Introduction

Significant amounts of initial reservoir oil remain unrecoverable during water flooding, often up to 60%, due to bypassing and improper oil sweeping1,2. For this reason, effective enhanced oil recovery (EOR) methods are crucial3. Nano-technology offers a novel approach to EOR, mainly through nanofluids-colloidal suspensions containing nanomaterials4,5,6,7. These nanofluids, created by dispersing nanoparticles (NPs) in water or a base fluid, exhibit high EOR performance owing to their small size. Unlike chemical EOR methods, nanofluid systems have minimal negative environmental impact and can be widely employed8. Therefore, this method has attracted more attention recently9,10. Nanofluids can reduce interfacial tension (IFT)11, reduce asphaltene deposition12, modify the mobility ratio13, and improve wettability alteration14,15 for better EOR. Also, they can improve polymer/surfactant flooding methods by optimizing the surfactant concentration16. Among the mentioned factors, IFT is vital in recovering residual oil in porous media17. NPs can improve the transfer of asphaltene to the interface of oil and brine and lead to decreased IFT18,19,20. Despite the volume of literature on IFT reduction by NPs, there are many problems regarding operational use and actual flooding conditions by nanofluids. NPs are sometimes greater than the reservoir’s pore-throat in highly dense formation, and if injected into the porous media, they block the fluid path. Additionally, research has shown that blockages in the pore channels occur swiftly when the size of NPs surpasses 10% of the pore channel’s diameter21. Developed nanocomposites also have good stability and IFT reduction, but producing them requires a lot of cost and energy. Accordingly, the synthesis method of NP should be easy and cost-effective for field-scale applications22. One of the best alternatives for addressing the challenges of large-size NPs is using smaller, more stable carbon NPs, known as graphene quantum dots (GQDs), which have received much attention recently.

GQDs have high stability and thermal conductivity, proper biocompatibility, and dispersal properties23,24. They are a family of carbon NPs with a lateral size under 20 nm25. These nano-particles have good characteristics, including high specific surface areas26, low toxicity, and high water solubility27,28. Proper GQDs are also compatible with water formation and maintain thermal stability over time29,30. These carbon NPs also have many available sites for functionalization31,32, which give them extraordinary properties that make them good candidates for drug delivery, biosensors, and nanofluids28,33,34,35. Moreover, these NPs can be easily doped with various dopants, tailoring them for EOR applications. So far, many investigations of EOR studies used silicon-based NPs, while other possibly suitable NP classes, such as GQDs, have been neglected36,37. Furthermore, with the new and simple methods, the production of GQDs has become very inexpensive and cost-effective. Researchers have proposed a low-cost synthesis method using a hydrothermal approach for producing GQDs27,38. Hydrothermal synthesis is a popular method for producing GQDs due to its simplicity, scalability, and cost-effectiveness39,40. Compared to other expensive methods, GQDs could be a more economically viable option for large-scale operations41,42. GQDs derived from ethylenediamine possess favorable attributes due to their relatively straightforward synthesis methods43 and could be a cost-effective solution for EOR applications. However, additional research and field trials are necessary to validate their performance and cost-efficiency in large-scale petroleum operations. Researchers must explore GQD performance under reservoir conditions and assess their effectiveness in enhancing oil recovery.

Several research papers investigated the effect of GQDs on IFT reduction for EOR applications. In 2020, Afzali Tabar et al.44 investigated using nanohybrid GQDs/MoS2 for EOR in carbonate rock. They illustrated that the performance of some surfactants in IFT reduction with these NPs shows a 70% improvement. In the same year, Zhou et al.45 used silicon quantum dots to enhance oil recovery in tight cores. In 2021, Shayan Nasr et al.36 evaluated the effect of nitrogen-doped graphene quantum dots (N-GQDs) on EOR performance. They reported considerable IFT reduction by these NPs in oil/water and n-heptane/water systems. In 2022, Ranjbar et al.46 used two types of carbon quantum dots (CQDs) with dual functions as a sensing and an enhancing agent for EOR operations. They improved the recovery factor on a micromodel pattern by 27.8% and 20.5% with only 200 ppm of two types of CQDs. Although, their CQDs decrease the IFT of oil and brine only from 38.6 to 32.7 and 35.8 mN/m. In 2023, Cao et al.47 used CQDs nanofluid to reduce injection pressure and improve oil recovery. They showed that 1000 ppm CQD solution has a pressure reduction rate of 35%. Also, they expressed that 2000 ppm of their CQDs could reduce the IFT from 30 mN/m to 17 mN/m. The same year, Mirzavandi et al.37 used silica-GQDs on IFT reduction and wettability alteration for EOR application on carbonate reservoirs. They presented that dispersing 1000 ppm silica-GQDs in deionized water enables an additional 14.4% oil recovery due to a significant reduction in the IFT value from 28.3 to 9.5 mN/m. The comparison between GQDs and NPs for EOR application is provided in Table 1. It highlights key parameters such as size, penetration depth, stability, cost-effectiveness, and surface functional groups, showcasing the advantages of GQDs over NPs in various aspects.

