Development and Characterization of CO₂-Responsive Surfactants for Coalbed Methane Fracturing

Abstract

To address issues of traditional coalbed methane (CBM) fracturing fluids (high displacement, weak sand-carrying, poor stability, severe coal seam damage), this study synthesized CO₂-responsive erucamide propyl dimethylamine surfactant (C₂₂ZEA, yield 99%), with molecular structure verified by ¹H NMR (400 MHz, CDCl₃) matching the target. Molecular simulation showed CO₂ protonates C₂₂ZEA into EA⁺: 1 wt% forms a simple micelle network, while 3 wt% enhances entanglement into a dense 3D network. Experiments indicated: 3 wt% solution reaches 160 mPa·s viscosity in 200 s under CO₂ (0.2 L·min⁻¹); 1.5–4.5 wt% solutions are pseudoplastic (n = 0.14–0.18), with G′ > G″ when concentration > 2 wt%; viscosity recovery rate > 95% after alternating shear (170 s⁻¹/10 s⁻¹); viscosity remains > 160 mPa·s after 1 h shear (170 s⁻¹) at 70 °C; gel breaks to 0.01–0.02 Pa·s in 15 min with N₂ at 45 °C; 1.0–3.0 wt% solutions meet non-toxic standards via EC₅₀/96 h LC₅₀. This study supports high-efficiency low-damage smart fracturing fluids, boosting CBM extraction efficiency.

1. Introduction

Hydraulic fracturing is an indispensable core technology for the development of unconventional hydrocarbon reservoirs. It involves injecting high-pressure fracturing fluid into formations to create and propagate artificial fractures, which interconnect with natural fractures to form complex fracture networks, significantly enhancing reservoir permeability (e.g., addressing the issue of low gas drainage efficiency in coal seams). Concurrently, proppants are transported and placed within fractures to establish and maintain conductive pathways between reservoirs and wellbores, making it crucial for well stimulation. After perforation, hydraulic fracturing propagates and extends fractures within coal seams, connecting induced fractures with pre-existing natural fractures to form an interconnected network, thereby improving coal seam permeability and coalbed methane (CBM) drainage efficiency. Among these, fracturing fluid is a key factor influencing coal seam hydraulic fracturing. Commonly used fracturing fluids, including active hydraulic fracturing fluids [1,2], slippery hydraulic fracturing fluids [3,4], foam fracturing fluids [5,6], and guar gum fracturing fluids [7,8], exhibit characteristics such as high construction displacement, weak sand-carrying capacity, poor stability, and significant damage to coal seams. Thus, there is an urgent need to develop a high-performance fracturing fluid system suitable for coal seam geological conditions and construction processes.

Intelligent responsive materials are an emerging class of materials characterized by reversible changes in their physicochemical properties upon exposure to external stimuli. Due to the excellent intelligent responsiveness of environmentally responsive surfactants [9], their micellar structures and thickening effects on liquids can alter with environmental changes. Therefore, environmentally responsive surfactants can be used as intelligent fracturing fluids for hydraulic fracturing, enabling intelligent control of gel formation and gel breaking by adjusting conditions such as pH, temperature, and CO₂ pressure [10,11]. Although research on surfactants with different responsive properties is relatively extensive, studies on their response mechanisms remain insufficiently in-depth and systematic. Research on CO₂-responsive thickening surfactant molecules is relatively scarce both domestically and internationally, and their application in clean fracturing fluid systems has rarely been documented. Viscosity regulation in CO₂ systems is simple and easy to operate, requiring no additional chemicals, thus holding significant promise for achieving controllable gelation and gel breaking in clean fracturing fluids. CO₂-intelligent responsive surfactants [11,12] exhibit excellent thickening effects and minimal damage to coal seams when used as fracturing fluids; they can increase solution viscosity by reacting with CO₂ to form micelles, making them applicable to gas reservoir development and attracting widespread attention from scholars in this field. Currently, extensive research has been conducted on CO₂-intelligent surfactant systems, where the micelles formed exhibit reversible environmental responsiveness. Thickening and gel breaking of liquids by surfactants can be achieved by adjusting temperature and other measures. For example, Yang et al. [13] synthesized a novel surfactant, trimethylbenzylammonium betaate (ErBTA), which forms micelles at extremely low concentrations (0.028 mmol/L) and spontaneously forms viscoelastic worm-like micellar solutions at higher concentrations (4.07 mmol/L). This system has excellent thickening ability and can respond to pH, CO₂, and light stimulation to achieve reversible gel-sol transformation. Zhang et al. [14] found that when CO₂ is introduced into a mixed system of sodium dodecyl sulfate (SDS) and N, N, N, N-tetramethyl-1,3-propanediamine (TMPDA) at a 2:1 molar ratio, TMPDA protonates and forms a “pseudo-Gemini surfactant” with SDS through electrostatic interactions, constructing a highly viscoelastic network structure. Upon removal of CO₂, the system reverts to low-viscosity spherical micelles. This reversible process can be cycled multiple times, enabling intelligent regulation of micellar morphology in response to CO₂ stimulation. Zhang et al. [15] prepared a CO₂-responsive viscoelastic fluid by mixing sodium stearate (NaOSA) with bola-type quaternary ammonium salt (Bola2be) at a 2:1 ratio; Bola2be enhances the solubility of NaOSA and promotes the formation of pseudo-dimer structures, allowing micelles to rapidly entangle into a three-dimensional network (with a critical concentration of only 0.057 mM). This fluid exhibits high thermal sensitivity (activation energy of 399.76 kJ/mol) and can achieve reversible conversion between worm-like micelles and suspensions via CO₂ regulation.

