Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook

CO2 foam injection has been shown to be effective under reservoir conditions for enhanced oil recovery. However, its application requires a certain stability and surfactant absorbability on rock surface, and it is also associated with borehole corrosion in the presence of water. Adding surfactants to CO2 can enhance the interaction between CO2 and crude oil and control the CO2 mobility, thereby improving the performance of CO2 flooding. This paper presents a review of the research of CO2-soluble surfactants and their applications. Molecular dynamics simulation is introduced as a tool for analyzing the behavior of the surfactants in supercritical CO2 (scCO2). The applications of CO2-soluble surfactants, including CO2 thickening, reducing miscibility pressure, and generating supercritical CO2 foam, are discussed in detail. Moreover, some opportunities for the research and development of CO2-soluble surfactants are proposed.


Introduction
Supercritical CO 2 is a carbon dioxide in a state above its critical point (304.13K and 7.3773 MPa), where it has properties of both a gas and a liquid [1].It exhibits high diffusivity, low viscosity, and low surface tension, making it an effective solvent for many organic compounds, such as oil, fat, and polymer.This environmentally friendly solvent is increasingly attractive to the petroleum industry as a replacement for traditional organic solvents [2,3].It can be used in extraction [4], enhanced oil recovery (EOR) [5][6][7][8], CO 2 sequestration [9], and cleaning processes [10,11].
CO 2 flooding has been used for decades and is a proven technique to enhance the recovery of low-and ultralow-permeability reservoirs [12][13][14][15].In the United States alone, CO 2 flooding contributes 5.6% of the country's total oil production, reaching 1371 × 10 4 t, accounting for 93% of the world's CO 2 -EOR oil production [16,17], and of the 136 CO 2 flooding projects, low-permeability reservoir projects account for 63.67% [18].Because of the low viscosity and low density of CO 2 , CO 2 is prone to causing viscous fingering, premature breakthrough, unfavorable mobility ratios, gravity differentiation, low swept volumes, and other problems underground, seriously affecting the effectiveness of CO 2 -EOR.To improve the performance of CO 2 flooding, it is critical to control the mobility of CO 2 .

Low viscosity
Viscous fingering Water alternating gas (WAG) It may cause tubing corrosion and scaling and not avoid gravity override.[19,20]

Premature breakthrough
Gel profile control It is not applicable to low-permeability reservoirs.

Carbonated water injection (CWI) or micro and nanobubbles (MNBs)
It leads to low CO 2 content and accelerated tubing corrosion.
[ 25,26] Low swept volume Surfactant-assisted or nanoparticle-assisted CO 2 foam It induces adsorption during injection and tubing corrosion.

