Reservoirs facing challenges such as high oil viscosity and high heterogeneity benefit from polymer flooding by decreasing unfavorable mobility ratios or improving crossflow. Despite this diversity of needs, polymer floods are often designed around a single point viscosity value, which doesn't account for variable shear rates in the porous media, in-situ viscosity, challenges with fluid injectivity, etc.
Polymers have diverse rheological characteristics. For example, synthetic polymers like HPAM are elastic and can range in their degree of shear sensitivity, while biopolymers like scleroglucan are inelastic and highly shear-thinning. We generated laboratory data for two such polymers, scleroglucan and an Acrylamide-ATBS Co-Polymer. We developed parameters to describe their rheological characteristics for a variety of models, including shear thinning, shear thickening and preshearing.
We then assessed the impact of these attributes on polymer flooding performance using UTCHEM, the University of Texas Chemical Flooding Reservoir Simulator. Polymer injectivity, viscosity, and shear rate were assessed for sensitivity to polymer rheology using simplified models of radial geometry. These results were correlated to laboratory experiments to ensure correct estimate of in-situ viscosities. Sensitivity to a variety of important parameters, including shear rate correction factor, simulation grid size, shear-thickening, and skin were also completed. A final sensitivity study was conducted using a heterogeneous reservoir model with the multiple patterns.
Simulations using the simplified radial model and the heterogeneous, commercial scale model support the same conclusion: the specifics of polymer rheology can have a significant impact on polymer flooding performance, and polymer rheology should be considered an important design attribute along with injection viscosity and retention. We show that injection of an elastic, HPAM-based polymer may require higher injection pressure or lower rates than a highly shear-thinning, inelastic biopolymer like scleroglucan. We also demonstrate how the differences in rheology impact reservoir pressure, in-situ viscosity distribution, in situ shear-rate distribution and flow into various reservoir layers.