A comprehensive review of the research and application of nano-fluorescent tracer material in oilfield

1. Introduction

Against the backdrop of the continuously increasing global energy demand, oil, a crucial energy resource, has drawn significant attention regarding its efficient extraction. The “14th Five-Year Plan for the Modern Energy System” clearly states that by 2025, the annual domestic crude oil production should be stabilized at around 200 million tons. During the oilfield exploitation process, accurately obtaining reservoir geological parameters and understanding the dynamic characteristics of water drive are essential for improving the crude oil recovery rate and addressing issues in water injection development. Tracer technology plays a vital role in this regard.

Traditional tracers can be divided into two categories: radioactive and non-radioactive [1,2]. Radioactive tracers, represented by tritium and its compounds, possess extremely high detection sensitivity, with the lowest detection limit reaching 10^-5 mg·L^-1, enabling the precise detection of trace amounts. However, as pointed out by R.D. Hutchins at the SPE International Symposium on Oilfield Chemistry [3], radioactive tracers are restricted due to their radioactivity, complex operation procedures, potential harm to human health, and environmental hazards. Non-radioactive tracers, such acidly used in the field of interwall tracing due to their low cost, low toxicity, thiocyanate (SCN^-), halide ions (Cl^-, Br^-, I^-), and Fluor benzoic acid (FBA) derivatives, have been widely used and diverse varieties [4-7]. Nevertheless, they have many limitations in practical applications. Their chemical properties make them prone to interact with reservoir minerals, resulting in severe adsorption phenomena. This makes the migration law of tracers in the reservoir complex and difficult to accurately track, thus affecting the accuracy of the tracing effect.

With the rapid development of materials science and nanotechnology, nano-fluorescent tracers have gradually become a research hotspot. Compared with traditional tracers, nano-fluorescent tracers have unique advantages [8]. Their fluorescent properties can effectively avoid the background concentration interference caused by the long-term use of traditional tracers, providing more accurate data for reservoir monitoring. This paper focuses on the in-depth research of carbon-based, silicon-based, and polymer-based nano-fluorescent tracers. The selection of these tracers is primarily based on their representation of the main research directions in the field of nano-fluorescent tracers, which cover different material systems, each with its own characteristics.

Carbon-based nano-fluorescent tracers are characterized by low cost, good stability, and low formation adsorption. Their raw materials are widely available, and common substances, such as sugars and biomass waste, can be used as carbon sources. The preparation methods have two main approaches: “top-down” and “bottom-up”. The ‘bottom-up’ hydrothermal and microwave synthesis methods are frequently employed. The hydrothermal synthesis method uses small molecular organic compounds as carbon sources and a high-temperature and high-pressure reaction in an autoclave. Through a series of complex processes, such as cross-linking, polymerization, carbonization, and dehydration of the carbon source, carbon quantum dots (CQDs) with fluorescent properties are formed. For example, using D (+)-xylose as a raw material and reacting at 200°C for 6 hrs, CQDs with an average particle size of ∼5 nm and good stability under high ionic strength and high temperature can be prepared. The microwave synthesis method instantaneously carbonizes carbon-containing organic compounds into carbon nanodots using the high microwave temperature. This method is efficient and environmentally friendly; however, the prepared carbon nanodots have an uneven particle size distribution, which brings difficulties in subsequent separation and application.

Silicon-based nano-fluorescent tracers usually have a core-shell structure. The core material can be quantum dots, organic fluorescent dyes, or rare earth luminescent materials, etc., and silicon dioxide is often used as the shell material to encapsulate and protect the core. For instance, coating a silicon dioxide shell layer on the surface of quantum dots by the improved Stöber method can significantly improve the stability of tracers in harsh reservoir environments. ZnO quantum dots, for example, have poor stability, low quantum yield, and unstable emission. However, after encapsulation to form ZnO@SiO₂NPs, they maintain a relatively high emission intensity at temperatures ranging from 0-100°C and salinities from 0-40 g/L.

The preparation techniques of polymer-based nano-fluorescent tracers cover physical and chemical methods. Physical methods include adsorption, embedding, self-assembly, etc., while chemical methods involve grafting and copolymerization. For example, the inverse emulsion polymerization method can introduce fluorescent monomers into polymer microspheres to prepare fluorescent polymer microspheres for detection. Polymer microspheres, mainly made from acrylamide, have strong hydrophilicity, good biocompatibility, excellent swelling, and viscoelasticity, and are widely used in oilfield development as a water control and plugging technology. However, it is difficult to accurately determine the concentration of traditional polymer microspheres during detection. The properties of fluorescent monomers can solve this problem.

However, the stability of nano-fluorescent tracers is affected by multiple factors. In terms of temperature, the fluorescence intensity of some carbon-based tracers decreases at high temperatures, while the stability of silicon-based ZnO@SiO₂NPs is enhanced after encapsulation. High salinity can degrade the performance of polymer tracers, while silicon-based tracers show better salt resistance. When the pH value exceeds a certain range, the stability of both carbon-based and polymer-based tracers is affected. In addition, although surface modification and functionalization can enhance stability, the interaction between tracers and reservoir substances may interfere with their stability.

