Page 42 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 31

the oil to come to the core surface and the oil saturation gradient is higher in
the earlier cycles.
Yu and Sheng (2015) did 10 cycles of huff-n-puff experiments under
different pressure depletion rates, using Eagle Ford outcrop samples, the
mineral oil Soltrol 130, and nitrogen. Their cumulative oil recovered
continued to increase with the cycle. One of the example results is presented
in Table 2.2.
Wan et al. (2015) history matched Yu and Sheng’s experiments and their
models also predicted the continuous increase with the cycle. Their simula-
tion data showed that the cumulative oil recovered increased with the cycle
almost linearly when the diffusion was not included in the model.
Sheng (2017b) simulated the huff-n-puff gas injection with 300 days of
huff and 300 days of puff time but no soak time, for 32,850 days (about 90
years). The cumulative oil recovery factor keeps increasing, although the oil
rate decreases with time as shown in Fig. 2.22. These results indicate that the
huff-n-puff process in shale and tight reservoirs can be continued until an
economic rate cut-off is reached. In a practical application, an economic
cut-off may not allow too many cycles. Artun et al. (2011) did a parametric
simulation study of a naturally fractured reservoir (a conventional reservoir).
They found that the optimum number of cycles was 2 to 3. Sanchez-Rivera
et al.’s (2015) simulation data shows that only the first cycle of huff-n-puff
CO 2 injection was profitable. They assumed the oil price is $90/STB and

the CO 2 cost is $2/Mscf. Reinjection of separator gas (about 50% CO 2
and 50% produced gas) make a project more profitable.

2.8 Effect of injected gas composition
N 2 ,CO 2 , and C 1 are separately used by different researchers to study
huff-n-puff gas injection in laboratory. To compare the performance of
these gases, Li et al. (2017a) did experiments and simulation work at the
same experimental setup and similar conditions. In their experiments, Wolf-
camp dead oil was used. The injection pressure was 2000 psi. More exper-
imental details are shown in Table 2.3. To check repeatability, two cores are
used. Note that the experimental conditions for CO 2 are not the same as
those for N 2 and C 1 which have the same experimental conditions. For
Core 1, N 2 and C 1 oil recovery factors are similar in the first three cycles,
but N2 is better than C 1 (Fig. 2.23a). For Core 2, N 2 was always better
than C 1 . It seemed that N 2 is better (Fig. 2.23b). Note the dead oil is