Preparation and Performance Evaluation of Modified Amino-Silicone Supercritical CO₂ Viscosity Enhancer for Shale Oil and Gas Reservoir Development

Abstract

Against the backdrop of global energy transition and strict environmental regulations, supercritical carbon dioxide (scCO₂) fracturing and oil displacement technologies have emerged as pivotal green approaches in shale gas exploitation, offering the dual advantages of zero water consumption and carbon sequestration. However, the inherent low viscosity of scCO₂ severely restricts its sand-carrying capacity, fracture propagation efficiency, and oil recovery rate, necessitating the urgent development of high-performance thickeners. The current research on scCO₂ thickeners faces a critical trade-off: traditional fluorinated polymers exhibit excellent philicity CO₂, but suffer from high costs and environmental hazards, while non-fluorinated systems often struggle to balance solubility and thickening performance. The development of new thickeners primarily involves two directions. On one hand, efforts focus on modifying non-fluorinated polymers, driven by environmental protection needs—traditional fluorinated thickeners may cause environmental pollution, and improving non-fluorinated polymers can maintain good thickening performance while reducing environmental impacts. On the other hand, there is a commitment to developing non-noble metal-catalyzed siloxane modification and synthesis processes, aiming to enhance the technical and economic feasibility of scCO₂ thickeners. Compared with noble metal catalysts like platinum, non-noble metal catalysts can reduce production costs, making the synthesis process more economically viable for large-scale industrial applications. These studies are crucial for promoting the practical application of scCO₂ technology in unconventional oil and gas development, including improving fracturing efficiency and oil displacement efficiency, and providing new technical support for the sustainable development of the energy industry. This study innovatively designed an amphiphilic modified amino silicone oil polymer (MA-co-MPEGA-AS) by combining maleic anhydride (MA), methoxy polyethylene glycol acrylate (MPEGA), and amino silicone oil (AS) through a molecular bridge strategy. The synthesis process involved three key steps: radical polymerization of MA and MPEGA, amidation with AS, and in situ network formation. Fourier transform infrared spectroscopy (FT-IR) confirmed the successful introduction of ether-based CO₂-philic groups. Rheological tests conducted under scCO₂ conditions demonstrated a 114-fold increase in viscosity for MA-co-MPEGA-AS. Mechanistic studies revealed that the ether oxygen atoms (Lewis base) in MPEGA formed dipole–quadrupole interactions with CO₂ (Lewis acid), enhancing solubility by 47%. Simultaneously, the self-assembly of siloxane chains into a three-dimensional network suppressed interlayer sliding in scCO₂ and maintained over 90% viscosity retention at 80 °C. This fluorine-free design eliminates the need for platinum-based catalysts and reduces production costs compared to fluorinated polymers. The hierarchical interactions (coordination bonds and hydrogen bonds) within the system provide a novel synthetic paradigm for scCO₂ thickeners. This research lays the foundation for green CO₂-based energy extraction technologies.

1. Introduction

The commercial development of shale oil and gas reservoirs heavily relies on reservoir fracturing technologies, where the fracturing fluid system is often regarded as the “lifeline” of engineering operations, directly determining the efficiency of fracture network propagation and the proppant-carrying capacity. However, traditional water-based fracturing fluids face significant environmental constraints. Hydraulic fracturing requires enormous freshwater resources, with water consumption per well reaching high levels. Although slickwater reduces friction through low viscosity, its insufficient proppant-carrying capacity leads to excessive proppant settlement near the wellbore, severely limiting the formation of complex fracture networks [1]. Meanwhile, foam fracturing fluids encounter technical bottlenecks: N₂/CO₂ foam systems reduce water usage, but their poor stability under high-temperature formation conditions—manifested by insufficient half-life and viscosity and retention—hinders effective proppant transportation [2]. Amid global efforts to address climate change, the oil and gas industry is undergoing a green transition, driving innovations in CO₂ fracturing, enhanced oil recovery (CO₂-EOR) and carbon sequestration technologies [3,4,5,6]. Studies have shown that CO₂-based waterless fracturing and CO₂-EOR [7,8] can save tens of millions of cubic meters of freshwater annually and achieve carbon emission reduction through geological storage, aligning with sustainable goals of “resource conservation” and “carbon neutrality” [9]. However, the inherent properties of CO₂ as a non-polar fluid—particularly its ultra-low viscosity under formation conditions—significantly restrict its engineering performance [10]. These properties exacerbate gas channeling and viscous fingering effects during displacement, reducing crude oil recovery in heterogeneous reservoirs and affecting long-term sequestration stability [11].