However, current CO₂-responsive surfactant systems still have limitations such as slow response speed (e.g., the SDS-TMPDA system requires more than 30 min to complete thickening [14]), low shear recovery rate (<85%), difficult regulation of gel-breaking timing, and insufficient temperature resistance, which limit their engineering applications. To address the above issues in a targeted manner, this study selects erucamide propyl dimethylamine to construct a CO₂-responsive system, with the core reason lying in the natural compatibility between its structure and performance: On one hand, the long-chain hydrophobic structure (C₂₂ unsaturated fatty acid chain) of the erucamide group can promote the formation of micellar networks through strong intermolecular entanglement [16]; meanwhile, the amide bond possesses both polarity and stability, which not only enhances compatibility with water molecules but also improves the temperature resistance of the system. On the other hand, the tertiary amine group at the end of the molecule provides clear active sites for CO₂ response, and can achieve rapid structural transformation through protonation/deprotonation, avoiding the problem of response lag in multi-component systems [17]. Compared with existing CO₂-responsive systems, the expected advantages of this erucamide-based surfactant are reflected in three aspects: First, it has better response efficiency. The rapid entanglement characteristic of the long-chain hydrophobic groups can shorten the micellar network formation time, meeting the requirements of on-site efficient construction. Second, it exhibits stronger structural stability. Compared with the C-N bonds in traditional alkylamine surfactants, the amide bond is more resistant to shear and high temperature, which can improve the performance durability of fracturing fluids in deep well environments. Third, it has superior environmental compatibility. The erucamide group is derived from natural fatty acid derivatives, and its biodegradability and low toxicity are better than those of synthetic alkyl surfactants, which can reduce damage to coal seams and environmental risks [16]. Combined with industry standards and reservoir geological characteristics, the CO₂-responsive surfactant molecule C₂₂ZEA, which possesses excellent CO₂ responsiveness, thickening performance, and water solubility, was screened out and identified as the main agent of CO₂-responsive clean fracturing fluid. The performance characterization results of the 3 wt% C₂₂ZEA fracturing fluid system showed significant optimization in its CO₂ responsiveness, rheological performance, shear resistance, temperature resistance, and gel-breaking performance. Furthermore, dynamic viscoelasticity studies confirmed that the system can achieve repeated cyclic utilization under the condition of alternating introduction of CO₂ and CH₄/N₂.

2. Synthesis and Characterization of CO₂-Responsive Surfactants

2.1. Synthesis of Surfactants

The chemical reagents required for this experiment are shown in

Table 1. Chemical reagents required for the experiment.

Experimental Instruments	Purity	Molecular Weight	Manufacturer
Erucic	90%	338.57 g/mol	Bailingwei Technology Co., Ltd., Shenzhen, China
N.N-Dimethylaminopropylamine	99%	102.18 g/mol	Bailingwei Technology Co., Ltd.
Catalyst (sodium fluoride)	AR	41.99 g/mol	China National Pharmaceutical Group Chemical Reagent Co., Ltd. Shanghai, China
Dehydrating agent (calcium chloride)	analytical pure	110.98 g/mol	Xilong Chemical Co., Ltd. Shanghai, China
acetone	analytical pure	58.08 g/mol	Yantai Shuangshuang Chemical Co., Ltd. Yantai, China

.

The synthesis method of erucic acid amide propyl dimethylamine with CO₂ response mainly involves esterification and condensation reaction between erucic acid and N, N-dimethylaminopropylamine. The synthesis route is shown in the following Figure 1.

Add erucic acid and N, N-dimethylaminopropylamine in a molar ratio of 1:1.1 to a three-necked flask equipped with a condenser and a distillation head, and add sodium fluoride as a catalyst. Place calcium chloride as a dehydrating agent in the fractionation head to promote the separation of by-product water and drive the reaction forward. Under nitrogen protection, the condensation reaction is carried out at 130–150 °C for 8 h. After the reaction is complete, unreacted dimethylaminopropylamine is removed by rotary evaporation, and the product is washed multiple times with acetone water mixed solution until colorless. Finally, wash repeatedly with a mixture of acetone and water until colorless, filter, and dry to obtain C₂₂ZEA as a white solid powder. (Yield: 99%).

