Page 204 - Fundamentals of Enhanced Oil and Gas Recovery

192 Mohammad Ali Ahmadi

Another issue that might occur during surfactant flooding is surfactant partitioning
from injected fluid into the reservoir oil.
The last but not the least concern regarding surfactant flooding is surfactant stabil-
ity in terms of thermal stability and salinity stability. In the case of surfactant flooding
in high temperature or oil reservoirs with high value of salinity, surfactant instability
could damage the reservoir permanently and reduce the oil production drastically. As
a result, before any field application, different experiments should be done to
figure out the stability of the surfactant versus temperature and salinity.

6.2.2 Alkaline Flooding
When an alkaline solution is injected in a reservoir, it reacts with the acid component
of the crude oil and a surfactant (called soap to differentiate it from injected synthetic
surfactants) is generated in situ. Therefore, most of surfactant-related mechanisms such
as reduction of IFT apply to alkaline flooding. Injected alkalis can react with divalents
so that insoluble precipitates are generated. The precipitates reduce permeability; thus
sweep efficiency is improved [28]. Other mechanisms include emulsification, oil
entrainment, bubble entrapment, and wettability reversal [28 30].
Mayer et al. summarized the research and laboratory results on the alkaline reac-
tions with oil, water, and rocks in terms of fundamental theories and mechanisms.
Here new or important findings are summarized [31]. One important alkali oil reac-
tion is to generate soap. It can be understood that if a high enough concentration or
enough amount of alkali is injected, all the acid components in the crude oil will be
converted to soap, as assumed by Delshad et al. and Karpan et al. in their numerical
simulation models [32,33]. However, Sheng did a simulation study and found that
only 25% of acids are converted into soap at a practical injection concentration of
2 wt.%. In other words, under practical conditions, not all acids can be converted into
soap [4]. This result was confirmed by Wang and Gu’s experimental data [34]. Sheng’s
results also showed that the amount of generated soap was small. From a particular
model, the soap concentration was only 0.1%. To be able to generate soap, pH must
21 21
be at least higher than 9.5 [4]. Multivalent cations (such as Ca and Mg ) cause
soap to form greasy, water-insoluble soap curds that are ineffective in recovering oil.
However, the precipitates generated from alkali water reaction may divert water flow
into less-permeability zones, thus improving sweep efficiency [28]. If this mechanism
is designed to be the main objective of alkaline injection, a high divalent content is
beneficial. For this purpose, sodium silicate is better than sodium carbonate or sodium
hydroxide, because the reaction between sodium silicate and calcium or magnesium
will generate highest amount of insoluble precipitants and lead to the highest perme-
ability reduction. Alkalirock reactions are complex depending on minerals and alkalis
[4,29,35].