Currently, most of the research on nano-fluorescent tracers is still at the laboratory stage. Industrial applications face problems such as high costs and immature large-scale preparation technologies. The actual reservoir environment is extremely complex, and the geological conditions of different reservoirs vary greatly. How to make nano-fluorescent tracers better adapt to such complex environments remains a major challenge in current research. Therefore, in-depth research on the preparation technology of nano-fluorescent tracers, optimization of their performance, and expansion of practical applications have important practical significance. Compared to previous articles, this paper aims to comprehensively review the research and application status of nano-fluorescent tracers, analyze their advantages and disadvantages, provide a reference for subsequent research, and promote the further development of this field.

2. Carbon-based Nano-fluorescent Tracers

Carbon-based nano-fluorescent particles consist of a carbon core, with particles smaller than 10nm known as carbon dots [9]. Carbon dots were first discovered in 2004 as a new type of carbon nanomaterial with fluorescent properties [10], they have been widely applied in biomedical imaging [11-13], photocatalytic reactions [14,15], water treatment [16,17], and oilfield tracing [18-21]. The nanomaterial is composed of carbon core and surface groups. The carbon core is a framework made up of sp2-hybridized graphite microcrystalline carbon and sp³-hybridized amorphous carbon [22], the surface typically has many oxygen-containing functional groups, such as hydroxyl and carboxyl groups, which provide excellent stability and water solubility to carbon dots. Moreover, extensive research [23-26] has shown that carbon dots shave strong migration capabilities and are superior to traditional organic dyes in terms of luminescence intensity and stability. Carbon sources for preparing carbon dots are widely available, and the usage of carbon dots in tracing processes is cost-effective.

The preparation of carbon dots falls into two categories: “top-down” and “bottom-up” methods [27]. The bottom-up approach primarily involves the combination of small molecule carbon precursors into larger carbon dots through methods like microwave, combustion [28-31], hydrothermal synthesis [32-36], and pyrolysis [37,38]. Compared to the top-down method, the bottom-up preparation method is simpler and easier to operate. Although the yield is relatively low, its luminous efficiency is comparable to that of traditional semiconductor quantum dots [39]. Currently, researchers mostly use hydrothermal synthesis and microwave synthesis, both bottom-up methods, to prepare carbon dots for application in oilfield tracing.

2.1. Hydrothermal synthesis method

The hydrothermal synthesis method mainly involves using small molecule organic compounds as carbon sources to synthesize water-soluble carbon dots through high temperature and high-pressure conditions in an autoclave [35]. The general preparation method can be summarized as follows: under hydrothermal conditions, carbon-containing precursors cross-link and polymerize with each other, then carbonize and dehydrate to form a crystalline or amorphous carbon core. The outer surface of the carbon core is attached to uncarbonized small molecules, which ensure the stability of the carbon core and endows it with rich fluorescent properties [40].

Zhong Liang Hu et al. [41] used D (+)-xylose (C₅H₁₀O₅) as the raw material, heat water at 200°C for 6 hrs in a reaction vessel, filter and purify to obtain a solution of CQDs. As shown in Figure 1, CQDs exhibit their maximum emission intensity when excited at 360 nm, with an average particle size of approximately 5nm. Experimental studies have shown that due to the presence of multiple highly polar functional groups on the surface of xylose CQDs, they exhibit excellent stability under high ionic strength and high temperature, as well as outstanding performance in absorbance and fluorescence intensity. They also demonstrate good migration ability in glass bead-filled columns and sandstone cores. However, this is not the case for all CQDs. The fluorescence properties of some CQDs will decline at high temperature. Studies have shown that when the temperature exceeds a certain threshold, the surface structure of CQDs will change, resulting in the reduction of fluorescence intensity and poor stability. This may be because high temperature destroys the chemical bonds on the surface of CQDs and affects its luminescence mechanism. In the calcite-filled column experiment, compared to solutions without CQDs, 37.8% of CQDs were retained in the calcite-filled column, the author believes that the surface charge of calcite is positive, contrary to the surface of CQDs in mineralized water, under the action of electrostatic force, making carbon dots adhere to the surface of calcite grains. Yan Vivian Li et al. [42] also found that, when conducting calcite sand column experiment using charge neutral CQDs (M-dots) synthesized from malic acid and ethanolamine, when M-dots were dispersed in NaCl and mixed saline solutions, the packed column only retained 5.6% and 7.3% of M-dots, respectively. This is because the near-zero charges, and strong hydrophilic decoration of CQDs M-dots make them significantly inert.

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Figure 1.