At present, the polymer thickeners under study are mainly divided into three categories: fluorinated polymers, hydrocarbon polymers, and polysiloxanes. Although fluorinated polymer thickeners exhibit good solubility and thickening performance, their high cost and serious environmental pollution problems limit their practical application fields [12]. Low-molecular-weight hydrocarbons show good solubility but a poor thickening effect. Long-chain high-molecular-weight hydrocarbons can dissolve in CO₂ under cosolvent conditions, but the impact of cosolvents on formations cannot be ignored [13]. In addition, scholars have developed three types of poly (ionic liquid) thickeners based on quaternary ammonium salts. Under the condition of adding toluene additive, the viscosity of the supercritical CS₂ (alternative to scCO₂) system increased by 26.9 times, with the maximum thickening value reaching 8.9 MPa, but their high synthesis cost and extensive use of cosolvents remain problematic [14].

Traditional silicon-containing polymers rely on the formation of Lewis acid-base hydrogen bonds between cosolvents and CO₂, as well as the similar compatibility between cosolvents and siloxanes to improve solubility. They have low cohesive energy, good economy, and huge development potential. S. Kili et al. [15]. introduced ether groups into the side chains of polysiloxanes using noble metal Pt catalysts. Theoretical calculations showed that this structure reduced the cloud point pressure by 25%, but the thickening effect was insignificant. Follow-up structure–activity relationship studies by Michael J. O’Brien’s team [16] proved that aromatically end-capped polydimethylsiloxane (PDMS) has limited solubility in scCO₂—even with the addition of 20 vol% hexane cosolvent, the viscosity increase is less than 3-fold, revealing the critical influence of terminal group polarity on thickening performance. Notably, the copolymer system of the hydrosilane and acrylate developed by Bin Liu’s team [17] achieved cosolvent-free thickening (8–10-fold viscosity increase), but the synthesis process relied on 0.5 wt.% chloroplatinic acid catalyst, significantly increasing production costs. To address this limitation, Yongfei Zhang et al. [18] designed an epoxy and silane hydrosilylation route to synthesize phenyl-modified polyether siloxanes, but due to the rigidity of the molecular chain and insufficient degree of freedom, their CO₂ thickening factor only reached 4–5-fold (15 MPa, 45 °C), highlighting the importance of flexible segments for forming dynamic network structures. Mingwei Zhao’s team [19] reported that when commercial dimethicone or PDMS was used with kerosene/ethanol as cosolvents, the viscosity of scCO₂ increased to 4.67 mPa·s and 6.52 mPa·s, respectively, which were 54-fold and 37-fold higher than those of pure scCO₂ and liquid CO₂ under the same conditions [20]. Hun-Soo Byun reported [21] that in the supercritical carbon dioxide system, with the addition of different auxiliary agents, three molecular weights of PDMS were mixed with the solvent mixture, respectively, and their phase behavior exhibited LCST-type curves. The pressure difference in the phase behavior between its weight-average molecular weight and solvent mass was observed, but the cloud point pressure was relatively high, reaching 55.52 MPa. However, the use of cosolvents increases carbon emissions, conflicting with green goals. The main problems of existing thickeners are as follows: the trade-off between the thickening efficiency and environmental friendliness, the unclear balance mechanism of molecular rigidity and flexibility, and insufficient structural stability under high-pressure shear fields. Therefore, innovative molecular design strategies are urgently needed to address these issues. Based on this, this paper proposes a novel modification method for amino silicone oil (AS), in which polyether-based acrylate is polymerized with maleic anhydride, followed by side-chain amidation to connect polydimethylsiloxane. The influencing factors of the rheological properties of modified amino silicone oil thickeners under the condition of no cosolvent were studied in this paper, and the micro-mechanism of their thickening was analyzed. The research results provide new ideas for the molecular design of carbon dioxide thickeners.

