Polysiloxane thickeners and supercritical carbon dioxide thickening systems and methods of making

Abstract

The application belongs to the technical field of oil and gas stimulation, and provides a polysiloxane thickening agent and a supercritical carbon dioxide thickening system and a preparation method. The preparation raw materials of the polysiloxane thickening agent include single-terminated polydimethylsiloxane macromolecular initiators and polymerized monomers, and the polymerized monomers include acrylamide containing a temperature-sensitive unit and acrylate containing a tertiary amine group. The preparation raw materials of the supercritical carbon dioxide thickening system include, according to mass percentage, 3% to 5% of the polysiloxane thickening agent, 20% to 25% of water, 0.2% to 0.4% of an organic crosslinking agent, and the rest of carbon dioxide. The application can greatly improve the macroscopic viscosity of Sc-CO2 under a lower polysiloxane thickening agent dosage, and solves the problems of a large Sc-CO2 thickening agent dosage, insufficient thickening, and difficulty in meeting higher sand ratio sand fracturing.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas production enhancement technology, and relates to a polysiloxane thickener and a supercritical carbon dioxide thickening system and preparation method. Background Technology

[0002] Anhydrous fracturing technology, as a highly promising and forward-looking reservoir stimulation technology, has attracted much attention in the field of unconventional oil and gas development in recent years due to its advantages such as water conservation and environmental friendliness. Supercritical carbon dioxide (Sc-CO2) fracturing is one of the typical technologies of anhydrous fracturing, with advantages such as strong fracture creation capability, easy flowback, water conservation, no water-sensitive / water-locking damage to the reservoir, good matrix adsorption strength and methane replacement capability, and the ability to achieve carbon dioxide burial, showing great application potential. However, due to the low viscosity of Sc-CO2 alone (only 0.02~0.05 mPa·s), it is difficult to achieve proppant carrying and has a large filtration loss, which limits the development and application of Sc-CO2 fracturing technology. With the deepening research and understanding of Sc-CO2 thickening, introducing thickeners that can be solubilized in Sc-CO2 to significantly increase its viscosity has become the most direct and effective method to improve thickening ability. Siloxane thickeners have become a focus of attention due to their good compatibility with Sc-CO2, environmental friendliness, and significant thickening effect.

[0003] Chinese patent CN107236091B discloses a supercritical carbon dioxide thickener that can increase the viscosity of Sc-CO2 by 100 times. Chinese patent CN110862541B discloses an acrylate / siloxane polymer supercritical carbon dioxide thickener, which increases the viscosity of Sc-CO2 by up to 187 times with a 1 wt% thickener addition. Chinese patent CN110862543B discloses a hyperbranched hydrogen-containing siloxane supercritical carbon dioxide thickener, which increases the viscosity of Sc-CO2 by up to 205 times with a 1 wt% thickener addition. Chinese patent CN112679663B discloses a supercritical carbon dioxide thickener, which increases the viscosity of Sc-CO2 by up to 197 times with a 0.5 wt% thickener addition. Chinese patent CN112961360B discloses a method for preparing a polygonal cage-like siloxane supercritical carbon dioxide thickener, with a maximum Sc-CO2 viscosity increase of 256 times when 3 wt% thickener is added. Chinese patent documents CN116948159A, CN116948177A, CN116948178A, CN116948200A, CN116948201A, CN116987216A, CN117024746 and CN116874789B disclose a series of supercritical carbon dioxide thickeners that can be used for oil-based drill cuttings treatment, with Sc-CO2 viscosity increases of more than 188 times when 3 wt% or more thickener is added. Chinese patent application CN116082617A discloses a phenyl polyether carbonate, its preparation method, and its application as a supercritical carbon dioxide thickener, showing that adding 1 wt% of the thickener can increase the Sc-CO2 viscosity by up to 25 times. Chinese patent application CN117510699A discloses a liquid carbon dioxide thickener, showing that adding 4 wt% of the thickener can increase the Sc-CO2 viscosity to a maximum of 12.17 mPa·s. Chinese patent CN118126260B discloses a POSS-based hybrid supercritical CO2 thickener, showing that adding 5 wt% of the thickener can increase the Sc-CO2 viscosity to a maximum of 8.63 mPa·s at a pressure of 7.92 MPa.

[0004] In summary, existing technologies primarily aim to increase the viscosity of Sc-CO2 by improving the compatibility (i.e., the solubilizing ability of the thickener in Sc-CO2) and the inherent thickening properties of the thickener itself after solubilization. However, even with high thickener dosages exceeding 5%, the increase in Sc-CO2 viscosity does not reach more than 13 mPa·s, exhibiting problems such as large dosage, insufficient thickening, and difficulty in meeting the requirements of high sand-to-coal ratio fracturing. Therefore, there is an urgent need to develop thickeners and thickening systems with excellent compatibility with Sc-CO2 that can increase the viscosity of Sc-CO2 to the tens of millipascals per second after solubilization at lower dosages, in order to meet the practical needs of high sand-to-coal ratio fracturing. Summary of the Invention

[0005] To address the problems of existing Sc-CO2 thickeners, such as large dosage, insufficient viscosity increase, and difficulty in meeting the requirements for high sand ratios in sand fracturing, this invention provides a polysiloxane thickener, its supercritical carbon dioxide thickening system, and its preparation method.

[0006] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a polysiloxane thickener, the raw materials for which include: a mono-terminated polydimethylsiloxane macromolecular initiator and a polymeric monomer, wherein the polymeric monomer includes acrylamide containing a temperature-sensitive unit and acrylate containing a tertiary amine group; the mass ratio of the mono-terminated polydimethylsiloxane macromolecular initiator, the acrylamide containing a temperature-sensitive unit, and the acrylate containing a tertiary amine group is 1:(0.7~0.8):(0.3~0.4); the raw materials for preparing the mono-terminated polydimethylsiloxane macromolecular initiator include: mono-terminated polydimethylsiloxane, an acid-binding agent, and a halogenating agent; the mono-terminated polydimethylsiloxane is a monoamino-terminated polydimethylsiloxane, a monohydroxy-terminated polydimethylsiloxane, or a mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane.

[0007] Preferably, the acrylamide containing the temperature-sensitive unit is one or both of N-isopropylacrylamide and diacetone acrylamide.

