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

Abstract

Abstract 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 injectivity 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. Introduction Recent successful applications of crosslinked polymer gels for reducing produced water have renewed the interest in this technology within the industry. The near-wellbore treatment of matrix formations with polymer gels has been shown to be an effective means for achieving the total shutoff of selected formation intervals, and has been used with success on producer wells to reduce both water and gas production. Near-wellbore treatments can also be used to modify the conformance of injected fluids where crossflow is not a consideration. Other impressive results have been achieved in water-flooded formations, where polymer gels have been used to close conductive fractures. The limited control over the gelation time and the limited stability at elevated temperature of the current gel-forming compositions (hereafter "gelants"), however, pose constraints on the broader application of this technology. Our research program has focused closely on these issues with the objective of extending gel technology to the treatment of high temperature reservoirs. Current evidence suggests that polymer gel stability is largely determined by the stability of the polymer molecules themselves, and several copolymers of acrylamide have been developed that offer enhanced stability to harsh conditions (high temperature, high divalent cation brine concentrations) relative to polyacrylamide (PAAm), Sydansk, on the other hand, has shown that it is possible to form polymer gels of high stability even with PAAm, if a low (eg., 5x105) molecular weight form is used in high concentration together with a high concentration of Cr(III) crosslinker. We have recently reported that additives capable of sequestering divalent cations can be used to extend the stability of polymer solutions and gels to higher temperature. By means of these approaches, we find that polymer gels can be made to resist extended aging in seawater brine to temperatures as high as 140 C. The rate at which a fluid polymer solution undergoes gelation is controlled by the crosslinking chemistry. Two principal types of crosslinkers, metal ions [Al (III) and Cr(III)] and organic systems (particularly phenol-formaldehyde) have been employed in the field with PAAm and acrylamide copolymers, Whereas the Al(III) crosslinkers appear to be suitable only for low temperature applications on account of their fast reaction with polymer, Cr(III) crosslinkers offer much better control over the gelation time. The widely-employed Cr(III)-acetate crosslinker can provide gelation delays of up to a few hours at temperatures as high as 70 C; P. 403