Construction of Carbon Dioxide Responsive Graphene Point Imbibition and Drainage Fluid and Simulation of Imbibition Experiments

Abstract

The global oil and gas exploration targets are gradually moving towards a new field of oil and gas accumulation with nanopore throats, ranging from millimeter scale to micro-nano pore throats. The development method of tight oil reservoirs is different from that of conventional oil reservoirs, and the development efficiency is constrained. Therefore, it is necessary to construct a nanoscale fluid with strong diffusion and dispersion and improve its permeability, suction, and displacement capabilities. Under the background of CCUS, carbon dioxide flooding is a better way to develop tight reservoirs. However, in order to solve the problem of gas channeling, this paper developed a carbon dioxide-responsive graphene point type surfactant, which has a good gas–liquid synergistic effect. At the same time, graphene nanomaterials are carbon-based and create no environmental damage in oil reservoirs. In this study, graphene quantum dots (GQDs) were prepared using the hydrothermal method, and functional graphene quantum dots (F-GQDs) responsive to carbon dioxide stimulation were synthesized by covalent grafting of amidine functional groups. By characterizing its structure and physical and chemical properties, and by conducting imbibition simulation experiments, its imbibition and drainage ability in nanopore throats is elucidated. Infrared spectrum measurement shows that after functional modification, the quantum dots exhibited new characteristic peaks at 1600 cm⁻¹ to 1300 cm⁻¹, considering the N-H plane-stretching characteristic peak. The fluorescence spectra showed that the fluorescence intensity of F-GQDs was increased after functional modification, which indicated that F-GQDs were successfully synthesized. Through measurements of interfacial activity and adhesion work calculations, the oil–water interfacial tension can achieve ultra-low values within the range of 10⁻² to 10⁻³ mN/m. Oil sand cleaning experiments and indoor simulations of spontaneous imbibition in tight cores demonstrate that F-GQDs exhibit effective oil-washing capabilities and a strong response to carbon dioxide. When combined with carbon dioxide, the system enhances both the rate and efficiency of oil washing. Imbibition recovery can reach more than 50%. The research results provide a certain theoretical basis and data reference for the efficient development of tight reservoirs.

1. Introduction

As is well known, the recovery rate of oil and gas is closely related to reservoir permeability and porosity. For conventional oil reservoirs, the water flooding recovery rate can generally reach 30–40% [1]. For tight oil reservoirs with a median porosity of 20% and a permeability range of μD to mD, the open flow rate of crude oil can be taken as 5–15%. The porosity of shale oil reservoirs is generally less than 15%, and the permeability is less than 1 mD, so their recovery rate will be smaller. The recovery rate of shale oil is generally between 1% and 10% [2,3,4]. In order to improve the crude oil recovery rate of tight and even shale oil reservoirs, it is necessary to further clarify the solid–liquid relationship, improve fluid migration ability, and strengthen the injectability and crude oil utilization ability. Compared to conventional pore throats, nanopore throats exhibit significant nanoconfinement effects. Fluid and rock walls have strong adsorption properties. The confinement effect places high demands on the wetting effect of the injected fluid. In recent years, nanotechnology has gradually been widely applied in the field of oil and gas. Nano oil displacement agents can solve some engineering problems in the traditional oil and gas reservoir extraction process. Practical problems exist, such as poor reservoir injectability, poor environmental adaptability, and significant reservoir damage. Nanomaterials have small size effects, surface effects, wetting properties, and shear thickening properties. These characteristics have broad application prospects in improving the oil and gas recovery rate of tight reservoirs. The retention and transport of carbon-based nanomaterials in porous media have received widespread attention [5].

Silicon dioxide nanoparticles have received significant attention in the development of tight reservoirs in recent years due to their low cost. However, there is no consensus about their diverse modification methods and structural performance mechanisms, and there is still a need to increase scientific research efforts in terms of their dispersibility, which limits their applications [6,7,8,9]. Sheet-shaped nanomaterials have a larger specific surface area and exhibit superior interfacial activity at the oil–water interface compared to three-dimensional spherical particles. Researchers are gradually shifting their focus towards two-dimensional nanosheets [10]. This article is based on the physical properties of unconventional reservoirs and further refines the size of graphene nanosheets to develop graphene quantum dots with better functional characteristics and sizes below 10 nm. The application concentration is reduced to one-thirtieth of traditional surfactants (the application concentration of conventional surfactants is 0.3 wt%, and the experimental concentration of the surfactant developed in this article is 0.01 wt%) for systematic evaluation.

