Rod-like micelles thicken CO(2)

A comprehensive review direct methods to overcome the limitations of gas injection during the EOR process

Among the Enhanced Oil Recovery (EOR) methods, gas-based EOR methods are very popular all over the world. The gas injection has a high ability to increase microscopic sweep efficiency and can increase production efficiency well. However, it should be noted that in addition to all the advantages of these methods, they have disadvantages such as damage due to asphaltene deposition, unfavorable mobility ratio, and reduced efficiency of macroscopic displacement. In this paper, the gas injection process and its challenges were investigated. Then the overcoming methods of these challenges were investigated. To inhibit asphaltene deposition during gas injection, the use of nanoparticles was proposed, which were examined in two categories: liquid-soluble and gas-soluble, and the limitations of each were examined. Various methods were used to overcome the problem of unfavorable mobility ratio and their advantages and disadvantages were discussed. Gas-phase modification has the potential to reduce the challenges and limitations of direct gas injection and significantly increase recovery efficiency. In the first part, the introduction of gas injection and the enhanced oil recovery mechanisms during gas injection were mentioned. In the next part, the challenges of gas injection, which included unfavorable mobility ratio and asphaltene deposition, were investigated. In the third step, gas-phase mobility control methods investigate, emphasizing thickeners, thickening mechanisms, and field applications of mobility control methods. In the last part, to investigate the effect of nanoparticles on asphaltene deposition and reducing the minimum miscible pressure in two main subsets: 1- use of nanoparticles indirectly to prevent asphaltene deposition and reduce surface tension and 2- use of nanoparticles as a direct asphaltene inhibitor and Reduce MMP of the gas phase in crude oil was investigated.

Synergy between surfactants' stiffness and concentration on their self-assembly into reverse micelles as water droplet carriers in nonpolar solvents

A study of the self-assembly process into reverse micelles (RMs) of linear surfactants and monomeric aqueous solutes dissolved in nonpolar solvents, varying the concentration (cs) and the persistence length (Lp) of the surfactants is presented here. The influence of cs and Lp on the structural and dynamic properties of the aggregates is investigated through mesoscopic simulations carried out with the dissipative particle dynamics method. All simulations are performed at a fixed water/surfactant molecular ratio of 2:1, varying the surfactant concentration from c = 6 wt% up to c = 12 wt%, for increasing surfactants' rigidity from Lp = 0.73 nm up to Lp = 44.99 nm. It is found that there exists a collaborative interplay between cs and Lp that enhances the number of RMs assembled and their diffusion as carriers of water droplets. These results should be useful as guidelines to understand and improve processes where the RMs are implemented to carry aqueous solutes in nonpolar solvents.

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.

Modulating shape transition in surfactant stabilized reverse microemulsions

The formation of reverse microemulsions (RMs) of spherical shape in the oil/water/surfactant ternary mixture at high molar ratio of water to surfactant (ω) is well established. Using dynamic light scattering, small-angle X-ray and neutron scattering, we elucidate the formation of non-spherical reverse microemulsions stabilised by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at ω = 10 and volume fractions of the dispersed phase, Φ, ranging from 0.005 to 0.20. In addition, we propose a strategy to tune the aspect ratio of non-spherical droplets and colloidal interactions by (i) varying the volume fraction of the dispersed phase (ii) changing the temperature, and (iii) by substituting the aliphatic oil with a mixture of aliphatic and aromatic hydrocarbons. This tunability of anisotropy along with a precise control of the interactions in the RMs, their ability to form spontaneously and their thermodynamic stability is crucial to provide a handle on reaction kinetics, synthesis of anisotropic nanoparticles as well as for their application as lubricants and viscosity modifiers.

Affecting analysis of the rheological characteristic and reservoir damage of CO2 fracturing fluid in low permeability shale reservoir

The fracturing property of liquid CO₂ fracturing fluid varies greatly due to the rheology of fracturing fluid during fracturing process. The main objective of this investigation is to study the rheology property of thickened liquid CO₂ by measuring the viscosity of thickened liquid CO₂ in different physical parameters of this prepared thickener and explain the causes of rheological changes. The results show that thickener content, branching content, and molecular weight of a thickener for all could significantly improve the rheology of liquid CO₂; the consistency coefficient K increased as they rose, but the rheological index n presented a decreased trend. Meanwhile, the mesh structure is proposed as a model to explain the rheological changes, and the large wetting angle means an excellent backflow, low reservoir damage, and low adsorption property. These results herein provide a basic reference to improve the CO₂ fracturing technology and molecular design of CO₂ thickener.