[0008] Preferably, the acrylate containing a tertiary amine group is ethyl 2-(dimethylamino)acrylate, propyl 3-(dimethylamino)acrylate, ethyl 2-(diethylamino)acrylate, propyl 3-(diethylamino)acrylate, ethyl 2-(dimethylamino)methacrylate, propyl 3-(dimethylamino)methacrylate, ethyl 2-(diethylamino)methacrylate, or propyl 3-(diethylamino)methacrylate.

[0009] Preferably, the mass ratio of the single-end capped polydimethylsiloxane, the acid-binding agent, and the halogenated reagent is 1:(0.06~0.09):(0.05~0.08).

[0010] Preferably, the acid-binding agent is anhydrous triethanolamine, triethylamine, pyridine, 3,4-dimethylaminopyridine, or N,N-diisopropylethylamine.

[0011] Preferably, the halogenated reagent is 2-bromoisobutyryl bromide or 2-chloroisobutyryl chloride.

[0012] When the single-terminated polydimethylsiloxane is a monoamino-terminated polydimethylsiloxane, a monohydroxy-terminated polydimethylsiloxane, or a mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane and the halogenating agent is 2-bromoisobutyryl bromide or 2-chloroisobutyryl chloride, the structural formula of the single-terminated polydimethylsiloxane macromolecular initiator is: ,or ,or

[0013] Where a is an integer between 45 and 115.

[0014] When the single-terminated polydimethylsiloxane is a monoamino-terminated polydimethylsiloxane, a monohydroxy-terminated polydimethylsiloxane, or a mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane and the halogenating agent is 2-bromoisobutyryl bromide or 2-chloroisobutyryl chloride, the structural formula of the polysiloxane thickener is: ,or ,or ; Wherein, a is an integer from 45 to 115, b is an integer from 30 to 120, c is an integer from 15 to 60, d is an integer from 0 to 1, e is an integer from 0 to 1, f is an integer from 2 to 3, and g is an integer from 0 to 1; when the acrylamide containing the temperature-sensitive unit is N-isopropylacrylamide, d is 0; when the acrylamide containing the temperature-sensitive unit is diacetone acrylamide, d is 1; when the acrylate containing the tertiary amine group is ethyl 2-(dimethylamino)acrylate, e is 0, f is 2, and g is 0; when the acrylate containing the tertiary amine group is propyl 3-(dimethylamino)acrylate, e is 0, f is 3, and g is 0; when the acrylate containing the tertiary amine group is 2-(diethylamino)acrylate... When the acrylate is ethyl acrylate, e is 0, f is 2, and g is 1; when the acrylate containing the tertiary amine group is propyl 3-(diethylamino)methacrylate, e is 0, f is 3, and g is 1; when the acrylate containing the tertiary amine group is ethyl 2-(dimethylamino)methacrylate, e is 1, f is 2, and g is 0; when the acrylate containing the tertiary amine group is propyl 3-(dimethylamino)methacrylate, e is 1, f is 3, and g is 0; when the acrylate containing the tertiary amine group is ethyl 2-(diethylamino)methacrylate, e is 1, f is 2, and g is 1; when the acrylate containing the tertiary amine group is propyl 3-(diethylamino)methacrylate, e is 1, f is 3, and g is 1.

[0015] Secondly, the present invention provides a method for preparing the polysiloxane thickener, wherein the polysiloxane thickener is prepared by atom transfer radical polymerization (ATRP) method, comprising: A polydimethylsiloxane macromolecular initiator with a single end cap, an acrylamide containing a temperature-sensitive unit, an acrylate containing a tertiary amine group, and an organic ligand are mixed in an organic solvent, a transition metal catalyst is added, and a polymerization reaction is carried out at 75~90℃ to obtain a polysiloxane thickener.

[0016] Preferably, the preparation method of the polysiloxane thickener specifically includes the following steps: Step S01: Add organic solvent to the polymerization tube, then add the calculated amount of single-terminated polydimethylsiloxane macromolecular initiator, acrylamide containing temperature-sensitive units, acrylate containing tertiary amine groups, and organic ligand in sequence. Stir thoroughly to dissolve evenly, then perform liquid nitrogen freezing, vacuuming, thawing and degassing cycles three times, and then purge with high-purity inert gas for 15-20 minutes. Step S02: Under inert gas protection, add a transition metal catalyst into the polymerization tube, evacuate / charge the tube 2-3 times, seal the polymerization tube quickly, and place it in an oil bath at 75-90℃ for stirring and reaction for 12-15 hours. Step S03: After the reaction is completed, the polymerization tube is exposed to air, the reaction solution is diluted with an appropriate amount of tetrahydrofuran to terminate the polymerization reaction, the obtained diluted solution is eluted with a neutral alumina column, the obtained filtrate is collected and concentrated by rotary evaporation, the supernatant is removed by washing with cold methanol several times and the precipitate is collected, and the precipitate is dried in vacuum at 45~50℃ for 8~12h to obtain polysiloxane thickener.

[0017] Preferably, in the preparation method of the polysiloxane thickener, the organic solvent is one or more of anisole, trifluorotoluene, and isopropanol / water mixed solution (the volume ratio of isopropanol to water is 1:1); the mass ratio of the organic solvent to the single-terminated polydimethylsiloxane macromolecular initiator is (5~10):1; and the high-purity inert gas is one or two of high-purity nitrogen and high-purity argon.

[0018] Preferably, in the preparation method of the polysiloxane thickener, the transition metal catalyst is cuprous bromide or cuprous chloride. The mass ratio of the transition metal catalyst to the single-terminated polydimethylsiloxane macromolecular initiator is (0.03~0.04):1.

[0019] Preferably, in the preparation method of the polysiloxane thickener, the organic ligand is pentamethyldiethylenetriamine, hexamethyltriethylenetetramine, or tris(2-dimethylaminoethyl)amine, and the mass ratio of the organic ligand to the single-terminated polydimethylsiloxane macromolecular initiator is (0.04~0.05):1.

[0020] Preferably, the preparation method of the single-terminated polydimethylsiloxane macromolecular initiator is as follows: Step S1: Mix the single-end capped polydimethylsiloxane and the acid-binding agent in a solvent to obtain mixed solution A; Step S2: Under conditions of -3~5℃ and inert gas protection, a halogenated reagent is added dropwise to mixed solution A to obtain mixed solution B; Step S3: Heat the mixed solution B to 20~25℃ to carry out the reaction and obtain the reaction solution C; Step S4: Remove the solid from the reaction solution C. The resulting liquid is washed, dried, and concentrated to obtain concentrated solution D. Step S5: The concentrate D is repeatedly precipitated in methanol, and the resulting precipitate is dried to obtain a single-terminated polydimethylsiloxane macromolecular initiator.