CO 2 -Soluble Surfactants
CO 2 is a weak solvent with very low polarizability and a very low dielectric constant (ε = 1.1.0280at 2.58 MPa and 25.05 • C [51]).Previous simulation studies showed that most surfactants are slightly or not at all soluble in scCO 2 [52].This finding is believed to be attributable to the stronger hydrophobic chain-to-hydrophobic chain interaction of the surfactant than the CO 2 hydrophobic chain interaction.Hoefling et al. [53] enabled the solubility of a surfactant in CO 2 by introducing functional groups with low polarity and low solubility parameters or those acting as Lewis bases on the CO 2 -hydrophobic tail chain [54].
A well-performed CO 2 -soluble surfactant is characterized by improved solubility and viscosity and enhanced interaction with crude oil to reduce oil-water interfacial tension or lower minimum miscibility pressure (MMP).Compared to conventional surfactants, CO 2 -soluble surfactants can reduce the mobility of the injected CO 2 and the adsorption of surfactants on the rock surface significantly [55,56].
It is found that the addition of co-solvents (e.g., ethanol, 1-pentanol, F-pentanol, and tri-n-butylphosphate) increases the solubility of polar compounds in CO 2 [57,58].Liu et al. [59] studied the influence of alcohols on the phase behavior of a nonionic surfactant Ls-54/CO 2 system.The results showed that the cloud point pressure (CPP) of the surfactant reduces significantly in the presence of 1-propanol, n-pentanol, and n-heptanol, and the reduction is more remarkable in the case of short-chain alcohols.Conversely, the addition of benzyl alcohol resulted in a CPP increase and solubility decrease of the surfactant.Chennamsetty et al. [60] investigated how alcohols affect the self-assembly of surfactants in scCO 2 using lattice Monte Carlo simulations.They found that short-chain alcohols were concentrated in the surfactant layer of the aggregates, replacing the surfactant molecules, so they acted as co-surfactants, which directly affected the performance of the aggregates, while long-chain alcohols acted as co-solvents, changing the properties of the solvent.
Table 2 provides the main CO 2 -soluble surfactants discussed in this section.It is found that the addition of co-solvents (e.g., ethanol, 1-pentanol, F-pentanol, and tri-n-butylphosphate) increases the solubility of polar compounds in CO2 [57,58].Liu et al. [59] studied the influence of alcohols on the phase behavior of a nonionic surfactant Ls-54/CO2 system.The results showed that the cloud point pressure (CPP) of the surfactant reduces significantly in the presence of 1-propanol, n-pentanol, and n-heptanol, and the reduction is more remarkable in the case of short-chain alcohols.Conversely, the addition of benzyl alcohol resulted in a CPP increase and solubility decrease of the surfactant.Chennamsetty et al. [60] investigated how alcohols affect the self-assembly of surfactants in scCO2 using lattice Monte Carlo simulations.They found that short-chain alcohols were concentrated in the surfactant layer of the aggregates, replacing the surfactant molecules, so they acted as co-surfactants, which directly affected the performance of the aggregates, while long-chain alcohols acted as co-solvents, changing the properties of the solvent.
Table 2 provides the main CO2-soluble surfactants discussed in this section.It is found that the addition of co-solvents (e.g., ethanol, 1-pentanol, F-pentanol, and tri-n-butylphosphate) increases the solubility of polar compounds in CO2 [57,58].Liu et al. [59] studied the influence of alcohols on the phase behavior of a nonionic surfactant Ls-54/CO2 system.The results showed that the cloud point pressure (CPP) of the surfactant reduces significantly in the presence of 1-propanol, n-pentanol, and n-heptanol, and the reduction is more remarkable in the case of short-chain alcohols.Conversely, the addition of benzyl alcohol resulted in a CPP increase and solubility decrease of the surfactant.Chennamsetty et al. [60] investigated how alcohols affect the self-assembly of surfactants in scCO2 using lattice Monte Carlo simulations.They found that short-chain alcohols were concentrated in the surfactant layer of the aggregates, replacing the surfactant molecules, so they acted as co-surfactants, which directly affected the performance of the aggregates, while long-chain alcohols acted as co-solvents, changing the properties of the solvent.
Table 2 provides the main CO2-soluble surfactants discussed in this section.It is found that the addition of co-solvents (e.g., ethanol, 1-pentanol, F-pentanol, and tri-n-butylphosphate) increases the solubility of polar compounds in CO2 [57,58].Liu et al. [59] studied the influence of alcohols on the phase behavior of a nonionic surfactant Ls-54/CO2 system.The results showed that the cloud point pressure (CPP) of the surfactant reduces significantly in the presence of 1-propanol, n-pentanol, and n-heptanol, and the reduction is more remarkable in the case of short-chain alcohols.Conversely, the addition of benzyl alcohol resulted in a CPP increase and solubility decrease of the surfactant.Chennamsetty et al. [60] investigated how alcohols affect the self-assembly of surfactants in scCO2 using lattice Monte Carlo simulations.They found that short-chain alcohols were concentrated in the surfactant layer of the aggregates, replacing the surfactant molecules, so they acted as co-surfactants, which directly affected the performance of the aggregates, while long-chain alcohols acted as co-solvents, changing the properties of the solvent.
Table 2 provides the main CO2-soluble surfactants discussed in this section.It is found that the addition of co-solvents (e.g., ethanol, 1-pentanol, F-pentanol, and tri-n-butylphosphate) increases the solubility of polar compounds in CO2 [57,58].Liu et al. [59] studied the influence of alcohols on the phase behavior of a nonionic surfactant Ls-54/CO2 system.The results showed that the cloud point pressure (CPP) of the surfactant reduces significantly in the presence of 1-propanol, n-pentanol, and n-heptanol, and the reduction is more remarkable in the case of short-chain alcohols.Conversely, the addition of benzyl alcohol resulted in a CPP increase and solubility decrease of the surfactant.Chennamsetty et al. [60] investigated how alcohols affect the self-assembly of surfactants in scCO2 using lattice Monte Carlo simulations.They found that short-chain alcohols were concentrated in the surfactant layer of the aggregates, replacing the surfactant molecules, so they acted as co-surfactants, which directly affected the performance of the aggregates, while long-chain alcohols acted as co-solvents, changing the properties of the solvent.
Table 2 provides the main CO2-soluble surfactants discussed in this section.

