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Review

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

by
Shisheng Liang
1,2,*,
Wenli Luo
2,3,*,
Zhixing Luo
4,
Wenjuan Wang
3,
Xiaohu Xue
5 and
Bo Dong
1,2
1
School of Engineering Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
2
Institute of Porous Flow and Fluid Mechanics, Chinese Academy of Sciences, Langfang 065007, China
3
PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
4
Research Institute of Exploration and Development, PetroChina Xinjiang Oilfield Company, Karamay 834000, China
5
China Petroleum Technology and Development Corporation, Beijing 100028, China
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(24), 8042; https://doi.org/10.3390/molecules28248042
Submission received: 18 September 2023 / Revised: 8 December 2023 / Accepted: 8 December 2023 / Published: 11 December 2023
(This article belongs to the Section Applied Chemistry)
Versions Notes

Abstract

:
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.

1. Introduction

Supercritical CO2 is a carbon dioxide in a state above its critical point (304.13 K 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], CO2 sequestration [9], and cleaning processes [10,11].
CO2 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, CO2 flooding contributes 5.6% of the country’s total oil production, reaching 1371 × 104 t, accounting for 93% of the world’s CO2-EOR oil production [16,17], and of the 136 CO2 flooding projects, low-permeability reservoir projects account for 63.67% [18]. Because of the low viscosity and low density of CO2, CO2 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 CO2-EOR. To improve the performance of CO2 flooding, it is critical to control the mobility of CO2.
The methods for controlling CO2 mobility mainly include water alternating gas (WAG) [19,20], gel profile control [21,22,23], direct thickeners (polymer and surfactant) [24], carbonated water injection (CWI) [25], micro and nanobubbles (MNB) [26], and surfactant-assisted or nanoparticle-assisted CO2 foam [27,28,29,30]. However, the contact of water with CO2 in these systems can lead to tubing corrosion and submersible pump corrosion, causing construction, sustainability, and cost issues [19]. The measurement of polymer and surfactant solubility is restricted by the uncertainty in cloud point determination [31], and highly soluble polymer and surfactants are often accompanied by high toxicity and high cost. WAG does not avoid gravity override and gel profile in not applicable to low permeability reservoirs. Table 1 provides the adverse effects of CO2 and mobility control methods.
CO2 foam injection has been shown to be effective in a variety of reservoir conditions, including high temperature and high salinity [7,8]. However, its application for EOR requires a prior confirmation of several properties, including the stability of CO2 foam under reservoir conditions [32,33,34], the surfactant absorbability on rock surface [35,36,37], and the ability to maintain a stable foam in the presence of CO2 [38,39,40,41]. To address these challenges, researchers have developed various CO2-soluble surfactants with different molecular structures and properties. CO2-soluble surfactants are able to dissolve in scCO2 to produce a transparent, stable phase [42,43], which can increase the viscosity of scCO2 to a certain extent [44]. They are injected through cost-effective facilities, with no water produced [45]. Underground, surfactants and CO2 interact with crude oil to improve the sweep volume and efficiency.
Eastoe et al. [46,47] reviewed the role of fluorocarbon surfactants in water-in-carbon dioxide (W/C) microemulsions. Enick et al. [29] reviewed the application of some surfactants in carbon dioxide thickening, but they were limited to fluorocarbon and silicone surfactants. Eastoe et al. [48] reviewed low-fluorine surfactants and non-fluorine surfactants in subsequent articles, but they did not mention the application of CO2-soluble surfactants in EOR. Therefore, we review the research of CO2-soluble surfactants through laboratory-scale and field-scale tests of EOR and propose the application prospects and challenges.
The adverse effects of CO2 and mobility control methods are listed in Table 1.
Table 1. Adverse effects of CO2 and mobility control methods.
Table 1. Adverse effects of CO2 and mobility control methods.
Adverse EffectMobility Control MethodShortcomingReference
Low viscosityViscous fingeringWater alternating gas (WAG)It may cause tubing corrosion and scaling and not avoid gravity override.[19,20]
Premature breakthroughGel profile controlIt is not applicable to low-permeability reservoirs.[21,22,23,49,50]
Unfavorable mobility ratioDirect thickeners
(polymer and surfactant)		High solubility is impossible under reservoir conditions.[24]
Low densityGravity overrideCarbonated water injection (CWI) or micro and nanobubbles (MNBs)It leads to low CO2 content and accelerated tubing corrosion.[25,26]
Low swept volumeSurfactant-assisted or nanoparticle-assisted CO2 foamIt induces adsorption during injection and tubing corrosion.[27,28,29,30]

