Page 286 - Fundamentals of Enhanced Oil and Gas Recovery

274 Mohammad Ali Ahmadi

9.3.1 Gas Injection
Gas injection in shale reservoirs can be applied either in continuous mode or huff-n-
puff mode [1]; these schemes described in following sections.

9.3.1.1 Continuous Gas Flooding
General speaking, continuous gas injection is most popular EOR scenario in conven-
tional oil reservoirs compared to huff-n-puff gas injection method. According to the
results gained by reservoir simulation studies, because of very low permeability of
shale reservoirs the gas cannot simply propagate in a area between injection and pro-
duction wells; this case happens when there is no fracture in the injector-producer
well space [11]. Moreover, numerical simulation and experimental results reveal that
the most portion of oil can be recover before the gas breakthrough time; this means
that in a case of gas channeling through natural/induced fractures or fingering due to
the heterogeneous nature of the shale formation, there is no significant oil production
[12]. In practical speaking, the occurrence of gas channeling or gas fingering will
make a big doubt on the applicability of such method in shale oil reservoirs. In Yu
et al. carried out both experimental and numerical studies on the capability of N2
injection as an EOR method in shale oil reservoirs. Based on their results, they con-
cluded that at certain conditions N2 injection could improve the oil production factor
[12]. Kovscek et al. [13] and Vega et al. [14] performed several experiments on shale
core samples with fracture to figure out the applicability of CO 2 injection in such
reservoirs; core samples used in their studies had permeabilities in range of 0.2 to
1.3 mD [13 14].
Most of the research on tight and shale oil EOR scenarios have been done in the
Bakken field. Wang et al. [15] performed numerical simulation method on the
Bakken reservoir in the Saskatchewan province to assess the potential of CO 2 injec-
tion. They considered various injection plans including water injection alternating
CO 2 injection, CO 2 injection, water injection, and CO 2 huff-n-puff injection. Based
on their simulation results for this reservoir, continuous CO 2 injection provides better
oil recovery factor compared to the other proposed scenarios. It is worth to mention
that the configuration of injection and production wells in their simulations are not an
actual huff-n-puff mode. Because they employed four injection wells and nine wells as
continuous production wells. However, in a case of huff-n-puff scenario the injection
and production well are the same. Another possible reason to make such a decision
that continuous CO 2 injection has a higher oil recovery factor in comparison with
other methods could be: (1) Using uncommon well configuration, especially, in CO 2
huff-n-puff scenario, (2) Considering long soaking time in that case (almost 5 years),
(3) Rock properties, especially, permeability values used in their model are not
ultra low as tight and shale reservoirs; permeability varies in their model from 0.04 to