CO2 thickening-downmixing-tracing nano carbon needle as well as preparation method and application thereof

Abstract

The preparation method of the CO2 thickening-downmixing-tracing nano carbon needle comprises the following steps: step 1, modifying a nitrogen-doped activated carbon quantum dot by adopting a silane coupling agent to obtain a modified activated carbon quantum dot; 2, mixing methyl silicone oil, a silane coupling agent and a thickening functional monomer to obtain a mixture, and activating; adding a catalyst and fully reacting to obtain a fluorine-containing siloxane polymer; 3, mixing the activated carbon quantum dots obtained in the step 1 and the fluorine-containing silane polymer solution obtained in the step 2, and fully reacting to obtain the CO2 thickening-downmixing-tracing nano carbon needle; according to the nano carbon needle, the viscosity of supercritical carbon dioxide is increased, the miscible phase pressure is reduced, meanwhile, in-situ emulsification tackifying can be carried out in high-water-content and high-permeability pores, and CO2 pressure driving-tracing-burying integration of an oil and gas field can be achieved, so that a win-win mode of green low-carbon development and economic benefits is constructed.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field exploitation, and particularly relates to a CO2 thickening - demixing - tracing nano - carbon needle, a preparation method and an application thereof. Background Art

[0002] In recent years, the supercritical carbon dioxide method (SC - CO2) for oil displacement has become a promising technology in reservoir development. It can not only be used for fracturing and oil displacement, but also achieve geological sequestration of CO2. SC - CO2 exhibits unique properties such as high density, low viscosity and high diffusivity. Compared with other fluids, it can achieve a higher penetration rate and enter pores better than water. The viscosity of supercritical carbon dioxide is usually between 0.02 and 0.5 mP·s. This relatively low viscosity causes some problems in the process of reservoir development. For example, the too - low viscosity leads to ineffective sand carrying during the fracturing process and difficulty in controlling fracture propagation, difficulties in miscibility during the mixing process with crude oil, a relatively high minimum miscibility pressure, and problems such as bypass flow and premature gas breakthrough in supercritical carbon dioxide flooding in heterogeneous reservoirs. How to increase the viscosity of supercritical carbon dioxide, reduce the miscibility pressure between it and crude oil, and solve the bypass flow problem in preferential channels is an urgent problem to be solved. Currently, the commonly used CO2 thickeners mainly include three types: fluorine - containing, siloxane - based, hydrocarbons and their derivatives.

[0003] For example, 202011130856 proposes a thickener for supercritical carbon dioxide flooding prepared by polymerizing fluorinated acrylate, acrylate containing tertiary amino group and styrene as polymerization monomers. This thickener has certain viscosity - increasing performance and a relatively low cloud point pressure, but it does not pay attention to the problems of heterogeneous bypass flow and breakthrough during the carbon dioxide flooding process. Summary of the Invention

[0004] The present invention provides a CO2 thickening - demixing - tracing nano - carbon needle, a preparation method and an application thereof in view of the problems existing in the prior art.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A preparation method of a CO2 thickening - demixing - tracing nano - carbon needle, comprising the following steps:

[0007] Step 1: Modify the nitrogen - doped activated carbon quantum dots with a silane coupling agent to obtain modified activated carbon quantum dots;

[0008] Step 2: Mix methyl silicone oil, silane coupling agent and thickening functional monomers to obtain a mixture, and activate it; after adding a catalyst and fully reacting, a fluorosilicone polymer is obtained; the mass ratio of methyl silicone oil, silane coupling agent and thickening functional monomers is 3 - 4:1 - 2:2.5 - 4;

[0009] Step 3: Mix the activated carbon quantum dots obtained in Step 1 and the fluorosilane polymer solution obtained in Step 2, and react fully to obtain CO2 thickening-decreasing mixing-tracer nano carbon needles; the mass ratio of the activated carbon quantum dots to the fluorosilane polymer is 1:2.

[0010] Furthermore, the preparation process of the nitrogen-doped activated carbon quantum dots in Step 1 is as follows:

[0011] React the organic carbon source and the nitrogen source by hydrothermal method or solid method to obtain nitrogen-doped activated carbon-based quantum dots;

[0012] wherein the mass ratio of the organic carbon source to the nitrogen source is 4-6:2-3.

