Page 252 - Enhanced Oil Recovery in Shale and Tight Reservoirs

234 Enhanced Oil Recovery in Shale and Tight Reservoirs

0.3 mN/m). Therefore, the additional oil recovery from POA was higher
and faster than that from CAC. Austad and Milter (1997) had the same
observation. To make use of wettability alteration, IFT should not be too
low. But IFT should not be high either, as high IFT cannot reduce residual
oil saturation (thus cannot increase relative permeabilities). There may exist
an optimum combination of IFT reduction and wettability alteration. In
principle, IFT affects the oil rate because it determines the magnitude of
capillary pressure (drive force), and it also affects the ultimate oil recovery
because it determines the capillary number which determines the residual
oil saturation (thus oil recovery).
Chen and Mohanty (2015) studied the synergy of wettability alteration and
IFT reduction on spontaneous imbibition oil recovery by simulation. They
concluded that the effect of low IFT on imbibition oil recovery in the presence
of wettability alteration is not significant, and the low IFT becomes extremely
important when the wettability alteration is not involved in oil recovery, based
on their simulation results shown in Fig. 9.12. However, low IFT should be
important, if any, despite whether the wettability alteration exists or not. As
shown in the figure the oil recovery is about 70% when the IFT is
0.003 mN/m with and without wettability alteration. In Fig. 9.12A,as the
IFT becomes higher, the oil recovery becomes lower, although the wettability
alteration is included, indicating the positive effect or the assistance to IFT pro-
vided by wettability alteration is not significant. This is probably if the IFT
values in this case are not high. In Fig. 9.12B, as there is no wettability alter-
ation, the only effect is from IFT. As the IFT becomes higher, the oil recovery

Figure 9.12 Effects of IFT on oil recovery with (A) wettability alteration or (B) without
wettability alteration (Chen and Mohanty, 2015).