Introduction

Shale bubble foam drainage gas production technology, as an effective method for enhancing oil recovery, relies on surfactant-generated foam to reduce fluid density, thereby facilitating shale gas recovery. However, existing technology faces challenges such as the inability to reuse surfactants and defoamers, along with the requirement for substantial quantities of treatment agents1,2. This becomes particularly significant when gas wells yield substantial amounts of water, leading to considerable usage of treatment agents.

To tackle this challenge, the development of a reusable surfactant leveraging the intelligent response properties of surfactants has emerged as a potential solution3. This advancement not only promises to lower the costs associated with shale gas foam drainage gas production but also enhances technical sustainability and environmental protection. Consequently, this study aims to investigate the utilization of intelligent response characteristics of surfactants to design a novel surfactant capable of cost reduction and enhancing efficiency.

Environmentally responsive surfactants are categorized based on their reaction to temperature, light, pH, magnetism, and redox changes4,5,6,7. Among these, redox-responsive surfactants stand out due to their robust controllability, broad applicability, ease of product treatment, and swift reaction times. These properties render them highly suitable for solving the problem of liquid loading, offering potential cost reductions and efficiency improvements. Consequently, redox-responsive surfactants have become the subject of extensive research.

The initial discovery of surfactants possessing redox capabilities was made by Baumgartner and Fuhrhop8, with the primary redox groups identified as ferrocene, selenium-containing, and disulfide bonds9,10,11. Saji et al.12 introduced the first ferrocene surfactants with redox functionalities, capable of reversible micelle formation or decomposition into monomers through redox reactions. Han et al.13 were pioneers in synthesizing selenium-containing surfactants using diphenyl diselenium, creating self-assembled micelle solutions with amphiphilic block copolymers that decompose into smaller monomers under mild oxidation conditions. Ghosh et al.14 revealed that surfactants with disulfide bonds could form redox-responsive micelles, connecting hydrophilic heads to hydrophobic tails through disulfide bonds, allowing for micelle decomposition in the presence of a reducing agent, dithiothreitol. Currently, ferrocene derivatives are the most widely used, particularly in the petroleum industry for oil displacement. However, while existing redox-reactive surfactants mainly focus on oil phase emulsification, their application in foam drainage gas production faces challenges, including maintaining redox performance stability under complex formation conditions, foam stability, and compatibility with formation water.

Leveraging the findings of previous studies, this paper introduces the application of oxidation-reduction surfactants in shale gas foam drainage gas production. Addressing challenges encountered with existing surfactants, such as their limited reusability and suitability in the shale gas foam drainage gas production process, we developed an oxidation-reduction surfactant based on the self-assembly mechanism of intermolecular electrostatic interaction. The redox response characteristics and mechanism of action were examined through surface tension, absorbance, particle size, and Zeta potential analyses. Additionally, the adaptability issues encountered during shale gas foam drainage gas production application were investigated through foaming performance tests.

Experimental condition

Materials

H2O2 (30 wt% in H2O), sodium sulfite (≥ 98%), 1–1’-ferrocene dicarboxylic acid and ethyl acetate were obtained from China National Pharmaceutical Group Chemical Reagent Company. N, N-dimethyldodecylamine was sourced from Shanghai McLean Biochemical Technology Co., Ltd., with purity confirmed to be over 96% through gas chromatography and titration methods.

Preparation of Redox responsive surfactants

At ambient temperature, 1–1’ ferrocene dicarboxylic acid was blended and agitated with N, N-dimethyldodecylamine in a molar ratio of 1:2.5, employing a stirring velocity of 300 revolutions per minute, and maintaining the reaction for 5 h. The addition of hydrogen peroxide, comprising 0.3% of the surfactant’s molar mass, facilitates the preparation of an oxidized surfactant. Separating the reactants using ethyl acetate yields an aqueous solution of the oxidized surfactant, which can then be obtained by drying.

By incorporating an equimolar mass of Na2SO3 directly into the aqueous solution of the oxidized surfactant for reduction treatment, a reducing surfactant can be produced. The preparation procedure is illustrated in Fig. 1.

Fig. 1
figure 1

Preparation mechanism of redox surfactants.

