The Mechanism of Microgel Formation in Partially Hydrolyzed Polyacrylamide

Abstract

The reduction of water mobility by solutions of partially hydrolyzed polyacrylamides is brought about partially hydrolyzed polyacrylamides is brought about not only by an increase in viscosity but also by a decrease in effective permeability to water. This decrease in water permeability is due to retention of the polymer within the pore channels so that there is a marked reduction of flow. In addition to this resistance effect it was found that the polymer retained within the pore channels causes pseudo dilatant flow at high rates of shear. This increase in viscosity with increasing rate of flow may be important in displacement of oil, particularly in systems with a markedly heterogeneous permeability. However, until recently it was not understood how the polymer retained within the pore structure could restrict flow since even though these polymer molecules have molecular weights of the order of several million there is still a pronounced disparity between molecular size and the pronounced disparity between molecular size and the diameters of the main flow channels in a typical porous medium. porous medium. In a recent paper it was shown that solutions of partially hydrolyzed polyacrylamides may contain partially hydrolyzed polyacrylamides may contain gellike complexes of a size comparable with that of the pores in a porous medium. It was suggested that this pores in a porous medium. It was suggested that this microgel, which should be of proper size to give optimum results, is largely responsible for the observed reduction in mobility and is formed by a crosslinking of some of the polymer molecules. It was shown experimentally that the size of the microgel not only was a function of the degree of hydrolysis of the polyacrylamide but also was greatly dependent on the polyacrylamide but also was greatly dependent on the intensity of drying employed in the preparation of the solid polymer. The purpose of this paper is to describe other methods of controlling microgel size and to present new experimental evidence that seems to indicate present new experimental evidence that seems to indicate that the mechanism of polymer crosslinking is largely due to anhydride linkages formed by elimination of water from COOH groups present along the polymer chain. A factor that has been found to have a profound effect on the formation of microgel is the pH of the solution from which the polymer is precipitated prior to drying. A homopolymer of acrylamide (MW = 3.5 x 10 (6)) was hydrolyzed in the usual manner by heating a solution containing the required amount of sodium hydroxide to accomplish the desired degree of hydrolysis. The pH was adjusted to either 8.0 or 11.5; then the polymer was precipitated in alcohol and both samples were rigorously dried for equal times in an oven at 95C. A 500 ppm solution of each polymer was then prepared in 2 percent NaCl and the resistance factor determined by flow through a Berea sandstone disc. The polymer that was precipitated from the solution with the higher pH exhibited a lower resistance factor in the porous medium. For example, an oven-dried polymer that was 3 6 percent hydrolyzed had a resistance factor of 12.4 if precipitated from a solution having a pH of 8.0, but only 3.04 if precipitated from a solution having a pH of 11. 5. precipitated from a solution having a pH of 11. 5. The degree of association due to crosslinking, and consequently the resistance factor, could be reduced, especially when the pH of the solution was increased concurrently, simply by heating dilute solutions of the crosslinked polymers. For example, a 500 ppm solution of an oven-dried 43.8 percent hydrolyzed polyacrylamide was prepared in 2 percent NaCl. Before this solution was heated the resistance factor was essentially infinity in Berea sandstone (the core was completely surface plugged). After the solution was heated for about 4.5 hours at 95C the resistance factor was reduced to 2.16. Similar results were achieved with a solution of Pusher (supplied by the Dow Chemical Co,), which decreased the resistance factor from 4.7 to 2.6 following heating. P. 373