[0021] Furthermore, in the preparation method of the single-terminated polydimethylsiloxane macromolecular initiator, in step S1, the solvent is anhydrous dichloromethane; the mass ratio of the single-terminated polydimethylsiloxane to anhydrous dichloromethane and the acid-binding agent is 1:(5~8):(0.06~0.09).

[0022] Furthermore, in the preparation method of the single-terminated polydimethylsiloxane macromolecular initiator, in step S2, the halogenated reagent is slowly added dropwise at a rate of 0.1~0.15 mL / min, and stirring is continued until the addition is complete; the mass ratio of the halogenated reagent to the single-terminated polydimethylsiloxane is (0.05~0.08):1.

[0023] Furthermore, in the preparation method of the single-end capped polydimethylsiloxane macromolecular initiator, in step S3, the reaction is carried out under stirring conditions, and the reaction time is 12~24h.

[0024] Furthermore, in the preparation method of the single-terminated polydimethylsiloxane macromolecular initiator, step S4 specifically involves: filtering the reaction solution C under reduced pressure to remove solids, taking out the filtrate and washing it repeatedly 3-5 times with 1 mol / L dilute hydrochloric acid, saturated sodium bicarbonate solution and saturated sodium chloride brine, and drying the obtained organic phase with anhydrous sodium sulfate and then concentrating it under reduced pressure in a rotary evaporator to obtain concentrated solution D; step S5 specifically involves: precipitating concentrated solution D repeatedly in an appropriate amount of cold methanol 5-6 times, and drying it under vacuum at 75-80℃ for 2-4 hours to obtain the single-terminated polydimethylsiloxane macromolecular initiator.

[0025] Thirdly, the present invention provides a supercritical carbon dioxide thickening system, the raw materials for which, by mass percentage, are: 3% to 5% of the polysiloxane thickener as described above, 20% to 25% of water, 0.2% to 0.4% of an organic crosslinking agent, and the balance being carbon dioxide.

[0026] Preferably, the organic crosslinking agent is adipic acid dihydrazide, N,N'-methylenebisacrylamide, or glutaraldehyde.

[0027] Fourthly, the present invention provides a method for preparing the supercritical carbon dioxide thickening system, comprising: Step S11: Disperse the polysiloxane thickener in water, add the organic crosslinking agent solution prepared in advance with the remaining water, and stir until a semi-transparent colloidal solution is formed. Step S12: Inject the semi-transparent colloidal solution into the reactor, fill the reactor with CO2 gas until the pressure reaches the preset pressure, and carry out the reaction at 50~55℃ to obtain the supercritical carbon dioxide thickening system.

[0028] Preferably, in the preparation method of the supercritical carbon dioxide thickening system, step S11 specifically involves: adding an appropriate amount of dilute hydrochloric acid to water to adjust the pH value to 4.5-5, weighing a calculated amount of polysiloxane thickener and slowly adding it to the water under vigorous stirring, continuing to stir for 30-40 minutes, then adding a calculated amount of organic crosslinking agent solution prepared in advance with the remaining water, and continuing to stir until a uniform semi-transparent colloidal solution is formed.

[0029] Preferably, in the preparation method of the supercritical carbon dioxide thickening system, step S12 specifically involves: injecting the translucent colloidal solution into a reaction vessel, introducing CO2 gas into the reaction vessel at room temperature until the pressure reaches 10 MPa, starting stirring and heating, gradually raising the temperature to 32°C, so that the translucent colloidal solution and CO2 are initially mixed to form a crude emulsion, continuing to pump in CO2 gas, raising the pressure and temperature to 15~20 MPa and 50~55°C respectively, and stirring the reaction at constant temperature and pressure for 2~3 hours to obtain the supercritical carbon dioxide thickening system.

[0030] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a polysiloxane thickener, prepared by atom transfer radical polymerization of acrylamide containing temperature-sensitive units and acrylate containing tertiary amine groups initiated by a single-terminated polydimethylsiloxane macromolecular initiator. The polysiloxane thickener can be used as a supercritical carbon dioxide thickener to improve the viscosity of Sc-CO2 by crosslinking with an organic crosslinking agent in water / Sc-CO2 microemulsions. Firstly, the nonpolar polydimethylsiloxane segments in the polysiloxane thickener possess CO2-loving properties. Utilizing the Sc-CO2-loving anchoring effect of the polydimethylsiloxane segments, the steric stability and dispersibility of the polysiloxane thickener in Sc-CO2 (i.e., ensuring good Sc-CO2 compatibility) can be improved, thereby enhancing the solubility of the polysiloxane thickener in Sc-CO2. Secondly, the polar acrylate segments containing tertiary amine groups in polysiloxane thickeners tend to aggregate at low temperatures to minimize contact with non-polar Sc-CO2, forming a water microdomain "core" (referred to as "water core"). The acrylamide segments containing temperature-sensitive units in polysiloxane thickeners tend to hydrophobically collapse after the temperature exceeds the lower critical solution temperature (LCST), thus synergistically stabilizing the interface with polydimethylsiloxane segments, reducing water core swelling, and increasing particle hardness. The organic crosslinking agent crosslinks the polysiloxane polymer chains into a nanogel network inside the water core, thereby significantly thickening and significantly increasing the viscosity of Sc-CO2. At the same time, the acrylate segments containing tertiary amine groups can provide steric hindrance, and the tertiary amine groups can provide pH responsiveness, assisting the crosslinking reaction and regulating hydrophilicity, preventing excessive particle aggregation and precipitation.

[0031] The preparation method of the polysiloxane thickener of the present invention introduces organic ligands to stabilize the metal center of the transition metal catalyst and adjust its redox potential, thereby precisely controlling the activity controllable characteristics of the polymerization reaction, making the polysiloxane thickener have a narrow molecular weight distribution and controllable structure.

[0032] Furthermore, in the preparation of single-end capped polydimethylsiloxane macromolecular initiators, "initiation sites" capable of ATRP polymerization are introduced into the polydimethylsiloxane chain ends by halogenating agents. Acidic byproducts generated in the reaction are neutralized by acid-binding agents, protecting the reactants and products and improving the yield of macromolecular initiators.

