Page 23 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 13

4. Remove the core from the vessel, measure the weight (W exp ), and calcu-
late the cumulative recovery factor as (W sat W exp )/W sat .
5. Repeat the procedures 1 to 4 for a set of times (cycles).
In Akita et al.’s (2018) experimental setup, crushed shale samples instead
of core plugs were used. In their experiments, the amount of fluid produced
during each cycle was obtained by the difference between the NMR vol-
umes before and after each cycle.
The oil recovery factor may also be derived from CT numbers. Accord-
ing to Akin and Kovscek (2003), the CT number of a core lies on the
straight line connecting phase 1 to phase 2. They stated that the CT number
of a core has a linear function with the attenuation coefficients of the consti-
tuting materials:

CT gor ¼ð1 fÞm þ fS o m þ fS g m gr (2.6)
r
or
where CT gor represents the CT number for a system of gas, oil, and rock, m r ,
m or , and m gr are the attenuation coefficients for the rock only, for the core
fully saturated with oil, and for the core fully saturated with gas, respectively
S o and S g are the oil and gas saturations, respectively. Note that all the
attenuation coefficients m or , and m gr are not the attenuation coefficients for
oil only and gas only, although our intuition or logic think they are.
If only gas is in the pores, the above equation can be written as
CT gr ¼ð1 fÞm þ fm gr (2.7)
r
If only oil is in the pores, the above equation can be written as

CT or ¼ð1 fÞm þ fm or (2.8)
r
For a pure fluid, oil or gas, f ¼ 1. From the above two equations, we
can see that CT is equivalent to m. Then from Eqs. (2.7) and (2.8), we have
CT or CT gr
f ¼ (2.9)
m m gr
or
The derived Eq. (2.9) is different from Eq. (2.5). Eq. 2.9 may be incorrect
as it is derived based on Eq. 2.6. We think Eq. 2.6 should be written as Eq.
2.6’:CT gor ¼ (1 f)CT r þ fS o CT o þ fS g CT g , as it will be further dis-
cussed later.
From Eqs. (2.6) and (2.7), we have
CT gor CT gr ¼ fS o ðCT o CT g Þ (2.10)