Page 203 - Enhanced Oil Recovery in Shale and Tight Reservoirs

186 Enhanced Oil Recovery in Shale and Tight Reservoirs

resulting images are the subtraction of the CT numbers after and before test.
For EF-1, it was under isotropic stresses. No fractures were visible for both oil
and water imbibition tests. For EF-2, it was under anisotropic stresses, no frac-
tures were visible either, probably because the stress difference was not large
enough. Compared with EF-1, there were more darker points indicating
more pore space. For EF-4, the difference between two stresses was larger.
One visible fracture was closed during the oil imbibition test, but it reopened
and grew during the water imbibition test. For EF-7, although the axial stress
and the confining stress were the highest, they had the same value, and no
fracture was visible at the end of water imbibition. Two conclusions can be
made from these tests: (1) it is easier to generate fractures by water than oil;
(2) it is easier to generate fractures under anisotropic stress conditions than
isotropic stress conditions.

8.4 Effect of bedding

Bedding or lamination is ubiquitous in shale rocks. It has been observed
that fractures prefer to form along the beddings (Moradian et al., 2017; Liu and
Sheng, 2019). Fluid imbibition is faster in the direction parallel to the lamina-
tion than that is perpendicular (Ghanbari and Dehghanpour, 2015). Guo et.al.
(2012) analyzed the drilling performance of more than 200 Eagle Ford shale
wells drilled by 31 operators in 22 countries from 2008 to early 2011. They
found that the water-based mud-shale interaction resulted in fracturing and
delamination along the bedding and enlargement of preexisting fractures.

8.5 Effect of existing natural fractures

In contrast to conventional propped hydraulic fracture treatments, water
fractures rely on reactivation of natural fractures to induce permanent shear
induced dilation, which enhances reservoir permeability (Chen et al., 2000;
Weng et al., 2015). Hydraulic fracturing is performed where shear failure is
anticipated to dominate (Zoback et al., 2012) in shale under anisotropic stress.
In the core EF-4 in Fig. 8.12, the existing fractures after water imbibition
were propagated and connected to form a longer fracture. Lei et al. (2017)
also observed that new fractures could propagate from the tips of natural
fractures driven by tensile failure and connection between other small frac-
tures nearby.