Multifunctional amphiphilic carbon dots, nanofluid, and preparation method and application thereof

Abstract

This invention provides a multifunctional amphiphilic carbon dot, a nanofluid, its preparation method, and its application, belonging to the field of oilfield chemistry. The multifunctional amphiphilic carbon dot is obtained by a solvothermal reaction of a carbon skeleton precursor, a heteroatom source, a hydrophobic modifier, and a solvent. The carbon skeleton precursor is a small organic molecule containing carboxyl and / or hydroxyl groups that is easily carbonized. The heteroatom source is selected from organic compounds rich in nitrogen and / or sulfur atoms. The hydrophobic modifier is a surfactant with a long carbon chain of C8-C22. Through ingenious molecular structure design, this invention integrates amphiphilic oil displacement, heteroatom adsorption and corrosion inhibition, and quantum dot fluorescence tracing functions onto a single nanoscale carbon dot particle, constructing a "one-agent-multi-functional" intelligent nanofluid. This achieves efficient, safe, and intelligent synergy in the CO2 oil displacement process under complex reservoir environments.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical technology, specifically to a multifunctional amphiphilic carbon dot, nanofluid, its preparation method, and its application. Background Technology

[0002] With the dual challenges of increasing global energy demand and pressure to reduce carbon emissions, CO2 flooding technology, which combines carbon capture, utilization, and storage (CCUS) with enhanced oil recovery (EOR), has become the mainstream development direction in the industry. After CO2 is injected into the reservoir, it can achieve miscible or immiscible oil displacement through mechanisms such as expanding crude oil, reducing crude oil viscosity, and reducing interfacial tension, while also achieving geological CO2 sequestration. However, in practical field applications, especially in low-permeability and complex fault-block reservoirs, the CO2-WAG (water-gas alternating injection) process faces severe technical challenges, significantly limiting its application scale and economic benefits.

[0003] First, the gas channeling problem caused by the extremely unfavorable mobility ratio is the core pain point. Under reservoir conditions, the viscosity of supercritical CO2 is typically only 0.02~0.1 mPa·s, far lower than the viscosity of formation crude oil and water (usually >1 mPa·s). This huge viscosity difference leads to an extremely unfavorable mobility ratio, making CO2 highly susceptible to viscous fingering during displacement. Coupled with the prevalent heterogeneity and fracture development in the formation, CO2 tends to rapidly penetrate into the production well along high-permeability channels, forming ineffective circulation, resulting in low gas utilization and difficulty in diverting residual oil in the medium-to-low permeability matrix. To control gas channeling, the industry has widely attempted to use surfactants or polymers to stabilize CO2 foam. However, traditional small-molecule organic surfactants are prone to thermal chain breakage at high temperatures (>100℃) and salting out in high-salinity environments (especially high calcium and magnesium ion environments), leading to rapid loss of foam stabilization ability. Conventional polymers, on the other hand, face problems such as poor shear resistance and easy clogging of low-permeability pore throats. The emerging nanoparticle-stabilized Pickering emulsion technology has shown potential in recent years. However, ordinary inorganic nanoparticles (such as SiO2) without special modification are prone to irreversible aggregation in saline water, making it difficult to migrate to the deep part of the reservoir and to spontaneously adsorb to the CO2-water interface to form an effective foam-stabilized film.

[0004] Secondly, the severe corrosion caused by CO2 in contact with water increases operational risks and costs. After CO2 is injected into the well bottom, it comes into contact with formation water or injected water, forming carbonic acid and lowering the pH of the aqueous phase to an acidic range of 3-4. Under high temperature, high pressure, and high flow rate conditions, this acidic environment causes severe electrochemical corrosion of carbon steel (such as commonly used pipe materials like N80 and P110), manifesting as uniform corrosion and localized pitting corrosion, significantly shortening pipe string life and even triggering blowouts. Current technology typically addresses this by adding corrosion inhibitors. However, commercially available high-efficiency organic corrosion inhibitors (such as imidazoline derivatives and quaternary ammonium salts) are mostly cationic or oil-soluble. They readily react with anionic surfactants used for oil displacement via electrostatic attraction, causing precipitation and rendering both ineffective. Oil-soluble corrosion inhibitors are difficult to disperse effectively in the aqueous phase and cannot protect the water-wetted pipe walls. This incompatibility issue of conflicting inhibitors often leaves mines in a dilemma: "ensuring oil displacement results in excessive corrosion, while ensuring corrosion prevention leads to severe gas leakage."

[0005] Finally, there is a lack of effective deep fluid flow tracking and monitoring methods. To optimize injection and production strategies, it is essential to understand downhole fluid transport patterns, the distribution of high-permeability channels, and the advance velocity of the waterflood front. Traditional monitoring methods rely on the additional injection of tracers. Radioactive tracers are limited in application due to environmental and approval issues; while chemical tracers (such as halogenated hydrocarbons and fluorinated benzoic acid) are available, the detection process is cumbersome, requiring expensive chromatography-mass spectrometry (GC-MS) instruments, and they are "passive" tracers, lacking inherent oil displacement or corrosion protection capabilities. This separation of monitoring operations from production enhancement measures increases construction procedures and costs.

[0006] In summary, current technologies lack an integrated working fluid that simultaneously meets the requirements of high temperature and high salt resistance, efficient foam stabilization and sealing, strong inhibition of acid corrosion, and self-tracing intelligent monitoring. Developing an integrated solution based on novel material structures is a key technology urgently needing breakthroughs in the field of CO2 enhanced oil recovery. Summary of the Invention

[0007] In view of the technical problems existing in the background art, the present invention provides a multifunctional amphiphilic carbon dot, nanofluid, preparation method and application thereof, aiming to solve the technical problems in the existing CO2 flooding technology that cannot simultaneously achieve high temperature resistance and high salt resistance, efficient foam stabilization and sealing, strong inhibition of acid corrosion and have self-tracing intelligent monitoring function.

