Page 55 - Fundamentals of Enhanced Oil and Gas Recovery

Screening Criteria of Enhanced Oil Recovery Methods 43

On the other hand, in SWAG, gas and water is injected simultaneously with a bubble
structure using multistage pump systems; however, this method requires precise moni-
toring for injection of the two-phase (water gas) fluid. Also, sometimes, gas is
injected in a tapered mode; in this method, gas injection is changed to water injection
once injection gas is detected in a production well (once breakthrough occurs).

2.2.1 CO 2 Injection
Since the beginning of the industrial revolution, human activities have produced
about a 40% increase in the atmospheric concentration of carbon dioxide, which con-
tributes to global warming [1]. Because of this, CO 2 capture and sequestration were
proposed as a solution for environmental problems. On the other hand, not only the
worldwide increase in demand for oil but also low recovery factor of oil reservoirs in
primary production stage showed the necessity of implementation of EOR methods.
Based on this fact, in recent decades, CO 2 injection has been used as an EOR method
in oil reservoirs. The idea of using CO 2 in EOR is becoming more popular because
of its aim to reduce greenhouse emission, its relatively lower cost, and high displace-
ment efficiency; particularly in the United States, CO 2 injection is so cost effective

because of available large pipeline of natural CO 2 . Based on this, number of CO 2
EOR projects shows an increasing trend especially in the United States; Cranfield
field, Lazy Creek field in Mississippi, and Sussex field in Wyoming are some recent
CO 2 EOR projects in sandstone reservoirs [2].CO 2 injection can be implemented in
oil reservoirs in both miscible and immiscible conditions, and generally, miscible CO 2
injection is more efficient than the immiscible one. The dominant occurring mecha-
nism for oil recovery by gas flooding is through mass transfer of components between
oil and gas phases. Actually, gas is to be contacted with oil as much as the system
reaches miscibility through condensing (condensing of heavy components of gas into
oil phase) and vaporizing (vaporizing of light components of oil into gas phase)
mechanisms. It should be noted that, although CO 2 is in gas form at atmospheric
pressure and temperature, it may convert to supercritical fluid at some reservoir condi-
tions [3]. In addition, the density decreases with temperature; thereby, the solubility of
CO 2 in oil decreases, and as a result the required MMP increases. Therefore, the dee-
per the reservoir is, the higher the MMP of the gas oil system (since the reservoir
temperature usually increases with depth). Therefore, accurate determination of
MMP, which can be carried out by experimental methods or correlations, is a must
for a precise prediction of the efficiency of a CO 2 EOR process. In the case of asphal-
tenic crude oils, miscibility condition (MMP) of CO 2 oil system is adversely affected
by asphaltene precipitation which may occur during CO 2 flooding [4,5].
Based on suitable fluid, rock, and reservoir properties, some criteria are defined
for screening of CO 2 EOR method. Suitable reservoirs for CO 2 EOR are selected