Table 1 The comparison between GQDs and NPs for EOR application23,45,48,49.
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Although the impact of GQDs on IFT is recognized, there has been a lack of comprehensive studies exploring the influence of various GQD precursors and their functional groups on IFT changes and the underlying mechanisms. Additionally, no investigations have examined the impact of GQD synthesis time and precursor mole fractions on IFT reduction. Moreover, most literature indicates a slight decrease in IFT due to GQDs. However, certain studies suggest a more significant IFT reduction when GQDs are combined with surfactants, as GQDs alone do not significantly reduce IFT.

This study represents the first systematic investigation into synthesizing various types of N-GQDs and characterizing their functional groups for GQDs surface activity and IFT reduction between crude oil and water. GQDs were produced using the hydrothermal method with ethylenediamine/urea/thiourea and citric acid. The IFT of nanofluid and crude oil in different GQD concentrations was investigated. Innovatively, the characterization tests were done to identify the GQDs’ chemical composition and functional group on their surface to evaluate the IFT reduction results. Two factors, synthesis duration and the molar composition of the precursors have been investigated to explore the effect of the synthesis procedure. Moreover, the effect of salt ions on the IFT combined with GQDs was evaluated. The results of this study provide valuable insights into overcoming the challenges of enhancing oil recovery in unconventional reservoir development. This is achieved by significantly reducing IFT without the use of any surfactant.

Materials and methods

Materials

In this research, sodium chloride (NaCl) and precursor materials like citric acid (C6H8O7), ethylenediamine (C2H4(NH2)2), urea (CH4N2O), thiourea (CH4N2S) were obtained from Merck Co. (Germany) without any impurity. The prepared brines in the IFT tests have salinities ranging from 0 to 200,000 ppm. The crude oil used in this research was obtained from one of the Iranian oil fields (18.61 API @ 60 °F). The dynamic viscosity of the crude oil at the experimental pressure (14.7 psi) and temperature (20 °C) was measured to be 142 centipoise (cP). The dialysis membrane for material purification was purchased from Spectra/Por with a retained molecular weight of 1000 Da.

Methods

Syntheses of N-GQDs and nanofluids preparation

N-GQDs were synthesized by a one-pot hydrothermal carbonization process43,50,51 using citric acid as a carbon (C) source and ethylenediamine, urea, and thiourea as doping material precursors for nitrogen (N) and Sulphur (S) source50,52,53,54. To prepare the N-GQDs (EDA31_5h), ethylenediamine and citric acid, by the molar ratio of 3, are added in 50 ml ultrapure water and mixed for 20 min using magnetic stirring. After that, the solution was transferred to a 200 mL (total volume) Teflon-lined hydrothermal autoclave and was kept in an electronic oven for 5 h at 180 °C. A light yellow solution was obtained after cooling it naturally at room temperature. The solution was filtered through a 0.22-micron syringe. Further, the solution was centrifuged for 10 min at 10,000 rpm, and the supernatant was collected to get N-GQDs. Finally, a pale-yellow powder denoted as EDA31_5h was obtained after dialysis and freeze-drying. The other N-GQDs were prepared in the same ways shown in Table 2. The N-GQDs produced by urea and thiourea are blue and green, respectively. In these syntheses, if the mole fractions of urea or thiourea decrease, the nanofluids’ color blue or green fade to black.

Table 2 The N-GQDs synthesized by different sources and conditions.
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In addition, a certain quantity of N-GQDs powder was added to a specific volume of deionized water for nanofluid preparation. Then, NaCl was added to the nanofluid at pre-defined concentrations to obtain the desired nanofluid. This article prepared the nanofluids at various ratios with N-GQDs and NaCl concentrations of 0–4000 ppm and 0–200,000 ppm, respectively. The extrinsic feature and preparation simulations of three GQDs are exhibited in Fig. 1.

