Page 206 - Fundamentals of Enhanced Oil and Gas Recovery

194 Mohammad Ali Ahmadi

increased, the sweep efficiency is improved by reducing viscous fingering. According
to the fractional flow equation [45],
1 1
f w 5 5 (6.2)
1 1 k ro =k rw μ =μ
w o 1 1 λ o =λ w
where, μ w is water viscosity, μ o denotes the oil viscosity, k rw represents the water rela-
tive permeability, k ro represents the oil relative permeability, λ o is oil mobility and λ w
is water mobility [46].
A secondary mechanism is related to polymer viscoelastic behavior. Because of
polymer viscoelastic properties, there is normal stress between oil and polymer solu-
tion. Thus, polymer exerts a larger pull force on oil droplets or oil films. Oil is
“pushed and pulled” out of dead-end pores. Thus, residual oil saturation is decreased
[45,47,48].
Presence of salt in a polymer aqueous solution results in decreasing of the solution
viscosity. Flory-Huggins model [49] can be used to evaluate the behavior of polymers
in the presence of salt in an aqueous solution [50].
Polymer solution viscosity may decrease as the pH is increased owing to the
increased salt effect of an alkali [51,52]. However, Mungan [53] reported that the
HAPM viscosity at 50 seconds 21 shear rate significantly decreased when lowering pH.
And Szabo [54] reported the increase in the viscosity of AM/2-acrylamido-2-methyl
propane sulfonate copolymer solution when NaOH was added. Those observations
are probably related to the early-time hydrolysis effect. The effect of pH is complex
considering different hydrolysis effect and salt effect [45].
One economic impact of polymer flooding that has been less discussed is the
reduced amount of water injected and produced, compared with water flooding [4].
Because polymer improves the sweep efficiency, less water is produced and injected.
In some situations, like an offshore environment and a desert area, water and the treat-
ment of water could be costly [45].

6.2.4 Alkaline Surfactant Polymer Flooding
An important mechanism of alkaline surfactant polymer (ASP) flooding is the
synergy between in situ generated soap and an injected surfactant. Generally, the
optimum salinity for the soap is unrealistically low. To satisfy the low optimum
salinity, the injected alkaline concentration must be so low that the injected alkali
is lower than the amount of consumption; thus, the alkali cannot propagate
forward. To solve this problem, a synthetic surfactant is added because the opti-
mum salinity for a surfactant is high. When the soap and the surfactant are mixed,
the optimum salinity range in which IFT reaches its low values is increased and is
widened [55].