CO2-switchable foams stabilized by a long-chain viscoelastic surfactant

CO2 induced phase transition on a self-standing droplet studied by X-ray scattering and magnetic resonance

HYPOTHESIS

Acoustic levitation is a suitable approach for studying processes occurring at the gas-liquid interfaces, as it allows its investigation in a contact-free manner while providing control over the gas phase. Here, we hypothesize that phase transitions induced by a CO2 rich atmosphere can be examined, at different length scales, in a contact-free manner.

EXPERIMENTAL

A system consisting of 12-hydroxysteric acid (HSA) soaps mixed with different ratios of monoethanolamine (MEA) and choline hydroxide, was prepared. Microliter droplets of the samples were acoustically levitated and monitored with a camera, while exposed to CO2 to modify the pH through diffusion at the air-liquid interface and inside the droplet. The phase transition and water mobility in the levitated droplets were evaluated through X-ray scattering (SAXS/WAXS) and magnetic resonance studies, in real-time. Finally, the droplets were collected and examined under the microscope.

FINDINGS

The introduction of CO2 gas induced a phase transition from micelles to multi-lamellar tubes, resulting in a gel-like behavior both in the bulk and at the interface. The high stability of the acoustic levitator allowed the investigation of this dynamic phenomenon, in real-time, in a contact-free environment. This study showcases the suitability of acoustic levitation as a tool to investigate complex chemical processes at interfaces.

CO2 Foamed Viscoelastic Gel-Based Seawater Fracturing Fluid for High-Temperature Wells

This study investigates the development of a novel CO2-foamed viscoelastic gel-based fracturing fluid to address the challenges of high-temperature formations. The influence of various parameters, including surfactant type and concentration, gas fraction, shear rate, water salinity, temperature, and pressure, on foam viscosity was systematically explored. Rheological experiments were conducted using a high-pressure/high-temperature (HPHT) rheometer at 150 °C and pressures ranging from 6.89 to 20.68 MPa. To simulate field conditions, synthetic high-salinity water was employed. The thermal stability of the CO2 foam was evaluated at a constant shear rate of 100 1/s for 180 min. Additionally, foamability and foam stability were assessed using an HPHT foam analyzer at 100 °C. The results demonstrate that liquid phase chemistry, experimental conditions, and gas fraction significantly impact foam viscosity. Viscoelastic surfactants achieved a peak foam viscosity of 0.183 Pa·s at a shear rate of 100 1/s and a 70% foam quality, surpassing previous records. At lower foam qualities (≤50%), pressure had a negligible effect on foam viscosity, whereas at higher qualities, it increased viscosity by over 30%. While a slight increase in viscosity was observed with foam qualities between 40% and 60%, a significant enhancement was noted at 65% foam quality. The addition of polymers did not improve foam viscosity. The generation of viscous and stable foams is crucial for effective proppant transport and fracture induction. However, maintaining the thermal stability of CO2 foams with minimal additives remains a significant challenge in the industry. This laboratory study provides valuable insights into the development of stable CO2 foams for stimulating high-temperature wells.

Effect of CO2 Concentration on the Performance of Polymer-Enhanced Foam at the Steam Front

This study examines the impact of CO2 concentration on the stability and plugging performance of polymer-enhanced foam (PEF) under high-temperature and high-pressure conditions representative of the steam front in heavy oil reservoirs. Bulk foam experiments were conducted to analyze the foam performance, interfacial properties, and rheological behavior of CHSB surfactant and Z364 polymer in different CO2 and N2 gas environments. Additionally, core flooding experiments were performed to investigate the plugging performance of PEF in porous media and the factors influencing it. The results indicate that a reduction in CO2 concentration in the foam, due to the lower solubility of N2 in water and the reduced permeability of the liquid film, enhances foam stability and flow resistance in porous media. The addition of polymers was found to significantly improve the stability of the liquid film and the flow viscosity of the foam, particularly under high-temperature conditions, effectively mitigating the foam strength degradation caused by CO2 dissolution. However, at 200 °C, a notable decrease in foam stability and a sharp reduction in the resistance factor were observed. Overall, the study elucidates the effects of gas type, temperature, and polymer concentration on the flow and plugging performance of PEF in porous media, providing reference for fluid mobility control at the steam front in heavy oil recovery.

