RIASSUNTO
Abstract
This paper describes a material derived from natural sources that can be used to crosslink a variety of polymers over a broad temperature range to produce gels for conformance applications.
Delayed crosslinked polymer systems have been used for many years in conformance applications. For the past decade, the most widely used system has been based on chromium (+3) crosslinked polyacrylamide. Organic crosslinkers, such as phenol/formaldehyde and polyethyleneimine (PEI) have also been used with a variety of polymers. However, these systems are being scrutinized by governmental agencies and have been scheduled for phase-out in some countries. Because of these issues, a single, environmentally friendly crosslinker that could be used with a variety of polymers over a broad temperature range was the focus of this study.
This paper details the laboratory development of an environmentally friendly, natural polyamine crosslinker system. This crosslinker can be used with a variety of polymers, such as polyacrylamide, AMPS/acrylamide, or alkylacrylate polymers. Gels ranging from stiff and ringing type to ""lipping"" gels have been obtained. The data illustrate a simple, commercially available system that can be applied to field operations. Potential crosslinking mechanism(s) of the system will be discussed.
Introduction
Water production in oil-producing wells becomes a more serious problem as the wells mature. Remediation techniques for controlling water production, generally referred to as conformance control, are selected on the basis of the water source and the method of entry into the wellbore. Treatment options include sealant treatments and relative permeability modifiers (also referred to as disproportionate permeability modifiers). This paper primarily discusses water control with water-based gels for applications in wells in which the oil- and water-producing zones are clearly separated and can be mechanically isolated.
Chromium (+3) crosslinked polyacrylamide gels can be choice materials for matrix-fluid shut-off systems.1-4 The crosslinking reactions in these gel systems take place by the complexation of Cr (+3) ions with carboxylate groups on the polymer chains (Fig. 1).
Because of the nature of the chemical bond between Cr (+3) and the pendant carboxylate groups, formation of insoluble chromium species can occur at high pH levels. Other problems with these systems include thermal instability, unpredictable gel times, and gel instability in the presence of chemical species that are potential ligands. The gel times are controlled by the addition of materials, which chelate with chromium in competition with the polymer-bound carboxylate groups.5,6 Perhaps the most important drawback for the chromium-linked gels is the toxicity concern with the metal, despite the fact that a +3 oxidation state is less toxic than a +6 oxidation state.
Another popular water-based gel system for water-control applications is based on a phenol/formaldehyde crosslinker system for homo-, co-, and ter-polymer systems containing acrylamide.7-11 Depending on the polymer composition, these gels are thermally stable, and the gel times are controllable over a wide temperature range. The crosslinking mechanism involves hydroxymethylation of the amide nitrogen, with the subsequent propagation of crosslinking by multiple alkylation on the phenolic ring (Fig. 2).12,13 Several variations of the same technology were created to overcome the toxicity issues associated with formaldehyde and phenol. These processes generally involve replacing formaldehyde and phenol with less toxic derivatives that generate phenol and formaldehyde in situ, or are themselves active components of the crosslinking system. For example, formaldehyde can be replaced with hexamethylene tetramine (HMTA), glyoxal, or 1-, 3-, or 5-trioxane. Substitutions for phenol included phenyl acetate, phenyl salicylate, or hydroquinone, among others.12,13 Extensive patent literature for this technology exists.14-22