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Recycling of surfactants

The novel redox surfactant features a dual hydrophobic structure, enabling the separation of hydrophobic materials post-reduction treatment through gravity settling or centrifugation. This mitigates the influence of accumulated external additives on solution compatibility during repeated redox processes.

Characterization

Test of FT-IR

The functional group structure of surfactants was analyzed using a Nicolet 6700 Fourier transform infrared spectrometer. Employing the compression technique, 1 mg of dried surfactant sample was uniformly mixed with 200 mg of potassium bromide powder. The mixture was pressed to generate thin sheets with a thickness of less than 0.5 mm for testing purposes.

Test of surface tension

The K100 surface tension meter is utilized in this test. Calibration is performed using deionized water via the hanging plate method before each test. Following each measurement, they are thoroughly rinsed with deionized water, followed by drying through incineration. The test temperature is 25 ℃.

Test of absorbance

A UV-visible spectrophotometer is utilized to examine the transmittance of surfactant solutions. Initially, the concentration of oxidized and reduced surfactants is set to 0.5 mmol/L, with a scanning wavelength range of 350 nm to 650 nm. Subsequently, the variance in absorption peaks between the two states is identified. Following this, the impact of cyclic redox processes on the surfactant absorbance under characteristic peak conditions is assessed. The test temperature is 25 ℃.

Test of particle size distribution (PSD)

The BI-200SM dynamic light scattering particle size analyzer is used in this test. The surfactants are tested at a concentration twice that of the critical micelle concentration (CMC). The test temperature is 25 ℃.

Test of Zeta potential

The surfactant solution is examined utilizing a ZetaPALS-type Zeta potentiometer. The test parameters are scanned for 1.5 min, with three scanning passes. The test temperature is 25 ℃. According to the evaluation criteria for foaming performance (Q/SY 1815–2015), the test concentration is 1.5 g/L (3.08 mM).

Test of foaming performance

Test foaming performance of treatment agent by gravity impact foaming method between solutions based on Ross-Miles foam instrument. During testing, 200 mL of the sample was added to the drip tube and 50 mL of the sample to the collection tube.

During the experiment, open the piston of the drip tube to allow the solution to flow down. When the solution in the drip pipe runs out, immediately start the stopwatch to measure the height of foam. The foam height of the surfactant solution was assessed at the start and after 5 min. Surfactants were dissolved in simulated formation water at a dosage of 1.5 g/L (3.08 mM). The simulated mineralization level of the formation water was set at 50 g/L, comprising 40 g/L (683.76 mM) of NaCl and 10 g/L (90.09 mM) of CaCl2. The test temperature is 90 ℃5,7.

Results and discussions

Analysis of FT-IR

By conducting infrared analysis on surfactants, it is possible to examine the composition of functional group structures in oxidation-reduction surfactants.

Fig. 2
figure 2

Results of FT-IR.

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Figure 2 illustrates the infrared spectrum of the 1,1’-ferrocene dicarboxylic acid, showcasing the peak of C = O at 1669 cm−1, characteristic peaks of C-O at 1296 cm−1 and 1168 cm−1, a distinctive peak of Fe-C at 575 cm−1, and a characteristic peak of Fe-O at 752 cm−1. The resultant surfactant also displays characteristic peaks of C = O and C-O. Furthermore, there are evident peaks of methyl and methylene at 2922 cm−1 and 2852 cm−1, along with CH3-N in the quaternary ammonium group at 1481 cm−1, originating from N, N-dimethyldodecylamine15. The successful electrostatic interaction between 1,1’-ferrocene dicarboxylic acid and N, N-dimethyldodecylamine led to the formation of an oxidation-reduction surfactant.

Analysis of surface tension

The correlation between surface tension and surfactant concentration is examined in both oxidized and reduced states, and the impact of response characteristics on the critical micelle concentration is assessed.

