Page 58 - Fundamentals of Enhanced Oil and Gas Recovery

46 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

the oil produced by CO 2 EOR is from two oil fields: Permian Basin in the United
States and Weyburn in Canada. However, some oil fields in other countries are pro-
ducing oil as a result of CO 2 EOR as well, such as Bati Raman in Turkey, Ghawar in
Saudi Arabia, and some oil fields in Trinidad.

2.2.2 Hydrocarbon Gas Injection

Hydrocarbon gas flooding is known as the oldest EOR method [8]. In this method, a
surplus of light associated or free hydrocarbon gases are injected to increase oil recov-
ery. Usually, this method is used where large natural gas resources are available, but
there is no transportation system in markets such as North Slope of Alaska (United
States) [2]. It is worthwhile to note that HC gas injection can be implemented by first
contact miscibility (FCM) or multiple contact miscibility (MCM) process. In FCM
condition, the injected fluid forms a single phase with oil upon first contact. Actually,
when liquefied high molecular weight HC gases (called liquefied petroleum gas) is
injected, oil is displaced through a FCM process. However, when light gases such as
methane are injected into oil reservoirs, they are not miscible in first contact; hence,
miscibility is reached through multiple contacts and mass transfer between gas and oil
phases. On the other hand, the required pressure to achieve miscible condition for
HC method is greater than that one for CO 2 injection, while the highest miscibility
pressure is required for N 2 injection process [18]. However, particularly for shallow
reservoirs with low pressure, under a reasonable economic condition, light HC gas
injection can be enriched by adding heavier HC gases (ethane, propane, and butane)
to make miscibility easier to achieve [19]. There are some performed HC gas floods
in sandstone reservoirs; in addition to miscible and immiscible HC gas flooding in
Alpine [20], Kuparuk [21], and Prudhoe Bay [22,23] oil fields (all in the North Slope
of Alaska, United States), a miscible HC gas flooding project has been implemented
in Brassey field in Canada [17]. In the case of carbonate reservoirs, miscible HC
gas flooding was carried out in South Swan Hills field in Canada [24]; furthermore,
this EOR method has been reported in some carbonate reservoirs of the Middle
East [25,26].

2.2.3 N 2 -Flue Gas Injection
Availability and cost of the injected fluid are main restrictions for implementing EOR
processes. Nitrogen and flue gas are the cheapest gases with widespread availability
that can be used for improving oil recovery [8]. Although N 2 and flue gas are
suitable for application as MCM displacement fluids like CO 2 , they usually require
much higher pressures to reach miscibility condition. On the other hand, this charac-
teristic makes N 2 and flue gas more suitable for deep reservoirs with light oil where
high-pressure conditions can be achieved without any concern about fracturing of