Page 214 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 197

Schmitt et al. (1994) laboratory analysis and the numerical results of Wang and
Rahman (2015) water leak-off model, the first shale microfailure mode is the
tension crack mode. Another failure mode is the sliding crack between the
interfaces of different solid components, which can be represented similarly
by Fig. 8.21. Before fracture treatment, the stress state of shale is in equilib-
rium. After water leak-off into the shale matrix, an additional pressure is
induced to the pore in each component. Because the pore pressures are
different now in each component, the effective stress of each component is
different, and this difference will result in a slip at the interface (shear failure);
thus microcracks or fractures are generated. Rahman et al. (2002) also stated
that the offset of two rough interfaces due to shear stress perturbation can
greatly increase the permeability of the reservoir. From this point of view,
swelling due to water imbibition may create microfractures.
As mentioned earlier, it has been observed that fractures can be created
without confinement in laboratory. In an organic-rich shale reservoir, the
tensile fractures induced by water imbibition are possible, because
the organic matter may provide the displacement space, and the capillary
pressure and clay swelling pressure may lead to tensile stresses to exceed
the tensile strength to grow the fractures (Yang et al., 2015). They further
used a discrete particle model to explain the fracturing mechanisms in
different boundary conditions. Without confinement, shale matrix expands
freely, and disordered microscopic failures occur (Fig. 8.22A). The fracture
networks will be able to grow and cause macroscopic failures. Under
confinement, the displacement space is provided by organic materials, but
tensile failures or created fractures will be smaller (Fig. 8.22B). Under
confinement in a standard triaxial test, the cracks will grow in the direction
of maximum stress S 1 , leading to the global failure plane (Fig. 8.22C). This
feature differs largely from conventional rocks.
Even if microfractures are formed, these microfractures may not form a
network; therefore, whether the rock permeability is enhanced remains
questionable. Apparently, using clay stabilizers in fracturing fluids is more
practiced in the oil industry. Typical clay stabilizers are choline chlorite

þ
((CH 3 ) 3 NCH 2 CH 2 OHCl), KCl, and TMAC ((CH 3 ) 4 N Cl often abbre-
þ

viated further as Me 4 N Cl , tetramethyl ammonium chloride).
8.9 Effect of low-pH and carbonated water
Shale is generally considered to be composed of material that has little or
noacidsolubility.Clay, finequartz, and organicmaterial make upthe “default”