.2because the pressure upstream is determined primarily by the well FTP, and the pressure downstream is determined by the pressure control valve on the first separator in the process.
Crude oil Production System
CRUDE OIL PRODUCTION
OMITOLA Oluwatobiloba. P
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– What is Crude oil ?
– How is crude Oil formed in place?
• Exploration & Drilling
• Surface Production Operations
– Wellhead and Manifold
Types of Separator
Effect of Separator Pressure on separation
– Gas Processing
– Oil Treatment
– Produced Water Treatment
– Waste Utilization & Disposal
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• What is Crude oil?
• Crude oil also known as petroleum is a naturally
occurring fossil fuel which contains mixture of HCs
and inorganic compounds such as oxygen, sulphur,
nitrogen, etc. and found in geologic formations
beneath the earth surface.
• It is formed when large quantities of dead organisms
are buried underneath rocks and undergo intense
heat and pressure.
• The major classes of hydrocarbons in crude
– Paraffins (Alkane family)
– Aromatics (Benzene family)
– Napthenes or Cycloalkanes
– Other hydrocarbons types are the Alkenes
• How is crude Oil formed in place?
• Petroleum is a fossil fuel derived from ancient
fossilized organic materials, such as zooplankton and
• Vast quantities of such remains settle on land, sea,
lake bottoms, mixing with sediments thereby getting
buried under anoxic conditions.
• As further layers settle to the sea or lake bed,
intense heat and pressure build up in the lower
• This process causes the organic matter to change,
first into a waxy material known as kerogen and then
with more heat into liquid and gaseous hydrocarbons
via a process known as catagenesis.
• Tectonic movements further push the crude oil until
they are trapped.
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• Crude oil-gas mixture once produced
from oil wells drilled moves through two
main distinctive processing operations in
order to obtain useful petroleum
• Surface Operations; where gases are
separated from oil with further
treatment of the separated fluids.
• Refining Operations; where crude oil is
fractionated into cuts with physical and
further chemical conversion processes to
produce different HC components of
Fig 1-0. A schematic Illustration of the three different
production operations of the oil industry
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Exploration & Drilling
• Exploration involves the search for rock formations associated with oil or natural gas deposits. It includes
geophysical prospecting and exploratory drilling. Before exploration can be done in an area of possible or
probable reserves, an Oil Prospecting License (OPL) has to be given by the authorized body.
• After prospecting and a region has been tagged a proven reserve, then an Oil Mining License (OML) is
needed to begin drilling operations, be it exploratory, appraisal wells or developmental wells.
• Drilling is the act of boring holes into the earth crust.
• Once a promising geological structure has been identified;
Exploratory wells also known as “wildcat” are drilled to confirm the presence or absence of HCs and also
to determine the internal pressure of the reservoir
Appraisal wells also known as “outstep” are drilled to determine the size, extent and quantify the
Development or Production wells exploit and transport oil and gas from the reservoir through formation
pressure, artificial lift, and possibly advanced recovery techniques, until economically feasible reserves
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Surface Production Operations (SPO)
• SPO covers the processing of all three streams
from the well head as indicated in Fig. 3-0 while
Fig 3-1 illustrates a typical process flow diagram
• The need for field processing of crude oil-gas
mixture is justified for four main reasons:
These mixtures are very difficult to handle,
meter, or transport.
It is unsafe and uneconomical to ship or
transport such two-phase mixtures
overseas to refineries and gas plants.
Oil producers have to abide with the
specifications set for shipping and refining
Environmental constraints established for
safe handling of HCs and the disposal of
produced salt water
Fig. 3-0 An illustration of the Surface field operations
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Fig 3-1. A typical flow sheet of SPO
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Wellhead and Manifold
• The production system begins at the wellhead installed
on the cased hole to seal the annular space between
casing and tubing, control wellhead pressure, adjust
well flow rate and transport oil downstream.
• The size of the opening in its choke (valve) determines
the flow rate.
• For high-pressure wells, it is desirable to have a positive
choke in series with an adjustable choke.
• Due to the high-risk situations, an automatic shutdown
valve should be installed on the wellhead.
• The flow-lines from several wells are gathered in a
manifold and routed into a separator
Fig 3-2 A wellhead
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• Well effluents flowing from producing wells are usually identified as turbulent, high velocity
mixtures of gases, oil and salt water.
• As these streams reach the surface , they undergo continuous reduction in temperature
and pressure forming a two phase fluid-flow: gas and liquid.