Graphene quantum dots (GQDs) are nanomaterials with a size of less than 10 nm, a nearly spherical structure, and stable luminescence. GQDs feature unique fluorescence properties, along with low toxicity and excellent biocompatibility [11,12,13]. Consequently, they find a wide range of applications in biological imaging, fluorescence sensing, photocatalysis, and organic photovoltaic devices [14,15,16,17]. Due to its inherent carbon-based composition, these GQDs do not need to be separated from oil and gas products. This article primarily explores the potential applications of GQDs in oil and gas field development. GQDs are an emerging particle with rich advantages in oil and gas applications. A structural diagram of graphene dots is shown in Figure 1.

From Figure 1, it can be seen that the center of the graphene quantum dot structure is a hydrophobic grid-like structure, and the periphery is composed of various hydrophilic groups. Graphene quantum dots have amphiphilic structural characteristics, which give them the potential to adjust the oil–water interface [18,19,20,21]. It has good water solubility and biocompatibility, which can minimize pollution from environmental transport to the food chain. GQDs are graphite layered structures, which are carbon dots with a large network of sp²-conjugated island structures. The surface functional groups include hydroxyl, carboxyl, amino, and carbonyl groups. Compared with one-dimensional graphene nanoribbons and two-dimensional graphene nanosheets, zero-dimensional GQDs exhibit stronger quantum confinement and boundary effects below 10 nm in size, and high-concentration nanofluids can also exhibit stronger desorption ability. Stimulation-responsive GQDs have advantages such as excellent interfacial activity, temperature and salt resistance, strong desorption, intelligence, and green environmental protection, and have broad application prospects in the field of tight oil and gas development for achieving the dual carbon goal and combining the mechanism of carbon dioxide displacement to improve oil recovery.

Common carbon dioxide-responsive functional groups include tertiary amino groups, amide groups, amidine groups, and phenolic hydroxyl groups [22,23]. The functional group structure is shown in Figure 2. Amino, hydroxyl, and ester groups have lone electron pairs, and these groups have good protonation ability in water-dispersed fluids. Under the Lewis acid-base reaction, the hydrophilicity of the hydrophilic head group in the molecule is enhanced [24,25]. Furthermore, the dispersion of solutes in the solution is altered, resulting in characteristic changes in the physical and chemical properties of the system. Numerous studies have demonstrated the advantage of amidine in binding CO₂ because amidine is able to promote the formation of relatively stable carbamate salts and the efficient binding of CO₂ by the deprotonation of the hydroxyl group. By comparing the characteristics of different carbon dioxide-responsive functional groups through the system [26,27,28], as shown in

Table 1. Comparison of functional group performance in response to carbon dioxide stimulation.

Serial Number	Functional Group	Advantage	Boundedness
1	Amidine	Strong sensitivity to carbon dioxide; A very small amount of carbon dioxide can quickly respond	The amidine group is alkaline and undergoes a certain degree of hydrolysis in water. But cyclic amidines have the characteristics of simple preparation process and good stability
2	Amine	Weakly alkaline functional group; Boramine absorbs carbon dioxide at room temperature 2.8 times more than the commonly used reagent ethanolamine, but its response speed is slow; Tertiary amines are more sensitive to carbon dioxide response than primary amines, with better repeatability and efficiency	The protonation reaction between the tertiary amino group and carbon dioxide is slow and requires continuous introduction of carbon dioxide for stimulation and regulation.
3	Guanidine	Guanidine and amidine groups have similar properties	Organic strong base
4	Imidazole	The alkalinity is weak, and the protonated imidazole functional group is more stable compared to the amidine and tertiary amine groups	High viscosity, with certain biological toxicity

, this article uses tertiary amidine groups to modify the functionality of GQDs. This study prepared graphene quantum dots using a hydrothermal method and synthesized functional graphene quantum dots responsive to carbon dioxide stimulation through the covalent grafting of amidine functional groups. The successful fabrication of the targeted functional graphene quantum dots was confirmed through characterization using infrared FTIR spectroscopy, particle size analysis, transmission electron microscopy, and fluorescence spectrophotometry. Evaluations were conducted to enhance crude oil recovery through tests measuring surface tension, oil–water interfacial tension, adhesion work calculations, wetting reversal experiments, oil sand washing rates, and indoor physical simulation of imbibition. This further assessed the potential application of the prepared carbon dioxide-responsive graphene quantum dots in the development of tight oil and gas fields. The carbon dioxide-stimulated responsive graphene point surfactant developed in this study has a positive effect on carbon application and tight oil reservoir development in CCUS, and its environmental characteristics have good promotion significance.