One-pot synthesis and characterization of CQDs using D (+)-Xylose as raw material (Experimental conditions: Using D (+)-xylose as the raw material, heating at 200°C for 6 hrs in a reaction vessel, filtering and purifying to obtain a carbon quantum dot solution); (a) Synthesis process; (b) UV-Vis absorption spectra and fluorescence emission spectra of CQDs; (c) Transmission electron microscopy (TEM) images of CQDs with the same lattice fringes (Upper) and (d) High-resolution transmission electron microscopy (HR-TEM) images (lower).

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Xinjiong Shi et al. [43] used anhydrous citric acid rich in carboxyl groups as a carbon source, ethylenediamine rich in amino groups as a nitrogen source, heat water at 200°C for 6 hrs in a reaction vessel, obtain carbon dot solution, purify and dry to obtain N-doped CQDs (CA CQDs), as shown in Figure 2. The average particle size of CA CQDs material is 5.36 nm, with a fluorescence quantum yield of 23.52%, and excellent fluorescence performance at 10^-6gL^-1. In a series of oilfield tracer applicability evaluation experiments, CA CQDs exhibit minimal variation under different temperatures, salt solutions, and pH conditions, demonstrating excellent stability, the adsorption effect of different minerals (quartz sand, oil sand, montmorillonite, and artificial core fragments) on CA CQDs is relatively small, after adsorption for 5 days, the concentration retention rate remained stable at over 90%, and the compatibility with the formation water sample was also good, with a concentration deviation within 2%. The sand-filled tube simulation injection experiment showed a significant breakthrough in the elution process, and has strong migration ability, but the retention and adsorption capacity is weak, making it easy to monitor and analyze in real time, compared with the initial permeability, the reservoir damage rate is generally below 6%, and the degree of damage is relatively weak, which meets the requirements for on-site tracer use.

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Figure 2.

Preparation process and fluorescence phenomenon of N-doped CQDs (CA CQDs) (Experimental conditions: Using anhydrous citric acid as the carbon source and ethylenediamine as the nitrogen source, heating at 200°C for 6 hrs in a reaction vessel, purifying and drying to obtain CA CQDs).

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Saba Ranjbar et al. [44] used m-phenylenediamine (m-PD) and o-phenylenediamine (o-PD)as raw materials, respectively, in the presence and absence of tartaric acid (TA), a fluorescence quantum yield of 18% was synthesized by hydrothermal treatment at 180°C for 10 hrs in a reaction vessel, yellow luminescent CDs (y-CDs) with an average particle size of 8nm and green luminescent CDs (g-CDs) with a fluorescence quantum yield of 30% and an average particle size of 5 nm. In a one-month fluorescence response experiment using EOR nanofluids prepared with 20ppm g-CD and 200ppm y-CDs, the fluorescence intensity attenuation of nanofluids can be ignored, and they exhibit long-term emission stability under high temperature and high salinity conditions.

Su Rigu et al. from China National Petroleum Corporation [45] proposed a method for preparing dual emission wavelength red fluorescent CQDs, using guanidine hydrochloride and o-phenylenediamine as nitrogen and carbon sources, red carbon quantum dot solution was synthesized by hydrothermal synthesis in a stainless steel high-pressure reactor lined with polytetrafluoroethylene, after separation, purification, and drying, carbon quantum dot powder with a size of 3-6 nm is obtained. This synthesis method has fewer by-products, lower cost, and the resulting CQDs have high fluorescence intensity, good stability, and low toxicity.

In recent years, due to the fluorescence properties of CQDs [46], the research and application of CQDs have provided new research ideas, by introducing carbon quantum dot monomers into the fracturing fluid to give it fluorescence properties, this fluorescence characteristic can be accurately detected, thereby achieving the determination of the concentration of fracturing fluid in the produced fluid, solved the problems of low detection accuracy and complex operation caused by the influence of formation background concentration in traditional detection methods.

Chaozong Yan et al. [47] used citric acid, borax, ethylenediamine, and diethanolamine as raw materials, reacted at 180°C for 5 hrs in a reactor, after dialysis purification, a yellow-brown solution of B and N co-doped CQDs was obtained (see Figure 3), the particle size distribution of its CQDs is between 1.20-1.42 nm, with most concentrated at 1.30nm. The particle size is small and evenly distributed, with a relative fluorescence quantum yield of 53.8%. As shown in Figure 4, CQDs co-doped with B and N were used as crosslinking agents to crosslink with guar gum fracturing fluid to prepare carbon dot crosslinked guar gum fracturing fluid (CDG). In the simulated fracturing fluid backflow process of core displacement experiment, the fracturing fluid increases with the increase of backflow time, the fluorescence intensity decreases from the initial value of 1463a. u. to 181a. u., with a fluorescence retention rate of 12.3%, and the color of the reverse discharge gradually became lighter. Experiments have shown that based on the fluorescence characteristics of carbon dots, their unique fluorescence can be used to characterize the discharge of fracturing fluid, thus serving as a fluorescent tracer, and different fluorescence retention rates can be used to describe the backflow rate of different sections of horizontal fracturing.