• The physical separation of these phases is one of the basic operations in the production,
processing, and treatment of oil and gas and it’s achieved by a Separator.
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Types of Separator
Separators are designed in either horizontal, vertical, or spherical configurations
Fig. 3-4 Vertical separator
Fig. 3-3.Horizontal separator
Fig. 3-5 Spherical separator
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Horizontal Separator Vs. Vertical Separator
• In the gravity settling section of a horizontal vessel, the liquid droplets fall
perpendicular to the gas flow and thus are more easily settled out of the gas
• Horizontal separators offer greater liquid capacity and are best suited for liquid-liquid
separation and foaming crudes.
• Horizontal vessels require more plan area to perform the same separation as vertical
vessels but its large cross-sectional area allows more settling.
• The relief valve and some of the controls of the vertical separator may be difficult to
service without special ladders and platforms.
• Smaller, horizontal vessels can have less liquid surge capacity than vertical vessels
sized for the same steady-state flow rate.
• Since the interface area is larger in a horizontal separator than a vertical separator, it
is easier for the gas bubbles, which come out of solution as the liquid approaches
equilibrium, to reach the vapour space.
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• Separators are classified as "two-phase" if they separate gas from the total liquid stream
and "three-phase" if they also separate the liquid stream into its crude oil and water
Fig. 3-6 Two-phase horizontal separator
Fig. 3-7 Three-phase horizontal separator
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Advantages of the three-phase over the two-phase separator are:
• It separates free water from oil-water mixture.
• The three phase also has a larger liquid section allowing more retention time for the
oil and water to separate.
• Unlike two-phase units, three-phase separators have two liquid-level controllers that
are often combined with internal baffles & weirs to regulate the oil-gas and oil-water
• At the oil/water interface there is a pneumatic displacement type level control which
actuates the water dump valve.
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Effects of Separator pressure on separation
• If the pressure for initial separation is too high, too many light components will stay in
the liquid phase at the separator and be lost to the gas phase at the tank.
• If the pressure is too low, not as many of these light components will be stabilized into
the liquid at the separator and they will be lost to the gas phase.
• The changes in separation pressure give rise Stage separation.
Fig. 3-8 Stage separation
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The actual practice of processing natural gas to pipeline dry gas quality levels can be quite
complex, but usually involves four main processes to remove the various impurities:
Crude-oil carry over Removal
• A scrubber which is a two-phase separator designed to recover liquids carried over
from the gas outlets of production separators or to catch liquids condensed due to
cooling or pressure drop is used at the first stage of gas processing.
• There are two techniques for removing NGLs from the natural gas stream: the
absorption method and the cryogenic expander process
• Absorption process involves the passage of the crude oil through an absorption tower in
which a ‘lean’ absorption oil makes contact with the oil thereby absorbing the NGLs
present in the oil.
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• The ‘rich’ absorption oil exits the tower through the bottom of the tower and is fed
into lean oil stills, where it is heated to a temperature above the boiling point of the
NGLs, but below that of the oil;
• Cryogenic processes are required for high and economical recovery rates. Essentially,
cryogenic processes consist of dropping the temperature of the gas stream to around -
120 degrees Fahrenheit.
• In the turbo expander process, external refrigerants are used to cool the natural gas
stream. Then, an expansion turbine is used to rapidly expand the chilled gases, which
causes the temperature to drop significantly. This rapid temperature drop condenses
ethane and other hydrocarbons in the gas stream, while maintaining methane in
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There are two main processes of water removal in gas processing: Glycol(absorption) and Solid
• Glycol absorption method of dehydration is very similar to using absorption for NGL
extraction but the main difference is the use of a glycol instead of an absorption oil.
• It also entails the oil making contact with the glycol (e.g. TEG) , an hygroscopic substance in a
glycol tower, the rich glycol re-boiled and stripped of its water component in form of steam,
flashed of its dissolved gas in a flash tank and fed again into the tower.
• Solid desiccants like activated alumina, silica gel are filled into adsorption towers
• As the wet gas passes through the tower, water molecules are retained on the surface of
these desiccant beds leaving the dry gas to exit the bottom of the tower.
• To regenerate the desiccant, a high temperature gas is passed through the saturated desiccant
bed and vaporizes the water in the desiccant tower, leaving it dry for further use.
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Fig. 3-9 Simple Gas processing flow-sheet
Sweetening and Acid Gas Removal
• One of the most important operations of
gas processing is the removal of sulphur
and acid gases such as carbon dioxide.
