Summary
An experimental study of shear stability of several high-molecular-weight
polymers used as mobility-control agents in EOR projects has been performed in
well-controlled conditions. The shearing device was made of a capillary tube
with an internal diameter (ID) of 125 μm, through which polymer solution was
injected at a controlled rate. The setup enables a precise measurement of the
shear rate to which the polymer macromolecule is submitted. The degradation
rate was measured by the viscosity loss induced by the passage into the
capillary tube. The shear rate was gradually increased up to 106
sec–1 while checking degradation rate at each stage.
Different commercial EOR polymer products were submitted to the test with
polyacrylamide backbone and different substitution monomer groups. All
macromolecules behave as flexible coils in solution. The parameters
investigated were
- Molecular weight (between 6 and 20x106)
- Nature of substitution group (acrylate, ATBS/sulfonate,
nVP/vinyl-pyrrolidone)
- Salinity
Polymer shear degradation increases with molecular weight and salinity, but
decreases with the presence of acrylate, ATBS, and nVP. All results can be
interpreted in terms of chain flexibility. The highly flexible polyacrylamide
homopolymer is the most sensitive to shear degradation. Introduction of
acrylate groups in the polymer chain induces some stability because of the
rigidity provided by charge repulsion, which vanishes in the presence of high
salinity because of the screening of acrylate negative charges. ATBS and VP
groups, which are larger in size, provide significant chain rigidity, and thus
better shear stability. It is also shown that some very-high-molecular-weight
polymers, after passing the shearing device, attain a final viscosity lower
than lower-molecular-weight products with the same chemical composition. This
factor has to be taken into account in the final choice of a polymer for a
given field application.
As a comparison, although less popular today than 2 decades ago, xanthan gum
(XG), which behaves like a semirigid rod, is shown to be much less sensitive to
the shear-degradation test than the coiled polyacrylamides (Sorbie 1991).
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
21 June 2011
- Meeting paper published:
12 April 2011
- Revised manuscript received:
6 August 2011
- Manuscript approved:
11 August 2011
- Published online:
16 April 2012
- Version of record:
11 June 2012
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