Page 211 - Enhanced Oil Recovery in Shale and Tight Reservoirs

194 Enhanced Oil Recovery in Shale and Tight Reservoirs

8.8 Further discussions and summary of views and
hypotheses

From the above discussions, we can conclude that if a clay-bearing rock
is not confined, microfractures may be generated. More microfractures may
be generated owing to shear failures under anisotropic stress conditions
than under isotropic stress conditions. Fracture opening, closing, reopening,
and reclosing are a dynamic process, as the hydration conditions are changed.
The views on the interactions of water-clay bearing rock are not agreed in
terms of microfractures or permeability. Clays are fine crystalline particles
with two-dimensional arrays of either silicon/oxygen tetrahedral or
aluminum (or magnesium)/oxygen hydroxyl octahedral. Clays can be classi-
fied into five categories: montmorillonite, illite, kaolinite, chlorite, and atta-
pulgite (van Olphen, 1977). All natural clays have hydrophilic surfaces that
can adsorb water and some ion species. Hydration of clays in a suspension
brings about a repulsive force between particles. In addition, the water being
adsorbed on clays in a formation dissolves the existing salts and results in the
increase of swelling pressure, which causes the particles clusters to be dispersed
in the water medium. Therefore, when water contacts swelling clay, the rock
will swell. But whether the swelling will generate microfractures or reopen
existing natural fractures remains unconfirmed. Groisman and Kaplan
(1994) studied the influence of the bottom friction on the size and pattern
of created fractures during desiccation experiments. Fig. 8.20 shows the
formed fractures. In their experiment, rock samples lost water and
contracted, so that tension was reduced, and fractures were created. Such
experiments are opposite to water imbibition and rock swelling. From their
experiments, swelling may not create microfractures.

Figure 8.20 Pictures of experiment showing fracture creation during desiccation:
(A) glass plate uncoated, (B) bottom coated with 2 mm of grease, and (C) bottom
coated with 6 mm of Vaseline (Groisman and Kaplan, 1994).