Page 74 - Fundamentals of Enhanced Oil and Gas Recovery

62 Ramin Moghadasi et al.

to positively impact global warming and assist in providing to the world’s energy
demand. Hopefully, there are number of ongoing field practices to CO 2 sequestration
and EOR. Research has resulted a good understanding of associated mechanisms,
effective screening parameters, and also operational conditions for an optimized pro-
cess. But yet there is an amount of uncertainty about how efficiently this process
could be implemented [9 12].
This chapter demonstrates the fundamentals of CO 2 injection process in both mis-
cible and immiscible modes, explains how CO 2 EOR process could be facilitated in
practice, discusses laboratory tests, illustrates some examples of reservoir simulation
during CO 2 injection, details applicability of CO 2 EOR for unconventional resources,
and finally depicts the environmental aspects of CO 2 injection.

3.2 CO 2 INJECTION FUNDAMENTALS

When CO 2 is injected to the reservoir, it interacts physically and chemically
with reservoir rock and the existing hydrocarbon fluid. Such interactions are the base
mechanisms to explain why and how injected CO 2 recovers the remained oil in place
[13]. Majorly, these mechanisms are categorized as follows [8,14 16]:
1. Oil volume swelling
2. Oil and water density reduction
3. Oil viscosity reduction
4. Reducing the interfacial tension (IFT) between the reservoir rock and oil, which
has previously inhibited oil flow through the pores
5. Vaporization and extraction of the trapped of oil portions (mostly light
components).
Carbon dioxide has high solubility in oil, causing the oil to swell and consequently
reducing the oil viscosity and density. Additionally, there is almost always some water
in the reservoir, which is left from previous water flood; thereby injecting CO 2 will
result in reduced water density because it is soluble in water to some extent.
Eventually, it causes water and oil densities to be mostly similar, resulting in reduction
of gravity segregation effects, less override flow, and lower occurrence of the fingering
phenomenon [17].
The importance of each mechanism depends on the pressure and temperature of
the reservoir. The miscible process occurs at high temperatures and pressures, and the
immiscible process at lower pressure and temperature conditions. This makes a clear
distinction between these processes, which in turn leads to different performances
considering the incremental oil recovery associated with each of them [18].