Surfactant Partitioning and Foam Behavior for Different Gas Composition, Salinity, Pressure, and Temperature

Foam mobility control for gas-enhanced oil recovery (GEOR) methods is an attractive solution to several conformance issues faced by heterogeneous reservoirs. A novel nonionic foaming surfactant was characterized for its ability to partition into light hydrocarbons. The impact of surfactant partitioning on foam stability and propagation in limestone cores was evaluated through high-pressure foam drainage and coreflooding experiments. It was found that the surfactant micelle size in electrolyte solutions increased with temperature and the presence of divalent ions, which influenced surfactant partitioning into hydrocarbon phases. For ethane-brine systems, surfactant partitioning coefficient (kp) decreased exponentially with increasing surfactant concentration and temperature. However, it increased logarithmically with the pressure and concentration of monovalent salts. Divalent cations were found to depress the partitioning. Both solubility and partitioning of surfactant in different hydrocarbons increased nonlinearly with hydrocarbon phase density. An increase in surfactant partitioning at 0.5 wt % surfactant concentration significantly improved foam stability for all hydrocarbon gases, including methane, methane–ethane, and ethane, as indicated by the measured foam half-life. Moreover, foam propagation rates and steady-state foam viscosities in limestone improved remarkably with surfactant partitioning. Methane foam propagated about 3 times faster and reached a higher steady-state viscosity as the kp value increased from 0 to 0.06 at 3 wt % surfactant concentration. Enriching methane with ethane (at 75 mol % methane and 25 mol % ethane) could obtain a similar kp value at reduced surfactant concentration (0.5 wt %) and enhance moderately foam propagation and viscosity. Further increasing surfactant partitioning with 100% ethane in the gas phase (a kp value of about 0.5 at the same surfactant concentration of 0.5 wt %) resulted in a remarkable improvement in both foam propagation rate and steady-state foam viscosity.