Rheology of fumed silica nanoparticles/partially hydrolyzed polyacrylamide aqueous solutions under small and large amplitude oscillatory shear deformations

Partially hydrolyzed polyacrylamide (HPAM) is one of the most widely used polymers for enhanced oil recovery operations. However, high temperature and high salinity in oil reservoirs restrict its functionality and performance. To alleviate this, incorporating fumed silica nanoparticles (NPs) in HPAM solutions was found to be very effective in harsh oil reservoir conditions to improve the efficiency of polymer flooding. Studying the flow behavior of hybrid polymer and fumed silica NP solutions under real reservoir conditions can be very challenging and hard to achieve due to continuously converging and diverging flow through porous structures. In this regard, rheological analysis of such systems under well-controlled flow histories within the capability of rotational rheometers can be of great importance to fully understand the mechanical response of these hybrid solution systems. In this study, two types of fumed silica NPs with different surface chemistries and two types of HPAM polymers with different molecular weights were dispersed/dissolved in deionized water. Linear viscoelastic properties of the hybrid solution systems were studied based on their step-stress (creep) and small amplitude oscillatory shear responses. As deformation in porous media can be rapid and large, consideration of nonlinear viscoelastic properties can be very crucial. The stress decomposition method and Lissajous–Bowditch curves were used to describe the intercycle and intracycle shear-thickening and strain-stiffening ratios quantitatively and qualitatively. In brief, linear and nonlinear rheology conjugated with thermogravimetric analysis and cryo-scanning electron microscopy imaging enabled us to characterize viscoelastic properties of the hybrid systems and link our observations to microstructural features. Through polymer bridging, the slightly hydrophobic fumed silica NPs (AEROSIL R816) had a unique ability to form interconnected, predominately elastic network structures in contrast to large agglomerated structures formed by highly hydrophilic AEROSIL 300. This has led to observing very different rheological behaviors, regardless of the HPAM polymer molecular weight, below and above a critical fumed silica NPs concentration.