Interfacial Tension Required for Significant Displacement of Residual Oil
Authors: Oh Soo Gun , Slattery John C. 
Published: December 2007

Abstract
A static analysis of displacement in a single irregular pore partially filled with oil is used to investigate the effects of interfacial tension and wettability on the tertiary recovery of residual oil by a low interfacial tension waterflood. There are several results.
For the most efficient displacement of residual oil, either the porous structure should be water-wet or intermediate-wet. There is a critical value for the interfacial tension above which the residual oil cannot be displaced, but instead will assume a static configuration. Although the computation describes the static configuration of an oil globule in a pore, it suggests the underlying mechanism for the episodic motion or jump of a globule as it is displaces.
The discussion of displacement in a single pore is extended by an estimate for the value of the capillary number, required for recovery of residual oil from those pores whose neck radii are larger than the mean pore neck radius. A comparison is offered with data taken from the literature.

Introduction
Petroleum is found in the microscopic pores of sedimentary rocks such as sandstones and limestones. Not all pores will be filled with petroleum. Some pores contain water or brine that petroleum. Some pores contain water or brine that is saturated with the minerals in the local rock structure.
In the primary stage of production, oil and brine are driven into a well from the surrounding rock by the relatively large difference between the initial field pressure and the pressure in the well. Perhaps 10 to 20% of the oil originally in place is recovered in this manner.
In the secondary stage of production, water or steam is pumped into a selected pattern of wells in a field, forcing a portion of the oil into other production wells. The void volume in a permeable production wells. The void volume in a permeable rock may be thought of as many intersecting pores of varying diameters. Consider two parallel pore spaces of unequal permeabilities. A blob of oil (residual oil) will be trapped in that pore space, through which the oil is displaced by the water more slowly. In this manner, 10 to 40% of the oil initially in place will be recovered, but 40 to 80% of the oil originally in place is left behind in the field.
Several tertiary recovery techniques have been proposed. We focus our attention here on the proposed. We focus our attention here on the recovery of residual oil by a low interfacial tension waterflood. One can distinguish between at least two different types of such waterfloods.
One type uses a small volume (3 to 20% PV) of a relatively concentrated surfactant solution. The concentration of the surfactant solution is high enough to ensure that it is miscible with the crude oil in all proportions. As a small slug of this surfactant solution moves through the porous structure mixing with and displacing the crude oil, surfactant is lost by adsorption on the rock. The solution is diluted further with the connate water present in the structure. As the concentration of present in the structure. As the concentration of surfactant falls, the water/crude oil/surfactant mixture can move from a single-phase region to a multiphase region on its phase diagram. We should expect that a miscible displacement conducted with a small slug of surfactant solution will revert to an immiscible displacement at some point in the reservoir.
In the type of waterflood with which we are concerned here, a large volume (15 to 60% PV or more) of a dilute surfactant solution is used. The crude oil is nearly insoluble in the surfactant solution, and we speak of an immiscible displacement.
It has been estimated that in carefully selected, well-designed, well-performing operations, an additional 30% of the oil originally in place might be recovered by a low interfacial tension waterflood. The crude oil/water interfacial tension can be reduced by orders of magnitude when either a mixture of surfactants or an alkaline solution is added to the waterflood.
SPEJ
P. 83