Preparation method of chlorella water-thermal green synthesis carbon quantum dot nanofluid and oil displacement technology application

Abstract

The application discloses a preparation method of Chlorella hydrothermal green-synthesized carbon quantum dot nanofluid and oil displacement technology application, and belongs to the technical field of biological nano oil displacement, and the preparation method comprises the following steps: preparing Chlorella biomass, inoculating Chlorella in a culture medium for light culture, and collecting Chlorella algae powder by precipitation and freeze-drying; mixing pure water and Chlorella powder in a reaction kettle according to a certain liquid-solid mass ratio, placing the reaction kettle in a muffle furnace at 270 DEG C for 6h, taking out the product after reaction, and dialyzing the product for 24h; and the obtained solution is the Chlorella green-synthesized carbon quantum dot nanofluid. The preparation method and the oil displacement agent use low-cost Chlorella as a biomass raw material which has high industrial practicability, can be cultured on a large scale under low-energy-consumption conditions, has environmental friendliness, and has the potential to significantly improve the recovery rate.

Description

Technical Field

[0001] This invention belongs to the field of bio-nano-oil displacement technology, specifically relating to a method for preparing and applying a hydrothermal green synthetic carbon quantum dot nanofluid from Chlorella to oil displacement technology. Background Technology

[0002] Due to continuous large-scale exploration and production worldwide, traditional oil and gas reserves can no longer meet daily needs, and the development of unconventional oil and gas resources such as low-permeability reservoirs has become a research hotspot. Low-permeability reservoirs are characterized by severe heterogeneity, low permeability, and low porosity, and traditional EOR methods such as water drive, gas drive, fracturing, and chemical flooding cannot be developed economically and effectively.

[0003] Using nanomaterials to improve crude oil extraction efficiency has become a mainstream research direction in petroleum development. Current research has shown that carbon-based nanomaterials, SiO2 nanomaterials, and metal oxide nanomaterials have mechanisms such as reducing interfacial tension (IFT), changing reservoir rock wettability, increasing the viscosity and rheological properties of injected fluids, and reducing crude oil viscosity and asphaltenes precipitation.

[0004] Carbon quantum dots possess biocompatibility, high chemical stability, unique optical properties, excellent water solubility, low toxicity, smaller size, abundant surface functional groups, and larger modified surface area, which can significantly improve reservoir crude oil production and enhance fluid displacement efficiency.

[0005] Currently, the synthesis methods of carbon quantum dots are mainly divided into two categories: "top-down" and "bottom-up". Among them, the bottom-up method for synthesizing carbon quantum dots can use inexpensive and renewable biomass materials as widely available carbon sources, and therefore has attracted more attention from researchers. Biomass hydrothermal method, as a typical bottom-up green synthesis method, has advantages such as simple operation, strong controllability, low cost and environmental friendliness.

[0006] In existing published invention patent application literature, for example, Chinese invention patent application number CN201611128316.4 discloses a method for preparing fluorescent carbon quantum dots using seaweed as raw material, including the following steps: (1) cleaning the seaweed raw material to remove impurities, crushing the seaweed and water together to obtain seaweed slurry; (2) placing an appropriate amount of seaweed slurry into a sealed system for high-temperature hydrothermal reaction and carbonizing the seaweed slurry; (3) post-processing the carbonized product in step (2) to remove insoluble matter and large-sized components to obtain fluorescent carbon quantum dot solution; (4) freeze-drying the fluorescent carbon quantum dot solution in step (3) to obtain fluorescent carbon quantum dot powder.

[0007] For example, Chinese invention patent application number CN202110313835.2 discloses a microalgae-based nano-carbon quantum dot solid powder, its preparation method, and its uses; a method for preparing microalgae-based nano-carbon quantum dot solid powder, characterized in that: the method uses Chlorella as a carbon precursor, and prepares nitrogen- and sulfur-doped microalgae-based nano-carbon quantum dot materials through hydrothermal reaction, centrifugation, and dialysis treatment. The specific steps are as follows: First step: deionized water and Chlorella are added to a high-pressure reactor simultaneously, or the microalgae slurry mixture is directly added to the high-pressure reactor, and the air in the reactor is replaced with nitrogen gas and the reactor is sealed; Second step: the materials are stirred and mixed evenly, and a hydrothermal carbonization reaction is carried out. During the hydrothermal carbonization process, the nitrogen and sulfur functional groups contained in Chlorella combine with the functional groups on the surface of carbon quantum dots to form nitrogen- and sulfur-doped microalgae-based nano-carbon quantum dots.

[0008] For example, Chinese invention patent application number CN202310298902.7 discloses a green synthesis method for Chlorella carbon dots used in fluorescence imaging, including the following steps: 1) Chlorella is put into a grinder and ground into powder; 2) A certain amount of Chlorella powder is weighed and dispersed in deionized water and stirred thoroughly in a magnetic stirrer; 3) The mixed solution is then transferred to a polytetrafluoroethylene-lined stainless steel high-pressure reactor for heating reaction; 4) After the heating reaction is completed, the mixture is naturally cooled to room temperature, and the cooled sample is centrifuged to remove impurities, then filtered using a needle filter and dried overnight at a certain temperature; 5) The overnight dried sample is transferred to a freeze dryer for drying to obtain biomass carbon dots.