Fluorocarbon Surfactants
Fluorocarbon surfactants were first discovered to be soluble in scCO2 [79].CO2-soluble surfactants could be synthesized using perfluoroalkyl polyether carboxylate as tail groups instead of hydrocarbon carboxylate [80].

Fluorocarbon Surfactants
Fluorocarbon surfactants were first discovered to be soluble in scCO2 [79].CO2-soluble surfactants could be synthesized using perfluoroalkyl polyether carboxylate as tail groups instead of hydrocarbon carboxylate [80].

Fluorocarbon Surfactants
Fluorocarbon surfactants were first discovered to be soluble in scCO2 [79].CO2-soluble surfactants could be synthesized using perfluoroalkyl polyether carboxylate as tail groups instead of hydrocarbon carboxylate [80].Harrison

Fluorocarbon Surfactants
Fluorocarbon surfactants were first discovered to be soluble in scCO 2 [79].CO 2 -soluble surfactants could be synthesized using perfluoroalkyl polyether carboxylate as tail groups instead of hydrocarbon carboxylate [80].Harrison et al. [81] synthesized the first doublechain hybrid CO 2 -soluble surfactant composed of fluorocarbon (F) and hydrocarbon (H) chains.Cummings et al. [61] synthesized a series of semifluorinated F-H hybrid surfactants based on pentadecafluoro-5-dodecyl (F 7 H 4 ) sulfate anion (M-F 7 H 4 , where M may be Li, Na, K, or Rb), which can generate more anisotropic micelles in scCO 2 and exhibit a solubility up to 4.4 wt% in certain ranges of pressure and temperature.
Temtem et al. [62] investigated the interaction between fluorine atoms and CO 2 molecules through nuclear magnetic resonance (NMR) and molecular simulation.They found that the higher solubility of perfluorinated molecules may be related to the fundamental differences in the nature of their interaction with CO 2 compared to non-fluorinated molecules.In perfluorinated molecules, the carbon atom of CO 2 acts as a Lewis acid, and the fluorine atoms act as Lewis bases in non-fluorinated molecules; the oxygen atom of CO 2 acts as a Lewis base, and the proton of the hydrocarbon chain acts as a Lewis acid.Dardin et al. [82] used NMR spectroscopy to investigate the interactions between fluorocarbon solutes and scCO 2 solvent.They found evidence of specific solute-solvent interactions, such as hydrogen bonding and van der Waals forces, which were different from the interactions observed in traditional solvents, such as water and organic solvents.They attributed this excess magnetic shielding to van der Waals interactions between the fluorinated sites in the solute and carbon dioxide.
Beckman [83] attributed the high solubility of fluorinated compounds in scCO 2 to the following points: (1) the presence of fluorine creates molecules with weak self-interaction, making the miscibility with CO 2 possible at low pressures; (2) electronegative fluorine may interact with electron-poor carbon of CO 2 , reducing the miscibility pressure; or (3) the presence of fluorine affects the acidity of adjacent protons, allowing the possibility of specific interactions between these protons and the oxygen atoms of CO 2 .
Mohamed et al. [63] synthesized a new hybrid surfactant CF 2 /AOT 4 [sodium (4H,4H, 5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate], which has one hydrocarbon chain and one fluorocarbon chain.This hybrid H-F chain structure strikes a fine balance of properties, thus minimizing the fluorine content and maintaining a sufficient level of CO 2 solubility.Its solubility can reach 2.59 wt% at 34 MPa and 40 • C. Therefore, this structure and fluorination level can be used as a benchmark when designing low-fluorine surfactants.
Fluorocarbon surfactants are expensive and may cause environmental issues during subsurface applications [84][85][86], and those with perfluoroalkylpolyether tails also have toxicity concerns [87].Therefore, they cannot be applied on a large scale.

