(564c) Mixtures and Nanostructures in Enhanced Oil Recovery

Fluids utilized in Enhanced Oil Recovery (EOR) often have complex nanostructure in the process of achieving ultra-low interfacial tension (IFT) and/or mobility control. Examples are illustrated from alkaline surfactant polymer (ASP) flooding and carbon dioxide foam.

Synergism can occur with blending of two dissimilar surfactants. N67-7PO sulfate is a C16-C17 slightly branched propyoxylated sulfate surfactant that is tolerant of divalent ions. IOS15-18 is a C15-C18 internal olefin sulfonate that is randomly branched but intolerant of divalent ions. Mixtures of these two surfactants have a single-phase region with either NaCl or CaCl2 that is larger than with either surfactant alone.

Naphthenic soaps are generated in situ as a co-surfactant in ASP flooding. These soaps are generally more lipophilic that the usual sulfate and sulfonate surfactants used in EOR. This results in the optimal salinity for minimum IFT being a function of the soap/surfactant ratio or fraction. In addition, it was discovered that in under-optimum conditions (Winsor I), the lower-phase microemulsion is not a homogeneous phase. Rather it has a composition gradient with a more oil-rich microemulsion (colloidal dispersion) in contact with the excess-oil phase at gravity equilibrium. This oil-rich microemulsion is a middle layer with respect to the lower-phase microemulsion and excess oil. If this oil-rich microemulsion is present to coat the drop of excess-oil in a spinning-drop measurement, the IFT is ultralow (below 10-2 mN/m) over a much wider salinity range than in its absence.

Polymer is included in the surfactant slug of the ASP process for mobility control. If the salinity of the injected surfactant slug is near the optimal salinity of the pure surfactant, the solution may separate into surfactant-rich and polymer rich phases. This can result in loss of mobility control. However, the surfactant slug can be injected well below the optimal salinity of the pure surfactant and the ultra-low IFT region generated in situ as the surfactant mixes with the naphthenic soaps.

Carbon dioxide is widely used for EOR but often has poor sweep efficiency because of its low viscosity of about 0.03 cp. However, CO2 can have much higher apparent viscosity as it flows through porous media if it becomes dispersed in water as foam (or emulsion if below Tc). There are surfactants that can be injected solubilized in CO2 but yet generate in situ a CO2 in water foam (emulsion) as it mixes with water in the formation.