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Surfactant flooding is an Enhanced Oil Recovery technique in which the mobility of residual oil in the reservoir is increased by reducing the Interfacial Tension (IFT) between the injected fluid and the reservoir oil.
It is a chemical Enhanced Oil Recovery technique (cEOR) in which a small amount of surfactant (0.3-1.0 volume%) is mixed with the injected fluid (water) to sweep the reservoir oil. Surfactants are blended with co-surfactants which acts as active agents to enhance the properties of surfactant solution and helps to sustain optimal conditions with respect to pressure, temperature and salinity.
The presence of surfactants also improves the wettability (oil-wet to water-wet) of reservoir rock to enhance the recovery of oil. Surfactants also improve the recovery of residual oil through other mechanisms like micro-emulsification of trapped residual oil and by changing the interfacial rheological properties.Sometimes, the surfactants and co-surfactants, along with other components of the aqueous solution form a micro emulsion.
In primary and secondary recovery less than 50% of the oil can be produced and the residual oil gets trapped in the pores due to capillary forces. Therefore, in chemical enhance oil recovery (cEOR), surfactant solution is injected into the reservoir which reduces the capillary forces by greatly lowering the interfacial tension.
Surfactants types used
The surfactants are primarily divided into four types
- Cationic surfactant
- Anionic surfactant
- Non-ionic surfactant
Mechanisms of surfactant flooding
Micelle formation in surfactant flooding
Micro emulsions are formed in surfactant flooding between water and oil as the two immiscible phases resulting in drastically reduced IFT which mobilizes the trapped residual oil.
Adsorption process occurs when the surfactant molecules attach on the surface of reservoir rock. This hinders the propagation of surfactant and increases the amount of surfactant that must be injected to maintain the low IFT throughout the life of the flood.
Carbonate reservoirs are generally not suitable for surfactant flooding since carbonate reservoir are composed of calcite (CaC03), magnesite (MgCO3), anhydrites (CaSO4) etc. which have a low permeability matrix and mostly has oil wet or mixed wet conditions along with high concentration of divalent ions in their connate water. Sandstone Reservoirs are homogeneous and are more suitable of cEOR.
- High temperature, High Salinity, fractured Carbonate reservoirs.
- Low temperature and low salinity conditions are optimal, but sulfonate surfactants can be used for high temperature and low salinity conditions.
- High Adsorption and precipitation of surfactant is challenging for high salinity sandstone reservoirs.
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