Influence of Polymer Structural Conformation and Phase Behaviour on In-situ Viscosity

Polymer flooding is primarily implemented to accelerate oil production by sweep improvement and/or to reduce oil-water mobility ratio in heavy oil fields. During the last decade it has been evaluated for an increasing number of fields, both offshore and onshore. Although it is a mature EOR (Enhanced Oil Recovery) method, there are still a number of challenges with respect to describing polymer flow in porous media. This increases uncertainty and risk in planning and evaluation of polymer flood projects. A key issue is the prediction of in-situ viscosity. This study evaluates in-situ viscosity from the perspective of the different conformational states or phase behaviour classes of a typical synthetic polymer, HPAM (partially hydrolysed polyacrylamide). In-situ viscosity is determined as a function of concentration, molecular weight and salinity. The main objective is to improve the description and prediction of in-situ viscosity based on a classification of polymer phase behaviour. The experiments were performed using low and high molecular weight HPAM polymers dissolved in two different brines. In-situ viscosity was determined by core floods for 4 different concentrations in the dilute, semi-dilute, concentrated and gel solution regimes. The results reveal how salinity is a dominating factor for in-situ viscosity in the dilute and semi-dilute concentration regime, but is less important at higher concentrations. In the upper semi-dilute and concentrated regimes, molecular weight is the dominating factor for in-situ viscosity, but show an unexpected trend with regard to shear. The results also give insight on the influence of polymer elasticity on resistance factor (RF) and the influence of molecular weight, salinity and polymer concentration on residual resistance factor (RRF). The presented work show that the polymer conformational state should be considered when designing field implementation of polymer flooding.