Page 314 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Spontaneous imbibition 287

Figure 10.3 Surfactant phase saturation for 122 mD and 0.24 porosity at 20 days.

surfactant cannot diffuse into the low-permeability rock as fast as in the
high-permeability rock, although the capillary pressure is 412 times higher.
A higher capillary pressure means higher imbibition force, and thus higher
imbibition rate. However, there is another force: viscous force. We need to
consider these two forces together.

10.3.2 Theoretical considerations
According to Eq. (10.1), the imbibition velocity into a smaller pore is lower
than that into a larger pore. A core of smaller radii has lower permeability as r
p ffiffiffiffiffiffiffiffi
is proportional to k=f. Although the capillary force is high in a shale core,
the viscous force is also high; the resultant effect of two forces makes the
imbibition velocity in a low-permeability rock lower than that in a high-
permeability rock.
Note that Eq. (10.1) ignores the slip flow. For the slip flow to take place,
the capillary diameter needs to be smaller than approximately 3 nm (Sharp