[0013] Furthermore, the organic carbon source is one or two or more of sodium citrate, citric acid, lemon, glucose, cellulose, water-soluble chitosan, etc., mixed in any proportion;

[0014] The nitrogen source is composed of one or two or more of urea dimethylformamide, melamine, asparagine, arginine, ethylenediamine, benzylamine, dodecylamine, hexadecylamine, oleylamine, etc., mixed in any proportion;

[0015] In Step 1, the silane coupling agent is one of β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and γ-aminopropyltriethoxysilane;

[0016] The mass ratio of the activated carbon quantum dots to the silane coupling agent is 1-2:0.5-1.

[0017] Furthermore, in Step 2, the methyl silicone oil is composed of one or two or more of octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, etc., mixed in any proportion;

[0018] The silane coupling agent is composed of one or two of γ-aminopropyltrimethoxysilane and 1,3-bis(aminopropyl)tetramethyldisiloxane, mixed in any proportion;

[0019] The thickening functional monomer is composed of one or two or more of polytrifluoropropylmethylsiloxane, trifluoropropylmethylcyclotrisiloxane, 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane, methyl acrylate, methyl methacrylate, γ-butyrolactone, etc., mixed in any proportion.

[0020] Furthermore, the activation condition of the mixture in Step 2 is to react at 80-90 °C for 30-45 min.

[0021] Furthermore, the catalyst is obtained by mixing one or two or more of NH3, KOH, isopropanol, tetrabutyl titanate, etc., in any proportion;

[0022] In step 2, the reaction temperature is 120 - 130 °C and the reaction time is 30 - 45 min.

[0023] Furthermore, after the reaction in step 2 is completed, a capping agent is added to end the reaction. The capping agent is one or two or more of dimethyldimethoxysilane, trimethylsilane, cyclohexenyltriethoxysilane, silicon trifluoride, trichlorosilane, mixed in any proportion; after the reaction is completed, it is kept warm at 150 - 160 °C for 30 - 50 min.

[0024] Furthermore, in step 3, the reaction temperature is 70 - 80 °C and the reaction time is 4 - 4.5 h.

[0025] A CO₂ thickening - demixing - tracing nano - carbon needle.

[0026] An application of a CO₂ thickening - demixing - tracing nano - carbon needle, and the nano - carbon needle is used for supercritical carbon dioxide enhanced oil recovery.

[0027] The beneficial effects of the present invention are as follows:

[0028] (1) The nano - carbon needle obtained by the present invention can effectively increase the viscosity of supercritical carbon dioxide, reduce the miscibility pressure between supercritical carbon dioxide and crude oil, and improve the performance of supercritical carbon dioxide in the fields of enhanced oil recovery and fracturing;

[0029] (2) The nano - carbon needle obtained by the present invention, when used as a supercritical carbon dioxide thickening agent, while having the ability to thicken and reduce the miscibility pressure, can form a high - viscosity emulsion by in - situ emulsification with water, playing a role in profile control;

[0030] (3) The nano - carbon needle obtained by the present invention utilizes the photoluminescence of carbon quantum dots to monitor the flow path and distribution of fluids in the miscibility process in real time;

[0031] (4) The preparation method of the nano - carbon needle obtained by the present invention is simple, has high repeatability, is easy to produce on a large scale, and has broad industrial application prospects. Description of the Drawings

[0032] Figure 1 It is the infrared characterization diagram of the nano - carbon needle obtained in Example 1 of the present invention.

[0033] Figure 2 It is the transmission electron microscope diagram of the carbon quantum dots obtained in Comparative Example 1 of the present invention.

[0034] Figure 3 It is the SEM diagram of the nano - carbon needle obtained in Example 1 of the present invention.

[0035] Figure 4 It is the X - ray photoelectron spectrum of the carbon quantum dots obtained in Comparative Example 1 of the present invention.

[0036] Figure 5 These are the surface tension test results of the substances obtained in Examples 1-3 and Comparative Examples 1 and 2 of the present invention.

[0037] Figure 6 This is a schematic diagram of the emulsification of the nano-carbon needles obtained in Example 1 of the present invention.

[0038] Figure 7 This is the fluorescence characteristic diagram of the dispersant of the nano-carbon needles obtained in Example 1 of the present invention in different solvents.

[0039] Figure 8 This is a schematic diagram of the nano-carbon needles obtained in Example 1 of the present invention in a porous medium.

[0040] Figure 9 This is a schematic diagram of the nano-carbon needles obtained in Example 1 of the present invention in a porous medium. Detailed implementation manners

[0041] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0042] A preparation method of CO2 thickening-de-mixing-tracer nano-carbon needles includes the following steps:

[0043] Step 1: Modify the nitrogen-doped active carbon quantum dots with a silane coupling agent to obtain modified active carbon quantum dots;

[0044] The silane coupling agent is one of β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and γ-aminopropyltriethoxysilane; the weight average molecular weight is greater than 170 g / mol.

