Page 60 - Enhanced Oil Recovery in Shale and Tight Reservoirs

48 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 2.40 Conceptual steps for CO 2 EOR in fractured shale and tight reservoirs (Haw-
thorne et al., 2013).

CO 2 -based EOR mechanistic processes, as explained in Fig. 2.40. These
processes are related to what occurs in a huff-n-puff process and are
reviewed here.
Hawthorne et al. (2013) used the experimental setup in Fig. 2.41 and
used very small rock samples to have conducted CO 2 extraction experi-
ments. In their experiments, a small core plug was inside the vessel and there
is an empty space between the vessel wall and the plug, mimicking the flow
through fractures in fractured shale and tight reservoirs. Fig. 2.42 shows the
oil recovery of different molecular weight alkanes. It shows that there was no
apparent lag in oil recovery even in the first 10 min of exposure. This obser-
vation indicates that the mechanism in Step 2 in Fig. 2.40 that CO 2 carries
oil into the matrix so that oil production is reduced in the early time
pressurization is not significant. Similarly, the absence of a very fast recovery
in the first few minutes indicates that the initial oil swelling is not a signifi-
cant recovery mechanism.
According to Step 3 in Fig. 2.40, oil swelling and lowered oil viscosity
caused by CO 2 dissolution into the oil can likely enhance oil recovery.
Fig. 2.40 shows that lower-molecular weight oil had a higher recovery,