Silicone Surfactants
Judicious side chain functionalization of oligomeric silicones has been shown to produce a material whose phase behavior in CO 2 resembles that of fluorinated polyethers [88].Hoefling [89] explored the relationship between the structure and solubility of siliconebased amphiphiles in CO 2 via high-pressure phase-behavior experiments.
Alzobaidi et al. [90] used a comb polymer surfactant with a polydimethylsiloxane (PDMS) backbone and pendant linear alkyl chains to generate emulsions.Trisiloxane surfactants with very short and bulky CO 2 -philic headgroup forms showed extremely high solubility in CO 2 .For trisiloxane M(D'E 7 )M with seven EO repeat units, the solubility reached 1 wt% at 30 MPa and 25 • C [91].
Silicone surfactants are also expensive and environmentally unfriendly.Efforts have been made to obtain low toxicity and less expensive CO 2 -soluble hydrocarbon-based surfactants, including hydrocarbon surfactants and oxygenated hydrocarbon surfactants [92][93][94][95].

Hydrocarbon Surfactants
Hydrocarbon surfactants developed for CO 2 offer significant advantages over costly fluorocarbon and silicone surfactants.Hydrocarbon agents are the most promising CO 2soluble surfactants because of their environmentally friendly nature.The solubility of hydrocarbon surfactants in CO 2 may be achieved by the addition of a polar co-solvent to CO 2 to improve solvent polarity.
Hydrocarbon surfactants were first synthesized by Traian [96].He used inexpensive propylene and CO 2 to synthesize a series of poly(ether-carbonate) copolymers that readily dissolve in CO 2 at low pressures.
Pitt et al. [97] pointed out that t-butyl chain tips promote the lowest aqueous surface tensions for hydrocarbon surfactants.Eastoe et al. [98] studied the solubility of octylphenol nonionic surfactants (TritonX-100, X-100 reduced, and X-45) chain end group structures (i.e., highly methylated tert-butyl units) in CO 2 and aggregation effects.The results showed that the solubility of Triton surfactants depends on temperature and pressure, and methylation at the chain ends can promote the solubility in scCO 2 .Therefore, the t-butyl end can be considered a CO 2 -compatible group.
Shi et al. [99] measured the solubility pressure of three nonionic hydrocarbon surfactants (TX45 and TX100, Guerbet alkyl polyoxyethylene ether, and linear alkyl polyoxyethylene ether).The results showed that the surfactant with methylated tails (TX45 and TX100) had the highest solubility, and the surfactants with branched tails were less soluble than linear surfactants.TX45 exhibited a higher solubility (up to 0.188 wt% at 17 MPa and 70 • C) than those of other surfactants.Moreover, the addition of hexanol improved the solubility, which might be due to the fact that n-hexanol molecules can intercalate between the tails of the surfactant to hinder the interaction between molecules.
Liebum et al. [100] tested the solubility of three alkylamine surfactants in scCO 2 and supercritical carbon dioxide-methane mixture (scCO 2 -scCH 4 ) under high-pressure conditions at 40 • C and 60 • C. It was observed that highly methylated surfactant structures had the highest solubility in scCO 2 , up to 1 wt% at 40 • C, and the solubility decreased exponentially with the addition of methane to the system.Because of the strength of the surfactant-scCO 2 intermolecular interaction, the scCO 2 preferred shorter tail groups.