Fig. 1
figure 1

The preparation schematic of the three quantum dots.

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Nanofluid and crude oil characterization

High-Resolution Transmission Electron Microscope (HR-TEM) images of N-GQDs were performed using TECNAI F20 instrument. Fourier transform infrared (FTIR) spectroscopy measurements were obtained on a Nicolet iS10 FTIR spectrometer at room temperature in the wavelength range of 400–4000 cm-1. UV–Vis absorption spectrum was determined under UV light using a DR6000 UV–Vis spectrometer in the absorption range of 190–1100 nm. Photoluminescence (PL) spectra of GQDs’ samples were recorded using Agilent-G980A/USA instrument. The XRD pattern of samples was performed using EQUIN3000X x-ray diffraction with a suitable amount of powder. The X-ray photoelectron spectroscope (XPS) was used to measure the surface composition of N-GQDs within a depth of b10 nm (BESTEC EA10, X-ray photoelectron spectrometer, Germany). In this paper, the crude oil used for the experiments was obtained from an Iranian oil field. SARA fraction of the crude oil is shown in Table 3. Heavy components, such as asphaltenes and resins, play a significant role in IFT reduction. These components tend to adsorb at the oil–water interface, creating a more stable interface and thereby reducing the IFT. The presence of heavy polar compounds in the crude oil and their interaction with the aqueous phase directly impacts the effectiveness of the IFT reduction. All solution and crude oil density were measured using the Anton-Paar instrument at ambient temperature and atmospheric pressure. It should be noted that for real field evaluation, the IFT test needs to be conducted at reservoir pressure and temperature with live oil.

Table 3 Used crude oil specification.
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Interfacial tension measurement

The IFT between the nanofluid and the crude oil was measured using the pendant drop shape method, as shown in Fig. 2, using drop shape analysis (HT-42-20P, Germany) at different GQD concentrations. A needle was used to create an oil droplet. A motor-driven piston loaded the crude oil into the syringe. Images captured from the system by the Dino digital microscope camera were analyzed to calculate IFT values.

Fig. 2
figure 2

Schematic diagram of the drop shape analyzer for IFT measurement.

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The amount of IFT was determined via the diameter shape analysis method using the following Eq. (1)55,56:

$$\gamma = \frac{{\Delta \rho gD^{2} }}{H}$$
(1)

where g is gravitational constant, Δρ is density difference between crude oil and nanofluid, D is maximum horizontal diameter, and H is shape-dependent parameter that can be obtained from the table provided by Bartell and Niederhauser57 as a function of the d/D ratio. d is diameter at distance D from the top of drop55,56,58. Afterward, Misak 196859 and Rotenberg et al.60 investigated the methods and formula for calculating IFT. The deionized water and air IFT at 20 °C was calculated to calibrate the setup and the technique, as shown in Table 4. In this study, all experiments were performed with software two (Image-J), and the other software (DSA) was used only to confirm the trend of IFT changes. The needle diameter was carefully chosen within an appropriate range. Reducing the needle’s diameter allows the oil droplet to float at very low levels, enabling the calculation of near-zero interfacial tension values61. Detailed methodologies for IFT experiments can be found in other sources62,63.

Table 4 Calculated IFT between deionized water and air at 20 °C using different software.
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Results and discussion

Characterization of N-GQDs and nanofluids

UV–Vis and PL tests

The UV–Vis and PL spectra are shown in Fig. 3. There are two absorption bands at about 340 nm and 240 nm for EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs (yellow line, blue line, and green line, respectively). These absorption bands, which are close to other studies52,54,64, are related to n → π* and π → π* transition of C=O and C=C bond in the GQDs. All the GQD samples show strong and bright blue emissions at about 450 nm, excitation-independent emissions in the PL spectrum (the red lines), and full width at half maximum of 65 nm, which proposes that GQD size and surface state are almost uniform53,65.

Fig. 3
figure 3

UV–Vis spectrum for EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs (yellow line, blue line, and green line, respectively) and their photoluminescence spectra at 360 nm (red lines).

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Figure 4 illustrates EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs solutions, which show a brown-yellow color under daylight and a bright blue color under a 365 nm UV lamp. These strong fluorescence are attributed to the nitrogen doping66.

