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

Abstract

Supercritical carbon dioxide (SC-CO₂) fracturing technology has the characteristics of a large amount of fixed CO₂ and anhydrous fracturing. It has great application potential for developing unconventional oil and gas resources and mitigating the greenhouse effect. However, the low viscosity of SC-CO₂ limits the development of this technology. In this work, HS series thickeners were prepared via a ring-opening polymerization and hydrosilylation reaction by a molecular simulation-aided design method. The simulation results of cohesive energy density, interaction energy, and radial distribution function are consistent with the visualization experimental results, which proves that HS (hyperbranched siloxane) series thickeners have excellent solubility in SC-CO₂. HS-3 is the best thickener in the HS series. At 305.15 K and 10 MPa, 5 wt% HS-3 (60 s⁻¹) increases the viscosity of SC-CO₂ by 151 times, and the apparent viscosity is 3.024 mPa s. The apparent viscosity of SC-CO₂ was positively correlated with the pressure and concentration but negatively correlated with the temperature and shear rate. The results indicate that it is feasible to introduce an aliphatic group and polysiloxane into a SC-CO₂ thickener by hydrosilylation.

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1. Introduction

The application of carbon dioxide has received extensive attention from scientists.¹ The two main reasons are as follows: (1) climatic changes caused by CO₂ cannot be overlooked, and (2) compared with hydrocarbon used as a raw material in the traditional industry, carbon dioxide can be economically beneficial and reduce environmental pollution.² CO₂ utilization was seen as a necessary cornerstone of successful large-scale carbon capture and storage.³ Currently, 5% of the US annual crude oil production relies on CO₂-EOR (enhanced oil recovery) technology.⁴ Under heterogeneous reservoir conditions, 1.1–3.3 barrels of crude oil can be produced for every ton of CO₂ injected.⁵ The CO₂-EOR technology has the potential to be applied to 90% of the world's oilfields,⁶ which means that 140 Gt CO₂ will be captured and fixed while enhancing the recovery rate.³ In short, SC-CO₂ fracturing technology can improve the efficiency of shale gas development and reduce carbon emissions (Fig. 1).

Fig. 1 CO₂ and CH₄ competitive adsorption.

At present, the core problem limiting this technology is that the viscosity is too low (0.02–0.05 mPa s) to carry the proppant into the formation, and hence, it is necessary to study an excellent thickener. The reported SC-CO₂ thickener^7–9 containing fluorine has two disadvantages: (1) fluorinated monomers are too expensive to be widely used and (2) fluorine enters the biosphere via water circulation and cannot be metabolized by organisms.¹⁰ The thickening mechanism of siloxane thickeners was reported:^11–15 (1) the formation of a Lewis acid hydrogen bond between a cosolvent and CO₂ and (2) the similar compatibility between the cosolvent and siloxane. The main characteristics of the reported hydrocarbon thickeners^16,17 are as follows: (1) low-molecular weight compounds have good solubility, but the thickening effect is poor and (2) under co-solvent conditions, long-chain polymer hydrocarbons are soluble in CO₂. Organosilicon thickeners often rely on cosolvents to improve the solubility, and cosolvent causes formation damage.¹² Silicone has the characteristics of low glass transition temperature, low binding energy, and good economy, which has great potential for the research and development of SC-CO₂ thickeners. In this work, a series of SC-CO₂ thickeners were designed and prepared to solve the high cost and high pollution of fluoropolymers and cosolvents. The thickener has excellent solubility, thickening property, heat resistance, and pressure resistance.

A series of SC-CO₂ thickeners were designed and prepared by molecular simulation. The chemical structures of the copolymers were characterized by FT-IR, ¹H-NMR, and GPC. The solubility of the thickener in SC-CO₂ was studied using a visual reactor. The response of the thickener to the apparent viscosity of SC-CO₂ under different conditions was studied using a self-designed capillary viscometer. The intermolecular interaction energy, radial distribution function (RDF), and cohesive energy density (CED) of the polymer were calculated to explain its solubility in SC-CO₂.

2. Experimental section

2.1 Synthesis and characterization

The materials used in the experiment were octamethylcyclotetrasiloxane (D4, 98%), 1,1,3,3-tetramethyldisiloxane (TMD, 98%), ethylene glycol dimethacrylate (98%), triethylene glycol dimethacrylate (95%), trimethylolpropane trimethacrylate (98%), and H₂Cl₆Pt·6H₂O (99.99%), which were purchased from Aladdin. H₂SO₄ (98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, and trimethylolpropane trimethacrylate were washed three times with a 3% NaOH alkali solution to remove MEHQ (p-hydroxyanisole) and then washed three times to remove NaOH. Anhydrous magnesium sulfate was dried, filtered to remove magnesium sulfate, distilled to remove residual alkali, and then refrigerated at low temperatures.

