Interplay of Capillary, Viscous And Buoyancy Forces In the Mobilization of Residual Oil
Authors: Morrow Norman R. 
Published: March 2010

Abstract
An experimental study has been made of conditions for mobilization of residual oil using as an experimental model an isolated blob of fluid held within pore spaces having well-defined geometry. The viscous or gravity forces required to overcome capillary forces and the interplay of these forces were determined for blobs of known volume and length. Results provide a direct test of capillary-number concepts which are assumed to apply in the recovery of residual oil by immiscible displacement from the swept zone of a waterflood. Comparative measurements were made/or three distinctly different wettability conditions: complete wetting, weak wetting and intermediate wettability.

Introduction
Keywords: Enhanced recovery, Capillary forces, Viscous forces, Bouyancy forces, Residual oil, Waterflooding, Displacement, Immiscible displacement, Wettability. In the displacement of oil from a reservoir by waterflooding, a significant fraction of oil is bypassed. The residual oil remaining in the swept zone is a prime target for enhanced recovery. In water-wet media, this oil is believed to remain as small discontinuous blobs or ganglia which are held by capillary forces at their positions in the pore space of the rocks. In a rock of, say, 200 millidarcy permeability, a barrel of oil might typically be distributed as 1011 blobs having average volumes of about 10−6 ccs. Laboratory displacement studies of residual oil from even relatively small core samples involve extremely large numbers of blobs. Although there has been considerable recent progress(1,2) in modelling the mobilization of large populations of blobs, interpretation of the recovery mechanism is usually based on simple models of single blobs. This paper presents results of a study of mobilization of single blobs under relatively well-defined conditions of pore geometry and wettability.

Background
Smith and Crane(3) noted that trapping of liquid by capillary forces can arise in tubes both through nonuniformity of tube diameter and through contact-angle hysteresis or some combination of the two effects. Movement of the trapped liquid occurred when there was sufficient pressure differential across the given tube. It was later demonstrated that the amount of oil trapped by capillary forces within a porous medium was sensitive to flow rate.(4) A comprehensive review of capillary trapping and mobilization in porous media has recently been given by Stegemeter.(5)
Laboratory studies also show that residual oil can be recovered if the pressure differential due to viscous flow is sufficient to overcome capillary forces so that some blobs, or at least parts of some blobs, are mobilized.(6,8). If the ratio of viscous to capillary forces is raised sufficiently, almost complete recovery of residual oil can be achieved. Various methods of expressing this ratio have been reviewed by Larson.(9). A widely accepted form is the capillary number, v µ/ σv; v is the superficial fluid velocity, µ the viscosity of the flowing fluid and σ the interfacial tension. Values of viscous pressure "gradient which can be achieved in the reservoir are limited by the injection pressures, which must not exceed the fracturing pressure of the formation.