To improve sweep efficiency for carbon dioxide (CO 2 ) enhanced oil recovery (EOR) up to 120 C in the presence of high-salinity brine (182 g/L NaCl), novel CO 2 /water (C/W) foams have been formed with surfactants composed of ethoxylated amine headgroups with cocoalkyl tails. These surfactants are switchable from the nonionic (unprotonated amine) state in dry CO 2 to cationic (protonated amine) in the presence of an aqueous phase with a pH less than 6. The high hydrophilicity in the protonated cationic state was evident in the high cloudpoint temperature up to 120 C. The high cloud point facilitated the stabilization of lamellae between bubbles in CO 2 /water foams. In the nonionic form, the surfactant was soluble in CO 2 at 120 C and 3,300 psia at a concentration of 0.2% (w/w). C/W foams were produced by injecting the surfactant into either the CO 2 phase or the brine phase, which indicated good contact between phases for transport of surfactant to the interface. Solubility of the surfactant in CO 2 and a favorable C/W partition coefficient are beneficial for transport of surfactant with CO 2 -flow pathways in the reservoir to minimize viscous fingering and gravity override. The ethoxylated cocoamine with two ethylene oxide (EO) groups was shown to stabilize C/W foams in a 30-darcy sandpack with NaCl concentrations up to 182 g/L at 120 C and 3,400 psia, and foam qualities from 50 to 95%. The foam produces an apparent viscosity of 6.2 cp in the sandpack and 6.3 cp in a 762-lm-inner-diameter capillary tube (downstream of the sandpack) in contrast with values well below 1 cp without surfactant present. Moreover, the cationic headgroup reduces the adsorption of ethoxylated alkyl amines on calcite, which is also positively charged in the presence of CO 2 dissolved in brine. The surfactant partition coefficients (0 to 0.04) favored the water phase over the oil phase, which is beneficial for minimizing losses of surfactant to the oil phase for efficient surfactant usage. Furthermore, the surfactant was used to form C/W foams, without forming stable/viscous oil/water (O/W) emulsions. This selectivity is desirable for mobility control whereby CO 2 will have low mobility in regions in which oil is not present and high contact with oil at the displacement front. In summary, the switchable ethoxylated alkyl amine surfactants provide both high cloudpoints in brine and high interfacial activities of ionic surfactants in water for foam generation, as well as significant solubilities in CO 2 in the nonionic dry state for surfactant injection.

Despite significant interest in CO 2 foams for EOR, very few studies have reported stable foams at high temperatures and high salinities, which are often encountered in the Middle East and elsewhere. Stable CO 2 /water (C/W) foams at high temperatures up to 120 o C and salinities have been achieved with ethoxylated cationic surfactants. The surfactants were shown to stabilize C/W foams with high salinity brine with NaCl concentration up to 182 g/L at 120 °C, 3400 psia, and to form unstable dodecane/water emulsions with the 120 g/L NaCl brine solutions. Thus, the foams have the potential to provide mobility control to prevent loses of CO 2 in high permeability regions, but simultaneously allow high permeability in the presence of residual oil. The surfactants are soluble in CO 2 and thus may be injected in the CO 2 phase to simplify the EOR process. The aqueous solubility of the surfactant at high temperatures is enhanced with the appropriate number of EO groups on the amine head group. Viscosities of high-pressure C/W foams (emulsions) formed with these surfactants were investigated by capillary rheology. These hybrid cationic/nonionic surfactants combine the high cloud points of ionic surfactants with high solubilities in CO 2 of nonionic surfactants. Furthermore, the variation of the tail length and the degree of ethoxylation offer great flexibility for stabilizing CO 2 foams for EOR at high temperatures and high salinities. Ethoxylated cocoamine exhibited lower adsorption on calcite than that on dolomite, given the presence of silica sites in the latter. High divalent ion concentrations in 22% total dissolved solids (TDS) brine contributed to the reduction of surfactant adsorption on silica sites in the dolomite powder.