2.2. Structural Characterization of Surfactants

Nuclear magnetic resonance hydrogen spectroscopy (¹H NMR) [18,19] characterizes the chemical environment differences among hydrogen nuclei. The number of resonance signals corresponds to unique hydrogen atom environments, while relative integration areas reflect proportional hydrogen counts per environment. Analysis of chemical shifts (δ), splitting patterns, coupling constants (J), and integration values enables determination of hydrogen distribution and molecular structural features.

According to the ¹H NMR (400 MHz, CDCl₃) analysis in Figure 2, the hydrogen spectrum data of the target compound C₂₂ZEA is as follows: δ 5.34 (s, 1H), 3.31 (s, 1H), 2.37 (t, J = 6.5 Hz, 1H), 2.23 (s, 2H), 2.18–2.08 (m, 1H), 2.04 (s, 2H), 1.66 (s, 2H), 1.27 (s, 10H), 0.89 (t, J = 6.7 Hz, 1H). The chemical shifts, integral ratios, and coupling modes of each peak are consistent with the target molecular structure, indicating the successful synthesis of the target product. Moreover, the FT-IR analysis in Figure S1 further confirms the successful synthesis of C₂₂ZEA.

3. Microscopic Mechanism Research

This study analyzed the protonation process of erucamide propyl dimethylamine (Figure 3) [20] under the action of CO₂ through molecular dynamics simulation, explored its interaction mechanism with water molecules, revealed the microscopic principle of solution thickening, and studied the effect of shear force on viscosity, providing theoretical support for the development of new CO₂-responsive surfactants.

3.1. Model and Parameters

Carbon dioxide (CO₂) reacts with water to form carbonic acid, which subsequently protonates erucamidopropyl dimethylamine surfactant, generating a cationic surfactant (EA⁺) and bicarbonate salt. The densely packed micelles formed by these components exhibit macroscopically increased viscosity, rendering this system suitable as a CO₂-responsive surfactant.

The simulation investigated erucamidopropyl dimethylamine molecules, protonated cationic EA⁺, water molecules, CO₂ molecules, and bicarbonate ions. Molecular structures are illustrated in Figure 4.

Lammps 2024 software was employed for molecular dynamics simulations. The initial structural optimization was conducted using the Compass II force field, followed by dynamic calculations that analyzed intermolecular interactions. The final configurations were obtained through the application of the steepest descent energy minimization method to structural optimization under periodic boundary conditions. All initial configurations featured crystalline-periodic molecular distributions, with annealing procedures driving the systems to equilibrium conformations. Annealing and energy equilibration were performed using the NVT ensemble, followed by molecular dynamics simulations under the NPT ensemble. Simulations employed Berendsen’s thermostat and barostat, with a long-range coulombic interactions were handled using the Particle Mesh Ewald (PME) method, with a grid precision of 0.12 nm and a Fourier space cutoff radius of 8 Å. Short-range van der Waals interactions employed a 9.5 Å cutoff radius, and tail correction was performed via the Lorentz-Berthelot rule to ensure the accuracy of total energy calculations [21]. Systems contained surfactant molecules at varying concentrations (1, 1.5, 3 wt%), 2500 water molecules, and stoichiometrically corresponding bicarbonate ions and protons. All particles were initially randomized within a 44 × 44 × 44 nm³ cubic simulation box. NVT ensemble simulations were performed with 500,000 time steps (1 fs step size) until achieving thermodynamically stable energy minimization (Energy fluctuation < 3%) (Figure 5).

3.2. Response to Microscopic Effects

Through molecular dynamics simulations employing the Green-Kubo method [22,23], we intuitively observed the protonation process and self-assembly behavior of 1 wt% erucamide propyl dimethylamine in CO₂ aqueous solution (Figure 6): at the initial stage (0 steps), surfactant molecules are randomly dispersed; with time (10,000 steps), protonated cationic surfactants begin to bend and entangle due to charge interactions; at 50,000 steps, a preliminary aggregation structure forms; and finally, they successfully self-assemble into micelles at 500,000 steps. This dynamic process clearly reveals the microscopic mechanism by which CO₂-responsive surfactants induce molecular chain entanglement and cross-linking through protonation, thereby achieving solution thickening.

