Page 156 - Primer on Enhanced Oil Recovery

146 Primer on Enhanced Oil Recovery

Salinity and water hardness have a significant effect on polymer flooding, espe-
cially when using hydrolyzed polyacrylamide (PAA, HPAM). The effect of miner-
alization and water hardness on the polymer solution is expressed in a decrease in
the electrostatic repulsion forces in the polymer. Salinity leads to the destruction of
the solution structure, a significant decrease in the viscosity of the polymer solution.
The effect in some way is proportional to the salt concentration. The overall effect
of water hardness has an even more significant effect on the polymer solution, as
exposure to bivalent calcium and magnesium ions, other things being equal, is more
significant than single-ion ions of sodium and potassium.
Polymer solution viscosity is somewhat proportional to polymer molecular
weight. But at the same time big molecules are not very stable and can be broken.
This leads to smaller molecular weight polymer solution with a lower viscosity and
polymer flooding efficiency reduction. Polymer molecule destruction can be
induced mechanically, chemically, thermally and by a bacteria.
Polymers usually delivered in powder form and need to be mixed with water at
the injection site. The solution making should include few stages and it is time con-
suming process. Mechanical mixing needs to be done with care as too vigorous
mixing can reduce solution viscosity below stated by the polymer manufacturer.
High pressure injection can add to this thinning process. Air oxygen and iron con-
tamination negatively affects polymer molecules. For this reason polymer mixing
should be undertaken in the air-tight and preferably iron-free mixers. Bacterial spe-
cies from the mixed water and anaerobic bacteria in the formation are usually feed
on polymer solutions. The reduction of polymer viscosity can be as high as 80%. In
many cases this leads to the necessity to use some biocides to suppress bacterial
activity.
As a polymer solution is injected its temperature rises to the formation tempera-
ture. It is universally accepted that the absolutely top temperature limit for the poly-
mer flooding is always below 130 C. As the molecule hydrolysis is thermally

activated process it is always happening. It is just above some critical temperatures
hydrolysis becomes catastrophic. Lately some polymers have been produced which
allow to work at slightly higher temperatures.
The injectivity of the polymer solution is an important property for several rea-
sons. Firstly, the rate of injection of the polymer solution directly affects the effi-
ciency of the project. Secondly, cleaning (drainage) of injection wells may be
necessary if the polymer solution reduces the injectivity. These cleaning works can
reduce the technological and economic efficiency of polymer flooding. Injectivity
decreases with increasing molecular weight of the polymer and full analysis and
optimization of all processes is highly recommended. The injectivity of the polymer
solution is more efficient when the polymer solution exhibits shear liquefaction.

12.1.1 Polymer flooding applicability criteria

The criteria for the polymer flooding applicability can be summarized as on
Fig. 12.4.