Polymer Flow Behaviour As Applied To Enhanced Recovery
Authors: Stanislav J.F. , Kabir C.S. 
Published: March 2010

Polymer solution flow behaviour through porous media has been studied at length over the last decade with a view to improve oil recovery. An attempt has been made in this paper to understand the basic flow behaviour mechanisms involved, along with pertinent aspects. Numerous field results are also summarized which reveal how flood success or failure is dependent on reservoir and fluid characteristics.

Introduction

IT IS A WELL-KNOWN FACT that during the water flooding used in secondary oil recovery processes, water follows the path of least resistance, which eventually results in "fingering" and early water breakthrough into the producers. At this point, the operation becomes uneconomical - yet up to one third or more of oil may remain in place.

Enhanced recovery could be achieved by improving the water-oil mobility ratio*, which ensures a more uniform flow pattern, resulting in increased sweep efficiency.

Mobility may be controlled by:

reducing the oil viscosity (application of heat, gas resaturation and miscible drives); increasing the water viscosity.

In this article, only the work dealing with the latter will be discussed.

Initially, one alternative considered was the use of materials such as glycerin, sugar or glycols as viscosifiers. However, the amount required to effect an appreciable viscosity increase is so high that their usage is economically prohibitive. Then serious consideration was given to the use of more efficient synthetic water-soluble polymers. Large size of macromolecules suggests that a great amount of energy will be dissipated, so that the Viscosity of solution could be significantly increased even at low polymer concentration.

Currently high-molecular-weight polymers are being used in:polymer flooding, where, by increasing the resistance to flow of water relative to that of oil, fingering is reduced;miscible flooding, where the displacing fluid (surfactants) must be prevented from fingering through the miscible fluid (for these processes, control of the mobility of the following aqueous displacement fluid is essential).

It should be pointed out, however, that the economical feasibility of using polymers in oil recovery is still being questioned. Furthermore, a major obstacle to the wider use of polymer flooding is the lack of a satisfactory method for predicting the performance of the oil recovery process. Such a difficulty stems from the lack of understanding of the behaviour of polymers flowing through porous media.

Essentially, water mobility is reduced in three ways:by virtue of high viscosity at flooding shear rates;by polymer-medium rock interaction;by a combination of (a) and (b).

A great number of water-soluble polymers have been used as water mobility agents. In essence, they fall into two groups: synthetic polymers and biopolymers (polysaccharides), the former being further divided into polyelectrolytes (partially hydrolyzed polyacrylamides) and nonionic materials (polyethyl-eneoxides).

Effect of Molecular Size

Generally speaking, the effectiveness of polymers in terms of oil recovery increases with the molecular size. The determining factor, however, is -the ratio of a macromolecular diameter to a pore opening*.