CO2 emulsion/foam is a promising method for controlling the mobility and improving the volumetric sweep efficiency in CO2 enhanced oil recovery (CO2-EOR) process. Recently, amine surfactants attract the attention of the researchers as CO2 emulsifiers/foamers, because of their switchable property: the surfactants are nonionic and CO2 soluble at high pH, and are cationic and water soluble at low pH. However, the efficiency of the commercial switchable amine surfactants is usually suppressed at high salinity (> 200 g/L TDS) and temperature (> 100 °C). Thus, novel switchable alkyl-amine surfactants are designed in house based on the hydrophilic and CO2-philic balance for rapidly generating strong and stable CO2 emulsions at high salinity and high temperature. These novel surfactants are evaluated and compared to a commercial one with respect to the solubility in brine and CO2, and emulsifying ability in bulk and in porous media at high temperature, high pressure and high salinity. The novel surfactants show outstanding performance: soluble in 220 g/L NaCl brine at pH≤8 from room temperature to 120 °C, soluble in CO2 at relatively low pressure (91 bar) and high temperature (110 °C). The surfactants are thermally stable at 110 °C and pH=4 in the absence of O2. Strong CO2 emulsion/foam is observed in both bulk test and in silica sandpack with 0.2 (wt)% of the surfactant in brine. Additionally, the apparent viscosity of the CO2 emulsion/foam at 110 °C is significantly higher than that at lower temperatures. Comparing to the commercial surfactants, the CO2 emulsion/foam is stronger and generated faster by the novel surfactants. These novel surfactants can be synthesized using commercially available feeds and simple industrial processes. Thus, the novel surfactants are promising for generating the CO2 emulsion/foam, especially in the hot and salty carbonate reservoirs.

Because many amine surfactants are soluble in both water and CO2 phases, they attract interest with regard to stabilizing CO2‐in‐water dispersion systems. In our recent research, we find that the solubility of alkyl‐amine surfactant in water can be significantly enhanced by salts, even though the salts are usually “salting‐out” to other surfactants. The influence of various anions (NO3−, Br−, Cl−, and SO42−) and cations (Na+, Ca2+, and Mg2+) on the alkyl‐amine surfactants is investigated. The results are contrary to Hofmeister series and show that all the anions can enhance the solubility (salting‐in) in the order of: NO3− > Br− > Cl− > SO42−, while the impact of the cations is insignificant. A physical–chemistry model based on the switchable property of the surfactant is proposed and well explains the experimental results. Therefore, the switchable alkyl‐amine surfactants have potential to be applied under high‐salinity and high‐temperature conditions, for example, in enhanced oil recovery processes for a hot and salty carbonate reservoir.