as
comes from the latin 'petra
No-one in USA would think of putting 'petroleum' in their car,
it would be tantamount to filling up with crude oil!
In the UK, many cars have been converted to dual fuel - they can
gasoline or on compressed natural gas. If a driver needs 'gas',
may need gasoline ('petrol') or they may need more natural
In the UK, the black liquid that comes out of the ground is
called 'crude', or simply 'oil'. Outside the oil industry, crude
called petroleum in the UK. The idea that you could get
petroleum - the
run your car on - straight out of the ground would seem like a
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