2. CO2-Soluble Surfactants

CO2 is a weak solvent with very low polarizability and a very low dielectric constant (ε = 1.1.0280 at 2.58 MPa and 25.05 °C [51]). Previous simulation studies showed that most surfactants are slightly or not at all soluble in scCO2 [52]. This finding is believed to be attributable to the stronger hydrophobic chain–to–hydrophobic chain interaction of the surfactant than the CO2 hydrophobic chain interaction. Hoefling et al. [53] enabled the solubility of a surfactant in CO2 by introducing functional groups with low polarity and low solubility parameters or those acting as Lewis bases on the CO2-hydrophobic tail chain [54].
A well-performed CO2-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, CO2-soluble surfactants can reduce the mobility of the injected CO2 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 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.

2.1. 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 et al. [81] synthesized the first double-chain hybrid CO2-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 (F7H4) sulfate anion (M-F7H4, where M may be Li, Na, K, or Rb), which can generate more anisotropic micelles in scCO2 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 CO2 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 CO2 compared to non-fluorinated molecules. In perfluorinated molecules, the carbon atom of CO2 acts as a Lewis acid, and the fluorine atoms act as Lewis bases in non-fluorinated molecules; the oxygen atom of CO2 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 scCO2 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 scCO2 to the following points: (1) the presence of fluorine creates molecules with weak self-interaction, making the miscibility with CO2 possible at low pressures; (2) electronegative fluorine may interact with electron-poor carbon of CO2, 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 CO2.
Mohamed et al. [63] synthesized a new hybrid surfactant CF2/AOT4 [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 CO2 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.
Recently, the aggregation behavior and interfacial properties of a hybrid surfactant sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2-sulfonates, FCm-HCn (FC length m = 4, 6, HC length n = 2, 4, 5, 6, and 8) were examined in water–CO2 (W/C) mixture as functions of pressure and temperature. FC6-HC4 exhibited a higher solubility (up to 1.08 wt% at 35 MPa and 45 °C) than other surfactants [64,65].
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.

2.2. Silicone Surfactants

Judicious side chain functionalization of oligomeric silicones has been shown to produce a material whose phase behavior in CO2 resembles that of fluorinated polyethers [88]. Hoefling [89] explored the relationship between the structure and solubility of silicone-based amphiphiles in CO2 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 CO2-philic headgroup forms showed extremely high solubility in CO2. For trisiloxane M(D’E7)M with seven EO repeat units, the solubility reached 1 wt% at 30 MPa and 25 °C [91].
Shi et al. [67] synthesized four siloxane polyether surfactants using allyl bromide, polyethylene glycol methyl ether, and heptamethyltrisiloxane (HMTS) as raw materials, which showed high solubility in CO2. In particular, HMTS demonstrated the highest solubility, around 2.4 mol%.
Silicone surfactants are also expensive and environmentally unfriendly. Efforts have been made to obtain low toxicity and less expensive CO2-soluble hydrocarbon-based surfactants, including hydrocarbon surfactants and oxygenated hydrocarbon surfactants [92,93,94,95].