[0009] In the development of low-permeability oil reservoirs, existing technologies often suffer from problems such as poor stability, aggregation, and pore blockage due to the complex pore structure, high temperature, and high salinity of traditional nanomaterials, thereby reducing their effectiveness. Based on the above-mentioned technical problems of existing technologies, this invention proposes a method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally and applying it to oil displacement technology. Summary of the Invention

[0010] To address the aforementioned technical problems in existing technologies, this invention provides a method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally and its application in oil displacement technology.

[0011] The present invention adopts the following technical solution:

[0012] The method for preparing the carbon quantum dot nanofluid synthesized by hydrothermal green synthesis of Chlorella is characterized by comprising:

[0013] Step 1: Prepare Chlorella biomass precursor solution;

[0014] Step 2: Inoculate the Chlorella biomass precursor liquid into BG-11 medium, and carry out light culture and purification treatment to obtain precipitate;

[0015] Step 3: Freeze and dry the obtained precipitate to collect Chlorella powder;

[0016] Step 4: Mix ultrapure water with Chlorella powder, place in a muffle furnace for hydrothermal synthesis of carbon quantum dot products, and centrifuge and dialyze the carbon quantum dot products to remove impurities, thereby obtaining a purified carbon quantum dot solution.

[0017] Step 5: Characterize the carbon quantum dot nanofluid.

[0018] Furthermore, in step 1, the Chlorella biomass precursor solution consists of BG-11 culture and trace element A5 solution, wherein the BG-11 culture medium includes: NaNO3, K2HPO4, MgSO4·7H2O, CaCl2·2H2O, citric acid, ferric ammonium citrate, EDTANa2, and Na2CO3.

[0019] Further, in step 1, the Chlorella biomass precursor liquid comprises, by mass ratio: NaNO3 10 mL / L, K2HPO4 10 mL / L, MgSO4·7H2O 10 mL / L, CaCl2·2H2O 10 mL / L, citric acid 10 mL / L, ferric ammonium citrate 10 mL / L, EDTANa2 10 mL / L, Na2CO3 10 mL / L, trace element A5 1 mL / L, H3BO3 2.86 g / L, MnCl2·4H2O 1.86 g / L, ZnSO4·7H2O 0.22 g / L, NaMoO4·2H2O 0.39 g / L, CuSO4·5H2O 0.08 g / L, Co(NO3)2·6H2O 0.05 g / L, with the remainder being water.

[0020] Furthermore, in step 2, the process of inoculating the Chlorella biomass precursor liquid into BG-11 medium for light cultivation and purification to obtain a precipitate includes: setting the light intensity of the BG-11 medium for light cultivation to 2000-3000 Lux, and using an aeration device to oxygenate and prevent algal seed precipitation; after the light cultivation is completed, the supernatant is removed by siphoning, the precipitate is centrifuged at 7000 rpm for 15 min, and washed with ultrapure water and centrifuged multiple times to remove residual salt ions.

[0021] Furthermore, in step 3, the freezing and drying treatment of the obtained precipitate to collect Chlorella powder includes: freezing the obtained precipitate in a -80°C freezer for 12 h, drying it in a vacuum freeze dryer for 24 h, and collecting Chlorella powder.

[0022] Further, in step 4, the process of mixing ultrapure water with Chlorella powder, hydrothermally synthesizing carbon quantum dot products in a muffle furnace, and then centrifuging and dialysis the carbon quantum dot products to remove impurities, thereby obtaining a purified carbon quantum dot solution, includes:

[0023] Step 4.1: Mix ultrapure water and Chlorella powder at a solid-liquid mass ratio of 40:1, place in a muffle furnace, and hydrothermally react at 270℃ for 6 h to obtain carbon quantum dot products. Centrifuge and dialyze the carbon quantum dot products to remove impurities and obtain purified carbon quantum dot solution.

[0024] Step 4.2: Centrifuge the carbon quantum dot product at 8000 r / min and collect the supernatant. Filter the supernatant using a 0.45 μm filter membrane and transfer the filtered solution to a 1000 Da dialysis bag for dialysis for 24 h to remove small molecule impurities and salt ions. The resulting solution is the purified carbon quantum dot nanofluid.

[0025] Furthermore, in step 5, the carbon quantum dot nanofluid undergoes characterization testing, including:

[0026] Step 5.1: Prepare Fourier transform infrared (FTIR) samples using the KBr pellet method with the collected carbon quantum dot powder to determine the functional group composition of the carbon quantum dots. Set the scanning range of the infrared spectrometer to 4000–400 cm⁻¹. -1 The resolution is 4cm. -1 The number of scans was 32.

[0027] Step 5.2: XRD is used to determine the crystal structure, grain size and interatomic spacing of carbon quantum dots; monochromatic Cu Kα1 radiation is used as the X-ray source, and the diffraction angle range of 5-90° is scanned at a scanning speed of 2° / min to obtain XRD images;

[0028] Step 5.3: XPS was used to accurately analyze the elemental composition and chemical valence state of carbon quantum dots. Al·Kα was selected as the excitation source and the working parameters were 6 mA × 12 kV. The carbon quantum dot sample was scanned in its entirety, and the target elements C, O and N were scanned in detail.