[0033] This invention utilizes the aforementioned polysiloxane thickener to form a water / Sc-CO2 microemulsion with water and an organic crosslinking agent. The organic crosslinking agent then forms an internal phase micro-crosslinked thickening system. Through a quadruple synergistic effect—the affinity for Sc-CO2 by non-polar polysiloxane segments, the thermosensitive hydrophobic effect of acrylamide segments containing thermosensitive units, the micro-crosslinking effect of the organic crosslinking agent, and the pH responsiveness and steric hindrance effect of acrylate segments containing tertiary amine groups—this system not only ensures excellent solubility of the polysiloxane thickener in Sc-CO2 but also significantly increases the macroscopic viscosity of Sc-CO2 with a relatively low dosage of polysiloxane thickener. This solves the problems of existing Sc-CO2 thickeners, such as high dosage, insufficient thickening, and difficulty in meeting the requirements of high sand-to-sand ratio fracturing. It has great potential for application in Sc-CO2 water-reduced / low-water fracturing. The present invention has at least the following technical effects: under pressure ≥15MPa and temperature ≥50℃, 3wt%~5wt% polysiloxane thickener can be completely and uniformly dissolved in Sc-CO2, so that the maximum viscosity of the supercritical carbon dioxide thickening system reaches 67.4mPa·s, which is 1347 times higher than that of pure Sc-CO2. Attached Figure Description

[0034] Figure 1 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 19-36 and Comparative Examples 1-3 with a thickener dosage of 3.0 wt%.

[0035] Figure 2 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 19-36 and Comparative Examples 7-9 with a thickener dosage of 3.0 wt%.

[0036] Figure 3 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 19-36 and Comparative Examples 13-15 with a thickener dosage of 3.0 wt%.

[0037] Figure 4 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 37-54 and Comparative Examples 4-6 with a thickener dosage of 5.0 wt%.

[0038] Figure 5 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 37-54 and Comparative Examples 10-12 with a thickener dosage of 5.0 wt%.

[0039] Figure 6 Viscosity comparison chart of the supercritical carbon dioxide thickening systems prepared in Examples 37-54 and Comparative Examples 16-18 with a thickener dosage of 5.0 wt%. Detailed Implementation

[0040] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0041] Unless otherwise specified, all chemical reagents and chemical products mentioned in this invention are known and commonly used chemical reagents and chemical products in the prior art; unless otherwise specified, the solutions in this invention refer to aqueous solutions with water as the solvent, for example, hydrochloric acid solution is an aqueous solution of hydrochloric acid; room temperature in this invention generally refers to a temperature between 15°C and 25°C, and is generally defined as 25°C.

[0042] The present invention will be further described below with reference to embodiments: (I) Preparation of polysiloxane thickener Example 1 The mass ratio of monoamino-terminated polydimethylsiloxane to 2-bromoisobutyryl bromide was 1:0.05. Under magnetic stirring, 10g of monoamino-terminated polydimethylsiloxane was added to a reaction vessel containing 50g of anhydrous dichloromethane and stirred thoroughly. Then, 0.6g of anhydrous triethanolamine was added and stirred until homogeneous. The reaction system was cooled to 0°C in an ice-salt bath. Under nitrogen protection, 0.5g of 2-bromoisobutyryl bromide was slowly added dropwise at a rate of 0.15mL / min using a constant pressure dropping funnel, and stirring was continued until the addition was complete. The ice-salt bath was removed, the temperature was raised to 25°C, and the reaction was stirred for 12h. After the reaction was completed, the resulting reaction solution was filtered under reduced pressure to remove solids. The filtrate was washed three times in succession with 1mol / L dilute hydrochloric acid, saturated sodium bicarbonate solution, and saturated sodium chloride brine. The resulting organic phase was dried with anhydrous sodium sulfate and concentrated under reduced pressure in a rotary evaporator. The concentrated solution was repeatedly precipitated five times in an appropriate amount of cold methanol. The precipitate was dried under vacuum at 75°C for 2h to obtain the monoamino-terminated polydimethylsiloxane macromolecular initiator.

[0043] The mass ratio of monoamino-terminated polydimethylsiloxane macroinitiator, N-isopropylacrylamide, and ethyl 2-(dimethylamino)acrylate was fixed at 1:0.7:0.3. 50g of anisole was added to a 100mL polymerization tube, followed by 10g of monoamino-terminated polydimethylsiloxane macroinitiator, 7g of N-isopropylacrylamide, 3g of ethyl 2-(dimethylamino)acrylate, and 0.4g of pentamethyldiethylenetriamine. The mixture was stirred thoroughly until dissolved and homogeneous. The tube was degassed three times using liquid nitrogen, followed by freezing, vacuuming, and thawing, and then bubbled with high-purity nitrogen for 15 minutes. Under high-purity nitrogen protection, 0.3g of cuprous bromide was rapidly added, followed by vacuuming / … After three gas-circulation cycles, the polymerization tube was quickly sealed and placed in a 90℃ oil bath for 12 hours of stirring. After the reaction, the tube was exposed to air, and the polymerization reaction was terminated by diluting the reaction solution with 25g of tetrahydrofuran. The diluted solution was eluted using a neutral alumina column, and the filtrate was collected and concentrated by rotary evaporation. The solution was washed repeatedly with cold methanol to remove the supernatant and the precipitate was collected. The precipitate was dried under vacuum at 50℃ for 8 hours to obtain the monoamino-terminated polysiloxane thickener PDMS-AG-1. Gel permeation chromatography (GPC), using polystyrene as a standard and tetrahydrofuran as the mobile phase, determined the number-average molecular weight of PDMS-AG-1 to be 12200 g / mol, the weight-average molecular weight to be 14000 g / mol, and the polydispersity index to be 1.15, indicating a narrow molecular weight distribution characteristic of ATRP (Action-Touch Reactive Polymerization).

[0044] Example 2 The only difference from Example 1 is that the monoamino-terminated polydimethylsiloxane is replaced with a monohydroxy-terminated polydimethylsiloxane, resulting in a monohydroxy-terminated polysiloxane thickener PDMS-AG-2.

[0045] Example 3 The only difference from Example 1 is that the monoamino-terminated polydimethylsiloxane is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, resulting in mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-3.

