Page 46 - Fundamentals of Enhanced Oil and Gas Recovery

34 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

comes into contact with oil, contains less residual oil (5% 10%). Steam mobility is
high in this area, so mobility and steam presence decreases in production wells [1].
An oil reservoir naturally starts production using the potential energy saved within it.
After a while, depending on the conditions of the reservoir, energy level declines and the
reservoir will not have any production. Finally, it would reach a point where production
is no longer economically worthwhile. Oil production through this natural mechanism is
called initial exploitation, and EOR processes start at this point. EOR is a general term
that differentiates oil recovery processes from the initial exploitation. Flooding and gas
injection into the oil reservoirs are two commonplace gravity methods commonly known
as secondary recovery. Gas or water is injected into the reservoir prior to tertiary recovery
to save the internal pressure of the reservoir. Any other technique employed for further
production after the secondary recovery is called tertiary recovery.

1.15.3.1 A Review on Enhanced Oil Recovery Methods From Reservoirs
The main purpose of the new EOR methods is to employ ways to recover more oil.
Gas or water injection prevents the inner pressure of the reservoir from dropping
quickly. Reservoir engineers have been looking for methods that would allow them
to recover more or even all of the initial oil of a reservoir for years. Oil recovery
methods fall into these three categories:
1. Natural deletion
2. Secondary recovery
3. Tertiary recovery
EOR methods include recovery techniques that can be classified under the second
and third categories. EOR process is used as a supplementary mechanism for the nat-
ural thrust mechanism of the reservoir such as pressure fixation, wetness changes, and
mobility coefficient control.
As shown in Figs. 1.17 and 1.18, the general categorization of enhanced recovery
methods is as follows:
1. Natural production
a. Water flooding
b. Dissolved gas flooding

Figure 1.17 Distribution of methods employed in enhanced recovery.