[0045] The mass ratio of the active carbon quantum dots to the silane coupling agent is 1-2:0.5-1. The modification temperature is 70-80 °C, and the reaction time is 4-4.5 h.

[0046] The preparation process of the nitrogen-doped active carbon quantum dots is as follows:

[0047] React the organic carbon source and the nitrogen source by hydrothermal method or solid method (one-step method) to obtain nitrogen-doped active carbon-based quantum dots; the organic carbon source is one or two or more of sodium citrate, citric acid, lemon, glucose, cellulose, water-soluble chitosan, etc. mixed in any proportion;

[0048] The nitrogen source is composed of one or two or more of urea dimethylformamide, melamine, asparagine, arginine, ethylenediamine, benzylamine, dodecylamine, hexadecylamine, oleylamine, etc. mixed in any proportion;

[0049] Among them, the mass ratio of the organic carbon source to the nitrogen source is 4-6:2-3.

[0050] The hydrothermal method process is as follows:

[0051] Seal the solution containing organic carbon source and nitrogen source in a polytetrafluoroethylene reaction kettle, and then seal the reaction kettle in a muffle furnace. Under the condition of a reaction temperature of 120 - 150 °C, react for 60 - 90 min. Filter, dialyze, purify, and dry the synthesized solid to obtain pure activated carbon quantum dot powder.

[0052] The solid-phase method process is as follows: Put the organic carbon source and nitrogen source into a crucible, grind and stir evenly, use nitrogen to remove the air in the muffle furnace, put the crucible into the muffle furnace and seal it. Under the condition of a reaction temperature of 120 - 180 °C, react for 45 - 60 min. Filter, dialyze, purify, and dry the synthesized solid to obtain pure activated carbon quantum dot powder.

[0053] Step 2: Mix methyl silicone oil, silane coupling agent, and thickening functional monomer to obtain a mixture, and activate it; add a catalyst based on ring-opening polymerization reaction and hydrosilylation reaction, and after sufficient reaction, obtain a fluorosilicone polymer; the mass ratio of methyl silicone oil, silane coupling agent, and thickening functional monomer is 3 - 4:1 - 2:2.5 - 4.

[0054] The methyl silicone oil is composed of one or two or more of octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecamethylcyclohexasiloxane mixed in any proportion; the weight-average molecular weight is greater than 290 g / mol.

[0055] The silane coupling agent is composed of one or two of γ-aminopropyltrimethoxysilane and 1,3-bis(aminopropyl)tetramethyldisiloxane mixed in any proportion; the weight-average molecular weight is greater than 170 g / mol.

[0056] The thickening functional monomer is composed of one or two or more of polytrifluoropropylmethylsiloxane, trifluoropropylmethylcyclotrisiloxane, 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane, methyl acrylate, methyl methacrylate, and γ-butyrolactone mixed in any proportion. The weight-average molecular weight is greater than 190 g / mol.

[0057] The activation condition is under the condition of 80 - 90 °C, react for 30 - 45 min. The catalyst is composed of one or two or more of NH3, KOH, isopropanol, and tetrabutyl titanate mixed in any proportion; the catalyst dosage is 0.02 - 0.05% of the mass of the fluorosilicone polymer (DPS).

[0058] The reaction temperature is 120 - 130 °C, and the reaction time is 30 - 45 min.

[0059] After the reaction is completed, a capping agent is added to terminate the reaction. The capping agent is one or two or more of dimethyldimethoxysilane, trimethylsilane, cyclohexenyltriethoxysilane, silicon trifluoride, trichlorosilane, mixed in any proportion; after the reaction is completed, it is kept warm at 150-160 °C for 30-50 min to deactivate the catalyst. The dosage of the capping agent is 0.01-0.05% of the mass of the fluorosilicone polymer (DPS).

[0060] Step 3: Mix the activated carbon quantum dots obtained in Step 1 and the fluorosilane polymer solution obtained in Step 2, and carry out a nucleophilic ring-opening reaction to obtain CO2 thickening-decreasing mixing-tracer nano carbon needles; the mass ratio of the activated carbon quantum dots to the fluorosilane polymer is 1:2. The reaction temperature is 70-80 °C, and the reaction time is 4-4.5 h.