[0046] Example 4 The only difference from Example 1 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain a monoamino-terminated polysiloxane thickener PDMS-AG-4.

[0047] Example 5 The only difference from Example 2 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain a monohydroxy-terminated polysiloxane thickener PDMS-AG-5.

[0048] Example 6 The only difference from Example 3 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-6.

[0049] Example 7 The only difference from Example 1 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.4:0.3, resulting in a monoamino-terminated polysiloxane thickener PDMS-AG-7.

[0050] Example 8 The only difference from Example 2 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.4:0.3, resulting in a monohydroxy-terminated polysiloxane thickener PDMS-AG-8.

[0051] Example 9 The only difference from Example 3 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.4:0.3, resulting in a mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-9.

[0052] Example 10 The only difference from Example 1 is that the mass ratio of the monoamino-terminated polydimethylsiloxane macromolecular initiator, N-isopropylacrylamide, and ethyl 2-(dimethylamino)acrylate is 1:0.8:0.4, resulting in the monoamino-terminated polysiloxane thickener PDMS-AG-10.

[0053] Example 11 The only difference from Example 10 is that the monoamino-terminated polydimethylsiloxane is replaced with a monohydroxy-terminated polydimethylsiloxane, resulting in a monohydroxy-terminated polysiloxane thickener PDMS-AG-11.

[0054] Example 12 The only difference from Example 10 is that the monoamino-terminated polydimethylsiloxane is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, resulting in mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-12.

[0055] Example 13 The only difference from Example 10 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain a monoamino-terminated polysiloxane thickener PDMS-AG-13.

[0056] Example 14 The only difference from Example 11 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain a monohydroxy-terminated polysiloxane thickener PDMS-AG-14.

[0057] Example 15 The only difference from Example 12 is that N-isopropylacrylamide is replaced with diacetone acrylamide to obtain mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-15.

[0058] Example 16 The only difference from Example 10 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.5:0.3, resulting in a monoamino-terminated polysiloxane thickener PDMS-AG-16.

[0059] Example 17 The only difference from Example 11 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.5:0.3, resulting in a monohydroxy-terminated polysiloxane thickener PDMS-AG-17.

[0060] Example 18 The only difference from Example 12 is that N-isopropylacrylamide is replaced with a mixture of N-isopropylacrylamide and diacetone acrylamide in a mass ratio of 0.5:0.3, resulting in a mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-18.

[0061] (II) Preparation of supercritical carbon dioxide thickening system based on polysiloxane thickener Example 19 The mass percentages of fixed polysiloxane thickener, water, organic crosslinking agent, and carbon dioxide were 3%, 20%, 0.2%, and 76.8%, respectively. Add 15g of water to a beaker, and adjust the pH to 4.5 with 0.1mol / L dilute hydrochloric acid. Weigh 3g of monoamino-terminated polysiloxane thickener PDMS-AG-1 and slowly add it to the water under vigorous stirring. Continue stirring for 30 minutes, then add an aqueous solution containing 0.2g of adipic acid dihydrazide prepared with 5g of water. Continue stirring until a uniform semi-transparent colloidal solution is formed. Pour the entire semi-transparent colloidal solution into a sealed high-pressure reactor with a sapphire window. At room temperature, pressurize the reactor with CO2 gas until the pressure reaches 10MPa. Start stirring and heating, and gradually raise the temperature to 32℃ to allow the semi-transparent colloidal solution to initially mix with CO2 to form a water / Sc-CO2 microemulsion. Continue pumping CO2 gas to raise the pressure and temperature to 20MPa and 50℃, respectively. Stir and react at constant temperature and pressure for 3 hours to obtain the supercritical carbon dioxide thickening system Sc-ZCT-1.

[0062] Example 20 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-2, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-2.

[0063] Example 21 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-3, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-3.

[0064] Example 22 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-4, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-4.

[0065] Example 23 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-5, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-5.

[0066] Example 24 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-6, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-6.

[0067] Example 25 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-7, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-7.

[0068] Example 26 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-8, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-8.

[0069] Example 27 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-9, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-9.

[0070] Example 28 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-10, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-10.

[0071] Example 29 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-11, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-11.

[0072] Example 30 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-12, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-12.

[0073] Example 31 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-13, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-13.

[0074] Example 32 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-14, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-14.

[0075] Example 33 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with a mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-15, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-15.

[0076] Example 34 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-16, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-16.

[0077] Example 35 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-17, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-17.

[0078] Example 36 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-18, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-18.

[0079] Example 37 The only difference from Example 19 is that the mass percentages of polysiloxane thickener, water, organic crosslinking agent, and carbon dioxide are changed to 5%, 25%, 0.4%, and 69.6%, respectively, to obtain the supercritical carbon dioxide thickening system Sc-ZCT-19.

[0080] Example 38 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-2, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-20.

[0081] Example 39 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-3, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-21.

[0082] Example 40 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-4, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-22.

[0083] Example 41 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-5, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-23.

[0084] Example 42 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-6, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-24.

[0085] Example 43 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-7, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-25.

[0086] Example 44 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-8, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-26.

[0087] Example 45 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-9, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-27.

[0088] Example 46 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-10, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-28.

[0089] Example 47 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-11, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-29.

[0090] Example 48 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-12, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-30.

[0091] Example 49 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-13, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-31.

[0092] Example 50 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-14, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-32.

[0093] Example 51 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-15, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-33.

[0094] Example 52 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polysiloxane thickener PDMS-AG-16, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-34.

[0095] Example 53 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with the monohydroxy-terminated polysiloxane thickener PDMS-AG-17, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-35.

[0096] Example 54 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polysiloxane thickener PDMS-AG-18, resulting in the supercritical carbon dioxide thickening system Sc-ZCT-36.

[0097] Comparative Example 1 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-1.

[0098] Comparative Example 2 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-2.

[0099] Comparative Example 3 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-3.

[0100] Comparative Example 4 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-4.

[0101] Comparative Example 5 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-5.

[0102] Comparative Example 6 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, resulting in the thickening system DB-ZCT-6.

[0103] Comparative Example 7 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, and adipic dihydrazide is not added. The mass percentages of monoamino-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 3%, 20%, and 77%, respectively, to obtain the thickening system DB-ZCT-7.

[0104] Comparative Example 8 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, and adipic dihydrazide is not added. The mass percentages of monohydroxy-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 3%, 20%, and 77%, respectively, to obtain the thickening system DB-ZCT-8.