2.3. Hydrocarbon Surfactants

Hydrocarbon surfactants developed for CO2 offer significant advantages over costly fluorocarbon and silicone surfactants. Hydrocarbon agents are the most promising CO2-soluble surfactants because of their environmentally friendly nature. The solubility of hydrocarbon surfactants in CO2 may be achieved by the addition of a polar co-solvent to CO2 to improve solvent polarity.
Hydrocarbon surfactants were first synthesized by Traian [96]. He used inexpensive propylene and CO2 to synthesize a series of poly(ether-carbonate) copolymers that readily dissolve in CO2 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 CO2 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 scCO2. Therefore, the t-butyl end can be considered a CO2-compatible group.
Sagisaka et al. [68] customized an isostearyl surfactant, sodium 2-(4,4-dimethylpentan-2-yl)-5,7,7-trimethyloctyl sulfate (SIS1), with a highly methyl-branched alkyl isostearyl group as a CO2-philic tail and observed the water–CO2 phase behavior at a fixed concentration of 0.08 mol%.
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 scCO2 and supercritical carbon dioxide–methane mixture (scCO2-scCH4) under high-pressure conditions at 40 °C and 60 °C. It was observed that highly methylated surfactant structures had the highest solubility in scCO2, 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–scCO2 intermolecular interaction, the scCO2 preferred shorter tail groups.

2.4. Oxygenated Hydrocarbon Surfactants

CO2 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 CO2 than branched alkanes. Accordingly, a series of acetylated sugars and cyclodextrins are highly soluble in scCO2 [92,107,108,109]. These surfactants, branched alkylphenol ethoxylates, branched alkyl ethoxylates, and a fatty acid–based surfactant, are also soluble in CO2 under the condition of good foaming performance [110].
The surfactant bis(2-ethylhexyl)sodium sulphosuccinate (AOT) is generally thought to be completely insoluble in scCO2 [81]. Ihara et al. [69] found that AOT can be completely dissolved in scCO2 containing 7.5% ethanol. Eastoe et al. [70,71,72] investigated the phase behaviors of AOT and its homologs and found that surfactants with trimethylpentyl or trimethylhexyl tails, sodium bis(2,4,4-trimethyl-1-pentyl) sulfosuccinate, and sodium bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate are soluble in CO2.
Zhang et al. [111] used AOT as the CO2-soluble surfactant to stabilize CO2 foam in the presence of ethanol and demonstrated by core-flooding experiment that AOT dissolved in scCO2 can interact with formation water in situ to form scCO2 foam and thus control the mobility of CO2. Some scholars believe that the dissolution of surfactants in CO2 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 scCO2 by adding a small quantity of F-pentanol [112].
Gold et al. [113] found that the addition of a third chain to sulphosuccinate surfactants significantly increases the activity in CO2. Hollamby et al. [73] designed a triple hydrocarbon chain surfactant, sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14). TC14, with three CO2-philic chains, efficiently formed hydrated reverse micelles in scCO2, while conventional twin-tailed surfactants could not.
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 CO2 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 scCO2. They found that the oligo(vinyl acetate)-functionalized surfactants were highly soluble in scCO2, 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 scCO2. 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 scCO2 [94] and that an increase in the number of EO groups reduced the solubility in CO2 [98].
Burrows et al. [75] evaluated the CO2 solubility of three nonionic surfactants (branched tridecyl ethoxylate Indorama SURFONIC TDA-9, branched nonylphenol ethoxylate Indorama SURFONIC N-100, and linear dodecyl ethoxylate Indorama SURFONIC L12-6). The results showed that each surfactant could dissolve in CO2 up to 1 wt% at pressures and temperatures commensurate with CO2 EOR.
Zhang et al. [57] investigated the dissolution of surfactants in scCO2 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 CO2, and the dissolution of surfactants and co-solvents in CO2 solutions could greatly increase the viscosity of the mixture.
Chen et al. [76,77,78] proved that the thermally stable amine ethoxylate C12–14N(EO)2 is a well-performed candidate foam agent that can dissolve in CO2 even in high salinity at high temperature. They indicated that C12–14N(EO)2 is switchable from a nonionic to a cationic state by lowering pH and soluble in brine when it is cationic and in scCO2 when it is nonionic. They also reported that 0.2 wt% C12–14N(EO)2 and C12–14N(EO)5 can still be dissolved in CO2 at a pressure ˂23 MPa and a temperature up to 120 °C. Zhang et al. [114] used the tallow ethoxylated amine surfactant, C16–18N(EO)5, to generate and stabilize CO2 foam at high pressure and high temperature. They found that C16–18N(EO)5 is soluble in scCO2 up to 0.5 wt%, and it is effective in improving foam stability when it is added during the CO2 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.