Experimental Investigations and MD Simulation on Nanoparticle-Enhanced CO2-Responsive Foam (NECRF): Implications on CO2-EOR

CO2-responsive foam (CRF) is a highly promising candidate for CO2-enhanced oil recovery (CO2-EOR) because it displays higher stability than the surfactant-stabilized foam owing to the formation of robust wormlike micelles (WLMs) upon exposure to CO2. In this work, the nanoparticle-enhanced CO2-responsive foam (NECRF) was properly prepared using lauryl ether sulfate sodium (LES)/diethylenetriamine/nano-SiO2, and its interfacial properties and EOR potential were experimentally and numerically assessed, aiming to explore the feasibility and effectiveness of NECRF as a novel CO2-EOR technique. It was found that the interfacial expansion elastic modulus increased 6-fold after CO2 stimulation. The modulus continued to increase with the introduction of nano-SiO2 owing to the pronounced synergistic effect of WLMs and nanoparticles. In addition to increasing the viscosity of the foaming liquid, WLMs and nano-SiO2 enhanced the shearing resistance of the NECRF as well. Calculations demonstrated that both the coarsening rate and the size distribution uniformity coefficient of NECRF were markedly lower than that of the LES foam, which subsequently inhibited NECRF decay and greatly improved its dynamic stability. Besides, molecular dynamics simulation revealed that adding inorganic salts to NECRF could notably enhance the foaming performance due to the intensified hydration of surfactant head groups and reduced binding energy of neighboring molecules. Nuclear magnetic resonance-assisted core flooding experiments validated the exceptional capacity of NECRF to sweep the low-permeability region and improve the conformance profile. Overall, these findings may provide valuable insights into the development and application of novel materials and strategies for the CO2-EOR.

Effects of Different Types of Stabilizers on the Properties of Foam Detergent Used for Radioactive Surface Contamination

Compared with high-pressure water and reagent washing decontamination, foam decontamination has a promising application due to its ability to significantly reduce the volume of radioactive waste liquids and effectively decontaminate the inner surface of the pipes, the interior of the large cavities, and the vertical walls. However, the foam is less stable, leading to a low decontamination rate. Currently, three main types of stabilizers with different stabilizing mechanisms, namely nanoparticles, polymers, and cosurfactants, are used to improve foam stability and thus increase the decontamination rate. Nanosilica (NS), xanthan gum (XG), and n-tetradecanol (TD) were used as typical representatives of nanoparticles, polymers, and cosurfactants, respectively, to improve the stability of the foam detergent with pH < 2 and chelating agents. The differences in the effects of these three types of stabilizers on foam properties were investigated. Although NS, XG, and TD all increase the half-life of the foam from 7.2 min to about 40 min, the concentration of TD is much lower than that of NS and XG in the foaming solution, and TD foaming solution has the highest foaming ratio. Moreover, TD can markedly lower the surface tension, resulting in a significant reduction of the wetting contact angle on the surfaces of glass, ceramic tile, stainless steel, and paint, while NS and XG cannot signally change the surface tension and have no obvious effect on the wetting contact angle. At low shear rates, TD can increase the apparent viscosity of foam by two orders of magnitude, and the wall-hanging time of the foam on the vertical wall is more than 30 min. In contrast, NS and XG cause a limited increase in the apparent viscosity of the foam, and the wall-hanging times are both less than 5 min. In addition, TD foaming solution has excellent storage stability, and the storage time has no obvious effect on the performance of the foam. And after only three days of storage, NS undergoes severe agglomeration and precipitation in the foaming solution, resulting in a complete loss of the stabilizing effect. After 90 days of storage, the half-life of XG foam decreases by 26%. For simulated radioactive uranium contamination on both horizontal and vertical surfaces, TD can significantly improve the decontamination rate, especially for vertical surfaces, where TD can increase the single decontamination rate by more than 50%.

