Page 319 - Enhanced Oil Recovery in Shale and Tight Reservoirs

292 Enhanced Oil Recovery in Shale and Tight Reservoirs

time for the surfactant imbibition to alter wettability; it takes 1 million days
to reach 42.2%.
The above table also shows that if the oil-wetness is gradually altered to
completely water-wetness by surfactant, the shale rock can have an oil
recovery of 13%, whereas the conventional rock has 46.9%. The former
case has much lower oil recovery because the surfactant solution has to
slowly imbibe into the rock to change the wettability. In other words, if a
shale or tight reservoir is initially oil-wet, it is not an effective method to
use surfactant to change wettability. Note that the water-oil interfacial ten-
sion (IFT) is ultralow (0.008 mN/m).
A surfactant may only change wettability but not reduce IFT. Such sur-
factants are generally cationic and nonionic surfactants (Sheng, 2013a). In
such a situation, the water-oil IFT is maintained at 20 mN/m, but the
wettability is altered to intermediately water-wet and water-wet completely
by the surfactant. The simulation results for these two cases are also shown in
the above table. When the oil-wetness is changed to intermediate water-
wetness, the oil recovery from the shale core is 0.01% when the IFT equal
to 0.008 mN/m, but it is 5% when the IFT is 20 mN/m. When the oil-
wetness is changed to complete water-wetness, the oil recovery from the
shale rock is 13% when the IFT is equal to 0.008 mN/m, but it is 41.1%
when the IFT is 20 mN/m. These results show that when the wettability
is altered, the IFT should be higher for a shale rock. To make this conclusion
visually clear, the oil recovery curves for the case of IFT ¼ 0.008 mN/m by
1 million days and for the case of IFT ¼ 20 mN/m by 138 days are
compared in Fig. 10.6, when the shale rock wettability is changed from
initially oil-wet to water-wet by surfactant. Such results are consistent

Figure 10.6 Comparison of oil recovery curves of different IFT values when the shale
wettability is changed from initially oil-wet to water-wet by surfactant.