Description of the material. Stable CO2/water (C/W) foams at high temperatures and salinities have been achieved with substituted amines in limestone, sandstone and glass bead packs with permeabilities from 1 to 78 Darcy. Foams were formed upon injection of the CO2 soluble surfactant in the CO2 phase and would be beneficial for improving sweep efficiency in EOR process.
Application. Despite significant interest in CO2 foams for EOR, very few studies have reported stable foams at high temperatures (120 °C) and high salinities, which are often encountered in the Middle East and elsewhere. The foams provide mobility control and stabilize the displacement front in CO2flooded zones to improve sweep efficiency.
Results, Observations, Conclusions. The amine surfactants are switchable between the nonionic and cationic states with pH or the nature of the solvent. They exhibit nonionic behavior when introduced in the CO2 phase, which favors injectivity, and cationic in the presence of concentrated brine with dissolved CO2. The hydrophilic/lipophilic balance of the amines was tuned by modification of the amine head group or tail length to design strong foams. It was important to increase the basicity of surfactants to enhance the solvation in the aqueous phase over a pH range of 4 to 7. These surfactants were effective in lowering the interfacial tension between water and CO2 at high temperature and salinity. They generated viscous C/W foams in limestone, sandstone and glass bead packs at 120 °C in the presence of 22% TDS brine when surfactants were injected from either the aqueous or CO2 phase. At pH below 6, these surfactants exhibited low oil/water partition coefficients on the order of 0.1 which suggests that these surfactants will have minimal retardation due to partitioning into oil in the EOR process.
Significance of Subject Matter. These surfactants stabilized C/W foam at high temperature and salinity, and partitioned to the water phase over dodecane phase for efficient surfactant utilization. The high solubility in CO2 is beneficial for the surfactant to be available along CO2 flow pathways in a reservoir to minimize viscous fingering and gravity override.