pH-Responsive Viscoelastic Fluids of a C22-Tailed Surfactant Induced by Trivalent Metal Ions

pH-responsive viscoelastic fluids are often achieved by adding hydrotropes into surfactant solutions. However, the use of metal salts to prepare pH-responsive viscoelastic fluids has been less documented. Herein, a pH-responsive viscoelastic fluid was developed by blending an ultra-long-chain tertiary amine, N-erucamidopropyl-N, N-dimethylamine (UC₂₂AMPM), with metal salts (i.e., AlCl₃, CrCl₃, and FeCl₃). The effects of the surfactant/metal salt mixing ratio and the type of metal ions on the viscoelasticity and phase behavior of fluids were systematically examined by appearance observation and rheometry. To elucidate the role of metal ions, the rheological properties between AlCl₃- and HCl-UC₂₂AMPM systems were compared. Results showed the above metal salt evoked the low-viscosity UC₂₂AMPM dispersions to form viscoelastic solutions. Similar to HCl, AlCl₃ could also protonate the UC₂₂AMPM into a cationic surfactant, forming wormlike micelles (WLMs). Notably, much stronger viscoelastic behavior was evidenced in the UC₂₂AMPM-AlCl₃ systems because the Al³⁺ as metal chelators coordinated with WLMs, promoting the increment of viscosity. By tuning the pH, the macroscopic appearance of the UC₂₂AMPM-AlCl₃ system switched between transparent solutions and milky dispersion, concomitant with a viscosity variation of one order of magnitude. Importantly, the UC₂₂AMPM-AlCl₃ systems showed a constant viscosity of 40 mPa·s at 80 °C and 170 s^-1 for 120 min, indicative of good heat and shear resistances. The metal-containing viscoelastic fluids are expected to be good candidates for high-temperature reservoir hydraulic fracturing.

CO2-responsive gels

CO₂-responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO₂. The use of CO₂ as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO₂-responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO₂-responsive polymers, the study of CO₂-responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO₂ such as swelling or a transformed morphology. In the past 15 years, CO₂-responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO₂-triggered aggregation and their capacity for reversible CO₂ absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO₂ sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO₂-responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO₂-induced self-assembled gels (including poly(ionic liquid) gels).

Advances in CO2-switchable surfactants towards the fabrication and application of responsive colloids

CO₂-switchable surfactants have selective surface-activity, which can be activated or deactivated either by adding or removing CO₂ from the solution. This feature enables us to use them in the fabrication of responsive colloids, a group of dispersed systems that can be controlled by changing the environmental conditions. In chemical processes, including extraction, reaction, or heterogeneous catalysis, colloids are required in some specific steps of the processes, in which maximum contact area between immiscible phases or reactants is desired. Afterward, the colloids must be broken for the postprocessing of products, solvents, and agents, which can be facilitated by using CO₂-switchable surfactants in surfactant-stabilized colloids. These surfactants are mainly cationic and can be activated by the protonation of a nitrogen-containing group upon sparging CO₂ gas. Also, CO₂-switchable superamphiphiles can be formed by non-covalent bonding between components at least one of which is CO₂-switchable. So far, CO₂-switchable surfactants have been used in CO₂-switchable spherical and wormlike micelles, vesicles, emulsions, foams, and Pickering emulsions. Here, we review the fabrication procedure, chemical structure, switching scheme, stability, environmental conditions, and design philosophy of such responsive colloids. Their fields of application are wide, including emulsion polymerization, catalysis, soil washing, drug delivery, extraction, viscosity control, and oil transportation. We also emphasize their application for the CO₂-assisted enhanced oil recovery (EOR) process as a promising approach for carbon capture, utilization, and storage to combat climate change.

