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

Abstract

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 corefloods are shown to be separable into two components: (1) an initial pressure drop associated with the entrance of polymer into the sandstone and (2) a constant pressure gradient 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.