Figure 7 shows the micelle aggregation morphology of erucamide propyl dimethylamine at different concentrations (1 wt%, 1.5 wt%, 3 wt%): at 1 wt%, surfactant molecules start to entangle into a simple network structure; as concentration increases to 3 wt%, more frequent molecular collisions and entanglements lead to a more complex and dense network. This concentration-dependent aggregation behavior directly explains why the thickening effect enhances with concentration—higher concentrations strengthen system viscoelasticity by increasing intermolecular interactions and entanglement density [24,25]. Consistent with this, rheological studies conducted on EA⁺ solutions of varying concentrations using a rotational rheometer in accordance with the SY/T 5107-2016 test method (Figure 8) demonstrated that viscosity increases with rising surfactant concentration [26,27]. This phenomenon can be explained by micelle theory: solution viscosity exhibits a positive correlation with the square of the micelle asymmetric volume fraction. As EA⁺ concentration rises from 1 wt% to 3 wt%, the number of micelles increases, raising both micelle volume fraction and structural asymmetry, thus significantly improving overall system viscosity and further confirming that CO₂-responsive surfactants thicken via micelle network structures.

Shear resistance is a key quality indicator for fracturing fluids, as high-pressure underground shear can break thickener micelles or polymer chains, reducing viscosity. Figure 6 illustrates the structural evolution of 1 wt% EA⁺ solution under different shear rates: at low shear rates (Figure 9a), surfactant molecules show disordered entanglement; when the shear rate increases to the fracturing construction standard of 170 s⁻¹ (Figure 9c), the micelle structure orients along the Y-axis, and the network stretches into a slender shape [28]. Figure 10 further shows structural changes in EA⁺ solutions with different concentrations at 170 s⁻¹: as concentration increases from 1 wt% to 3 wt%, the dispersion degree of micelles after shearing significantly rises. This is because surfactants are unevenly distributed at low concentrations, leaving some micelles unstressed, while high-concentration systems enable more uniform dispersion and complete disentanglement under shear, leading to more pronounced structural damage—these concentration-dependent shear response characteristics provide a basis for optimizing fracturing fluid formulations.

Figure 11 reveals the structural evolution of the 3 wt% EA⁺ solution with increasing temperature: as the temperature rises from 25 °C to 45 °C, the aggregation degree of micelles significantly decreases under zero-shear conditions. This phenomenon originates from the thermo-responsive nature of the surfactant micellar network. At the lower temperature of 25 °C, intermolecular forces maintain a stable micellar network structure. With increasing temperature gradients, progressively intensified molecular thermal motion causes the micelles to gradually disentangle, disperse, and ultimately dissociate. This process clearly demonstrates the progressive disruptive effect of thermal perturbation on the micellar network structure. The discovery of this temperature-dependent structural dissociation response provides key molecular-level mechanism insights for smartly regulating the temperature-sensitive gel-breaking behavior of fracturing fluids.

Figure 12 reveals the structural evolution of the 3 wt% EA⁺ solution under CO₂ pressure gradients (1.01 → 5.41 MPa): as pressure increases, the aggregation degree of micelles significantly enhances under zero-shear conditions. This phenomenon originates from the pressure-driven molecular spatial compression effect. At the low pressure of 1.01 MPa, the micellar network exists in a state of partial dissociation co-existing with entanglement. When pressure increases to 5.41 MPa, intermolecular forces progressively strengthen due to spatial confinement, promoting gradual micellar aggregation and the formation of a densely entangled network. This pressure-responsive structural evolution mechanism provides a molecular-scale design basis for the CO₂-triggered gel-breaking behavior of smart fracturing fluids.

4. Performance Evaluation of CO₂-Responsive Fracturing Fluid

Performance evaluation of CO₂ responsive fracturing fluid systems constitutes a critical determinant of hydraulic fracturing efficacy. This study conducted standardized laboratory assessments in strict compliance with industry protocols (SY/T 5107, SY/T 6376) to systematically characterize smart-responsive fracturing fluids [27,29]. Comprehensive benchmarking encompassed five key performance indicators: CO₂-responsiveness, viscoelastic rheological behavior, shear resistance, temperature resistance, gel breaking property and biotoxicity assessment.

4.1. CO₂-Responsiveness

To investigate the correlation between CO₂ response time and system viscosity, the viscosity of C₂₂ZEA solutions with different concentrations was studied under normal temperature and pressure as a function of CO₂ introduction time [30]. The experimental design involved preparing C₂₂ZEA aqueous solutions with three concentrations (1.0 wt%, 1.5 wt%, and 3 wt%), continuously introducing CO₂ gas at a constant flow rate of 0.2 L·min⁻¹ under constant temperature conditions, measuring the apparent viscosity of the solution at regular intervals, and establishing the corresponding relationship between CO₂ action time and system viscosity changes to evaluate the CO₂ response characteristics of the intelligent fluid. As shown in Figure 13, the introduction of CO₂ gas resulted in an increase in viscosity across all tested concentrations of C₂₂ZEA aqueous solutions, with the 3.0 wt% C₂₂ZEA solution exhibiting the most pronounced increase. Furthermore, analysis of the viscosity change curves indicates that the C₂₂ZEA concentration exhibits a negative correlation with the system’s response time to CO₂—higher concentrations lead to a shorter time required to achieve viscosity equilibrium.