[0008] In a first aspect, the present invention provides a multifunctional amphiphilic carbon dot, which is obtained by a solvothermal reaction of a carbon skeleton precursor, a heteroatom source, a hydrophobic modifier and a solvent. The carbon skeleton precursor is a small organic molecule containing carboxyl and / or hydroxyl groups that is easily carbonized; The heteroatom source is selected from organic compounds rich in nitrogen and / or sulfur atoms; The hydrophobic modifier is a surfactant with a long carbon chain of C8~C22; The carbon skeleton precursor includes at least one of citric acid, malic acid, glucose, and ethylenediaminetetraacetic acid; the heteroatom source includes at least one of polyethyleneimine, ethylenediamine, cysteine, and thiourea; and the hydrophobic modifier includes at least one of dodecylamine, hexadecylamine, oleylamine, and perfluorooctanoic acid. The mass ratio of carbon framework precursor, heteroatom source, and hydrophobic modifier is 1:(0.5~1.5):(0.1~1.0).

[0009] Preferably, the solvent is a mixture of water and an organic solvent; the organic solvent includes at least one of ethanol, dimethylformamide, and dimethyl sulfoxide.

[0010] Preferably, the temperature of the solvothermal reaction is 160~220℃, and the reaction time is 6~12 hours.

[0011] Secondly, the present invention provides a method for preparing multifunctional amphiphilic carbon dots, comprising the following steps: mixing a carbon skeleton precursor, a heteroatom source, a hydrophobic modifier and a solvent, and carrying out a solvothermal reaction at 160~220°C, cooling to room temperature after the reaction is completed, and obtaining multifunctional amphiphilic carbon dots after filtration, dialysis and freeze-drying.

[0012] Thirdly, the present invention provides a nanofluid obtained by dispersing the multifunctional amphiphilic carbon dots described in the first aspect in a highly mineralized brine; the total mineralization of the highly mineralized brine is 100,000 to 250,000 mg / L; and the mass concentration of the multifunctional amphiphilic carbon dots in the nanofluid is 0.05% to 0.5%.

[0013] Fourthly, the present invention provides an application of the nanofluid as described in the third aspect in oil displacement in low-permeability high-temperature reservoirs and / or oil displacement in high-salinity low-permeability reservoirs.

[0014] Fifthly, the present invention provides a CO2-WAG synergistic oil displacement method, comprising the following steps: S1. Disperse the multifunctional amphiphilic carbon dots described in the first aspect in simulated formation water of the target reservoir to form a nanofluid; S2. Nanofluid is used as a liquid slug and alternately injected with carbon dioxide gas slugs into the target oil-bearing formation to generate Pickering foam in situ to block gas channeling. S3. Collect a sample of the produced liquid at the extraction end and detect the emission fluorescence intensity at a specific excitation wavelength.

[0015] Preferably, in step S1, the environmental conditions for the simulated formation water in the target reservoir are: temperature of 80℃~150℃, total salinity of 100,000~250,000 mg / L; and the mass concentration of multifunctional amphiphilic carbon dots in the nanofluid of 0.05%~0.5%. In step S2, the size of a single liquid slug is 0.1~0.3 PV, the size of a single gas slug is 0.1~0.3 PV, the gas-liquid volume ratio is (1~2):1, and the number of alternating injection rounds is 3-5 rounds; In step S3, the excitation wavelength is set to 340~400 nm, and the wavelength range for receiving emitted fluorescence is set to 420~500 nm.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention integrates three functions of oil displacement, corrosion inhibition and tracing on the same nanoparticle carrier to achieve "one agent with multiple functions". This molecular-level integration completely avoids the incompatibilities such as precipitation, stratification and self-decomposition caused by traditional multi-component chemical agent compounding, simplifies the on-site construction process and reduces the total amount of chemical agent used and the operating cost.

[0017] (2) The amphiphilic carbon dots provided by the present invention have excellent temperature resistance, salt resistance and foam stability. Their thermal stability and chemical stability are far superior to those of organic small molecule surfactants and polymers, effectively solving the problem of gas channeling control in harsh reservoir environments.

[0018] (3) The CO2-WAG synergistic oil recovery method provided by this invention significantly improves oil recovery and safety. Indoor core experiments have confirmed that, compared with conventional waterflooding, the synergistic method of this invention can increase crude oil recovery by more than 15-20%. At the same time, this nanofluid can reduce the corrosion rate of N80 steel in saturated CO2 brine to <0.05 mm / a, with a corrosion inhibition efficiency of more than 95%, significantly extending the service life of the tubing and ensuring operational safety.

[0019] (4) The CO2-WAG synergistic oil displacement method provided by the present invention has intelligent and visualized dynamic monitoring capabilities. It does not require additional tracer injection and can achieve low-cost and high-sensitivity online monitoring by utilizing the stable fluorescence of carbon dots themselves, providing an intuitive basis for reservoir engineers to adjust injection and production schemes in real time. Attached Figure Description

[0020] Figure 1 This is a transmission electron microscope image of the multifunctional amphiphilic carbon dot N-ACD prepared in Example 1 of the present invention; Figure 2 This is a particle size distribution diagram of the multifunctional amphiphilic carbon dot N-ACD prepared in Example 1 of the present invention; Figure 3This is a comparison chart of the high-temperature and high-pressure dynamic corrosion evaluation of nanofluids in Example 1 and Comparative Example 1 of the present invention. Figure 3 (a) shows the state of the N80 standard steel sheet before corrosion. Figure 3 (b) Corrosion photographs of N80 standard steel sheets after immersion in different nanofluids. Detailed Implementation

[0021] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention.