Oxygenated Hydrocarbon Surfactants
CO 2 has a substantial quadrupole moment that operates over a much shorter distance than dipolar interactions [101,102].It has been shown to have strong Lewis acid-Lewis base interactions with oxygen atoms of some ethers or carbonyls on solute molecules [103][104][105][106].There are several oxygenated hydrocarbon groups that exhibit more favorable thermodynamic interactions with CO 2 than branched alkanes.Accordingly, a series of acetylated sugars and cyclodextrins are highly soluble in scCO 2 [92,[107][108][109].These surfactants, branched alkylphenol ethoxylates, branched alkyl ethoxylates, and a fatty acid-based surfactant, are also soluble in CO 2 under the condition of good foaming performance [110].
Zhang et al. [111] used AOT as the CO 2 -soluble surfactant to stabilize CO 2 foam in the presence of ethanol and demonstrated by core-flooding experiment that AOT dissolved in scCO 2 can interact with formation water in situ to form scCO 2 foam and thus control the mobility of CO 2 .Some scholars believe that the dissolution of surfactants in CO 2 can lead to the formation of microemulsions and emulsions [71].The low-molecular weight surfactant bis (3,5,5-trimethyl-l-hexyl) sodium sulfosuccinate (AOT-TMH) could stabilize diluted W/C emulsions with average diameters of 50 nm and 4 µm, respectively, for miniemulsions and macroemulsions.Also, AOT W/C reverse microemulsions could be formed in scCO 2 by adding a small quantity of F-pentanol [112].
Ryoo et al. [74] investigated the formation of W/C microemulsions by nonionic methylated branched hydrocarbon surfactants, poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ethers.They found that methylation and branching increased the solubility of surfactants in CO 2 by weakening the interactions between the tails.
Fan et al. [92] synthesized a series of oxygenated hydrocarbon-tailed ionic surfactants composed of acetylated sugar, poly-p-phenylene oxide, or oligo(vinyl acetate) and evaluated their solubility in scCO 2 .They found that the oligo(vinyl acetate)-functionalized surfactants were highly soluble in scCO 2 , with single-tailed surfactants having the solubility of 7 wt% at 25 • C and 48 MPa and twin-tailed surfactants having the solubility of 3 wt%.
Liu et al. [93] studied the solubility of the acetylene glycol-based nonionic surfactant Dynol-604 (a non-fluorine and non-silicone surfactant) in scCO 2 .They observed that Dynol-604 had a solubility that increased with rising pressure and decreased with elevating temperature, which reached 5 wt% at 26 MPa and 60 • C.They also investigated non-fluorine and non-silicone nonionic surfactants Ls-36 and Ls-45 containing propylene oxide (PO) and ethylene oxide (EO) groups and found that both Ls-36 and Ls-45 were highly soluble in scCO 2 [94] and that an increase in the number of EO groups reduced the solubility in CO 2 [98] .
Zhang et al. [57] investigated the dissolution of surfactants in scCO 2 in the presence of co-solvents, nonionic surfactants (N-NP-10c, branched alkylphenol ethoxylates and APG-1214, Alkyl polyglucoside), and anionic surfactants (N-NP-15c-H, sulfonated alkylphenol ethoxylates).The results showed that increasing the pressure and adding co-solvents could effectively improve surfactants' dissolution in CO 2 , and the dissolution of surfactants and co-solvents in CO 2 solutions could greatly increase the viscosity of the mixture.
Chen et al. [76][77][78] proved that the thermally stable amine ethoxylate C 12-14 N(EO) 2 is a well-performed candidate foam agent that can dissolve in CO 2 even in high salinity at high temperature.They indicated that C 12-14 N(EO) 2 is switchable from a nonionic to a cationic state by lowering pH and soluble in brine when it is cationic and in scCO 2 when it is nonionic.They also reported that 0.2 wt% C 12-14 N(EO) 2 and C 12-14 N(EO) 5 can still be dissolved in CO 2 at a pressure <23 MPa and a temperature up to 120 • C. Zhang et al. [114] used the tallow ethoxylated amine surfactant, C 16-18 N(EO) 5 , to generate and stabilize CO 2 foam at high pressure and high temperature.They found that C 16-18 N(EO) 5 is soluble in scCO 2 up to 0.5 wt%, and it is effective in improving foam stability when it is added during the CO 2 phase.
The dissolution of oxygenated hydrocarbon surfactants requires a large amount of co-solvent, which adds cost.In addition, co-solvent reduces the viscosity of the system, which is not conducive to mobility control.It is necessary to seek an optimal addition of co-solvent to minimize cost while meeting the required solubility.

