The aim of surfactant flooding is to recover the capillary-trapped residual oil after waterflooding.By means of surfactant solutions, the residual oil can be mobilized through a strong reduction in the interfacial tensions between oil and water.
The success of surfactant flooding depends on different factors:
- Formulation of surfactant
- Cost of Surfactants
- Availability of Chemicals
- Environmental Impacts
- Oil Price
All of these factors above are critical due to the high volumes usually required to flood one field. Therefore, in order to minimize the transportation costs, it is critical to have plants, big enough to accommodate the capacity needed to satisfy the demand in close proximity to the field being flooded and that the cost of the chemicals be low enough to make the sizable initial investment in chemicals profitable in the long terms.
Working mechanism of surfactant molecule to reduce IFT
When an anionic surfactant is dissolved in an aqueous phase, molecules of the surfactant start to dissociate into a cation and an anionic monomer.
Due to dual nature of surfactant molecule, it tends to accumulate at the interface with lipophilic (hydrophobic) ‘‘tails’’ placed in the oil phase and hydrophilic ‘‘heads’’ in the aqueous phase.
The increased concentration of the surfactant at the interface results in dramatic reduction of IFT between the phases.
For EOR always prefer to use low salinity water for injection because increasing salinity causes reduction in electrical double layer so reduction in IFT will be low and hence more difficult to recover oil.
The magnitude and nature of interfacial charge and surface charge on minerals and clays present in the reservoir rocks in the oil-displacement can contribute significantly to the design of surfactant formulations for optimum performance under given reservoir conditions.
The sign an magnitude of the charge will influence the adsorption of the surfactant on minerals and clays of the reservoir.
Polar constituents (asphaltenes and resins) in crude oil play an important role in determining the reservoir wettability due to their adsorption onto the rock surface.
The oil-wetting surfaces lead to poor oil displacement, whereas the water-wetting surfaces lead to efficient oil displacement for surfactant flooding.
The proper choice of surfactant can selectively alter the rock wetting from oil to water (brine) and can create favorable conditions for efficient oil displacement. Also the use of additives (salt, acid or base) to alter the rock wettability is a promising approach for EOR.
Sodium hydroxide (NaOH) can effectively change rock surfaces from oil-wet to water-wet
Effect of Surfactant concentration on IFT:
IFT decreases with increasing surfactant concentration and at a critical concentration the IFT approaches its minimum value. Beyond this critical concentration, the IFT increases with an increase in surfactant concentration.
Effect of salinity:
A specific surfactant concentration and salinity is required for the formation of ultra-low IFT. As the salt concentration varies in aqueous phase, the partition coefficient of the surfactant between oil and water is altered which seems to be responsible for achieving ultralow IFT.
The surfactant concentration in the oil phase increases with increasing salt concentration in the aqueous phase and vice versa. Select optimal salinity in such a way that surfactant concentration is highest at the oil-water interface which produces the lowest IFT.
The partition coefficient at optimal salinity was found to be unity
SURFACTANT FLOODING
Variations
Surfactant-Polymer Flood (SP)
Low Tension Polymer Flood (LTPF)
Adsorption on rock surface
Slug dissipation due to dispersion
Slug dilution by water
Formation of emulsions
Treatment and disposal problems
Mobility Control:
For an efficient oil displacement by surfactant flooding, the mobility controlling polymer should be less mobile than the surfactant slug and oil bank.
Loss of mobility control in the fluid sequence causes fingering which, in turn, reduces oil displacement efficiency. Mobility control is one of the most important considerations in designing a surfactant polymer formulation.
Many polymers such as polyacrylamides and polysaccharides have been used as effective mobility control agents
Types of interfaces
Interface is the boundary between two or more phases
exist together
The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interfacial phase.
Several types of interface can exist depending on whether the two adjacent phases are in solid, liquid or gaseous state.
Important of Interfacial phenomena in our daily life:
- Adsorption of drugs onto solid adjuncts in dosage forms
- Penetration of molecules through biological membranes
- Emulsion formation and stability
- The dispersion of insoluble particles in liquid media to form suspensions.
Physical origin of surface tension
A molecule at the surface, is missing half its attractive interactions, which is why when segregated to the surface, a liquid molecule is in an unfavorable energy state. This is the fundamental reason that liquid adjust their shape in order to expose the smallest possible surface area.
The interior molecules are equally attracted by all neighboring molecules with the same force, so that the resulting force is zero.
To keep the equilibrium, an equal force must be applied to oppose the inward tension in the surface.
The surface tension then causes drops of liquid to be round since the surface of the liquid is being minimized that way.
The molecules at the surface are attracted by molecules from inside, whereas no force from outside the surface. Hence the resulting force is towards the interior of the liquid.