1. Introduction

Table 1. Comparison of functional group performance in response to carbon dioxide stimulation [16,17,18].

2. Material and Methods

Figure 3. Mechanism of graphene dot synthesis by citric acid pyrolysis.

Figure 3. Mechanism of graphene dot synthesis by citric acid pyrolysis.

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 μ m². Soak the core in water and 0.1wt% 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. Monitor the changes of the wetting angle. Experimental conditions: temperature: 20 ℃, pressure: 0.1MPa.

Figure 4. Contact angle tester (right: measuring sample).

Figure 4. Contact angle tester (right: measuring sample).

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.Calculate the adhesion energy of the prepared graphene quantum dots on the target rock core 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.

$$\Delta \mathrm{W}={\mathrm{W}}_0-{\mathrm{W}}_1$$

(1.1)

(1.2)

(1.3)

The calculation formula for the decrease in adhesion energy is shown in 1.1 and 1.2, 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. Calculate using formula 1.3.

2.7. Oil Washing Experiment

To test the oil washing ability of the nanorepellent solution and to characterize the properties of the soluble oil and dissolved organic blockage of the F-GQDs.

Make oil sands and aging: first weigh 7g kerosene and 20 / 40 mesh 42g quartz sand, the oil and quartz sand fully stir evenly. Place the oil sand mixture in a 50℃ water bath for 2h (Consider the temperature loss set 50℃ simulated reservoir 45℃) for aging. After aging, weigh 7g of oil sands and load it into the oil washing test tube with scale, and pour the 0.1% F-GQDs into the test tube with oil sands to observe the oil washing efficiency. Make parallel experiments to observe the effect of washing oil. Under the same experimental conditions, carbon dioxide gas was continuously input into the test tube, and a set of parallel tests were done to observe the experimental effect. As a blank control, water was poured into the oil sands and then fed into carbon dioxide to test the oil washing effect, and two groups of experiments were conducted.

Figure 5. Instrument diagram of Oil washing effect experiment.

Figure 5. Instrument diagram of Oil washing effect experiment.

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 hours. 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 formulas 1.4are as follows:

(1.4)

R:Imbibition recovery rate; ρ_o: Oil phase density; ρ_w: Density of reagent dispersion system

M₀:Zero time oil leakage and suction; M_t: t time oil leakage and suction;$△$M: Weight difference before and after core saturation

3. Results and Discussion

3.2. Wetting Reversal Characteristic

The oil-treated core was placed in tap water and nano repellent (F-GQDs) liquid phase pool respectively, and the contact angle change of clean water and nano repellent on the core surface was determined.Studies show that F-GQDs have good wetting inversion ability. The results are shown in Figure 8,after soaking in F-GQDs liquid, the oil drop on the surface of the core were changed from oily to hydrophilic.

Figure 10. Contact angle determination results.

Figure 10. Contact angle determination results.

3.3. Surface Tension and Interfacial Tension Determination

The nano repellent (F-GQDs) was prepared according to the concentration of 0.1%, and the results of the interface tension are shown in Table 1. According to the experimental data, it can be found that F-GQDs have strong interface activity, and the interfacial tension of oil and water can reach the medium-high temperature range of 10^-3mN/m.

Table 1. Interface tension of F-GQDs.

Table 1. Interface tension of F-GQDs.

Number	T/℃	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

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 2.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, the theoretical adhesion work is reduced, and the adhesion work is reduced to increased. The calculated value of adhesion work indicates that the F-GQDs have a good ability to start crude oil.

Table 2. Data table of calculation of theoretical adhesion and actual adhesion.

Table 2. Data table of calculation of theoretical adhesion and actual adhesion.

No.	IFT (10-2mN/m)	contact angle θ (°)	cosθ	W0(10-2mN/m)	W1(10-2mN/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

3.5. Oil Washing Experiment Analysis

The experimental results are shown in Table 3, which was 86%, and the average oil washing rate of 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 is significantly greater than the washing efficiency of a single mode. Considering 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.

Table 3. Effect results of F-GQDs (20℃).

Table 3. Effect results of F-GQDs (20℃).

Number	Sample	Oil sand contains oil volume(ml)	Wash oil volume(ml)	Oil washing rate(%)	Average value(%)
1	F-GQDs	1.25	1.10	88.00	86.00
2	F-GQDs	1.25	1.05	84.00
3	CO2+F-GQDs	1.25	1.20	96.00	97.00
4	CO2+F-GQDs	1.25	1.22	98.00
5	CO2+Water	1.25	0.05	4.00	4.80
6	CO2+Water	1.25	0.07	5.60

4. Conclusions

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