• The removal of sulphur which exists in the
form of H2S in ‘sour’ gas streams is
achieved by a Sweetening process
• The sour gas is run through a tower, which
contains the amine solution. This solution
has an affinity for sulfur, and absorbs it
much like glycol absorbing water.
• Like the process for NGL extraction and
glycol dehydration, the amine solution used
can be regenerated (that is, the absorbed
sulfur is recovered), allowing it to be
reused to treat more sour gases
• It is also possible to use solid desiccants like
iron sponges to remove the sulfide and
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• Oil Treatment
After free water removal, produced oil often contains excessive impurities which are required
to be reduced to a value acceptable for transportation or sales:
• It is usually the first process in crude oil processing. It involves removal of salt dissolved
in the water in the crude oil. It is achieved by a process unit called desalter
• There are also electrostatic dehydrators which enhance coalescing of small water
droplets and assist in settling
• For an emulsion to exist there must be two mutually immiscible liquids, an emulsifying
agent, and sufficient agitation to disperse the discontinuous phase into the continuous
• A common method for separating this emulsion is to heat the stream thereby
deactivating the emulsifying agent, allowing the dispersed water droplets to collide.
This is achieved by heater treaters
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• The process of coalescence is also required in
emulsion treatment. Hence, electrostatic
coalescers subject emulsions to a high
voltage electric field thereby causing the
small droplets dispersed in the oil phase to
coalesce and settle.
• Demulsifying agents: Chemicals sold under
trade names such as Tretolite™, Visco™, and
Breaxit™ are examples which act as surface
acting agents to neutralize the effect of
• It refers to lowering the vapour pressure to a
value that will allow safe handling and
• It is achieved by stage separation, reboiled
distillation and sometimes the combination of
the two methods
Fig. 3-10 Simple Oil processing flow-sheet
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• Water Treatment
In producing operations it is often necessary to handle produced water properly before reuse
or disposal. The water must be separated from the crude oil and disposed of in a manner that
does not violate established environmental regulations.
• Table 3-11 lists the various methods employed in produced-water treating systems and
the type of equipment that employ each method
Method Equipment Type
Approximate minimum Drop size
Skimmer Tanks and Vessels
Parallel Plate Interceptors
30-50Corrugated Plate Interceptors
Cross Flow Separators
Enhanced Gravity separation
Filtration Multi-Media 1+
Table 3-11 Produced-Water Treating Equipment
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• Produced water will always have some
form of primary treating prior to disposal.
• This could be a skim tank, skim vessel or
• Most of these devices employ gravity
• Depending upon the severity of the
treating problem, secondary treating,
utilizing a CPI, crossflow separator, or a
flotation unit may be required.
• Liquid-liquid hydrocyclones are often used
either in a single stage or with a
downstream skim vessel or flotation unit.
Fig. 3-12 Typical produced-water treating system.
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• Waste Utilization and Disposal
SPO generate dangerous wastes just like the drilling operations further upstream and can be harmful to life
and the environment especially in offshore situations. These wastes include: produced water, gas flares, oil
spills, deck drains, chemicals which contain heavy metals and radioactive materials, etc.
Some of these wastes can be reused after further treatment while some are subject to disposal
• Onshore, produced water will normally be re-injected in the formation to serve as artificial lift for
wells which cannot further achieve optimal production by its natural drive mechanism
• It can also be pumped into a disposal well when not needed
• As shown in Fig. 3-13, compressed gas could also be re-injected into the formation through
injection wells to lighten the column of fluid and allow the reservoir pressure to force the fluid to
• At high pressure, the gas could also be used in Industrial power plants to generate electric power in
large quantities that can be supplied to end users
• At low pressure, it can be used in internal combustion engines to power locomotives.
Other wastes like deck drains are collected in a gathering system, treated and disposed overboard or
added to the treated produce water for reinjection.
Waste lube oil and waste lube oil filters are usually sent to offsite reclamation plant
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Fig. 3-13 Gas lift System
o Other forms of artificial lift include electric pumps which could be Positive Displacement
in near-surface production or Submersible pumps where the well flowing pressure is very
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• The significance of a production facility is to separate the well stream into three
components, typically called phases (oil, gas, and water), and process these phases into
some marketable product(s) or dispose of them in an environmentally acceptable manner.
• To achieve optimal production each process must be carried out efficiently.