Propagation of Cr(III) in porous media and its effect on polymer gelant performance

A simple thermodynamic interpretation of the aqueous Cr(III)-ligand/polymer system is used to explain its chemical behavior, both in static tests and during flow in porous media. Three aspects of this behavior are elaborated and their practical implications discussed. First, the extent to which Cr(III) is lost from solution, e.g., by hydrolysis, depends upon the stability of the Cr(III)-ligand complex, which in turn depends strongly upon pH, temperature and the chemical nature and concentration of the ligands. This stability dictates how well Cr(III) propagates through porous media, although the Cr(III) concentrations observed on time scales relevant to some applications are rate limited rather than equilibrium limited. Second, polymer functional groups are themselves strong ligands, so that gelation time reflects the competition for Cr(III) between polymer, ligand and hydroxide ions. Interaction between the solution and a porous medium can strongly influence this competition, resulting in crosslinking rates different from those observed in bulk. Third, Cr(III) retention due to hydrolysis is predicted to be reversible. Experiments confirm this prediction, and retained Cr(III) is also shown to react with polymer. The interpretation presented here also pemits an assessment of the performance limits of the system at high temperatures. The multi-faceted competition for Cr(III) within a porous medium makes the balance between too little ligand and too much very delicate at high temperature, implying that forming gels at large distances from the wellbore will be difficult in high-temperature formations. In contrast, for near-wellbore treatments Cr(III)/polymer gelants are robust, because the reactivity of retained Cr(III) renders them self-correcting for poor propagation.