Page 327 - Enhanced Oil Recovery in Shale and Tight Reservoirs

300 Enhanced Oil Recovery in Shale and Tight Reservoirs

Using the base shale model, the effect of gravity is also investigated by
changing oil density. The oil and water densities in the base model are
3
0.88 and 1.0 g/cm , respectively. A low IFT of 0.008 mN/m and a high
IFT of 20 mN/m are studied. Table 10.4 shows that at the IFT of
0.008 mN/m, no oil can be recovered by 138 days, when the oil density is
3
0.88 or 1.0 g/cm ; and the incremental oil recovered from the gravity is about
8.7% (¼ 22e13.3) by 1.33 million days. At the IFT of 20 mN/m, the oil
3
recovered is 5% by 138 days, when the oil density is 0.88 or 1.0 g/cm ;
the incremental oil recovered is 5.5% (¼ 33.5e28) by 1.33 million days.
From these data, we can see that within a realistic time, the gravity cannot
play an important role in increasing oil recovery. To realize the gravity effect
on improving oil recovery, a very long time is needed (millions of days for the
small shale core).
Using the parameter values for the small shale core, the calculated Bond
number is 4.41 10 11 for the IFT equal to 0.008 mN/m:
2
3 19
Drgk 120kg m 9:8m s 3 10
N B ¼ ¼ ¼ 4:41 10 11
6
s ð8 10 Þ
14
And it is 1.764 10 if the IFT is 20 mN/m. These data imply that in
a shale reservoir, the gravity cannot play an important role in fluid flow
compared with the capillary force. When the permeability is too low in shale
and tight formations, the gravity cannot overcome the flow resistance
(viscous force); a high IFT and water-wetness are needed to lead a high
capillary drive force to displace oil out. In such a situation, the gravity is
not a dominant factor; and the higher the IFT, the higher the oil recovery
by spontaneous imbibition will be.

Table 10.4 Effect of gravity on RF.
By 138 days By 1.33 million days

IFT ¼ 0.008 mN/m
RF at oil density 0.88 0.01 22
RF at oil density 1.0 0.01 13.3
IFT ¼ 20 mN/m

RF at oil density 0.88 5.0 33.5
RF at oil density 1.0 5.0 28