[0029] Step 5.4: Use TEM to observe the morphology, size and distribution of C-CQDs material. After sonicating the purified C-CQDs solution for 5 min, use a micropipette to drop the liquid sample onto the ultrathin carbon film copper grid, let it dry naturally, leaving a thin layer of liquid adhering to the carbon film, and observe and analyze the sample morphology at 200 kV.

[0030] Furthermore, in step 5, the carbon quantum dot fluid is regularly spherical and monodisperse, with a particle size range of 0.5-4 nm and an average diameter of 1.54±0.56 nm. The surface contains NH bonds, OH bonds, C=O bonds, and CO bonds. Among the C, O, and N elements, the C content is 74.86%±5%, the O content is 17.45%±3%, and the N content is 7.69%±2%.

[0031] This invention also provides an application of oil displacement technology using carbon quantum dot nanofluid synthesized hydrothermally from Chlorella. The carbon quantum dot nanofluid is injected into a low-permeability reservoir, reducing the oil-water interfacial tension to 0.17-9.32 mN / m and changing the wettability of the reservoir rock surface from oil-wet to water-wet, thereby achieving a spontaneous adsorption recovery rate of over 46.9%.

[0032] Compared with the prior art, the superior effects of the present invention are as follows:

[0033] 1. The method for preparing carbon quantum dot nanofluids synthesized by hydrothermal green synthesis of Chlorella described in this invention innovatively selects Chlorella as the sole carbon and nitrogen source. Chlorella is a microorganism that grows rapidly, has low cultivation costs, and can be industrially cultivated on a large scale in a photobioreactor. The raw material source is wide-ranging and renewable. This method abandons the traditional method of using fossil fuel derivatives or expensive chemical reagents, and achieves green, sustainable and low-cost raw material substitution from the source.

[0034] 2. The preparation method of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally according to the present invention adopts a one-step hydrothermal method, using only Chlorella powder and water as reactants, and reacting at a specific temperature. This process does not require complex equipment, does not involve toxic or harmful solvents or harsh post-processing, is simple to operate, has relatively low energy consumption, and is an environmentally friendly green synthesis route that is easy to scale up and implement industrially.

[0035] 3. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally in this invention produces carbon quantum dots with small particle size and uniform distribution, effectively avoiding clogging in the micropores of low-permeability reservoirs. Benefiting from the biological composition of Chlorella, the carbon quantum dots are naturally modified with abundant hydrophilic functional groups such as hydroxyl, carboxyl, and amino groups on their surface; this endows them with excellent water dispersion stability and good interfacial activity. The proteins and nucleic acids inherent in Chlorella allow nitrogen to be naturally incorporated into the carbon quantum dot structure during synthesis, eliminating the need for additional nitrogen sources. This self-doping further modulates their surface properties, enhancing their interaction with crude oil components.

[0036] 4. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to the present invention provides carbon quantum dots that, due to their small size, good dispersibility, high chemical stability and abundant surface active groups, can better migrate in complex pores and effectively act on the oil-water-rock interface, comprehensively exerting the dual mechanism of reducing interfacial tension and changing wettability, thereby more economically and effectively extracting crude oil in low-permeability reservoirs. Attached Figure Description

[0037] Figure 1 This is a Fourier transform infrared diagram of the carbon quantum dot material in the preparation method of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally according to the present invention.

[0038] Figure 2 This is an X-ray energy dispersive electron image of the carbon quantum dot material in the preparation method of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally according to the present invention;

[0039] Figure 3 This is the X-ray photoelectron spectrum of the carbon quantum dot material in the preparation method of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally according to the present invention;

[0040] Figure 4 This is a transmission electron microscope image of the carbon quantum dot material in the preparation method of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally according to the present invention;

[0041] Figure 5 This is a graph showing the interfacial tension changes of the carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 7 at different concentrations.

[0042] Figure 6 This is a graph showing the contact angle variation of the carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 8 at different concentrations;

[0043] Figure 7 This is a graph showing the change in the oil recovery rate of carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 9. Detailed Implementation

[0044] To better understand the above-mentioned objectives, features and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0045] Example 1

[0046] The method for preparing the Chlorella biomass precursor liquid includes:

[0047] Chlorella was inoculated into a culture medium containing specific components and cultured under light. The culture medium was BG-11 medium. The Chlorella biomass precursor solution consisted of BG-11 medium and trace element A5 solution. The BG-11 medium contained the following components in the following mass ratio: NaNO3 10 mL / L, K2HPO4 10 mL / L, MgSO4·7H2O 10 mL / L, CaCl2·2H2O 10 mL / L, citric acid 10 mL / L, ferric ammonium citrate 10 mL / L, EDTANa2 10 mL / L, Na2CO3 10 mL / L, and trace element A5 1 mL / L.