2. Material and Methods

Uniformly sized graphene quantum dots were prepared using citric acid high-temperature cracking. Efficient binding of CO₂ was performed through the deprotonation of the hydroxyl group. This study used tertiary amidine groups to modify the functionality of GQDs. The preparation process is shown in Figure 3. During the preparation process, citric acid molecules first underwent a dehydration reaction to form a benzene ring, which gradually grew and eventually generated GQDs. This method can effectively adjust the particle size of GQDs by controlling the reaction time, thus enabling the preparation of GQDs with suitable particle sizes as needed.

2.1. Experimental Materials

Main chemical reagents: citric acid (99%), sodium hydroxide (chemical analysis pure), and deionized water (self-made in this laboratory). Size and physical property parameters of cores: Three naturally dense core columns, each 5 cm in length, 2.5 cm in diameter, and with a porosity of 8%. The core permeabilities are 0.39 × 10⁻³ μm², 0.45 × 10⁻³ μm², and 0.34 × 10⁻³ μm², respectively.

Experimental instruments: Fourier transform infrared spectrometer; X-ray diffraction spectrometer; laser particle size analyzer; heated digital display constant temperature magnetic stirrer; quasi micro-electronic balance, surface thermometer, pH meter, and so on. A parameter list representing the main equipment is shown in

Table 2. List of parameters for experimental structural characterization equipment.

No.	Name of Instrument	Model Specifications	Supplier	Purpose
1	Fourier transform infrared spectrometer	Nicolet iS50	Thermo Fisher Scientific Molecular Spectroscopy Company, Shanghai, China	Structural characterization of synthetic products
2	Laser particle size analyzer	Mastersizer (from 3.8 nm to 100 µm)	Pudi Biotechnology Co., Ltd., Shanghai, China	Evaluation of particle size distribution in dispersed systems
3	Fluorescence spectrophotometer	LS55	Perkin-Elmer Co., Ltd., Shanghai, China	fluorescence spectroscopy measurement
4	UV Visible Absorption Spectrometer	U-3010	Hitachi High-Tech Co., Ltd., Shanghai, China	UV Visible Absorption Spectroscopy Determination
5	Transmission Electron Microscope	H-7650B TEM (80 kV)	Hitachi High-Tech Co., Ltd., Shanghai, China	GQDs morphology observation
6	Contact angle tester	OCA200	Otellino Instrument Co., Ltd., Beijing, China	Wetting reversal characteristic test

.

2.2. Experimental Procedures

Graphene quantum dots were prepared using the citric acid pyrolysis method. The specific preparation process involves placing 2 g of citric acid into a 25 mL beaker and then heating the beaker on a digital constant temperature platform (the bottom temperature of the beaker was measured to be 200 °C). After 3 min, citric acid becomes liquid, and within 15 min, its color changes from colorless to light yellow, orange, and orange-red. The liquid obtained from the reaction was added dropwise into 100 mL of 4 mg/mL NaOH solution while vigorously stirring the solution with a magnetic stirrer, and then the pH of the solution was adjusted with NaOH. During this process, a series of color changes are accompanied, ultimately resulting in a solution of graphene quantum dots.

In order to functionalize two GQDs with amino groups in amidine, GQDs (1 g) were dispersed in chloroform (50 mL), and then amidine (1 mL) was added. Then the mixture was refluxed and stirred at 60 °C for 5 h. After removing amidine and chloroform by rotary evaporation, F-GQDs were obtained by freeze drying. Highly dispersed functionalized GQDs were synthesized through the formation reaction of amide bonds between carboxyl groups and amines in F-GQDs.

2.3. Structural Characterization

Infrared Fourier transform infrared spectroscopy, X-ray diffraction (XRD), particle size analysis, transmission electron microscopy, and fluorescence spectroscopy were used to characterize the structure of the developed functional graphene quantum dots.

2.4. Wetting Reversal Characteristic Experiment

Using dense natural rock core as the experimental physical model, the permeability of the rock core is 0.1 × 10⁻³ μm². Soak the core in water and 0.01 wt% F-GQD separately, and use a contact angle tester to add aviation kerosene to the surface of the core in reverse using a three-phase contact method. The changes of the wetting angle were monitored. When the contact angle between the rock core, oil, and soaking solution is greater than 90°, it indicates that the oil phase cannot spread on the surface of the rock core, and the rock core has hydrophilicity. Conversely, the rock core has hydrophilicity. Experimental conditions: temperature: 20 °C, pressure: 0.1 MPa. The contact angle measuring instrument and method are shown in Figure 4.