Effective viscosification of supercritical carbon dioxide by oligomers of 1-decene

Viscosification of carbon dioxide by polymers can make large scale CO₂ sequestration safe and efficient. We present solubility of branched hydrocarbon oligomers in CO₂ and viscosification measurements at relevant subsurface conditions. Polymers of 1-decene (P1D) with about 20 repeating units are found to be effective in CO₂ viscosification, increasing it by 6.5-fold at 1.8 wt% concentration at 308 K and 31 MPa. We reason that methyl groups and branching promote solubility and viscosification. Low molecular weight oligomers can have lower solubility in CO₂ than higher molecular weight ones and the trend in solubility is non-monotonic at constant pressure and temperature. Analysis of solubility trend of P1D oligomers in CO₂ advances our understanding of molecular structure and functionality and opens the path to engineering of oligomers effective in viscosification and widespread use of CO₂.

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Increasing P1D size has non-monotonic solubility trend in CO₂ at moderate pressures

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P1D solubility in CO₂ has monotonic trend at higher pressures

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Methyl groups enhance solubilization in CO₂

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An exponential trend observed for relative viscosity with polymer molecular size

Research progress on supercritical CO2 thickeners

According to the thickening principle and molecular structure of thickeners, supercritical carbon dioxide (scCO2) thickeners have been summarized and introduced by dividing into polymers, small molecular compounds and surfactants. The properties such as solubility, thickening effect, thickening condition and existing problems of scCO2 thickeners are analyzed and assessed, and the research progress and prospects of scCO2 thickeners are proposed. ScCO2 is used in both CO2 fracturing and CO2 flooding for enhanced oil recovery (EOR). However, due to its low viscosity, the proppant carrying ability and filtration control ability of scCO2 are too weak for fracturing. Also, in the process of CO2 flooding, its low viscosity not only exacerbates the gravity override but also leads to an unfavorable mobility ratio that results in viscous fingering, early breakthrough, and poor sweep efficiency. Therefore, scCO2 thickeners have good application prospects in oil and gas production for improved oil recovery (IOR).

Achieving solubility alteration with functionalized polydimethylsiloxane for improving the viscosity of supercritical CO2 fracturing fluids

Thickened carbon dioxide flow state.

Anisotropic reversed micelles with fluorocarbon-hydrocarbon hybrid surfactants in supercritical CO2

Previous work (M. Sagisaka, et al. Langmuir 31 (2015) 7479-7487), showed the most effective fluorocarbon (FC) and hydrocarbon (HC) chain lengths in the hybrid surfactants FCm-HCn (sodium 1-oxo-1-[4-(perfluoroalkyl)phenyl]alkane-2-sulfonates, where m = FC length and n = HC length) were m and n = 6 and 4 for water solubilization, whereas m 6 and n 6, or m 6 and n 5, were optimal chain lengths for reversed micelle elongation in supercritical CO₂. To clarify why this difference of only a few methylene chain units is so effective at tuning the solubilizing power and reversed micelle morphology, nanostructures of water-in-CO₂ (W/CO₂) microemulsions were investigated by high-pressure small-angle neutron scattering (SANS) measurements at different water-to-surfactant molar ratios (W₀) and surfactant concentrations. By modelling SANS profiles with cylindrical and ellipsoidal form factors, the FC6-HCn/W/CO₂ microemulsions were found to increase in size with increasing W₀ and surfactant concentration. Ellipsoidal cross-sectional radii of the FC6-HC4/W/CO₂ microemulsion droplets increased linearly with W₀, and finally reached ∼39 Å and ∼78 Å at W₀ = 85 (close to the upper limit of solubilizing power). These systems appear to be the largest W/CO₂ microemulsion droplets ever reported. The aqueous domains of FC6-HC6 rod-like reversed micelles increased in size by 3.5 times on increasing surfactant concentration from 35 mM to 50 mM: at 35 mM, FC6-HC5 formed rod-like reversed micelles 5.3 times larger than FC6-HC6. Interestingly, these results suggest that hybrid HC-chains partition into the microemulsion aqueous cores with the sulfonate headgroups, or at the W/CO₂ interfaces, and so play important roles for tuning the W/CO₂ interfacial curvature. The super-efficient W/CO₂-type solubilizer FC6-HC4, and the rod-like reversed micelle forming surfactant FC6-HC5, represent the most successful cases of low fluorine content additives. These surfactants facilitate VOC-free, effective and energy-saving CO₂ solvent systems for applications such as extraction, dyeing, dry cleaning, metal-plating, enhanced oil recovery and organic/inorganic or nanomaterial synthesis.

Shape transition of water-in-CO2 reverse micelles controlled by the surfactant midpiece

Designing CO₂-philic surfactants for generating wormlike reverse micelles (RMs) is an effective approach to enhance the viscosity of supercritical CO₂ (scCO₂), however this remains challenging.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO₂), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C₈Benz). It is found that adding the C₈Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C₈Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C₈Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C₈Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO₂.

