Kinetics of the Chromium(VI)/Thiourea Reaction in the Presence of Polyacrylamide

Abstract

Abstract The formation of chromium(III)-polyacrylamigels was followed kinetically using spectrophotometric methods. The rate of chromium(VI)species conversion by thiourea and bisulfite was monitored to find a kinetic model for the oxidation/reduction reaction. The third order model found here shows first-order dependence on chromium(VI) concentration on thiourea and on pH. Additional results of the thiourea study show that a constant amount of chromium(VI) is consumed at gelation regardless of the initial concentration of that species, indicating that the CR(VI) reduction reaction may be the rate-determining step in gelation. No such model or constant conversion for the bisulfite-reduced reaction could be determined. Introduction The use of high viscosity polymeric fluids and gels for water flow control is well established, with the primary application in enhanced oil recovery. Injection of such fluids can result in selective permeability reduction in formations where secondary recovery techniques have become ineffective. Because injected solutions of high molecular weight polymers are not retained after large quantities of water flush the formation, more durable gelled polymers have been developed. The gelling solution typically consists of a sufficient concentration of metal ion and the polymer, in aqueous solution. Gels made with chromium(III) and polyacrylamide were found to be particularly stable and polyacrylamide were found to be particularly stable and resistant to elution. Because the CR(III) ion is a strong complexing agent, the necessary bonding with the polymer is initiated only by controlled increase in the PH of the solution — a process difficult to achieve in-situ. The preferred method of gelling such solutions is to generate CR(III)from the CR(VI) form using a reducing agent. Huang also found that the type of reducing agent used in such a gelling solution radically affected the time required for gelation after mixing. Thus, it was hypothesized that the oxidation/reduction reaction is the rate-determining step in the overall gelation sequence. The Cr(III)-polyacrylamide gelling, system was studied by Terry, et al., and Huang to obtain useful process design data. They found the gelling reaction rate increases when the concentration of any of the three reactants is increased. The time required for gelation was found to be very sensitive to temperature. Jordan found that the gelation time of a specified system decreases as the temperature is increased. Plots of the logarithm of gelation time versus the reciprocal of the absolute reaction temperature were found to be linear for the systems studied. This correlation showed that the system apparently followed the Hurdand Letteron model, which assumed rate dependence on only one species. The usefulness of gelled polymers in reservoir applications is dependent on having strong gel formation with proper timing so gelation occurs in the desired location. Chromium(III)/polyacrylamide solutions with gel times ranging from half an hour to several months can be made by varying the concentrations of the reactants. It is desirable to predict the rate of the redox reaction for any predict the rate of the redox reaction for any combination of reactants with a kinetic model. Such a model was developed, to describe the reaction of CR(VI) with thiourea as reducing agent. All work has been done in aqueous polyacrylamide solutions. BACKGROUND: CHEMICAL SYSTEM, PROCEDURE, AND EQUIPMENT Chemical System The reacting systems consisted of three components in aqueous solution. Solutions of chromium(VI) ion, polyacrylamide, and reducing agent were mixed and observed as the reaction progressed. The chromium ion was added as a solution progressed. The chromium ion was added as a solution of sodium dichromate dihydrate (Na2Cr2O7.2H2O) The exact nature of the entire gelling process is unknown, and probably consists of a number of reaction steps. P. 397