Effect of the concentration of commercial partially CO2-soluble surfactants on foam strength

Injection of CO₂ foam is an emerging technology for CO₂ mobility control. When searching for suitable surfactant system for maximum effect of the foam application it is necessary to consider deployment methods (dissolved in brine or in CO₂) and ways to avoid loss of foam strength away from the injection well. Loss of strength is partially related to concentration depletion caused by surfactant adsorption and partitioning into formation brine.

Here, two non-ionic surfactants with different CO₂ solubilities were studied in steady-state core flooding experiments with co-injection of CO₂ and surfactant solution with varying concentrations. For each surfactant concentration apparent viscosities were calculated from the measured differential pressures.

The surfactant Brij L23 had a low partition coefficient at the experimental conditions used, meaning that little surfactant was found in the CO₂. With this surfactant, foam was formed for low concentrations. The other surfactant, Tergitol TMN 10, was more CO₂ soluble, and the partition coefficient increased when the pressure was increased. As a result of the pressure increase more surfactant partitioned into the CO₂ and a higher concentration of surfactant was needed to form strong foam.

The experimental observations are explained as a result of the surfactant partitioning and are related to the critical micelle concentration of surfactant in brine. Additional measurements were made to characterise the rheology of the CO₂-brine interface for the different surfactant systems. It was shown that the interface was more flexible for the systems and conditions where strong foam was observed in the core flooding tests.

The observation that surfactant systems with stronger partitioning into CO₂ required higher concentrations to form strong foam, and also collapsed more rapidly for decreasing surfactant concentrations, seems to reject a proposition that high CO₂ solubility is favourable for expanding the reservoir zone where foam is generated. This indicates that the search for methods for effective CO₂ mobility control should concentrate on surfactants that are mainly water soluble.