Enhanced Oil Recovery at a glance



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:

          1. Thermal Injection
          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:

   - Steam (or hot water) injection
 - 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.



















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