Fig. 4
figure 4

Photos of EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs in water under daylight (brown yellow, left-side) and UV light (bright blue, right-side).

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FTIR tests

Functional groups on the surface of GQDs play a significant role in their surface activity, can interact with oil and water molecules, reducing the IFT between them. This helps in stabilizing the oil–water interface and improving the efficiency of oil recovery. To determine the functional groups on the surface of EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs, the FTIR spectra are investigated in Fig. 5.

Fig. 5
figure 5

FTIR spectra of EDA31_5h, Urea31_5h and Thiourea31_5h GQDs.

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The broad peaks at 3212 cm-1, 3204 cm-1, and 3223 cm-1 indicate the presence of O–H stretching vibrations for EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs, respectively. The broad absorption bands at 3056 cm-1, 3053 cm-1, and 3060 cm-1 are characteristic peaks of N–H stretching vibrations. Thus, the GQDs should have excellent water solubility due to these polar functional groups on their surface. The weak pick at 2540 cm-1 in the Thiourea31_5h is indicated to S–H stretching modes66,67. The strong picks at 1636, 1709, and 1714 are assigned to C=O stretching modes or carboxyl acid groups (–COOH) and picks at 1559, 1576, and 1570 are assigned to C=N stretching modes. The 1378, 1438, and 1407 cm-1 peaks indicate C–N stretching modes. The FTIR bands within the 1000–1400 cm-1 range are attributed to the stretching vibrations of C–O, C–S, and scissoring and bending of C–H68. The C–S bonds are noted by absorption peak at 1188 cm-1 for Thiourea31_5h GQDs53.

XRD tests

The XRD diagrams (supplementary Figure S1) show diffraction peaks for EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs at around 21.6°, 24.6°, 20.6° and also correspond to the lattice spacing of 0.41, 0.36, 0.43 nm, respectively. These values are higher than the results of HR-TEM due to the existence of functional groups on GQD surfaces43,69.

TEM tests

The morphology of EDA31_5h GQDs samples was characterized by TEM. The particle size was measured with the TEM image and ImageJ software (Fig. 6). As seen from the figure’s inset, the size distribution of the EDA31_5h GQDs is between 3 and 7 nm. The associated HR-TEM images reveal that EDA31_5h GQDs discloses a graphite nature. Also, the measured lattice fringe distance for the HR-TEM images is 0.24 nm, corresponding to the (1120) crystal phase of graphite50.

Fig. 6
figure 6

TEM image (left-side), HR-TEM image for lattice spacing (right-side) of EDA31_5h GQDs samples.

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XPS tests

XPS tests provided a more comprehensive picture of the chemical composition and type of chemical bonding of EDA31_5h, Urea31_5h, and Thiourea31_5h GQD materials. Figure 7 shows the wide full-scan XPS spectrum with four distinct peaks at about 284, 401, 532, and 164 eV, corresponding to C1s, N1s, O1s, and S2p, respectively.

Fig. 7
figure 7

XPS spectra of EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs. The first row is the full scan survey, and the second, third, fourth, and fifth (only for Thiourea31_5h) rows are the high-resolution C1s, N1s, O1s, and S2p XPS spectra.

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The high-resolution C1s spectrums in the figure (second row) are de-convoluted into three peaks with binding energies around 284.5 eV,286 eV, and 288.4 eV, which are assigned to C=C/C–C, C–O/C–N/C–S, C=O respectively. The N1s spectrums in the figure (third row) show two peaks with binding energies at 399.8 eV and 401.5 eV, which is attributed to pyrrolic N (H–N–C2) and graphite N (N–C3). The fourth row illustrates the O1s spectrums of bonds C=O (531.4 eV) and C–O (533.1 eV). Besides, the S2p spectrum peaks at 163.5 eV and 164.6 eV, corresponding to S2p3/2 and S2p1/2 for Thiourea31_5h GQDs. The results of XPS confirm the doping of N and O atoms in the prepared EDA31_5h and Urea31_5h GQDs; also, N, O, and S atoms in the Thiourea31_5h GQDs. The N peak intensity is strong in full scan XPS spectra of EDA31_5h. On the other hand, the N signal can be barely seen in the Urea31_5h and Thiourea31_5h GQDs. Moreover, the O peak for the Thiurea31_5h is stronger than others. Figure 8 illustrates several column charts drawn from the XPS tests that can be utilized to analyze the IFT results.

