This article describes a crosslinking material derived from natural sources that can be used with 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 regulatory agencies and have now 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 selected for study.
This article details the laboratory development of an environmentally friendly, natural polyamine crosslinker system, namely chitosan. This crosslinker can be used with a variety of polymers, such as polyacrylamide, AMPS/acrylamide (AMPS/AA), or alkylacrylate polymers. Gels ranging from stiff and "ringing" type to "lipping" have been obtained. Additionally, this article summarizes results from recent efforts targeted toward designing chitosan-based gel compositions with improved environmental rating for potential field use. These efforts included using chitosan as the base polymer in combination with a synthetic polymeric crosslinker, as well as developing gels derived completely from natural polymers.
This study primarily focused on conformance water-based gels designed for application in wells in which the oil- and water-producing zones were clearly separated and could be mechanically isolated. Water-based gel systems for conformance applications included both chromium (+3) crosslinked polyacrylamide and phenol/formaldehyde crosslinker systems for homo-, co-, and ter-polymer systems containing acrylamide.
Recently, a less toxic crosslinker tested extensively in field trials worldwide enjoyed a high success rate. This system is based on PEI crosslinker and a copolymer of acrylamide and t-butyl acrylate (PA-t-BA). PEI is a low-toxicity material approved for food contact. Recent test results indicate that a variety of polymers containing amide pendant groups are crosslinked by PEI, presumably through a transamidation reaction pathway to provide gels (Fig. 1).
However, in some countries, PEI is targeted for replacement by systems that are less corrosive, less toxic, less bioaccumulating, and which show good biodegradability. Base polymers used in conformance gel systems containing a carbon/carbon backbone, although of low toxicity, are resistant to biodegradation and tend to bioaccumulate or persist in the environment for long periods.