Page 328 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Spontaneous imbibition 301

10.8 Effect of viscosity ratio
Generally, the wetting phase viscosity (m w ) is used and the nonwetting
phase viscosity is ignored. Ma et al. (1997) included the effect of nonwetting
p
phase (oil) viscosity (m nw )by using m m . However, many experimental
ffiffiffiffiffiffiffiffiffiffiffiffiffi
w nw
studies (Behbahani and Blunt, 2005; Fischer and Morrow, 2006; Fischer
et al., 2006) and numerical results (Behbahani and Blunt, 2005) did not
support that such treatment could be general. Wang et al. (2015b) observed
in laboratory that as the oil viscosity was lower, the oil recovery from spon-
taneous imbibition was higher. Makhanov et al. (2014) found that the imbi-
bition rate of xanthan gum solution was significant despite its high viscosity.
This suggests that water uptake is mainly controlled through preferential
adsorption of water molecules by the clay particles, and high viscosity of
the solution can only partly reduce the imbibition rate.

10.9 Effect of initial water content
If the initial water saturation S wi is higher than the connate (immobile)
water saturation S wc , the capillary pressure will be lower but the water (wet-
ting) phase mobility will be higher. They affect the imbibition in different
directions. Most of experiments were done at S wc or S wi ¼ 0 so far.

10.10 Countercurrent flow versus cocurrent flow

In a case of water-wet core which has a high initial oil saturation, the
space between the outside core and the wall of the imbibition cell is full of
water. The water and oil phase pressures in the imbibition cell is the same,
but the oil phase pressure inside the core is higher because the capillary pres-
sure is positive. Then oil will flow from the inside of the core to the outside.
Meanwhile the water outside the core will flow toward the inside. Thus
countercurrent flow occurs by the capillary force. In laboratory, oil can be
seen to flow out of the core from all the faces of the core.
In the above system, if chemicals, a surfactant as an example, are added in
the water so that the water-oil interfacial tension is reduced, the water-oil
capillary pressure is significantly reduced, and the gravity force can over-
come the capillary force. Then water will push oil from the bottom of
the core. Both oil and water will flow out of the top face of the core in
the same vertical direction. This flow is called cocurrent flow.