3. Molecular Dynamics Simulation

Molecular dynamics (MD) simulations provide us with an atomic-level insight into the surfactant solubility in scCO2. Salaniwal et al. [115] reported the first molecular simulations of the self-assembly of di-chain surfactants in scCO2 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 CO2 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 CO2-soluble surfactant. Nan et al. [122,123] investigated how alcohols with different tail lengths (C2OH-C16OH) and concentrations affect the water–AOT–scCO2 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 CO2-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 CO2 for methyl (CH3) rather than for methylene (CH2) groups.
MD simulations allow the in-depth analysis of the interactions between CO2-soluble surfactants and gases in order to understand the behaviors of surfactants in the gas phase and thus optimize their performance.

4. Applications

CO2-soluble surfactants can be applied for CO2 thickening, reducing minimum miscibility pressure (MMP), and generating scCO2 foam. This section focuses on the critical roles of surfactants in controlling the CO2 properties in the reservoir, thereby enhancing oil recovery. Table 3 provides the summary of studies on the applications of CO2-soluble surfactants for EOR.

4.1. CO2 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 scCO2 [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 CO2, to 0.22 MPa·s, suggesting that surfactant self-assembly can control the viscosity of CO2.
Zhao et al. [126] investigated the solubility and thickening properties of three polysiloxanes modified with different functional groups in scCO2. 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 scCO2 system to reach 12.57 MPa·s.
Other researchers discussed factors such as molecular weight, concentration, shear rate, temperature, and pressure that affect CO2 thickening. Table 4 [24,139] presents a summarization of these factors.

4.2. Reducing Miscibility Pressure

CO2-soluble surfactants improve CO2-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 scCO2 microemulsion system by adding CO2-soluble surfactant AOT and co-solvent ethanol to CO2 and found that the MMP of the CO2-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 CO2 flooding than the polyoxyethylene ether. In other words, PO is more soluble in CO2 than EO, and it is more CO2-philic.
Guo et al. [129] synthesized an oil-soluble surfactant CAE and evaluated its effects on the IFT and MMP of the CO2–oil system through experiments. The results showed that CAE is soluble in scCO2 but insoluble in water, reducing the MMP by 22.34% at a concentration of 0.2 wt%.
Zhang et al. [130] investigated the synergy between ethanol and surfactants in lowering the IFT and MMP of the CO2–oil system. They indicated that, under the optimal conditions, that is, 7 wt% ethanol + 0.3 wt% TXIB(2,2,4-trimethyl-1,3,-pentaerediol diisobutyl ester), the MMP was reduced by as much as 30.2%.
Lv et al. [131,132] treated different nonionic polyether surfactants for their solubility in CO2 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% C4PO3 could reduce MMP from 17.75 to 13.6 MPa.
Kuang et al. [140] dealt with five surfactants in terms of their solubility in CO2 and their improvements in CO2-oil miscibility. The results indicated that surfactants could reduce the viscosity of oil to a certain extent and lower the IFT between CO2 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 CO2 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 CO2–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.

