What
comes out of an oil field?
Major
Crude oil. (Texas tea, Black gold)
Minor
Gas (Natural gas, Methane).
Condensate and natural gas liquids (only sometimes found in an
oil
field)
Crude Oil
Crude oil comes in a variety of colours and thicknesses.
It may
be black, it may be brown, it may be greenish. It may be
relatively
thin, it may be relatively thick.
The 'thickness', or density, of crude is measured against a
scale
developed by the American Petroleum Institute. The denser and
heavier
the oil, the lower on the 'API' scale it is.
Regardless of what it looks like, the raw crude oil as it is
pumped out
of the ground is not much
use. To be able to
use crude oil, the different components that make it up need to
be
seperated out. Crude oil run through various processes in oil
refineries will seperate out various proportions of the
constituents of
that particular crude. But when crude is refined, it yields
differing
amounts
of the various component parts ('fractions'), depending on how
the
crude was formed, and what kind of organic material it
originated from.
Crude from one field may have a different 'yeild profile' to
crude from
a field somewhere else.
Several thousand different chemicals can be identified in the
various
crude oils.
The huge pressure and high temperature in the deep underground
oil
reservoir means that some chemical compounds which would be
gases at
normal temperatures and pressures above the ground remain liquid
under
the ground, and form a part of the liquid crude. When the crude
is
pumped up from the depths, the pressure is released and
temperature
drops. The liquified gases in the crude are then released.
This is
normally methane, and is not usually associated with any other
volatile
compounds which liquify at normal above-ground temperature and
pressure. The gas usually associated with oil wells is therefore
called
'dry gas'. Historically, the gas that bubbles out as the crude
comes to
the
surface has simply been diverted and burnt ('flared off') from
the top
of a tall pipe at the well head.
Light
crude
Depending on the field, some crude oils are naturally 'runny'
and
light. They are easy to refine, and are highly sought after.
These are
lower density oils.
Sweet
crude
Crude oil with very little sulfur in it. Excess sulfur has to be
removed from crude at the refinery, a process that costs extra
money.
Heavy
crude
Others are very thick, viscous, and heavy. Heavier oils are
often found
relatively close to the surface. Any lighter more volatile
components
that might have been formed have vaporised and found their way
to the
surface and disappeared. They are made up of large molecules
such as
hexadecane (16 carbon & 34 hydrogen atoms, or C
16H
34)
and octadecane (18 carbon & 38 hydrogen atoms or C
18H
38)
They have to be refined in
specialised refineries especially built to handle them. These
large
molecules are split or 'cracked' into smaller molecules.
Hexadecane -
in effect fuel oil - can be cracked down to a mix of octane,
hexane,
and a little ethylene. Octane and hexane are components of
gasoline.
This is done by heating.
Sour crude
Some crudes are naturally high in sulphur. If there is more than
2.5%
sulfur present, they are called 'sour'
crudes.
Condensate
See below
Natural gas liquids
See below
What comes out of a
gas field?
Major
Gas (Natural gas, Methane).
Minor
Condensate and natural gas liquids (frequently associated with a
gas
field)
Gas
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
methane.
These 'other' hydrocarbons in the natural gas (methane) stream
are
either-
(1)
gaseous hydrocarbons
that can be relatively easily
turned
into a liquid with application of moderate pressure or
freezing,
or :
(2)
liquid at normal
temperatures and pressures (known as natural gas condensate, or
'natural gasoline'),
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
processing plant.
The term 'Natural gas liquids' includes
both the
'condensed' gaseous liquids captured at specialised natural gas
plants,
and
the 'normally liquid' lease condensate, often removed right at
the wellhead.
The
gaseous liquids
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 -42
oC.
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
useful.
(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
Condensate refers to
a specific portion of the 'Natural Gas
Liquids'. It is more usually referred to as 'lease condensate'.
'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.
It
excludes the
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?
Break-downs
of
the crude oil that went in.
Crude is heated through a defined temperature range that causes
the
liquids in the desired fraction to boil off. For 'straight run'
gasoline, for example, as the crude is heated at the bottom of a
tall
'still' the low boiling point liquids that make up gasoline all
vaporise. These 'gasoline vapors' reach the top of the still
where
temperatures are not more than 200
oC, and the vapors
of the
low boiling point fraction of the crude are drawn off and
condensed .
It is hotter lower down the column, and higher boiling point
liquids
are first vaporised, then drawn off and condensed at this lower
point.
These tall column stills are known as 'fractionating towers'.
The distilled liquid fractions may then be further refined by
removing
impurities with various chemicals.
The less desirable higher boiling point liquids that make up
'kerosene'
are usually further broken down using catalysts to more
desirable
liquids
suitable for gasoline and for making aviation gas. The gasoline
liquids
from this process are blended with 'straight run' gasoline. A
variety
of other chemical techniques, such as isomerisation and
dehydrogenation, are used to improve some refinery liquids for
use in
gasoline.
The light gas oils, or fuel oils, can either be further split
for
manufacture of more gasoline, or retained and refined for when
demand
for furnace oil is high (winter), or when extra quantities of
deisel
fuel are needed. Refineries have some flexibility in changing
the
product mix between more or less proportions of gasoline versus
fuel
oils, but it can't be done instantly.
