A nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid and a preparation method thereof

Abstract

This invention discloses a nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids and its preparation method. The dispersant uses citric acid as the carbon source, 1-hexadecylamine as the nitrogen source, and a surface modifier. Nitrogen-doped carbon quantum dots are synthesized via solid-phase pyrolysis. These quantum dots are then added to water-based drilling fluid at a ratio of 0.01 wt% to 1.2 wt% and stirred until homogeneous. This dispersant, at an addition of only 0.01%, can reduce the average particle size of asphalt from 247.7 μm to below 100 μm; it can reduce oil sand adhesion by more than 30%. The dispersant of this invention has advantages such as low addition amount, high dispersion efficiency, excellent compatibility with drilling fluid systems, environmental friendliness, and simple preparation and application processes. It is suitable for controlling asphalt pollution during oil sand drilling and has broad engineering application prospects.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of oilfield chemistry and nanomaterials; in particular, it relates to a nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids and its preparation method. Background Technology

[0002] Oil sands, as an important unconventional oil and gas resource, are crucial for global energy supply. However, the high-viscosity bitumen in oil sands easily infiltrates drilling fluid systems during drilling. When these bitumen aggregates mix with water-based drilling fluids, they disrupt the three-dimensional network structure of the drilling fluid, leading to severe deterioration in its performance. This manifests as decreased gel strength, barite sedimentation, increased filtration loss, and reduced cuttings carrying capacity. It also significantly increases the risk of oil sands adhering to the drill string surface, causing complex downhole situations such as stuck pipe and annular blockage.

[0003] my country possesses considerable oil sands resources, primarily distributed in the Tarim Basin and Junggar Basin. As an important component of the unconventional energy system, these resources have not yet received sufficient attention from energy companies, nor have they been exploited and utilized on a large scale or systematically. Existing bitumen dispersion technologies are mostly concentrated in the oil production and transportation stages, focusing on solving problems such as sedimentation, wellbore adhesion, and fluidity during crude oil pipeline transportation, with improving crude oil fluidity as the core objective. The potential impact on the drilling fluid system is typically less considered. Currently, specialized research addressing the complex issues of bitumen dispersion, wellbore stability, and tool adhesion during oil sands drilling operations is lacking in China, and related technological reserves and theoretical exploration urgently need to be strengthened.

[0004] Although various bituminous treatment technologies have been developed in the past heavy oil extraction process, oil production and drilling operations belong to two distinct construction stages in the petroleum industry chain, with significantly different objectives, environmental conditions, and technological requirements. Oil production primarily focuses on the long-term stability and flow assurance of well production, while drilling emphasizes wellbore safety, efficiency, and wellbore stability. Therefore, the performance requirements for bituminous treatment agents differ fundamentally between these stages. Existing bituminous dispersion technologies for oil production are often difficult to directly apply to drilling conditions and must be systematically adapted and redeveloped to suit the characteristics of drilling fluid systems and the dynamic high-temperature, high-pressure environment downhole. From the perspective of the drilling engineering field as a whole, only Iran and the Tarim Basin of Sinopec Northwest Oilfield Company have published a limited number of analyses and solutions regarding related issues globally, but to date, a complete and systematic technical system has not yet been established.

[0005] In recent years, nanomaterials have shown potential in improving oil recovery and solving oilfield chemical problems due to their unique properties. Carbon nanomaterials such as carbon nanotubes and graphene have been studied for adsorbing and dispersing asphaltene, but their preparation cost is high, their dispersibility is poor, and their applicability in complex drilling fluid systems needs to be verified.

[0006] Carbon quantum dots, as an emerging carbon-based zero-dimensional nanomaterial, possess advantages such as small size, large specific surface area, abundant surface functional groups, good chemical stability, excellent biocompatibility, and low toxicity. Currently, research on carbon quantum dots mainly focuses on bioimaging, sensing, photocatalysis, and energy fields, while their application as a functional additive in oilfield chemistry, particularly in addressing bitumen contamination in oil sands drilling fluids, remains largely unexplored. Therefore, developing a highly efficient and environmentally friendly bitumen dispersant based on carbon quantum dots to restore and enhance the performance of contaminated drilling fluids has significant theoretical and engineering application value. Summary of the Invention

[0007] This invention addresses the problems of existing technologies, such as the significant deterioration of drilling fluid performance due to asphalt contamination leading to rheological degradation and increased filtration loss; and the limitations of traditional asphalt dispersants, such as insufficient dispersion efficiency at high temperatures and poor compatibility with drilling fluid systems. This invention provides a highly efficient, environmentally friendly asphalt dispersant for water-based drilling fluids with excellent compatibility with drilling fluid systems, as well as its preparation method, and further discloses its specific application method in improving the overall performance of drilling fluids.

