Page 205 - Fundamentals of Enhanced Oil and Gas Recovery

Chemical Flooding 193

The acid number or total acid number is the mass of potassium hydroxide
(KOH) in milligrams that is required to neutralize 1 g of crude oil. It is intuitive
that if a crude oil has a higher acid number, more soap will be generated. Then
more oil can be recovered. Therefore, for alkaline flooding to be effective, some
minimum acid numbers may exist. Cooke et al. reported that a number of crude
oils and synthetic oils had been flooded by alkaline water in the laboratory [36].No
oil with an acid number less than about 1.5 had been successfully flooded to a resid-
ual oil saturation much below that for a normal water flood. Therefore, Cooke et al.
proposed a minimum acid number of 1.5 [36]. Sheng proposed a minimum acid
number of 0.3 mg KOH/g oil [37]. However, Ehrlich and Wygal reported that
crude oils with acid numbers higher than 0.1 0.2 mg KOH/g oil or IFT at 0.1%
NaOH less than 0.5 mN/m gave significant caustic-flood oil [38]. There was no fur-
ther correlation of increased oil at higher acid numbers or lower IFTs. Sheng calcu-
lated the reduction in residual oil saturation by alkaline flood at different acid
numbers from the data presented by Ehrlich and Wygal [4,38]. The alkali used was
0.1% NaOH. The calculated data show that these two variables were not correlated.
The current conclusions are that there is no minimum acid number for a successful
alkaline flooding, and there is no relationship between the oil recovery and acid
number [29].
No simple relationship has been observed between the amount of oil recovered
and the measured value of IFT [36,38]. Cooke et al. pointed out that low IFT is
a necessary but not sufficient condition for a successful alkaline flood [36].
However, the alkaline flooding results from Castor et al. showed that the recovery
efficiencies could be better correlated with the stability of emulsions and wettabil-
ity alteration rather than with the IFT [39]. Interestingly, Li’s data showed that
even when the acid number was zero, the IFT decreased with alkaline concentra-
tion [40]. The currently accepted statement is that oil recovery is not correlated to
IFT for alkaline flooding, because other factors like wettability alteration may play
the role [29].
The average values of the parameters that are important to an alkaline process
from the real projects are also listed. Each parameter is analyzed using the method of
rank and percentile, and the average value (median) is taken at 50% percentile. The
most important parameters are formation of water divalent contents, clay contents, oil
acid number, and oil viscosity [29].

6.2.3 Polymer Flooding
The polymer flooding process is defined as an addition of polymer with injected water
to increase the viscosity of the injected water and reduce the mobility ratio between
oil and injected water. It is well known that when the viscosity of polymer solution is