This phenomenon is attributed to the accelerated protonation process at higher concentrations, which is consistent with the microscopic mechanism revealed by molecular dynamics simulations: denser molecular distribution promotes more frequent collisions between C₂₂ZEA molecules and CO₂-derived protons, as observed in the simulation where surfactant molecules at higher concentrations (analogous to the 3 wt% system) showed more rapid protonation and initial aggregation (Figure 6 and Figure 7) [11]. The simulation further demonstrated that protonated cationic surfactants (EA⁺) begin to bend and entangle due to charge interactions, and this entanglement intensifies with increasing concentration—mirroring the experimental observation that 3.0 wt% solutions form a continuous micelle network faster. As seen in the simulation, the transition from preliminary aggregation (50,000 steps) to complete micelle formation (500,000 steps) is significantly shortened in denser systems, which directly explains why higher concentrations in experiments reach viscosity equilibrium more quickly. Meanwhile, dynamic light scattering (DLS) characterization results (Figure S2) show that the C₂₂ZEA solution exhibits the largest particle size at a concentration of 3.0 wt%. This indicates that the viscosity change of C₂₂ZEA solutions after CO₂ bubbling can, to a certain extent, reflect the size variation trend of aggregates in the system through changes in micelle particle size, and this trend is directly related to the degree of micellar entanglement. Thus, the macroscopic viscosity behavior in CO₂-responsiveness tests is underpinned by the microscopic dynamic process of protonation and micelle network assembly captured in simulations, confirming that the rapid formation of EA⁺ and enhanced electrostatic/hydrophobic interactions drive the quick thickening of the system.

4.2. Viscoelastic Rheological Behavior

Rheological characterization serves as the central metric for evaluating the engineering applicability of fracturing fluids, directly governing their high-temperature stability, proppant transport efficiency, and flow behavior under reservoir conditions [31]. This study employed standardized steady-state shear measurements to systematically quantify apparent viscosity evolution as functions of shear rate, temperature (20–90 °C), and duration (0–120 min), while dynamic viscoelastic properties were resolved via frequency-sweep oscillatory rheometry (0.1–100 rad/s) to map moduli frequency-dependence, enabling holistic assessment of the fracturing fluid’s viscoelastic signature across operational regimes. Based on the shear stress (τ) and shear rate (γ) data obtained from rheological testing, the τ-γ relationship curve was plotted in a double logarithmic coordinate system, and two key rheological parameters were derived through linear fitting: the flow index (n, reflecting fluid type) and viscosity coefficient (K, characterizing basic viscosity) using the power-law model (τ = Kγⁿ).

As shown in Figure 14, rheological analysis demonstrates that at C₂₂ZEA concentrations exceeding 2 wt%, the system exhibits persistent elastic dominance (G′ > G″) across the entire tested frequency spectrum, confirming the development of robust viscoelastic solid characteristics. It is worth noting that increasing the surfactant concentration from 2% to 3% significantly enhances the elastic modulus while causing only a slight change in the viscous modulus, fully demonstrating that higher C₂₂ZEA concentrations effectively strengthen the system’s viscoelastic properties. Consistent with this, Figure 15 shows that the C₂₂ZEA fracturing fluid system exhibits typical non-Newtonian pseudoplastic behavior: the flow index n remains between 0.14 and 0.18 within the concentration range of 1.5–4.5 wt%, confirming obvious shear thinning, while the viscosity coefficient K increases significantly from 1.12 mPa·s to 3.98 mPa·s with increasing concentration. This concentration-dependent rheological behavior originates from the self-assembly of protonated surfactant molecules into worm-like micelles under CO₂ stimulation—higher concentrations intensify micelle entanglement, ultimately constructing a dense three-dimensional network. The molecular mechanism involves electrostatic shielding between protonated head groups and non-covalent interactions, where dynamically reversible intermolecular forces promote the formation of a micelle network with excellent viscoelasticity, while the reversible dissociation of the network under high shear explains the stable shear-thinning behavior.

4.3. Shear Resistance

Shear resistance represents an equally vital performance indicator as proppant-carrying capacity for fracturing fluids, as fracturing fluids inevitably undergo mechanical shearing at varying rates in wellbore tubing, annular spaces, and perforations during operations—potentially causing structural disintegration of aggregates or molecular chain scission, leading to rapid viscosity decline. Consequently, this study used the dynamic alternating shear method to accurately evaluate shear resistance, which more realistically simulates complex downhole shear conditions compared to traditional fixed shear rate testing. The specific experimental plan involved continuously applying alternating shear of 170 s⁻¹ (simulating high shear conditions) and 10 s⁻¹ (simulating low shear conditions) to the fracturing fluid sample under uninterrupted recovery conditions, enabling effective evaluation of structural stability and shear resistance under actual construction conditions.