A Comparative Study on CO2-Switchable Foams Stabilized by C22- or C18-Tailed Tertiary Amines

The CO2 aqueous foams stabilized by bioresource-derived ultra-long chain surfactants have demonstrated considerable promising application potential owing to their remarkable longevity. Nevertheless, existing research is still inadequate to establish the relationships among surfactant architecture, environmental factors, and foam properties. Herein, two cases of ultra-long chain tertiary amines with different tail lengths, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and N-oleicamidopropyl-N,N-dimethylamine (UC18AMPM), were employed to fabricate CO2 foams. The effect of temperature, pressure and salinity on the properties of two foam systems (i.e., foamability and foam stability) was compared using a high-temperature, high-pressure visualization foam meter. The continuous phase viscosity and liquid content for both samples were characterized using rheometry and FoamScan. The results showed that the increased concentrations or pressure enhanced the properties of both foam samples, but the increased scope for UC22AMPM was more pronounced. By contrast, the foam stability for both cases was impaired with increasing salinity or temperature, but the UC18AMPM sample is more sensitive to temperature and salinity, indicating the salt and temperature resistance of UC18AMPM-CO2 foams is weaker than those of the UC22AMPM counterpart. These differences are associated with the longer hydrophobic chain of UC22AMPM, which imparts a higher viscosity and lower surface tension to foams, resisting the adverse effects of temperature and salinity.

Synthesis of d-Gluconic Acetal Surfactants and Their Foaming Behaviors

Sugars are natural and environmentally benign substances, which can offer various hydroxyl groups. The understanding of details of the hydroxyl interactions in the hydrophilic groups of sugar-based surfactants, as well as the related properties, is still indistinct. Here, novel d-gluconic acetal surfactants with bicyclic and monocyclic structures in the head group were designed and synthesized. The obtained surfactant with a bicyclic architecture exhibited excellent foamability and a multistimulus-responsive behavior toward foam stabilization. In addition, the control of foamability from defoaming and foaming could be achieved by changing pH values or bubbling gas of CO₂/N₂. To explore the structural effects such as hydroxyl groups and rigidity of the head group on the properties of sugar-based surfactants, another kind of amphiphilic molecule with various OH- groups and a monocycle in the head group was designed for comparison. These two series of amphiphilic molecules both exhibited good surface activity. However, only the d-gluconic acetal surfactant with a bicyclic structure and a smaller number of OH- groups exhibited excellent foamability. Further studies showed that the foam behaviors were attributed to the conformation and arrangement of the surfactant molecule at the surface layer with the assistance of hydrogen bonds formed by hydroxyl groups and H₂O molecules. In addition, the surfactant could provide an environmentally friendly foamer in many potential applications.

Synthesis and stability of switchable CO2-responsive foaming coupled with nanoparticles

CO₂-responsive foaming has been drawing huge attention due to its unique switching characteristics in academic research and industrial practices, whereas its stability remains questionable for further applications. In this paper, a new CO₂-switchable foam was synthesized by adding the preferably selected hydrophilic nanoparticle N20 into the foaming agent C₁₂A, through a series of analytical experiments. Overall, the synergy between cationic surfactants and nanoparticles with a contact angle of 37.83° is the best. More specifically, after adding 1.5 wt% N20, the half-life of foam is 14 times longer than that of pure C₁₂A foam. What’s more, the C₁₂A-N20 solution is validated to own distinctive CO₂-N₂ switching features because very slight foaming degradations are observed in terms of the foaming volume and half-life time even after three cycles of CO₂-N₂ injections. This study is of paramount importance pertaining to future CO₂ foam research and applications in energy and environmental practices.

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Cationic surfactants have the best synergy with NPs with a contact angle of 37.83°

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The foam stability increased with the increase of NPs concentration

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CO₂/N₂ can control the foaming properties of C₁₂A-N20 solution and are reversible

Liquid Foam Stabilized by a CO2-Responsive Surfactant and Similarly Charged Cellulose Nanofibers for Reversibly Plugging in Porous Media

