Page 329 - Enhanced Oil Recovery in Shale and Tight Reservoirs

302 Enhanced Oil Recovery in Shale and Tight Reservoirs

The countercurrent flow and cocurrent flow described above can occur
in spontaneous imbibition for any pair of wetting and nonwetting phases, for
example, in a gas-liquid system.
It has been observed that oil comes from all the faces of the core in the
early time, then comes from the top face of the core only at later time
(Schechter et al., 1994; Chen and Mohanty, 2015). In other words, the capil-
lary force dominates in the early phase, while the gravity plays the dominant
role in the late phase. The gravity works all the time, but the capillary force is
higher in the early time than in the late time, because the capillary pressure
gradient becomes lower in the late time because the imbibition distance
becomes longer, while the capillary pressure at the imbibition front remains
the same. The fact that the capillary pressure plays the important role in
the early time has been verified by the simulation work by Sheng (2013b)
which is represented in Fig. 10.13. Look at the two oil recovery curves
marked by OW þ IFT and WW þ IFT. The OW þ IFT curve represents
the imbibition oil recovery when the wettability is kept oil-wet but a low
water-oil IFT is maintained; the WW þ IFT curve represents the oil recov-
ery when the wettability is changed from oil-wet to water-wet and a low
water-oil IFT is maintained. It can be seen that the oil recovery from
WW þ IFT is much higher than that from OW þ IFT in the early time

50
Only kr altered
WW+IFT
45 IW+IFT
OW+IFT
40
Only Pc altered
35
Recovery factor, % 25
30

20
15
10
5

0
0 20 40 60 80 100 120 140 160
Imbibition time, day
Figure 10.13 Effect of the combined wettability alteration and IFT reduction in spon-
taneous imbibition.