Page 196 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 179

failure, even without clay swelling inside the rock that initiates the creation of
microfractures.

8.3 Effect of confining stress
The above reported lab results were observed at ambient conditions
(without a confining pressure). Based on the reports in the literature and
our work, no doubt, microfractures can be generated when a clay-bearing
rock contacts with water without confinement. During hydraulic fracturing
the shale reservoir matrix is in contact with the fracturing fluid under
confining conditions, and the shale rock interaction with fracturing fluid
will be influenced by in situ stresses. Therefore, we are more interested
whether microfractures can be formed under confining stresses.
It can be understood that when water enters the inner structure of rock
grains, swelling causes the rock grains to be disintegrated without confine-
ment; with confinement, the swelling pressure increases. Behnsen and Faulk-
ner (2011), Duan and Yang (2014), and Faulkner and Rutter (2000) reported
that with isotropic confining pressure, significant reduction was observed on
clay-bearing rocks or montmorillonite sample permeability measured with
water. Whether or not fractures can be induced to increase permeability
during water imbibition in shale under an isotropic compressive stress remains
controversial.
Onaisi et al. (1993) studied the swelling and swelling-induced fracturing
of cylindrical drained Pierre shale samples in contact with water-based mud
placed in a central cylindrical borehole. When the water activity of the mud
was greater than that of the shale, swelling, large deformation of the well-
bore and fractures appeared, the fracture pattern depending on the confine-
ment conditions. When a sample was not confined, fractures were mainly
radial; and when a sample was confined, fractures were mainly circular,
with some well-developed slip lines around the borehole. When water
activities of mud and shale were in balance, virtually no alteration of the
wellbore was seen. Their borehole configuration can be considered equiv-
alent to the fractured shale condition.
Santos and da Fontoura (1997) also stated that swelling only occurs if the
water equilibrium inside the rock is disturbed, being subsequently contacted
by different fluids. Santos et al. (1997a) observed that dehydrated shale cores
were more reactive to water than preserved cores; wellbore instability was
more caused mechanically (mud weight) than chemical reaction.