There are three primary techniques of EOR: gas injection, thermal injection, and chemical injection. Gas injection, which uses gases such as natural gas, nitrogen, or carbon dioxide (CO2), accounts for nearly 60 percent of EOR production in the United States. Thermal injection, which involves the introduction of heat, accounts for 40 percent of EOR production in the United States, with most of it occurring in California. Chemical injection, which can involve the use of long-chained molecules called polymers to increase the effectiveness of waterfloods, accounts for about one percent of EOR production in the United States. In 2013, a technique called Plasma-Pulse technology was introduced into the United States from Russia. This technique can result in another 50 percent of improvement in existing well production.

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Enhanced oil recovery
1.1 Introduction
Enhanced oil recovery (abbreviated EOR) is the implementation of various
techniques for increasing the amount of crude oil that can be extracted from
an oil field. Enhanced oil recovery is also called improved oil recovery or
tertiary recovery (as opposed to primary and secondary recovery). According
to the US Department of Energy, there are three primary techniques for EOR:
thermal recovery, gas injection, and chemical injection. Sometimes the term
quaternary recovery is used to refer to more advanced, speculative, EOR
techniques. Using EOR, 30 to 60 percent, or more, of the reservoir's original
oil can be extracted, compared with 20 to 40 percent using primary and
secondary recovery.
Reservoir recovery operations divided into 3 phases:
1. Primary recovery.
2. Secondary recovery.
3. Tertiary recovery.
Primary recovery
It’s the first phase of oil production begins with the discovery of oilfield using
the natural stored energy to move the oil to the well by the expansion of
volatile components and reservoir rocks , also the force exerted by the water
Secondary recovery
When energy is depleted, production declines and secondary phase of
production begins supplemental energy is added to the reservoir by injection
of well.
Tertiary recovery
As the water to oil production ratio of the field approaches an economic limit
of operation, when the net profit diminishes because the difference between
the value of the produced oil and the cost of water treatment and injection
becomes ton narrow, the tertiary period of production begins. Since this last
period in the history of the field commences with the introduction of chemical
and thermal energy to enhance the production of oil, it has been labeled as
enhanced oil recovery (EOR).

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Figure 1:Primary , Secondary and Tertiary recovery.
No single EOR process has yet emerged which is applicable to all petroleum
reservoirs as a general method for enhancement of oil recovery; however,
specific processes that are quite distinct from each other have been developed
to address reservoirs with special characteristics. The criteria for selection of
a particular EOR process are complex because of the large number of
petrophysical, chemical, geologic, environmental, and fluid properties that
must be considered for each individual case.
1.2 Techniques
There are three primary techniques of EOR: gas injection, thermal injection,
and chemical injection. Gas injection, which uses gases such as natural gas,
nitrogen, or carbon dioxide (CO2), accounts for nearly 60 percent of EOR
production in the United States. Thermal injection, which involves the
introduction of heat, accounts for 40 percent of EOR production in the United
States, with most of it occurring in California. Chemical injection, which can
involve the use of long-chained molecules called polymers to increase the
effectiveness of waterfloods, accounts for about one percent of EOR
production in the United States. In 2013, a technique called Plasma-Pulse

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technology was introduced into the United States from Russia. This technique
can result in another 50 percent of improvement in existing well production.
Gas injection
Gas injection or miscible flooding is presently the most-commonly used
approach in enhanced oil recovery. Miscible flooding is a general term for
injection processes that introduce miscible gases into the reservoir. A miscible
displacement process maintains reservoir pressure and improves oil
displacement because the interfacial tension between oil and water is reduced.
This refers to removing the interface between the two interacting fluids. This
allows for total displacement efficiency. Gases used include CO2, natural gas
or nitrogen. The fluid most commonly used for miscible displacement is
carbon dioxide because it reduces the oil viscosity and is less expensive than
liquefied petroleum gas. Oil displacement by carbon dioxide injection relies
on the phase behavior of the mixtures of that gas and the crude, which are
strongly dependent on reservoir temperature, pressure and crude oil
composition.
Figure 2:CO2 injection process.

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Thermal injection
In this approach, various methods are used to heat the crude oil in the
formation to reduce its viscosity and/or vaporize part of the oil and thus
decrease the mobility ratio. The increased heat reduces the surface tension and
increases the permeability of the oil. The heated oil may also vaporize and
then condense forming improved oil. Methods include cyclic steam injection,
steam flooding and combustion. These methods improve the sweep efficiency
and the displacement efficiency. Steam injection has been used commercially
since the 1960s in California fields. In 2011 solar thermal enhanced oil
recovery projects were started in California and Oman, this method is similar
to thermal EOR but uses a solar array to produce the steam.
Steam flooding
Steam flooding (see sketch) is one means of introducing heat to the reservoir
by pumping steam into the well with a pattern similar to that of water
injection. Eventually the steam condenses to hot water, in the steam zone the
oil evaporates and in the hot water zone the oil expands. As a result, the oil
expands the viscosity drops and the permeability increases. To ensure success
the process has to be cyclical. This is the principal enhanced oil recovery
program in use today.
Figure 3:Steam flooding process.

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Fire injection
Fire flooding works best when the oil saturation and porosity are high.
Combustion generates the heat within the reservoir itself. Continuous
injection of air or other gas mixture with high oxygen content will maintain
the flame front. As the fire burns, it moves through the reservoir toward
production wells. Heat from the fire reduces oil viscosity and helps vaporize
reservoir water to steam. The steam, hot water, combustion gas and a bank of
distilled solvent all act to drive oil in front of the fire toward production wells.
There are three methods of combustion: Dry forward, reverse and wet
combustion. Dry forward uses an igniter to set fire to the oil. As the fire
progresses the oil is pushed away from the fire toward the producing well. In
reverse the air injection and the ignition occur from opposite directions. In
wet combustion water is injected just behind the front and turned into steam
by the hot rock. This quenches the fire and spreads the heat more evenly.
Chemical injection
The injection of various chemicals, usually as dilute solutions, have been used
to aid mobility and the reduction in surface tension. Injection of alkaline or
caustic solutions into reservoirs with oil that has organic acids naturally
occurring in the oil will result in the production of soap that may lower the
interfacial tension enough to increase production. Injection of a dilute solution
of a water-soluble polymer to increase the viscosity of the injected water can
increase the amount of oil recovered in some formations. Dilute solutions of
surfactants such as petroleum sulfonates or biosurfactants such as
rhamnolipids may be injected to lower the interfacial tension or capillary
pressure that impedes oil droplets from moving through a reservoir. Special
formulations of oil, water and surfactant, microemulsions, can be particularly
effective in this. Application of these methods is usually limited by the cost
of the chemicals and their adsorption and loss onto the rock of the oil
containing formation. In all of these methods the chemicals are injected into
several wells and the production occurs in other nearby wells.
Polymer flooding
Polymer flooding consists in mixing long chain polymer molecules with the
injected water in order to increase the water viscosity. This method improves
the vertical and areal sweep efficiency as a consequence of improving the

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water/oil Mobility ratio. Surfactants may be used in conjunction with
polymers; They decrease the surface tension between the oil and water. This
reduces the residual oil saturation and improves the macroscopic efficiency of
the process. Primary surfactants usually have co-surfactants, activity boosters,
and co-solvents added to them to improve stability of the formulation. Caustic
flooding is the addition of sodium hydroxide to injection water. It does this by
lowering the surface tension, reversing the rock wettability, emulsification of
the oil, mobilization of the oil and helps in drawing the oil out of the rock.