2. Experimental Section

2.1. Experimental Materials

Methoxy polyethylene glycol acrylate (MPEGA) was purchased from Liangzhi Chemical (Guangzhou, China) Co., Ltd. Maleic anhydride (MA), toluene, and anhydrous ethanol were supplied by Chengdu Kelong Chemical Co., Ltd. (Chengdu, China). Amino silicone oil (AS) was obtained from Jiashan Jiangnan Textile Materials Co., Ltd. (Jiaxing, China), Glacial acetic acid was sourced from Chengdu Jinshan Chemical Reagent Co., Ltd. (Chengdu, China). 2,2′-Azobis (2-methylpropionitrile) (AIBN) was procured from Sichuan Yousifu Biotechnology Co., Ltd. (Chengdu, China). CO₂ gas (99.99% purity) and N₂ gas were supplied by Chengdu Xindu Zhengrong Gas Co., Ltd. (Chengdu, China).

2.2. Measurement Device and Calculation Method

2.2.1. FI-IR Measurements

The Fourier transform infrared spectrometer used, model WQF520, was manufactured by Beijing Rayleigh Analytical Instrument Co., Ltd. (Beijing, China). Specific parameter indices were as follows: wave number range, 7000–400 cm⁻¹; wave number accuracy, ±0.5 cm⁻¹ and optimal resolution, 0.5 cm⁻¹.

2.2.2. Cloud Point and Viscosity Measurement

As shown in Figure 1, the capillary viscometer was conducted in a self-designed viscosity measuring device. It has two processes to test viscosity: the drying process and the viscosity testing process. Drying Process: Slowly open the valve of the CO₂ cylinder (1) to allow CO₂ gas to flow into the desiccator (2) for drying. Start the compression pump (3), adjust the flow rate, and let the dried CO₂ purge the pressure-resistant gas cylinder (6). Viscosity Testing Process: Deliver the thickener from the thickener tank (4) at the proportion set by the experiment. It is mixed with CO₂ in the pressure-resistant gas cylinder (6) or the pipeline of the six-way valve to form the fluid system to be tested. The data acquisition and processing system (7) collects data such as the pressure drop and flow rate of the fluid flowing in the capillary. The viscosity of the system is automatically calculated according to Poiseuille’s law. The calculation formula for viscosity is shown in (1) [19].

$$\mathrm{\eta }={\displaystyle \frac{{\tau }_W}{{\gamma }_W}}={\displaystyle \frac{\mathrm{D}\mathrm{\Delta }\mathrm{P}/4\mathrm{L}}{8\mathrm{\upsilon }/\mathrm{D}}}$$

(1)

In the Formula (1), η (Pascal-seconds) represents viscosity; τ_w and γ_w represent wall shear stress and apparent shear rate, respectively. D is the diameter of the capillary, L is the length of the capillary, ∆p is the pressure difference across the ends of the capillary, and ν is the fluid flow velocity.

As shown in Figure 2, Thickener System Mixing: Start the ISCO pump (4) to deliver the thickener from the thickener tank (3) at the proportion set in the experiment. It mixes with CO₂ in the pipeline to form the scCO₂-thickener fluid system to be tested. The uniformity of fluid mixing is preliminarily observed through the viewing windows (5, 6). The small ball falls along the center of the visible area of the viewing window (6). Turn on the high-speed camera (7) to capture the movement trajectory of the falling ball inside the viewing window (6). Calculate the falling speed of the small ball in the uniform-speed section and identify the displacement–time relationship of the falling ball within the viewing window (6). Viscosity Calculation: The viscosity of the system is calculated using Stokes’ law, the principle of the falling-ball viscosity measurement method. The calculation formula for viscosity is shown in (2) [17].

$$\mathrm{\eta }={\displaystyle \frac{2{\mathrm{\gamma }}^2({\rho }_s-{\rho }_f)g}{9\mathrm{\nu }}}$$

(2)

where η represents the fluid viscosity, γ is the radius of the small ball, ρ_s denotes the density of the small ball, ρ_f stands for the density of the fluid, g is the gravitational acceleration, and υ is the falling velocity of the small ball. Substitute these parameters into the formula to calculate the shear viscosity of the scCO₂-thickener system.