Molecular Dynamics Simulation
Molecular dynamics (MD) simulations provide us with an atomic-level insight into the surfactant solubility in scCO 2 .Salaniwal et al. [115] reported the first molecular simulations of the self-assembly of di-chain surfactants in scCO 2 into stable, spherical aggregates.Rocha [116] investigated the structural properties of the W/C binary fluid−fluid interface by means of MD simulation.
Lísal et al. [117] modified Larson's lattice model and used it to study micellar behavior in a supercritical solvent-surfactant system via large-scale Monte Carlo simulations.Li et al. [118] performed simulations on model homopolymer/solvent systems with varying interaction strengths and explored the influence of surfactant structure (head and tail lengths) on phase transition.Ren et al. [119] studied the microstructure of CO 2 microemulsions via MD simulations, revealing the origin of the synergistic effect between hydrotropes and surfactants.Zhu et al. [120] reported the dynamic process and interface equilibrium structure in self-assembled aggregates based on MD simulations.
Zhang et al. [121] used MD simulations to clarify the AOT-AOT and AOT-solvent interactions and their effects on interfacial properties, such as interfacial tension (IFT) and interfacial thickness at the molecular level, which indicated that ethanol enhances the foam stability and regeneration capacity of CO 2 -soluble surfactant.Nan et al. [122,123] investigated how alcohols with different tail lengths (C 2 OH-C 16 OH) and concentrations affect the water-AOT-scCO 2 interface system through MD simulations.Kobayashi et al. [124] discussed the mutual solubility of heavy n-alkanes (typically, n-decane, n-hexadecane, n-eicosane) and their structural isomers in CO 2 -rich and hydrocarbon-rich phases using continuous fractional component Gibbs integrated Monte Carlo simulations and suggested that the improvement of solubility is due to a higher coordination number of CO 2 for methyl (CH 3 ) rather than for methylene (CH 2 ) groups.
MD simulations allow the in-depth analysis of the interactions between CO 2 -soluble surfactants and gases in order to understand the behaviors of surfactants in the gas phase and thus optimize their performance.

Applications
CO 2 -soluble surfactants can be applied for CO 2 thickening, reducing minimum miscibility pressure (MMP), and generating scCO 2 foam.This section focuses on the critical roles of surfactants in controlling the CO 2 properties in the reservoir, thereby enhancing oil recovery.Table 3 provides the summary of studies on the applications of CO 2 -soluble surfactants for EOR.Alter the wettability of unconventional rock 75% Haeri [137] * "\" indicates that this reference does not provide this part of the parameters or results.

CO 2 Thickening
Self-assembly of surfactants can form linear or rod-like micelles, which, at a certain concentration, will intertwine to form a network structure, thereby increasing the viscosity of scCO 2 [138].
Trickett et al. [125] synthesized a fluorinated surfactant Ni(di-HCF4) by ion exchange and measured its viscosity using a falling ball viscometer (10 wt% Ni(di-HCF4) at 25 • C and 35 MPa).The results showed that the viscosity of the system increased by 90% compared to pure CO 2 , to 0.22 MPa•s, suggesting that surfactant self-assembly can control the viscosity of CO 2 .
Zhao et al. [126] investigated the solubility and thickening properties of three polysiloxanes modified with different functional groups in scCO 2 .The results showed that vinyl polysiloxane had the highest solubility and best thickening capacity, and the higher the kinematic viscosity and concentration of vinyl polysiloxane, the better the thickening capacity.It was also indicated that, at 40 • C and 39.24 MPa, adding 8 wt% (1000 centistokes) vinyl polysiloxane enabled the viscosity of the scCO 2 system to reach 12.57 MPa•s.
Other researchers discussed factors such as molecular weight, concentration, shear rate, temperature, and pressure that affect CO 2 thickening.Table 4 [24,139] presents a summarization of these factors.

