Page 305 - Enhanced Oil Recovery in Shale and Tight Reservoirs

278 Enhanced Oil Recovery in Shale and Tight Reservoirs

(Alvarez et al., 2018). But those cores were aged for 4 weeks or 6 months.

Eagle Ford shale cores were also found oil-wet (179 ), after 1 day of aging

in brine followed by 7 days of aging in oil at 80 C (Mohanty et al., 2017).
Apparently, shale rocks were reported oil-wet or mixed wet (Odusina et al.,
2011; Akbarabadi et al., 2017). However, it is possible that data were mis-
interpreted, as discussed in the preceding section.
Some authors stated that the wettability cannot be determined by
measuring contact angles, because the measured contact angles are not
consistent with imbibition experiments (Xu and Dehghanpour, 2014;
Ghanbari and Dehghanpour, 2015). The inconsistency can be explained
by the preceding discussions in this chapter.
Several authors (Odusina et al., 2011; Dehghanpour et al., 2013; Makha-
nov et al., 2014) observed that more water than oil imbibed into shales.
They ascribed this phenomenon to absorption of water molecules by clays,
as water adsorption may generate microfractures and thus increase sample
permeability. But, those microfractures are generally generated in experi-
ments when no confining pressure is applied. Under confinement, created
microfractures may not be generated or may be closed later (Zhang and
Sheng, 2017a,b; Zhang et al., 2017). However, it has been observed that
more microfractures are generated under water imbibition than under oil
imbibition (Makhanov, 2013). It is possible that more water imbibed may
be caused by more microfractures generated under the experiments without
confinement. Of course, it can be caused by water-wetness. Singh (2016)
reviewed some theories that discuss the effect of liquid droplet size, and
he stated this effect must be considered to determine wettability. Marmur
(1988) proposed that if the water drop radius r w satisfies the following con-
dition, water can imbibe into oil-wet shares:
r c
r w < (9.67)
cosq
where r c is the capillary radius, and q is the macroscopic contact angle.
Several researchers (Habibi et al., 2016; Yassin et al., 2017) including us
observed that oil spread on shale rock surfaces in the presence of air, indi-
cating oil-wetness. However, for the same rock samples, it was observed
that the water contact angles in the presence of air could be acute, indicating
water-wetness. The actual wettability could be opposite!