[0008] This invention is achieved through the following technical solution:

[0009] This invention relates to a nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids. This dispersant can be directly added to the water-based drilling fluid for use; the amount of nitrogen-doped carbon quantum dots added is 0.01 wt% to 1.2 wt%. This dispersant is used to disperse intruding asphalt and maintain the rheological properties and filtration properties of the drilling fluid. The addition of nitrogen-doped carbon quantum dots reduces the adhesion of oil sand to metal surfaces by more than 30% after hot tumbling aging at 50–70°C.

[0010] The addition of nitrogen-doped carbon quantum dots effectively disperses infiltrated asphalt without significantly affecting the basic properties of the water-based drilling fluid, and improves its ability to inhibit adhesion and disperse. Specifically, at an addition concentration of 0.01 wt%, compared with the reference drilling fluid without the dispersant, the rheological properties of the drilling fluid remain stable after aging: apparent viscosity (AV) changes by less than 10%, plastic viscosity (PV) changes by less than 10%, and dynamic shear force (YP) changes by less than 10%. It has a significant dispersing and deagglomeration effect on asphaltene agglomerates: after treatment with this dispersant, the average particle size of the infiltrated asphaltene particles in the drilling fluid decreases. Filtration performance remains good: the API filtration loss of the drilling fluid after aging decreases by no less than 20%.

[0011] Preferably, the nitrogen-doped carbon quantum dots are synthesized by solid-phase pyrolysis using citric acid as the carbon source, 1-hexadecylamine as the nitrogen source and surface modifier.

[0012] Preferably, the method for preparing the nitrogen-doped carbon quantum dots is as follows: citric acid and 1-hexadecylamine are mixed in a molar ratio of 1:1 to 1:5, reacted at 160 to 200°C for 1 to 5 hours, cooled, and a crude product is obtained; after solvent dispersion, filtration, dialysis, and drying, nitrogen-doped carbon quantum dot powder is obtained.

[0013] The nitrogen-doped carbon quantum dot powder involved in this invention has the following characteristics and properties:

[0014] The surface is rich in hydroxyl (-OH), carbonyl (C=O), amino (-NH2), and long-chain alkyl (-CH2-) groups, with an average particle size of 10–30 nm. It improves the overall performance of drilling fluids: it can significantly reduce the adhesion of oil sands to metal surfaces; it optimizes the rheological properties of drilling fluids after high-temperature aging, such as adjusting dynamic shear force (YP); it can significantly reduce API filtration loss at specific concentrations (e.g., 0.01%); it is environmentally friendly and has good compatibility: the carbon-based material is low in toxicity, compatible with polymer networks, and does not affect the stability of the system.

[0015] Preferably, the molar ratio of citric acid to 1-hexadecylamine is 1:3; the reaction temperature is 180°C and the reaction time is 3 hours.

[0016] Preferably, the water-based drilling fluid comprises water, a gelling agent, a filtration reducer, a reducing agent, and a weighting material.

[0017] Preferably, the gelling agent is xanthan gum; the filtration loss reducing agent is modified starch; the reducing agent is sodium bisulfite; and the weighting material is calcium carbonate.

[0018] Preferably, the water-based drilling fluid is prepared by adding 0.7 wt% reducing agent, 0.2 wt% gelling agent, and 0.75 wt% filtration loss reducer to 350 mL of fresh water and stirring at 12000 rpm for 20 minutes; then slowly adding 8.6 wt% weighting material and continuing to stir for 20 minutes.

[0019] Preferably, the water-based drilling fluid is a slightly acidic water-based drilling fluid with a pH of 4 to 6.

[0020] The present invention also relates to the aforementioned method for preparing nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid, comprising: adding nitrogen-doped carbon quantum dots with a weight percentage content of 0.01 wt% to 1.2 wt% to water-based drilling fluid and stirring evenly.

[0021] Preferably, the nitrogen-doped carbon quantum dots have an average particle size of 10–30 nm.

