Page 67 - Primer on Enhanced Oil Recovery

58 Primer on Enhanced Oil Recovery

spring, displacing oil to the producing wells. The implementation of gas injection
requires the use of high pressure compressors.
Injection of immiscible gases is inferior to the efficiency of water flooding, but
in some cases it is the only possible method of secondary oil recovery. This is espe-
cially evident in low permeability reservoirs and in the reservoirs containing high
amounts of swelling clays.

7.3 Why enhanced oil recovery (EOR) methods

Secondary oil recovery methods increase volumes of recovered oil. Nevertheless,
despite the increase in oil recovery rate as a result of the use of secondary oil recov-
ery methods, in particular the most effective oil flooding, a significant amount of
residual oil remains in the reservoir. According to the accumulated around the globe
work data for reservoirs: with low permeability (tight oil reservoirs) or containing
heavy oils, the final oil recovery when using primary and secondary oil recovery is
5 10%; 10 25% of oil in place is recovered in the drive mode of dissolved gas;
25 40% oil in place is recovered at partially water-pumping mode, gas injection or
gravitational mode; more, 40 55% oil in place, is recovered by the ubiquitous
waterflooding. To increase the oil recovery rate above the indicated values, tertiary,
so named Enhanced oil recovery (EOR) Methods of hydrocarbon extraction are
used. The purpose of the EOR use is to increase the final oil recovery by:

Increase in sweep efficiency due to:
reducing the ratio of the mobility of the injected and displaced fluids,
blocking of the washed highly permeable water-saturated zones and the re-direction of

the injected fluid into the low-permeable oil-saturated zones of the reservoir.
Surface forces modification in the reservoir due to:
reducing the interfacial surface tension between the oil and the displacing fluid,
reduce the effect of capillary forces,
changes in reservoir rock wettability,
disjoining pressure changes.
Combinations of the above processes.
Ternary extraction processes of hydrocarbons are carried out after the processes
of primary and secondary extraction have already been executed. At the same time,
fluids other than those used in secondary recovery (ordinary water and immiscible
gas) are introduced into the formation (see Fig. 7.6). As the above listed set of
methods, EOR process is a broader concept because EOR methods can be applied
at any stage of field development, including primary, secondary and tertiary hydro-
carbon recovery. Thus, an EOR can be implemented as a tertiary process if it fol-
lows a flooding or injection of immiscible gas, but it can be a secondary process if
it directly follows the primary recovery of hydrocarbons. Nevertheless, majority of
EOR projects are implemented after water flooding. At this stage, it is important to
establish the difference between EOR and IOR (improved oil recovery) to avoid
misunderstandings. The term IOR refers to the use of any EOR operation or any