[0061] The obtained CO2 thickening-decreasing mixing-tracer nano carbon needles are used for supercritical carbon dioxide flooding, as a thickening agent, a decreasing mixing agent, and a tracer.

[0062] The usage process is as follows:

[0063] 1) Use supercritical CO2 to prepare a CO2 thickening-decreasing mixing-tracer multifunctional nano carbon needle with a mass concentration of 1-1.5 wt.%, and stir to make it fully dispersed in supercritical CO2.

[0064] 2) Inject 0.1-0.3% times the pore volume of the target reservoir of the CO2 thickening-decreasing mixing-tracer multifunctional nano carbon needle into the formation.

[0065] 3) After injecting the CO2 thickening-decreasing mixing-tracer multifunctional nano carbon needle, inject a certain volume of oilfield injection water slug for displacement.

[0066] Example 1

[0067] A preparation method of CO2 thickening-decreasing mixing-tracer nano carbon needles includes the following steps:

[0068] Step 1: Put 20 g of citric acid and 10 g of dodecylamine into a crucible and stir evenly. Use nitrogen to remove the air in the muffle furnace, put the crucible into the muffle furnace and seal it, set the reaction temperature to 180 °C, and the reaction time to 45 min; filter, dialyze, purify, and dry the synthesized solid to obtain pure nitrogen-doped activated carbon quantum dot powder.

[0069] Add 15 g of coupling agent γ-aminopropyltriethoxysilane and 30 g of absolute ethanol to a three-necked flask, then add the activated carbon quantum dot powder to the three-necked flask and stir evenly. Carry out a condensation reaction at 70 °C in an oil bath for 4 h to obtain coupling agent-modified activated carbon quantum dots, and obtain coupling agent-modified activated carbon quantum dot powder through filtration, dialysis, purification, and drying.

[0070] Step 2: Add 30 g of octamethylcyclotetrasiloxane, 10 g of γ-aminopropyltrimethoxysilane, and 25 g of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane into a three-necked flask, stir evenly, heat up to 80 °C in an oil bath, and activate for 45 min; add 0.04 g of isopropanol into the three-necked flask, raise the temperature to 90 °C and catalyze for 30 min, then continue to raise the temperature to 120 °C and react for 4 h; finally, add 0.03 g of dimethyldimethoxysilane into the three-necked flask and react for 60 min, then raise the oil bath temperature to 160 °C for 30 min to obtain a fluorosilicone polymer.

[0071] Step 3: Stir evenly the activated carbon quantum dot powder modified with the coupling agent obtained in Step 1 and the fluorosilicone polymer obtained in Step 2, lower the temperature to 70 °C and react for 4 h to obtain the final product, a CO2 thickening-de-mixing-tracer multifunctional nano-carbon needle.

[0072] Example 2

[0073] A preparation method of a CO2 thickening-de-mixing-tracer nano-carbon needle includes the following steps:

[0074] Step 1: Put 20 g of sodium citrate and 15 g of asparagine into a crucible, stir evenly, use nitrogen to remove the air in the muffle furnace, put the crucible into the muffle furnace and seal it, set the reaction temperature to 120 °C, and the reaction time to 60 min; filter, dialyze, purify, and dry the synthesized solid to obtain a pure nitrogen-doped activated carbon quantum dot powder.

[0075] Add 12.5 g of coupling agent γ-aminopropyltriethoxysilane and 30 g of absolute ethanol into a three-necked flask, then add the activated carbon quantum dot powder into the three-necked flask and stir evenly, carry out a condensation reaction at 80 °C in an oil bath for 4.5 h to obtain an activated carbon quantum dot modified with the coupling agent, and obtain an activated carbon quantum dot powder modified with the coupling agent through filtration, dialysis, purification, and drying.

[0076] Step 2: Add 40 g of dodecamethylcyclohexasiloxane, 10 g of 1,3-bis(aminopropyl)tetramethyldisiloxane, and 40 g of trifluoropropylmethylcyclotrisiloxane into a three-necked flask, stir evenly, heat up to 90 °C in an oil bath, and activate for 30 min; add 0.04 g of isopropanol into the three-necked flask, raise the temperature to 90 °C and catalyze for 30 min, then continue to raise the temperature to 130 °C and react for 4 h; finally, add 0.03 g of dimethyldimethoxysilane into the three-necked flask and react for 60 min, then raise the oil bath temperature to 150 °C for 50 min to obtain a fluorosilicone polymer.