First, add 1 g TMD and 50 g D4 to the hydrothermal reactor, and use 3–4 drops of sulfuric acid (90 °C, N₂ atmosphere) as a catalyst to prepare the crude product of Si–H terminal chain siloxane by ring-opening polymerization.¹⁸ Then, sodium carbonate was added to the crude product to remove the catalyst, and the impurities were removed by vacuum distillation to obtain pure Si–H-terminated polysiloxane. In the third step, 4 g of ethylene glycol dimethacrylate was added to a 250 mL three-necked flask equipped with a thermometer, condenser, and magnetic stirring device, 40 ppm chloroplatinic acid was activated at 70 °C for 2 h, and hydrosilylation¹⁹ was reacted for 4 h. To remove chloroplatinic acid, 1 g activated carbon was added to the product, the hydrosilylation reaction was stopped, and the mixture was repeatedly washed with distilled water. Under the vacuum conditions of 370 K and 0.06 MPa, small-molecule compounds and water were removed using a rotating evaporator, and the final product HS-1 was obtained. The Si–H-terminated polysiloxane was reacted with unsaturated acrylate, in turn, to obtain thickeners HS-2 and HS-3. The synthetic route is shown in Fig. 4a.

2.1.1 FI-IR measurements. The polymer was characterized by FT-IR using a Nicolet IS50 instrument. After washing the ATR (attenuated total reflection) crystal with ethanol, 0.10 mL of the sample was spread evenly on the surface of the ATR. The scan number of the spectrum is 128 and the resolution is 8 cm⁻¹. The wavenumber ranges from 4000 to 500 cm⁻¹.²⁰

2.1.2 ¹H NMR measurements. A Bruker-400 MHz NMR was used to record the ¹H NMR spectrum^21–23 of the polymer with deuterated chloroform as a solvent.

2.1.3 GPC measurements. The measurement of the polymer molecular weight was completed using a WATERS 2414 refractive index detector. First, 5 mg mL⁻¹ polymer solution of chromatographic grade tetrahydrofuran was prepared, and then, the solution was passed to the gel chromatography column at a flow rate of 1 mL min⁻¹ at 35 °C.²⁴

2.1.4 Cloud point and viscosity measurement. Fig. 2a shows the cloud point measuring device. First, 1 wt% HS thickener (305.15 K) was added to the visualization reactor, switches (1) and (4) were turned on, and a vacuum pump was used to empty the visualization container (the switches not mentioned in each step were closed), and the right side of the middle container II was filled with water. Second, switches (2), (5), and (6) were turned on, an advection pump was used to compress CO₂ into the liquid in the intermediate container I, switches (2), (4), and (6) were turned on, liquid CO₂ was transferred to the visible container and this was repeated several times until the container was full (every time compress 5 MPa CO₂ gas once and fill up the 500 mL visualization container with (4)–(5) operations). (3) After the thickener and CO₂ were mixed evenly, switches (3), (4), and (6) were turned on, the homogeneous liquid in the visible container was slowly pumped out by a hand pump, and the cloud point phenomenon was observed (pressure when the homogeneous solution is turbid). Throughout the experiment, the advection pump controlled the pressure increase and the manual pump controlled the pressure reduction.

Fig. 2 (a) SC-CO₂ phase behavior test device. (b) Capillary viscometer flow chart.

The capillary viscometer (Fig. 2b) consisted of an advection pump, intermediate containers, a differential pressure sensor, and a back-pressure system. In the beginning, the heating area was opened to make the system reach 305.15 K, CO₂ gas entered the intermediate container I containing the thickener under the action of the pump, and CO₂ was repeatedly injected and pressurized until thickened CO₂ reached the supercritical state²⁵ (T > 304.25 K, P > 7.38 MPa). Then, the experiment was carried out at a flow rate of 0.18 mL min⁻¹ (τ = 60 s⁻¹). The back pressure on the end of the system is 8 MPa. When the system was in equilibrium, the differential pressure in the sensor was recorded and the viscosity was calculated using eqn (1). Eqn (1) is generally suitable for laminar fluids:²⁶

where η is the fluid apparent viscosity (Pa s), τ_w the wall shear stress (Pa), γ_w the apparent shear rate (s⁻¹), D the capillary diameter (m), Δp the pressure difference of capillary (MPa), L the capillary length (m), and v the flow velocity of thickened liquid CO₂ (m s⁻¹).