Fig. 8
figure 8

The ratio of peak intensity N1s /C1s (× 10 for better exhibition) and O1s /C1s in the full scan XPS spectra, sp3 C/sp2 C in high-resolution C1s XPS spectra, and pyrrolic N and graphite N/pyrrolic N in high-resolution N1s XPS spectra for EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs, respectively.

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The peak intensity ratios of N1s/C1s (RN/C) for EDA31_5h GQDs is about 0.12, whereas RN/C for Urea31_5h and Thiourea31_5h GQDs is about 0.4 and 0.5, respectively. This indicates that the N doping degree of EDA31_5h GQDs is higher than other N-doped GQDs in this report70. The relative intensity ratio of sp3 C/sp2 C and pyrrolic N/graphite N shows more value for EDA31_5h GQDs because more N atoms exist in its graphene surface50,71. The peak intensity ratio of O1s/C1s (RO/C) for Thiourea31_5h GQDs is 0.54, which is higher than the others. Thus, the Thiourea31_5h GQDs may have a more polarity in water solutions.

Effect of N-GQDs types on IFT

To evaluate the effect of ethylenediamine-derived, urea-derived, and thiourea-derived GQD concentrations on the IFT between the nanofluid and crude oil, deionized water containing 15, 50, 150, 500, 1000, and 4000 ppm GQDs was used. All IFT tests were conducted over a sufficient duration (30 min) to allow the oil drop to reach static equilibrium. For each IFT value, at least three tests were performed at every data point, and the reported values represent averages. Additionally, in some cases, repeated syntheses were carried out under identical conditions to verify test accuracy. Two factors (synthesis duration and the molar composition of the precursors) were investigated to assess the impact of the synthesis procedure. The tree diagram illustrating all the IFT tests performed can be found in supplementary Figure S2. The IFT values between crude oil and nanofluid for different concentrations of EDA31_5h (with error bars), Urea31_5h, and Thiourea31_5h GQDs are shown in Fig. 9.

Fig. 9
figure 9

IFT changes of crude oil with different concentrations of EDA31_5h (with error bars), Urea31_5h, and Thiourea31_5h GQDs.

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The graph shows that the yellow curve, associated with EDA31_5h, exhibits the most significant decrease in slope. These dots can effectively absorb non-polar oil molecules with their nitrogen elements and the ability to form bonds on both sides (pyrrolic N) and non-polar carbon branches50. Furthermore, GQDs are equipped with polar atoms (such as oxygen and nitrogen) on their surface, which allows them to absorb polar water molecules. This unique property enables them to play a pivotal role in bridging two phases, similar to surfactants, and attracting asphaltene and resins to the interface between the two phases. Therefore, EDA31_5h GQDs have a significant impact on the reduction of IFT, a key factor in various industrial processes. This description can be confirmed based on XPS tests, which reveal a high RN/C ratio in EDA31_5h GQDs. The N doping degree of EDA31_5h GQDs is higher than other N-doped GQDs, and the ratio of sp3 C/sp2 C and pyrrolic N/graphite N shows more value for EDA31_5h GQDs. Pyrrolic N forms two bonds with carbon atoms, creating straight-chain hydrocarbon branches with nonpolar properties. This leads to better polar-nonpolar phase bonding. These unique properties of EDA31_5h GQDs are a testament to their potential in the IFT reduction, marking a significant advancement in our understanding of EOR field operation. In contrast, the blue graph shows a slight decrease in the Urea31_5h IFT change. According to the XPS tests, Urea31_5h GQDs have low pyrrolic N and sp3 C/sp2 C content. Unlike ethylenediamine, urea molecules have three binding ports during synthesis. These structural differences increase the polarity of Urea31_5h GQDs, reducing their ability to be present at the interface of two fluids and eliminating IFT reduction. The green curve represents the IFT value of Thiourea31_5h GQDs. These GQDs contain not only O and N atoms but also S atoms. Their increased polarity arises from the high RO/C ratio in their molecular structure. Consequently, they cannot create a strong connection between the polar and nonpolar phases, resulting in unchanged IFT. In conclusion, the functional groups on GQDs enhance their surface activity, improve dispersion stability, and facilitate molecular interactions, all of which contribute to effective IFT reduction in oil and nanofluids.