[0048] Trace element A5, in terms of mass content, includes: H3BO3 2.86 g / L, MnCl2·4H2O 1.86 g / L, ZnSO4·7H2O 0.22 g / L, NaMoO4·2H2O 0.39 g / L, CuSO4·5H2O 0.08 g / L, Co(NO3)2·6H2O 0.05 g / L, with the remainder being water;

[0049] The light intensity was set to 2000-3000 Lux, and an aeration device was used to oxygenate the algae and prevent them from settling.

[0050] After the light culture was completed, the supernatant was removed by siphoning. The precipitate was centrifuged at 7000 rpm for 15 min, the supernatant was removed, and then washed with ultrapure water and centrifuged several times to remove residual salt ions.

[0051] The ultrapure water washing and centrifugation are repeated to remove residual salt ions, specifically including:

[0052] Add sufficient ultrapure water (usually 5-10 times the volume of the precipitate) to the centrifuge tube containing the algae sludge. Use a vortex mixer or pipette to repeatedly blow and agitate the water to completely and evenly disperse the algae sludge in the water to form a suspension. The ultrapure water has a high resistivity, usually > 18 MΩ·cm. Washing with it can maximize the concentration difference between the water and the salt in the algae sludge, and efficiently "extract" salt ions through diffusion without introducing new impurities.

[0053] The resuspended suspension was placed in a centrifuge and centrifuged again.

[0054] Remove the centrifuge tubes from the centrifuge, avoiding shaking. Use a pipette or vacuum aspiration device to remove all the supernatant, leaving only the algae at the bottom of the tube. Repeat the above steps several times.

[0055] The precipitate was frozen at -80℃ for 12 h and dried using a vacuum freeze dryer for 24 h to obtain Chlorella powder.

[0056] During the light cultivation process, the light intensity and aeration rate should be checked and adjusted regularly to ensure the optimal growth conditions for Chlorella.

[0057] Chlorella powder was taken from the Chlorella biomass precursor liquid. Pure water was mixed with the Chlorella powder at a solid-liquid mass ratio of 40:1. The mixture was placed in a muffle furnace and hydrothermally reacted at 270°C for 6 h to obtain carbon quantum dot products. The carbon quantum dot products were centrifuged and dialyzed to remove impurities and obtain a purified carbon quantum dot solution.

[0058] The carbon quantum dot product was centrifuged at 8000 r / min, and the supernatant was collected. The supernatant was filtered through a 0.45 μm filter membrane. The filtered solution was transferred to a 1000 Da dialysis bag and dialyzed for 24 h to remove small molecule impurities and salt ions. The resulting solution was the purified carbon quantum dot nanofluid.

[0059] It should be noted that the muffle furnace used in the method for preparing Chlorella biomass precursor liquid described in this invention is a general-purpose box-type high-temperature heating device. Originally, a muffle referred to a closed cavity used to isolate the heating material from the fuel, prevent pollution and oxidation, and maintain the atmosphere inside the furnace. The muffle furnaces used today have evolved into sealed furnace chambers that use electric heating elements (such as silicon carbide rods or resistance wires) for heating and are wrapped with insulation materials (ceramic fibers or refractory bricks).

[0060] Example 2

[0061] Scanning 400-4000 cm using a Thermo Fisher Scientific Nicolet iS20 instrument -1 The chemical structure and functional group information of carbon quantum dots in the wavenumber range were determined by preparing Fourier transform infrared (FTIR) samples using the KBr pellet method with collected Chlorella and carbon quantum dot powder. The functional group composition of the carbon quantum dots synthesized by Chlorella was determined, and the scanning range of the infrared spectrometer was set to 4000–400 cm⁻¹. -1 The resolution is 4cm. -1 The number of scans was 32.

[0062] FTIR spectra of the Chlorella biomass precursor liquid prepared in Example 1 and the carbon quantum dot nanofluid synthesized by Chlorella hydrothermally in the green process are plotted using Origin, as shown below. Figure 1 As shown; FT-IR analysis is a highly sensitive method for characterizing the formation of solid dispersion systems, capable of demonstrating structural changes through shifts or intensities of molecular absorption peaks. Figure 1 Chlorella and carbon quantum dots were shown at 4000-400 cm⁻¹ -1 The FT-IR spectra within the scanning range show that the carbon quantum dots have a complex surface structure and retain various active groups from Chlorella during hydrothermal synthesis. (3405 cm⁻¹) -1 The absorption peak at 2925-2855 cm⁻¹ originates from the NH bending vibration and the OH stretching vibration. The asymmetric and symmetric OH stretching vibrations are typical characteristics of carbon quantum dots synthesized via biological methods. -1 The absorption peaks within this range originate from the stretching vibrations of CH in aliphatic hydrocarbons or aldehydes, at 1650 cm⁻¹. -1 The absorption peak at 1300-1000 cm⁻¹ originates from the stretching vibration of C=O in the anhydride configuration, while the absorption peak at 1300-1000 cm⁻¹ originates from the stretching vibration of C=O in the anhydride configuration. -1 The absorption peaks within the range are attributed to the stretching vibration of CO in the amide group. Compared with the FT-IR spectrum of Chlorella, the peak shapes of CO and COC in the carbon quantum dot spectrum are weakened. This is because COC is cracked during the high-temperature hydrothermal carbonization process, and the OH group is oxidized to -COOH, causing the CO vibration mode to shift to a higher frequency, resulting in the dispersion or weakening of the original CO peak shape. The N and O elements in Chlorella have been successfully modified on the surface of carbon quantum dots to form abundant oxygen-containing functional groups and NH bonds, which is beneficial to enhancing the dispersibility of carbon quantum dots in solution.