2.2 Materials studio simulation

The polymer simulated in this work was named HS-1, HS-2, and HS-3, and a CO₂ system with 2000 CO₂ molecules, a polymer system with 8 polymer chains, and a polymer with 8 polymer chains 2000 CO₂ molecules were established using the Material Studio software; the all-atom molecular model of the CO₂ system is shown in Fig. 3.

Fig. 3 Nine systems are built on the all-atom model: (a) CO₂ system composed of 2000 CO₂, (b) polymer system composed of 8 polymer chains, and (c) polymer–CO₂ system composed of 8 polymer chains and 2000 CO₂.

In recent years, molecular simulation technology has been widely used in the research field of CO₂ thickeners.^9,27 In this work, three kinds of polymer–CO₂ boxes (305.15 K, 10 MPa) were constructed with amorphous cells, and through the optimization of the Forcite model and annealing calculation, the dynamic balance system was obtained. The thermostat and barostat were Andersen and Berendsen, respectively. The running time for all the systems was 400 ps, and the trajectories were saved at 1 ps intervals.^23,28,29

3. Result and discussion

3.1 Structural characterization of HS series thickeners

The four test samples all contain 1256 cm⁻¹ (Si–CH₃), 1054 cm⁻¹ (Si–O),³⁰ and 792 cm⁻¹ (Si–C),³¹ as shown in Fig. 4. After the hydrosilylation reaction, HS-1, HS-2, and HS-3 showed C=C and C=O tensile vibration peaks at 1638 cm⁻¹ (ref. 32) and 1736 cm⁻¹,³³ which were not in 50 : 1 Si–H-terminated polysiloxane, and the Si–H stretching vibration peak (2160 cm⁻¹)³⁴ vanished.

Fig. 4 (a) Synthetic route of HS series thickener (b) Infrared spectrum of HS series thickener(c)HS series thickener hydrogen NMR spectrum.

The chemical shifts of the four samples shown in Fig. 4 are detailed in Table 1. The results indicate that: (1) the Si–H peaks at 3.6 ppm in 50 : 1 polysiloxane do not appear in HS series thickeners, (2) C=C hydrogen at 6.48–6.50 ppm appears in HS series thickeners. Combining the results of Fig. 5 and Table 1, it was proved that HS series thickeners were successfully prepared.

Table 1 Chemical shift of siloxane and HS series thickeners

Polymer category	δ/ppm	Types of H
50 : 1 Si–H terminated polysiloxane	0.17	Si–CH3
	3.60	Si–H
	7.28	CDCl3
HS-1	0.16	Si–CH3
	1.17	O=C–CH(CH3)–CH2
	2.04	CH2=C(CH3)–C=O
	2.6	O=C–CH(CH3)–CH2
	4.41	O–CH2–CH2–O
	6.48–6.50	CH2=C(CH3)–C=O
	7.26	CDCl3
HS-2	0.15	Si–CH3
	1.09–1.18	O=C–CH(CH3)–CH2
	2.01	CH2=C(CH3)–C=O
	2.4	O=C–CH(CH3)–CH2
	3.6–3.8	CH2–O–CH2–CH2–O–CH2
	4.38	O=C–O–CH2
	6.48–6.50	CH2=C(CH3)–C=O
	7.28	CDCl3
HS-3	0.17	Si–CH3
	1.14–1.26	O=C–CH(CH3)–CH2–Si
		(CH2)3–C–CH2–CH3
	1.60	(CH2)3–C–CH2–CH3
	2.04	CH2=C(CH3)–C=O
	2.4	O=C–CH(CH3)–CH2
	3.8–4.03	(CH2)3–C–CH2–CH3
	6.48–6.50	CH2=C(CH3)–C=O
	7.26	CDCl3

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Fig. 5 HS series thickener phase behavior: (a) pure SC-CO₂ at 298.15 K, 7.48 MPa; (b) thickener and SC-CO₂ mixed phase at 298.15 K, 7.48 MPa; (c) thickener and SC-CO₂ mixed phase at 305.15 K, 7.66 MPa; (d) thickener and SC-CO₂ mixed phase at 305.15 K, 7.32 MPa.