[0022] To address the technical challenges of existing CO2 flooding technologies that cannot simultaneously achieve high-temperature and high-salt resistance, efficient foam stabilization and sealing, strong inhibition of acid corrosion, and self-tracing intelligent monitoring, this invention provides a multifunctional amphiphilic carbon dot, nanofluid, its preparation method, and its applications. Through ingenious molecular structure design, amphiphilic flooding, heteroatom adsorption and corrosion inhibition, and quantum dot fluorescence tracing functions are integrated into a single nanoscale carbon dot particle, constructing a "one agent, multiple functions" intelligent nanofluid. This enables efficient, safe, and intelligent synergy in the CO2 flooding process under complex reservoir environments.

[0023] In a first aspect, embodiments of the present invention provide a multifunctional amphiphilic carbon dot, which is obtained by a solvothermal reaction of a carbon skeleton precursor, a heteroatom source, a hydrophobic modifier and a solvent; The carbon skeleton precursor is a small organic molecule containing carboxyl and / or hydroxyl groups that is easily carbonized; The heteroatom source is selected from organic compounds rich in nitrogen and / or sulfur atoms; The hydrophobic modifier is a surfactant with a long carbon chain of C8~C22; The carbon skeleton precursor includes at least one of citric acid, malic acid, glucose, and ethylenediaminetetraacetic acid; the heteroatom source includes at least one of polyethyleneimine, ethylenediamine, cysteine, and thiourea; and the hydrophobic modifier includes at least one of dodecylamine, hexadecylamine, oleylamine, and perfluorooctanoic acid. The mass ratio of carbon framework precursor, heteroatom source, and hydrophobic modifier is 1:(0.5~1.5):(0.1~1.0).

[0024] In the technical solution of this invention embodiment, small organic molecules rich in carboxyl groups (-COOH) and / or hydroxyl groups (-OH) are selected as carbon sources, and sp is formed by high-temperature carbonization. 2 / sp 3While the carbon core framework is hybridized, the incompletely carbonized surface groups endow the material with excellent water solubility and dispersion stability in salt water. Polyamines or thiols rich in nitrogen (N) and sulfur (S) lone-pair electron heteroatoms participate in the reaction. These heteroatoms are doped into the carbon dot lattice or modified on the surface. On the one hand, the lone-pair electrons of N and S atoms can form stable coordination bonds with the empty d orbitals of metallic iron (Fe), allowing the carbon dots to be tightly adsorbed onto the metal surface, forming a dense protective film that blocks H. + The presence of CO2 acts as a corrosion inhibitor; on the other hand, the introduction of heteroatoms introduces defect energy levels into the π-π conjugated system of carbon dots, significantly improving the fluorescence quantum yield and red-shifting the emission wavelength, thus enhancing the tracing performance. The introduction of hydrophobic molecules with long alkyl chains (C8~C22) to co-react with the carbon source utilizes the polarity difference between the hydrophobic chain and the hydrophilic group of the carbon precursor in the reaction solvent to form an asymmetric "Janus" structure, where one side of the particle is hydrophilic and the other is oleophilic / CO2-loving. This structure endows the carbon dots with an appropriate hydrophilic-oleophilic balance (HLB) value, enabling them to spontaneously and strongly adsorb and oriented at oil-water or CO2-water interfaces. The mass ratio of carbon framework precursor, heteroatom source, and hydrophobic modifier plays a key regulatory role in the structural formation and functional realization of amphiphilic carbon dots. Through extensive experimental verification, the preferred mass ratio of the three is 1:(0.5~1.5):(0.1~1.0). This ratio range can achieve precise construction of the asymmetric Janus amphiphilic structure of carbon dots, effective doping of heteroatoms, and optimal control of hydrophilic and oleophilic balance, ensuring that carbon dots simultaneously possess the triple functions of ultra-stable bubble sealing, efficient corrosion inhibition, and highly stable fluorescence tracer. If the ratio deviates from this range, it will lead to the failure of the amphiphilicity of carbon dots, insufficient / excessive heteroatom doping, or decreased dispersion stability, thereby causing the core function to decay or even fail.

[0025] Furthermore, in some embodiments, the solvent is a mixture of water and an organic solvent; the organic solvent includes at least one of ethanol, dimethylformamide, and dimethyl sulfoxide.

[0026] Furthermore, in some embodiments, the volume ratio of water to organic solvent is (1~5):1.

[0027] Furthermore, in some embodiments, the temperature of the solvothermal reaction is 160~220°C, and the reaction time is 6~12 hours.

[0028] Secondly, embodiments of the present invention provide a method for preparing multifunctional amphiphilic carbon dots, comprising the following steps: A carbon skeleton precursor, heteroatom source, hydrophobic modifier and solvent were mixed and subjected to a solvothermal reaction at 160~220℃. After the reaction was completed, the mixture was cooled to room temperature, filtered, dialyzed and freeze-dried to obtain multifunctional amphiphilic carbon dots.

[0029] Thirdly, embodiments of the present invention provide a nanofluid obtained by dispersing the multifunctional amphiphilic carbon dots described in the first aspect in a highly saline solution; the total salinity of the highly saline solution is 100,000~250,000 mg / L.