Fig. 3
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Results of surface tension

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As depicted in Fig. 3(a), the increase in concentration of two monomers has little effect on surface tension. While, there is a notable decrease in surface tension with increasing concentration in the oxidized state, with the critical micelle concentration and corresponding surface tension significantly lower compared to those in the reduced state. Following oxidation, the iron atoms acquire charge. Consequently, in the oxidized state, one end of the surfactant adopts a hydrophilic structure, influencing the arrangement and interaction of molecules on the liquid surface. This enhances the propensity of molecules to form micelles on the liquid surface, thereby leading to a reduced CMC in the oxidized state16. Conversely, in the reduced state, the hydrophobic molecular structure of surfactants impedes the arrangement and interaction of molecules on the liquid surface, rendering it less favorable for micelle formation. Consequently, higher concentrations are necessary to form micelle structures, resulting in a relatively elevated critical micelle concentration.

Apart from alterations in molecular structure, discrepancies in intermolecular interactions among surfactants in oxidized and reduced states are also pivotal factors contributing to variations in critical micelle concentrations17. In an oxidized state, modifications in molecular structure can induce shifts in the interaction mode between molecules, thereby influencing micelle formation and diminishing CMC.

Based on Gibbs adsorption model, calculate the maximum adsorption capacity and minimum area of gas-liquid interface through surface tension curve (Eqs. 1 and 2).

$${\Gamma _{\hbox{max} }}= - \frac{1}{{nRT}}{\left( {\frac{{\partial \gamma }}{{\partial \ln c}}} \right)_T}$$
(1)
$${A_{\hbox{min} }}=\frac{1}{{{\Gamma _{\hbox{max} }}{N_A}}}$$
(2)

Where, Γmax is the maximum adsorption capacity, mol/m2. R is the gas constant, J/mol/K. T is the absolute temperature, K. γ is the surface tension, N/m. c is the concentration of surfactant, mol/L. \({\left( {\frac{{\partial \gamma }}{{\partial \ln c}}} \right)_T}\) is the calculated slope. Amin is the minimum area of gas-liquid interface, m2. NA is Avogadro’s constant.

Table 1 Calculation results of surfactant related parameters.
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The surface tension associated with the CMC signifies the compactness of arrangement of surfactant molecules at gas-liquid interface. In oxidized state, the hydrophilic nature of one end of the surfactant’s molecular structure prompts the formation of a closely packed hydrophilic film on the liquid surface, resulting in a reduction in surface tension18. Oxidized surfactants have a larger maximum adsorption capacity, which can significantly improve their adsorption efficiency. It also has a small minimum area of gas-liquid interface, so it requires less energy to form foam, thus significantly reducing the surface tension (Table 1).

To further examine the recycling efficacy of the colloidal solution through repeated redox cycles, the impact of oxidation and reduction conditions on its surface tension was assessed at twice the critical micelle concentration. As depicted in Fig. 3 (b), following 6 successive redox reactions, the variation in surface tension did not surpass 10%. Introducing reducing agents leads to an escalation in surface tension. The addition of oxidizing agents converts neutral ferrocene groups into positively charged ferrocene cations; however, upon reuse, the conversion efficiency of ferrocene groups is compromised, with some failing to convert into hydrophilic groups, thereby impeding the effectiveness of surfactants13.

Analysis of absorbance

Redox surfactants exhibit variations in molecular structure and electronic configuration under distinct oxidation states, leading to diverse absorption characteristic peaks or intensities in the UV-visible absorption spectrum. Through comparative analysis of the UV-visible absorption spectra of redox surfactants under varying conditions, insights into their redox properties, reaction mechanisms, and transition patterns between redox states can be interpreted.

Fig. 4
figure 4

Results of absorbance

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In the reduced state depicted in Fig. 4 (a), the central iron atom of ferrocene adopts a + 2 oxidation state, with the π electrons in the ferrocene group forming coordination bonds with the iron atom, leading to the formation of a stable complex. Consequently, stemming from π electron transition within cyclopentadienyl group, the ferrocene molecule displays a characteristic peak at 447 nm. Conversely, in the oxidized state, neutral ferrocene groups undergo oxidation to form positively charged ferrocene cations. This process results in the disruption of coordination bonds between the π electrons in the ferrocene group and the iron cation, leading to alterations in the molecular structure of ferrocene. Consequently, the characteristic peak diminishes as the π electron transition within the cyclopentadienyl group ceases due to the breakdown of coordination bonds.

