Page 193 - Enhanced Oil Recovery in Shale and Tight Reservoirs

176 Enhanced Oil Recovery in Shale and Tight Reservoirs

Eagle Ford and Barnett recovered 20% and 24%, respectively. The cracks
were induced over time in Barnett samples when exposed to the distilled
water. As a result, more oil was recovered. Although no fractures were
visually seen in the Eagle Ford sample, the high oil recovery factor was
obtained. It is believed that the Eagle Ford sample had better-connected
pores. Marcellus sample showed the lowest recovery of about 2% somehow.
It can be seen that the imbibition oil recovery was closely related to the degree
of hydration. More hydration led to higher oil recovery.
Other researchers also observed microfractures generated by hydration.
Dehghanpour et al. (2012) observed that water did not physically damage
organic shales. Water altered shale samples much more than oil (Dehghan-
pour et al., 2013). Gomaz and He (2012) observed secondary fractures
generated along bedding, and more fractures observed in fresh water than
saturated salt mud. Ji and Geehan (2013) conducted studies on shale samples
immersed in fresh water and saturated with salt water and found that shale
hydration swelling stress could cause formation of secondary fractures that
enhance shale oil and gas recovery.
Actually, a few operators have suggested that water adsorbed by minerals
in the rock creates localized clay swelling that may serve to hold open small
fractures and fissures (Hu et al., 2013). In contrast to conventional propped
hydraulic fracture treatments, slick water fracturing relies on the reactivation
of natural fractures to induce permanent shear-induced dilation, which
enhances reservoir permeability (Zoback et al., 2012; Weng et al., 2015).
Sharma and Manchandra (2015) listed five evidences of the existence of
induced unpropped fractures. Although the conductivity of unpropped,
shear-induced fractures is relatively low compared to that of propped frac-
tures, such conductivity has played an important role in enhancing the
productivity of ultralow-permeability rocks like shale (Weng et al., 2015;
Jansen et al., 2015).
Water absorption in shale is often accompanied by a change in the crystal
dimension of clay minerals: this manifests as a swelling of the rock and leads to
cracks and fractures. The swelling pressure may break the natural cementation
of shale and thus secondary fractures may be formed (Ji and Geehan, 2013).
Generally, shale reservoirs have laminated beddings in the form of heavy
disklike cores from vertical wells and small broken cores from deviated wells.
Beside dominant bedding planes, shale also shows networks of smaller weak
planes and natural fractures (Abousleiman et al., 2010). These weak planes
could serve as the sites for secondary fracture creation.