A New Gelation Technology for In-Depth Placement of Cr+3/Polymer Gels in High-Temperature Reservoirs

Abstract

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.