Tuning Micellar Structures in Supercritical CO2 Using Surfactant and Amphiphile Mixtures

For equivalent micellar volume fraction (ϕ), systems containing anisotropic micelles are generally more viscous than those comprising spherical micelles. Many surfactants used in water-in-CO₂ (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO₂-philic surfactants with hydrotropes and cosurfactants may generate elongated micelles in w/c systems at high-pressures (e.g., 100-400 bar). A range of novel w/c microemulsions, stabilized by new custom-synthesized CO₂-phillic, partially fluorinated surfactants, were formulated with hydrotropes and cosurfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure, and hydrotrope tail length were all investigated. Dispersed water domains were probed using high pressure small-angle neutron scattering (HP-SANS), which provided evidence for elongated reversed micelles in supercritical CO₂. These new micelles have significantly lower fluorination levels than previously reported (6-29 wt % cf. 14-52 wt %), and furthermore, they support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio; from this value a relative viscosity value can be estimated through combination with the micellar volume fraction (ϕ). Combining these new results with those for all other reported systems, it has been possible to "map" predicted viscosity increases in CO₂ arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.

Molecular dynamics study of di-CF4 based reverse micelles in supercritical CO2

Reverse micelles (RMs) in supercritical CO₂ (scCO₂) are promising alternatives for organic solvents, especially when both polar and non-polar components are involved. Fluorinated surfactants, particularly double-chain fluorocarbon surfactants, are able to form well-structured RMs in scCO₂. The inherent self-assembly mechanisms of surfactants in scCO₂ are still subject to discussion. In this study, molecular dynamics simulations are performed to investigate the self-aggregation behavior of di-CF4 based RMs in scCO₂, and stable and spherical RMs are formed. The dynamics process and the self-assembly structure in the RMs reveal a three-step mechanism to form the RMs, that is, small RMs, rod-like RMs and fusion of the rod-like RMs. Hydrogen-bonds between headgroups and water molecules, and salt bridges linking Na⁺ ions, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The results show that di-CF4 molecules have a high surfactant coverage at the RM interface, implying a high CO₂-philicity. This mainly results from bending of the short chain (C-COO-CH₂-(CF2)₃-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful in understanding surfactant self-assembly phenomena and designing new CO₂-philic surfactants.

The self-assembly structure and the CO2-philicity of a hybrid surfactant in supercritical CO2: effects of hydrocarbon chain length

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains, as effective CO₂-philic surfactants, could improve the solubility of polar substances in supercritical CO₂. Varying the length of the HC of hybrid surfactants is an effective way to improve the CO₂-philicity. In this paper, we have investigated the effects of the HC length on the self-assembly process and the CO₂-philicity of hybrid surfactants (F7Hn, n = 1, 4, 7 and 10) in water/CO₂ mixtures using molecular dynamics simulations. It is found that the self-assembly time of F7Hn exhibits a maximum when the length of the HC is equal to that of the FC (F7H7). In this case, the investigation of H-bonds between the water core and CO₂ phase shows that F7H7 has the strongest CO₂-philicity because it has the best ability to separate water and CO₂. To explain the origin of the differences in separation ability, the analysis of the structures of the reverse micelles shows that there are two competing mechanisms with a shortening HC. Firstly, the volume of F7Hn is reduced, which thus decreases the separation ability. Moreover, this also leads to the curved conformation of the FC. As a result, the separation ability is enhanced. These two mechanisms are balanced in F7H7, which has the best ability to separate water and CO₂. Our simulation results demonstrate that the increased volume and the curved conformation of the hybrid surfactant tail could enhance the CO₂-philicity in F7Hn surfactants. It is expected that this work will provide valuable information for the design of CO₂-philic surfactants.

Supercritical carbon dioxide: a solvent like no other

Supercritical carbon dioxide (scCO2) could be one aspect of a significant and necessary movement towards green chemistry, being a potential replacement for volatile organic compounds (VOCs). Unfortunately, carbon dioxide has a notoriously poor solubilising power and is famously difficult to handle. This review examines attempts and breakthroughs in enhancing the physicochemical properties of carbon dioxide, focusing primarily on factors that impact solubility of polar and ionic species and attempts to enhance scCO2 viscosity.

Shape transitions in supercritical CO2 microemulsions induced by hydrotropes

The ability to induce morphological transitions in water-in-oil (w/o) and water-in-CO2 (w/c) microemulsions stabilized by a trichain anionic surfactant 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) with simple hydrotrope additives has been investigated. High-pressure small-angle neutron scattering (SANS) has revealed the addition of a small mole fraction of hydrotrope can yield a significant elongation in the microemulsion water droplets. For w/o systems, the degree of droplet growth was shown to be dependent on the water content, the hydrotrope mole fraction, and chemical structure, whereas for w/c microemulsions a similar, but less significant, effect was seen. The expected CO2 viscosity increase from such systems has been calculated and compared to related literature using fluorocarbon chain surfactants. This represents the first report of hydrotrope-induced morphology changes in w/c microemulsions and is a significant step forward toward the formation of hydrocarbon worm-like micellar assemblies in this industrially relevant solvent.