2.3. Preparation Methods

2.3.1. Synthetic Routes

We designed a scCO₂ thickener and its synthesis route. The synthesis was divided into two steps: free radical polymerization of MA and MPEGA, and amide modification of the resulting polymer with AS side chains. The specific synthesis route is shown in Scheme 1:

2.3.2. Synthesis of MA-Co-MPEGA-AS

The first step is polymerization. Under nitrogen protection, add maleic anhydride (MA, 2.5 g, 25.5 mmol) and dehydrated toluene (147.5 g) into a 500 mL three-necked flask. Stir the mixture while heating until MA is completely dissolved. Subsequently, add methoxy polyethylene glycol acrylate (MPEGA, 50 g), raise the temperature to 80 °C, and maintain isothermal conditions at 80 °C. Using a constant-pressure dropping funnel, slowly add a 10 g toluene solution containing the initiator AIBN (0.525 g, 3.2 mmol) at a controlled dropping rate to ensure uniform addition within 30 min. After complete addition, continue stirring for 4 h to promote the formation of the copolymer via a free radical alternating copolymerization mechanism. Purification: Dissolve the precipitate in toluene, re-precipitate with n-hexane, and repeat this purification process twice. Drying: Vacuum-dry the purified product at a low temperature (40 °C, 24 h) to obtain 49.45 g of the final polymer (94.2% yield).

The second step is modification. In a 250 mL three-necked flask equipped with a Dean-Stark trap under nitrogen protection, add the MPEGA-co-MA copolymer (10 g, anhydride content: 4.2 mmol) and dehydrated toluene (80 g). Magnetically stir the mixture in a 60 °C oil bath until complete dissolution (within <30 min). Subsequently, add glacial acetic acid (0.1 g) as a protonic acid catalyst and heat to 85 °C, to form a homogeneous reaction system. Using a constant-pressure dropping funnel, slowly add a toluene solution containing AS (25 g, 50 g toluene) (amine value: 0.8 mmol/g) at a controlled dropping rate to ensure uniform addition over 30 min. After the dropwise addition is complete, maintain the system at 85 °C for 3 h of reflux reaction. Subsequently, raise the temperature to 120 °C and continue refluxing for 5 h. Remove low-boiling components from the system using a Dean-Stark trap. Upon completion of the reaction, dry the product under reduced pressure to obtain the modified aminosiloxane polymer.

2.3.3. Reaction Mechanism

The reaction mechanism of amidation modification (Scheme 2) is as follow: The anhydride groups in the polymer first interact with the proton (H+) in the acetic acid catalyst to form a truly active intermediate. The amino group in the amino silicone oil, with its lone pair of electrons, acts as a nucleophile to attack the carbonyl carbon atom of the active intermediate, forming a tetrahedral intermediate. Subsequently, this intermediate loses a proton and undergoes rearrangement to form amide and carboxylic acid molecules. A polymer unit is used to illustrate the specific mechanism, where R₁ and R₂ respectively polymerize the identical monomer unit portions [22].

3. Results and Discussion

3.1. Fourier Transform Infrared (FTIR) Structural Analysis

Figure 3a displays the FTIR spectrum of the copolymer MPEGA-co-MA. Key functional group vibrations include the following: 2871 cm⁻¹ (ν_s(C-H)), symmetric stretching of methylene (-CH₂-) in the PEG segment; 1731 cm⁻¹ (ν(C=O)), strong carbonyl stretching of the acrylate ester; 1250 cm⁻¹ (ν_as(C-O-C)), asymmetric stretching of the anhydride group; and 1107 cm⁻¹ (ν_s(C-O-C)), symmetric ether linkage stretching in the polyether segment, with peak symmetry reflecting chain ordering. These results confirm successful copolymerization. The infrared spectrum of the raw material side-chain amino silicone oil (AS) in Figure 3b shows the following: stretching vibration peaks of -CH₃ at 2965 cm⁻¹ and 2874 cm⁻¹; bending vibration peak of CH at 1411 cm⁻¹; deformation vibration peak of Si-CH₃, plane rocking vibration peak of CH, and stretching vibration peak of Si-C at 1259 cm⁻¹, 869 cm⁻¹, and 801 cm⁻¹; stretching vibration peaks of Si-O-Si at 1096 cm⁻¹ and 1016 cm⁻¹; bending vibration peaks of -NH₂ and -NH at 1593 cm⁻¹; and rocking vibration peak of -NH₂ at 702 cm⁻¹. Figure 3c illustrates the FTIR spectrum of the modified amino silicone oil polymer AM-AS, with peak assignments and structure–property correlations detailed as follows: 3465–3478 cm⁻¹,Broad absorption band, attributed to O–H stretching vibrations (ν(O–H)) in carboxylic acid groups, with a high-frequency shift suggesting intermolecular hydrogen bonding. 2966 cm⁻¹: Sharp peak corresponding to symmetric stretching of methylene (–CH₂–) in the polyoxyethylene segment (ν_s(C–H)), confirming structural integrity of the polyether backbone. 1732 cm⁻¹: Strong asymmetric stretching vibration of the ester carbonyl group (ν_as(O–C=O)), enhanced by conjugation effects due to copolymerization. 1641 cm⁻¹: Dual-characteristic peak arising from overlapping contributions of N–H bending vibration in primary amides (δ(N–H)) and carbonyl (C=O) stretching vibration in secondary amides (ν(C=O)). This confirms the successful grafting of amino silicone oil via amide bond formation. 1264 cm⁻¹: Region characteristic of siloxanes, showing in-plane bending vibrations of C–H in Si–CH₃ (δ(Si–CH₃)), further verifying silicone grafting. 1095 cm⁻¹ and 1019 cm⁻¹: Asymmetric stretching of the Si–O–Si backbone (ν_as(Si–O–Si)) and symmetric stretching of C–O–C in polyether chains (ν_s(C–O–C)), respectively. 797 cm⁻¹: Bending vibration of Si–C bonds (δ(Si–C)), confirming structural preservation of siloxane segments without degradation.

