Pressure Drop And Flow Distribution of A Polymer Solution In A NonmHomogeneous Porous Medium
Authors: Vossoughi S. , Seyer F.A. 
Published: March 2010

Introduction

In oil displacement from porous media by dilute polymer solutions, oil recoveries are often much higher than with conventional water floods. In this study, the porous medium consisted of rows of glass rods similar to those used in earlier studies, but with non-uniform spacing of the rods. Using both polynte1' solutions and Newtonian fluids, flow distributions have been measured in individual pores with streak photographs taken through a microscope. Pressure drops have also been recorded over a wide 1'a1lge of flow rates.

A simple parallel-plate capillary model fm' the bed was sufficient to interpret all of the Newtonian data, including the effect of pore size on flow rate through the pore. For the polymer solution, the data show that the dependence of flow rate on the size of a pore was much less than expected from theoretical considerations. As a consequence of this, the polymer solution is expected to inhibit dispersion and also reduce the tendency for viscous finger to form in large pores, which in turn may be one of the major effects in enhancing recovery.

Introduction

VISCOUS FINGERING and/or longitudinal dispersion in porous media is of primary importance in immiscible as well as miscible oil displacements. Although a considerable number of studies have been devoted to defining the effects of fingering(1–4) or attempting, for example, to analyze the instability of interfaces(5–8), it is far from clear what the actual mechanisms are. Indeed, given the ability to observe viscous fingering, it is at best a difficult statistical problem to describe the fingers. Given a simple model of n starting finger or incipient instability, it is not difficult to demonstrate how these might grown(1). However, this type of analysis yields little insight into the actual mechanism, nor particularly how this may be related (if at all) to local heterogeneities in the porous medium.

Of growing current interest are concepts of mobility control, in which additives are used to reduce the mobility of the displacing phase. It is well known that decreasing the mobility of the displacing phase effects greater recovery through improved areal sweep efficiency and at least qualitatively acts to stabilize the interface from the deleterious effects of dispersion and fingering.

In the case of polymer additives, several effects may occur which, at least in the case of reservoir types of porous media, are poorly understood. Pressure-drop measurements interpreted through Darcy's Law may show a much greater decrease in mobility from that of the solvent k/ µ than expected from considerations of the viscosity of the material(10,11). Because these materials are usually shear thinning or pseudoplastic, quantitative interpretation of results is complicated by the question of what is the appropriate value of viscosity. Separate from the question of viscosity, the apparent reductions in permeability have been variously attributed to pore blockage, absorption of polymer, fluid elasticity, etc., all of which may be important in certain situations (12,18). Similarly, visual observations in model experiments (18,19), for example, indicate that the interface may be markedly stabilized.