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Figure 3.

Preparation of B, N Co-doped CQDs (Experimental conditions: Using citric acid, borax, ethylenediamine, and diethanolamine as raw materials, reacting at 180°C for 5 hrs in a reactor, and dialyzing and purifying to obtain B, N co-doped CQDs).

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Figure 4.

Crosslinking mechanism of carbon dot crosslinked guar gum fracturing fluid (CDG) (Experimental conditions: Using B, N co-doped CQDs as crosslinking agents to crosslink with guar gum fracturing fluid to prepare CDG for the core displacement experiment simulating the fracturing fluid backflow process).

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Yake Fan et al. [48] used citric acid and ethylenediamine as raw materials, in a Polytetrafluoroethylene (PTFE) high-temperature and high-pressure reactor, a hydrothermal reaction at 180°C was carried out for 5 hrs, obtaining a dark brown solution of CQDs. Afterward, zirconia, 2wt%lactate, and 5wt% triethanolamine were added to react and generate Zr-N co-doped carbon quantum dot nano-crosslinking agent (see Figure 5). The particle size distribution of the crosslinking agent is between 1.78-2.06nm, with good stability and many cross-linking sites, it can be cross-linked with anionic polyacrylamide (HPAM) solution to form a high-strength hydrogel fracturing fluid and can be used as a nano fluorescent tracer to test the backflow rate of fracturing fluid. The synthesis of Zr-N co-doped carbon quantum dot nano-crosslinking agent solves the problems of aggregation, instability, and gel breakage of conventional polyacrylamide fracturing fluid crosslinking agents.

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Figure 5.

Preparation of Zr-N Co-doped carbon quantum dot nano-crosslinking Agent (Experimental conditions: Using citric acid and ethylenediamine as raw materials, performing a hydrothermal reaction at 180°C for 5 hrs in a PTFE high-temperature and high-pressure reactor to obtain a carbon quantum dot solution, and then adding zirconium oxide, 2wt% lactate, and 5wt% triethanolamine to react to generate a Zr-N co-doped carbon quantum dot nano-crosslinking agent).

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During the hydrothermal synthesis of CQDs, unreacted impurities may remain in the system after the reaction of raw materials, which may leach out during subsequent applications. In the calcite-filled column experiment, CQDs interact with calcite. Although the specific ion leaching situation is not clear, it is speculated that the interaction between the functional groups on the surface of CQDs and the ions on the surface of calcite may lead to ion release or exchange.

Overall, the hydrothermal synthesis method is simple, cost-effective, and can be used to synthesize heterogeneous reactions in one pot. The synthesized CQDs have a high fluorescence quantum yield, and the preparation of carbon dots as crosslinking agents is successful, providing new insights for the application of carbon dots [49]. However, there is still a lack of comprehensive and systematic research on the migration of carbon quantum dot fluids in fractured aquifers and oil and gas reservoirs, it is recommended to conduct further research on the sources of charges in carbon core particles and their interactions with the solute solid interface of reservoir rocks, while meeting the requirements of strong fluorescence characteristics and high quantum yield.

2.2. Microwave method

Microwave method is a novel, green, and efficient method for preparing carbon dots [50], this method involves the instantaneous carbonization of carbon-containing organic compounds into carbon nanodots during the microwave process at high temperatures. Compared with the hydrothermal method, the key advantage of the microwave method is [51], that during the synthesis process, energy directly acts on the interior of materials through the interaction between molecules and magnetic fields (non-contact heating).

In 2009, Zhu et al. [52] first adopted the microwave method. Dark brown carbon dots were rapidly prepared using sugar (glucose, fructose, etc.) and polyethylene glycol (PEG-200) as raw materials. By reacting under 500W in a microwave for 5 and 10 mins, carbon dots with particle sizes of 2.75 ± 0.45nm and 3.65 ± 0.60nm were obtained, respectively. As the reaction time increases, the particle size increases, and the fluorescence quantum yield decreases from 6.3% to 3.1%.

Carbon element is widely present in natural substances; therefore, carbon dot synthesis raw materials can also be prepared using numerous natural starting materials [53]. Although food waste is a byproduct of food processing, it is rich in carbohydrates. Orange peels, banana peels, quinoa shells, durian peels, etc. contain various functional compounds, such as vitamins, minerals, flavonoids, etc., making them excellent carbon sources. Carlos A Franco et al. [54] purified carbon quantum dot powder by heating the Mortiño tropical fruit extracts a carbon source and ethylenediamine as a nitrogen source in a 300W microwave oven for 6 mins. As can be seen in Figure 6, the average diameter of CQDs is 30 nm, ranging from 5 to 60 nm. In the core breakthrough experiment, CQDs show a significant breakthrough in the injected pore volume, and there is no significant interaction between carbon dots and porous media, this is because under experimental conditions (pH 7), the acidic functional groups of carbon dots carry negative charges, and there is a repulsive force between them and the negative charges on the surface of silica sand, which has good compatibility with water.

