Page 163 - Primer on Enhanced Oil Recovery

Chemical EOR 153

Figure 12.9 Impact of clay type on alkaline adsorption.

12.2.1 Alkaline flooding applicability criteria
Typically, alkaline flooding begins with pre-injection (preflush)ofsoftenedwater
(water with very low mineral content, especially with very low content of calcium
and magnesium) followed by injection of the plug of an alkaline solution. The
slug volume is at around 10 30%PV. The alkali slug is pushed (driven) through
the formation by the injected after the alkali water. In this case, as a rule, alkaline
flooding occurs after ordinary flooding, since It is designed to remove residual oil
from the surface of the rock due to created low interfacial tension and emulsifi-
cation of oil. This method is not intended to be carried out at the early stages of
field development due to the low potential for sweep efficiency increase (see
Fig. 12.10).
The following needs to be taken into the consideration

Gypsum. Reservoirs with layered anhydrite content of more than 0.1% should be rejected
as candidates. This is due to the high adsorption of alkali from the alkaline solution.
Kaolinite. In fields with a high content of kaolinite, alkaline flooding can be carried out
with alkalies with a low pH (8.2 10).
Montmorillonite. Montmorillonite, due to its high surface area and cation-exchange abil-
ity, can absorb most of the alkali introduced due to adverse precipitation reactions. High
montmorillonite rich collectors are generally not suitable for alkaline flooding.
Sandy formations are most preferred for alkaline flooding. Carbonate rocks are less pre-
ferred because alkalies with high pH react with carbonates.
The CO 2 content in the oil reservoir is currently considered as an important selection cri-

terion. Reservoirs with a high content of CO 2 , and the presence of formation waters with
a pH of less than 6.5 are not good candidates for alkaline flooding.
As noted above, in general, reservoir oil must have a high acid number to
achieve low interfacial tension under alkaline flooding especially in the case when
the injected solution does not contain a synthetic surfactant. Nevertheless, even in
fields containing low-active oil, alkaline water-flooding can be successful as a
result of the manifestation of other, in addition to a decrease in the interfacial ten-
sion, oil displacement mechanisms.