Page 60 - Fundamentals of Enhanced Oil and Gas Recovery

48 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

gum is a great resource used by bacteria. Even adding the combination of both bio-
cide and xanthan polymer to an oil well with bacteria issues will not be successful due
to molecular weight difference of xanthan and biocide. Moreover, xanthan biopoly-
mer has a lower molecular weight and higher price compared to PHPA which is less
economical for large-scale projects. Therefore, PHPA is the most used polymer in
field studies. On the other hand, high molecular weight PHPA increases water viscos-
ity, while in low permeability formations, it causes polymer retention on rock surface.
Bailey [29] and Taber [8] have adopted reservoir screening criteria for polymer
flooding. The reservoir properties that should be considered are reservoir type, per-
meability, oil viscosity, reservoir temperature, and formation water salinity. Table 2.1
shows the range and average value of some screening criteria for polymer flooding.
Based on the literature, formation water salinity must be lower than 10,000 ppm for a
successful polymer flooding.
Sandstone reservoirs are mostly the preferred type of reservoirs for polymer flood-
ing projects and Daqing (1996 2010) as a large-scale project is an example of success-
ful project with 10% 12% average recovery [30]. On the other hand, recent studies
also show that polymer flooding is an option for unconventional reservoirs as well
[31], while permeability of the reservoir is an essential factor in polymer solution
propagation as mentioned earlier. The average permeability reported for 40 successful
treatments was 563 mD compared to 112 mD for three discouraging projects [32].
Another sensible approximation is the pore throat radius that should at least be five
times greater than the root mean square radius of gyration of the polymer [33].
Polymer concentration is another important factor that should be considered for
successful polymer flooding, while low polymer concentration (213 ppm) [34] causes
viscosity reduction; moreover, inappropriate mixing mechanism results in a similar
effect [35]. In this regard, it is noted that special mixing equipment is required to mix
the polymer with the injected water to avoid forming fish-eye. Also, oxygen jeopar-
dizes the polymer stability while degrading the polymer structure; therefore, oxygen
scavenger is used as a solution for this problem. Likewise, high salinity formation
water has high negative effect on the PHPA structure; divalent and trivalent cations of
salt interact with the PHPA structure and precipitate. A well-known solution for this
problem is injecting a low salinity water preflush prior to polymer slug. It is worth-
while to note that Xanthan biopolymer can tolerate water salinity more than PHPA.
The maximum reservoir temperature reported for polymer flooding is reported to
be 237.2 F, but in general, 62% of the projects were implemented in 108 158 F

[14]; therefore, chemical EOR is not recommended for high-temperature wells. The
mobility ratio of water and oil phase depends on the viscosity of the oil. It has been
shown that incremental oil recovery increases with increasing oil viscosity lower than
30 cp, while at greater oil viscosities, the incremental oil recovery decreases [36];
moreover, based on data of 70 chemical projects (mainly polymer flooding), most of