4.3. Supercritical CO2 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]. CO2-soluble surfactants are soluble in scCO2, and their injection does not involve water, which can mitigate the risk of wellbore corrosion. The surfactants dissolved in CO2 can interact in situ with formation water in the presence of CO2 to form CO2 foam, which helps reduce the mobility of CO2 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 CO2 phase. SACROC [56] carried out a pilot test on a CO2 foam system with CO2-soluble surfactant, revealing an effective control on the CO2 mobility and reduction of the oil–gas ratio.
The high solubility of a surfactant in CO2 and a favorable W/C partition coefficient are beneficial for the transport of the surfactant along CO2-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 scCO2 and water on surfactant transport and foam propagation during two-phase flow. They found that, for all CO2-soluble surfactants studied, the core-scale CO2 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 scCO2 using a high-pressure and high-temperature visualized foam device. The results showed that the surfactant N-P-12 had good CO2 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 CO2.
The Baker Hughes researchers [143] suggested the use of CO2 foams for gas lift operations in conjunction with a cross-linked siloxane polymer (Dow Corning 1250 (poly(trimethylhydrosilylsiloxane), MW 5770, MN 3160) as the recommended surfactant.
Ramadhan [136] investigated the foaming behavior of a CO2-soluble, cationic, amine-based surfactant, N.N.N’-trimethyl-N’-tallow-1,3-diaminopropane (DTTM), which forms wormlike micelles at elevated salinity. They believed that the viscous fingering of surfactant-carrying CO2 causes a delay in foam generation and propagation.
Haeri et al. [137] measured the CO2 solubility of two nonionic, water-soluble, branched alkyl tail surfactants (Indorama SURFONIC®N-100, an ethoxylated nonylphenyl alcohol with 10 ethylene oxide groups, and Indorama SURFONIC® TDA-9, an ethoxylated branched tridecyl alcohol with nine ethylene oxide groups) through core-flooding experiments at 25–75 °C. The results indicated that the surfactants had a solubility of roughly 1 wt%.
Burrows et al. [75] evaluated the CO2 solubility of three nonionic surfactants (branched tridecyl ethoxylate Indorama SURFONIC TDA-9, branched nonylphenol ethoxylate Indorama SURFONIC N-100, and linear dodecyl ethoxylate Indorama SURFONIC L12-6). They found that each surfactant dissolved in CO2 up to 1 wt% at pressures and temperatures comparable to CO2 EOR and CO2-dissolved surfactants did not significantly affect CO2–oil IFT or generate CO2 foams. They also reported that SURFONIC TDA-9 achieved the highest oil recovery of 75%, compared to 71% by pure CO2.
The partitioning of surfactant into the CO2 phase results in faster foam propagation and stronger foam [144]. After the foam collapses in the transport process, CO2-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 CO2 mobility.

5. Outlook

The utilization of CO2-soluble surfactants for EOR presents both challenges and opportunities. Despite the promising results observed in experimental studies and field tests, CO2-soluble surfactants cannot be applied widely unless some limitations are addressed. Fluorinated and siloxane surfactants have notable solubility in CO2, but they come with high cost and toxicity. Hydrocarbon and oxygenated hydrocarbon surfactants exhibit low solubility in scCO2, so they can work only with the support of a large amount of co-solvent. These challenges hinder the large-scale field application of CO2-soluble surfactants.
Nonetheless, new surfactant molecules can be tailored for diverse applications. Such efforts should be made to enhance the solubility in CO2, minimize the necessity of co-solvent, and improve the performance in IFT and foam stability. Rigorous investigations into the molecular interactions between CO2 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 CO2-soluble polymers.
Additionally, there is a growing interest in the development of biomass-based CO2-soluble surfactants or renewable surfactants, which are cost-effective and environmentally friendly.