Reducing Miscibility Pressure
CO 2 -soluble surfactants improve CO 2 -oil miscibility mainly by mitigating the IFT of the oil-gas system, increasing the volume expansion of oil, and reducing the viscosity of oil.
Dong et al. [127] prepared a scCO 2 microemulsion system by adding CO 2 -soluble surfactant AOT and co-solvent ethanol to CO 2 and found that the MMP of the CO 2 -oil system was reduced from 24.55 to 22.02 MPa.
Wang [128] synthesized different fatty alcohol polyethers and demonstrated that the fatty alcohol polyoxypropylene ether performed better in reducing the MMP during CO 2 flooding than the polyoxyethylene ether.In other words, PO is more soluble in CO 2 than EO, and it is more CO 2 -philic.
Guo et al. [129] synthesized an oil-soluble surfactant CAE and evaluated its effects on the IFT and MMP of the CO 2 -oil system through experiments.The results showed that CAE is soluble in scCO 2 but insoluble in water, reducing the MMP by 22.34% at a concentration of 0.2 wt%.
Lv et al. [131,132] treated different nonionic polyether surfactants for their solubility in CO 2 and their potential to improve oil-gas miscibility.The results showed that polyoxypropylene alkyl ethers, especially those with low molecular weights and high PO groups, significantly improved the oil-gas miscibility.Under the conditions of 50 • C, adding 3 wt% C 4 PO 3 could reduce MMP from 17.75 to 13.6 MPa.
Kuang et al. [140] dealt with five surfactants in terms of their solubility in CO 2 and their improvements in CO 2 -oil miscibility.The results indicated that surfactants could reduce the viscosity of oil to a certain extent and lower the IFT between CO 2 and oil, thereby enhancing the miscibility of the two phases.This effect would be further enhanced after adding low-carbon alcohols.Under the conditions of 50 • C and 30 MPa, adding 0.5 wt% SPO5 and 0.25 wt% n-pentanol to CO 2 increased the recovery by 2.29% compared with CO2 alone, ultimately reaching 93.47%.
Li et al. [141] selected two nonionic alkoxylated surfactants (ethylene glycol butyl ether and Span 80) to analyze the MMP reduction in CO 2 -oil systems.The results showed that the addition of surfactants could accelerate the miscibility process and change the rock surface from oil-wet to water-wet, thereby promoting the oil flow in the reservoir and ultimately enhancing oil recovery.

Supercritical CO 2 Foam
Conventional foaming surfactants only traverse through the reservoirs in the aqueous phase [41].Surfactants required for foam generation and stabilization can be injected by dissolving in an aqueous solution [142].CO 2 -soluble surfactants are soluble in scCO 2 , and their injection does not involve water, which can mitigate the risk of wellbore corrosion.The surfactants dissolved in CO 2 can interact in situ with formation water in the presence of CO 2 to form CO 2 foam, which helps reduce the mobility of CO 2 and improve the sweep efficiency, thereby enhancing oil recovery.
Le et al. [133] proposed for the first time that foam can be generated in situ via the injection of surfactant in the CO 2 phase.SACROC [56] carried out a pilot test on a CO 2 foam system with CO 2 -soluble surfactant, revealing an effective control on the CO 2 mobility and reduction of the oil-gas ratio.
The high solubility of a surfactant in CO 2 and a favorable W/C partition coefficient are beneficial for the transport of the surfactant along CO 2 -flow pathways in the reservoirs to minimize the possibility of viscous fingering and gravity override.Ren et al. [16,134] studied the effect of surfactant partitioning between scCO 2 and water on surfactant transport and foam propagation during two-phase flow.They found that, for all CO 2 -soluble surfactants studied, the core-scale CO 2 displacement rates increased with decreasing surfactant partition coefficients.
Bi et al. [135] evaluated the solubility of 31 oilfield or industrial surfactants and their modified products in scCO 2 using a high-pressure and high-temperature visualized foam device.The results showed that the surfactant N-P-12 had good CO 2 foam stability at high temperature (120 • C) and the addition of alcohol as co-solvent could significantly increase the solubility of the surfactants in the CO 2 .
The Baker Hughes researchers [143] suggested the use of CO 2 foams for gas lift operations in conjunction with a cross-linked siloxane polymer (Dow Corning 1250 (poly(trimethy lhydrosilylsiloxane), MW 5770, MN 3160) as the recommended surfactant.
The partitioning of surfactant into the CO 2 phase results in faster foam propagation and stronger foam [144].After the foam collapses in the transport process, CO 2 -containing surfactants keep contact with formation water during the upward movement, which allows for the regeneration of foam and the resistance to foam collapse.This makes it possible to achieve effective control of CO 2 mobility.