[0063] The KBr pelleting method involves mixing and grinding a small amount of the sample to be tested with a large amount of spectrally pure potassium bromide powder until homogeneous. Then, in a special mold, high pressure is applied using a hydraulic press to press it into a transparent, thin disc.

[0064] Example 3

[0065] XRD patterns of carbon quantum dots were determined using a Rigaku SmartLab SE diffractometer. The carbon quantum dot powder was ground uniformly, placed in a sample cell, and pressed flat with a glass slide. Monochromatic Cu Kα1 radiation (λ=0.15418nm) was used as the X-ray source, and the diffraction angle range of 5-90° was scanned at a scanning speed of 2° / min to obtain XRD images. The obtained patterns were imported into Jade 6.5 software to analyze the structure of the carbon quantum dot material.

[0066] The XRD pattern of the carbon quantum dot nanofluid synthesized hydrothermally from Chlorella prepared in Example 1 is shown below. Figure 2 As shown, the XRD pattern results indicate that there is a broad diffraction peak near 2θ=24° in the diffraction pattern of carbon quantum dots. This is attributed to the small size effect and partial graphitization characteristics of carbon quantum dots, which also shows that carbon quantum dots have obvious amorphous structure and crystallinity.

[0067] Example 4

[0068] XPS data of the samples were measured using a Thermo Scientific K-Alpha spectrometer with Al·Kα as the excitation source and operating parameters of 6 mA × 12 kV. The full spectrum of the carbon quantum dot powder sample was scanned, and then the target elements C, O, and N were scanned in detail. The test data were then imported into Avantage software for charge correction and fine spectral analysis.

[0069] XPS spectra of the carbon quantum dot nanofluid synthesized hydrothermally from Chlorella prepared in Example 1 are as follows: Figure 3 As shown, XPS spectroscopy was used to further identify the chemical state and elemental properties of carbon quantum dots. Figure 3 Images (a) to (d) show the XPS full scan and high-resolution C1s, N1s, and O1s spectra of carbon quantum dots, such as... Figure 3 As shown in (a), there are three distinct peaks at 284.8, 399.8 and 532 eV, which belong to C1s, N1s and O1s respectively, proving that carbon quantum dots mainly contain three elements: C, O and N. The contents of C, O and N are 74.86%, 17.45% and 7.69% respectively. It can be seen that C and O are the main constituent elements of carbon quantum dots, and N is successfully doped into the surface of carbon quantum dots. Figure 3 (b) is the fine C1s spectrum after peak fitting, where the peak with a binding energy of 284.8 eV corresponds to sp2 hybrid CC / C=C, the peak with a binding energy of 286.1 eV corresponds to CO / CN, and the peak with a binding energy of 288.3 eV corresponds to C=O, indicating that carbon exists in different states such as quantum dots or organic biomass; Figure 3 (c) is the fine N1s energy spectrum of carbon quantum dots, mainly consisting of CNC (binding energy 399.6 eV) and NH (binding energy 400.9 eV). The overall content is low, indicating that nitrogen mainly originates from amino acids in biomass. The fine O1s spectrum is shown below. Figure 3 As shown in (d), the fitting results show two main peaks at 531.3 eV and 533.1 eV, which are attributed to the bonding energies of C=O and CO, respectively. XPS results indicate that the carbon quantum dot surface has abundant oxygen and nitrogen-related hydrophilic functional groups, which not only helps to improve the dispersion stability of carbon quantum dots in aqueous solution, but also enhances the ability of carbon quantum dots to change the wettability of porous media surfaces.

[0070] Example 5

[0071] The morphology, size and distribution of carbon quantum dot materials were observed using a JEOL F200 thermal field emission transmission electron microscope. After sonicating the purified carbon quantum dot solution for 5 min, the liquid sample was dropped onto an ultrathin carbon film copper grid using a micropipette. The sample was then dried naturally, leaving a thin layer of liquid adhering to the carbon film. The morphology of the sample was then observed and analyzed at 200 kV.

[0072] TEM image of the carbon quantum dot nanofluid synthesized hydrothermally from Chlorella prepared in Example 1 is shown below. Figure 4 As shown, the morphology of carbon quantum dots was characterized by TEM. The carbon quantum dot particles are regular round, uniformly dispersed monodisperse nanodots with a particle size distribution range of 0.5-4 nm. According to the size distribution histogram, the average diameter is 1.54±0.56 nm. Furthermore, according to the high-resolution TEM (transmission electron microscope) in the inset of the figure, clear lattice fringes can be observed with a lattice spacing of 0.22 nm, corresponding to the (100) crystal plane of graphite, confirming that the carbon quantum dots have a graphite-like carbon core structure.

