Page 213 - Enhanced Oil Recovery in Shale and Tight Reservoirs

196 Enhanced Oil Recovery in Shale and Tight Reservoirs

layeredpatternmade fromtetrahedral(T)silicatesheetssurroundingoctahedral
(O) aluminumsheetshave layers approximately 10 Å thick. The clays have the
tendency to expand up to 20 times their original volume (Park et al., 2016)
and a high swelling pressure exists due to water adsorption on smectite clays.
The amount of smectite clays in shale is the major factor influencing swelling
stress. Wang and Rahman’s (2015) water leak-off model shows that the total
water flux into shale matrix during a hydraulic fracturing treatment is
controlled by capillary pressure, osmotic pressure, and hydraulic pressure of
different solid components. The invaded water volume increases with the
increase of clay content and the decrease of organic matter.
As reported by Wong (1998), Zhang et al. (2016), Al-Bazali et al. (2007),
and Al-Bazali (2013), hydration swelling due to water imbibition can
weaken the mechanical strength of shale (i.e., water-weakening effect).
Therefore, hydration softening of shale can possibly be used to enhance
the shear failure of shale rocks and the generation of fractures.
Fu et al. (2004) studied the heterogeneity effect on fracture generation
when the temperature was elevated. They observed that heterogeneity pro-
moted fracture generation during temperature alterations (Fig. 8.21). Different
materials have different thermal expansion behaviors. Raising or reducing
temperature creates strain contrasts between the two adjacent materials.
The difference in strain may create internal shear stresses and promote fracture
generation. Shale is typically heterogeneous and laminated and contacts clays
which have different swelling properties from other minerals. Based on the

Figure 8.21 Fractures formed between two adjacent materials because the difference
in their strains created internal shear stress and promoted fracture creation (Fu et al.,
2004).