3.2 Molecular weight analysis

The GPC results of Fig. S1 (ESI†) indicate that the degree of polymerization of 50 : 1 Si–H-terminated polysiloxane is 5, and the molecular weights of HS-1, HS-2, and HS-3 show that the degree of polymerization is 5, 15, and 16, respectively. In a supercritical state, low-content HS-1 may have better solubility, and higher molecular weight HS-2 and HS-3 may have better thickening performance.

3.3 Cloud point

Fig. 5a shows that pure SC-CO₂ (298.15 K, 7.48 MPa) in the visualization reactor is transparent. When the visualized reactor system was composed of HS series thickeners and SC-CO₂, there were two situations: (1) HS-1 was added to SC-CO₂ (298.15 K, 7.48 MPa), and the liquid shows obvious turbidity under white background light and (2) after shaking, SC-CO₂ (298.15 K, 7.48 MPa) was thickened by HS-2, and HS-3 appeared slightly turbid. Finally, the visualization reactor was sealed and placed in a 305.15 K oven to stand for 12 h. After shaking, it can be observed that the three thickened SC-CO₂ (305.15 K, 7.66 MPa) fluids were all in a clear and bright state (Fig. 5c). Fig. 5c and d are the results of the phase behavior study. The instrument in Fig. 2b was used to study the phase behavior (maintaining 305.15 K as a constant temperature condition). According to the experimental results, the three thickening systems remain clear and translucent when the system state reaches the non-critical state of 305.15 K and 7.32 MPa. The HS thickener prepared for this study has no cloud point of supercritical conditions and is a thickener with excellent solubility in SC-CO₂ (no fluoropolymer and co-solvent added).

The introduction to multiple aliphatic groups of the thickener molecule is beneficial for enhancing the solubility. The mechanism is that CO₂ and lipid groups form hydrogen bonds via the action of Lewis acids and bases,^35–37 and the good chain flexibility^38,39 of polysiloxane itself also improves the solubility of polymers in SC-CO₂.

3.4 Thickening mechanism and performance

3.4.1 Effect of temperature on the apparent viscosity. The effect of temperature on the apparent viscosity of thickened SC-CO₂ was recorded, as shown in Fig. 7a. In the study range of 305.15–325.15 K, the overall trend was that as the temperature gradually increases, the apparent viscosity of thickened SC-CO₂ gradually decreases. This is consistent with the related research reported in the literature. The reason is that temperature has a certain influence on the network structure formed by polymer chains (Fig. 6a).

Fig. 6 Dissolution and viscosity-increasing mechanisms of SC-CO₂ mainly include (a) polymer–polymer action, (b) polymer–CO₂ action, and (c) flexibility polymer of a chain.

Fig. 7 Effect of different research conditions on apparent viscosity. (a) Influence of temperature on apparent viscosity, (b) influence of pressure on apparent viscosity, (c) influence of thickener content on apparent viscosity, and (d) influence of shear rate on apparent viscosity.

The mechanism (Fig. 6a and b) involves the following steps: (1) the HS series polymer chains themselves entangle with each other to form a network structure^40–42 and (2) the hydrogen bond formed by the aliphatic group and CO₂ in the polymerization unit via the action of Lewis acid and base further strengthens the network structure.^35–37 When the system temperature increases, the network structure that hinders the flow of CO₂ molecules becomes loose, and the macroscopic performance is that the viscosity decreases. The viscosity of the system will drop sharply until the temperature rises enough to break the hydrogen bond. However, it is worth mentioning that the HS-3 thickener exhibits a certain temperature resistance. This is because there are a certain number of side chains in the polymerization unit, which make the reticular structure formed by the HS-3 thickener more stable than those formed by HS-1 and HS-2.

3.4.2 Effect of pressure on the apparent viscosity. The effect of pressure on the apparent viscosity of thickened SC-CO₂ is depicted in Fig. 7b. Under the conditions where τ = 60 s⁻¹, C = 5 wt%, and T = 305.15 K, the lifting pressure has a positive effect on the viscosity of SC-CO₂ (8–16 MPa). At 16 MPa, the HS-3 thickener has the best thickening effect, reaching 3.26 mPa s.

As explained from the thickening mechanism, the tighter the network structure formed by polymer molecular chains and CO₂ (Fig. 6a), the greater the flow resistance to SC-CO₂, and the macroscopic manifestation shows an increase in apparent viscosity.⁴² As the pressure increased, the interaction between the electron-donating groups in the polymer and CO₂ increased, which promoted the increase in apparent viscosity.⁴³ During the pressurization process, the intermolecular distance gradually decreases, and hydrogen bonds (Fig. 6b) are easier to form.⁴⁴ The two effects complement each other and thus have a thickening effect.