[0030] Furthermore, in some embodiments, the mass concentration of the multifunctional amphiphilic carbon dots in the nanofluid is 0.05% to 0.5%.

[0031] Fourthly, embodiments of the present invention provide an application of the nanofluid as described in the third aspect in oil displacement in low-permeability high-temperature reservoirs and / or oil displacement in high-salinity low-permeability reservoirs.

[0032] Fifthly, embodiments of the present invention provide a CO2-WAG synergistic oil displacement method, comprising the following steps: S1. Disperse the multifunctional amphiphilic carbon dots described in the first aspect in simulated formation water of the target reservoir to form a nanofluid; S2. Nanofluid is used as a liquid slug and alternately injected with carbon dioxide gas slugs into the target oil-bearing formation to generate Pickering foam in situ to block gas channeling. S3. Collect a sample of the produced liquid at the extraction end and detect the emission fluorescence intensity at a specific excitation wavelength.

[0033] In the technical solution of this invention embodiment, the key mechanism for implementing the CO2-WAG synergistic injection process lies in the following: In the porous formation medium, when the nanofluid comes into contact with CO2, the amphiphilic ACDs particles rapidly migrate towards the gas-liquid interface. The hydrophobic end extends into the CO2 phase, while the hydrophilic end remains in the aqueous phase, forming a layer of highly viscoelastic solid particles on the bubble surface, like miniature "armor," thus generating an ultra-stable Pickering foam or emulsion in situ. This nanoparticle-stabilized foam has strength and lifespan far exceeding that of ordinary surfactant foams, effectively blocking gas channeling pathways such as large pores and fractures, increasing flow resistance, and forcing the subsequently injected fluid to divert into the medium-to-low permeability matrix, thereby significantly improving sweep efficiency and recovery rate. The foam stabilization mechanism of the amphiphilic carbon points containing nitrogen / sulfur heteroatoms is as follows: (1) Interfacial anchoring synergistic effect: N / S heteroatoms have strong polar lone pairs of electrons, which can react with high-valence metal ions (such as Ca) in the aqueous phase. 2+ Mg 2+) Forms dynamic coordination bridging, while the hydrophobic ends of long carbon chains are inserted into the CO2 phase. This "polar head-nonpolar tail" structure forms a more dense "solid-like particle film" at the interface than traditional surfactants. The presence of N / S atoms increases the charge density on the particle surface, and prevents the aggregation and detachment of nanoparticles at the interface through electrostatic repulsion under high temperature and high salt conditions, thereby significantly improving the mechanical strength of Pickering foam. (2) Synergistic enhancement of interfacial viscoelasticity: N / S doping changes the sp of carbon dots 2 The electron cloud distribution of the carbon nucleus enhances the hydrogen bond network between the functional groups (-COOH, -OH, -NH2) on the carbon dot surface and the water molecules at the interface. Experimental tests show that the viscoelastic modulus (G') of the interfacial film formed by N / S amphiphilic carbon dots is about 40%-60% higher than that of undoped amphiphilic carbon dots. This high viscoelastic film can effectively resist liquid film drainage during the bubble merging process and delay foam decay. (3) In-situ corrosion prevention-foam stabilization coupling: Traditional corrosion inhibitors often destroy the foam stabilizing properties of surfactants (such as precipitation). In this invention, while N / S atoms form Fe-N / Fe-S coordination protective film on the metal tube wall, the carbon dots that are not adsorbed on the metal surface still maintain perfect amphiphilicity and continue to play a foam stabilizing role. This dynamic balance between "adsorption film formation" and "interfacial foam stabilization" solves the technical problem of foam collapse caused by corrosion inhibitors in traditional compound systems.

[0034] Furthermore, in some embodiments, the environmental conditions for the simulated formation water in the target reservoir are: temperature of 80℃~150℃, total salinity of 100,000~250,000 mg / L; and mass concentration of multifunctional amphiphilic carbon dots in the nanofluid of 0.05%~0.5%.

[0035] Furthermore, in some embodiments, the size of a single liquid slug is 0.1~0.3 PV (pore volume), the size of a single gas slug is 0.1~0.3 PV, the gas-liquid volume ratio is (1~2):1, and the number of alternating injection rounds is 3-5 rounds.

[0036] In the technical solution of this invention embodiment, the injection method in step S2 can also be replaced by: pre-mixing the nanofluid with carbon dioxide gas in a wellhead or bottomhole mixer to generate foam and then continuously injecting it, or interspersing gas and liquid injection during gas-liquid alternating injection. All of the above are conventional injection methods in the field and can be flexibly adjusted according to the on-site process requirements to adapt to different reservoir characteristics.

[0037] Furthermore, in some embodiments, in step S3, the excitation wavelength is set to 340-400 nm, and the wavelength range for receiving emitted fluorescence is set to 420-500 nm.

[0038] In the technical solution of this invention embodiment, the tracer monitoring method described in step S3 can use the following pretreatment to eliminate interference when the background fluorescence interference is severe due to crude oil emulsification in the produced liquid: add a demulsifier to the produced liquid sample to demulsify and separate the layers, or add an organic extractant to extract the crude oil components and take the lower clear aqueous phase for fluorescence testing; or use synchronous fluorescence scanning technology to deduct background noise.

[0039] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0040] In the following embodiments of the present invention, the simulated formation water is a high-salinity brine prepared according to the water quality of a target oil reservoir, with a total salinity of 120,000 mg / L, wherein Na + 35000 mg / L, Ca 2+ 6000 mg / L, Mg 2+ 2000 mg / L, Cl - 73000 mg / L, HCO3 - 4000 mg / L.

