Page 278 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 255

from the above equations (Siebold et al., 1997). The thin layer wicking
method is also based on the same principle. In this method, glass slides
covered with particles are vertically immersed in a liquid and the height of
the rising liquid is measured using a video camera (Van Oss et al., 1992).

9.8.3 Spontaneous imbibition method
For shale and tight rocks, the wettability is commonly determined by
measuring contact angles and spontaneous imbibition volumes of water
and oil. The principle is that water imbibition volume will be higher if
one rock is more water-wet than the other (Zhou et al., 2000). The rock
is more water-wet if more water is imbibed than oil. Similarly, if the
water-wetting (contact) angle is smaller, it is more water-wet. But wetting
angle must be measured in a system of liquid-liquid not liquid-gas, as dis-
cussed later. Lan et al. (2015a) defined the wettability index of water
(WI w ) and the wettability index of oil (WI o ):
V w1
WI w ¼ (9.40)
V w1 þ V o2

V o2
WI o ¼ (9.41)
V w1 þ V o2
where V w1 is the normalized water volume imbibed into the dry plug 1, and
V o2 is the normalized oil volume imbibed into the dry plug 2; the plug 1 and
plug 2 are twin plugs. The normalized volumes are calculated by dividing
the final equilibrium volumes of water and oil by their plug pore volumes,
respectively. These normalized volumes V w1 and V o2 are actually imbibition
saturations S w1 and S o2 , respectively. In their experiments, the core plugs
were set vertically, and the bottom face of each plug contacted with
imbibing water or oil. Then the water gravity and oil gravity are different in
the experiments. Strictly speaking, the plugs should be set horizontally.
Note that it is implied in the literature that a higher imbibition volume
corresponds to a higher imbibition rate when the wettability is evaluated us-
ing water and oil for imbibition. This may not be necessarily true. When the
imbibition rate is compared, we need to consider the differences of fluid vis-
cosities and surface tensions of water and oil. Therefore, we propose to
conduct countercurrent imbibition experiments. For a twin plug initially
saturated oil, measure the imbibed water volume (saturation); for the other