Page 159 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Gas flooding compared with huff-n-puff gas injection 143

Other researchers also compared huff-n-puff gas injection with gas
flooding. Shoaib and Hoffman (2009) simulated CO 2 injection in different
injection schemes (continuous injection or flooding and huff-n-puff) in the
Elm Coulee field in Richland County, Montana. Oil is produced from the
Bakken formation. The reservoir porosity is 7.5%, the permeability is
0.01e0.04 mD, and the oil viscosity in the reservoir is about 0.3 cP. The
shale layers contain natural fractures that formed during the conversion
process of kerogen followed by generation and expulsion of oil.
The pressure buildup tests indicate a permeability of 2.5 mD in the upper
shale region. For the huff-n-puff mode, a cycle of 9 months is used
(3 months in each of injection, soaking, and production periods). The
huff-n-puff injection increased recovery over primary production by 2.5%
for 0.19 PV injection. The incremental oil recovery from the flooding
ranges 13%e15% for about 0.2 PV injection. They found that the gas flood-
ing is better than huff-n-puff. The good flooding performance may result
from the relatively high permeability (2.5 mD from the buildup) because
the injected gas and displaced oil can flow to the producers in the flooding
mode (Sheng and Chen, 2014). The lower huff-n-puff performance in this
case may be able to be improved by optimization of the huff, puff, and soak-
ing time, for example, by reducing the soaking time, as the optimization can
change the conclusion about the preference of huff-n-puff and flooding
(Sheng, 2015b), as also discussed earlier in this section.
Wang et al. (2010) assessed the CO 2 potential in the Bakken formation
in the Saskatchewan area. In their simulation model, the porosity is 7.5%,
and the permeabilities in the upper three layers andlower five layers are
2.5 mD and 0.04 mD, respectively. The oil viscosity in the reservoir is about
0.3 cP. They claimed that continuous CO 2 injection is better than the huff-
n-puff CO 2 injection. In their continuous mode, there are four injectors and
nine producers. In their huff-n-puff mode, two wells are in 10 years of huff
injection, while another group of two wells are in 5 years of soaking
followed by 5 years of production, and the rest of nine wells are in contin-
uous production mode. Three points may help understand why the contin-
uous injection is better than the huff-n-puff injection in this case. (1) In the
huff-n-puff mode, not all the wells are operated in such mode; (2) 5 years of
soaking time is too long, and thus some operation time is lost; (3) the perme-
ability from this model is not ultralow so that CO 2 is able to flood from an
injector to a producer (Sheng and Chen, 2014).
Kurtoglu (2013) simulated a three-well pattern. The three horizontal
wells are parallel with each other and produce in the first 450 days, the