6. Conclusions

This paper reviews the research of CO2-soluble surfactants and their applications for EOR and proposes the relevant challenges and opportunities.
The addition of CO2-soluble surfactants has been shown to be promising in mitigating the adverse effects of CO2 injection to enhance oil recovery. By modifying the structures and concentrations of CO2-soluble surfactants, it is possible to achieve CO2 thickening, reduce the miscibility pressure, and generate scCO2 foam. CO2-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 CO2-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 CO2-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.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

Authors Zhixing Luo, Wenjuan Wang and Xiaohu Xue were employed by the companies PetroChina Xinjiang Oilfield Company and China Petroleum Technology and Development Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Table 2. Structures of CO2-soluble surfactants discussed in this section.
Table 2. Structures of CO2-soluble surfactants discussed in this section.
Structure ConditionsSolubilityCo-SolventReference
Molecules 28 08042 i001
M-F7H4		60 MPa
45 °C		4.4 wt%-Cummings [61]
Molecules 28 08042 i002
Krytox 157 FSL Mw 2500		22 MPa
60.85 °C		1.72 wt%-Temtem [62]
Molecules 28 08042 i003
di-CF2		22.4 MPa
40 °C		2.77 wt%-Mohamed [63]
Molecules 28 08042 i004
Hybrid CF2/AOT4		34 MPa
40 °C		2.59 wt%-Mohamed [63]
Molecules 28 08042 i005
FC6-FC4		35 MPa
45 °C		1.08 wt%-Sagisaka [64,65]
Molecules 28 08042 i006
Sulfonated siloxane-functional sulfonate surfactants		31.7 MPa
65 °C		1.00 wt%-Fink [66]
Molecules 28 08042 i007
HMTS		9.25 MPa
50 °C		0.11 wt%-Shi [67]
Molecules 28 08042 i008
(PEG)3-TS		10.09 MPa
50 °C		0.003 wt%-Shi [67]
Molecules 28 08042 i009
SIS1		35–75 °C
10–40 MPa		0.007 wt%-Sagisaka [68]
Molecules 28 08042 i010
TMN-6		35–75 °C
10–40 MPa		0.004 wt%-Sagisaka [68]
Molecules 28 08042 i011
AOT		30 MPa
60 °C		DissolvedEthanol Ihara [69]
Molecules 28 08042 i012
sodium bis(2,4,4-trimethyl-1-pentyl)sulfosuccinate		25 MPa
33 °C		Dissolved-Eastoe [51,52,53]
Molecules 28 08042 i013
sodium bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate		25 MPa
33 °C		Dissolved-Eastoe [70,71,72]
Molecules 28 08042 i014
TC14		3 MPa
25 °C		Dissolved-Hollamby [73]
Molecules 28 08042 i015
Tergitol TMN ethoxylated nonyl ether		2.75 MPa
25 °C		1.0 wt%-Ryoo [74]
Molecules 28 08042 i016
SURFONIC TDA-9		29 MPa
77 °C		1.0 wt%-Burrows [75]
Molecules 28 08042 i017
SURFONIC N-100		38 MPa
77 °C		1.0 wt%-Burrows [75]
Molecules 28 08042 i018
SURFONIC L12-6		31 MPa
77 °C		1.0 wt%-Burrows [75]
Molecules 28 08042 i019
N-NP-10c		18.4 MPa
40 °C		1.76 wt%Ethanol Zhang [57]
Molecules 28 08042 i020
APG-1214		17.4 MPa
40 °C		1.66 wt%Ethanol and ethylene glycol (3:2)Zhang [57]
Molecules 28 08042 i021
N-NP-15c-H		17 MPa
40 °C		0.57 wt%Ethanol and ethylene glycol (4:1)Zhang [57]
Molecules 28 08042 i022
C12–14N(EO)2		22.76 MPa
120 °C		0.2 wt%-Chen [76,77,78]
Molecules 28 08042 i023
C12–14N(EO)5		17.93 MPa
120 °C		0.2 wt%-Chen [76,77,78]
Table 3. Summary of studies on the applications of CO2-soluble surfactants for EOR.
Table 3. Summary of studies on the applications of CO2-soluble surfactants for EOR.
Reservoir TypeConditionsSystem CompositionFunctionUltimate Oil