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Figure 6.

High-resolution transmission electron microscopy image (Left) and particle size distribution (Right) of CQDs after preparation (Experimental conditions: Using the extract of Mortiño tropical fruit as the carbon source and ethylenediamine as the nitrogen source, heating in a 300W microwave oven for 6 mins to purify and obtain carbon quantum dot powder).

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Liu Yao from Shaanxi University of Science and Technology [55] used triamine citrate and trisodium phosphate as raw materials. After heating in a 700W microwave oven for 150 seconds, carbon quantum dot powder was obtained by dialysis drying. The particle size distribution of this carbon quantum dot is 3-7nm, with a spherical morphology. The maximum lifetime of the carbon dot is about 10.95 ns, and its quantum conversion rate is 23%, making it a short-lived quantum dot. The aqueous solution prepared by carbon dots is used as the aqueous phase, and the water in oil type temperature resistant and salt resistant microsphere lotion P-(AM-MBA-SSS) is used as the oil phase, emulsification is carried out under the action of the emulsifying machine to form a stable fluorescent composite lotion (see Figure 7). In the indoor displacement test using sand-filled tubes instead of rock cores, after being irradiated with ultraviolet light, the produced liquid showed significant fluorescence, indicating that quantum dots were successfully labeled with fluorescence after binding with microspheres, which can be used for tracking during underground transportation.

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Figure 7.

Schematic diagram of fluorescent composite emulsion (Experimental conditions: Using an aqueous solution of CQDs as the aqueous phase and an oil-in-water temperature-resistant and salt-resistant microsphere emulsion P - (AM-MBA-SSS) as the oil phase, emulsifying under the action of an emulsifier to form a stable fluorescent composite emulsion).

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Through further research on the luminescence mechanism of CQDs, researchers have found that CQDs exhibit certain phosphorescence characteristics in addition to their ordinary fluorescence properties [56]. Phosphorescence refers to a delayed fluorescence phenomenon exhibited when the UV lamp is stopped, and the emission of phosphorescence is mainly related to the C=O triplet state on the surface of carbon dots.

Zhai Wei et al. from Suzhou Xingshuo Nanotechnology Co., Ltd. [57] provided a method for synthesizing quantum dot petroleum tracers with phosphorescent properties using microwave synthesis, among them, N, P, F, and other elements are introduced during the synthesis process to regulate the triplet energy level structure of CQDs and promote the formation of phosphorescent CQDs with stable phosphorescence properties; And introduce a loading agent after synthesis to prevent the quenching effect of petroleum tracer. Through experimental testing, the carbon quantum dot tracer synthesized by this method has phosphorescence properties, with a phosphorescence lifetime of up to milliseconds and a detection limit of up to 0.3 ppm. Using this tracer can effectively avoid interference from petroleum self-fluorescence, thereby improving detection sensitivity. This tracer is also environmentally friendly.

The use of microwave-assisted carbon source carbonization can produce CDs with good water solubility without the need for further passivation, which have the advantages of speed, greenness, and economy. This method has strong scalability, low cost, and ecological friendliness. The disadvantage is that the particle size distribution of CDs is uneven, and they are difficult to control, making separation and purification challenging. Biomass waste is a widely distributed, easily accessible, and eco-friendly carbon source. The synthesis of CQDs using biomass as a carbon source not only provides a new solution for synthesizing carbon dots but also proposes a sustainable development plan for solving global pollution problems.

2.3. Surface functionalization of carbon based nano-fluorescent tracer

CQDs are a new type of environmentally friendly material that combines strong fluorescence and high permeability and have been tested in oil reservoirs with simple experiments. The results show that CQDs have strong adaptability to changes in temperature, salinity, and pH [58,59] values. However, in more severe oil reservoir environments, the stability and migration capacity of quantum dots in the reservoir still face significant challenges. As a nanomaterial, carbon dots have abundant reaction sites on their surface, which can modify the functional groups of CDs through interactions with other compounds [60]. These functional groups can regulate the energy level structure of CDs, thereby controlling their optical properties, improving the stability of quantum dots, and enhancing their migration rate in oil reservoirs to adapt to increasingly harsh oil reservoir environments.

