Page 128 - Fundamentals of Enhanced Oil and Gas Recovery

116 Pouria Behnoudfar et al.

4.4 FIRST CONTACT MISCIBLE VERSUS MULTICONTACT MISCIBLE

In some cases, the injected gas cannot yield the FCM with the reservoir crude
oil once it is injected. However, it may gradually develop dynamic miscibility with
the residual oil through MCM under actual reservoir conditions [48,49].
The result of recovery governed by immiscible gas injection is limited basically by
three factors [50]:
• Areal sweep efficiency
• Volumetric sweep efficiency
• Microscopic sweep efficiency
Because of viscous fingering, gravity segregation, permeability stratification, IFT,
wettability, and pore structure, ultimate oil recovery is always much less than 100%.
Hence, the reason of the interest in the miscible injection method can be explained as
it is a more efficient recovery method [18,21,51,52]. In fact, it is usually not economi-
cal and in some cases, not technically feasible to inject a gas that create the FCM with
the oil; therefore, the injected gas is designed to develop miscibility by the net transfer
of components from the oil into the gas (a vaporizing gas drive) or from the gas to
the oil (a condensing gas drive) [7].
Many phenomena are studied as they can limit the efficiency of the miscible flood-
ing process. For instance, permeability heterogeneity is considered as a strong limita-
tion on recovery because it can lead to flow channeling and poor sweep efficiency.
Small-scale heterogeneities are particularly problematic for all secondary and tertiary
recovery processes, because they can have a significant effect on recovery, which can-
not be modeled explicitly in field-scale simulations [53].
Compositional simulation is usually utilized to estimate the performance of the
recovery schemes on the basis of equation-of-state properties developed from the
regression on data obtained from laboratory experiments. The accuracy of the mea-
surements is highly dependent on the validity of the assumptions used in these simula-
tions [54,55].

4.5 HEAVY OIL RECOVERY USING CO 2

Recently, recovery of heavy oil by CO 2 displacement methods has gained much
popularity worldwide. This approach especially received attention in reservoirs where
steam flooding is not applicable. First, efforts in large-scale field tests such as Lick
Creek field in Arkansas and Wilmington field in California have proved the applicabil-
ity of CO 2 immiscible displacement for heavy oil recovery [56,57]. The high oil