The Origins of Low Interfacial Tensions for Enhanced oil Recovery
Authors: Davis H. Ted , Scriven L. E. 
Published: December 2007

Advances in understanding of the stress and structure in fluid interfaces1 are employed to delineate the origins of ultralow interfacial tensions needed for the low-tension mechanism of enhancing oil recovery. We find the molecular theory known as gradient theory, having its roots in Van der Waals early work, is the most transparent currently. Interfacial tension, it reveals, depends on (1) the topography of the homogeneous fluid free energy surface fO in component density or composition space; (2) on this surface, the path . described by the composition sequence through the interface; and (3) the values of influence parameters cαβ which gauge effective ranges of intermolecular interaction effects that are peculiar to inhomogeneous fluid such as in the interface. The path . turns out, however, to depend solely on the influence parameters cαβ and the surface topography of fO. The influence parameters and free energy surface can be estimated in special cases from molecular models, or, in more practical situations, they can be estimated from empirical or semiempirical models based on theoretical functions which have constants that can be estimated from limited experimental data.

Tic-lines, binodals, and critical points (plait points) of two-phase regions and tie-triangles and critical endpoints of three-phase regions of course reflect the hill-and-dale topography of the free energy surface. This topography, especially in the composition ranges of metastable and unstable homogeneous fluid, is also a major determinant in the structure and tension of fluid interfaces – the existence of corridors or passes with little altitude change between coexisting phases favoring low tensions. The same topographical features that explain the commonly observed progression of phase diagrams2,3,4 are involved in the associated sequence of interfacial tensions. The commonality of the topography of low tension free energy surfaces manifests itself in what are known as critical point scaling laws.

Increasing or decreasing temperature towards an upper or lower critical point flattens the free energy surface with a consequent reduction of tension. Amphilic substances when introduced to brine and hydrocarbon may function as (i) miscibility agents that create a one-phase path around the amphiphile side of the free energy hill separating brine and oil phases; (ii) solubilizers and swellers that change the shape of the hill and the positions of its surrounding valleys to improve fractional flow and self-sharpening in displacement (and incidentally provide critical point low tensions near the plait point); (iii) inducers of a pair of low lying passes that cut short the amphiphile side of the hill and intersect in the dale of a third liquid phase, the passes providing low-tension paths leading from brine-rich to oil-rich phases; (iv) inducers of the same, except that the third phase is richer in surfactant-like amphiphile, is viscid (perhaps even liquid crystalline in some cases), and is easily dispersed; or (v) strong adsorbers in the brine-oil interface which cause influence-parameter combinations that make tension small without changing appreciably the free energy surface traversed by the interfacial path. Only the last is ordinary surface activity: ultralow tensions by this mechanism though widely expected have still not been proved, however.

Some of the practical fruits of this new knowledge are in hand and many more are ripening for use in enhanced oil recovery and other amphiphile-related science and technology.