A Practical Method to Calculate Polymer Viscosity Accurately in Numerical Reservoir Simulators

Abstract

Abstract Polymers play a key role in several EOR processes such as polymer flooding, surfactant-polymer flooding and alkaline-surfactant-polymer flooding due to the importance of mobility control in achieving high oil recovery. Numerical simulations are used to predict the performance of these processes and in particular the injection rate of the polymer solutions since the economics are sensitive to the injection rates. It is important to model the polymer viscosity accurately so that accurate predictions of injection rate can be made. Because the viscosity of the non-Newtonian polymer solution is a strong function of shear rate, the calculation of shear rate must be accurate. The shear rate depends on the local velocity in the reservoir. As the gridblock size increases, the calculation of this velocity is less accurate, especially close to the wells. We present a method to calculate the effective shear rate for the gridblock sizes that are practical for large field simulations. We also present improvements in the polymer viscosity model. At high temperature, the rate of hydrolysis of polyacrylamide polymers is significant over reservoir time scales. A simple scheme was developed to account for the hydrolysis and the corresponding change in viscosity as the polymer flows through the reservoir. This study demonstrates that accurately computing polymer viscosity is important for the proper representation of not only the well pressure, but also for the pressure distribution substantially away from the wellbore, thereby significantly influencing the performance of chemical EOR processes that use polymer.