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The general conditions for gel formation by phenolformaldehyde polymer solutions have been examined in studies with three acrylamide polymers, Contrary to an earlier repom 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. l%ese getants display good injectivity in coreflood and slim-tube experiments at temperatures'upto 140"C. On the other hant he 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.

A coordination chemical approach for delaying the gelation of Cr+3/polymer compositions has been explored. By screening organic ligands for their gelation delaying power with hydrated Cr+3/polymer and Cr(OAc)3/polymer solutions several powerful retarding ligands have been identified. Use of the pre-formed Cr+3 complexes of these ligands together with additional, uncomplexed ligand in the gelant solution provides outstanding control over the gelation time over the temperature range 60°-135°C. With these compositions gelation times spanning the range from several hours to one month or more have been obtained at temperatures up to 120°C; such gelation delays should be suitable for a number of near-well and in-depth gel treatments. In virtue of the low toxicity of Cr+3 and its complexes, the potential environmental impact of the new gelation compositions is notably reduced relative to those employing Cr+6 or formaldehyde. A number of variables have been evaluated for their influence on the gelation delay, and preliminary experimental results on the propagation of Cr+3 in porous media at high temperature are reported. Our current mechanistic understanding of the gelation delay chemistry is also described.

Summary Coordination chemistry concepts have been used in the development of Cr+3/polymer solutions that meet the delayed-gelation requirements for in-depth treatment of high-temperature reservoirs. Use of the malonate complex of Cr+3, Cr(malonate)3, provides gelation delays at elevated temperature much greater than those obtained with either the hydrated Cr+3 ion or Cr(acetate)3. Inclusion of additional, uncomplexed malonate ions in the formulations provides a means for further, and predictable, extension of the gelation time. The gelation delays obtained over the temperature range of 90 to 135°C are equal to or greater than those that can be achieved with the best gelation delay technologies described previously. The dependence of the gelation time on temperature, pH, and the Cr+3 and malonate ion concentrations has been investigated. Preliminary results indicate that the thermal decomposition of the delaying ligand, malonate, plays an important role in determining the gelation rate. The use of a Cr+3 complex of relatively low toxicity rather than the carcinogenic Cr+6 ion to control gelation rate is an attractive feature of the new technology. Introduction Development of aqueous polymer gel technology for EOR applications commands increasing research interest. Targeted applications include profile modification treatments of heterogeneous reservoirs to bring about better contact between injected fluids or gases and the reservoir strata containing residual mobile oil. Also under active development are treatments on producer wells to reduce water production. The extensive application of polymer gel technology, however, will require resolution of several technical problems, including (1) the currently limited possibilities to achieve delayed gelation in reservoirs at elevated temperature, particularly by means of an environmentally acceptable technology; (2) the limited possibilities for directing the gelant solutions to specific targeted zones within the reservoir; and (3) the limited stability of polymer gels under high-temperature, high-salinity reservoir conditions, owing to polymer hydrolysis and oxidative degradation. This paper addresses the first of these challenges. We previously described the results of our efforts to understand more fully the crosslinking chemistry of aqueous Cr+3/polymer gels. Those studies were motivated by our desire to develop the potential of this class of polymer gels for EOR applications. Here, we introduce a new and simple chemical approach to controlling the gelation reaction of Cr+3/polymer solutions that substantially meets the gelation delay requirements for in-depth treatments of high temperature reservoirs.

This paper introduces a new strategy for improving the high temperature stability of acrylamide polymer solutions and gels in aqueous solutions containing the divalent cations Ca2+ and M@+.Specifically, certain low molecular weight compounds capable of complexing with the divalent cations are shown to reduce signiticantly their negative impact on the solubility of acrylamide polymers and on the stability of crosslinked acrylamide polymer gels with regard to syneresis. Where the inhibitordivalent ion complexes fonned are soluble, it should be possible to propagate their polymer or gelant solutions through matrix rock; where insoluble divalent ion-inhibitor complexes are formed, these inhibitors may still be compatible with gel placement in fkactured reservoirs or in the immediate vicinity of the weubore. The results obtained offer the possibility to extend the upper temperature limit for the use of polyacrylamides and acrylamide copolymers in brines in both polymer floodq and polymer gel treatments.Related studies show that pH is a key factor influencing the solubility of acrylamide polymers in the presence of divalent cations. This fundamental variable has been overlooked in earlier studies correlating PAAm solubility with the degree of polymer hydrolysis, divalent cation concentration, and temperature.