[0077] Step 3: Take the raw materials according to the mass ratio of 1:2 of the coupling agent-modified activated carbon quantum dot powder obtained in Step 1 and the fluorosiloxane polymer obtained in Step 2, stir evenly, lower the temperature to 80 °C and react for 4.5 h to obtain the final product, the CO2 thickening-decreasing mixing-tracer multifunctional nano carbon needle.

[0078] Example 3

[0079] A preparation method of CO2 thickening-decreasing mixing-tracer nano carbon needle, comprising the following steps:

[0080] Step 1: Put 30 g of cellulose and 10 g of asparagine into a crucible and stir evenly. Use nitrogen to remove the air in the muffle furnace, put the crucible into the muffle furnace and seal it. Set the reaction temperature to 150 °C and the reaction time to 70 min; Filter, dialyze, purify and dry the synthesized solid to obtain pure nitrogen-doped activated carbon quantum dot powder.

[0081] Add 10 g of coupling agent γ-aminopropyltriethoxysilane and 30 g of absolute ethanol to a three-necked flask, then add the activated carbon quantum dot powder to the three-necked flask and stir evenly. Carry out a condensation reaction at 75 °C in an oil bath for 4.5 h to obtain the coupling agent-modified activated carbon quantum dot. Filter, dialyze, purify and dry to obtain the coupling agent-modified activated carbon quantum dot powder.

[0082] Step 2: Add 30 g of dodecamethylcyclohexasiloxane, 20 g of 1,3-bis(aminopropyl)tetramethyldisiloxane, and 40 g of polytrifluoropropylmethylsiloxane to a three-necked flask and stir evenly. Heat the oil bath to 85 °C and activate for 30 min; Add 0.04 g of isopropanol to the three-necked flask, raise the temperature to 90 °C and catalyze for 30 min, then continue to raise the temperature to 130 °C and react for 4 h; Finally, add 0.03 g of dimethyldimethoxysilane to the three-necked flask and react for 60 min, then raise the oil bath temperature to 155 °C and keep it for 40 min to obtain the fluorosiloxane polymer.

[0083] Step 3: Take the raw materials according to the mass ratio of 1:2 of the coupling agent-modified activated carbon quantum dot powder obtained in Step 1 and the fluorosiloxane polymer obtained in Step 2, stir evenly, lower the temperature to 75 °C and react for 4.5 h to obtain the final product, the CO2 thickening-decreasing mixing-tracer multifunctional nano carbon needle.

[0084] Comparative Example 1

[0085] Step 1: Put 20 g of citric acid and 10 g of dodecylamine into a crucible and stir evenly. Use nitrogen to remove the air in the muffle furnace, put the crucible into the muffle furnace and seal it. Set the reaction temperature to 180 °C and the reaction time to 60 min; Filter, dialyze, purify and dry the synthesized solid to obtain pure nitrogen-doped activated carbon quantum dot powder.

[0086] Add 15 g of coupling agent γ-aminopropyltriethoxysilane and 30 g of absolute ethanol into a three-necked flask, then add the activated carbon quantum dot powder into the three-necked flask and stir evenly. Carry out the condensation reaction at 70 °C in an oil bath for 4 h to obtain the activated carbon quantum dots modified by the coupling agent. The activated carbon quantum dot powder modified by the coupling agent is obtained through suction filtration, dialysis, purification, and drying.

[0087] Comparative Example 2

[0088] Add 30 g of octamethylcyclotetrasiloxane, 10 g of γ-aminopropyltrimethoxysilane, and 25 g of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane into a three-necked flask and stir evenly. Heat up the oil bath to 80 °C and activate for 45 min; add 0.04 g of isopropanol into the three-necked flask, raise the temperature to 90 °C and catalyze for 30 min, then continue to raise the temperature to 120 °C and react for 4 h; finally, add 0.03 g of dimethyldimethoxysilane into the three-necked flask and react for 60 min, then raise the oil bath temperature to 150 - 160 °C for 30 - 50 min to obtain the fluorosilicone polymer.

[0089] Perform Fourier transform infrared spectroscopy characterization on the CO₂ thickening-decreasing mixing-tracer multifunctional nano-carbon needles obtained in Example 1, and the results are as Figure 1 shown. It can be seen from the figure that the absorption peak at 797.85 cm -1 corresponds to the stretching vibration of the Si-C bond, which is a common vibration type in organosilicon compounds; the absorption peak corresponding to the range of 1000 - 1200 cm -1 corresponds to the stretching vibration of the Si-O-Si bond; the absorption peak at 1208 cm -1 corresponds to the stretching vibration of the C-O bond; the absorption peak at 1261 cm -1 corresponds to the stretching vibration of the C-F bond; the absorption peak at 2969 cm -1 corresponds to the stretching vibration of the C-H bond in CH₃; the absorption peak at 3337 cm -1 corresponds to the stretching vibration of the N-H bond in CH₃; the appearance of the above chemical characteristic peaks indicates the successful synthesis of the quantum tracer nano-carbon needles.

