Viscosity-driven stabilization of CO2-in-brine foams using mixtures of cocamidopropyl hydroxysultaine and sodium dodecyl sulfate

Foam stability is one of the key factors determining the success of any foam-field application, and for that, it is imperative to identify surfactant formulations able to reduce the extensive foam drainage and coarsening occurring in harsh environments, such as in high salinity brines. In this work, mixtures of cocamidopropyl hydroxysultaine (CAHS) and sodium dodecyl sulfate (SDS) at 1 wt% were evaluated as foaming agents to obtain CO2-in-brine foams with improved stability, compared to the single surfactant foams. The results showed that the mixture with excess of the zwitterionic surfactant (2:1, CAHS:SDS) produced a CO2-foam with a half-life four times higher than that of the foam formed using single components, at the same surfactant concentration. These foams exhibited a drastic reduction in both drainage and coarsening mechanisms, which was attributed to the significant increase (four orders of magnitude) in bulk phase viscosity due to the formation of large surfactant aggregates in the brine, as confirmed by dynamic light scattering (DLS) and rheological measurements. The analysis of the gas fraction in the foams formulated with the surfactant mixtures revealed that they were also able to comprise large amounts of CO2 (83–88% per volume of aqueous phase), as a result of retarding the diffusion of CO2 through the viscous aqueous phase of the foam. The results obtained in this work demonstrated that the synergy exhibited by CAHS and SDS in brine had a direct impact not only in the viscosity-driven stabilization of the CO2-foams, but also for the capture and retention of large amounts of CO2 inside the foam, which can have a direct impact in the sweeping efficiency, in the case of EOR, and in the CO2 storage, in the case of carbon capture and storage (CCS) technologies.