Abstract

This paper describes an experimental and theoretical study of the mechanisms governing polymer slug propagation through porous media. polymer slug propagation through porous media. An analytical model taking into account the macromolecule exclusion from pore walls is proposed to predict rodlike polymer velocity in porous media and thus the spreading out of polydispersed polymer slugs. Under conditions where this wall polymer slugs. Under conditions where this wall exclusion is maximum, i.e. at low shear rates and polymer concentrations, the experiments show that xanthan propagation is effectively predicted by this model. At higher flow rates and polymer concentrations, the effects of hydrodynamic dispersion and viscous fingering are analyzed. A new fractionation method for determining molecular weight distribution of polymers used in EOR is proposed. proposed

Introduction

The spreading out of polymer slugs during their porpagation through reservoirs is a very important problem because it car, lead to a loss in mobility control and thus to a decrease in polymer flooding efficiency. polymer flooding efficiency. Dawson and Lantzi observed that, in the presence of adsorption, polymer molecules move presence of adsorption, polymer molecules move through a porous medium more rapidly than water. They attributed this phenomenon to the existence of a pore volume fraction inaccessible to polymers because of the too large dimensions of macromolecules compared to pore throats. More recently, thermodynamic arguments predicting differences in polymer concentration at equilibrium between. bulk solution and pore wall vicinity and thus between pores of different sizes have been proposed to explain the differences in velocity through porous media between polymer and solvent. Since these mechanisms are strongly molecular size dependent, they are expected to cause a spreading out of polymer slugs depending on their polydispersity. However, no quantitative study of this phenomenon has been made.

This paper describes a theoretical and experimental study of polymer velocity through porous media in the absence of both adsorption and degradation. A theoretical model taking into account the pore wall exclusion mechanism is proposed for pore wall exclusion mechanism is proposed for calculating polymer velocity through porous media. Experiments were performed using xanthan solutions flowing through unconsolidated porous media. The respective influence of pore wall exclusion, hydrodynamic dispersion and viscous fingering on the spreading out at the leading edge and at the trailing edge of the polymer slug is analyzed.

From experiments carried out at different flow rates and polymer concentrations, experimental conditions for obtaining a maximum size fractionation of xanthan by wall exclusion chromatography are defined and provide a method for characterizing high-molecular-weight water-soluble polymers.

THEORETICAL CONSIDERATIONS
Mechanisms Determining Polymer Velocity

If we except the case of low permeability natural cores, the pore volume fraction which is strictly inaccessible to polymer molecules is too small to explain the significant differences in velocity, through porous media between polymer and solvent. However, the pore wall exclusion which is a quite general phenomenon, is expected to increase polymer velocity more strongly and thus to be able to explain the experimental observations.

Effects of Pore Wall Exclusion

In a polymer solution near a nonattractive wall such as a nonadsorbent solid surface, macromolecule centers of mass are entropically excluded from the wall vicinity over a distance related to macromolecule size. This phenomenon has been theoretically treated under static conditions for both flexible and rodlike polymers.

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