Page 238 - Fundamentals of Enhanced Oil and Gas Recovery

226 Mohammad Ali Ahmadi

7.6.2.6 Salt-in Effect
The solubility of the organic material in water can be drastically decreased by adding
salt to the solution, that is, the salting-out effect, and the solubility can be increased
by removing salt from the water, that is, the salting-in effect [54]. Therefore, a
decrease in salinity below a critical ionic strength can increase the solubility of the
organic material in the aqueous phase, so that oil recovery is improved [45].

7.6.2.7 Osmotic Pressure
Sandengen and Arntzen [71] demonstrated using experiments that oil droplets acted
as semipermeable membranes; oil droplets could move under an osmotic pressure gra-
dient. They proposed that such osmotic gradient relocate oil by expanding an other-
wise inaccessible aqueous phase in a porous rock medium. This mechanism cannot
explain the need for the existence of crude (polar) oil and clays [45].

7.6.2.8 Wettability Alteration
As mentioned earlier, brine films are more stable at a lower salinity. This suggests that
LS water will cause cores to become mixed-wet (less water-wet). Mixed-wet cores
show lower residual oil saturation or higher oil recoveries than strongly water-wet or
oil-wet cores [72,73]. Buckley et al. [70] explained wettability alteration as a result of
the interaction between crude oil and reservoir rock. Berg et al. [74] experimentally
showed that LS water could achieve wettability alteration. Nasralla et al. [75] showed
that LS water could decrease contact angles. Yousef et al. [76] and Zekri et al. [77]
reported that LS water injection could change wettability to more water-wet in car-
bonates. Vledder et al. [78] even provided a proof of wettability alteration in a field
scale [45].
Drummond and Israelachvili [79] showed that the wettability was altered from oil-
wet to water-wet at pH . 9 and from water-wet to intermediate-wet at pH , 9, as
shown in Fig. 7.9. Fig. 7.9 depicts the wettability map as a function of pH and Na 1
concentration. In LS waterflooding, pH is most likely below 9. This can also explain
why connate water is needed for the LS effect because the existence of connate water
makes water-wettability possible. The wettability alteration is the most frequently sug-
gested mechanism [45,53].

7.6.3 Field Tests of Low-Salinity Waterflooding

Four different SWCTT were carried out in Alaska North slope; using low salinity
water injection increased the oil production rate in all the tests [44,45].
Another field test of low salinity water injection was performed in Endicott off-
shore oil field located on the North Slope of Alaska. Field observations show the sig-
nificantly reduction in water cut after using low salinity water injection process [80].