Page 162 - Primer on Enhanced Oil Recovery

152 Primer on Enhanced Oil Recovery

reservoir with water, i.e., hydrophilize the rock. In this case, the approaching wet-

ting angle in the sandy rock can be reduced from 70 to 10 . When the wettability
of a porous medium changes, two cases should be considered.
when displacement occurs in a hydrophobic (oil-wet) reservoir, where residual oil is a

continuous phase, the alkaline agent changes the pH of the injected water, and the wetta-
bility of the rock changes from hydrophobic to hydrophilic. As a result, the coefficient of
mobility of the displacement front (injected liquid) decreases, which contributes to an
increase in oil recovery.
even in hydrophilic reservoirs under certain conditions (the reservoir temperature, pH and
salinity of the alkaline solution) an intermittent, non-wetting residual oil phase can
become a wetting continuous phase. The presence of water droplets in the continuous
phase in a hydrophobic formation increases the pressure gradient. The increased pressure
gradient allows to displace more oil from smaller pores and the residual oil saturation
decreases.
In inhomogeneous formations, emulsification of oil and its capture by small
pores, contributes to the deviation of the flow of alkaline solution to areas of the
water not covered by the simple waterflood. This significantly increases the areal
sweep efficiency and leads to a significant increase in oil recovery.
The presence of salts in the solution significantly changes the process of inter-
action of oil with an alkaline solution. The presence of calcium chloride increases
interfacial tension, reducing the performance of alkaline flooding. The presence
of carbon dioxide also leads to reduction the solution activity by to the formation
of soda ash, which does not allow to significantly reduce the interfacial tension.
Sodium chloride has a positive effect on alkali activity. It allows, other things
being equal, to significantly reduce the alkali concentration in the alkaline
solution.
The presence of clay in the rock has a significant effect on alkaline flooding.
Clays have a negative impact on the process of alkaline flooding. This is mostly
produced by the process of the ion exchange. The process leads to a decrease in
pH. Alkali adsorption is also different in different rock formations with clays (see
Fig. 12.9). As can be seen, on pure quartz sands, the adsorption is absent, but it is
very high in montmorillonites and especially anhydrites.
Clays swell in the solution of alkali. Some clays, like montmorillonite,
increase their volume more than two folds. Clays also start to produce fines
small clay particles. In effect, the presence of clay significantly affects the
efficiency of alkaline flooding. Accordingtothe experimental studies, whenthe
content of clays of the montmorillonite group is more than 20%, the anhydrous
oil displacement rate with an alkaline solution is the same as with water displace-
ment, while the anhydrous displacement ratio increases due to swelling of the
clays, which ensures uniform displacement. In contrast to sand in carbonate reser-
voirs, the effectiveness of alkaline flooding is associated with the presence of
nitrogen-containing compounds in oil. Moreover, the isolation of these com-
pounds on the surface of the carbonate rock contributes to its hydrophilization
with all the following processes.