3.2. Thickener Performance Evaluation

Based on continuum mechanics theory, scCO₂ under reservoir temperature and pressure conditions (45–80 °C, 8–25 MPa) exhibits typical Newtonian fluid behavior, where its viscosity (0.01–0.04 mPa·s) remains independent of shear rate [23]. However, upon introducing MA-co-MPEGA-AS, the rheological behavior of the system undergoes a fundamental transition to non-Newtonian fluid characteristics [23]. The thickened system demonstrates pronounced shear-thinning behavior, conforming to the power-law model, which signifies the formation of a dynamic physical crosslinked network. Drawing from the work of Benedito J et al. [24,25], scCO₂ undergoes significant phase transitions in solvation capacity and transport properties when crossing the critical point (T = 31.15 °C, P = 7.38 MPa). Guided by this, we designed a gradient temperature–pressure experimental system (30 °C/8 MPa, 55 °C/14 MPa, 75 °C/20 MPa) covering typical shale reservoir temperature–pressure windows to systematically evaluate the performance of the MA-co-MPEGA-AS. Part of the experimental data is presented in

Table 1. Thickening performance of modified amino siloxane thickener in supercritical CO₂.

Temperature (°C)	Pressure (MPa)	Polymer (wt%)	CO2 (mPa·s)	System Viscosity (mPa·s)	Viscosity Increase Factor
35	8	1.5	0.026	1.637	63
35	8	3.0	0.026	2.791	107
35	14	1.5	0.051	4.249	83
35	14	3.0	0.051	5.546	109
35	20	1.5	0.067	4.765	71
35	20	3.0	0.067	5.811	87
55	8	1.5	0.023	1.485	65
55	8	3.0	0.023	2.361	103
55	14	1.5	0.045	3.608	80
55	14	3.0	0.045	5.150	114
55	20	1.5	0.062	3.933	63
55	20	3.0	0.062	5.093	82
75	8	1.5	0.021	1.137	54
75	8	3.0	0.021	2.143	102
75	14	1.5	0.042	2.581	61
75	14	3.0	0.042	3.938	94
75	20	1.5	0.060	3.018	50
75	20	3.0	0.060	4.385	73

.

Optimal Window: At 55 °C/14 MPa, the viscosity enhancement factor reaches 114-fold (the highest historical value). Critical Transition (

Table 2. Coupling effects of temperature and pressure fields.

Temperature (°C)	Pressure (MPa)	Viscosity Enhancement Factor Trend	Key Data Examples
35	8 → 20	63 → 71 (Δ + 13%)	3.0 wt.%, 8 → 20 MPa
55	8 → 14	103 → 114 (Δ + 11%)	3.0 wt.%, 8 → 14 MPa
75	8 → 20	102 → 73 (Δ − 28%)	3.0 wt.%, 8 → 20 MPa

): When the temperature exceeds 55 °C, the thickening efficiency under high pressure (20 MPa) significantly decreases (from 102-fold to 73-fold). Pressure-Dependent Mechanism: As pressure increases from 8 MPa to 14 MPa, the density of carbon dioxide (ρ) rises from 650 kg/m³ to 850 kg/m³. This macroscopic increase in density leads to an enhanced binding energy of the system, further strengthening the Lewis acid–base interactions between the polyether segments and CO₂ molecules. This synergistically promotes the increase in system viscosity.