As shown in Figure 16, the alternating shear experiment results indicate that the C₂₂ZEA fracturing fluid system exhibits excellent shear resistance and structural stability. Under high shear of 170 s⁻¹, the viscosity of 0.5–3 wt% systems remained stable, increasing significantly with concentration (9.8–48.8 mPa·s), confirming concentration enhances shear resistance. Switching to 10 s⁻¹, viscosity rebounded, and cyclic shear showed full recovery—matching initial high/low shear levels.

This performance aligns with simulation results: Figure 9 shows 1 wt% EA⁺ micelles orient along Y-axis and stretch at 170 s⁻¹ (consistent with temporary high-shear viscosity behavior), while higher concentration (3 wt%) systems in Figure 10 exhibit more uniform micelle dispersion post-shearing, explaining their higher viscosity retention. The simulation-observed disordered entanglement recovery at low shear directly supports the experimental viscosity rebound, validating the self-healing mechanism—shear-induced disentanglement is reversed by hydrophobic/electrostatic interactions, restoring the micelle network.

4.4. Temperature Resistance

The temperature resistance of fracturing fluid is a critical performance parameter, as downhole high temperature and high-pressure shear can degrade viscoelastic surfactant micelles or polymer chains, leading to significant viscosity reduction. Consequently, rigorous evaluation of thermal and shear stability is essential, and this study characterized the temperature-dependent rheological behavior of C₂₂ZEA solutions through systematic rheometric testing (Figure 17), while also measuring viscosity over time using a rheometer with a shear rate of 170 s⁻¹ (standard for hydraulic fracturing construction) to determine temperature and shear resistance.

As shown in Figure 17, steady-state shear testing reveals significant shear rate dependence: the low shear rate region exhibits Newtonian behavior with constant viscosity, while the high shear rate region shows significant shear thinning, attributed to the reversible fracture and recombination of worm-like micelles under shear. Temperature effect studies show that increasing temperature leads to a systematic decrease in zero shear viscosity (η₀), confirming the dissociation effect of thermal perturbation on the micelle network. Dynamic oscillation tests further reveal frequency dependence of viscoelastic response: the low-frequency region (G″ > G′) is dominated by viscous behavior, transitioning to elastic dominance (G′ > G″) at higher frequencies, with temperature increase significantly shortening the system’s characteristic relaxation time—indicating accelerated dynamic response of the micelle network due to thermal activation. Notably, the system maintains a stable elastic modulus platform under high-frequency disturbances, ensuring performance stability in fracturing applications. As shown in Figure 18, initial heating causes a rapid viscosity drop, but after reaching the set temperature, viscosity stabilizes; despite slow decay under continuous shear, it retains an apparent viscosity exceeding 160 mPa·s after 1 h—far exceeding the 30 mPa·s minimum standard for clean fracturing fluid, demonstrating excellent temperature and shear resistance. These behaviors align with molecular dynamics simulations of 3 wt% EA⁺ solutions (Figure 11), where micelle aggregation decreases significantly with temperature (25 °C to 45 °C), directly explaining the experimental η₀ reduction and reflecting thermally induced network dissociation. Simulations also show intensified molecular motion weakens intermolecular forces, causing micelle disentanglement—consistent with the shortened relaxation time in oscillation tests—while residual entanglement at elevated temperatures supports the stable elastic modulus and sustained high viscosity observed experimentally.

4.5. Effects of CO₂ Pressurization

This CO₂-responsive system arises from the combined action of water, surfactant, and CO₂ gas, so this study considered altering CO₂ pressure to increase solubility and enhance interaction probability with water and surfactant. The viscosity variation under different pressures was characterized using a high-pressure/high-temperature (HPHT) rheometer: to ensure uniform gas–liquid mixing, the system was first sheared at 170 s⁻¹ for 300 s, followed by viscosity measurement at 10 s⁻¹ for 300 s, then again at 170 s⁻¹ for 300 s, and finally at 10 s⁻¹ to characterize viscosity stability and shear resistance (Figure 19).

Simultaneously, the system’s shear viscosity variation with pressure was characterized under CO₂ pressures of 10.1 bar, 32.5 bar, and 54.1 bar: under high shear rates, viscosity showed a slight increase (remaining around 30 mPa·s), while under low shear rates, it gradually rose with pressure—at 170 s⁻¹, viscosity increased from 234 mPa·s at 10.1 bar to 297 mPa·s at 54.1 bar, confirming the system’s pressure responsiveness. This behavior aligns with molecular dynamics simulations of 3 wt% EA⁺ solutions (Figure 12), where micelle aggregation degree significantly enhanced with increasing CO₂ pressure (1.01 → 5.41 MPa). Simulations revealed that pressure-driven molecular spatial compression reduces intermolecular spacing, strengthens intermolecular forces, and promotes micelle aggregation into a densely entangled network—directly explaining the experimental viscosity increase with pressure. The simulated transition from partial dissociation to dense entanglement under elevated pressure corresponds to the macroscopic viscosity enhancement observed, validating that pressure-induced network densification is the core mechanism for the system’s pressure responsiveness.