Outlook
The utilization of CO 2 -soluble surfactants for EOR presents both challenges and opportunities.Despite the promising results observed in experimental studies and field tests, CO 2 -soluble surfactants cannot be applied widely unless some limitations are addressed.Fluorinated and siloxane surfactants have notable solubility in CO 2 , but they come with high cost and toxicity.Hydrocarbon and oxygenated hydrocarbon surfactants exhibit low solubility in scCO 2 , so they can work only with the support of a large amount of co-solvent.These challenges hinder the large-scale field application of CO 2 -soluble surfactants.
Nonetheless, new surfactant molecules can be tailored for diverse applications.Such efforts should be made to enhance the solubility in CO 2 , minimize the necessity of cosolvent, and improve the performance in IFT and foam stability.Rigorous investigations into the molecular interactions between CO 2 and surfactants are crucial for engineering formulations that effectively lower IFT.The systematic screening of surfactant structures and functional groups can help define the configurations yielding the highest IFT reduction under reservoir conditions.Foam stability can be optimized by adjusting the surfactant concentration, formulation composition, and inclusion of additives such as CO 2 -soluble polymers.
Additionally, there is a growing interest in the development of biomass-based CO 2soluble surfactants or renewable surfactants, which are cost-effective and environmentally friendly.

Conclusions
This paper reviews the research of CO 2 -soluble surfactants and their applications for EOR and proposes the relevant challenges and opportunities.
The addition of CO 2 -soluble surfactants has been shown to be promising in mitigating the adverse effects of CO 2 injection to enhance oil recovery.By modifying the structures and concentrations of CO 2 -soluble surfactants, it is possible to achieve CO 2 thickening, reduce the miscibility pressure, and generate scCO 2 foam.CO 2 -soluble surfactants can help enhance oil recovery by improving viscosity, reducing IFT, and increasing sweep efficiency in the reservoir.Molecular dynamics simulations provide researchers with theoretical and predictive insights into the screening and design of CO 2 -soluble surfactants with lower costs of trial and error for the purpose of efficient application.
While experimental studies demonstrated significant improvements in oil recovery, some CO 2 -soluble surfactants are challenging for application because of their high cost and toxicity, as well as solubility-related issues and co-solvent usage.Future research should focus on the development of new surfactant molecules, optimization of formulations, and determination of environmentally friendly alternatives.

Author Contributions:
Conceptualization, S.L. and W.L.; formal analysis, B.D.; investigation, Z.L.; writing-original draft preparation, S.L. and W.W.; writing-review and editing, S.L. and W.L.; visualization, W.W.; supervision, X.X.; funding acquisition, W.L. All authors have read and agreed to the published version of the manuscript.Funding: This research is funded by CNPC's major scientific and technological project: fine description of CCUS oil flooding geological body and key technology research of reservoir engineering, project number 2021ZZ01-03.

Table 2 .
Structures of CO 2 -soluble surfactants discussed in this section.
CO2-56]uble surfactants can reduce the mobility of the injected CO2 and the adsorption of surfactants on the rock surface significantly[55,56].

Table 2 .
Structures of CO2-soluble surfactants discussed in this section.
CO2-56]uble surfactants can reduce the mobility of the injected CO2 and the adsorption of surfactants on the rock surface significantly[55,56].

Table 2 .
Structures of CO2-soluble surfactants discussed in this section.
CO2-56]uble surfactants can reduce the mobility of the injected CO2 and the adsorption of surfactants on the rock surface significantly[55,56].

Table 2 .
Structures of CO2-soluble surfactants discussed in this section.

Table 2 .
Structures of CO2-soluble surfactants discussed in this section.
Harrison et al. [81]synthesized the first double-chain hybrid CO2-soluble surfactant composed of fluorocarbon (F) and Harrison et al. [81]synthesized the first double-chain hybrid CO2-soluble surfactant composed of fluorocarbon (F) and

Table 3 .
Summary of studies on the applications of CO 2 -soluble surfactants for EOR.

Table 4 .
General effects of factors on the solubility and viscosity of CO 2 .