Page 191 - Enhanced Oil Recovery in Shale and Tight Reservoirs

174 Enhanced Oil Recovery in Shale and Tight Reservoirs

(reduce formation permeability). However, it was observed that the water
may help to generate microfractures or open existing microfractures in shale
formations if no confining pressure is applied (Dehghanpour et al., 2013;
Morsy et al., 2013a-c, 2014a-b, Morsy and Sheng, 2014a).
We studied the effect of water hydration swelling on microfracture gen-
eration without confining pressure and observed that shale rocks could be
fragmented or cracked. Fig. 8.1 shows that Mancos shale samples had cracks
and became fragmented by different degrees depending on the salinity. At 5%
and 10% NaCl, the rock samples were fragmented; however, when the
concentration was above 15%, the sample had fewer cracks. Imbibition tests
showed that higher oil recovery was obtained as the salinity concentration
decreased (Morsy and Sheng, 2014a).
In Fig. 8.2, Barnett shale samples showed clear fractures parallel to the
bedding when immersed in distilled water. The imbibition oil recovery
was enhanced, and the measured permeability was significantly increased
owing to the fractures generated.
Fig. 8.3 shows that the Mancos sample exposed to fresh water during
spontaneous imbibition was most sensitive to fresh water as it was highly
damaged due to severe hydration; the Barnett sample showed several cracks
when exposed to fresh water (distilled water); there were cracks on the
Marcellus shale, although they are not clearly seen in the figure; the Eagle
Ford sample was least sensitive to water salinity with no cracks seen (Morsy
and Sheng, 2014a).

Figure 8.1 Mancos rock samples in 5%, 10%, and 15% NaCl solutions.