Page 321 - Enhanced Oil Recovery in Shale and Tight Reservoirs

294 Enhanced Oil Recovery in Shale and Tight Reservoirs

0
Oil recovery, % OOIP 0 σ, mN/m 41 14 10 6.6 3.7 IMBIBITION IN AQUEOUS SOLUTION
0

- Mantes chalk
0
- Conjugate phases of the ternary
1.5
0.8
system : n-C / Et OH / Brine
0
- Mantes chalk
- T = 20°C 6
0 Extrapolations
0
0 10 1 10 2 10 3 10 4 10 5
Time, min.
Figure 10.7 Average recovery curves for various IFTs (Cuiec et al., 1994).

because the gravity segregation played an important role at a longer time
when the IFT was low. This also implies that the gravity effect is slower
than the capillary effect in low-permeability porous media. Such results
are in agreement with those from Schechter et al. (1991) for low-
permeability samples. Austad and Milter (1997) used surfactants to reduce
the oil-water IFT to conduct imbibition tests on chalk. The imbibition
rate was lower than that of water.
Cuiec et al. (1994) also showed that after a higher IFT fluid system was
used, a subsequent lower IFT fluid system resulted in an increase in oil re-
covery. Their interpretation was that there was a capillary pressure threshold
caused by the contact angle hysteresis and short oil ganglia. A high IFT sys-
tem caused the raised threshold and at some time, oil ganglia movement
stopped, and no further oil could be recovered. Note that in Cuiec et al.’s
experiments, the low IFT values of 0.8 and 1.5 mN/m might not be low
enough to cause the significant increase in capillary number or the significant
decrease in residual oil saturation. In other words, the increase in oil recov-
ery may not be caused by capillary desaturation.
Wang et al. (2015b) reported that the imbibition oil recovery increased as
the IFT became lower, but decreased as the IFT became further lower, as
shown in Fig. 10.8. They interpreted that the increase of the interfacial ten-
sion enlarged the emulsified oil droplets, which increased the resistance to
displace oil by gravity segregation. They concluded that there existed an
optimal IFT that led to the highest imbibition recovery. The exact core