According to the data analysis (

Table 3. Concentration dependence.

Concentration (wt.%)	Average Viscosity Enhancement Factor (Fold)	Enhancement Slope (Fold/wt.%)
1.5	66 ± 11	-
3.0	96 ± 13	20.5 (R2 = 0.23)

), increasing the thickener concentration from 1.5 to 3.0 wt% resulted in an average 30.7-fold increase in system viscosity. Doubling the concentration significantly enhances the thickening efficiency, but the enhancement is non-linearly modulated by temperature and pressure conditions. The low R² value (0.23) further indicates the limited individual effect of concentration, highlighting the need for synergistic optimization with temperature and pressure parameters. For instance, under optimized conditions (55 °C, 14 MPa), the viscosity gain slope reaches 30-fold per wt.%, demonstrating the critical role of multi-variable coupling in maximizing viscosity enhancement.

3.2.1. Effect of Shear Rate on the Viscosity of CO₂ Thickening

Shear Thinning Behavior and Critical Shear Rate Analysis: Experimental data confirm that shear rate critically governs the viscosity decay of modified amino silicone oil thickeners (Figure 4 and Figure 5). Below the critical shear rate (γ < 500 s⁻¹), the 3.0 wt.% system exhibits a viscosity decay rate of Δη/Δγ = −0.023 mPa·s/(s), while the 1.5 wt.% system shows a steeper decline of −0.028 mPa·s/(s). Above the critical shear rate (γ ≥ 500 s⁻¹), both systems stabilize, with decay rates moderating to −0.007 mPa·s/(s) (3.0 wt.%) and −0.011 mPa·s/(s) (1.5 wt.%), respectively. This phase transition reflects shear-induced disruption of the thickener network, with higher concentrations demonstrating superior structural resilience. Concentration-Dependent Shear Resistance: At 14 MPa/200 s⁻¹, the 3.0 wt.% system achieves 6.398 mPa·s, 1.17 times higher than the 1.5 wt.% system (5.488 mPa·s). When shear rates escalate to 850 s⁻¹, the 3.0 wt.% system retains 72.7% of its initial viscosity (4.65 mPa·s vs. initial 6.398 mPa·s), outperforming the 1.5 wt.% system’s 57.4% retention (3.15 mPa·s vs. initial 5.488 mPa·s). This highlights the enhanced shear stability of high-concentration networks. Viscosity Coupling of Pressure and Temperature: Optimal performance occurs at 14 MPa/55 °C, where the 3.0 wt.% system reaches peak viscosity (6.398 mPa·s at 200 s⁻¹), exceeding the performance at 8 MPa/35 °C (4.131 mPa·s) and 20 MPa/75 °C (5.75 mPa·s) by 54.9% and 11.3%, respectively. This bimodal behavior correlates with CO₂ density optimization (ρ = 780 kg/m³ at 14 MPa), which balances the solvation of polyether chains and interactions between siloxane and CO₂.

3.2.2. Effect of Temperature on the Viscosity of CO₂ Thickening Systems

In unconventional oil and gas development, as the depth of fractured wells increases, the reservoir temperature rises. To evaluate the thermal stability of modified amino silicone oil viscosifiers (Figure 6), experiments were conducted within the temperature range of 35–80 °C under constant pressure (14 MPa), viscosifier concentrations (1.5 wt.% and 3.0 wt.%), and shear rate (500 s⁻¹).

Non-Linear Viscosity Decay Behavior:For the 1.5 wt.% system, viscosity decreased by 44.5% from 35 °C to 80 °C (Δη = −0.043 mPa·s/°C). For the 3.0 wt.% system, viscosity decreased by 30.1% over the same temperature range (Δη = −0.0317 mPa·s/°C), indicating enhanced thermal stability in the high-concentration network. Critical Transition Temperature (T = 55 °C): Below 55 °C, the 3.0 wt.% system exhibited better viscosity retention compared to the 1.5 wt.% system (92.8% vs. 82.6%). Above 55 °C, the viscosity decay rate sharply increased: 3.0 wt.%, −0.051 mPa·s/°C; and 1.5 wt.%, −0.047 mPa·s/°C. Possible Mechanism Explanation: The different viscosity decay rates between the two systems arise from their distinct network structures. Increasing the viscosifier concentration enhances inter-chain entanglement density, partially offsetting thermally induced network disruption. Theoretical studies suggest that high temperatures accelerate the thermal expansion and free volume growth of CO₂ thickened systems, leading to a reduced viscosity due to weakened intermolecular interactions [26].