[0105] Comparative Example 9 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, and adipate dihydrazide is not added. The mass percentages of mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 3%, 20%, and 77%, respectively, to obtain the thickening system DB-ZCT-9.

[0106] Comparative Example 10 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, and adipic acid dihydrazide is not added. The mass percentages of monoamino-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 5%, 25%, and 70%, respectively, to obtain the thickening system DB-ZCT-10.

[0107] Comparative Example 11 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, and adipic dihydrazide is not added. The mass percentages of monohydroxy-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 5%, 25%, and 70%, respectively, to obtain the thickening system DB-ZCT-11.

[0108] Comparative Example 12 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, and adipate dihydrazide is not added. The mass percentages of mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, water, and carbon dioxide are changed to 5%, 25%, and 70%, respectively, to obtain the thickening system DB-ZCT-12.

[0109] Comparative Example 13 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, and water and adipic dihydrazide are not added. The mass percentages of monoamino-terminated polydimethylsiloxane and carbon dioxide are changed to 3% and 97%, respectively, to obtain the thickening system DB-ZCT-13.

[0110] Comparative Example 14 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, and water and adipate dihydrazide are not added. The mass percentages of monohydroxy-terminated polydimethylsiloxane and carbon dioxide are changed to 3% and 97%, respectively, to obtain the thickening system DB-ZCT-14.

[0111] Comparative Example 15 The only difference from Example 19 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, and water and adipate dihydrazide are not added. The mass percentages of mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane and carbon dioxide are changed to 3% and 97%, respectively, to obtain the thickening system DB-ZCT-15.

[0112] Comparative Example 16 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monoamino-terminated polydimethylsiloxane, and water and adipic acid dihydrazide are not added. The mass percentages of monoamino-terminated polydimethylsiloxane and carbon dioxide are changed to 5% and 95%, respectively, to obtain the thickening system DB-ZCT-16.

[0113] Comparative Example 17 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with monohydroxy-terminated polydimethylsiloxane, and water and adipic dihydrazide are not added. The mass percentages of monohydroxy-terminated polydimethylsiloxane and carbon dioxide are changed to 5% and 95%, respectively, to obtain the thickening system DB-ZCT-17.

[0114] Comparative Example 18 The only difference from Example 37 is that the monoamino-terminated polysiloxane thickener PDMS-AG-1 is replaced with mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane, and water and adipate dihydrazide are not added. The mass percentages of mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane and carbon dioxide are changed to 5% and 95%, respectively, to obtain the thickening system DB-ZCT-18.

[0115] (III) Viscosity testing of supercritical carbon dioxide thickening system The viscosity of the supercritical carbon dioxide thickening system was tested using an online high-pressure viscometer under constant temperature and pressure, and the appearance changes of the system (whether it became cloudy and phase separation occurred) were observed through a high-pressure viewing window.

[0116] The viscosity of the supercritical carbon dioxide thickening systems obtained in Examples 19-54 and the thickening systems obtained in Comparative Examples 1-18 were tested under constant pressure and temperature of 20 MPa and 50 °C using the above method, and their appearance was observed. The results are listed in Table 1.

[0117] Table 1. Viscosity test results of different thickening systems under 20 MPa pressure

[0118] As shown in Table 1 and Figures 1-3As shown in Examples 19-36, a single-terminated polydimethylsiloxane was prepared as a macromolecular initiator, and atom transfer radical polymerization was initiated between acrylamide (N-isopropylacrylamide and diacetone acrylamide) containing temperature-sensitive units and acrylate containing tertiary amine groups to obtain a polysiloxane thickener. This thickener was then formulated into a water / Sc-CO2 microemulsion and formed into an internal phase micro-crosslinked thickening system (Sc-ZCT-1~Sc-ZCT-18) using an organic crosslinking agent. The viscosity of the thickening system formed by 3.0 wt% polysiloxane thickener at 20 MPa and 50 °C reached 28.4~48.3 mPa·s, which is higher than that of the thickening systems obtained in Comparative Examples 1-3 (DB-ZCT-1~DB-ZCT-3, without the introduction of acrylamide containing temperature-sensitive units and acrylate containing tertiary amine groups). The viscosity is 26.9 mPa·s or higher (i.e., an increase of 1793% or more), which is 27.0 mPa·s or higher (i.e., an increase of 1929% or more) higher than the thickening systems obtained in Comparative Examples 7 to 9 (DB-ZCT-7 to DB-ZCT-9, without the introduction of acrylamide containing temperature-sensitive units, acrylate containing tertiary amine groups, and without the introduction of organic crosslinking agents), and 27.65 mPa·s or higher (i.e., an increase of 3687% or more) higher than the thickening systems obtained in Comparative Examples 13 to 15 (DB-ZCT-13 to DB-ZCT-15, without the introduction of acrylamide containing temperature-sensitive units, acrylate containing tertiary amine groups, and without the introduction of organic crosslinking agents and water), and 28.35 mPa·s or higher (i.e., an increase of 567 times or more) higher than pure supercritical carbon dioxide (0.05 mPa·s).

[0119] Similarly, as shown in Table 1 and Figures 4-6As shown in Examples 37-54, a single-ended polydimethylsiloxane was prepared as a macromolecular initiator, and a thermosensitive acrylamide (N-isopropylacrylamide and diacetone acrylamide) was initiated to undergo atom transfer radical polymerization with acrylate containing tertiary amine groups to obtain a polysiloxane thickener. This thickener was then formulated into a water / Sc-CO2 microemulsion and formed into an internal phase micro-crosslinked thickening system (Sc-ZCT-19~Sc-ZCT-36) using an organic crosslinking agent. The viscosity of the thickening system formed by 5.0 wt% polysiloxane thickener at 20 MPa and 50 °C reached 39.1~67.4 mPa·s, which is higher than that of the thickening systems obtained in Comparative Examples 4-6 (DB-ZCT-4~DB-ZCT-6, without the introduction of thermosensitive acrylamide and acrylate containing tertiary amine groups). The viscosity is 36.4 mPa·s or higher (i.e., an increase of 1348%), which is 36.6 mPa·s or higher than the thickening system obtained in Comparative Examples 10-12 (DB-ZCT-10-DB-ZCT-12, without the introduction of acrylamide containing temperature-sensitive units, acrylate containing tertiary amine groups, and without the introduction of organic crosslinking agents) (i.e., an increase of 1464%), and 38.12 mPa·s or higher than the thickener obtained in Comparative Examples 16-18 (DB-ZCT-16-DB-ZCT-18, without the introduction of acrylamide containing temperature-sensitive units, acrylate containing tertiary amine groups, and without the introduction of organic crosslinking agents and water) (i.e., an increase of 3890%), and 39.05 mPa·s or higher than pure supercritical carbon dioxide (0.05 mPa·s) (i.e., an increase of 781 times or more).

