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Abstract
The problems associated with current chemical flooding technologies are based around inadequate sweep efficiencies and unfavorable mobility ratios which leave much of the recoverable oil left untouched in the pores of the reservoir. In order to address the low sweep efficiency and unfavorable mobility ratio issues, numerous formulations of polymer and surfactant base fluids have been used for enhanced oil recovery (EOR) applications with varying degree of success. The use of Colloidal Gas Aphrons (CGA) as an alternative chemical EOR technique is investigated in this study. Colloidal Gas Aphrons (CGA) are described as micro-bubbles which are 10 to 100 microns in size with a gas containing inner core encapsulated by a thin surfactant film. Aqueous CGA fluids are comprised of water, polymer and surfactant solutions.

An experimental study was conducted to determine the optimum surfactant and polymer concentrations which would yield stable micro-bubbles. The formulations of stable micro-bubbles were analyzed in terms of rheology, bubble size distribution and time stability.

In order to determine the displacement efficiency of CGA fluid in the EOR process, flooding experiments were conducted using a 2D linear model and 3D radial model, both packed with glass beads and saturated with mineral oil. Flooding experiments were performed using a) water, b) aqueous polymer solution, c) aqueous polymer and surfactant solution mixed at low shear rate, d) CGA fluid, e) water followed by CGA fluid, and f) water followed by polymer solution. 
Efficiency of oil recovery using the CGA fluid was compared to that of other fluids. All experiments were repeated to ensure consistent results. Less than 3 % variation in results was observed in all cases.

Pressure drop, ultimate recovery and injected fluid retention time data were measured during the flooding experiments. In addition, time-lapse images taken at regular intervals were analyzed to study frontal displacement patterns observed in 2-D experiments.

The results indicated that the CGA fluids showed more stable frontal displacement as compared to water flooding. The cumulative oil recovery performance of CGA fluids was comparable but slightly less than that of aqueous polymer solutions. CGA fluids, however, required significantly lower injection pressure as compared to aqueous polymer solutions. The breakthrough time of CGA fluids was longer than that of any of the other fluids tested indicating that CGAs have longer retention time.

Results from preliminary experiments encourage the further investigation of colloidal gas aphrons as an alternative EOR technique. The results will also be useful in designing an EOR process as an alternate to polymer, surfactant-polymer or WAG flood with particular importance to carbon sequestration as CO2 / flue gas can also be used in micro-bubble generation in place of air.

Introduction
Aphrons were first described by Sebba (1987) who later introduced the term Colloidal Gas Aphron (CGA). Colloidal gas aphrons are micron sized bubbles composed of a gaseous inner core encapsulated by an inner and outer surfactant layer. Between these surfactant layers there is viscous water layer which is important to aphron stability. Figure 1 shows the structure of an aqueous colloidal gas aphron. CGA are termed aqueous when they are dispersed in an aqueous surfactant solution.