2.5. Surface Tension and Interfacial Tension Determination Experiments

The surface and interfacial tension of the nanoflooding agent was determined Experiment condition: temperature 45 °C and 95 °C; pressure: 0.1 MPa. Nanoflooding agents (F-GQDs) were prepared according to the stock solution concentration of 0.01 wt% The experimental oil is aviation kerosene. The equipment adopts a rotating droplet high-temperature interfacial tension meter.

2.6. Calculation of Adhesion Work Reduction Value

The adhesion skill reflects the degree of bonding between rocks and crude oil, and is also the work that needs to be overcome to activate the surface of rocks and crude oil. The greater the viscous force of crude oil on the surface of oil wet rocks, the greater the adhesion work. The adhesion work is related to the interfacial tension of the displacement fluid and the interfacial tension of the rock. When chemical agents act on the surface oil of rock cores, the lower the interfacial tension, the smaller the contact angle until the oil is peeled off. The entire process is also a process of decreasing adhesion work.

In theory, reducing the contact angle to 90° is just enough to activate crude oil, and at this point, the adhesion work is the activation adhesion work. In fact, the contact angle that can stably start crude oil should be less than 90°, and the adhesion work is defined as the actual adhesion work. The adhesion energy of the prepared graphene quantum dots on the target rock core is calculated based on the formula for reducing adhesion energy. This part of the study lays a theoretical foundation for the mechanism of graphene quantum dots initiating core oil production.

$$∆W=W_0{-W}_1$$

(1)

$$\mathrm{\Delta }W={\sigma }_0(1-\cos {\theta }_0)-{\sigma }_1(1-\cos {\theta }_1)$$

(2)

$$\begin{array}{l}{\theta }_0=90°\\ \mathrm{\Delta }W={\sigma }_0-{\sigma }_1(1-\cos {\theta }_1)\\ {\sigma }_0={\sigma }_1\\ \mathrm{\Delta }W={\sigma }_1\cos {\theta }_1\end{array}$$

(3)

The calculation formula for the decrease in adhesion energy is shown in (1) and (2), which is the difference between the theoretical adhesion energy and the actual adhesion energy. In order to simplify the testing process, it is assumed that the interfacial tension remains unchanged before and after the change in contact angle and it is calculated using Formula (3).

2.7. Oil Washing Experiment

To test the oil washing ability of the nano flooding agent and to characterize the properties of the soluble oil and dissolved organic blockage of the F-GQDs, the following procedure was carried out.

To make and age oil sands: First, weigh 7 g kerosene and 20/40 mesh 42 g quartz sand, and then fully stir the oil and quartz sand evenly. Place the oil sand mixture in a 50 °C water bath for 2 h (the temperature loss is set to 50 °C and the simulated reservoir to 45 °C) for aging. After aging, weigh 7 g of oil sands and load into the oil washing test tube with a scale, and pour the 0.01 wt% F-GQDs into the test tube with oil sands to observe the oil washing efficiency. Make parallel experiments to observe the effect of the washing oil. Under the same experimental conditions, continuously input carbon dioxide gas into the test tube, and perform a set of parallel tests to observe the experimental effect. As a blank control, pour water into the oil sands and then feed into carbon dioxide to test the oil-washing effect, and conduct two groups of experiments. Instrument diagram of oil-washing effect experiment are shown in Figure 5

2.8. Imbibition Experiment

The mass method was employed to assess the permeability and absorption efficiency of various fluids acting upon the crude oil within rock cores, with the experiment spanning a total duration of 240 h. The detailed procedure for the permeability experiment is as follows [18]: The rock core, saturated with crude oil, is suspended beneath a high-precision balance using copper wire, which allows for real-time recording of the rock’s weight. Concurrently, it is ensured that the core is fully submerged in a beaker of exudate, preventing any contact between the core and the beaker.

By documenting the weight changes of the rock core at various time points, the quantity of oil permeated from the core and its absorption efficiency can be computed. The calculation Formula (4) is as follows:

$$R={\displaystyle \frac{\raisebox{1ex}{${\rho }_0(M_0-M_{\mathrm{t}})$}\!\left/ \!\raisebox{-1ex}{$({\rho }_w-{\rho }_0)$}\right.}{\mathrm{\Delta }M}}$$

(4)

R: Imbibition recovery rate; ${\rho }_0$: Oil phase density; ${\rho }_w$: Density of reagent dispersion system; $M_0$: Zero time oil leakage and suction; $M_{\mathrm{t}}$: t time oil leakage and suction; $\mathrm{\Delta }M$: Weight difference before and after core saturation.