[0022] The present invention has the following advantages:

[0023] (1) Efficiently breaking down asphaltene aggregation: The dispersant involved in this invention incorporates nitrogen-doped carbon quantum dots. The purpose is to utilize the polar functional groups (-OH, -NH2) on the surface of nitrogen-doped carbon quantum dots to form hydrogen bonds with asphaltene molecules. At the same time, its long-chain alkyl groups generate steric hindrance. The dual effect effectively breaks down the π-π stacking between asphaltene layers, achieving nanoscale efficient dispersion and fundamentally alleviating the damage of asphaltene to the drilling fluid system structure.

[0024] (2) Significantly improves key properties of drilling fluid: By dispersing asphaltene and inhibiting wax crystals, the carbon quantum dots of the present invention can directly improve the engineering properties of the contaminated drilling fluid: reduce viscosity to facilitate pumping, reduce filtration loss to enhance wellbore stability, reduce oil sand adhesion to prevent stuck drill bit, and achieve "one agent with multiple effects".

[0025] (3) Low addition amount and high cost-effectiveness: The carbon quantum dots described in this invention can exert significant effects at extremely low concentrations (0.01wt%-1.2wt%), which is far lower than the amount of traditional polymer dispersants used, thus reducing the processing cost. Furthermore, due to their high efficiency, the total amount of chemical agents added is reduced.

[0026] (4) Environmentally friendly and temperature-stable: The synthesis route is green, using citric acid and alkylamine as raw materials. The prepared carbon quantum dots have good thermal stability and are stable under the normal operating temperature of drilling fluids, making them suitable for use in high-temperature oil sand drilling environments.

[0027] (5) Simple preparation and application process: The synthesis method of carbon quantum dots is simple and easy to scale up. Its application as an additive is simple, it can be directly mixed into the drilling fluid system without complicated pretreatment, which is convenient for field implementation and promotion. Attached Figure Description

[0028] Figure 1 The Fourier transform infrared (FT-IR) spectrum of nitrogen-doped carbon quantum dots (N-CQDs) prepared in Example 1 of this invention;

[0029] Figure 2 The images show the absolute fluorescence quantum yield (QY) spectra of the N-CQDs prepared in Example 1 of this invention; the left image shows the quantum yield calculation results, and the right image shows the absorption and emission spectra.

[0030] Figure 3 Transmission electron microscope (TEM) image of N-CQDs prepared in Example 1 of the present invention;

[0031] Figure 4 The particle size distribution histogram of N-CQDs prepared in Example 1 of this invention;

[0032] Figure 5 Thermogravimetric analysis (TGA) curve of N-CQDs prepared in Example 1 of this invention;

[0033] Figure 6 This is a comparison photo of the state of a blank drilling fluid sample and a drilling fluid sample with 0.01% N-CQDs added after high-temperature aging.

[0034] Figure 7 The graph shows the effect of different concentrations of N-CQDs on the adhesion of oil sand to steel rods at different temperatures.

[0035] Figure 8 The graph shows the effect of N-CQDs concentration on the apparent viscosity (AV), plastic viscosity (PV), and dynamic shear force (YP) of drilling fluid.

[0036] Figure 9 Comparison photos of the filter cake morphology formed by the blank sample (left) and the filter cake formed after adding dispersant (right);

[0037] Figure 10 A graph showing the effect of N-CQDs concentration on API filtration loss in drilling fluid.

[0038] Figure 11 This is a comparison of the dispersion state of oil droplets in a blank oil sample and an oil sample with 0.01% N-CQDs added, as observed under a polarizing microscope. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are merely further illustrations of the present invention, but the scope of protection of the present invention is not limited to the following embodiments.

[0040] Example 1

[0041] This embodiment relates to the preparation of nitrogen-doped carbon quantum dots (N-CQDs), and the specific steps are as follows:

[0042] Weigh citric acid (1.92 g, 0.01 mol) and 1-hexadecylamine (7.2 g, 0.03 mol) into a mortar and grind them together until homogeneous.

[0043] The mixture was heated at 180°C for 3 hours. After the reaction, it was allowed to cool naturally to room temperature to obtain a brownish-yellow solid. The solid was ground into powder, and 1.0 g of the powder was dispersed in 50 mL of anhydrous ethanol and sonicated for 30 minutes to ensure thorough dispersion. Subsequently, the mixture was filtered through a 0.22 μm microporous membrane, and the filtrate was transferred to a dialysis bag with a molecular weight cutoff of 1000 Da. Dialysis was performed in deionized water for 24 hours, with the water changed every 6 hours. The liquid in the dialysis bag was collected, concentrated by rotary evaporation at 55°C, and finally dried in a vacuum drying oven at 60°C for 12 hours to obtain a brownish-yellow N-CQDs powder.

