RIASSUNTO
Abstract
This paper describes a crosslinking material derived from natural sources. This material can be used with a variety of polymers over a broad temperature range to produce gels for conformance applications.
Delayed cross-linked 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+) cross-linked polyacrylamide. Organic crosslinkers, such as phenol/formaldehyde and polyethyleneimine 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 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. Recent efforts have targeted designing chitosan-based gel compositions. These efforts included not only using chitosan as the base polymer in combination with a synthetic polymeric cross-linker, but also developing gels derived completely from natural polymers. Results from these studies will be presented.
Introduction
As oil-producing wells mature, water production becomes a more serious problem. Remediation techniques for 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. These gels are designed for applications in wells in which the oil- and water-producing zones are clearly separated and can be mechanically isolated.
In the past, chromium (+3) crosslinked polyacrylamide gels have been used as matrix-fluid shut-off systems.1-4 The crosslinking reactions in these gel systems occur through the complexation of Cr (+3) oligomers with carboxylate groups on the polymer chains. 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 values. 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 that chelate with chromium in competition with the polymer-bound carboxylate groups.5,6
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. 1).12,13 Several variations of the same technology were created to overcome 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.12-22
Recently, a less toxic crosslinker was tested extensively in field trials worldwide and has enjoyed a very high success rate.23-27 This system is based on polyethyleneimine (PEI) crosslinker and a copolymer of acrylamide and t-butyl acrylate (PA-t-BA). PEI is a low-toxicity material that is approved in the United States for food contact.28-31 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. 2).32
However, due to recent changes in European environmental regulations, PEI is targeted for phase-out from the Norwegian section of the North Sea within the next few years. Additionally, the base polymers used in conformance gel systems typically contain a carbon-carbon backbone. Such polymers, even though they contain low toxicity, are resistant to biodegradation and tend to bio-accumulate or persist in the environment for long periods.