Anisotropic dynamics of guest molecules in aerosol OT lamellar structures

The present work is focused on developing a description of an anisotropic microheterogeneous medium, exploiting the dynamics of a guest molecule. The medium in question is the lamellar structures formed in the aqueous layer of ternary mixtures containing aerosol OT (AOT), water and n-heptane. The guest used in this study is the fluorescent probe, coumarin 153 (C153). The dynamics of this molecule, within the lamellar structure, have been studied using a combination of steady state and time resolved fluorescence, as well as fluorescence correlation spectroscopy (FCS). The fluorophore is strongly solvatochromatic and so, the wavelength of excitation can be tuned so as to selectively excite fluorescent molecules residing in different regions of the microheterogeneous media, even if the spatial separation between these regions is below the diffraction limit. The excitation wavelength in the present experiments is chosen so as to exclusively excite those C153 molecules that reside in the hydrophobic region of the lamellar structures. This triggers two different modes of diffusion, one along and the other perpendicular to the bilayers of the AOT. Thus, the dynamics of the fluorescent probe provide an elegant manifestation of the anisotropy of the host medium.

Smart wormlike micelles

A major scientific challenge of the past decade pertaining to the field of soft matter has been to craft 'adaptable' materials, inspired by nature, which can dynamically alter their structure and functionality on demand, in response to triggers produced by environmental changes. Amongst these, 'smart' surfactant wormlike micelles, responsive to external stimuli, are a particularly recent area of development, yet highly promising, given the versatility of the materials but simplicity of the design-relying on small amphiphilic molecules and their spontaneous self-assembly. The switching 'on' and 'off' of the micellar assembly structures has been reported using electrical, optical, thermal or pH triggers and is now envisaged for multiple stimuli. The structural changes, in turn, can induce major variations in the macroscopic characteristics, affecting properties such as viscosity and elasticity and sometimes even leading to a spontaneous and effective 'sol-gel' transition. These original smart materials based on wormlike micelles have been successfully used in the oil industry, and offer a significant potential in a wide range of other technological applications, including biomedicine, cleaning processes, drag reduction, template synthesis, to name but a few. This review will report results in this field published over the last few years, describe the potential and practical applications of stimuli-responsive wormlike micelles and point out future challenges.

Amphiphiles for supercritical CO2

A semi-fluorinated hybrid amphiphile, pentadecafluoro-5-dodecyl (F7H4) sulfate, has been shown to form reversed micelles in dense CO(2); the aggregates evolve to form water-in-CO(2) (w/c) microemulsion droplets on addition of water. Aggregation structures in these w/c phases have been characterised by small-angle neutron scattering (SANS), showing the presence of cylindrical droplets, which change into dispersed lamellar phases at even higher water loadings. Other systems are also introduced, being high internal phase emulsions (HIPEs) with brine, and liquid and supercritical CO(2), stabilized by certain commercially available nonylphenol ethoxylates (Dow Tergitol NP-, and Huntsman Surfonic N- amphiphiles). These dispersions have been characterised by SANS for the first time. Quantitative analyses of the HIPEs SANS profiles show that they behave similarly to hydrocarbon-water emulsion analogues, with regard to total interfacial areas and the effects of amphiphile concentration on the underlying structures. Finally, the advantages and disadvantages of both approaches for controlling the physico-chemical properties of liquid/supercritical CO(2) in potential applications are compared and contrasted. These results highlight the importance of using specially designed CO(2)-philic amphiphiles for generating self-assembly structures in dense CO(2).

CO2: a wild solvent, tamed

This article reviews approaches for modification of solvent properties of supercritical carbon dioxide (scCO(2)), with particular reference to self-assembly of oligomeric and polymeric solute additives. Of special interest are viscosity modifiers for scCO(2) based on molecular self-assembly. Background on polymers and surfactants with CO(2)-compatible functionalities is covered, leading on to the attempts made so far to increase the scCO(2) viscosity, which are described in detail. The significance of this field, and the implications a breakthrough could bring environmentally and economically will be addressed.

Hydrocarbon metallosurfactants for CO2

Cobalt and nickel salts of the highly branched trichain anionic surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) are shown to be soluble in dense CO(2) at concentrations up to 6 wt % at 500 bar pressure. This is a remarkably high solubility for such hydrocarbon transition metal surfactants in CO(2). High-pressure small-angle neutron scattering (HP-SANS) has been used to study the surfactant aggregates in a normal organic solvent, cyclohexane, dense CO(2), and also mixtures of these two pure solvents. The results show that transition metal TC14 derivatives are viable compounds for incorporating reactive and functional metal ions into CO(2).