[0041] Example 1 The preparation of a nitrogen-containing amphiphilic carbon dot (N-ACD) involves the following specific steps: (1) Weigh 2.0g of citric acid (carbon source) and 1.5g of polyethyleneimine (PEI, nitrogen source and hydrophilic corrosion inhibitor) and dissolve them in 20mL of deionized water, stirring until completely dissolved; (2) Add 0.8 g cetylamine (HDA, hydrophobic modifier) ​​and 15 mL anhydrous ethanol (cosolvent) to the solution and sonicate for 45 minutes until a uniform milky white emulsion is formed; (3) Transfer the above mixed emulsion to a 100mL stainless steel high-pressure reactor lined with polytetrafluoroethylene, seal it and place it in a homogeneous reactor, and react at a constant temperature of 180℃ for 8 hours. After the reaction is completed, let it cool naturally to room temperature. (4) The resulting dark brown product solution was first filtered with a 0.22 μm microporous membrane to remove large particulate impurities, and then placed in a dialysis bag (molecular weight cutoff MWCO=3500Da) and dialyzed in deionized water for 48 hours, with the dialysis water being changed every 8 hours to remove unreacted small molecule precursors. (5) Freeze-dry the purified solution in the dialysis bag for 24 hours to obtain a fluffy brownish-yellow solid powder, which is the multifunctional nitrogen-containing amphiphilic carbon point, denoted as N-ACD.

[0042] Figure 1 This is a transmission electron microscope image of the multifunctional amphiphilic carbon dot N-ACD prepared in this embodiment. Figure 2 The image shows the particle size distribution of the multifunctional amphiphilic carbon dot N-ACD prepared in this embodiment. TEM characterization shows that the N-ACD is quasi-spherical with an average particle size of about 3.5 nm and good dispersibility.

[0043] N-ACD was dissolved in simulated formation water to prepare a nanofluid with an N-ACD concentration of 0.3 wt%.

[0044] Example 2 The preparation of a nitrogen-sulfur co-doped amphiphilic carbon dot (NS-ACD) involves the following steps: (1) Weigh 1.0g of glucose (carbon source) and 1.0g of L-cysteine ​​(nitrogen-sulfur co-doped source) and dissolve them in 25 mL of deionized water; (2) Add 1.0g oleylamine (OLA, a long-chain hydrophobic agent containing unsaturated double bonds) and 10mL ethanol, and ultrasonically disperse to form a uniform system; (3) Transfer the above system to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene, seal it and place it in a homogeneous reactor, and react at 200 °C for 12 hours; after the reaction is completed, allow it to cool naturally to room temperature. (4) The subsequent filtration, dialysis and freeze-drying steps are the same as in Example 1, and a dark brown solid powder is obtained, which is the multifunctional nitrogen-sulfur co-doped amphiphilic carbon point, denoted as NS-ACD.

[0045] NS-ACD was dissolved in simulated formation water to prepare a nanofluid with an NS-ACD concentration of 0.3 wt%.

[0046] Example 3 The difference between this embodiment and Example 1 is that the amount of hexadecylamine is adjusted to 0.2g, while the rest of the synthesis and purification steps are the same as in Example 1. The carbon dots obtained are denoted as N-ACD-L (low hydrophobicity).

[0047] N-ACD-L was dissolved in simulated formation water to prepare a nanofluid with an N-ACD-L concentration of 0.3 wt%.

[0048] Example 4 The difference between this embodiment and Example 1 is that the amount of hexadecylamine is adjusted to 1.5g, while the rest of the synthesis and purification steps are the same as in Example 1. The carbon dots obtained are denoted as N-ACD-H (highly hydrophobic).

[0049] N-ACD-H was dissolved in simulated formation water to prepare a nanofluid with an N-ACD-H concentration of 0.3 wt%.

[0050] Example 5 The difference between this embodiment and Example 1 is that the amount of PEI is adjusted to 1g, while the rest of the synthesis and purification steps are the same as in Example 1.

[0051] Amphiphilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with an amphiphilic carbon dot concentration of 0.3 wt%.

[0052] Example 6 The difference between this embodiment and Example 1 is that the amount of PEI is adjusted to 3g, while the rest of the synthesis and purification steps are the same as in Example 1.

[0053] Amphiphilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with an amphiphilic carbon dot concentration of 0.3 wt%.

[0054] Comparative Example 1 Nanofluids containing 0.3 wt% alkylbenzene sulfonate and 0.1 wt% imidazoline quaternary ammonium salt were prepared by dissolving heavy alkylbenzene sulfonate and imidazoline quaternary ammonium salt in simulated formation water.

[0055] Comparative Example 2 This comparative example uses only citric acid and PEI to synthesize hydrophilic carbon dots without hydrophobic chains. The specific steps are as follows: (1) Weigh 2.0g of citric acid (carbon source) and 1.5g of polyethyleneimine (PEI) and dissolve them in 20mL of deionized water, stirring until completely dissolved; (2) Add 15 mL of anhydrous ethanol (co-solvent) to the solution and ultrasonically disperse to form a homogeneous system; (3) Transfer the system to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene, seal it and place it in a homogeneous reactor, and react at 180 °C for 8 hours; after the reaction is completed, allow it to cool naturally to room temperature. (4) The subsequent filtration, dialysis and freeze-drying steps are the same as in Example 1 to obtain hydrophilic carbon dots without hydrophobic chains.

[0056] The hydrophilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with a hydrophilic carbon dot concentration of 0.3 wt%.

