Page 248 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 231

45
40

35
30
Recovery factor, % 25

20
15
10
Base adsorb.
5 2x adsorp.
1/2 adsop.
0
0 20 40 60 80 100 120 140
Time, Day
Figure 9.8 Effect of surfactant adsorption.

pressures, as shown in Fig. 9.9. It shows that the oil recovery factors corre-
lated with contact angles better than with IFTs; with a lower contact angle,
the recovery factor was higher; but the recovery factors did not show a cor-
relation with the IFTs. It implies that the wettability alteration effect of a sur-
factant solution is more important than the IFT reduction effect.

9.5.3 Effect of IFT on spontaneous oil recovery with and
without wettability alteration
As discussed earlier, the dominant mechanism of wettability alteration is to
change the capillary pressure. The capillary pressure is directly proportional
to IFT. Therefore, the IFT reduction actually mitigates the effect of wettability
alteration, and the oil rate or oil recovery is reduced, as shown in Fig. 9.10.In
the figure, the incremental oil recovery is defined by the oil recovery with
wettability alteration minus the oil recovery without wettability alteration
for the same IFT value. Thus, the incremental oil recovery represents the
oil recovery due to the wettability alteration mechanism only. As the IFT
becomes lower, the oil recovery due to wettability alteration is lower.
Fig. 9.11 shows that a high IFT does increase oil recovery without wetta-
bility alteration. This result is consistent with the theory and it also confirms
that the used simulation model correctly captures the mechanisms. A surfac-
tant may reduce IFT and change wettability. IFT reduction can increase oil