Microemulsions
Microemulsions can be considered as swollen micelles - but not all micellar
solutions can be swollen to the extent of forming microemulsions.
Microemulsions are apparently homogeneous mixtures of water and
oil with large amounts of surfactants. They are thermodynamically stable
(as opposed to emulsions), are formed spontaneously and contain particles
that are extremely small.
Droplet diameters in microemulsions typically range from 100-1000 Å.
Microemulsions are usually transparent/translucent as opposed to emulsions
which are turbid (dirty muddy).
This structure arises when the concentration of surfactant is high.
The properties that prove useful over other structures :
- thermodynamic stability
- spontaneous formation
- possibility of near uniform size of droplets of suspension formed
- possibility of nearly homogeneous mix of oil and water soluble
substances.
Microemulsions in enhanced oil recovery:
Some 40-60% of the original oil in a field typically remains trapped in the
reservoir after primary and secondary recoveries.Enhanced oil recovery
is the attempt to recover this remaining oil reserves through different techniques like
surfactant flooding . It is important economically to recover as much of the
remaining oil as possible.
Oil can be recovered in the initial primary process by using the
natural energy of the reservoir. This is followed by the secondary
process of water injection into the porous rocks to maintain the pressure
and facilitate oil displacement.
Most of the oil remaining after these processes is left behind in the
pores of the rocks - capillary retention force
is essentially responsible for resistance to
further recovery displacement by these techniques. A quick calculation of the
pressure gradient across the hemispherical meniscus in the capillary
for typical values of pore radius and interfacial tension before the addition
of surfactants gives us values that are at least a couple of orders of
magnitude greater than typically maintainable in an oil reservoir - hence
the oil drop gets stuck in the capillaries.
Therefore reduction of the interfacial tension using surfactants becomes the
most practical way of recovering the oil left after water-flooding.
Two major techniques to do so are:
- Microemulsion flooding : A fluid which under normal circumstances is immiscible in oil but the addition of
sufficient concentration of a surfactant to causes the fluid and oil to become
miscible (this requires lowering of the interfacial tension to values less than
0.001 dynes/cm). Once the two are miscible the oil flows with the injected
water without getting trapped further and can be recovered once it comes out
of the well.
- Low concentration micellar surfactant flooding -- this method reduces the
oil/water interfacial tension (without making them miscible). A reduction in
the interfacial tension decreases the capillary
retention force. Consequently the capillary retention force becomes
typically less than the pressure gradient maintainable by water-flooding hence
the droplets do not get stuck in the capillaries anymore.
Thus by the addition of surfactants
capillaries no longer restrict the motion of the oil along with the flow
of injected water.
-
Surfactants not only influence static properties of interfaces
but also influence rheological properties which becomes an important
factor when trying to control the motion of the oil/water interface
through pores in the oil reservoir.
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