[0057] Comparative Example 3 This comparative example uses only citric acid and hexadecylamine to synthesize amphiphilic carbon dots, without nitrogen or sulfur heteroatom doping. The specific steps are as follows: (1) Weigh 2.0g of citric acid (carbon source) and dissolve it in 20mL of deionized water, stirring until completely dissolved; (2) Add 0.8 g cetylamine (HDA, hydrophobic modifier) ​​and 15 mL anhydrous ethanol (cosolvent) to the solution and disperse by ultrasonication to form a uniform system; (3) Transfer the above system to a 100mL stainless steel high-pressure reactor lined with polytetrafluoroethylene, seal it and place it in a homogeneous reactor. React at 180℃ for 8 hours. After the reaction is completed, allow it to cool naturally to room temperature. (4) The subsequent filtration, dialysis and freeze-drying steps are the same as in Example 1 to obtain amphiphilic carbon dots without nitrogen and sulfur doping, denoted as ACD.

[0058] Amphiphilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with an amphiphilic carbon dot concentration of 0.3 wt%.

[0059] Comparative Example 4 The difference between this comparative example and Example 1 is that the amount of PEI is adjusted to 0.2g, while the rest of the synthesis and purification steps are the same as in Example 1.

[0060] Amphiphilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with an amphiphilic carbon dot concentration of 0.3 wt%.

[0061] Comparative Example 5 The difference between this comparative example and Example 1 is that the amount of PEI used is adjusted to 6g, while the rest of the synthesis and purification steps are the same as in Example 1.

[0062] Amphiphilic carbon dots were dissolved in simulated formation water to prepare a nanofluid with an amphiphilic carbon dot concentration of 0.3 wt%.

[0063] Performance testing Test 1: Hydrophilic-Oil Balance (HLB) Test Test method: Three-phase contact angle tests were conducted using a contact angle measuring instrument (Model: Dataphysics OCA20). N80 steel sheets were sequentially ultrasonically cleaned with acetone, anhydrous ethanol, and deionized water for 15 min each, and then dried to serve as a solid substrate. The steel sheet was placed on a testing platform, and 2 μL of simulated formation water (containing nanofluid) and simulated crude oil were added dropwise at room temperature (25±1℃) to form a water-oil-solid three-phase system. The droplet morphology was recorded using a high-speed camera system, and the contact angle was calculated using the Young-Laplace equation. Each sample was tested in triplicate, and the average value was taken. The contact angle measurement results show that the water-oil-solid contact angle of the nanofluid prepared in Example 1 is 88° (neutral wetting), the contact angle of the nanofluid prepared in Example 3 is 55° (slightly hydrophilic), the contact angle of the nanofluid prepared in Example 4 is 115° (slightly oleophilic), and the water-oil-solid contact angle of the nanofluid prepared in Comparative Example 2 is 32° (strongly hydrophilic). The test results show that by adjusting the ratio of carbon source and hydrophobic modifier, the wettability of carbon dots can be precisely controlled, so as to achieve a balance between hydrophilicity and hydrophobicity, thereby meeting the requirements for stable adsorption at the gas-liquid interface.

[0064] Test 2 Compatibility and Long-Term Stability Evaluation The nanofluids obtained from each of the examples and comparative examples were placed in sealed glass bottles and aged in a constant temperature oven at 120°C. The results are shown in Table 1 below. Table 1

[0065] Table 1 shows that the carbon dots prepared in the embodiments of the present invention exhibit excellent temperature and salt resistance stability and compatibility. In contrast, the traditional surfactant compound system of Comparative Example 1 completely decomposed after 24 hours of aging, and the nitrogen- and sulfur-free amphiphilic carbon dots of Comparative Example 3 showed flocculent precipitation after 7 days of aging. This fully demonstrates the role of N / S heteroatom doping and the synergistic design of the amphiphilic structure in improving the stability of nanofluids. Comparative Examples 4 and 5 both showed varying degrees of precipitation after 30 days of aging, indicating that when the PEI dosage is too low, the number of hydrophilic groups on the carbon dot surface is insufficient, making it difficult to disperse stably in highly salinized water; while when the PEI dosage is too high, excessive polymer chain segments are prone to intermolecular entanglement, leading to a decrease in the effective repulsive force between particles, thereby inducing agglomeration and precipitation.

[0066] Test 3: High Temperature and High Pressure CO2 Foam Stability Test Using a high-temperature, high-pressure visible foam apparatus, the half-life of foams generated by stirring nanofluids and CO2 at a volume ratio of 1:1 with each of the examples and comparative examples was tested under the conditions of 120℃, 15MPa, CO2 purity of 99.9%, gas-liquid volume ratio of 1:1, and high-shear stirring at 3000rpm. The results are shown in Table 2 below. Table 2

[0067] Table 2 shows that the amphiphilic carbon-dot-based nanofluids prepared in the embodiments of the present invention exhibit excellent foam stabilization performance under high temperature and high salt conditions. In particular, the half-life of the foam formed by the nitrogen-sulfur co-doped amphiphilic carbon-dot nanofluid in Example 2 and CO2 is greater than 48 hours. The amphiphilic carbon dots in the embodiments of the present invention significantly improve the stability of CO2 foam through the electronic effects and structural modification of heteroatoms. Specifically, the foam stabilization performance of carbon dots is significantly improved through a triple mechanism: first, by utilizing lone pairs of electrons and polar groups to enhance the adsorption strength and directional arrangement of the CO2-water interface, inhibiting bubble aggregation; second, by enriching surface functional groups to enhance the viscoelasticity of the interfacial film, resisting bubble deformation and rupture; and third, by increasing the surface charge density, enhancing the electrostatic repulsion under high temperature and high salt conditions, preventing aggregation and ensuring continuous foam stabilization.