When it was converted back to the reduced state, pertinent characteristic peaks resurfaced, denoting its favorable reversible conversion attributes. As depicted in Fig. 4 (b), the characteristic absorption peak of the surfactant continues to display commendable reversible traits even after undergoing secondary alternating reduction and oxidation treatments. However, owing to the disruption of coordination bonds in the oxidized state, the re-establishment of these bonds becomes more challenging. Consequently, upon reverting to the reduced state, the formation of coordination bonds may not attain the original strength, resulting in a reduction in the intensity of the characteristic absorption band19.

Analysis of PSD

Employing a nanoparticle size analyzer facilitates the assessment of particle size distribution, enabling the identification of variations in particle size during the redox process. The effect of retarding liquid membrane separation to maintain the stability of foam can be analyzed.

Fig. 5
figure 5

Results of PSD

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As illustrated in Fig. 5(a), the average particle size of the surfactant in its oxidized state measures approximately 310 nm. Conversely, in the reduced state, the particle size of the surfactant decreases, with an average size of around 124 nm. Large fluid dynamic diameters may have vesicular structures20,21. In the oxidized state, the molecular arrangement of surfactants leads to one end exhibiting hydrophilic properties while the other end displays hydrophobic characteristics. Through hydrophobic aggregation, a core is formed, with the hydrophilic portion facing outward to create a surface. This arrangement allows surfactants to form micelle structures in the oxidized state, contributing to larger overall molecular dimensions22. Conversely, in the reduced state, both ends of the surfactant molecules adopt hydrophobic structures, resulting in the loss of micelle-forming capabilities. Consequently, surfactants may exist as single molecules or in dispersed forms in the reduced state due to the inability to form micelle structures.

As shown in Fig. 5 (b), the PSD increases when it transitions to the reduced state. Upon reverting back to the oxidized state, PSD of the surfactant slightly decreases. Although the majority of molecules exist in a dispersed single-molecule state, a minority can still aggregate to form micelles23,24. Thus, when it converts to the reduced state, certain molecules fail to form micelles, resulting in a reduction in PSD.

Analysis of Zeta potential

Alterations in the charge state of surfactants during the redox process can influence their interaction forces with particles or micelles, consequently impacting the stability of surfactant. Measuring Zeta potential allows the impact of oxidation-reduction surfactants on the stability of dispersed systems to be assessed.

Fig. 6
figure 6

Results of Zeta potential.

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As depicted in Fig. 6, after 6 cycles, the Zeta potential value of the surfactant in the reduced state measures 19.6 mV, whereas in the oxidized state, it rises to 27.1 mV, which is more stable than the same type of surfactant (reduced state 15 mV, oxidized state 23 mV)25. This observation suggests that the surfactant exhibits good stability when oxidized. In an oxidized state, one end of the surfactant molecule bears a positive charge while the other end is hydrophobic. This distribution of charge renders the molecule more inclined to forming a stable micelle structure within the solution26. Conversely, in the reduced state, both ends of the molecule adopt hydrophobic structures, resulting in a more uniform charge distribution which may diminish the stability of the micelle.

In the oxidized state, surface active agent molecules exhibit a hydrophilic structure at one end, facilitating the formation of hydrogen bonds and electrostatic interactions with water molecules in the solvent. This enhances the dispersion stability of molecules in the solution27. Conversely, in the reduced state, both ends of the molecule adopt a hydrophobic structure, which hinders interaction with water molecules, resulting in comparatively lower stability in the solution.

Analysis of foaming performance

The redox properties of redox surfactants significantly impact their stability within foam systems. Conducting foaming performance tests allows for the evaluation of surfactant stability during foam formation, providing crucial parameters and performance assessment criteria for the design, optimization, and implementation of intelligent foam drainage and gas production systems.

Fig. 7
figure 7

Results of foaming performance.