The type of irreducible residues that remain from distillation
depends
on the makeup of the crude. It may be tars, or 'asphalt'
Some crudes - heavy crudes - have no appreciable gasoline
components.
They have to be subjected to heating cycles to crack down into
useful
hydrocarbon liquids to be able to make gasoline. This requires
more
heating than light crudes. Heavy crudes therefore cost more to
process.
As a result, heavy crudes are less expensive to buy as the cost
of
dealing with them is significantly higher and there is less
profit in
them. In addition, they require specially configured refinery
processes requiring more capital to build. Once built,
refineries
handling heavy crudes make more gross profit (at least) than
refineries
handling light crude - but only so long as heavu crudes sell
cheaper.
As more refineries are converted to handle heavy crudes, demand
will go
up, and the price advatage reduce.
Heavy crude takes more energy to process than light crude.
Overall,
there is roughly 14%-18% more energy needed to refine a barrel
of heavy
crude than to refine a barrel of light crude.
The gasoline-burning national car fleet of the USA means that
USA
refineries try to maximise
gasoline production, whereas the increasingly diesel-powered
European
car fleet means European refineries try to maximise light gas
oil and
diesel. European refineries typically break a barrel of crude
oil down
into about 25% gasoline, 50%
light gas oil/diesel where USA refineries typically break a
barrel of
crude down into around 50% gasoline and 25%
diesel/heating oil.
Gasoline
Crude oil (UK) or Petroleum (USA) is heated to extract the
components
with the lowest boiling point. The smaller molecule hydrocarbons
that
are usually a liquid at ambient temperatures are (smallest
molecules to
largest) pentane, hexane, heptane,octane, decane, and dodecane.
Pentane
is a liquid until the temperature hits 36
oC, when it
volatises into a gas. Dodecane is a liquid until temperatures
hit just
over 215
oC. The other liquids turn into a gas at
temperatures between these ranges. When these six liquid
hydrocarbons
are put into a mixture together, the mixture is called
'gasoline'. Some
of the lighter liquids are chemically 'reformed' to make them
more
suitable as a car fuel.
The heptane component is a straight chain hydrocarbon molecule
tending
to combust very quickly in high compression engines, causing
pre-ignition or 'knocking'. A chemically 're-formed'
branch-chained
form of octane (C
8H
18),
'iso octane', is considered the ideal fuel, as it combusts at a
slower
pace, giving better compression. Iso-octane is given a nominal
rating
of 100, as the perfectly combusting fuel for modern high
compression
car engines. Heptane, on the other hand, is given an 'octane
rating' of
zero.
Blends of gasoline are measured against a standard comprised of
a
defined proportionate blend of iso-octane and heptane (the
'Research
Octane'). The closer to
100 the 'octane rating'* of the gasoline blend is, the better
the
explosion profile in the engine and the less tendency to
knocking.
Many modern engines have tended to reduce the importance of the
octane
rating as electronics automatically adjust the timing and
fuel:air mix
for optimal combustion regardless of the octane rating (to a
point).
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
in USA.
Kerosene
The two heaviest and least volatile components of gasoline,
decane and
dodecane, when mixed together are known as 'kerosene'. A
specially
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
and
octadecane. Heavy crudes have little else. These molecules are
heated
up and further split apart (with the aid of a catalyst) to make
them
usable. Hexadecane (C
16H
34),
for example, can be split into various proportions of octane (C
8H
18),
hexane
(C
6H
12) and a small amount of ethylene (C
2H
4).
The
octane and hexane liquids are used as components of gasoline.
Light gas oil is also further refined into grades for home
heating fuel
oil, and
highly refined grades for diesel.
A barrel of crude ultimately yields about 36.5% fuel oil.
Lubricating oils
The fraction of the crude that has very many carbon atoms is
used as
liquid lubricant.
A barrel of crude ultimately yields about 2% lubricating oils.
Grease
The fraction of the crude that has even more carbon atoms is
used
semi-solid grease. High paraffin crudes are best for grease
production.
Paraffin wax
The heaviest molecules in the crude are solids at normal
temperatures.
Paraffin wax is used for candles.
Averaged, a barrel of crude ultimately yields about 11.5%
grease,
paraffin wax,
tars, ethylene and other miscellaneous products.
The cultural divide
In America, the fuel we put in our cars tank is 'gasoline',
always
shortened to 'gas'. The tank is called a 'gas tank'.
In the UK and former colonies, the fuel we put in our cars tank
is
'petroleum', always shortened to 'petrol'. The tank is called a
'petrol
tank'. (
But the
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',
or it
might be called 'petroleum'. (Petroleum is literally correct, as
it
comes from the latin '
petra',
stone,
and '
oleum', oil).
No-one in USA would think of putting 'petroleum' in their car,
because
it would be tantamount to filling up with crude oil!
In the UK, many cars have been converted to dual fuel - they can
run on
gasoline or on compressed natural gas. If a driver needs 'gas',
they
may need gasoline ('petrol') or they may need more natural
gas.....
In the UK, the black liquid that comes out of the ground is
either
called 'crude', or simply 'oil'. Outside the oil industry, crude
is
never
called petroleum in the UK. The idea that you could get
petroleum - the
stuff you
run your car on - straight out of the ground would seem like a
bizarre
joke.
©
Copyright
2006
Sustainable Living Organisation
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and partly opinion of Sustainable Living
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in this page being current, correct or factual.
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