Slow-gelling Cr+3/polyacrylamide solutions for reservoir profile modification: Dependence of the gelation time on pH



Systematic studies of the gelation of both buffered and unbuffered aqueous Cr+3/polyacrylamide solutions show that the gelation time is a strong function of pH, increasing by about one order of magnitude per unit decrease in pH for polyacrylamides less than about 7.5% hydrolyzed. The gelation rate also depends strongly on the Cr+3 concentration, the degree of polymer hydrolysis, and temperature. At 25°C, gelation delays of more than 10 months have been obtained; the maximum delay observed at 60°C is about 1 month and at 90°C 1 d. The resistance of buffered gel solutions to change in pH results in significantly longer gelation times at 90°C and low pH than for identical unbuffered solutions; at lower temperatures, however, the differences are insignificant. Most importantly, the use of low pH to control the gelation time of Cr+3/polymer solutions provides an attractive, inexpensive substitute for the environmentally unacceptable Cr+6/reductant method currently employed for profile modification treatments.