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

R.S. Seright – 1983

Abstract

This paper investigates the influence of mechanical degradation and viscoelasticity on the injectivity of partially hydrolyzed polyacrylamide solutions. Injection pressures in linear corefloods are shown to be separable into two components: (1) an entrance pressure drop associated with polymer entering the sandstone, and (2) a constant pressure gradient throughout the remainder of the core. Entrance pressure drop is zero until polymer flux reaches the mechanical degradation threshold; thereafter it increases with flux. A new umax/dgr² correlation is developed to predict entrance pressure drop and degradation level directly from sandface flux, permeability, and porosity. This correlation eliminates the need for iterative procedures and is applicable to any flow geometry. A model is developed to estimate polyacrylamide injectivity in both linear and radial flow, accounting for entrance pressure drop and dilatant behavior near the wellbore.

Key Takeaways

  • Linear coreflood injection pressure is separable into two components: entrance pressure drop (at the sandface, associated with mechanical degradation) and a constant bulk core pressure gradient — a physically meaningful decomposition for injectivity modeling.
  • Entrance pressure drop is zero until polymer flux exceeds the mechanical degradation threshold; beyond that threshold, both entrance pressure drop and degree of degradation increase with increasing flux.
  • A umax/dgr² correlation predicts entrance pressure drop and mechanical degradation level directly from sandface flux, permeability, and porosity — eliminating the need for iterative calculations and applicable to any flow geometry.
  • Polymer solutions that undergo high entrance pressure drop and high mechanical degradation during first injection show zero entrance pressure drop and no further degradation upon reinjection into the same core at the same flux — confirming degradation is an irreversible, entrance-dominated process.
  • A comprehensive injectivity model for polyacrylamide solutions is developed that combines the entrance pressure drop correlation with the dilatant (shear-thickening) near-wellbore behavior — applicable to both linear and radial flow geometries.
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