Effect of synthesis time of N-GQDs on IFT

To investigate the effect of synthesis time on IFT, ethylenediamine, urea, and thiourea were synthesized with the help of citric acid as the carbon source in 5, 10, and 15 h. Figure 10 is related to the IFT of ethylenediamine-derived GQDs at different synthesis durations and GQDs concentrations.

Fig. 10
figure 10

IFT changes of crude oil and different concentrations of ethylenediamine-derived GQDs solutions with various synthesis times (EDA31_5h, EDA31_10h, EDA31_15h).

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Based on the graph, GQDs derived from ethylenediamine exhibit the lowest IFT values at 5, 10, and 15 h of synthesis durations. It’s important to note that GQDs are not synthesized in less than 5 h due to their low quantum yield values, resulting in limited GQD formation. According to the results, the decrease in IFT for EDA31_5h is more significant than for EDA31_10h GQDs. However, the 10-h and 15-h graphs are almost equal and have no meaningful differences. Extended synthesis times can sometimes lead to increased agglomeration and larger nanoparticle sizes36. The high IFT observed for EDA31_10h compared to EDA31_5h may be attributed to the larger sizes of GQDs. Interestingly, when GQDs transform into NPs and grow in size, their ability to reside at the interface between two phases diminishes, resulting in reduced power for IFT reduction. Another contributing factor could be the variation in pyrrolic N content within ethylenediamine-derived GQDs. Specifically, as synthesis time increases, the proportion of graphite N increases while pyrrolic N decreases50. Graphite N, which is embedded within the graphene structure rather than on the surface, does not significantly impact IFT reduction. Consequently, EDA31_10h GQDs exhibit a lesser effect on reducing IFT. Figure 11 is related to the IFT of urea-derived GQDs at different synthesis durations and GQD concentrations.

Fig. 11
figure 11

IFT changes of crude oil and different concentrations of urea-derived GQDs solutions with various synthesis times (Urea31_5h, Urea31_10h, Urea31_15h).

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As the GQDs concentration increases, the curves for urea-derived GQDs exhibit a lower slope than those of ethylenediamine-derived GQDs. In contrast to ethylenediamine-derived GQDs, the urea-derived GQDs synthesized for 10 h show IFT values lower than those synthesized for 5 h, with a variation of approximately one unit. Therefore, extending the synthesis time for urea GQDs reaches an optimal point, and further increases lead to a growth in IFT. According to existing research, the IFT of GQDs occasionally decreases or increases with longer synthesis times and larger particle sizes36. Under constant salinity and nanofluid concentration, varying nanoparticle sizes from 10 to 20 and 30 nm increases IFT72. Smaller nanoparticle sizes correlate with larger IFT reduction. As nanoparticle size decreases, the specific surface area increases. This increase in surface area enhances the effectiveness of the structure formed between the nanofluid interface and crude oil, consequently reducing IFT72,73. On the other hand, for GQDs, increased synthesis time and larger GQD sizes result in non-constant surface functional groups (unlike non-carbonic nanoparticles such as TiO2 or MgO). This variability may release more polar atoms like O and S or alter the proportion of graphite-N and pyrrolic-N50, affecting GQD surface activity. Thus, IFT for GQDs may either decrease or increase with longer synthesis times and larger GQD sizes, as other researchers have also reported36. Thus, different precursors may have different IFT-changing behaviors. Figure 12 is related to the IFT of thiourea-derived GQDs at different synthesis durations and GQD concentrations.

Fig. 12
figure 12

IFT changes of crude oil and different concentrations of thiourea-derived GQDs solutions with various synthesis times (Thiourea31_5h, and Thiourea31_10h).

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Consequently, thiourea-derived GQDs exhibit the least capability for IFT reduction. Additionally, the IFT value of Thiourea31_5h GQDs is nearly identical to that of Thiourea31_10h GQDs.

Generally, the synthesis duration significantly influences the IFT reduction of GQDs solution and crude oil. This duration varies depending on the specific N-doping precursors used. Our study found that the choice of N-doping precursor has a greater impact on IFT reduction than the actual synthesis duration. It’s essential to note that both insufficient and excessive synthesis time can lead to a decrease in the quantum yield of GQDs and unnecessary energy consumption. Considering industrial projects, cost-effectiveness is crucial. Therefore, in addition to optimizing IFT by synthesis time, the quantity of GQDs produced should also be considered.

