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.

The interfacial properties for surfactants at the supercritical CO2-water (C-W) interface at temperatures above 80°C have very rarely been reported given limitations in surfactant solubility and chemical stability. These limitations, along with the weak solvent strength of CO2, make it challenging to design surfactants that adsorb at the C-W interface, despite the interest in CO2-in-water (C/W) foams (also referred to as macroemulsions). Herein, we examine the thermodynamic, interfacial and rheological properties of the surfactant C12-14N(EO)2 in systems containing brine and/or supercritical CO2 at elevated temperatures and pressures. Because the surfactant is switchable from the nonionic state to the protonated cationic state as the pH is lowered over a wide range in temperature, it is readily soluble in brine in the cationic state below pH 5.5, even up to 120°C, and also in supercritical CO2 in the nonionic state. As a consequence of the affinity for both phases, the surfactant adsorption at the CO2-water interface was high, with an area of 207Å(2)/molecule. Remarkably, the surfactant lowered the interfacial tension (IFT) down to ∼5mN/m at 120°C and 3400 psia (23MPa), despite the low CO2 density of 0.48g/ml, indicating sufficient solvation of the surfactant tails. The phase behavior and interfacial properties of the surfactant in the cationic form were favorable for the formation and stabilization of bulk C/W foam at high temperature and high salinity. Additionally, in a 1.2 Darcy glass bead pack at 120°C, a very high foam apparent viscosity of 146 cP was observed at low interstitial velocities given the low degree of shear thinning. For a calcium carbonate pack, C/W foam was formed upon addition of Ca(2+) and Mg(2+) in the feed brine to keep the pH below 4, by the common ion effect, in order to sufficiently protonate the surfactant. The ability to form C/W foams at high temperatures is of interest for a variety of applications in chemical synthesis, separations, materials science, and subsurface energy production.

The utilization of nonionic surfactants for stabilization of CO2 foams has been limited by low aqueous solubilities at elevated temperatures and salinities. In this work, a nonionic surfactant C12–14(EO)22 with a high degree of ethoxylation resulted in a high cloud point temperature of 83 °C even in 90 g/L NaCl brine. Despite the relatively high hydrophilic–CO2-philic balance, the surfactant adsorption at the C–W interface lowered the interfacial tension to ∼7 mN/m at a CO2 density of ∼0.85 g/mL, as determined with captive bubble tensiometry. The adsorption was sufficient to stabilize a CO2-in-water (C/W) foam with an apparent viscosity of ∼7 cP at 80 °C, essentially up to the cloud point temperature, in the presence of 90 g/L NaCl brine in a 30 darcy sand pack. In a 1.2 darcy glass bead pack, the apparent viscosity of the foam in the presence of 0.8% total dissolved solids (TDS) brine reached the highest viscosity of ∼350 cP at 60% foam quality (volume percent CO2) at a low superficial velocity of 6 ft/day. Shear-thinning behavior was observed in both the glass bead pack and the sand pack irrespective of the permeability difference. In addition, C12–14(EO)22 stabilized C/W foam with an apparent viscosity of 80–100 cP in a 49 mdarcy dolomite core formed through a coinjection and a surfactant-alternating-gas process. The dodecane–0.8% TDS brine partition coefficient for C12–14(EO)22 was below 0.1 at 40 °C and 1 atm. The formation of strong foam in the porous media and the low oil–brine partition coefficient indicate C12–14(EO)22 has potential for CO2-enhanced oil recovery.

We demonstrated for the first time stable CO/water foams at temperatures up to 120 °C and CO volumetric fractions up to 0.98 with a single diamine surfactant, CN(CH)CN(CH). The foam stability was increased by increasing the packing parameter of the surfactant with a long tail and methyl substitution on the amine to form entangled viscoelastic wormlike micelles in the aqueous phase. The foam was more viscous and stable compared to foams with spherical micelles in the aqueous lamellae as seen with CN(EO) and CN(EO)CN(EO).

The design of surfactants for stabilizing CO 2 -in-water (brine) (C/W) foams at high temperature is challenging given the low density (solvent strength) of CO 2 , limited surfactant solubility in brine, and a lack of knowledge of the interfacial and rheological properties. Herein, the tail length of trimethylammonium cationic surfactants was optimized to provide the desired phase behavior and interfacial properties for formation and stabilization of the C/W foams. The headgroup was properly balanced with a C 12−14 hydrocarbon tail to achieve aqueous solubility in 22% total dissolved solids (TDS) brine up to 393 K (120°C) along with high surfactant adsorption (area/surfactant molecule of 154 Å 2 ) at the CO 2 − water (C−W) interface which reduced the interfacial tension from ∼40 mN/m to ∼6 mN/m. For C 12−14 N(CH 3 ) 3 Cl, these properties enabled stabilization of a C/W foam with an apparent viscosity of 14 mPa·s at 393 K in both a crushed calcium carbonate packed bed (75 μm 2 or 76 Darcy) and a capillary tube downstream of the bed. In addition, the partition coefficient of the surfactant between oil and 22% TDS (255 kg/m 3 ) brine was less than 0.15, which would be beneficial for minimizing the loss of the surfactant to an oil phase in applications including enhanced oil recovery and hydraulic fracturing.