The Mechanism of Reduction of Water Mobility by Polymers in Glass Capillary Arrays
Authors: Thomas C.P. 
Published: December 2007

Abstract
Flow experiments with polymer solutions in arrays of glass capillaries with diameters of 2,5, and 25 microns have produced physical evidence of the mechanism by which polymers and polymeric gels reduce water mobility in porous media. The results indicate that polyacrylamides and polysaccharides reduce the permeability of straight glass polysaccharides reduce the permeability of straight glass capillary arrays by forming an adsorbed layer of polymer on the capillary wall that reduces the polymer on the capillary wall that reduces the effective size of the capillary. The effective thickness of the polymer layer on the glass surface was about 0.2 to 0.3 micron. The thickness was slightly less than the molecular diameter of 0.3 to 0.8 micron of the molecules in 1,200-ppm brine and was found to be independent to pore size for pores with diameters larger than three to four times the average molecular diameter of the polymer in solution. it was concluded that the reduction of water mobility by polymers in a porous medium, over and above the effect of increasing the viscosity, results form an adsorbed layer of polymer on the pore walls, and from mechanical entrapment in pore constrictions and small pores. It was shown that a previously reported sequential polyacrylamide-cation treatment method could polyacrylamide-cation treatment method could be repeated to achieve very large decreases in mobility to brine. The cross-linked layer established during the first sequence was three to five times more effective in reducing the pore diameter than the initial polyacrylamide layer. For example, the average thickness of the cross-linked layer formed with each sequence of particular polyacrylamide (19-percent hydrolyzed) was 1.5 microns, compared with 0.3 microns for the initial polyacrylamide layer. No mixing of polacrylamide and crosslinking solutions was required for this sequential system to be effective, since extensive flushing with water between the solutions in the sequence did not alter the results. Hence, it should not be necessary to propagate highly viscous solutions or gels through a reservoir to develop large residual resistance factors in depth with the sequential system.

Introduction
Investigators of polymer effects on water mobility in porous media normally have used outcrop sandstones, reservoir cores, glass bead packs, and sand packs, all of which have complex flow paths and surface characteristics that vary somewhat from sample to sample. As a result, it has been difficult to produce direct physical evidence of the mechanism by which polymers and polymeric gels reduce the permeability of these porous media, and researchers have postulated various mechanisms based on the combined interaction of polymer and geometry to explain the observed effects. We used fused-glass capillary arrays to investigate the mechanism of permeability reduction. The glass] arrays provided a means for determining effects of polymer in geometrically simple pores of the same polymer in geometrically simple pores of the same size as those in reservoir porous media; since they could be used repeatedly, the same geometry and surfaces were available for each test. The main purpose of this research was to provide direct physical evidence of how and where provide direct physical evidence of how and where polymers interact with surfaces of porous media. polymers interact with surfaces of porous media. We were primarily concerned with effects caused by polymer adsorption and retention remaining after polymer solutions had been displaced from porous polymer solutions had been displaced from porous media by water. Other aspects of polymer flooding, such as non-Newtonian viscosity effects, inaccessible pore volume, behavior attributed to pore volume, behavior attributed to viscoelastic properties of polymer solutions, salt concentration, pH, polymer molecular weight, and degree of hydrolysis, have been investigated previously in more conventional porous media. previously in more conventional porous media. Additional evidence is provided on how the combination polyacrylamide-cation treatment method,]composed of sequential injection of solutions of polyacrylamide and aluminum citrate, produces large polyacrylamide and aluminum citrate, produces large residual resistance factors to water and brine. This evidence provides a basis for tailoring in-situ polymer networks to the needs of a particular polymer networks to the needs of a particular reservoir
SPEJ
p. 130