3.4.3 Effect of thickener contents on the apparent viscosity. The effect of HS series thickener contents on the apparent viscosity of thickened SC-CO₂ is depicted in Fig. 7c. The research results show that in the concentration range of 1–5 wt% (10 MPa, 305.15 K), the apparent viscosity of the system gradually increases with the increase in the thickener content. At a concentration of 5 wt%, the HS-3 thickener has the best thickening effect, reaching 3.024 mPa s.

The mechanism of action is that with the increase in the thickener content of a closed system, the number of polymer molecular chains that can form a network structure and hydrogen bonds⁴² increases multiple times, and the tighter structure greatly hinders the flow of CO₂ molecules. Nevertheless, it takes a lot of energy to destroy the network structure and hydrogen bonds.⁴⁵ Under the experimental conditions in this article, there was not enough external energy to destroy the structure. Therefore, the CO₂ molecules still flowing in the system were captured by the expanding polymer network (Fig. 6a) and macroscopically manifested as an increase in apparent viscosity.

3.4.4 Effect of share rate on the apparent viscosity. Under the conditions where C = 5 wt%, T = 305.15 K, and P = 10 MPa, the increase in shear rate hurts the apparent viscosity of SC-CO₂ (60–140 s⁻¹). It shows that after thickening, SC-CO₂ is a power-law fluid with shear thinning characteristics.^40,46,47 It can be seen from Fig. 7d that the apparent viscosity of HS-1 decreases with the increase in shear rate, which is lower than that of HS-2 and HS-3. The shorter molecular chain of HS-1 is less affected by the increased shear rate. The shorter molecular chain can avoid the shear thinning characteristics, but the short molecular chain often exhibits poor thickening performance. Thickeners HS-2 and HS-3 are more sensitive to the effect of shear rate on apparent viscosity because of longer molecular chains. At 140 s⁻¹, the HS-3 thickener has the best thickening effect, keeping it at 0.86 mPa s.

The HS thickener is a functionalized polysiloxane, which has inherently good chain flexibility (Fig. 6c). With the condition of low shear rate, the shear force is not enough to completely destroy the network structure formed by the entanglement with polymer molecular chains, so that the SC-CO₂ fluid maintains a higher viscosity at a low shear rate. When the shear rate gradually increases, the network structure is gradually destroyed, resulting in a highly oriented arrangement of polymer molecular chains along the shear direction, which reduces the flow resistance of CO₂ (decreases the apparent viscosity).

3.5 Materials studio computational simulation

3.5.1 Dynamic simulation equilibrium conformation. Fig. 8 shows the distribution of three kinds of polymer chains (305.15 K, 10 MPa) in SC-CO₂, where the polymer backbone was marked as purple, and the CO₂ molecules were marked with red oxygen atoms and black carbon atoms. From Fig. 8, it can be observed that after Forcite dynamics simulation, the poly HS-1 and poly HS-2 polymer chains were distributed among a curled state in the amorphous box, while the poly HS-3 polymer chains were distributed in a semi-curved state. The CO₂-philic group in poly(HS-1) and poly(HS-2) have a single chain structure, while the branched-chain CO₂ affine monomers in poly(HS-3) increase the richness of the side chains of the polymers. The appropriate amount of side chains makes the polymer chains present wide distribution of simulation results, which was more helpful in the viscosity of SC-CO₂.

Fig. 8 Dynamic simulation equilibrium conformation of HS series thickeners.

3.5.2 Interaction energy calculation. Solving the low viscosity problem of SC-CO₂ depends on the interaction between the polymer and CO₂. The interaction energy (E_inter) can be used to measure the strength of this effect. The greater the absolute value of E_inter was, the stronger the polymer–CO₂ interaction was. First, the total energy E_{polymer–CO2} of CO₂–polymer system in a certain frame was calculated. Then, energy E_CO2 after removing the polymer chains and energy E_polymer after removing the CO₂ molecules were calculated. Finally, interaction energy E_inter between CO₂ and polymer chains was obtained using eqn (2) as follows:⁴⁸

E_inter = E_{polymer–CO2} − (E_polymer + E_CO2) (2)

The E_{polymer–CO2}, E_polymer and E_CO2 parameters of HS series thickeners are shown in Table 2. From the research data in Table 2, it can be predicted that the solubility of poly HS-2 and poly HS-3 in SC-CO₂ was close and stronger than that of poly HS-1.

