Page 236 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 219

water-wet case is higher than that in the oil-wet case; the water relative
permeability has the opposite behavior. Therefore, a water-wet system is
more favorable to oil recovery than an oil-wet system. This is another mech-
anism of wettability on oil recovery.
From the above listed mechanisms, a strongly water-wet system should
be preferred for oil recovery. However, early researchers found that the
highest waterflooding oil recovery occurs at intermediate wetting conditions
(Moore and Slobod, 1956; von Engelhardt and Lubben, 1957; Kennedy
et al., 1955; Loomis and Growell, 1962; Morrow and McCaffery, 1978),
as presented by Tiab and Donarld (2004) in Fig. 9.5.
Alhammadi et al.’s (2017) microscale study of waterflooding performance
showed that the optimal recovery was obtained for a rock that appeared
neither strongly water-wet nor strongly oil-wet at the pore scale. In strongly
water-wet pores, snapoff in small pores traps oil; in strongly oil-wet pores, oil
is confined to layers which flow too slowly to provide significant oil recovery.
Jadhunandan and Morrow (1995) core-scale experiments also showed such
result. More generally, how wettability affects oil recovery or gas recovery
is process-dependent. For example, in depleting condensate reservoirs, one
may think gas wetting is not preferred to maximum gas production. Howev-
er, fluorocarbon surfactants are used to alter the rock wettability from liquid
wetting to favorably gas-wetting condition, because the condensate saturation
trapped in the near wellbore region can be reduced to mitigate the condensate
blockage (Sharma et al., 2018).

0.5

.4

.3
S or
.2

.1

0
–1.0 0.0 1.0
Wetting index (cos )
Figure 9.5 Ultimate oil recovery as a function of wettability index (Tiab and Donarld,
2004).