Page 208 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 191

through some water-wet pores. In addition, she reported higher imbibition
oil recovery in low-salinity water than in high-salinity water. Low-salinity
water will have higher reactivity, resulting in more swelling. Then most
likely, more fractures would be created, if swelling could. But her data
did not indicate swelling created fractures or resulted in higher permeability.
Clay swelling may decrease matrix permeability and natural fracture
permeability, but induced fracture may result in permeability increase.
Whether the permeability is increased or decreased depends on the balance
between the two factors, as Singh (2016) described in Fig. 8.16.

8.7 Effect on rock mechanical properties
Hydration swelling due to water imbibition can weaken the mechan-
ical strength of shale (even more than 60%) (Wong, 1998; Al-Bazali, 2013;
Cheng et al., 2015), and it can reduce the shear-induced fracture conductiv-
ity (Pedlow and Sharma, 2014; Jansen et al., 2015).
Zhang and Sheng (2018) used experimental setup shown in Fig. 8.8 to
find the stress difference (s 1 e s 2 ) required to generate shear-induced frac-
tures in a shale rock before and after water imbibition under some isotropic
pressure. The stress differences required were 58.7, 40, and 24.4 MPa,
respectively, for Mancos cores after water imbibition without confining
pressure, at the confining pressure of 20 and 2 MPa. When the isotropic
confining is zero (at an atmospheric condition), no stress difference was
required. See Fig. 8.17. The data show that the rock was more difficult to
fracture under water imbibition at a higher isotropic confining pressure.
These data also suggest that refracturing would be easier as the rock was
already imbibing water.

Figure 8.16 Schematic to describe permeability change due to water imbibition for
(A) matrix and natural fractures, (B) microfractures, and (C) the whole rock sample
(Singh, 2016).