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As depicted in Fig. 7 (a) and (b), the initial foaming height of the oxidized surfactant aqueous solution upon first use is 131 mm, and after 5 min, it reaches 94 mm. According to the enterprise standard Q/SY 1815–2015, which specifies initial and 5-minute foam heights greater than 120 mm and 80 mm, respectively, these values meet the requirements for liquid loading. Conversely, the initial foaming height of the reduced surfactant aqueous solution is 12 mm, with no foaming observed after 5 min. Even after 6 cycles, it maintained consistent performance in terms of repeated foaming and defoaming in the reduced state. Specifically, the reduction in bubble height in the oxidized state was less than 10%, while the foam height remained at 0 for 5 min in the reduced state. Figure 7 (c) shows that the foaming performance was significantly improved under the condition of no salt solution. Figure 7 (d) indicates that at a concentration of 1.5 g/L (3.08 mM), the foaming performance reaches stability, and excessive concentration can affect the defoaming performance.

The action mechanism of oxidation-reduction surfactant during foam drainage gas production is illustrated in Fig. 8.

Fig. 8
figure 8

Mechanism of action of redox surfactants.

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Surfactants in their oxidized state exhibit enhanced capability in facilitating bubble formation and sustaining their stability through several mechanisms. By adjusting surface tension, forming large micelles, and minimizing liquid film drainage, oxidized surfactants demonstrate superior foaming performance. Typically, surfactants possess hydrophilic and hydrophobic ends in their oxidized state. The hydrophilic end interacts with water molecules, facilitating micelle formation, while the hydrophobic end interacts with gas, promoting stable gas-liquid interfaces and bubble formation. Surface tension test results indicate that oxidized surfactants effectively reduce liquid surface tension, facilitating bubble formation and stabilization28,29. This occurs because surfactant molecules create an adsorption layer on the liquid surface in the oxidized state, weakening intermolecular forces between liquid molecules and promoting bubble expansion and stability. Moreover, particle size testing reveals that oxidized surfactants tend to form larger micelle structures, which envelop gas and maintain bubble stability30,31. These large micelles offer ample space and structural support, allowing bubbles to persist without rupturing. Additionally, large micelle particles reduce liquid film drainage, resulting in smoother and more stable bubble surfaces, thereby prolonging bubble lifespan.

Foam drainage gas production typically necessitates the application of defoamers at the wellhead to facilitate gas-liquid separation. Defoamers work by lowering surface tension, disrupting the stability of the film formed on bubble surfaces, and facilitating bubble rupture and dissipation. When oxidation-reduction surfactants transition to their reduced state, factors such as increased surface tension and decreased particle size adversely affect foaming performance, leading to foam elimination. Conventional surfactant solutions containing organosilicon defoamers struggle to recover and reuse foaming agent components. However, oxidation-reduction surfactants can be restored to their oxidized state through reoxidation, thereby regaining their favorable foaming characteristics. This conversion process can be rapidly achieved through chemical means.

Conclusion

  1. (1)

    An electrostatic binding approach was utilized to develop a surfactant with redox-responsive properties, employing the interaction between amino and carboxyl groups. Specifically, 1,1’-ferrocene dicarboxylic acid and N, N-dimethyldodecylamine were utilized in the synthesis process. This surfactant demonstrates swift responsiveness to oxidation and reduction states facilitated by H2O2 and Na2SO3.

  2. (2)

    The redox response properties of the novel surfactants can be observed through changes in surface tension, absorbance, particle size, and Zeta potential. In the oxidized state, the molecular structure features both hydrophilic and hydrophobic ends, resulting in a surface tension of 26.4 mNm−1 and a critical micelle concentration of 1mmol/L. Under these conditions, the molecules autonomously assemble into micelles with a size of 310 nm, maintaining dispersion stability with a Zeta potential of 28.7 mV. Conversely, in the reduced state, the UV visible absorption spectrum displays a characteristic peak of ferrocene molecules at 447 nm.

  3. (3)

    The new surfactant offers notable advantages, displaying robust foaming capabilities in its oxidized state and effective defoaming properties in the reduced state. Moreover, it facilitates easy recovery, separation, and reuse. In its oxidized state, the surfactant exhibits excellent dispersion stability, leveraging its ability to reduce surface tension and form large micelles, thereby stabilizing foam. Foam height can reach 131 mm within 5 min. Upon conversion to the reduced state, surface tension increases while particle size decreases, facilitating rapid defoaming. Its hydrophobic properties enable recovery, after which it can be oxidized to restore foaming performance. Even after 6 cycles of reuse, the foam height remains above 120 mm for 5 min.