[0090] Characterize the carbon quantum dots synthesized in the comparative example by transmission electron microscopy, and the results are as Figure 2 shown. It can be seen from the figure that the size of the carbon quantum dots is within 10 nm.

[0091] Characterize the nano-carbon needles synthesized in the example by scanning electron microscopy, and the results are as Figure 3 shown. It can be seen from the figure that the size of the CO₂ thickening-decreasing mixing-tracer multifunctional nano-carbon needles grafted with the thickening unit becomes larger, proving the successful synthesis of the material.

[0092] Figure 4 X-ray photoelectron spectroscopy of the carbon quantum dots obtained in Comparative Example 1 of the present invention. It can be seen from the figure that the full-spectrum analysis shows the same three elements: C1s (285.08 eV), N1s (400.08 eV), and O1s (532.08 eV). The peak-fitting analysis shows that the forms of carbon existence are carbon-carbon single bonds, carbon-carbon double bonds (284.5 eV), and carbon-oxygen double bonds (287.8 eV). The form of oxygen existence is carbon-oxygen double bonds, indicating that the material contains carboxyl groups or carbonyl groups. The form of nitrogen existence is pyrrole N (399.7 eV). This characterization can illustrate the chemical structure and composition of the quantum dots and also prove that nitrogen atoms have been successfully doped into the active carbon quantum dots.

[0093] The systems prepared in Examples 1-3 and Comparative Examples 1-2 were added to supercritical carbon dioxide at a concentration of 1% to form a mixture. The viscosity was measured using a high-temperature and high-pressure resistant closed Haake rheometer. The test temperature was 45 °C and the test pressure was 10 Mpa. Before the test, a low-speed pre-shear was performed for 10 min to make the CO2 thickening-de-mixing-tracer multifunctional carbon nanoneedles and supercritical carbon dioxide mix evenly, and then the rotation speed was increased to 170 S -1 The shear viscosity was measured. Pure supercritical carbon dioxide was set as the control group, and the test results are shown in Table 1.

[0094] It can be seen from the table that the thickener obtained in Example 1 has the best thickening effect on supercritical carbon dioxide, with a viscosity reaching 9.18 mPa·s and a viscosity ratio reaching 306. The viscosity ratios of the thickeners obtained in Example 2 and Example 3 also exceed 240. The viscosity ratio of the supercritical carbon dioxide mixed with pure quantum dots is 63, indicating that the quantum dots have a relatively small effect on the thickening performance of supercritical carbon dioxide. The viscosity ratio of the fluorosiloxane polymer is 148, indicating that the viscosity ratio of the nanocarbon needles prepared by mixing nitrogen-doped carbon quantum dots and fluorosiloxane polymers is significantly increased. There is a synergistic effect between nitrogen-doped carbon quantum dots and fluorosiloxane polymers, and neither of them can be absent. Each substance is indispensable for the thickening performance and has an indispensable influence.

[0095] Table 1. Test results of viscosity performance

[0096]

[0097] The systems prepared in Examples 1-3 and Comparative Examples 1-2 were dissolved in crude oil at a concentration of 1%. The surface tension of the oil droplets of supercritical CO2 under different pressure conditions was measured using a high-temperature and high-pressure interfacial tensiometer at a test temperature of 45 °C. The linear relationship curve between the interfacial tension and the pressure was extrapolated to the pressure when the interfacial tension was zero, which is the minimum miscibility pressure between the crude oil and supercritical CO2. Pure supercritical carbon dioxide was set as the control group, and the test results are shown in Table 2.

[0098] The results are similar to those of the thickening test. The thickening agent obtained in Example 1 has the best effect on reducing the miscibility pressure, and the reduction degree is 26.8%. The reduction degree of the fluorosiloxane polymer for the miscibility pressure is 2.7%. The effect of carbon quantum dots on reducing the miscibility pressure is also limited. The effect of reducing the miscibility pressure is brought about by the synergy of the two.

