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The Eakring Oilfield Section
What is Oil
 | What is Oil?
Oil is a generic
term for fluids that are not miscible with water. The name comes from Latin oleum
for olive oil.
 | Crude oil
consists of a mixture of petroleum liquids and gases
(together with associated impurities) pumped out of the ground through
oil wells.
 | Petroleum
(from Latin petrus–rock and oleum–oil) or mineral
oil. It can be shortened to the prefix petro-, as
in "petrodiesel". Petroleum is a thick, dark brown or
greenish inflammable liquid, which, at certain points, exists in the
upper strata of Earth's crust. It consists of a complex mixture of
various hydrocarbons, largely of the methane
series, but may vary much in appearance, composition, and
properties. |
| In chemistry, a hydrocarbon is an
organic compound consisting only of carbon and hydrogen. They all
consist of carbon backbone and atoms of hydrogen attached to that
backbone.
The simplest hydrocarbon, methane
is a gas with a chemical formula of CH4.
Lewis Structure:
H
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H-C-H
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H |
| Strictly speaking, petroleum consists entirely of
aliphatic hydrocarbons, those composed of nothing but hydrogen and
carbon.
The four lightest hydrocarbons -- CH4
(methane), C2H6 (ethane), C3H8
(propane) and C4H10 (butane) -- are all gases,
boiling at -107°C, -67°C, -43°C, and -18°C, respectively
(-161°, -88°, -46°, and -1° degrees F).
The chains in the C5-7 range are all
light, easily vaporized, clear naphthas. They are used as solvents,
dry cleaning fluids, and other quick-drying products. The chains
from C6H14 through to C12H26
are blended together and used for Petrol (gasoline). Kerosene is made up of
chains in the C10 to C15 range, followed by
diesel fuel/heating oil (C10 to C20) and
heavier fuel oils as the ones used in ship engines. These petroleum
compounds are all liquid at room temperature.
Lubricating oils and semi-solid greases (including
Vaseline®) range from C16 up to C20.
Chains above C20 form solids, starting
with paraffin wax, then tar and asphaltic bitumen.
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Boiling ranges of petroleum atmospheric pressure
distillation fractions in degrees Celsius: |
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| petrol ether: 40 - 70 (used as solvent)
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| light petrol: 60 - 100 (automobile fuel)
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| heavy petrol: 100 - 150 (automobile fuel)
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| light kerosene: 120 - 150 (household solvent
and fuel)
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| kerosene: 150 - 300 (jet engine fuel)
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| gas oil: 250 - 350 (Diesel fuel/ heating)
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| lubrication oil: > 300 (engine oil)
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| remaining fractions: tar, asphalt, residual
fuel |
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| Formation
Millions of years ago much of the earth was covered by
seas and swamps. As microscopic animals and plants in these seas died, their
remains were deposited in the sea bed, they mixed with rock materials eroded
from higher ground.
Gradually, sediments many thousands of feet thick, built
up and became compressed to form sedimentary rocks such as sandstone and
limestone.
Because of the lack of oxygen, the small organisms did not
decay but were transformed by heat and high pressure into oil and gas. This
biological material in rocks starts off largely as a waxy material known as kerogen.
As more new rocks formed above, gravity, capillary action
and water pressure forced the oil and gas out of this source rock and
upwards through porous rocks.
If further upward movement was eventually stopped by rock
layer and if a side ways movement was prevented by a fold or fault in the
earth's surface, then oil and gas are trapped in a reservoir. This is not
an underground lake, as the name suggests: oil and gas are held in the pores
between grains the same as water is held in a sponge.
A few scientists, notably Thomas Gold, have
suggested other, abiogenic, theories for the origins of crude oil.
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A Typical Anticline |
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| Exploration and Drilling - The Basics
| Exploration
The task of finding oil is assigned to geologists,
whether employed directly by an oil company or under contract from a
private firm. Their task is to find the right conditions for an oil trap
-- the right source rock, reservoir rock and entrapment. Many years ago,
geologists interpreted surface features, surface rock and soil types,
and perhaps some small core samples obtained by shallow drilling. Modern
oil geologists also examine surface rocks and terrain, with the
additional help of satellite images. However, they also use a variety of
other methods to find oil. They can use sensitive gravity meters
to measure tiny changes in the Earth's gravitational field that could
indicate flowing oil, as well as sensitive magnetometers to
measure tiny changes in the Earth's magnetic field caused by flowing
oil. They can detect the smell of hydrocarbons using sensitive
electronic noses called sniffers. Finally, and most commonly,
they use seismology, creating shock waves that pass through
hidden rock layers and interpreting the waves that are reflected back to
the surface.
Photo
courtesy Institute of Petroleum
Searching for oil over water using seismology
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In seismic surveys, a shock wave is created by the
following:
| Compressed-air gun - shoots pulses of air
into the water (for exploration over water)
| Thumper truck - slams heavy plates into the
ground (for exploration over land)
| Explosives - drilled into the ground (for
exploration over land) or thrown overboard (for exploration over
water), and detonated |
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The shock waves travel beneath the surface of the Earth
and are reflected back by the various rock layers. The reflections
travel at different speeds depending upon the type or density
of rock layers through which they must pass. The reflections of the
shock waves are detected by sensitive microphones or vibration detectors
-- hydrophones over water, seismometers over land. The
readings are interpreted by seismologists for signs of oil and
gas traps.
Although modern oil-exploration methods are better
than previous ones, they still may have only a 10-percent success rate
for finding new oil fields. Once a prospective oil strike is found, the
location is marked by GPS coordinates on land or by marker buoys on the
sea.
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| Drilling
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Typical Drilling Rig |
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SPUDDING IN |
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During drilling a liquid
mixture containing clays and other natural materials, called 'Mud'
is pumped down the drill string forcing the rock cuttings up to the
surface. These cuttings are analysed for indications of oil or gas. |
The start of a well is known as 'Spudding
in' the origins of this phrase are now lost but the phrase came from the
early days of drilling in the USA.
A well is cut by a drill bit rotated on the
end of lengths of pipe called a 'drill string'. The 'Mud' forces
the rock cuttings up to the surface and also helps to cool the drill bit - it
also acts as a safety device against oil and gas pressure that may be
encountered. Drill bits are usually made of hard tungsten but can also be
diamond tipped. As drilling progresses deeper, 'Mud weight' is increased to
counterbalance the pressure in the formations.
Only if extensive initial testing of the well
indicates the presence of oil in commercial quantities is a system installed for
the Production of oil.
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Production |
Because of the low down-hole pressures
encountered in the East Midlands fields, oil is pumped to the surface. This
involves installing a small well cellar and concrete pumping base at the
surface. A small 'rig' completes the well by running tubing down the
borehole. The borehole casing and the cement sheath around it, is then
perforated.
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Oil enters the borehole from the
reservoir through the perforations and is drawn to the surface by means
of a sub-surface pump, connected by sucker rods to the pumping jack or
'nodding donkey' on the surface. The nodding action draws the oil to the
surface on the up-stroke. Today donkeys are electrically driven,
although earlier versions were diesel powered. On the surface, oil is
separated from any gas and water before despatch to the refinery. |
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Pump Jack 144
One of the five 'Nodding Donkeys' that remain preserved on
the Dukes Wood Nature Trail
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