An Approach To Determine Polymer Viscoelasticity Under Flow Through Porous Media by Combining Complementary Rheological Techniques
Authors: Hincapie R. E. , Duffy J.. , O'Grady C.. , Ganzer L.. 
Published: August 2015

Polymer viscoelastic effects associated with flow through porous media and its influence on residual oil mobilization are still widely debated. A number of measurement techniques have been develop recently to characterize viscoelastic parameters. However, how these results should be related to the in-situ rheological behavior is still an area of research. This paper investigates the relationship between first normal stress difference (N1), relaxation time and extensional parameters. Furthermore it proposes a new approach that can best describe N1 behavior.

HPAM (Hydrolyzed Polyacrilamide) and Xanthan solutions were extensively characterized using complementary rheological measurement techniques. Effects of concentration, temperature and hardness of reservoir brines were considered. Techniques applied included: rotational rheometry, DLS (Dynamic light scattering) microrheology and microfluidics; the latter allows the determining of shear and extensional components. The measurements included shear viscosity, SAOS (relaxation), N1 (streamline tension) and extensional thickening (strain hardening). In addition, polymer in-situ rheological behavior was measured using a microfluidics device (to preliminary validate the apparent viscosity). In the process, polymer is injected through Glass-Silicon-Glass micromodels (GSG) based on CT scan of core samples and the associated pressure drop was recorded.

First, different measurement techniques were cross-validated against each other. Data obtained from microrheology was correlated to oscillatory measurements, showing an excellent agreement for viscous and elastic modulus. Similarly, rotational and oscillatory measurements of viscosity comparison using Cox-Merz model showed that the steady state viscosity matches well with the complex viscosity. Furthermore, N1 was determined using Laun's rule (from oscillatory and microrheology data) which was found to qualitatively agree with N1 measurements at mid-high shear rates and the extensional normal stresses by shear components. The second part of the work studies the relationship between N1 and the apparent shear-thickening. Comparing the N1 observed with the apparent elongational viscosity, it was found that the onset of both parameters occurs at similar shear rate range. As well N1 comparisons depicted a strong dependence on polymer concentration and apparent rate. Sensitivity analysis on the effect of temperature and salinity on N1 parameters have shown a strong dependency. Finally it was found that using an Extended Cox-Merz Plot (rheological parameters relation) will improve the interpretation of the phenomenon's taking place, showing better results when polymer solutions are pre-sheared.

These insights allow for better understanding of polymer solutions behavior in flooding applications. Moreover it helps defining key rheological techniques and tests for assessing their behavior under flow through porous media. This, in turn, provides crucial understanding of displacement mechanisms that take place at microscale. Ultimately this approach helps parameterizing the possible effect of polymer viscoelastic properties.