Page 114 - Fundamentals of Enhanced Oil and Gas Recovery

102 Pouria Behnoudfar et al.

displacing phases during vaporizing/condensing gas drive, oil swelling, and oil viscos-
ity reduction leading to reservoir repressurization and alleviation of capillary forces
[6,7]. Various forms of injected gases consist of hydrocarbon gases such as natural gas,
a liquefied petroleum slug driven by natural gas and enriched natural gas, and nonhy-
drocarbon gases, including nitrogen, carbon dioxide, and flue gas, are extensively uti-
lized to reduce the residual oil saturation.
The thermal and chemical projects are frequently used in sandstone reservoirs
rather than other lithologies (e.g., carbonates and turbiditic formations) [8].
Since the 1970s, CO 2 injection has been documented as an encouraging operation
for EOR [9]. Selection of CO 2 as an injection gas is more desired as it could be
employed for the goals of both CO 2 sequestration resulting in the greenhouse gas
reduction and also as an EOR agent. CO 2 as an EOR agent is suitable for recovering
light-to-medium oils from deep reservoirs [10]. Furthermore, CO 2 storage into
underground hydrocarbon reservoirs can result in declining the greenhouse gas emis-
sions. For CO 2 injection processes, various difficulties including large requisite per
incremental barrel of CO 2 , corrosion problem in surface facilities, precipitation and
deposition of asphaltene leading to formation damage and wettability alteration, and
separation of CO 2 from the valuable hydrocarbons have been reported in the literature
[10 13].
N 2 -contaminated lean hydrocarbon gases or simply nitrogen injection are also
another applicable EOR agents for high pressure and deep reservoirs with light or
volatile crude oils containing hydrocarbon components of C 2 C 5 . The advantages of
N 2 gas are the availability, low cost, and abundance. By cryogenic processes, nitrogen
is produced from air for a long time [14]. Based on the injection pressure at oil com-
position and reservoir temperature, injection of nitrogen process could be implemen-
ted in both miscible and immiscible means which is discussed subsequently [5].

4.2 IMMISCIBLE AND MISCIBLE PROCESSES

In immiscible flooding, an interface between the trapped crude oil and the
injected fluid exists, and consequently, a capillary pressure also will be established due
to the prementioned interface. The immiscible flood benefits are principally as a result
of reservoir pressure maintenance. Residual oil saturations can probably be higher
than that of miscible flood since the two fluids are immiscible [15]. Therefore, the
miscible flood attains higher recovery factors than the immiscible flood. The miscibil-
ity is theoretically referred to the cases in which there is no interface between the two
phases involved (i.e., zero equilibrium IFT). In other words, the miscible condition is
when two phases can be mixed with each other at any ratio [16].