[0068] In Comparative Example 1, the traditional surfactant system rapidly degraded its foam after stirring stopped, completely defoaming within 30 minutes. Comparative Example 2, with its hydrophilic carbon dots lacking hydrophobic groups, produced almost no stable foam. Comparative Example 3, with its amphiphilic carbon dots lacking nitrogen and sulfur doping, relied solely on long carbon chains for amphiphilicity, resulting in weak interfacial adsorption, easy particle aggregation, and an inability to form a stable solid particle film, thus significantly reducing foam stability. Comparative Examples 4 and 5, with their poor foam stability generated by nanofluids, indicate that if the nitrogen and sulfur heteroatom source content is too low, insufficient interfacial charge density leads to particle aggregation at high temperatures, reducing foam stability. Conversely, if the nitrogen and sulfur heteroatom source content is too high, the excessive hydrophilicity of the carbon dots makes them difficult to orient themselves at the gas-liquid interface, similarly reducing foam stabilization efficiency.

[0069] Test 4: High Temperature and High Pressure Dynamic Corrosion Evaluation The weight loss method combined with electrochemical testing was used. N80 standard steel sheets were placed in a high-temperature and high-pressure autoclave, and the nanofluids to be tested obtained from each of the examples and comparative examples were added. CO2 was introduced to a partial pressure of 2 MPa and a total pressure of 10 MPa. The samples were dynamically stirred and corroded at 120°C for 168 hours. The corrosion conditions of each group were statistically analyzed, and the results are shown in Table 3 below.

[0070] Table 3

[0071] Figure 3 This is a comparison chart of the high-temperature and high-pressure dynamic corrosion evaluation of nanofluids in Example 1 and Comparative Example 1 of the present invention. Figure 3 (a) shows the state before corrosion. Figure 3 (b) are corrosion photographs of N80 standard steel sheets after immersion in different nanofluids, where 627 is blank simulated water, 660 is Example 1, and 672 is Comparative Example 1. It can be seen that the steel sheet in the blank simulated water showed severe ulcer-like pitting corrosion after aging; the steel sheet in the nanofluid of Example 1 remained bright after aging, forming a dense and uniform adsorption film; the steel sheet in the fluid of Comparative Example 1 showed more obvious local corrosion pits and uneven corrosion areas on its surface after aging, indicating that the corrosion inhibition and protection effect of the traditional surfactant system on metals is not as good as that of the amphiphilic carbon dot nanofluid of the present invention.

[0072] Test results show that the amphiphilic carbon dots prepared in the embodiments of the present invention have excellent corrosion inhibition performance, with corrosion inhibition efficiency all >96%, which meets the requirements for reservoir corrosion prevention. Comparative Example 1 showed a corrosion rate of 0.4215 mm / a, far exceeding the industry standard, indicating partial failure of the corrosion inhibitor. Comparative Example 2, with nitrogen-doped carbon dots possessing corrosion-inhibiting active sites, did not affect the adsorption and film formation on the metal surface due to the absence of hydrophobic chains. Its corrosion rate was 0.1568 mm / a, with a corrosion inhibition efficiency of 88.4%. An adsorption film formed on the steel sheet surface, but its density was only moderate. Comparative Example 3, with amphiphilic carbon dots lacking nitrogen and sulfur doping and therefore lacking corrosion-inhibiting active sites, showed a corrosion rate of 1.2867 mm / a. Significant pitting corrosion occurred on the steel sheet, with a corrosion inhibition efficiency of only 4.8%. Comparative Example 4 showed a significant decrease in the corrosion inhibition performance of the nanofluid, indicating that when the PEI dosage was too low, the nitrogen-containing functional groups on the carbon dot surface were insufficient, making it difficult to form a complete adsorption film on the metal surface. Comparative Example 5, due to its high PEI dosage, resulted in an excessive amount of polymer chains forming a loose and uneven adsorption film structure on the metal surface, leading to a decrease in the density of the adsorption film and ultimately a decline in corrosion inhibition performance.

[0073] Test 5: Evaluation of Fluorescence Stability and Tracing Feasibility The fluids were aged at 120℃ in simulated formation water for different times, and their fluorescence spectra were measured under 365 nm ultraviolet light excitation. Simultaneously, 10% crude oil was added to the fluids for emulsification, and the fluorescence signal of the aqueous phase was measured. The test results are shown in Table 4 below. Table 4

[0074] Test results show that the amphiphilic carbon dots prepared in the embodiments of the present invention have excellent fluorescence tracer capabilities. After 30 days of aging, the maximum emission fluorescence intensity retention rate exceeds 70%. After crude oil emulsification, the fluorescence signal in the aqueous phase is still clearly distinguishable, and the resistance to environmental interference is strong, meeting the requirements for long-term downhole tracer. The nanofluid of Comparative Example 1 has no fluorescence characteristics and cannot detect fluorescence signals, thus lacking tracer capability. Although Comparative Example 2 has some fluorescence, it lacks amphiphilicity and cannot migrate with the oil displacement fluid, making tracer meaningless. The amphiphilic carbon dots of Comparative Example 3 have extremely low fluorescence quantum yield (<5%) due to the lack of nitrogen and sulfur heteroatom doping. The initial fluorescence signal is weak, and the fluorescence intensity is almost completely quenched after 7 days of aging. After crude oil emulsification, there is no obvious fluorescence signal in the aqueous phase, making tracer feasibility completely impossible. The nanofluid of Comparative Example 4 has a low PEI content, resulting in insufficient nitrogen-containing functional groups on the carbon dot surface, leading to a decrease in fluorescence quantum yield. The fluorescence signal of the nanofluid of Comparative Example 5 is quenched due to excessive heteroatom doping, resulting in a decrease in fluorescence quantum yield and a significant decrease in tracer sensitivity.

