Role of Capillary Forces In Detennining Microscopic Displacement Efficiency For Oil Recovery By Waterflooding
Authors: Melrose J.C. 
Published: March 2010

The factors controlling the distribution of two immiscible fluids, such as oil and water, within the interstices of a porous solid are reviewed. It is shown that an improved understanding of these factors forms the basis of much recent work aimed at the development of new methods for reducing capillary forces and, consequently, eliminating residual oil saturations.

Introduction

THE PETROLEUM INDUSTRY has for many years devoted much research effort to the development of new processes for achieving improved oil recovery efficiencies. The current status of the most promising of these new processes has been reviewed by Elkins(l) and by Arnold(2). For the most part, the research leading to these processes has developed along lines pointed out by Muskat(3) in a review published about 20 years ago. In addition to thermal processes, which are used primarily to overcome adverse crude oil viscosity characteristics, a variety of processes are designed to eliminate or reduce the so-called "surface forces" within the crude-oil-displacing fluid-reservoir rock system.

When displacement is carried out by flooding with an immiscible fluid such as water or gas, the surface forces (or. more accurately, the capillary forces) are responsible for trapping a large portion of the oil phase within the interstices or pores of the rock. For typical, homogeneous sandstone rocks, under usual waterflooding conditions, trapping will occur within an appreciable fraction of the total number of pores constituting the pore space of the rock. The fraction of the pore volume which is involved may range from 10 to 50 per cent. Thus, capillary forces play a major role in limiting the recovery efficiency of such displacement processes.

Capillary forces will clearly be eliminated if the drive fluid is preceded by a slug or bank of fluid which is miscible both with the reservoir oil and with the displacing phase(4). However, the costs of the solvent materials required in processes based on this principle may be prohibitive. Alternatively, water-soluble surfactants, such as petroleum sulphonates, can be employed to eliminate the tendency of the oil phase to remain trapped in the pores of the rock. Details of several processes based on the use of such surfactants have been recently reported(5–9).

In this paper, the nature of the surface or capillary forces encountered in petroleum reservoirs, together with the microscopic features of the associated trapping mechanism, will first be reviewed. Some features of a new recovery process based on the use of surfactants, i.e., the low-tension waterflooding process (7–9), will then be described. This discussion will emphasize one of the principal microscopic mechanisms on which the process is based. An analysis of this mechanism indicates that ultra-low values of the oil-water interfacial tension are required in order to achieve improved recovery.

Hydrostatics, Capillarity and Residual Saturations

HYDROSTATIC PRINCIPLES IN MULTIPHASE FLUID SYSTEMS

Two or more fluid phases confined within a porous solid phase of very small average pore size wilt generally be microscopically commingled. The fluid-fluid interfacial areas associated with fluid distributions of this type are large and characterized by high curvatures.