4.6. Gel Breaking Property

The breaking performance of fracturing fluid is a key factor determining proppant conductivity, with ideal breaking requiring fast and thorough reaction, low residue, and appropriate timing—constrained by residue blockage, time matching (optimal 1–2 h window post-construction), and low viscosity for reduced flowback resistance. Given the intelligent properties of C₂₂ZEA, the introduction of N₂ into the system under elevated temperature conditions tends to cause issues such as interfacial performance degradation. Accordingly, this study innovatively constructed and investigated a N₂-triggered gel-breaking mechanism: under different temperature conditions, N₂ was introduced into the CO₂-saturated C₂₂ZEA solution, and the intelligent gel-breaking performance was systematically evaluated by real-time monitoring the variation law of the system’s viscosity over time (as shown in Figure 20).

The experimental results indicate that temperature significantly affects the N₂-triggered gel breaking process of the C₂₂ZEA-CO₂ system (Figure 20). At 25 °C, viscosity decreases slowly in the first 5 min—attributed to high initial viscosity delaying CO₂ displacement; at 35 °C, the rate of viscosity decrease accelerates significantly; at 45 °C, the system achieves complete gel breaking within 15 min (final viscosity 0.01–0.02 Pa·s). This temperature dependence stems from two mechanisms: higher temperatures reduce initial system viscosity, promoting N₂ displacement efficiency for CO₂, and weaken intermolecular forces, accelerating the transformation of three-dimensional networks into spherical micelles. Notably, the fastest structural disintegration and viscosity decay at 45 °C provide important reference for on-site construction temperature control.

4.7. Biotoxicity Assessment

4.7.1. Biological Toxicity Testing

The biocompatibility of fracturing fluids serves as a core indicator for assessing environmental risks. This study adheres to SY/T 6787-2010 and SY/T 6788-2010 standards, providing cross-trophic scientific evidence for environmental risk assessment through coupled analysis of bacterial luminescence inhibition (acute molecular toxicity) and mysid survival response (chronic biological effects) [32,33].

Based on toxicological inhibition principles (luminescence intensity ∝ biological activity), six concentration-gradient sample solutions (10⁶ mg/L, 10⁵ mg/L, 10⁴ mg/L, 10³ mg/L, 10² mg/L, 10 mg/L) were prepared (30 g/L NaCl matrix) [34]. Luminometer measurements quantified luminescence intensity in blank tubes (E₀) versus sample tubes (Eᵢ), calculating relative luminescence: E = (E_i/E₀) × 100%. A univariate linear regression equation correlating concentration (C) and luminescence (E) was established, with the EC₅₀ value (concentration at 50% luminescence inhibition) determining toxicity classification (

Table 2. Biological toxicity classification.

EC50/(mg·L−1)	<1	1~100	101~1000	1000~10,000	>10,000
Toxicity level	Highly toxic	Moderately toxic	Toxic	Slightly toxic	Non-toxic

).

Following API/ASTM recommended protocols (using Neomysis awatschensis as a substitute for Mysidopsis bahia with equivalent sensitivity), six concentration-gradient solutions (10⁶ mg/L, 10⁵ mg/L, 10⁴ mg/L, 10³ mg/L, 10² mg/L, 10 mg/L) were prepared in artificial seawater [35]. Each concentration group included parallel samples (two 800 mL beakers), stocked with 20 healthy mysids (5 ± 1 days). After 96 h exposure, survival rates were recorded. The LC₅₀ value (median lethal concentration) was derived from the concentration (C)–survival rate (S) regression equation, with toxicity graded according to

Table 3. LC₅₀ and 96 h LC₅₀ values for different samples.

96 h LC50/(μg·L−1)	<1	1~100	101~1000	1000~10,000	>10,000
Toxicity level	Highly toxic	Moderately toxic	Toxic	Slightly toxic	Non-toxic

standards.

This study evaluated the biological toxicity of aqueous C₂₂ZEA systems at varying concentrations. According to the toxicity classification criteria in Table 2 and Table 3, experimental data in

Table 4. EC₅₀ and 96 h LC₅₀ values for different samples.

Evaluation Sample	EC50/(mg·L−1)	96 h LC50/(μg·L−1)	Biological Toxicity Classification
1.0 wt% C22ZEA aqueous solutions	1.94 × 106	8 × 105	non-toxic
1.5 wt% C22ZEA aqueous solutions	1.83 × 106	6 × 105	non-toxic
3.0 wt% C22ZEA aqueous solutions	8.24 × 105	3 × 105	non-toxic

demonstrate consistent results from both bioassay methods. The C₂₂ZEA aqueous system is classified as non-toxic, complying with environmental regulations and exhibiting high environmental compatibility and practical applicability [36,37].

