Injectivity Behavior of Copolymer and Associative Polymers Decoded Using Extensional Viscosity Characterization: Effect of Hydrophobic Association
Authors: Azad Madhar Sahib , Trivedi Japan J 
Published: April 2017

Abstract
Associative polymer (AP) solutions in general exhibit higher resistance factors and subsequently lower injectivity than hydrolyzed polyacrylamide (HPAM) because of strong extensional flow characteristic in porous media despite having similar shear viscosity. From a scientific point of view, the challenge is to understand and quantify these properties in terms of the nature of their association in water. The kind of hydrophobic association (intramolecular or intermolecular) that AP exhibit is concentration dependent and will influence not only the shear but also the extensional properties and therefore elongational flow as well in the porous media. Therefore, the role of hydrophobic association on shear and extensional rheology and its effect on the injectivity in porous media requires comparative investigation over its counterpart non-associating HPAM.
Unlike shear rheology, measurement of bulk extensional properties for relatively low viscous enhanced oil recovery polymer solutions remains a challenge. In this study, extensional rheology measurements are performed using capillary breakup extensional rheometer (CaBER). The CaBER setup uses a stepstrain to stretch a droplet of liquid placed between the two plates and follows its midplane diameter that declines exponentially in the intermediate time scale where the filament breakup is governed by the balance between driving surface tension and resisting elastic force. The midpoint diameter is fitted with the upper-convected Maxwell model to determine the extensional relaxation time. Extensional viscosity calculated using the axial force balance at the critical Deborah number (Decr) determined using finitely extensible nonlinear elastic model would be indicative of polymer's elasticity along with polymer's relaxation time. These extensional parameters are further used for correlating with resistance factors and injectivity experimental data.
Extensional relaxation time of AP and HPAM solutions correspond to 1.2s and 1s at 2000 ppm, whereas at 1000 ppm these values are 0.45s and 0.53s, correspondingly. At Decr, AP demonstrated much higher extensional viscosity (~760 Pa·s) for 2000 ppm solution than HPAM (~470 Pa·s), however, this difference was marginal at 1000 ppm concentration. This indicates intermolecular interaction at higher concentration for AP solution is dominant whereas at lower concentration intramolecular attraction dominates extensional properties. These results are in accordance with porous media observations where AP solution at 2000 ppm showed much higher resistance factors and reduced injectivity when compared with HPAM at low to intermediate rates and similar value of resistance factors and injectivity with HPAM at 1000 ppm throughout all the rates. Further at 2000 ppm, extensional thickening in AP solutions at intermediate rate is followed by strong extensional thinning at high rates. This could be attributed to the transformation of intermolecular attraction to the intramolecular attraction at higher extension rates. AP exhibits comparable injectivity with HPAM in porous media at higher rates due to this transformation. However similar shear behavior shown by these polymers even at 2000 ppm indicates that shear rheology can't predict the intermolecular hydrophobic association of AP in porous media. This is attributed to the lesser inter-chain interactions that occur during shear flow.
Characterization method presented here can be used for quantifying the elongation flow in porous media and predicting the injectivity behavior of associative and non-associative HPAM polymers. The method can be used for quick screening of polymers having similar shear rheology.