Effect of precursors molar ratio of N-GQDs on IFT

To investigate the impact of precursor mole fraction on IFT, ethylenediamine, urea, and thiourea were synthesized using citric acid as the doping material at 3, 1, and 0.5 mol fractions. Then, their IFTs were analyzed in the presence of different GQD concentrations. Figure 13 illustrates the IFT of various GQDs concentrations with varying ratios of ethylenediamine to citric acid, considering a fixed synthesis time of 5 h.

Fig. 13
figure 13

IFT changes of crude oil and different concentrations of ethylenediamine-derived GQDs with various mole fractions of precursor (EDA31_5h, EDA11_5h, and EDA21_5h).

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Based on the graph, the lowest IFT value corresponds to EDA31_5h GQDs. Additionally, the changes in IFT for EDA11_5h and EDA12_5h GQDs are negligible, as evidenced by the downward-sloping graphs with a minimal difference of less than two units. The reason behind this behavior lies in the surface bonds of EDA31_5h GQDs and their formation mechanisms during synthesis. Specifically, EDA31_5h GQDs contain more nitrogen (N) atoms in their structure due to the higher ethylenediamine content as a precursor. Consequently, these GQDs are effectively absorbed at the interface, enhancing their connection to the nonpolar phase and leading to reduced IFT. Supplementary Figure S3 and S4 displays graphs illustrating IFT changes with varying concentrations of urea-derived and thiourea-derived GQDs at different molar ratios of precursors, respectively. The graph indicates that the IFT values for these diagrams have no significant changes.

Concerning the results in this section, a higher molar ratio of ethylenediamine to citric acid precursors enhances the potential of GQDs to reduce IFT. Conversely, a high molar ratio of ethylenediamine to citric acid results in a lower quantum yield. For instance, when synthesizing pure ethylenediamine, the yield point of GQDs obtained is minimal43. Thus, employing excessively high molar ratios (more than 3) of ethylenediamine for GQD production in EOR operations is impractical due to the limited volume of GQDs obtained.

Effect of N-GQDs with salt on IFT

To investigate the synergy effect of GQDs combined with salt ions on IFT, 1000 ppm of EDA31_5h, Urea31_5h, and Thiourea31_5h GQDs in different NaCl concentrations was used (as shown in Fig. 14). Specifically, NaCl concentrations of 0, 1000, 5000, 10,000, 25,000, 50,000, 100,000, and 200,000 ppm were employed to demonstrate IFT changes both with and without GQDs across a wide range.

Fig. 14
figure 14

IFT value of crude oil and different NaCl concentrations with 1000 ppm of EDA31_5h, Urea31_5h, Thiourea31_5h GQDs, and without GQDs.

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As depicted in the line graph, the changes in IFT for NaCl solutions without GQDs are insignificant. The IFT decreases as the NaCl concentration increases to 10,000 ppm. This reduction occurs because, at low salt concentrations, resins and asphaltenes migrate to the phase interface, resulting in decreased IFT. However, when the NaCl concentration is further increased from 10,000 to 200,000 ppm, the electrical double layer becomes thinner. This leads to asphaltene depletion near the interface and an increase in IFT74. Compared to the IFT of deionized water and crude oil, the IFT of 1000 ppm EDA31_5h GQDs without salt shows a reduction of approximately 8 mN/m. Furthermore, as the NaCl concentration increases, the IFT sharply decreases, reaching a minimal value of 0.70 mN/m. This is a crucial finding as it underscores the unique properties of ethylenediamine-derived GQDs. The interaction between EDA31_5h GQDs and NaCl draws asphaltene and resin from the oil bulk to the interface of the two phases, resulting in a stronger bond with the nonpolar phase and significant IFT reduction. While GQDs (typically NPs) are commonly used in conjunction with surfactants to reduce IFT75,76,77,78, this exceptional reduction demonstrates that N-GQDs can effectively and independently reduce IFT. To illustrate the drop changes resulting from the addition of EDA31_5h GQDs, two samples of crude oil in water with 200,000 ppm NaCl concentration without/with 1000 ppm EDA31_5h GQDs are shown in Fig. 15.

Fig. 15
figure 15

Crude oil droplet which is immersed in water with 200,000 ppm NaCl concentration (a) without GQDs (b) with 1000 ppm EDA31_5h GQDs.