In 2011, Liu et al. [61] synthesized blue carbon dots with a size of 2-7 nm and a fluorescence quantum yield of 12% using glycerol as a carbon source and 4,7,10-trioxy-1,13-tetradecanediamine (TTDDA) as a surface passivator. TTDDA plays a role in dehydration and surface modification. It was discovered that the addition of nitrogen significantly enhances its fluorescence properties, but the reaction yield of carbon dots is relatively low. Qian et al. [62] prepared various N-doped carbon dots using the hydrothermal method. The experiment used 1,2-ethylenediamine, 1,3-propanediamine, 1,3-butanediamine, and ethylene glycol as raw materials to investigate the effect of precursors with different nitrogen contents on the fluorescence properties of carbon dots. The experimental results show that the quantum yield of CQDs doped with Nitrogen ranges from 20.4% to 36.3%, which is much higher than that of undoped synthesized CQDs. N atoms rely on their advantages of having a radius similar to carbon atoms, high electronegativity, and a pair of high-affinity lone pair electrons. By injecting electrons into carbon dots and changing their internal environment, their fluorescence performance can be improved. Nitrogen doping can significantly improve the quantum yield of CQDs and enhance their stability in complex environments. Surface functionalization can not only adjust the optical properties of CQDs but also change their surface charge and chemical activity, reducing the interaction with other substances to improve stability.

Wei Wang et al. [63] used citric acid as the carbon source and ethanolamine as the nitrogen source, and introduced organic compounds with different functional groups (fluorinated, sulfonated, and zwitterionic), such as 2,4,6-trifluoroaniline, 8-aminonaphthalene-1,3,6-trisulfonic acid disodium salt (ANTS), 4-amino-3-hydroxy-1-naphthalenesulfonic acid (AHNS), 1- (3-sulfopropyl) -2-vinylpyridine betaine (SPVPB), and cocoamidopropyl hydroxy sulfonamide (CAHS) to synthesize CQDs via hydrothermal synthesis. According to Table 1, compared with undoped C-dots, the retention rate of 2,4,6-FluoroBenzoic acid (FBA) containing multiple fluorine groups and phosphatidylcholine-doped C-dots containing phosphonic acid groups slightly increased in limestone. The adsorption of other doped C-dots on rocks can be ignored, with a residual concentration of about 100% in the suspension. In the molecular dynamics simulation experiment shown in Figure 8, the retention mechanism of the reservoir rock surface with ordinary and modified carbon dots was further elucidated, and the adhesion size was G-CA>G-ANTS>G-SPVPB>G-2FBA. Among them, the adhesion of modified carbon dots was smaller than that of ordinary carbon dots, which was consistent with the experimental results.

Table 1. Determination of the retention rates of undoped and doped carbon dots in limestone in 35 kppm brine by fluorescence spectrometry (Experimental Conditions: Measuring the retention rates of different doped carbon dots in limestone in 35 kppm brine using fluorescence spectrometry).

Adulterant	Functional group	Detention rate
no		0.25%
2-FBA	-F	∼0
2, 4-FBA	-F	∼0
2, 4, 6-FBA	-F	0.8%
AHNS	-SO3-	∼0
ANTS	-SO3-	∼0
APTS	-SO3-	∼0
L-α-Lecithin	-N+—PO4-	0.5%
CAHS	-N+—SO3-	∼0
CAPB	-N+—COO-	∼0
SPVPB	-N+—SO3-9	∼0

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Figure 8.

Representative simulation device of 2-FBA functionalized carbon quantum Dot analogue (G-2FBA) on the surface of Calcite (Experimental conditions: Studying the retention mechanism of ordinary and modified CQDs on the surface of reservoir rocks through molecular dynamics simulation. In the simulation, the atomic colors are set as Ca = green; c = cyan; o = red; h = white; f = pink. The blue and red arrows indicate the approaching and retracting directions in the force measurement simulation).

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Jinjiang Chen et al. [64] designed a novel water-soluble silicon-doped carbon quantum dot fluorescent tracer (CQD-W) with a particle size of approximately 2 nm. Research has shown that CQD-W has good water solubility because it has a surface rich in various functional groups, such as polycarboxylate, hydroxyl, and siloxane. It exhibits a negative charge in pure water, making the adsorption of CQD-W on the rock surface very low, which can meet the fluidity and injection capacity of reservoirs with a permeability above 0.005 mD. During the flow process, it is unlikely to change the internal pore structure of the core, resulting in a sturdy, damage-resistant reservoir. The cumulative recovery rates of low-permeability and high-permeability reached 97.41% and 96.57%, respectively, mainly due to the negative charge of CQD-W and the repulsive effect between CQD-W and negatively charged cores. In the synthesis of carbon quantum dot nano-fluorescent tracers, amine molecules such as ethanolamine, ethylenediamine, 1,2-phenylenediamine, etc. are mostly chosen as precursors. This is because Nitrogen has a synergistic effect on enhancing the photo-luminescent performance, which can make CDs have stronger photo luminescence [65]. Exploring the introduction of more different types of functional groups, further exploring the synergistic effect between CDs and doped hetero atoms, further studying the luminescence mechanism of different CDs, summarizing the luminescence mechanism, and synthesizing carbon dots with better stability and stronger transport ability [66] to cope with more complex and harsh underground environments, and applying them to oilfield tracing and other fields, will have broad development prospects [67].

