Page 54 - Fundamentals of Enhanced Oil and Gas Recovery

42 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

methane, ethane, and propane is injected into the reservoir in order to achieve a mis-
cible or immiscible gas oil system in the reservoir. In the latter method, nonhydro-
carbon gases such as carbon dioxide (CO 2 ), nitrogen (N 2 ), and also some exotic gases
are used as displacing fluid in the reservoir. It is noted that gas is also used as a second-
ary recovery method in oil reservoirs; in this method, HC gas is injected into the gas
cap to compensate the reservoir’s pressure decline. Based on some parameters such as
operating condition and oil composition, gas flooding can be carried out in miscible
or immiscible conditions. The primary mechanism of oil recovery in a gas flooding
process is mass transfer between oil and gas phases. Under miscible condition, mass
transfer increases and forms a miscible slug in front of gas phase. Moreover, swelling
and viscosity reduction of oil phase are activated during gas flooding due to condens-
ing of intermediate components of gas into the oil phase. Regarding gas flooding pro-
cess, minimum miscibility pressure (MMP) is defined as the minimum operating
pressure in which gas can reach miscibility with oil at reservoir conditions. Therefore,
the efficiency of oil displacement by gas can be explained by the concept of MMP.
During immiscible gas flooding, gas is injected below MMP; the gas increases reser-
voir pressure, and as a result the macroscopic displacement, efficiency is enhanced and
cause oil swelling slightly. It is noted that mass transfer occurs not only in miscible gas
flooding but also during immiscible process. Actually, gas always extracts some com-
ponents of oil phase. Therefore, to be more precise, solubility of oil gas system in
immiscible gas flood is not zero, but it is very small and negligible. Higher efficiency
of miscible gas flooding compared to immiscible process is attributed to the higher
mass transfer between oil and gas phases causing oil viscosity reduction and oil swell-
ing; thereby, greater viscous forces are obtained leading to better macroscopic and
microscopic efficiencies.
As long as a gas flooding operation is implemented above MMP, high oil recovery
factor is achieved. However, MMP is not the same for different gases, and usually it is
much greater for N 2 than for CO 2 ; so for a N 2 oil system, miscibility condition is
harder to be achieved. Usually, CO 2 injection is more efficient than the injection of
other nonhydrocarbon gases, and the number of potential target reservoirs for CO 2
EOR is much greater than for other gas methods.
Gas methods can be implemented based on the reservoir rock and fluid parameters
by different strategies as follows:
• Continuous gas injection
• Water alternating gas (WAG) injection
• Simultaneous water alternating gas (SWAG) injection
• Tapered WAG injection
The alternative methods were proposed and implemented in the recent decades to
overcome the problem of an undesirable mobility ratio and low displacement effi-
ciency of gas flooding. In WAG injection, gas and water slugs are injected alternately.