Page 180 - Enhanced Oil Recovery in Shale and Tight Reservoirs

164 Enhanced Oil Recovery in Shale and Tight Reservoirs

Joslin et al. (2017), and by Dong and Hoffman (2013) for the Bakken for-
mation in the Sanish field, North Dakota. Sheng and Chen’s (2014) simu-
lation results show that the average pressure during water injection cannot
be increased much higher than that before water injection, as the high pres-
sure is only near the injector. Their results also show that the oil recovery
from waterflooding is lower than that from gas flooding. Dong and Hoff-
man’s (2013) simulation results show that the oil recovery from their contin-
uous CO 2 injection is four times higher than that from waterflooding in the
Middle Bakken in the Sanish field (tight formation of 0.04 mD). Kurtoglu’s
(2013) simulation results also show that oil production from CO 2 injection
outperforms waterflooding. However, water-rock interactions are not
considered in the modeling work.
The above paragraphs compare the water and gas injection in the flood-
ing mode. Yu and Sheng (2017) compared the huff-n-puff mode by exper-
iments. The experimental setup for water injection has been presented
earlier in this chapter. For the huff-n-puff gas injection tests, the KCl solu-
tion (water) was replaced by nitrogen as the injection medium. Nitrogen
could be directly introduced into the vessel from the gas cylinder without
using the accumulator.
Fig. 7.8 compares the oil recovery performance from water huff-n-puff
and nitrogen huff-n-puff under different soaking times, with other condi-
tions being the same. It presents consistent trends that the nitrogen huff-
n-puff injection had much higher oil recovery than the water huff-n-puff
injection. Under the same operating conditions, a similar amount of oil
was produced in the first cycle from the two IOR processes. Their difference
increased with the cycle. After 12 cycles, the cumulative oil recovery factors
from water injection were all 10% higher than those from gas injection.
Kong et al. (2016) simulation results show that the CO 2 huff-n-puff
performance is much better than waterflooding in the tight Cardium oil
reservoir (0.2 mD).
However, Song and Yang (2013) compared waterflooding with huff-n-
puff CO 2 injection. In the waterflooding case, a core of 0.27 mD and the
initial oil saturation of 0.55 were flooded by 1.2 pore volumes of water until
no more oil was produced. The ultimate oil recovery was 51.5%. In the
huff-n-puff case, a core of 0.56 mD and the initial oil saturation of 0.43

were used. The oil viscosity was 2.17 cP at 20 C. CO 2 was injected for
1 h at a constant pressure of 7 MPa, soaked for 6 h and produced for 1 h.
The oil recovery was 42.8% after six cycles. It was under an immiscible