4.7.2. Environmental Fate and Accumulation Effects Analysis

The environmental safety of C₂₂ZEA is closely linked to its bio-based molecular structure. Its degradation potential primarily derives from two types of microbially recognizable degradable moieties: the erucamide group (derived from C₂₂ unsaturated fatty acid) and the amide bond (-CONH-). Based on the degradation patterns of bio-based surfactants reported in References [16,36], its environmental degradation pathway is inferred as follows: Amidases secreted by microorganisms such as Pseudomonas and Bacillus in the environment preferentially hydrolyze the amide bond, decomposing C₂₂ZEA into low-toxic erucic acid (which can be further degraded via β-oxidation) and dimethylpropylamine. Simultaneously, microbial oxidases attack the C=C double bond on the hydrophobic chain, gradually shortening the carbon chain and ultimately breaking it down into CO₂ and H₂O. Although direct degradation experiments were not performed in this study, the good biodegradation potential of C₂₂ZEA can be indirectly confirmed by referring to degradation data of structurally similar erucic acid-based surfactants (e.g., ErBTA in Reference [13], with a 28-day biodegradation rate of 78% ± 4%, significantly higher than that of synthetic alkylamine surfactants (<50%)). It is important to note that the “70 °C shear stability” described in Section 4.4 only reflects performance durability during construction. After construction, the low-viscosity solution (0.01~0.02 Pa·s) formed via N₂-induced gel breaking (Section 4.6) is more accessible to microorganisms for degradation, further reducing residual risks. Based on the coupled analysis of “toxicity threshold—degradation rate—exposure concentration”, C₂₂ZEA poses extremely low environmental accumulation risk, with the core evidence as follows: The experimentally determined EC₅₀ (>8.24 × 10⁵ mg·L⁻¹) is far higher than the potential residual concentration (<500 mg·L⁻¹) in fracturing flowback fluids (calculated based on construction displacement and flowback rate), and even if not fully degraded, the residual amount remains below the toxic effect threshold. Referring to the 3~7-day degradation half-life of similar bio-based surfactants [16], its degradation rate is much faster than the 1~3-day construction cycle and the environmental migration speed of flowback fluids, thus preventing accumulation; Additionally, its octanol-water partition coefficient (logKow = 5.2, calculated via ChemBioDraw) falls within the “low bioaccumulation range” (logKow < 6), and degradation products such as erucic acid are common metabolic substrates for organisms, which will not be enriched and amplified through the food chain.

5. Conclusions

This study developed a novel CO₂-responsive erucamide propyl dimethylamine surfactant (C₂₂ZEA) and verified its mechanism and performance as an intelligent CBM fracturing fluid. Its core novelty lies in integrating long carbon chain hydrophobic association and amine-based responsiveness into a single molecular structure, enabling CO₂-triggered micelle network formation and gel breaking without blending—distinct from SDS-TMPDA (blending-dependent), ErBTA (short-chain) and NaOSA/Bola2be (gemini) in the literature. MD simulations showed C₂₂ZEA protonates to EA⁺ under CO₂, self-assembling into 3D micelle networks; denser networks in 3 wt% solutions (due to increased molecular collisions) explain the 3-fold viscosity rise in 1~3 wt% solutions. Experiments confirmed: 3 wt% C₂₂ZEA reached 160 mPa·s within 200 s under CO₂ injection (0.2 L·min⁻¹), with flow index (n) 0.14~0.18 (pseudoplastic) and G′ > G″ across frequencies (meets sand-carrying needs); viscosity recovery rate > 95% in alternating shear (170/10 s⁻¹), gel-breaking to 0.01~0.02 Pa·s in 15 min (45 °C, N₂) with minimal residue; viscosity remained > 160 mPa·s after 1 h shearing (70 °C, 170 s⁻¹), increasing with CO₂ pressure (1.01 → 5.41 MPa); 1.0~3.0 wt% solutions had EC₅₀ > 8.24 × 10⁵ mg·L⁻¹ and 96 h LC₅₀ > 3 × 10⁵ μg·L⁻¹ (non-toxic). Comparison with 3 responsive systems and guar gum (8 indicators, Table S1) confirmed its advantages in response speed and shear recoverability.

Limitations include confinement to laboratory tests (no field trials), unevaluated compatibility with proppants (ceramic/quartz sand) and performance in high-salinity environments. Future work should focus on proppant compatibility and high-salinity performance verification, conduct field pilots, improve structure-activity relationships via multi-scale characterization, and assess industrial feasibility to advance its theoretical improvement and engineering application.