Page 83 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

As a general statement, heavy oil could only be assisted to flow by those solvents that
contain more intermediates than lean components [12].

3.2.1.3 Screening Factors for Miscible Flooding
Theoretically, compared with other methods of EOR, miscible gas flooding results in
more oil production. However, not all reservoirs can be candidates for a miscible gas
flood. Generally, miscible flooding process require a deep depth for injection. At shal-
low depths miscibility pressure cannot be attained as it proceeds to formation fracture.
There are other several screening criteria for evaluation of miscible flooding efficiency.
As these criteria are met, a quick performance evaluation can depict how efficient a
miscible process could be. As a key screening criteria, for instance, reservoirs with an
API gravity of 30 and above are more appropriate for a successful miscible flooding.
This is because high API crude oils are less viscous, and richer crude oils in interme-
diate components are required for miscibility to be reached through VGD or CGD
process. Less viscosity also provides a more favorable mobility ratio. Moreover, it has
been also reported that viscosity should be lower than 12 cP, residual oil saturation
should be higher than 300 STB/acre-ft, and reservoir heterogeneity should be very
low for CO 2 breakthrough to be delayed [31 34].

3.2.1.4 Miscible Flooding in Actual Fields
A large number of field applications of miscible CO 2 flooding have been implemented
worldwide. Most of them had promising results. In 1989, Brock and Bryan [35] sum-
marized their field tests with CO 2 flooding as an EOR candidate. They categorized
projects into three classes: (1) field cases, (2) producing pilots, and (3) nonproducing
pilots. Field cases are focused here.
In some of these projects, both continuous CO 2 flood and water alternative gas
(WAG) were implemented. For instance, in Dollarhide field, CO 2 breakthrough was
occurred only after 17 months. For a better mobility control process, a WAG process
was then implemented.

3.2.2 Immiscible Flooding
Generally, immiscible flooding has been used for pressure maintenance. However,
considerable numbers of immiscible gas injections have been applied directly as the
EOR agents. Although in an immiscible process, injection gas does not mix with
reservoir oil, but it partially dissolves in the oil phase, causing the crude oil to swell,
and reduces its viscosity. The degree of swelling and viscosity reduction depends
on the gas solubility; therefore it is essential to investigate the gas solubility into
the reservoir oil. In literature, three other mechanisms have also been presented
for immiscible flooding performance: (1) IFT reduction, (2) blowdown recovery,
and (3) injectivity increase. These mechanisms are known as compositional effects,