Nano-fluorescent tracers have great potential in oilfield applications, but their stability is affected by multiple factors. The fluorescence intensity of some carbon-based tracers decreases at high temperatures, while the stability of silicon-based ZnO@SiO₂NPs is enhanced after encapsulation. High salinity can degrade the performance of polymer tracers, while silicon-based tracers show better salt resistance. When the pH value exceeds a certain range, the stability of both carbon-based and polymer tracers is affected. Surface modification and functionalization can enhance stability, but the interaction between tracers and reservoir substances can interfere with their stability. Current research has many limitations. The synthesis methods have defects. The hydrothermal synthesis method has a low yield, and the microwave synthesis method has an unclear mechanism that makes it difficult to control particle size. Moreover, most of the research is still in the laboratory stage, and there is a lack of industrial test data. Future challenges include adapting to complex reservoir environments, developing large-scale preparation technologies, and optimizing fluorescence performance.

3. Silicon-based Nano-fluorescent Tracers

Silicon-based nanoparticles are composed of functionalized cores, biologically modifiable shells, and surface-modified biomolecules, exhibiting distinct core-shell structures. Its core materials can be quantum dots, organic fluorescent dyes, rare earth luminescent materials, etc.

Quantum dots have garnered considerable attention due to their potential application value [68], but some quantum dots cannot play an important role in practical applications due to their poor stability [69,70]. To address this issue, researchers have proposed a method of using a protective layer to protect quantum dots [71,72]. Choosing the appropriate shell material is not only the key to achieving the encapsulation and protection of quantum dots in harsh environments of oil and gas reservoirs, but also to realizing the surface properties and functionalization of quantum dots and thus preparing new tracers that can be used for oil-water two-phase distribution. Silicon dioxide (SiO₂) is currently the most popular core-shell encapsulation material [73], as shown in Figure 9. It not only has good physical, chemical, and mechanical properties but also better transparency and a moderate refractive index. It can be modified by functional groups such as amino, thio, or carboxyl groups [74] and has non-toxic and good biocompatibility.

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Figure 9.

(a-b) Transmission electron microscopy image of silica (Experimental conditions: Not mentioned, showing the microstructural image of silica).

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ZnO quantum dots have tunable emission, low cost, and relatively low toxicity, but their poor stability, low quantum yield, and unstable emission limit their application in the field of oilfield tracing [75]. Zain H. Yamani et al. [76] synthesized fluorescent ZnO quantum dots in the green to yellow region by sol-gel method, with an average diameter of about 5 nm (see Figure 10). Encapsulate the synthesized quantum dots using an improved St ö ber method to produce the desired size ZnO@SiO₂NPs. During the synthesis process, tetraethyl orthosilicate was used to obtain hydrophilic functional groups on ZnO quantum dots, and a combination of tetraethyl orthosilicate, tetraethyl orthosilicate (TEOS) and dimethyl dimoxyline was used to achieve partial hydrophobic functionality. Ionic impurities get introduced during the synthesis and modification processes. For example, when Zain H. Yamani et al. [76] encapsulated ZnO quantum dots, multiple reagents were used, which might have left residual ions. The change in stability under different temperatures and salinity conditions may cause internal ions to leach out. Ions in the silane reagents used for surface functionalization also have the potential to leach. In a series of fluorescence stability experiments, the encapsulated silicon-based nanoparticles showed better stability; this is because the packaging layer effectively isolates the core quantum dot from the external environment, reducing the impact of external factors. The PH NPs tracer maintained initial emission intensities of 55% and 80% at temperatures of 0∼100°C and salinities of 0∼40g/L, respectively. The improvement in emission stability was mainly due to the methyl group in DMDES (silica precursor).

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Figure 10.

(a-d) Scanning electron microscopy images of ZnO@SiO₂NPs (Experimental Conditions: synthesizing fluorescent ZnO quantum dots by the sol-gel method and encapsulating them with the improved Stöber method to obtain ZnO@SiO₂NPs, and taking SEM images).

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Safyan Akram Khan et al. [77] used a co-condensation method with different silanes (DMDES, OTES, and APTS) with tetraethyl orthosilicate (TEOS) as shown in Figure 11. ZnO@SiO₂NPs The surface was functionalized to form partially hydrophobic (pH NPs, S2), hydrophobic (HP NPs, S3), and hydrophilic (H-NPs, S4) silica nanoparticles (see Figure 12). The synthesized ZnO quantum dots are small-sized spherical particles with an average diameter of about 5.3 nm, the average particle size of encapsulated silica nanoparticles is between 50-70 nm, exhibiting an ordered mesoporous structure with hexagonal pore. ZnO quantum dots are uniformly distributed on the surface with slight aggregation. In the comparative experiment of photoluminescence stability, ZnO quantum dots only retained 13% of their original intensity after exposure to aqueous solution. By comparison, ZnO@SiO₂NPs (S1) maintained 72% fluorescence intensity in water even after 30 days of synthesis. Under harsh reservoir conditions, ZnO@SiO₂NPs The PL stability of (S2, S3) is higher than ZnO@SiO₂NPs PL stability of (S1, S4). Compared with other tracer materials (S1, S3, and S4), ZnO@SiO₂NPs (S2) exhibits the highest emission stability due to the correct balance of hydrophobic hydrophilic interactions.