CO₂ foams are of great importance in oil recovery but challenging in some aspects like long-term stabilization and time-separated conflict. In this work, a stability-enhanced switchable foam was fabricated using bis-(2-hydroxyethoxy) olefine amine (BOA) and trace amounts (0.05 wt %) of cationic-modified cellulose nanofibers (CCNFs). The CCNF was developed using sequentially functionalized CNF with diamine groups, which were essential to promote the aqueous dispersibility and a key for strengthening the stabilization of foam. The combination of similarly charged CCNFs and BOA in the presence of CO₂ contributed to both surface activity and viscoelasticity. It was demonstrated that CCNFs were entangled and stacked to form the compact films and possessed the ability to costabilize the lamellae, as observed by microscopic studies. In addition, the intermolecular H-bonds were promoted in the binary system after being protonated by CO₂ and thus balancing the electrostatic forces, as explored by spectroscopy characterizations. The soft fibrous structure of the CCNF was also capable of wrapping gas bubbles in the form of a functional membrane with both low gas permeability and high surface potential, which slowed down the coarsening and coalescence. Of particular interest is that the reversible protonation state of CCNF-BOA complexes upon the alternate treatment with CO₂/N₂ led to reversible fast foaming/defoaming, which would be beneficial to construct the steerable plugging in the sand pack. This work is expected to provide a new direction and application of the CO₂ responsive foam stabilized by similarly charged nanocellulose fibers in oilfield development.

Soybean-Oil-Based CO2-Switchable Surfactants with Multiple Heads

Oligomeric surfactants display the novel properties of low surface activity, low critical micellar concentration and enhanced viscosity, but no CO2 switchable oligomeric surfactants have been developed so far. The introduction of CO2 can convert tertiary amine reversibly to quaternary ammonium salt, which causes switchable surface activity. In this study, epoxidized soybean oil was selected as a raw material to synthesize a CO2-responsive oligomeric surfactant. After addition and removal of CO2, the conductivity analyzing proves that the oligomeric surfactant had a good response to CO2 stimulation. The viscosity of the oligomeric surfactant solution increased obviously after sparging CO2, but returned to its initial low viscosity in the absence of CO2. This work is expected to open a new window for the study of bio-based CO2-stimulated oligomeric surfactants.

Molecular dynamics simulation of CO2-switchable surfactant regulated reversible emulsification/demulsification processes of a dodecane-saline system

CO2-Switchable surfactants are of great potential in a wide range of industrial applications related to their ability to stabilize and destabilize emulsions upon command. Molecular dynamics simulations have been performed to reveal the fundamental mechanism of the reversible emulsification/demulsification processes of a dodecane-saline system by a CO2-switchable surfactant that switches between active (i.e., N'-dodecyl-N,N-dimethylacetamidinium (DMAAH+)) and inactive (i.e., N'-dodecyl-N,N-dimethylacetamidine (DMAA)) forms. The density profiles indicate that DMAAH+ could increase the oil-water interfacial thickness to a greater extent compared to DMAA. DMAAH+ could sharply reduce the interfacial tension of the dodecane-saline system, while DMAA only exhibits a limited decrease, which is in accordance with the experimental observation that DMAAH+/DMAA can reversibly emulsify/demulsify alkane-water systems. Our simulations showed that both the number and lifetime of hydrogen bonds (HBs) between DMAA and water are almost equal to those between DMAAH+ and water. In DMAA, the N atom connecting with the alkyl tail acted as a HB acceptor, while the N atom attached by a proton in DMAAH+ acted as a HB donor. Furthermore, the HBs between DMAAH+ and HCO3- at the interfaces are relatively limited. Hence, it is deduced that the HBs are insufficient to achieve the CO2-switchability of DMAA/DMAAH+. The Lennard Jones and coulombic potentials between DMAA/DMAAH+ and other species show that the coulombic potentials between DMAAH+ and water or anions (i.e., Cl- and HCO3-) sharply decrease with the increase of DMAAH+ and are much lower than those in models with DMAA. The enhanced coulombic interactions between DMAAH+ and anions lead to a remarkable reduction in interfacial tension and the emulsification of the alkane-saline system. Therefore, coulombic interactions are of crucial importance to the reversible emulsification/demulsification processes regulated by CO2-switchable surfactants, namely DMAAH+/DMAA.