3.2.3. Effect of Pressure on the Viscosity of CO₂ Thickening Systems

Under constant temperature (55 °C) and shear rate (500 s⁻¹), the modified amino silicone oil-thickened CO₂ system exhibits nonlinear pressure-dependent viscosity evolution, characterized by distinct regimes of structural response to pressure variations (

Table 4. Nonlinear pressure-dependent viscosity behavior.

Pressure (MPa)	1.5 wt.% Viscosity (mPa·s)	3.0 wt.% Viscosity (mPa·s)	Viscosity Increase (%)
8	1.485	2.361	-
14	3.608	5.150	1.5%: 128.4%
20	3.393	5.093	3.0%: 118.1%

).

A detailed analysis based on experimental data (Figure 7) reveals a rapid thickening zone (8–14 MPa). For the 1.5 wt.% system, the viscosity growth slope Δη/ΔP is 0.379 mPa·s/MPa. For the 3.0 wt.% system, Δη/ΔP is 0.428 mPa·s/MPa, indicating that the high-concentration network reaches crosslinking saturation earlier due to denser entanglement of chain segments. Saturation Plateau Zone (14–20 MPa): Compression Limitation Effect: Viscosity growth significantly slows down (Δη/ΔP = 0.058 mPa·s/MPa and Δη/ΔP = 0.032 mPa·s/MPa). Response of Concentration Dependent: At 20 MPa, the 3.0 wt.% system retains 96.3% of its viscosity, significantly higher than the 1.5 wt.% system (88.7%), demonstrating superior pressure tolerance of high-concentration networks. Possible Microscopic Explanation: The pressure-dependent viscosity evolution of scCO₂ thickening systems originates from the dynamic balance between solubility capacity and molecular interactions. A pressure-driven increase in CO₂ density weakens quadrupole–quadrupole interactions between CO₂ molecules, reducing self-association tendencies [27]. Simultaneously, it enhances directional attraction between polar groups in amino silicone oil (such as Si–O dipoles) and the quadrupoles of CO₂, promoting the formation of polymer–CO₂ complexes.

3.2.4. Effect of Thickener Concentration on the Viscosity of CO₂ Thickening Systems

Experimental data (Figure 8) show that viscosity rapidly increases with the addition of thickener concentration from 0 to 3 wt.%, especially within the low concentration range (0–1 wt.%), where viscosity rises from 0 to 5.46 mPa·s. This phenomenon may be due to the initial formation of a network structure by the thickener, effectively enhancing system viscosity. As the concentration increases to 1.5–3 wt.%, viscosity continues to rise, but at a slower rate, indicating that the network structure gradually matures and approaches a critical concentration, beyond which the thickening effect plateaus. Beyond 3 wt.%, viscosity stabilizes with minimal fluctuations (Δη < 0.1 mPa·s), suggesting that the system has reached the percolation threshold, where thickener molecules form a continuous three-dimensional network. Further increasing the concentration does not significantly enhance viscosity, possibly due to spatial hindrance. Mechanism Explanation: As the amount of thickener increases, it acts as a nucleation point for CO₂-affinity interactions, initiating local network clusters up to their maximum extent. Subsequently, excess thickener molecules occupy gaps without strengthening the network, leading to diminishing returns in viscosity improvement.

