(307g) CO2-Soluble Surfactants for Enhanced Oil Recovery Mobility Control Via Thickening or in-Situ Foam Generation

The very low viscosity of scCO2 diminishes the effectiveness of CO2 flooding of oil reservoirs. There are two strategies for using CO2-soluble compounds to decrease the mobility of supercritical carbon dioxide. The first involves the ?direct thickening? of CO2, which is accomplished by dissolving an associative thickener in the scCO2 that forms viscosity-enhancing macromolecules in solution. The second strategy is to inject a CO2 surfactant solution into the porous media (which contains both brine ands oil) that will generate a low mobility system of CO2 droplets separated by surfactant-stabilized brine lamellae that bridge pore throats.

Direct thickening was accomplished with surfactants that formed cylindrical, rather than spherical, micelles in scCO2. The surfactants employed divalent cations (Ni, Co) rather than a monovalent cation (Na). Therefore, each surfactant had two tails (rather than one). Further, each tail was a double-tail or triple-tail that was tailored to be CO2-philic, consisting of either highly fluorinated alkanes or highly branched hydrocarbon groups. High pressure SANS was employed to establish whether the micelles were cylindrical or spherical. Further, the dimensions of the micelles were determined. Cloud point pressures of surfactant solutions (1-10wt% surfactant) were determined for the dry and wet (W = 0 ? 15) systems using a non-sampling technique, and viscosity was determined using a falling cylinder technique. The CO2 viscosity was doubled using several weight percent of a fluorinated surfactant in the presence of water (viscosity results for the non-fluorous surfactants will also be presented). Direct thickening was also attempted with hydroxyaluminum disoaps. Although hydroxyaluminum di(2-ethyl hexanoate) is a prolific thickener for light alkanes at very dilute concentration, it is CO2-insoluble. Therefore the (2-ethyl hexyl) tails were replaced with highly branched, oxygenated hydrocarbon tails that have been shown to be CO2-philic when incorporated into other types of small compounds. Unfortunately, the aluminum disoaps tested to date have been CO2-insoluble, even in the presence of high concentrations of organic co-solvents.

With regard to our second technique, we have established the identity of several non-ionic, hydrocarbon-based, commercially available, inexpensive surfactants that can dissolve in CO2 at typical EOR reservoirs conditions to a high enough concentration (~0.01-0.1wt%) to form relatively stable emulsions/foams of liquid or supercritical CO2 droplets separated by films of brine. Although these surfactants are CO2-soluble, they are even more water-soluble, therefore they tend to form emulsions in which the brine is the low-volume (~15%) continuous phase. When mixed with dense CO2 and water, these surfactants can form a white emulsion of CO2 droplets that slowly collapses over a period of time encompassing tens of hours. The emulsions are less stable as the dissolved solids content of the brine (e.g. the concentration of NaCl) increases over the range of values associated with EOR projects. The performance of the best surfactants identified in this study was contrasted with that of the DOW Tergitol TMN surfactants previously identified by Keith Johnston of the University of Texas at Austin for this EOR application. The surfactants identified in this study appear to form more stable emulsions than Tergitol TMN, especially when tested with highly saline brine.

At this point in time, the use of CO2-soluble nonionic surfactants to form in-situ foams (as opposed to the direct thickening) appears to be the more promising and commercially viable mobility control technology.