Mobility Control with Partially Hydrolyzed PolyacrylamideA Reply to Emil Burcik
Authors: Lynch E.J. , Macwilliams D.C. 
Published: December 2007

The April, 1969, JPT Forum contained an article by Burcik in which it was suggested that "microgels" were the principal, if not the sole agent responsible for the mobility control that is obtained with polyacrylamide solutions. These gels were described as polyacrylamide solutions. These gels were described as chemically linked aggregations of polymer molecules. Although we agree with much of the data that he presents, having observed these phenomena also, we presents, having observed these phenomena also, we cannot agree with the conclusions, both real and implied, that are drawn therefrom. In particular, we feel that the presence of microgels is not only unnecessary, but actually detrimental to the performance of a polyacrylamide solution. polyacrylamide solution. A linear polymer molecule in solution is usually considered to be a series of segments, each moving freely and randomly, subject to the restrictions of other segments. Conversely, a nonlinear, multi-molecular system may have structures resulting from entanglement, branching, hydrogen bonding, and/or chemical crosslinking by anhydride, imide, or methylene bisamide formation. The resistance factor of a polymer solution, as measured with a short core, can polymer solution, as measured with a short core, can be made very high by introducing the proper amount of nonlinearity into the molecules. The true nature of this polymer is revealed when a long core is used, however, for then it is discovered that the nonlinear material is permanently removed from solution by filtration through the first inch or two of rock, There may be no evident gel buildup on the core face. Because Burcik used discs only 1/8-in. thick for measuring resistance factor, he was not able to observe this effect. To translate this into practical terms, a lightly cross-linked polymer will limit the injection rate of a well, and will do little for mobility control in the center of the field. For this reason, suppression of chemical cross-linking is of constant concern in the manufacture of Pusher brand polymer. As discussed in another paper, Burcik calculated the root-mean-square end-to-end distance to be 500for a polyacrylamide molecule having a molecular weight of a 3 x 10(6). He also found that some polymer could be strained from a solution of this type of polymer could be strained from a solution of this type of material with a Millipores filter having 0.45 mu (4,500 ) pore openings. His logical conclusion was that the polymer must exist in solution as aggregates of many polymer must exist in solution as aggregates of many molecules. The error in this approach lies in the calculation of molecular size, since the equation which was used is valid only for a polymer molecule that is a non-electrolyte in a Theta solvent. Partially hydrolyzed polyacrylamides dissolved in water or dilute salt solutions do not satisfy these requirements. A more reasonable calculation of polymer size may be made from intrinsic viscosity, [eta], which can be estimated for unhydrolyzed polyacrylamide from the following correlation:

(1)

where Mw is the weight average molecular weight. For Mw = 3 x 10(6), [eta] = 7.7. The mean square end-to-end distance square root of r can then be calculated from the relation given by Flory:

(2)

Measurements on partially hydrolyzed polymer in 3 percent salt solutions indicate that repulsion between charged groups on the polymer backbone creates a viscosity at least double that of an unhydrolyzed polymer. Therefore, the molecular size would be at polymer. Therefore, the molecular size would be at least 2,870 for partially hydrolyzed material. Polymerization does not produce molecules of uniform size. A typical vinyl-type polymerization yields a significant fraction of material having a molecular weight that is 2 1/2 times the average.

P. 1247