Page 320 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Spontaneous imbibition 293

with those from an earlier work (Sheng, 2013b). Kathel and Mohanty’s
(2013) experimental data also showed that the rate of oil recovery in tight
(10 mD) oil-wet or mixed wet sandstone cores increased with high IFT
by spontaneous imbibition.
Generally, cationics and nonionics can change wettability by maintaining
the IFT higher. Particularly, cationic surfactants can form ion pairs with
adsorbed organic carboxylates of the crude oil and stabilize them into the
oil thereby changing the rock surface to water-wet (Tabatabal et al.,
1993; Stadnes and Austad, 2000; Austad and Standnes, 2003; Xie et al.,
2005). Generally, the IFT between such surfactant solution and oil is not
low (>0.1 mN/m) (Adibhatla and Mohanty, 2008).
However, for the conventional rock, the preceding table shows that the
oil recovery factors for the 20 mN/m IFT cases are not as high as those in
their respective 0.008 mN/m IFT cases, when the wettability is changed
from oil-wet to intermediately water-wet and completely water-wet.

10.5 Effect of interfacial tension (IFT)
It can be seen from the preceding section that the IFT plays an impor-
tant role in imbibition and in wettability alteration. This section further dis-
cusses its role in spontaneous imbibition.

10.5.1 Theoretical and experimental analysis

Capillary pressure (p c ) can be calculated from
2s cos q
p c ¼ (10.19)
r
Here q is the contacting angle, r is the pore radius, and s is the IFT
between a wetting phase and a non-wetting phase. When s is higher, p c is
higher. Then the imbibition becomes stronger, and the imbibition rate be-
comes higher, which generally corresponds to a higher recovery of the non-
wetting phase (oil if the rock is water-wet). This claim can be verified
experimentally by Mattax and Kyte (1962), for example.
Cuiec et al. (1994) reported that the average recovery curves versus time
at different IFTs from oil recovery was higher in the early time as shown in
Fig. 10.7, which is consistent with the simulation results by Sheng (2013b)
who showed that the wettability alteration with higher capillary pressure is
effective in the early time of imbibition. But the oil recovery factors during
late time became lower than those from lower IFT experiments. This is