[0075] Test 6: Integrated Oil Displacement-Tracer Core Displacement Experiment A high-temperature, high-pressure long core displacement system with online fluorescence detection was constructed. Experimental conditions: artificial sandstone core (20 cm long, 2.5 cm in diameter, 50 mD permeability, 18% porosity), saturated simulated oil (formation viscosity 5 mPa·s), temperature 90℃, back pressure 10 MPa, injection rate 0.2 mL / min. The experimental procedure and results are as follows: (1) Water drive stage: Simulated formation water is injected until the water cut at the outlet is 98%, and the baseline water drive recovery rate is about 35%; (2) CO2-WAG synergistic injection stage: 0.2PV nanofluid prepared in Example 1 and 0.2PV CO2 were injected alternately for 3 cycles. The injection pressure increased from 2MPa to 5.5MPa, indicating that the Pickering foam system can effectively block high-permeability channels. (3) Subsequent water flooding stage: continue to inject simulated formation water until no more oil is produced; the final cumulative recovery rate reached 54.6%, which is 19.6% higher than that of water flooding; (4) Fluorescent tracer monitoring: Streamline fluorescence is detected online throughout the process. The excitation wavelength is set in the range of 340~400nm and the emission wavelength is set in the range of 420~500nm. Fluorescent signal is detected when about 0.8PV is injected. The fluorescence intensity curve shows a typical tracer peak shape. The breakthrough time and the pressure rise time are highly consistent. The carbon point recovery rate is over 92%, and the tracer is accurate and reliable.

[0076] It should be noted that the present invention is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments that have the same structure and perform the same effects as the technical concept within the scope of the present invention are included within the scope of the present invention. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of the present invention, are also included within the scope of the present invention.

Claims

1. A multifunctional amphiphilic carbon dot, characterized in that, It is obtained by a solvothermal reaction of a mixture of carbon framework precursor, heteroatom source, hydrophobic modifier and solvent; The carbon skeleton precursor is a small organic molecule containing carboxyl and / or hydroxyl groups and that is easily carbonized. The heteroatom source is selected from organic compounds rich in nitrogen and / or sulfur atoms; The hydrophobic modifier is a surfactant with a long carbon chain of C8 to C22; The mass ratio of the carbon framework precursor, heteroatom source, and hydrophobic modifier is 1:(0.5~1.5):(0.1~1.0); The carbon skeleton precursor includes at least one of citric acid, malic acid, glucose, and ethylenediaminetetraacetic acid. The heteroatom source includes at least one of polyethyleneimine, ethylenediamine, cysteine, and thiourea; The hydrophobic modifier includes at least one of dodecylamine, hexadecylamine, oleylamine, and perfluorooctanoic acid.

2. The multifunctional amphiphilic carbon dot according to claim 1, characterized in that, The solvent is a mixture of water and an organic solvent; the organic solvent includes at least one of ethanol, dimethylformamide, and dimethyl sulfoxide.

3. The multifunctional amphiphilic carbon dot according to claim 1, characterized in that, The temperature of the solvothermal reaction is 160~220℃, and the time of the solvothermal reaction is 6~12 hours.

4. The method for preparing multifunctional amphiphilic carbon dots as described in any one of claims 1 to 3, characterized in that, Includes the following steps: A carbon skeleton precursor, heteroatom source, hydrophobic modifier and solvent were mixed and subjected to a solvothermal reaction at 160~220℃. After the reaction was completed, the mixture was cooled to room temperature, filtered, dialyzed and freeze-dried to obtain multifunctional amphiphilic carbon dots.

5. A nanofluid, characterized in that, The multifunctional amphiphilic carbon dots described in any one of claims 1 to 3 are dispersed in a highly saline solution; the total salinity of the highly saline solution is 100,000 to 250,000 mg / L; and the mass concentration of the multifunctional amphiphilic carbon dots in the nanofluid is 0.05% to 0.5%.

6. The application of the nanofluid as described in claim 5 in oil displacement in low-permeability high-temperature reservoirs and / or oil displacement in high-salinity low-permeability reservoirs.

7. A CO2-WAG synergistic oil displacement method, characterized in that, Includes the following steps: S1. Disperse the multifunctional amphiphilic carbon dots as described in any one of claims 1 to 3 in simulated formation water of the target oil reservoir to form a nanofluid; S2. The nanofluid is used as a liquid slug and alternately injected with a carbon dioxide gas slug into the target oil-bearing formation to generate Pickering foam in situ to block gas channeling. S3. Collect a sample of the produced liquid at the extraction end and detect the emission fluorescence intensity at a specific excitation wavelength.

8. The CO2-WAG synergistic oil displacement method according to claim 7, characterized in that, In step S1, the environmental conditions of the simulated formation water in the target reservoir are: temperature of 80℃~150℃, total salinity of 100,000~250,000 mg / L; and the mass concentration of the multifunctional amphiphilic carbon dots in the nanofluid is 0.05%~0.5%. And / or, in step S2, the size of a single liquid slug is 0.1~0.3 PV, the size of a single gas slug is 0.1~0.3 PV, the gas-liquid volume ratio is (1~2):1, and the number of alternating injection rounds is 3-5 rounds; And / or, in step S3, the excitation wavelength is set to 340~400 nm, and the wavelength range for receiving emitted fluorescence is set to 420~500 nm.

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