Page 62 - Fundamentals of Enhanced Oil and Gas Recovery

50 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

Figure 2.1 Mechanism of IFT reduction by in situ anionic surfactant.

Nowadays, alkali is less popular due to the complication it causes in field [42,43].
Divalent cations existing in clays and water interact with alkali and precipitates (scale
problems) and causes formation damage. It is worthwhile to note that large amount of
alkali is consumed during this reaction. Moreover, the emulsion mechanism that is
supposed to assist the alkali to increase oil recovery usually forms stable emulsion that
intensively increases operating and capital cost of project due to the required treatment
on the produced fluids using surface facilities. Therefore, alkali is mostly combined
with polymer and surfactant to obtain greater oil recovery factor.

2.3.4 Combination of Chemical Methods
2.3.4.1 Alkaline Polymer Flooding and Alkaline Surfactant Flooding
Mobility control is achieved due to the presence of polymer in the solution and alka-
line generates in situ surfactants in alkaline polymer (AP) flooding. On the other
hand, the interaction between both chemicals has known to reduce the viscosity of
polymer solution. However, polymer reduces the amount of alkaline consumption in
the well. AP flooding was implemented on few pilot trail and field cases with low vis-
cosity oil [44 46], and some were not considered as an economical treatment and
scale problems were observed. David Pool in Canada and Xing Long Tai in China are
examples of AP flooding that were reported in the literature [47,48].
Less pilot studies were conducted with alkaline surfactant (AS) flooding since the
mobility control is a crucial fact that is missing in this treatment. Moreover, salinity of
the system increases as the alkaline is added to the surfactant solution and changes the
optimum-salinity of the surfactant-alone sample. When the salinity of the system
exceeds the optimum value, the IFT will not be at its lowest values, and when the
salinity is lower than the optimum, the system requires salt addition to achieve the
best results. It is noted that AS flooding is only used in sandstone reservoirs.
Generally, AP and AS are not as popular as the other chemical combinations; there-
fore, there is no specific screening criteria developed or reported for them, and the limita-
tions mentioned earlier for AP and AS methods should be considered in these methods.