[0044] The N-CQDs prepared in this embodiment were characterized as follows:

[0045] See Figure 1 As shown, the FT-IR spectrum reveals stretching vibration peaks of -CH2- at 2926 cm⁻¹ and 2854 cm⁻¹, a stretching vibration peak of C=O at 1695 cm⁻¹, and a stretching vibration peak of CO near 1179 cm⁻¹. This indicates the successful introduction of alkyl chains and oxygen- and nitrogen-containing functional groups, demonstrating the presence of surface-characteristic functional groups (such as -CH2-, C=O, and CO) of N-CQDs.

[0046] See Figure 3 and Figure 4 As shown, TEM analysis indicates that the N-CQDs are uniformly dispersed, with a minimum particle size of approximately 11.0 nm and an average particle size of approximately 25.7 nm, demonstrating the morphology, dispersion state, and size distribution of the N-CQDs.

[0047] See Figure 2 As shown, its absolute fluorescence quantum yield was 7.00% as determined by fluorescence spectroscopy.

[0048] See Figure 5 As shown, TGA analysis revealed that the sample mass remained stable before 285℃, with the main thermal decomposition occurring between 285-506℃. The results also showed the weight change curve (TG) of the nitrogen-doped carbon quantum dot sample under a nitrogen atmosphere during a programmed temperature rise from room temperature to 800℃, along with its corresponding differential thermogravimetric curve (DTG).

[0049] Example 2

[0050] This embodiment relates to the preparation and performance testing of water-based drilling fluids containing N-CQDs.

[0051] Preparation of drilling fluid-based slurry: Add 0.7 wt% sodium bisulfite, 0.2 wt% xanthan gum, and 0.75 wt% modified starch sequentially to 350 mL of fresh water, and stir at 12000 rpm for 20 minutes. Then slowly add 8.6 wt% calcium carbonate and continue stirring for 20 minutes to obtain the drilling fluid-based slurry.

[0052] Several portions of the above-mentioned base slurry were taken, and different masses of N-CQDs powder prepared in Example 1 were added to them, so that their final concentrations in the drilling fluid were 0, 0.01%, 0.04%, 0.08%, 0.12%, 0.16%, and 0.20% (w / v), respectively. After each concentration sample was hot-rolled at 50°C for 1 hour, the following performance tests were performed:

[0053] Rheological properties: Measured using a six-speed rotational viscometer at 50°C. Apparent viscosity (AV), plastic viscosity (PV), and dynamic shear strength (YP) were calculated. See [link to relevant documentation]. Figure 8 As shown, the results indicate that with increasing concentration, apparent viscosity (AV) decreases slightly, reaching its lowest value at an addition of 0.12%; plastic viscosity (PV) increases slightly at low concentrations; and dynamic shear force (YP) reaches its lowest value at 0.04%.

[0054] API filtration loss: Filtration loss was determined at 0.69 MPa. The filtration loss of the blank sample was 8.5 mL. When 0.01% N-CQDs were added, the filtration loss dropped sharply to about 3.0 mL; when the concentration was increased to 0.12%, the filtration loss rebounded to about 7.0 mL, indicating that there is an optimal concentration range for the effect of N-CQDs on filtration loss.

[0055] Adhesion rate test: 30 g of simulated oil sand (containing bituminous material) and different concentrations of N-CQDs were added to the drilling fluid-based slurry, and the mixture was hot-rolled and aged at 50℃, 60℃, and 70℃ for 1 hour each. The mass of oil sand adhering to the surface of the steel rod was then measured. See [link / details]. Figure 6 , Figure 7 , Figure 9 As shown in the results, the addition of N-CQDs significantly reduced adhesion at all test temperatures. The reduction was particularly pronounced at a concentration of 0.01%.

[0056] Example 3

[0057] This embodiment relates to the influence of N-CQDs on wax crystal morphology.

[0058] Waxy crude oil samples were heated and melted, and then 0% and 1.0% N-CQDs were added respectively, followed by thorough stirring. The mixed oil samples were dropped onto glass slides, cooled, and solidified, and the wax crystal morphology was observed under a polarizing microscope. In the blank sample, the wax crystals were large, exhibiting obvious platy or needle-like aggregations. In the sample with 1.0% N-CQDs added, the wax crystal size was significantly reduced, the morphology was more rounded, and the distribution was more uniform. DSC testing showed that adding 1.0% N-CQDs lowered the pour point of this waxy crude oil.

