Considering the current
increasing global energy demand, the increase in number of brown oil fields and
the associated reduction in the amount of conventional oil reserves, it is not
surprising to witness the upsurge towards the engagement of the Enhanced Oil Recovery (EOR) techniques
with a view of recovering more of the oil in place.
It is fair to say the era
of “easy oil” is gradually coming to an end. Many of the largest producing
fields are now reaching their tail phase. However, this does not mean that the
residual oil in the reservoir is not worth recovering, it merely indicates that
the remains would be harder to recover.
In order to improve the
economic efficiency of residual oil recovery various types of oil recovery mechanisms
are considered throughout the life cycle of a field. After extraction of oil
through primary and secondary methods, which involve mechanisms such as field
natural rive, water drive, gas cap drive etc., EOR techniques will be
considered to potentially recover the remaining oil in the reservoir.
EOR
classification
It is essential to
embrace the candidate reservoir characteristics in order to choose and design
the suitable EOR technique. EOR
techniques are typically subdivided into four different categories of:
2. Miscible Gas Injection
3. Chemical Injection
4. Other
Figure
1: EOR Classification
Source:
Primera Reservoir
1. Thermal Method
Thermal methods have been
progressively used in the industry since the mid-1950s. It is fair to say that
in terms of technology and experience they are the most advanced among other
EOR methods. This method involves the addition of thermal energy or heat into
the reservoir in order to increase the temperature of the residual oil,
subsequently lowering the viscosity of the oil and therefore enhance its
mobility and ability to flow through the reservoir. Thermal methods are mainly
used for recovery of heavy oil with API ranging from 10-20ᵒ.
Popular thermal methods
are:
- In situ combustion
- Cyclic Steam Treatment (huff and puff)
- Steam Assisted Gravity Drainage (SAGD)
Other processes which are
not as widely use:
- Electrical/electromagnetic heating
Figure 2: Steam and water flooding
Source: DOE
2. Miscible Gas Flooding
This method is performed
by gases such as carbon dioxide (CO2). In this process the gas
injected will expand in the reservoir and therefore displaces part of the oil
to the producing wellbore. Some gases also tend to dissolve in the oil in order
to reduce its viscosity and enhance the mobility of the oil. The main
mechanisms behind this technique are:
- Generation of miscibility
- Swelling of the crude oil
- Lowering of the oil viscosity
- Lowering of the Interfacial tension
between oil and the gas
Figure
3: CO2 Flooding using CO2 (CO2-EOR) as the miscible gas for injection
Source:
DOE
3. Chemical
injection
This technique involves
the use of long-chained molecules called polymers as well as detergent-like
surfactants and alkaline as the displacing fluid. The mechanism behind this
process is reduction in the mobility ratio (Equation 1) and/or the increase in capillary
number. Water soluble polymers such as Polyacrylamides have proven to be
effective when it comes to reducing of the mobility ratio.
Surfactants would be
added to the displacing solution in order to lower the interfacial tension
between oil and water. Alkaline such as hydroxide react with the acid
components of the crude oil in order to produce natural surfactants within the
reservoir which would assist with the lowering of the interfacial tension
between the oil and water.
(Eq. 1)
Where:
M = Mobility Ratio
µo = Oil
viscosity
µw =
Displacing fluid (water)
Kro = End
point relative permeability to oil
Krw = End
point relative permeability to water
Popular types of Chemical
EOR are:
- Surfactant
flooding
- Polymer
Flooding
- Alkaline
Flooding
- Alkali-Surfactant-Polymer
Flooding (ASP)
Figure
4: Polymer-Surfactant Flooding
4. Other
There are other EOR
methods available in the market which include combination of techniques such as
Surfactant-Steam, Micellar-ASP etc. Couple of honourable mentions of such
techniques are Microbial EOR (MEOR) and Foam Flooding.
- Microbial EOR
In this process the
microbes tend to react with a carbon source such as oil in order to produce
surfactant, polymers or gases such as CO2, N2 and H2.
The mechanisms behind the recovery of oil in this particular type of EOR are
those of classic chemical methods which involve the lowering of interfacial
tension, change in wettability, reduction of mobility ratio and oil viscosity.
- Foam Flooding
This technique has been
used since mid-1960s. It involves a non-Newtonian fluid with characteristics
which are overseen by various variables. Foam is metastable dispersion of large
volume of gas in a continuous liquid phase which constitutes a small volume of
the foam. (Laura Romero-Zeron, 2012). Bulk foams are mainly formed when gas
becomes in contact with a liquid which contains a surfactant in the presence of
mechanical agitation (Sydansk & Romero-Zeron, 2011). Foams have been mainly
proposed for use as mobility control agents as well as to enhance the
conformance in the reservoir. Foams have the ability to reduce the permeability
of the swept zones and subsequently pushing the steam or CO2 or
other gas phase into the un-swept areas of the reservoir.
By Arash Farhadi
Lead Chemical EOR Reservoir Engineer
Primera Reservoir
Reference
- Laura Romero-Zeron (2012). Introduction
to Enhanced Oil Recovery (EOR) Processes and Bioremediation of Oil-Contaminated
Sites. Canada: University of New Brunswick.
31-34.
- Sydansk R. D. &
Romero-Zerón, L. Reservoir Conformance Improvement. Richardson, Texas: Society
of Petroleum Engineers, 2011.
Strong focus on decision making and uncertainty analysis combined with advanced, state of the art and complex workflows.
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