[0073] Example 6

[0074] The carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 1 was diluted at a certain ratio. The interfacial tension between the carbon quantum dot solution and the crude oil system was assessed using a rotating drop interfacial tensiometer. When two immiscible liquids form droplets in a rotating sample tube, the droplet shape is affected by both centrifugal force and interfacial tension. Under high-speed rotation, the droplet is elongated into an ellipsoid, and its shape is related to the balance between interfacial tension and centrifugal force. By measuring the droplet's shape parameters (such as the major and minor axes), the interfacial tension can be calculated. The specific experimental procedures are as follows:

[0075] Prepare crude oil and carbon quantum dot solutions of different concentrations for the experiment, measure the density of carbon quantum dot samples and the original density, and calculate the density difference between samples of different concentrations and crude oil.

[0076] Adjust the experimental temperature to ensure it remains constant and avoid experimental errors caused by temperature fluctuations;

[0077] Thoroughly clean the experimental sample tube with the carbon quantum dot solution to be tested, inject the carbon quantum dot solution to be tested into the sample tube using a microsyringe, and then inject the oil phase into the center of the sample tube using another microsyringe. The introduction of air bubbles should be avoided throughout the process.

[0078] The experimental rotation speed was set to 3000 rpm. The instrument was started to make the sample tube rotate at high speed. The image of the droplet in the rotating state was taken every 5 minutes. Then the major axis and minor axis of the droplet were measured. The interfacial tension value was calculated using the instrument software. The recording was stopped after the interfacial tension value stabilized after three measurements.

[0079] After the experiment, the sample tubes were cleaned with petroleum ether and then placed in a fume hood to air dry.

[0080] The interfacial tension results of the carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 1 at different concentrations are as follows: Figure 5 As shown, the interfacial tension gradually decreases with increasing carbon quantum dot concentration, and high concentrations of carbon quantum dots can significantly reduce the oil-water interfacial tension. When the carbon quantum dot concentration is 0 ppm, the interfacial tension is 9.32 mN / m. When the concentration increases to 320 ppm, the oil-water interfacial tension reaches its lowest value of 0.17 mN / m, a decrease of 98.17%. Therefore, this indicates that the interfacial tension decreases with increasing carbon quantum dot concentration, demonstrating that the interfacial tension is highly sensitive to the concentration of carbon quantum dots.

[0081] Example 7

[0082] To verify the ability of carbon quantum dot nanofluids to alter the wettability of reservoir rocks, a contact angle meter was used to assess wettability. The study investigated the ability of carbon quantum dot nanofluids at different concentrations to change rock wettability. The specific experimental steps for measuring the contact angle are as follows:

[0083] The surface of the quartz plate was cleaned with a mixture of ethanol and ultrapure water to remove contaminants and impurities, and then placed in a fume hood to air dry.

[0084] Quartz glass slides were immersed in crude oil and aged in a 60°C oven for 7 days.

[0085] Remove the aged quartz discs and clean the surface of the quartz discs with n-heptane. Then, air dry the n-heptane residue on the surface in a fume hood.

[0086] Carbon quantum dot nanofluid solutions of different concentrations were prepared, and quartz sheets were soaked for 24 h to modify their surface.

[0087] The change in contact angle was measured using the pendant drop method. The successfully modified quartz sheet was placed on the sample stage of the contact angle measuring instrument, and the position of the sample stage was adjusted to be horizontal. Carbon quantum dot nanofluid solution was injected into the sample cell, and the injection volume was approximately enough to cover the quartz sheet.

[0088] Use a curved microsyringe to draw up an appropriate amount of crude oil, fix it above the sample stage, adjust the distance between the syringe and the quartz plate, and slowly inject a drop of crude oil under the quartz plate. Adjust the intensity and distance of the instrument's light source to observe the clearly outlined oil droplet and quartz plate.

[0089] Record the morphology of oil droplets on the surface of a quartz plate and measure the contact angle value.

[0090] The contact angle of carbon quantum dot nanofluids synthesized from Chlorella hydrothermally in Example 1 was measured at different concentrations; wherein, C-1, carbon quantum dot nanofluid concentration 320 ppm; C-2, carbon quantum dot nanofluid concentration 160 ppm; C-3, carbon quantum dot nanofluid concentration 80 ppm; C-4, carbon quantum dot nanofluid concentration 40 ppm; C-5, carbon quantum dot nanofluid concentration 20 ppm; and C-6, formation water.

[0091] The contact angles of the carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 1 at different concentrations are as follows: Figure 6 As shown in the figure; the experimental results show that when the carbon quantum dot concentration is 0 ppm, the contact angle is 140.4°. As the carbon quantum dot concentration increases, the contact angle between the oil and solid phases gradually decreases. When the concentration increases to 320 ppm, the contact angle drops to a minimum of 19.5°. This indicates that carbon quantum dots can effectively change the wettability of rock surfaces from oil-wet to water-wet, and the higher the concentration, the stronger the ability to change the wettability of rock surfaces.