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The IFT changes of Urea31_5h and Thiourea31_5h GQDs doped with different atoms (O and S) are depicted using blue and green lines in Fig. 14. Urea can be considered a diamine, which is a superior dopant compared to monoamine50. Specifically, Urea31_5h GQDs exhibit a monotonic decrease in IFT (approximately 5 mN/m) with increasing NaCl concentration, although their values remain significantly higher than those of EDA31_5h GQDs. The underlying reason for this phenomenon lies in the agglomeration type, N-GQD formation during synthesis, and the structural characteristics influenced by urea, thiourea, and ethylenediamine molecules. Notably, urea, with its low pyrrolic N content in the chain and three functional ports for GQD formation, prevents the development of branched hydrocarbon chains with nitrogen, resulting in highly polarized GQDs. Additionally, the O–H and N–H functional groups observed in the FTIR absorption bands contribute to the enhanced polar properties and excellent water solubility of urea-derived and thiourea-derived GQDs53. In the same way, as discussed in the XPS test section, urea-derived GQDs exhibit a higher surface O content compared to ethylenediamine-derived GQDs. This elevated oxygen presence hinders the interaction of urea-derived GQDs with the oil phase, thereby diminishing their surfactant-like effect. Consequently, the IFT reduction capability of urea-derived GQDs is lower than that of their ethylenediamine-derived counterparts. The presence of thiourea-derived GQDs at the interface is restricted due to the additional S atoms and their high polarity. As a result, these GQDs mostly remain in the bulk fluid phase and do not show significant changes in IFT as the salt concentration increases.

Conclusions

This study investigates the IFT behavior between crude oil and N-GQD solutions at varying GQD and salt concentrations for EOR operation. Additionally, the impact of GQD synthesis time and precursor molar ratios on IFT are analyzed. To this end, ethylenediamine, urea, and thiourea are separately synthesized hydrothermally with citric acid, followed by characterization tests such as XPS, HR-TEM, XRD, and FTIR tests. Our key findings are as follows:

  1. 1)

    Based on XPS analysis, ethylenediamine-derived GQDs exhibit a higher N doping degree and sp3 C/sp2 C intensity ratio. These GQDs effectively connect with the nonpolar phase, reducing the IFT of nanofluid and crude oil from 18.71 to 6.84 mN/m at a concentration of only 1000 ppm.

  2. 2)

    Urea-derived and thiourea-derived GQDs possess greater polarity due to their functional group (high RO/C and low pyrrolic-N). Consequently, they exhibit less capability to interact with the oil phase, resulting in IFT reductions from 18.71 to 16.03 and 16.56 mN/m, respectively.

  3. 3)

    The interaction between EDA31_5h GQDs and NaCl leads to a stronger bond with the nonpolar phase, significantly reducing IFT. Remarkably, even at 1000 ppm EDA31_5h GQDs concentration, these NPs achieve an IFT of 0.70 mN/m in the presence of 200,000 ppm NaCl. This underscores the unique properties of ethylenediamine-derived GQDs, which can independently reduce IFT without surfactants. Notably, the corresponding values for Urea31_5h and Thiourea31_5h GQDs are 12.33 and 18.04, respectively.

  4. 4)

    Prolonging the synthesis time of ethylenediamine-derived GQDs beyond 5 h diminishes their IFT reduction effectiveness. Urea-derived GQDs perform optimally at approximately 10 h, while thiourea-derived GQDs show no significant IFT change with varying synthesis times.

  5. 5)

    Ethylenediamine-derived and urea-derived GQDs with a precursor molar ratio of 3 (mole ethylenediamine/urea per mole citric acid) significantly reduce IFT. Conversely, the higher citric acid content in the precursors produces more polar GQDs, rendering those with molar ratios of 1 and 0.5 less effective. On the other hand, thiourea-derived GQDs exhibit no significant IFT changes across different precursor molar ratios.

  6. 6)

    GQDs synthesized via the hydrothermal method from ethylenediamine for 5 h at a molar ratio of 3 (referred to as EDA31_5h) offer economic advantages and cost-effectiveness for EOR applications. Their favorable attributes position them as strong candidates for field trials and further research. These GQDs possess several desirable features, including low toxicity, good solubility, and high stability. Notably, their small particle size (typically less than 20 nm) enables effective penetration into micro-nano pores within low-permeability reservoirs. Given these promising characteristics, EDA31_5h GQDs represent excellent prospects for practical field applications.