3.3. Thickening Mechanism of Modified Amino Silicone Oil Polymer in CO₂ Thickening Systems

MA-co-MPEGA-AS achieves a balance between high solubility and thickening performance at the macro level. Its molecular design differs from other linear thickeners, as the side chains contain both polyether groups and siloxane groups, exhibiting significant performance characteristics: on the one hand, it increases the degree of freedom of the polymer, and the ether groups better interact with CO₂ through Lewis acid–base effects, which is conducive to improving solubility; on the other hand, the polydimethylsiloxane in the side chains forms a spatial network structure, increasing the system viscosity. The underlying micro-mechanisms may involve the following: Dipole–Quadrupole Orientation interactions [28]: Ether oxygen atoms (Lewis bases) in the polyether segments form C=O⋯C dipole–quadrupole interactions with CO₂’s quadrupole moment (Lewis acid). The C–H⋯O hydrogen bond between the carbonyl group and CO₂ provides additional binding sites, enhancing solvent stabilization [29,30]. Siloxane–CO₂ Bonding: Electron donor–acceptor interactions occur where the 3d empty orbitals of silicon atoms accept lone pair electrons from CO₂ oxygen atoms. Hydrophobic Effects: Low-polarity siloxane methyl groups (Si–CH₃) induce local CO₂ density enrichment through hydrophobic interactions. Synergistic Effects of Methyl Terminal Groups; The methyl groups at the MPEGA terminus participate in the formation of the solvation layer via weak C–H⋯O hydrogen bonds, expanding the polymer–CO₂ interaction interface. The construction of Hierarchical Network: The introduction of polyether acrylate disrupts the regularity of amino silicone molecular chains, increasing conformational freedom. The polymer’s disordered conformation facilitates the configuration of extended chains in scCO₂, contributing to system mixing entropy and strengthening thickening effects. Different Reactivities of Amine Groups: Primary amines (–NH₂) and secondary amines (–NH–) form covalent amide bonds with maleic anhydride, promoting the formation of a three-dimensional network. Therefore, modified amino silicone achieves thickening in supercritical CO₂ through synergistic hierarchical molecular interactions and dynamic network construction.

4. Conclusions

This study successfully utilized maleic anhydride to connect and construct modified amino siloxane polymers (MA-MPEGA-AS) through molecular design strategies, and investigated their polymer properties and rheological performance. The side chain of MA-co-MPEGA-AS contain both polyether groups and siloxane groups, which increases the degree of freedom of the polymer. The ether groups have better interactions with CO₂ through Lewis acid–base effects, which is beneficial to improving solubility. On the other hand, the polydimethylsiloxane in the side chain forms a spatial network structure, which enhances the system viscosity, thus achieving the balance between solubility and thickening ability. The synthetic process and properties described herein offer the following key values and significances:

• Mechanistic Elucidation of Anhydride–Amino Reactions: The nucleophilic acyl substitution mechanism is validated, where the reaction between maleic anhydride and amino silicone oil proceeds via a tetrahedral intermediate, eliminating the need for noble metal catalysts (e.g., chloroplatinic acid), as shown in Scheme 1 of the reference. This confirms the feasibility of Lewis acid–base catalysis for efficient amide bond formation.
• Green Synthesis and Low cost Conditions: The process operates at room temperature (25–110 °C) and ambient pressure, reducing energy consumption compared to traditional high-temperature processes. Noble-Metal-Free Catalysis: By leveraging the inherent reactivity of acid anhydrides, the synthesis avoids toxic catalysts, aligning with the “atom economy” principle of green chemistry.
• Rheological Performance and Solubility enhancement mechanism of molecular Design: The polyether segments in MA-co-MPEGA-AS interact with CO₂ through Lewis base coordination (ether oxygen as electron donor), improving solubility in supercritical fluid. Viscosity Modulation: Polydimethylsiloxane side chains form a three-dimensional network, increasing viscosity via intermolecular entanglement, as observed in NCA polymerization.
• Prospects of Industrial Application in Energy Extraction.The polymer enables a 114-fold viscosity increase in CO₂-based fluids, enhancing proppant transport efficiency in hydraulic fracturing without cosolvents. Cost Reduction: Eliminating precious metal catalysts reduces production costs, while mild conditions lower equipment investment and operational risks.

However, this study also has the following limitations: The maximum system temperature was only 80 °C, which cannot adapt to high-temperature formations. These issues provide potential directions for follow-up research. Insufficient Depth of Mechanism Verification: Restrictions are due to laboratory equipment. Although the “hierarchical molecular interaction” mechanism was proposed, in situ characterization data, such as molecular dynamics simulations [31], are lacking to directly validate the interaction process at the polymer–CO₂ interface. Limited Application Scenarios: Experiments were only carried out at a constant shear rate (500 s⁻¹) and within a single pressure/temperature range, failing to simulate the complex fluid shear fluctuations in actual reservoirs. Comparative studies are insufficient: research on heat resistance and stability has not been conducted, making it difficult to more accurately quantify the technical advantages of MA-MPEGA-AS. These shortcomings provide directions for follow-up research.