[0120] As shown in Table 1, the viscosity of the thickening systems in Comparative Examples 1-3 is basically the same as that in Comparative Examples 7-9. This means that without the introduction of acrylamide containing temperature-sensitive units or acrylate containing tertiary amine groups, the introduction of organic crosslinking agents has no significant effect on viscosity. This indicates that the organic crosslinking agents significantly improve viscosity mainly by promoting the crosslinking of acrylamide segments containing temperature-sensitive units and acrylate segments containing tertiary amine groups.

[0121] It can also be noted that the viscosity of the thickening system in Comparative Examples 13-15 is lower than that in Comparative Examples 7-9, indicating that the presence of water can increase viscosity. This is because the presence of water provides the fundamental conditions for the formation of the "water core", serving as a template for the self-assembly of the thickener and the physical basis for indirect thickening, and providing the necessary carrier for the temperature-sensitive response. This allows the hydrophilic temperature-sensitive units in the thickener to be located at the interface or shell of the water core. When the temperature changes, these acrylamide segments containing temperature-sensitive units interact with the water molecules in the water core, resulting in a hydrophilic-hydrophobic conformational transition.

[0122] The above results indicate that even under high temperature and high pressure conditions, the increase in supercritical carbon dioxide viscosity through single-end-capped polydimethylsiloxane alone is relatively small. However, by introducing acrylamides (N-isopropylacrylamide and diacetone acrylamide) containing thermosensitive units and acrylates containing tertiary amine groups to form amphiphilic block copolymers with single-end-capped polydimethylsiloxanes, and then preparing an internal phase micro-crosslinked thickening system in a water / Sc-CO2 microemulsion using an organic crosslinking agent, the viscosity of supercritical carbon dioxide can be significantly increased to the order of 50-60 millipascals / seconds through a quadruple synergistic effect: the Sc-CO2 anchoring effect of nonpolar polysiloxane segments + the thermosensitive hydrophobic effect of acrylamide segments containing thermosensitive units + the micro-crosslinking effect of organic crosslinking agents + the pH responsiveness and steric hindrance effect of acrylate segments containing tertiary amine groups.

[0123] Furthermore, as shown in Table 1, in Examples 19-54, a polysiloxane thickener was prepared by combining a single-terminated polydimethylsiloxane, acrylamide containing a temperature-sensitive unit (N-isopropylacrylamide and diacetone acrylamide), and acrylate containing a tertiary amine group. This thickener was then used to form an internal phase micro-crosslinked thickening system (Sc-ZCT-1~Sc-ZCT-36) in a water / Sc-CO2 microemulsion. The system exhibited a uniform and transparent appearance in supercritical carbon dioxide, indicating that the polysiloxane thickener was completely soluble in Sc-CO2. In contrast, the thickening system (DB-ZCT-1~DB-ZCT-18) formed by a single-terminated polydimethylsiloxane in a water / Sc-CO2 microemulsion or in pure Sc-CO2 all exhibited a turbid appearance, indicating that the thickener was not completely soluble in Sc-CO2. This indicates that the thickener can have excellent Sc-CO2 compatibility through the synergistic effect of the Sc-CO2 anchoring effect of the nonpolar polysiloxane segments and the acrylamide segments containing temperature-sensitive units and the acrylate segments containing tertiary amine groups.

[0124] The above data results indicate that the present invention's internal phase micro-crosslinked thickening system, obtained by introducing acrylamides (N-isopropylacrylamide and diacetone acrylamide) containing temperature-sensitive units and acrylates containing tertiary amine groups, forming an amphiphilic block copolymer thickener with a single-terminated polydimethylsiloxane, and then using an organic crosslinking agent in a water / Sc-CO2 microemulsion, exhibits excellent solubilizing ability in Sc-CO2. It can significantly increase the viscosity of Sc-CO2 to the order of fifty to sixty millipascals within a thickener dosage range of 3.0 wt% to 5.0 wt%, demonstrating significant thickening ability for Sc-CO2 and excellent compatibility.

[0125] Furthermore, Table 1 shows that the viscosity of the thickening systems in Examples 25-27 (using a mixture of N-isopropylacrylamide and diacetone acrylamide) is higher than that of the thickening systems in Examples 19-21 (using N-isopropylacrylamide) and Examples 22-24 (using diacetone acrylamide). This indicates that the combined use of N-isopropylacrylamide and diacetone acrylamide, both containing temperature-sensitive units, has a certain synergistic effect. With the same amount of acrylamide containing temperature-sensitive units, the combination of the two can achieve better results. The viscosity of the thickening systems in Examples 28-33 is higher than that in Examples 19-24. This is because the addition amounts of acrylamide containing temperature-sensitive units and acrylates containing tertiary amine groups were increased in Examples 28-33. This further demonstrates that the introduction of acrylamide containing temperature-sensitive units and acrylates containing tertiary amine groups is beneficial to increasing viscosity.

