Static and dynamic foam/oil interactions: Potential of CO₂-philic surfactants as mobility control agents

Examination of the nature and extent of CO₂ foam/oil interactions is essential for the successful design and application of surfactants in foam assisted water alternating gas (FAWAG) process. A structure-property relationship of surfactants based on theoretical method, static bulk tests, and dynamic displacements in porous media subjected to foam flooding is provided in this study. Surfactants with an increased affinity towards CO₂ are intended to improve foam mobility control mechanism in a Malaysian oil reservoir. Two anionic foaming agent surfactants (in-house developed FomaxII and industry benchmark alpha olefin sulfonate (AOS)), and two CO₂-philic surfactants (in-house developed FomaxVII and UTP-Foam) are selected to investigate the effect of surfactant tail groups on the phase interactions in a system of CO₂-oil-brine. The effect of surfactant presence on the CO₂/water and oil/water interfaces under reservoir conditions is measured by using a high pressure, high temperature tensiometer. Surfactants with higher tolerance against oil are selected based on static stability column tests and the equilibrium spreading coefficients model. The surfactant alternated with gas (SAG) displacements are then conducted by using the selected CO₂-philic surfactants dissolved in injection brine to study the interactions of foam and oil in porous media at the reservoir conditions. A relationship was observed between foam stability column tests and the prediction results of the spreading theory, which can be a useful preliminary screening of an oil-resistant foam. The performance of foam during injection of surfactants in sandstone cores was also supporting the preliminary screening results. However, it was shown to be dependent on different parameters (such as formation of macroemulsion once foam lamellae ruptures, and surfactant-rock surface interactions), which are not necessarily observed in the static tests results. Macroemulsion once formed hindered the performance of subsequent FomaxVII surfactant slugs to produce foam.