|Fig. 1: Oil production and consumption by region during 2014.  (Courtesy of British Petroleum)|
Despite the fluctuations in the oil prices in the past few decades, the global demand for energy as demonstrated in Fig. 1 has been steadily growing.
This clearly demonstrates the significance of sustaining the production of the major sources of energy, which is specifically true for crude oil as the world's leading energy resource. As time passes, it becomes even more difficult to discover new oil fields; hence, oil companies' main target in any field development is to reach the ultimate recovery while maintaining an economic oil rate.  Primary and secondary recovery methods, which includes but not limited to natural drive, artificial lift and water/ gas injection, typically recover as much as third to half of the oil in place.  There are three forces that govern the fluid flow in a reservoir which are viscous, gravity and capillary forces. The remaining oil in the reservoir is called "residual oil" and is trapped due to essentially capillary forces. Novel techniques were introduced over time to unlock this unrecoverable potential, otherwise known as Enhanced Oil Recovery (EOR) techniques. These techniques can significantly extend the global oil reserves/production and in turn the life time of the reservoir. Implementation and success of such projects always refer back to the economic feasibility element which is heavily dependent on oil prices. Broadly speaking, EOR schemes can be categorized into thermal and non- thermal depending on whether or not heat is employed in some form; with several methods residing within each category. Non Thermal EOR methods consist of chemical and miscible injection processes. One of the main Chemical EOR methods is surfactant flooding which is the focus of this paper. 
Before jumping into the mechanism of surfactant flooding, one must comprehend the goals of a given EOR method which can be summarized into:
The mobility ratio is defined as the mobility of the displacing fluid, which is water or surfactant enhanced water for the sake of the discussion, divided by the mobility of the displaced fluid which is oil, per
where κri and μi are the relative permeability and phase viscosity, respectively. It is quite intuitive by looking at the above equation that it is better to have a low mobility ratio (less than 1), since otherwise water or the displacing fluid will pass through oil creating fingering or channeling effects and thus reducing the sweep displacement efficiency as shown in Fig. 2.
|Fig. 2: A schematic demonstrating improvement in displacement efficiency at lower mobility ratio. (Source: S. Aldourasry. After Green and Willhite. )|
The capillary number (Ca) is defined as the ratio of viscous to capillary forces. It is given by
where v is the velocity, μ the displaced fluid viscosity, and σ is the surface tension. It has been demonstrated in the literature that residual oil saturation is a function of the capillary number.  In light of this, increasing the capillary number reduces the oil saturation - i. e. enhances oil recovery by essentially reducing the interfacial tension between oil and the injectant fluid.
Surfactant flooding, also known as detergent or micro emulsion flooding, uses low concentration of surfactants augmented water to lower the interfacial tension between oil and water and in some cases alter the wettability of the rock to create favorable conditions for efficient oil displacement.  It is the most complex and accompanied with high degree of uncertainty among other enhanced oil recovery methods. However, if designed optimally for the specific crude accounting for factors such as salinity, temperature, pressure and clay content, it would demonstrate a high potential for maximum oil recovery. Surfactants can be classified as anionic, cationic or non-ionic; among which anionic surfactants is widely used in the oil industry as an EOR injectant fluid due to its lower adsorption tendency on the reservoir rock. In the field implementation scale, other chemicals including polymer and alkali are also added to the surfactant slug in order to maximize the project design efficiency. Polymer increases the viscosity of the displacing fluid/water and thus generate a more desirable mobility ratio. Alkali, on the other hand, would lower the adsorption level of the surfactant and reacts with the acidic components of the oil to produce in-situ surfactant which as a result optimizes the injected surfactant concentration. 
Enhanced oil recovery techniques highly contributes to a sustainable oil production and thus ensures meeting the long term world's energy demands. Surfactant injection specifically is a promising technique that have been implemented on large scales and demonstrated a remarkable success for the cases under which proper and thorough evaluation/screening studies were conducted.
© Salem Aldousary. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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