[0092] Example 8

[0093] The study used an infiltration experiment to evaluate the effect of the carbon quantum dot nanofluid synthesized from Chlorella hydrothermally in Example 1 on improving oil recovery by simulating reservoir conditions. The specific operation steps of the infiltration experiment are as follows:

[0094] Record the length and diameter of the natural rock core used in the experiment using vernier calipers, and calculate the core volume; place the core in a 70℃ oven to dry to constant weight, and record the dry weight of the core;

[0095] Then, the core is placed in a stoppered conical flask connected to a vacuum pump. The vacuum pump is turned on and run for 24 hours to remove air from the core. The valve between the formation water and the vacuum device is opened so that the simulated formation water fills the pores of the entire core under negative pressure. The saturation time is 24 hours. After saturation, the wet weight of the core is measured.

[0096] The core was saturated with oil using a core displacement device. The flow rate of the advection pump was set to 0.3 mL / min. The saturation operation was stopped when oil was continuously discharged from the outlet. The crude oil content in the core was replaced by the volume of formation water displaced. The crude oil content after saturation was recorded as V0. The oil saturation of the core was calculated. The saturated core was then placed in crude oil and aged in a 60℃ constant temperature oven for 7 days for later use.

[0097] After the core aging process is complete, the crude oil is removed and the surface oil is wiped off. The core is then placed in an Amott percolation bottle, and the core is immersed in a carbon quantum dot nanosolution. The nanosolution is then added to the capillary scale of the percolation bottle, and the initial time of the percolation experiment is recorded.

[0098] The assembled percolation bottle was placed in a 60℃ constant temperature chamber. During the initial percolation stage, the volume of oil produced (Voil) was recorded every 1 hour. In the subsequent stage, data was recorded every 12 hours. After the crude oil production in the capillary of the percolation bottle stabilized, observation continued for 3 days. The percolation experiment was then stopped. The core percolation recovery rate (ER) was calculated based on the volume of oil produced (Voil, mL) using the formula ER = Voil / Vo × 100%. A spontaneous percolation experiment using simulated formation water as the percolation fluid was also conducted as a control group.

[0099] The hydrothermal green synthesis of carbon quantum dot nanofluids from Chlorella prepared in Example 1 showed the following adsorption recovery rate: Figure 7 As shown, under the same experimental conditions of 320 ppm nanoparticle concentration and 60℃ constant temperature water bath, carbon quantum dots (CQNs) exhibited superior percolation performance compared to SiO2 NPs. At the end of percolation, the percolation recovery rate of CQNs was 46.9%, while that of SiO2 NPs was 37.13%, and that of formation water was only 11.23%. CQNs significantly improved the percolation recovery rate by 9.77% compared to SiO2 NPs, demonstrating that CQNs are more effective at enhancing recovery than SiO2 NPs. Furthermore, the experiment revealed that CQNs reached their peak percolation recovery rate within the first 24 hours of percolation, while SiO2 NPs required 60 hours to reach their peak. This indicates that CQNs possess a higher percolation rate, thus demonstrating that CQNs have greater potential for improved percolation recovery than SiO2 NPs.

[0100] The Amott percolation test is a standard method for evaluating the ability of fluids (such as water, nanofluids, surfactants, etc.) to spontaneously draw into a core and displace crude oil under simulated reservoir conditions in a laboratory setting. It is mainly used to quantitatively determine the spontaneous percolation recovery rate of fluids. Specifically, a core saturated with crude oil and aged is completely immersed in the oil displacement agent solution to be tested. Under the action of capillary force, the fluid will spontaneously draw into the tiny pores of the core and displace the crude oil. By measuring the volume of crude oil extracted at different time intervals, the change in recovery rate over time can be calculated.

[0101] This invention also provides an application of oil displacement technology using carbon quantum dot nanofluid synthesized hydrothermally from Chlorella. By injecting the carbon quantum dot nanofluid into low-permeability reservoirs, the oil-water interfacial tension is reduced to 0.17-9.32 mN / m, and the wettability of the reservoir rock surface is changed from oil-wet to water-wet, achieving a spontaneous adsorption recovery rate of over 46.9%.

[0102] This invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims.

Claims

1. A method for preparing carbon quantum dot nanofluids synthesized hydrothermally from Chlorella, characterized in that, include: Step 1: Prepare Chlorella biomass precursor solution; Step 2: Inoculate the Chlorella biomass precursor liquid into BG-11 medium, and carry out light culture and purification treatment to obtain precipitate; Step 3: Freeze and dry the obtained precipitate to collect Chlorella powder; Step 4: Mix ultrapure water with Chlorella powder, place in a muffle furnace for hydrothermal synthesis of carbon quantum dot products, and centrifuge and dialyze the carbon quantum dot products to remove impurities, thereby obtaining a purified carbon quantum dot solution. Step 5: Characterize the carbon quantum dot nanofluid.

2. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to claim 1, characterized in that, In step 1, the Chlorella biomass precursor solution is composed of BG-11 culture medium and trace element A5 solution, wherein the BG-11 culture medium includes: NaNO3, K2HPO4, MgSO4·7H2O, CaCl2·2H2O, citric acid, ferric ammonium citrate, EDTANa2, and Na2CO3.

3. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to claim 1, characterized in that, In step 1, the Chlorella biomass precursor liquid comprises, by mass ratio: NaNO3 10 mL / L, K2HPO4 10 mL / L, MgSO4·7H2O 10 mL / L, CaCl2·2H2O 10 mL / L, citric acid 10 mL / L, ferric ammonium citrate 10 mL / L, EDTANa2 10 mL / L, Na2CO3 10 mL / L, trace element A5 1 mL / L, H3BO3 2.86 g / L, MnCl2·4H2O 1.86 g / L, ZnSO4·7H2O 0.22 g / L, NaMoO4·2H2O 0.39 g / L, CuSO4·5H2O 0.08 g / L, Co(NO3)2·6H2O 0.05 g / L, with the remainder being water.

4. The method for preparing carbon quantum dot nanofluids synthesized hydrothermally from Chlorella according to claim 1, characterized in that, In step 2, the Chlorella biomass precursor liquid is inoculated into BG-11 medium for light culture and purification to obtain a precipitate. The process includes setting the light intensity of the BG-11 medium for light culture to 2000-3000 Lux, and using an aeration device to oxygenate and prevent algal seed precipitation. After the light culture is completed, the supernatant is removed by siphoning, the precipitate is centrifuged at 7000 rpm for 15 min, and washed with ultrapure water and centrifuged multiple times to remove residual salt ions.

5. The method for preparing carbon quantum dot nanofluids synthesized hydrothermally from Chlorella according to claim 1, characterized in that, In step 3, the process of freezing and drying the obtained precipitate to collect Chlorella powder includes: freezing the obtained precipitate at -80°C for 12 hours, drying it using a vacuum freeze dryer for 24 hours, and collecting Chlorella powder.

6. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to claim 1, characterized in that, In step 4, the process of mixing ultrapure water with Chlorella powder, hydrothermally synthesizing carbon quantum dot products in a muffle furnace, and then centrifuging and dialysis the carbon quantum dot products to remove impurities, thereby obtaining a purified carbon quantum dot solution, includes: Step 4.1: Mix ultrapure water and the Chlorella powder at a solid-liquid mass ratio of 40:1, place in a muffle furnace, and hydrothermally react at 270°C for 6 h to obtain carbon quantum dot products. Centrifuge and dialyze the carbon quantum dot products to remove impurities and obtain purified carbon quantum dot solution. Step 4.2: Centrifuge the carbon quantum dot product at 8000 r / min and collect the supernatant. Filter the supernatant using a 0.45 μm filter membrane and transfer the filtered solution to a 1000 Da dialysis bag for dialysis for 24 h to remove small molecule impurities and salt ions. The resulting solution is the purified carbon quantum dot nanofluid.

7. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to claim 1, characterized in that, In step 5, the characterization tests of the carbon quantum dot nanofluid include: Step 5.1: Prepare Fourier transform infrared (FTIR) samples using the KBr pellet method with the collected carbon quantum dot powder to determine the functional group composition of the carbon quantum dots. Set the scanning range of the infrared spectrometer to 4000–400 cm⁻¹. -1 The resolution is 4cm. -1 The number of scans was 32. Step 5.2: XRD was used to determine the crystal structure, grain size and interatomic spacing of carbon quantum dots; monochromatic Cu Kα1 radiation was used as the X-ray source, and the diffraction angle range of 5-90° was scanned at a scanning speed of 2° / min to obtain XRD images; Step 5.3: XPS was used to accurately analyze the elemental composition and chemical valence state of carbon quantum dots. Al·Kα was selected as the excitation source and the working parameters were 6 mA × 12 kV. The carbon quantum dot sample was scanned in its entirety, and the target elements C, O and N were scanned in detail. Step 5.4: Use TEM to observe the morphology, size and distribution of C-CQDs material. After sonicating the purified C-CQDs solution for 5 min, use a micropipette to drop the liquid sample onto the ultrathin carbon film copper grid, let it dry naturally, leaving a thin layer of liquid adhering to the carbon film, and observe and analyze the sample morphology at 200 kV.

8. The method for preparing carbon quantum dot nanofluids synthesized from Chlorella hydrothermally according to claim 1, characterized in that, In step 5, the carbon quantum dot fluid is regularly spherical and monodisperse, with a particle size range of 0.5-4 nm and an average diameter of 1.54±0.56 nm. The surface contains NH bonds, OH bonds, C=O bonds, and CO bonds. The C, O, and N elements are: C content 74.86%±5%, O content 17.45%±3%, and N content 7.69%±2%.

9. An application of oil displacement technology for carbon quantum dot nanofluids synthesized hydrothermally from Chlorella, wherein the carbon quantum dot nanofluid prepared by the method for preparing carbon quantum dot nanofluids synthesized hydrothermally from Chlorella according to claim 1 is characterized in that... Injecting the carbon quantum dot nanofluid into low-permeability reservoirs reduces the oil-water interfacial tension to 0.17-9.32 mN / m and changes the wettability of the reservoir rock surface from oil-wet to water-wet, achieving a spontaneous adsorption recovery rate of over 46.9%.

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