[0126] In summary, this invention prepares a single-terminated polydimethylsiloxane as a macromolecular initiator. This macromolecular initiator then initiates the atom transfer radical polymerization of acrylamide containing temperature-sensitive units and acrylate containing tertiary amine groups to prepare a polysiloxane thickener. This thickener is then formulated into a water / Sc-CO2 microemulsion and formed into an internal phase micro-crosslinked thickening system using an organic crosslinking agent. The nonpolar CO2-loving polydimethylsiloxane segments enhance the Sc-CO2 anchoring effect, improving the thickener's steric stability and dispersibility in Sc-CO2 (i.e., ensuring good Sc-CO2 compatibility), while the polar tertiary amine-containing acrylate... At low temperatures, ester segments tend to aggregate to minimize contact with nonpolar Sc-CO2, forming water cores. Acrylamide segments containing temperature-sensitive units tend to hydrophobically collapse above the lowest critical solution temperature (LCST), synergistically stabilizing the interface with polydimethylsiloxane segments, reducing water core swelling, and increasing particle hardness. Organic crosslinking agents crosslink polysiloxane polymer chains into a nanogel network within the water core, thereby significantly thickening the mixture. Simultaneously, acrylate segments containing tertiary amine groups provide steric hindrance, and tertiary amine groups provide pH responsiveness, assisting the crosslinking reaction and regulating hydrophilicity, preventing excessive particle aggregation and precipitation. Therefore, this invention, through the quadruple synergistic effect of "the Sc-CO2-loving anchoring effect of non-polar polysiloxane segments + the thermosensitive hydrophobic effect of acrylamide segments containing thermosensitive units + the micro-crosslinking effect of organic crosslinking agents + the pH responsiveness and steric hindrance effect of acrylate segments containing tertiary amine groups," not only ensures the excellent solubility of the thickener in Sc-CO2, but also significantly increases the macroscopic viscosity of Sc-CO2 with a lower thickener dosage. This solves the problems of existing Sc-CO2 thickeners, such as large dosage, insufficient thickening, and difficulty in meeting the requirements of high sand ratio fracturing. The following technical effects can be achieved: under pressure ≥15MPa and temperature ≥50℃, 3wt%~5wt% polysiloxane thickener can be completely and uniformly dissolved in Sc-CO2, making the highest viscosity of the supercritical carbon dioxide thickening system reach 48.3~67.4 mPa·s, which is 965~1347 times higher than that of pure Sc-CO2.

[0127] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polysiloxane thickener, characterized in that, The raw materials for its preparation include: a single-terminated polydimethylsiloxane macromolecular initiator and polymerizable monomers, wherein the polymerizable monomers include acrylamide containing a temperature-sensitive unit and acrylate containing a tertiary amine group; the mass ratio of the single-terminated polydimethylsiloxane macromolecular initiator, the acrylamide containing a temperature-sensitive unit, and the acrylate containing a tertiary amine group is 1:(0.7~0.8):(0.3~0.4); the raw materials for preparing the single-terminated polydimethylsiloxane macromolecular initiator include: single-terminated polydimethylsiloxane, acid-binding agent, and halogenating agent; the single-terminated polydimethylsiloxane is a monoamino-terminated polydimethylsiloxane, a monohydroxy-terminated polydimethylsiloxane, or a mono(2-(2-hydroxyethoxy)propyl)-terminated polydimethylsiloxane.

2. The polysiloxane thickener according to claim 1, characterized in that, The acrylamide containing the temperature-sensitive unit is one or both of N-isopropylacrylamide and diacetone acrylamide.

3. The polysiloxane thickener according to claim 1, characterized in that, The acrylate containing the tertiary amine group is ethyl 2-(dimethylamino)acrylate, propyl 3-(dimethylamino)acrylate, ethyl 2-(diethylamino)acrylate, propyl 3-(diethylamino)acrylate, ethyl 2-(dimethylamino)methacrylate, propyl 3-(dimethylamino)methacrylate, ethyl 2-(diethylamino)methacrylate, or propyl 3-(diethylamino)methacrylate.

4. The polysiloxane thickener according to claim 1, characterized in that, The mass ratio of the single-end capped polydimethylsiloxane, acid-binding agent, and halogenated reagent is 1:(0.06~0.09):(0.05~0.08).

5. The polysiloxane thickener according to claim 1, characterized in that, The acid-binding agent is anhydrous triethanolamine, triethylamine, pyridine, 3,4-dimethylaminopyridine, or N,N-diisopropylethylamine.

6. The polysiloxane thickener according to claim 1, characterized in that, The halogenated reagent is 2-bromoisobutyryl bromide or 2-chloroisobutyryl chloride.

7. The method for preparing the polysiloxane thickener according to any one of claims 1 to 6, characterized in that, include: A polydimethylsiloxane macromolecular initiator with a single end cap, an acrylamide containing a temperature-sensitive unit, an acrylate containing a tertiary amine group, and an organic ligand are mixed in an organic solvent, a transition metal catalyst is added, and a polymerization reaction is carried out at 75~90℃ to obtain a polysiloxane thickener.

8. The method for preparing the polysiloxane thickener according to claim 7, characterized in that, The preparation method of the single-terminated polydimethylsiloxane macromolecular initiator is as follows: A mixed solution A is obtained by mixing a single-terminated polydimethylsiloxane and an acid-binding agent in a solvent; Under conditions of -3 to 5°C and inert gas protection, a halogenated reagent is added dropwise to mixed solution A to obtain mixed solution B; The mixed solution B is heated to 20-25℃ to carry out the reaction, and the reaction solution C is obtained. After removing the solids from reaction solution C, the resulting liquid is washed, dried, and concentrated to obtain concentrated solution D. The concentrate D was repeatedly precipitated in methanol, and the resulting precipitate was dried to obtain a single-terminated polydimethylsiloxane macromolecular initiator.

9. The method for preparing the polysiloxane thickener according to claim 7, characterized in that, The transition metal catalyst is cuprous bromide or cuprous chloride.

10. The method for preparing the polysiloxane thickener according to claim 7, characterized in that, The organic ligand is pentamethyldiethylenetriamine, hexamethyltriethylenetetramine, or tris(2-dimethylaminoethyl)amine.

11. A supercritical carbon dioxide thickening system, characterized in that, The raw materials for its preparation, by mass percentage, include: 3% to 5% polysiloxane thickener, 20% to 25% water, 0.2% to 0.4% organic crosslinking agent, and the balance being carbon dioxide; the polysiloxane thickener is the polysiloxane thickener according to any one of claims 1 to 6.

12. The supercritical carbon dioxide thickening system according to claim 11, characterized in that, The organic crosslinking agent is adipic acid dihydrazide, N,N'-methylenebisacrylamide, or glutaraldehyde.

13. The method for preparing the supercritical carbon dioxide thickening system according to claim 11, characterized in that, include: Disperse the polysiloxane thickener in water, add the organic crosslinking agent solution prepared in advance with the remaining water, and stir until a semi-transparent colloidal solution is formed; A translucent colloidal solution is injected into a reaction vessel, and CO2 gas is introduced into the reaction vessel until the pressure reaches the preset pressure. The reaction is carried out at 50~55℃ to obtain the supercritical carbon dioxide thickening system.

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