3. Results and Discussion

3.1. Characterization of Graphene Quantum Dots

3.1.1. Morphology and Particle Size of Graphene Quantum Dots

The prepared F-GQDs were observed and analyzed using TEM scanning electron microscopy. The result is shown in Figure 6. TEM imaging shows that the synthesized graphene quantum dots are monodisperse and uniform small spheres with a median particle size of 7 nm.

3.1.2. Infrared Spectra of Graphene Quantum Dots

To analyze the surface structure of graphene quantum dots, their infrared spectra were characterized; the results are shown in Figure 7. In the infrared spectrum, clear absorption peaks can be observed at 1630 cm⁻¹ and 1359 cm⁻¹, which are considered characteristic peaks of the C=O stretching vibration and the symmetric vibration of carboxylic acid. The peaks at 3438 cm⁻¹ and 1021 cm⁻¹ correspond to the stretching vibration characteristic peaks of hydroxyl -OH and C-O in unsaturated hydroxyl groups. The F-GQDs obtained after functional modification exhibit new characteristic peaks at 1600 cm⁻¹ to 1300 cm⁻¹, considering the N-H plane stretching characteristic peak. The appearance of characteristic peaks can confirm the successful preparation of the target product.

3.1.3. Determination of Fluorescence Intensity of Graphene Quantum Dots

The absorbance and fluorescence intensities were measured using an ultraviolet–visible absorption spectrometer and a fluorescence photometer, respectively. As shown in Figure 8, an absorption peak within 200–250 nm is attributed to the π–π* transition in the sp² domain of graphene quantum dots. The weak absorption band from 250 to 320 nm is associated with the fluorescence of carbon quantum dots [29], and the absorption peak within this range approximately aligns with the maximum excitation wavelength of the corresponding fluorescence excitation spectrum. The dispersion system of F-GQDs exhibits a significant red shift in the fluorescence emission peak (400 nm) and fluorescence enhancement under UV irradiation at 350–450 nm. The results are shown in Figure 8 and Figure 9. This wavelength range satisfies the blue light emission range, further proving the successful development of the target product F-GQDs. Based on the experimental results, future research can enhance the relative fluorescence quantum yield of blue light emission by adjusting the amino grafting ratio.

3.2. Wetting Reversal Characteristic

The oil-treated core was placed in tap water and a nanoflooding agent (F-GQDs) liquid phase pool, respectively, and the contact angle change of clean water and nano flooding agent on the core surface was determined. Studies show that F-GQDs have good wetting inversion ability.

The results are shown in Figure 10. From the experimental diagram, it can be observed that under the same experimental conditions, the three-phase contact angle in water is less than 90°, and the surface of the rock core is lipophilic. Immersed in F-GQDs with a three-phase contact angle greater than 90°, the core surface is hydrophilic. Experiments have shown that F-GQDs have a good ability to wet and reverse rock cores, and oil droplets have a better stripping effect on the surface of rock cores treated with F-GQDs.

3.3. Surface Tension and Interfacial Tension Determination

The nanoflooding agent (F-GQDs) was prepared at a concentration of 0.01 wt%; the results of the interface tension are shown in

Table 3. Interface tension of F-GQDs.

Number	T/°C	Detection Index/ mN/m	Results/ mN/m
1	45	surface tension	25.58
		interfacial tension	0.92 × 10−2
2	90	surface tension	20.35
		interfacial tension	0.11 × 10−2

. According to the experimental data, F-GQDs have strong interface activity, and the interfacial tension of oil and water can reach the medium-high temperature range of 10⁻³ mN/m.

3.4. Calculation of Adhesion Reduction Value

Combined with the interfacial tension test and wetting angle data, the adhesion work value was calculated, and the ability of F-GQDs nanofluiding to start core crude oil was further evaluated. The experimental data are shown in

Table 4. Data table of calculation of theoretical adhesion and actual adhesion. (The table data show the average of five test results. The standard deviation of IFT testing (0.92) is 0.052; the table data show the average of five test results. The standard deviation of IFT testing (0.11) is 0.041).