[0099] Table 2. Miscibility pressure test results

[0100]

[0101] The systems prepared in Examples 1-3 and Comparative Examples 1-2 were dissolved in crude oil at a concentration of 1%, and a high-temperature and high-pressure interfacial tensiometer was used to measure the surface tension between supercritical CO2 and oil droplets. The test temperature was 45 °C and the test pressure was 8 MPa. The surface tension of the crude oil without adding the system was set as the control group, and the test results are as Figure 5 shown.

[0102] It can be seen from the figure that the surface tension between supercritical carbon dioxide and crude oil is 24.1 mN / m. The interfacial activity of the nano-carbon needles obtained in Example 1 is the highest, and the surface tension is reduced to 20.2 mN / m. The higher interfacial activity makes the miscibility pressure of this system the lowest. Activated carbon quantum dots can also reduce the surface tension to a certain extent due to their amphiphilic structure, but this is not the most important factor; the synergy of the two is required to obtain the desired result.

[0103] The systems prepared in Examples 1-3 and Comparative Examples 1-2 were dissolved in white oil at a concentration of 1%, and an emulsification experiment was carried out using a homogenizing stirrer at 45 °C. After emulsification, the viscosity of the emulsion was measured. After measuring, CO2 was introduced from the bottom of the emulsion for a period of time, and the viscosity of the emulsion was measured again. The salinity of the water body used in the experiment was 10×10 4 mg / L. The white oil without adding the system was set as the control group, and the experimental data are shown in Table 3.

[0104] It can be seen from the table that when the water cut is 60%, the viscosity of the water-in-oil emulsion formed after emulsification with the thickening agent obtained in Example 1 is 121.2 mPa·s, and the viscosity rises sharply to 534.1 mPa·s after introducing carbon dioxide. As Figure 6 shown, it was found that the emulsion hung on the wall after inversion after emulsification. This thickening agent system showed excellent emulsifying and thickening properties in a carbon dioxide environment. As the water cut increased, the viscosity of the emulsion decreased to a certain extent. When the water cut was as high as 90%, the viscosity of the emulsion still remained at 79.1 mPa·s, and the viscosity rose sharply to 351.8 mPa·s after introducing carbon dioxide, indicating that this system could still exhibit excellent emulsifying and regulating effects under high water cut conditions, effectively solving the problem of fingering breakthrough in the process of supercritical carbon dioxide flooding in heterogeneous reservoirs.

[0105] Table 3. Emulsification test results

[0106]

[0107] The system prepared in Example 1 was dissolved in different solvents at a concentration of 1%, and from Figure 7 it can be seen that the system is insoluble in water, has good dissolution and dispersion properties in non-polar solvents such as methane and kerosene, and shows fluorescence characteristics under the irradiation of an ultraviolet lamp. The system prepared in Example 1 was dissolved in white oil at a concentration of 1% and injected into a microscopic model. Comparing Figure 8 with the pictures observed under an ordinary microscope, Figure 9 under the observation of a metallurgical microscope, the system exhibits excellent fluorescence characteristics in porous media, can accurately reflect the distribution characteristics of fluids in porous media, indicating that the modified CO2 thickening-decreasing miscibility-tracer multi-functional nano-carbon needles also have the same tracer characteristics as activated carbon quantum dots, which shows the tracer feasibility of nano-carbon needles in the oil displacement process.

[0108] The CO2 thickening-decreasing miscibility-tracer multi-functional nano-carbon needles obtained in the present invention have good dispersion properties in non-polar solvents such as CO2. This thickener effectively increases the viscosity of supercritical CO2. On the one hand, due to the fluorosiloxane polymer obtained in the present invention forming a special three-dimensional network structure with quantum dots as cross-linking sites, capturing CO2 molecules and weakening their flow characteristics, thereby increasing the viscosity of the fluid. The fluoropolymer obtained in the present invention makes the molecular chains of the thickener more soluble in supercritical CO2 by reducing the cohesive energy, and the mutual cross-winding between the molecular chains also increases the fluid viscosity. The mechanism by which this thickener reduces the miscibility pressure between supercritical CO2 and crude oil is that the strongly negatively charged fluorine atoms can generate a lower interfacial tension at the liquid-liquid interface, thereby reducing the miscibility pressure of supercritical CO2. When this thickener encounters water in high-permeability pores, in-situ emulsification and viscosity increase will occur, effectively preventing the problem of gas breakthrough during the supercritical CO2 oil displacement process.