Recovery		Reference
\*25 °C, 35 MPaNi(di-HCF4) (10 wt%) in CO2Thicken the system to 0.22 MPa·s\Trickett [125]
\40 °C, 39.24 MPaVinyl polysiloxane (8 wt%) in CO2Thicken the system to 12.57 MPa·s\Zhao [126]
Daqing crude oil45 °C, 22,7 MPaAOT (0.005 wt%–0.015 wt%), ethanol (13.76 wt%), water (0.41 wt%–1.61 wt%) in CO2Reduce the MMP from 23.8 to 22.7 MPaAbout 80%Dong [127]
Shengli crude oil60 °C, 13.22 MPaC12PO6 (0.6 wt%), ethanol (0.7 wt%) in CO2Reduce the MMP from 17.79 to 13.22 MPa\Wang [128]
\85 °C, 21.2 MPaCAE (0.2 wt%) in CO2Reduce the MMP from 27.3 to 21.2 MPa92.06%Guo [129]
Shengli crude oil60 °C, 11.41 MPaTXIB (0.3 wt%%), ethanol (7 wt%) in CO2Reduce the MMP from 16.79 to 11.41 MPa\Zhang [130]
\50 °C, 13.6 MPaC4PsO3 (3 wt%) in CO2Reduce the MMP from 17.75 to 13.6 MPa\Lv [131,132]
QS8 oilfield76 °C, 30 MPaSPO5 (0.5 wt%), n-pentanol (0.25 wt%) in CO2Reduce the IFT of the CO2-oil system93.47%Kuang [116]
Midland Farm (West Texas) crude oil26.67 °C, 10.34 MPaSurfactant (0.5 wt%) in CO2Generate foam, as indicated by the immediate increase of pressure drop at the start of CO2 injection60%Le [133]
SACROC Field60 °C, 24.13 MPaELEVATE™ CO2 EOR Conformance SolutionsGenerate foamOil production increased by 30% in one monthSanders [56]
Silurian-dolomite cores35 °C, 10.34 MPaCO2-soluble surfactants (S, 4 S, and 15 S) (0.15 wt%) in CO2Reduce the delay of foam propagation\Ren [16,134]
\120 °C, 20 MPaN-P-12 (2.99 wt%), ethylene glycol (16.97 wt%) in CO2Increase and then decrease the resistance92.5%Bi [135]
\40 °C, 11.72 MPaDTTM (0.5 wt%) in CO2Cause delay in foam generation and propagation by viscous fingering\Ramadhan [136]
Eagle Ford outcrop rock
Eagle Ford crude oil		80 °C, 27.6 MPaSURFONIC®N-100 (0.1 wt%) in CO2Alter the wettability of unconventional rock75%Haeri [137]
* “\” indicates that this reference does not provide this part of the parameters or results.
Table 4. General effects of factors on the solubility and viscosity of CO2.
Table 4. General effects of factors on the solubility and viscosity of CO2.
FactorGeneral Effects on CO2 of Increasing the Factor
SolubilityViscosity
Molecular weightDecreaseIncrease
ConcentrationIncrease and then decreaseIncrease
Shear rateIncreaseDecrease
TemperatureDecreaseDecrease
PressureIncreaseIncrease
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Liang, S.; Luo, W.; Luo, Z.; Wang, W.; Xue, X.; Dong, B. Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook. Molecules 2023, 28, 8042. https://doi.org/10.3390/molecules28248042

AMA Style

Liang S, Luo W, Luo Z, Wang W, Xue X, Dong B. Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook. Molecules. 2023; 28(24):8042. https://doi.org/10.3390/molecules28248042

Chicago/Turabian Style

Liang, Shisheng, Wenli Luo, Zhixing Luo, Wenjuan Wang, Xiaohu Xue, and Bo Dong. 2023. "Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook" Molecules 28, no. 24: 8042. https://doi.org/10.3390/molecules28248042

APA Style

Liang, S., Luo, W., Luo, Z., Wang, W., Xue, X., & Dong, B. (2023). Research of CO2-Soluble Surfactants for Enhanced Oil Recovery: Review and Outlook. Molecules, 28(24), 8042. https://doi.org/10.3390/molecules28248042

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