Table 2 E_{polymer–CO2}, E_polymer and E_CO2 parameters of HS series thickeners

System	Epolymer–CO2/kJ mol^−1	Epolymer/kJ mol^−1	ECO2/kJ mol^−1	Einter/kJ mol^−1
Poly HS-1 and SC-CO2	1157.25	−1089.06	1874.10	372.21
Poly HS-2 and SC-CO2	640.21	−3060.89	1910.25	1790.85
Poly HS-3 and SC-CO2	791.42	−2784.26	1880.17	1695.51

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3.5.3 Cohesive energy density and solubility parameter calculation. The cohesive energy density (CED) and solubility parameters were used to measure the interaction between polymer molecules. CED is the energy consumed by evaporating 1 mole of polymer per unit volume to overcome intermolecular forces, and the square root of CED was the solubility parameter. The reported research shows that polymers with high solubility in SC-CO₂ usually have lower CED,^38,49,50 and the solubility parameters of polymers and SC-CO₂ are very close.⁵¹ |Δδ| is calculated using formula (3). The CED and solubility parameters (δ) of polymers and SC-CO₂ at 10 MPa and 305.25 K are given in Table 3. According to the results presented in Table 3, HS series thickeners have good solubility in SC-CO₂, and |Δδ| of poly HS-3 and SC-CO₂ was only 0.1 (J cm⁻³)^1/2, indicating that the solubility of HS-3 in SC-CO₂ was the best among the HS series thickeners:

|Δδ| = |δ_{polymer–CO2} − δ_CO2| (3)

Table 3 CED and solubility parameter values of the HS series thickeners and CO₂ at 305.25 K and 10 MPa

System	CED/(J m^−3)	δ/(J cm^−3)^1/2	|Δδ|/(J cm^−3)^1/2
Poly HS-1 and SC-CO2	1.78 × 10^8	13.34	0.17
Poly HS-2 and SC-CO2	1.768 × 10^8	13.29	0.22
Poly HS-3 and SC-CO2	1.80 × 10^8	13.41	0.1
SC-CO2	1.825 × 10^8	13.51	0

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3.5.4 Radial distribution function calculation. The RDF values between HS series thickeners and SC-CO₂ molecules are represented by the intermolecular RDF of C–C pairs, which are recorded, as shown in Fig. 9. The equilibrium of 400 ps was simulated by force kinetics, and the RDF value of Inter_Cpolymer and was taken for analysis. If the RDF value of polymer CO₂ is large, it is proved that the HS series thickener is miscible with SC-CO₂.^{28,29,48,52,53}

Fig. 9 RDF between HS series thickeners and SC-CO₂.

The results shown in Fig. 9 indicate that the RDF values (305.15 K, 10 MPa) of are generally greater than those of Inter_Cpolymer. In the RDF diagram of HS-1, HS-2, HS-3, and CO₂, it can be seen that the three polymers can be dissolved in SC-CO₂, and the data show a similar trend. Among them, HS-1 showed a larger peak value of the range of 4–7 Å, which was the optimal solution to simulation calculation. This is because the molecular chain of HS-1 was shorter and the molecular weight was lower, resulting in better solubility.

4. Conclusion

In this study, the viscosity of HS series thickeners was studied using a self-designed capillary viscometer. The calculation results indicated that the HS series thickeners have an excellent dissolving effect on SC-CO₂, which provides a new type of feasibility for the design of SC-CO₂ thickeners. At first, polysiloxane had excellent chain flexibility to make HS series thickeners better dispersible in SC-CO₂. Then, the polymer molecular chains were cross-twisted via polymer–polymer interaction to form a spatial network structure, which trapped CO₂ molecules and weakened its fluidity to achieve a thickening effect. Finally, the introduction to multiple lipid groups of the polymerization units of the HS series resulted in (a) excellent solubility and (b) formation of hydrogen bonds between the carbonyl group and CO₂ via Lewis acid–base pairing. It was found that 5 wt% (10 MPa, 305.15 K) HS-3 has the best thickening performance, and the viscosity of SC-CO₂ increases 151 times, reaching 3.024 mPa s. The HS series thickeners prepared for this study can be dissolved in SC-CO₂ without the use of fluoropolymer and co-solvents, which has little damage to the formation and solves the problems of high price and serious pollution by traditional thickeners.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledgment the support of the National Natural Science Foundation of China (U1762212).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra02069b

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