[0109] Traditional CO2 thickeners mainly focus on increasing the viscosity of supercritical carbon dioxide and reducing the miscibility pressure, but fail to effectively solve the problem of gas breakthrough in high-permeability pores, and cannot monitor the miscibility distribution of fluids in the reservoir in real time during use. While increasing the viscosity of supercritical carbon dioxide and reducing the miscibility pressure, the CO2 thickening-decreasing miscibility-tracer multi-functional nano-carbon needles can also undergo in-situ emulsification and viscosity increase in high-water-cut and high-permeability pores, thus more effectively preventing gas breakthrough. The photoluminescence characteristics of carbon quantum dots enable them to monitor the flow path of fluids and the water flooding dominant channels in the miscibility process in real time, providing transparent and reliable data support for oilfield exploitation. In addition, carbon quantum dots overcome the disadvantages of traditional tracers such as large injection volume, high cost, and great environmental harm.

Claims

1. A preparation method of CO2 thickening-de-mixing-tracer nano-carbon needles, characterized in that, It includes the following steps: Step 1: Modify the nitrogen-doped activated carbon quantum dots with a silane coupling agent to obtain modified activated carbon quantum dots; Step 2: Mix methyl silicone oil, a silane coupling agent, and a thickening functional monomer to obtain a mixture, and activate it; after adding a catalyst and fully reacting, a fluorosilicone polymer is obtained; the mass ratio of methyl silicone oil, the silane coupling agent, and the thickening functional monomer is 3-4:1-2:2.5-4; Step 3: Mix the activated carbon quantum dots obtained in Step 1 and the fluorosilane polymer solution obtained in Step 2, and fully react to obtain CO2 thickening-decreasing mixing-tracer nano carbon needles; the mass ratio of the activated carbon quantum dots to the fluorosilane polymer is 1:

2.

2. The preparation method of a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, characterized in that The preparation process of the nitrogen-doped activated carbon quantum dots in Step 1 is as follows: React an organic carbon source and a nitrogen source by a hydrothermal method or a solid method to obtain nitrogen-doped activated carbon-based quantum dots; Wherein the mass ratio of the organic carbon source to the nitrogen source is 4-6:2-3.

3. The preparation method of a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 2, characterized in that, The organic carbon source is one or two or more of sodium citrate, citric acid, lemon, glucose, cellulose, and water-soluble chitosan, mixed in any proportion; The nitrogen source is composed of one or two or more of urea dimethylformamide, melamine, asparagine, arginine, ethylenediamine, benzylamine, dodecylamine, hexadecylamine, and oleylamine, mixed in any proportion; In Step 1, the silane coupling agent is one of β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and γ-aminopropyltriethoxysilane; The mass ratio of the activated carbon quantum dots to the silane coupling agent is 1-2:0.5-1.

4. A method for preparing a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, characterized in that, In Step 2, the methyl silicone oil is composed of one or two or more of octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecamethylcyclohexasiloxane, mixed in any proportion; The silane coupling agent is composed of one or two of γ-aminopropyltrimethoxysilane and 1,3-bis(aminopropyl)tetramethyldisiloxane, mixed in any proportion; The thickening functional monomer is composed of one or two or more of polytrifluoropropylmethylsiloxane, trifluoropropylmethylcyclotrisiloxane, 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane, methyl acrylate, methyl methacrylate, and γ-butyrolactone, mixed in any proportion.

5. A method for preparing a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, characterized in that, The activation condition of the mixture in Step 2 is at 80-90 °C for 30-45 min.

6. The preparation method of a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, wherein, The catalyst is obtained by mixing one or two or more of NH3, KOH, isopropanol, and tetrabutyltitanium in any proportion; In Step 2, the reaction temperature is 120-130 °C, and the reaction time is 30-45 min.

7. A method for preparing a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, characterized in that, After the reaction in Step 2, a capping agent is added to end the reaction. The capping agent is composed of one or two or more of dimethyldimethoxysilane, trimethylsilane, cyclohexenyltriethoxysilane, silicon trifluoride, and trichlorosilane, mixed in any proportion; after the reaction ends, it is kept warm at 150-160 °C for 30-50 min.

8. A method for preparing a CO2 thickening-de-mixing-tracer nano-carbon needle according to claim 1, characterized in that, In Step 3, the reaction temperature is 70-80 °C, and the reaction time is 4-4.5 h.

9. The CO2 thickening-decreasing mixing-tracer nano carbon needles obtained by the preparation method according to any one of claims 1-8.

10. The application of a CO2 thickening-de-mixing-tracer nano-carbon needle as described in claim 9, characterized in that, The nano-carbon needles are used for enhanced oil recovery by supercritical carbon dioxide.

Images