Page 24 - Enhanced Oil Recovery in Shale and Tight Reservoirs

14 Enhanced Oil Recovery in Shale and Tight Reservoirs

Combining Eqs. (2.9) and (2.10), we have

CT gor CT gr
S o ¼ (2.11)
CT or CT gr
Then the oil recovery factor (RF) is

S oi S o
RF ¼ 100% (2.12)
S oi
where S oi is the initial oil saturation. Although several groups of authors (Shi
and Horne, 2008; Li and Sheng, 2016; Meng et al., 2017) used the above
equation, the derivation lacks rigidity. An alternative derivation is proposed
below.
The mass balance equation for a core saturated with two fluids, gas and
oil, is

r gor ¼ð1 fÞr þ fS o r þ fS g r g (2.13)
o
r
Assume the density of a system or material is proportional to its CT
number,
CT gor ¼ð1 fÞCT r þ fS o CT o þ fS g CT g (2.14)
If the rock is saturated with oil or gas, we have

(2.15)
CT or ¼ð1 fÞCT r þ fCT o
and

CT gr ¼ð1 fÞCT r þ fCT g (2.16)
By combining Eqs. (2.14 and 2.16), Eq. (2.11) is derived.
Fig. 2. 5 shows the cumulative distribution of CT numbers for the dry
core, oil saturated core, and during eight cycles (Li and Sheng, 2016).
The CT numbers in the cycles were between the one for the dry core
and the one for the saturated core. The CT numbers decreased with cycle
number. From the CT number in each cycle, oil saturation was calculated
from Eq. (2.11), and the recovery factor was calculated from Eq. (2.12) as
shown in Fig. 2.6.
In the Tovar et al. (2014) experimental apparatus (Fig. 2.7), the frac-
turing space between a core plug and the wall of a container is packed
with glass beads to simulate hydraulic fractures. A CT scanner is used to
monitor the oil saturation changes during the huff-n-puff CO 2 injection
process. The oil recovery factor is calculated from CT numbers. The volume