[0059] Comparative Example 1

[0060] Contamination of drilling fluid without any added dispersants

[0061] The same water-based drilling fluid as in Example 2 was prepared, and simulated oil sand was added before hot rolling aging at 50°C for 1 hour. After aging, the slurry showed obvious stratification, with coarse oil droplets accumulating in the upper layer and the slurry at the bottom becoming lighter in color. A large amount of oil sand adhered to the surface of the steel rod. The filtration loss was high, and the filter cake was rough and porous.

[0062] Comparative Example 2

[0063] Use traditional surfactants (such as petroleum sulfonates).

[0064] Adding 0.1% petroleum sulfonate to contaminated drilling fluid. Tests showed that while petroleum sulfonate had some emulsifying and viscosity-reducing effect, asphalt still adhered in large quantities.

[0065] The comparison between the above examples and comparative examples shows that the nitrogen-doped carbon quantum dots (N-CQDs) provided by the present invention can achieve multiple functions such as efficient dispersion of asphaltene, reduction of metal surface adhesion, and improvement of drilling fluid rheology and filtration at extremely low addition levels (0.01wt%-1.2wt%). Its comprehensive performance is superior to that of traditional dispersants or surfactants, and it has good compatibility with water-based drilling fluid systems, demonstrating significant technical advantages and application potential.

[0066] This invention introduces nitrogen-doped carbon quantum dots as a dispersant. Utilizing the hydrogen bonding between their surface functional groups and asphaltene, as well as the steric hindrance effect of their alkyl chains, it effectively disrupts the π-π packing of asphalt, achieving efficient asphalt dispersion while maintaining the basic stability of drilling fluid performance. At an addition of 0.01%, this dispersant can reduce the average diameter of asphalt from 247.7 μm to below 100 μm; it also balances the reduction of oil sand adhesion with the stability of drilling fluid rheological and filtration properties after high-temperature aging. This product has advantages such as low addition amount, high dispersion efficiency, and good compatibility with drilling fluid systems, making it suitable for controlling asphalt contamination during oil sand drilling and possessing broad application prospects.

[0067] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids, characterized in that, The dispersant can be directly added to water-based drilling fluid for use; the amount of nitrogen-doped carbon quantum dots added is 0.01 wt% to 1.2 wt%.

2. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 1, characterized in that, The nitrogen-doped carbon quantum dots are synthesized by solid-phase pyrolysis using citric acid as the carbon source, 1-hexadecylamine as the nitrogen source and surface modifier.

3. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 2, characterized in that, The method for preparing the nitrogen-doped carbon quantum dots is as follows: citric acid and 1-hexadecylamine are mixed in a molar ratio of 1:1 to 1:5, reacted at 160 to 200°C for 1 to 5 hours, cooled, and a crude product is obtained; after solvent dispersion, filtration, dialysis and drying, nitrogen-doped carbon quantum dot powder is obtained.

4. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 3, characterized in that, The molar ratio of citric acid to 1-hexadecylamine is 1:3; the reaction temperature is 180°C and the reaction time is 3 hours.

5. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 1, characterized in that, The water-based drilling fluid contains water, gelling agent, filtration loss reducer, reducing agent, and weighting material.

6. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids as described in claim 5, characterized in that, The gelling agent is xanthan gum; the filtration loss reducing agent is modified starch; the reducing agent is sodium bisulfite; and the weighting material is calcium carbonate.

7. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids as described in claim 5, characterized in that, The water-based drilling fluid is prepared as follows: 0.7 wt% reducing agent, 0.2 wt% gelling agent, and 0.75 wt% filtration loss reducer are added sequentially to 350 mL of fresh water and stirred at 12000 rpm for 20 minutes; then 8.6 wt% weighting material is slowly added and stirring is continued for 20 minutes.

8. The nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 1, characterized in that, The water-based drilling fluid is a slightly acidic water-based drilling fluid with a pH of 4 to 6.

9. A method for preparing a nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluid as described in claim 1, characterized in that, include: Add nitrogen-doped carbon quantum dots with a weight percentage of 0.01 wt% to 1.2 wt% to the water-based drilling fluid and stir until homogeneous.

10. The method for preparing nitrogen-doped carbon quantum dot asphalt dispersant for water-based drilling fluids as described in claim 9, characterized in that, The average particle size of the nitrogen-doped carbon quantum dots is 10–30 nm.

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