CO2-responsive surfactants with tunable switching pH

HYPOTHESIS

One of the major challenges in applying CO₂-responsive surfactants concerns their tunable switchability and robustness under operating conditions. We hypothesize that combining monoethanolamine (MEA) with long-chain fatty acids (LCFAs) of variable chain lengths through electrostatic attraction could develop a series of CO₂-responsive surfactants with tunable switching pH.

EXPERIMENTS

The tunability of switching pH for this group of surfactants was demonstrated by in situ probing of the CO₂-responsive characteristics at the oil/water interface using dynamic interfacial tension (IFT) measurements. Two protocols were applied to distinguish interfacial response and solution response. The key importance of interfacial response was demonstrated by two essential applications of CO₂-responsive surfactants: demulsification of stable emulsions, and alternation of the interfacial properties of ultra-heavy crude oil-water interfaces.

FINDINGS

The switching pH of the CO₂-responsive surfactants was controlled by the hydrocarbon chain length of LCFAs. More importantly, their switching behaviour was found to be different at the interface and in the bulk solution, which is attributed to the enhanced molecular interactions at the interface. Since most applications require surfactants to be switched at the interface, it is thereby most appropriate to determine the switching pH through their interfacial responses.

Enhanced stability and high temperature-tolerance of CO2 foam based on a long-chain viscoelastic surfactant for CO2 foam flooding

CO₂ switchable foams have gained increasing attention recently for their smart properties. However, their performance at high temperature and high pressure has been less documented. In this study, a long-chain viscoelastic surfactant, N1-(3-aminopropyl)-N3-octadecylpropane-1,3-diamine bicarbonate (ODPTA) has been studied as a CO₂ foam agent for its application in CO₂ flooding in complex and harsh reservoir conditions, and the foam performance under static and dynamic conditions was tested up to 160 °C and 10.5 MPa using a visualized foam-meter and in sand-pack flooding experiments. The viscosity of the ODPTA and conventional surfactant solutions saturated with dissolved CO₂ was measured using a long coiled-tube viscometer at HTHP, and its effect on the high temperature-tolerance of CO₂ foams has been analyzed. The experimental results show that CO₂ foam generated using ODPTA is much more stable than the conventional surfactants (such as SDS and alkylphenol ethoxylates) and has high temperature-tolerance up to 160 °C, and has also exhibited excellent mobility control in CO₂ flooding experiments. The viscosity of the ODPTA–CO₂ bulk phase can be maintained as high as 12 mPa s under 160 °C and 10.5 MPa, which is much higher than that of the conventional surfactant solutions (similar to water). ODPTA's good foam performance with extremely high temperature-tolerance can be attributed to its high bulk phase viscosity in the brine water saturated with CO₂.

CO₂ switchable foams have gained increasing attention recently for their smart properties.

CO2-responsive aqueous foams stabilized by pseudogemini surfactants

HYPOTHESIS

To obtain surfactants with superior surface activity and responsive behavior, "pseudogemini" surfactants (short for D-LCFA) are synthesized by mixing long chain fatty acids (LCFA) and polyetheramine D 230 at fixed molar ratio (2:1). Non-covalently bonded building blocks indicate that CO₂-responsive aqueous foams can be obtained by utilizing such pseudogemini surfactants.

EXPERIMENTS

¹H NMR and FT-IR characterizations prove that the building blocks of these surfactants are associated by electrostatic interaction. The synthesis (Brønsted acid-base reaction) is simple and eco-friendly. "Pseudogemini" structure enables D-LCFA to reduce surface tension of aqueous solution effectively, thus facilitating foam generation. Rheograms, FF-TEM and Cryo-TEM results prove that different aggregates in D-LCFA aqueous solutions lead to different foam properties.

FINDINGS

Bubbling of CO₂ for about 30 s leads to the rupture of aqueous foams generated by D-LCFA, while removing CO₂ by bubbling of N₂ at 65 °C for 10 min enables re-generation of foams. The CO₂-responsive foaming properties can be attributed to dissociation of D-LCFA upon bubbling of CO₂ and re-association upon removal of CO₂. The effective CO₂-responsive foams can be applied to many areas, such as foam fracturing, foam enhanced oil recovery or recovering of radioactive materials.