The Effects of Mechanical Degradation and Viscoelastic Behavior on Injectivity of Polyacrylamide Solutions

Results of recent experiments that clarify the effects of mechanical degradation and viscoelastic behavior on the flow of partially hydrolyzed polyacrylamide solutions through porous media are presented. From these results, a simple model that may be used to predict injectivity of polyacrylamide solutions is developed. Injection pressures for linear core floods are shown to be separable into two components:an initial pressure drop associated with the entrance of polymer into the sandstone anda constant pressure gradient throughout the remainder of the core. Entrance pressure throughout the remainder of the core. Entrance pressure drop is zero until the polymer solution flux increases to the rate where mechanical degradation takes place. Thereafter, entrance pressure drop and the degree of polymer mechanical degradation increase with increasing flux. In addition, polymer solutions that undergo a large entrance pressure drop and a high degree of mechanical degradation when first injected into a core show no entrance pressure drop and no further degradation after reinjection into the same core at the same flux. These observations suggest that the entrance pressure drop is associated closely with the process of polymer mechanical degradation. A new correlation is developed that may be used to determine entrance pressure drop and the level of mechanical degradation directly as a function of sand face flux, permeability, and porosity. This correlation is more convenient to apply and less dependent on flow geometry than previous correlations. Based on these observations, a model is developed that may be used to estimate injectivity of polyacrylamide solutions in linear or radial flow geometries. This model takes into account the entrance pressure drop and the dilatant nature of the polymer near the wellbore. Predictions made with this model are compared with experimental results. Introduction This paper reports an investigation of the influence of mechanical degradation and viscoelasticity on the injectivity of partially hydrolyzed polyacrylamide solutions. The viscoelastic nature of the polymer is important primarily at high fluxes that occur near a wellbore. However, mechanical degradation affects the mobility of a polymer bank at all positions within a reservoir. The approach in this report is first to re-examine the process of predicting polymer mechanical degradation in porous media and to simplify the prediction process so that is may more readily be applied to field situations. Next, results of recent experiments that clarify the effects of mechanical degradation and viscoelastic behavior on the flow of polyacrylamide solutions through porous media are presented. Finally, these results are used to develop a simple model to estimate injectivity impairment during a polymer flood. Mechanical Degradation Mechanical degradation means that fluid stresses become large enough to fragment polymer molecules, resulting in an irreversible loss of viscosity and resistance factor. This may happen a porous medium or through a constriction. Resistance factor is defined as the ratio of brine mobility to the mobility of a polymer solution. It may be thought of as the apparent relative viscosity of a polymer solution in porous media. Resistance factors of polyacrylamide solutions are often greater than viscosities. This suggests that polyacrylamides reduce water mobility both by increasing solution viscosity and by reducing effective permeability to water.1,2 Part of the permeability reduction is retained after a polyacrylamide bank is displaced by brine.