Influence of Polymer-Molecule/Wall Interactions on Mobility Control

Abstract

Abstract This paper presents the results of a study of molecule/wall interactions on permeability modification of consolidated porous media by polymer solutions. The experiments were conducted with a newly developed low-shear porous media viscometer. This is a simple-to-use, versatile instrument that is particularly useful for measurements at the low shear rates characteristic of reservoir flooding. The key for obtaining reproducible, steadystate results was to expose the porous medium to several hundred pore volumes of polymer solution to saturate it with polymer. The effective permeability during polymer flow and the residual permeability were determined for xanthan gum and polyacrylamide solutions in Berea sandstone, Bradford sandstone, filter papers, and Nuclepore filters. A mechanistic interpretation of the coupling of adsorption, mechanical entrapment, shear rate, and inaccessible pore volume effects on the effective and residual permeabilities was developed. This is the first study to show that inaccessible pore volume can influence the residual permeability significantly. Introduction Solutions of high-molecular-weight polymers are being used as modified waterfloods and to control the mobility of the waterflood that follows the chemical slug in enhanced oil recovery. Currently, two distinctly different polymers are used most commonly for this application. The most popular mobility-control polymer is partially hydrolyzed polyacrylamide. This polyelectrolyte is sensitive to electrolytes and is susceptible to mechanical degradation. The second most frequently used mobility-control polymer is a polysaccharide called xanthan gum. This biopolymer is produced by a fermentation process and is less sensitive to electrolytes and shear degradation than polyacrylamide. Polyacrylamide increases the viscosity of aqueous solutions and causes changes in the permeability of porous media by adsorption and mechanical entrapment in pores whose dimensions are the same order of magnitude as the dimensions of the polymer in solutions. Numerous investigators have shown that polyacrylamide reduces the permeability of porous media during flow and that some of this permeability reduction is permanent. It generally is considered that xanthan gum reduces the mobility of a solution in porous media mainly by increasing the viscosity of the solution and that the action of the xanthan gum on the permeability is insignificant.The purpose of this study is to investigate the influence of polymer-molecule/wall interactions on mobility control. This investigation uses studies on the flow of xanthan gum and polyacrylamide solutions in various kinds of porous media with a wide range of characteristics. Although the permeability modification caused by xanthan gum molecules is not as pronounced as that caused by polyacrylamide, the polymer/wall interactions with this biopolymer are significant. Results of permeability-reduction studies during polymer flow and the residual permeability reduction as functions of shear rate, initial permeability, hydrodynamic size of polymer molecule in solution, electrolyte (NaCl) concentration, polymer concentration, and porous media characteristics are reported. The experiments were conducted with a newly developed low-shear porous media viscometer. Permeability modifications during and after polymer flow can be determined accurately with this simple instrument that eliminates the need for pumps and pressure measuring devices. The results of this investigation have been used to develop a mechanistic interpretation for the influence of molecule/wall interactions on mobility, which incorporates adsorption, mechanical entrapment, shear rate, and inaccessible pore volume effects. SPEJ P. 613^