Summary
Organically crosslinked gels have been used to control water production in high temperature applications. These chemical systems are based on the crosslinking of a polyacrylamide-based polymer/copolymer with an organic crosslinker. Polyethyleneimine (PEI) has been used as an organic crosslinker for polyacrylamide-based copolymers to provide thermally stable gels.
Literature reported that PEI can form aqueous gels with polyacrylamide (PAM) at room temperature. In this paper, we show for the first time the possibility of crosslinking polyacrylamide with PEI at temperatures up to 140°C (285°F) and pressures up to 30 bars (435 psi). This paper reports data both in bulk and in porous media.
The gelation time of the PAM crosslinked with PEI at high temperatures up to 140°C (285°F) and pressures up to 435 psi (30 bars) was measured. The effects of polymer concentration, crosslinker concentration, temperature, salinity, initial pH value, and the initial degree of hydrolysis of the polymer on the gelation time were examined in detail. All measurements were conducted in the steady shear mode. 13C Nuclear Magnetic Resonance Spectroscopy (13C NMR) was used to relate the gelation time to changes in the structure of the polymer and hence explain the variation in the gelation time in terms of the gelling system chemistry.
In bulk, thermally stable gels were obtained by crosslinking PAM with PEI at 130°C (266°F) for at least 8 weeks. The performance of the PAM/PEI system in sandstone cores at a temperature of 90°C (194°F) and pressure drops of 68.95 bars (1,000 psi) was examined. The system was found to be stable for 3 weeks, where the permeability was reduced by a factor of 100%.
Introduction
Water production is a serious problem in petroleum-producing operations. Additional costs are imposed by processing, treating, and disposing unwanted water. Of the available remediation techniques, chemical methods using polymer gels have been widely applied. The success rate of these chemical treatments depends, among other factors, on the understanding of gelation kinetics, gelant's compatibility with reservoir fluids, and thermal stability of the final gel.
Polymer gels have been used to reduce water production through the disproportionate permeability reduction (DPR) (Zaitoun and Kohler 1988; Liang et al. 1995). In DPR, the relative permeability to water is reduced to a greater extent than that to oil (or gas). Polymer gels were also used to totally block the pore space of the water producing zones in both matrix (Vasquez et al. 2003) and fractures (Alqam et al. 2001).
Polymer gels are generally classified into two categories based on the nature of polymer/crosslinker bonding chemistry. The first type is inorganic gel systems based on the crosslinking of the carboxylate groups on the partially hydrolyzed polyacrylamide chain (PHPA) with a trivalent cation like Cr(III) (Sydansk 1990; Lockhart 1994). This crosslinking is believed to rely on coordination covalent bonding. It should be mentioned that Cr(III)-carboxylate/acrylamide-polymer gels (CC/AP) were reported to be stable at temperatures up to 148.9°C (300°F) in Berea cores under pressure drops of 68.95 bars (1,000 psi) (Sydansk and Southwell 2000). The second class of polymer gels is based on covalent bonds between the crosslinker and the acrylamide-based polymer (Morgan et al. 1998; Moradi-Araghi 2000). High temperature applications require the use of thermally stable covalently bonded systems. However, these covalent bonds do not guarantee long-term stability. Literature reports (Moradi-Araghi 2000) highlight the importance of using a thermally stable polymer to produce thermally stable gels. Polyacrylamide-based polymers are known to hydrolyze at high temperatures causing gel syneresis (expulsion of water out of the gel structure due to over crosslinking) (Moradi-Araghi 2000), especially in brines with high contents of Mg+2 and Ca+2, where polymer precipitation may also occur (Moradi-Araghi and Doe 1984). Therefore, more thermally stable monomers are copolymerized with the acrylamide polymer to minimize excessive hydrolysis (Moradi-Araghi et al. 1987; Doe et al. 1987) and enhance thermal stability of the produced gel.