Martin Bartosek

Laboratory evaluation of phenol-formaldehyde/polymer gelants for high-temperature applications

Abstract Polymer gels are attractive tools for stopping unwanted fluid production from oil and gas wells, but conventional gelant formulations become increasingly difficult to apply at higher formation temperatures. In contrast phenol—formaldehyde cross-linked polymer gelants remain viable to at least 140°C. Choice of polymer composition permits control of the gelation delay, and the gelants display good injectivity even when the residence time approaches the gelation time. Cross-linking occurs over a wide range of pH and is insensitive to lithology. Although retention of the gelant components on formation rock is negligible, phenol strongly partitions into oil phases contacted by the gelant. Experiments and numerical modelling show that a preflush of phenol is an effective method of compensating for phenol partitioning in the formation.

Studies on Phenol-Formaldehyde Crosslinked Polymer Gels in Bulk and in Porous Media

The general conditions for gel formation by phenol-formaldehyde polymer solutions have been examined in studies with three acrylamide polymers. Contrary to an earlier report, polymer crosslinking is found to take place over a wide interval of pH. While the gelation time is relatively insensitive to the concentrations of phenol and formaldehyde or pH, it is strongly influenced by the temperature and the nature of the polymer. These gelants display good infectivity in corefloods and slim-tube experiments at temperatures up to 140 C. On the other hand, the partitioning of phenol into crude oil is found to be a significant issue for the propagation of these gelants. The use of a phenol pre-flush of the formation is shown by numerical modeling to be a potentially viable solution for this problem.

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.