No.	IFT (10−2 mN/m)	Contact Angle θ (°)	cosθ	W0 (10−2 mN/m)	W1 (10−2 mN/m)	Wettability
1	0.92	121.51	−0.52	0.92	1.40	lipophilicity
2	0.92	58.53	0.52	0.92	0.44	hydrophily
3	0.11	121.51	−0.52	0.11	0.17	lipophilicity
4	0.11	58.53	0.52	0.11	0.05	hydrophily

. The study proves that when the surface wetting of the core changes from oily wetting to water wetting, the adhesion work is greatly reduced, the interface tension is reduced, and the theoretical adhesion work is reduced. The calculated value of adhesion work indicates that the F-GQDs have a good ability to start crude oil.

3.5. Oil Washing Experiment Analysis

The experimental results are shown in

Table 5. Effect results of F-GQDs (20 °C).

Number	Sample	Oil Sand Contains Oil Volume (mL)	Wash Oil Volume (Ml)	Oil Washing Rate (%)	Average Value (%)	Standard Deviation
1	F-GQDs	1.25	1.10	88.00	86.00	2.828
2	F-GQDs	1.25	1.05	84.00
3	CO2 + F-GQDs	1.25	1.20	96.00	97.00	1.414
4	CO2 + F-GQDs	1.25	1.22	98.00
5	CO2 + Water	1.25	0.05	4.00	4.80	1.131
6	CO2 + Water	1.25	0.07	5.60

, revealing an overall average oil-washing rate of 86%, and the average oil washing rate of the F-GQDs was 97%. In the blank control test, carbon dioxide was used in the water, and the average oil-washing efficiency was 4.80%. The oil-washing efficiency of CO₂ + F-GQDs was significantly greater than the washing efficiency of a single mode. The carbon dioxide stimulation responsiveness of the surface functional groups of F-GQDs improves the mixing ability of crude oil, which then greatly improves the oil-washing efficiency.

3.6. Imbibition Experiment Analysis

The imbibition experimental curves of the three dispersion systems (CO₂ + Water, 0.01 wt% F-GQDs, and CO₂ + 0.01 wt% F-GQDs) are shown in Figure 11. During the imbibition process of F-GQDs, the introduction of carbon dioxide leads to an increase in the slope of the data line prior to the plateau phase of the initial imbibition curve, effectively showcasing the enhanced imbibition rate. Experimental data from the three curves reveal that F-GQDs exhibit a superior ability to prevent carbon dioxide gas migration compared to water and can produce a synergistic effect, significantly enhancing the imbibition efficiency.

The imbibition experiment demonstrates that the F-GQDs system exhibits effective oil-washing capabilities, achieving a crude oil recovery rate of 45% after 240 h. When combined with carbon dioxide, the system’s oil recovery rate accelerates under the same conditions, further increasing the recovery rate to over 50%. The recovery rate of the carbon dioxide and water system serving as the blank control is approximately 20%. The comprehensive experimental data reveal that the F-GQDs system shows promising application potential for enhancing oil recovery from tight reservoirs.

4. Conclusions

(1) This article developed a carbon dioxide-responsive graphene quantum dot-type surfactant (F-GQDs). The successful development of the target product was confirmed by infrared and fluorescence spectroscopy. Transmission electron microscopy characterization showed that the prepared F-GQDs were a highly dispersed system with a median particle size of 7 nm.

(2) The oil–water interfacial tension, surface tension, and adhesion work of 0.01 wt% F-GQDs were measured. Compared to conventional surfactants, F-GQDs can achieve ultra-low interfacial tension at a concentration 30 times lower, indicating strong interfacial activity and wetting reversal ability. The calculated value of adhesion work indicates that F-GQDs have good crude oil starting ability.

(3) Oil sand experiments show that F-GQDs have a good oil-washing effect of more than 86%. After introducing carbon dioxide, the efficiency of F-GQDs wash oil can reach 97%. F-GQDs can enhance the imbibition recovery rate, reaching 45% within 240 h. After combining with carbon dioxide, it is further increased to over 50%. The F-GQDs system has good application potential in the exploitation of tight oil reservoirs.

Based on the above research data, the prepared F-GQDs have good potential to improve oil recovery and achieve excellent performance. After compounding with carbon dioxide, the oil-washing ability is greatly improved. In the future, the comprehensive performance and oil displacement effect of F-GQDs in porous media will be studied. Tight reservoirs are usually subjected to acid fracturing measures. Subsequent research will focus on evaluating the effectiveness of different system combinations, as well as stability testing of factors such as temperature, salinity, and pH, in order to achieve a long-term stable and efficient nanofluidic oil displacement system for unconventional tight reservoirs.