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Figure 11.

Chemical structures of silanes functionalized on the surface of ZnO@SiO₂ nanoparticles: (a) Tetraethyl orthosilicate (TEOS); (b) 3-Aminopropyltriethoxysilane (APTS); (c) n-Octyltriethoxysilane (OTES); (d) Dimethyldiethoxysilane (DMDES) (Experimental conditions: Functionalizing the surface of ZnO@SiO₂ nanoparticles with different silanes to show the chemical structures of the used silanes).

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Figure 12.

Synthesis mechanism and surface functionalization of ZnO@SiO₂ nanoparticles to obtain partially hydrophobic (PH-NPs, S2), hydrophobic (HP-NPs, S3), and hydrophilic (H-NPs, S4) functions (Experimental conditions: Functionalizing the surface of ZnO@SiO₂ nanoparticles by the co-condensation of different silanes with tetraethyl orthosilicate to study the properties of nanoparticles with different functionalizations).

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Routing [72] of Hebei University of Technology used ammonia as the catalyst to grow silica shells on the surface of spherical and rod-shaped CdSe/ZnS quantum dots by the microemulsion method (see Figure 13). By comparison, it was found that this is QDs@SiO₂. The morphological characteristics of nanoparticles mainly depend on the morphological characteristics of quantum dots. After wrapping the silica shell, the absorption peaks of both morphologies of quantum dots became less obvious, and the half peak width is increased by 5 nm compared to the initial nanorod quantum dots, without significant changes. The impact on the optical properties of quantum dots can be ignored; These are two different forms QDs@SiO₂ The emission peak of nanoparticles exhibits a slight blue shift compared to the emission peak of initial quantum dots.

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Figure 13.

(a) Transmission electron microscopy image of CdSe/ZnS spherical QDs@SiO₂; (b) Transmission electron microscopy image of CdSe/CdS Nanorod QDs@SiO₂ (Experimental conditions: Growing silica shells on the surfaces of spherical and rod-shaped CdSe/ZnS quantum dots with ammonia as the catalyst by the microemulsion method and taking TEM images).

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Nicolas Agenet et al. [78] synthesized several silicas-based nanoparticles (nano-spheres), including rare earth element complexes and organic dyes, used as “intelligent” oilfield tracers. The author demonstrated that the tight binding of organic molecules and inorganic complexes (dye spacing<5 nm) leads to a “smart” optical signal from particles. In addition, the glycosylated amide surface coating of nanospheres has been proven to effectively reduce the interaction between rock and nanoparticles, achieving a recovery rate of 98% in sandstone. The above validates the concept of fluorescent nanospheres and demonstrates their propagation ability in porous media.

Pazini et al. [79] synthesized two types of silica nanoparticles, FSiNP and FSiNP-NH₂, with fluorescein 5 (6) - isothiocyanate (FITC) as the core and different surface (shell) functional groups (see Figure 14). Among them, FSiNP-NH₂ uses additional 3-aminopropyltriethoxysilane (APTES) in the final stage of synthesis to graft aminopropyl groups onto its surface. The nominal sizes of synthesized FSiNP and FSiNP-NH₂ nanoparticles are 64 ± 9 nm and 86 ± 23 nm, respectively; both sizes are below 100nm. Experiments have found that compared with FSiNP grafted with silanol, FSiNP-NH₂ containing amino propyl surface groups exhibits better stability and lower adsorption, with a fluorescence spectrum signal change of only 3% after 90 mins; In the packed column experiment, the adsorption capacity of FSiNP was 0.034 mg/g-beads, while the adsorption capacity of the nanofluid containing FSiNP-NH₂was much lower at 0.004 mg/g-beads. The author believes that the low colloidal stability of FSiNP may lead to additional NP aggregation and deposition in the column, but the relatively low adsorption properties of both nanofluids demonstrate the potential of these fluorescent NPs in transport and adsorption studies.

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Figure 14.

Core-shell structure of fluorescent silicon nanoparticles (SiNPs) (Experimental conditions: Synthesizing fluorescent silicon nanoparticles FSiNP and FSiNP - NH₂ with fluorescein 5(6)-isothiocyanate (FITC) as the core and different surface (shell) functional groups to show their core-shell structures).

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Silicon-based nano-tracers show certain advantages in oilfield applications, and their stability is affected by multiple factors. In terms of temperature, the poor stability of ZnO quantum dots limits their application, while ZnO@SiO₂NPs formed by encapsulation with the improved Stöber method can maintain a certain emission intensity at 0-100°C, with significantly enhanced stability. ZnO@SiO₂NPs can also maintain good emission stability at a salinity of 0-40 g/L. Surface modification and functionalization have a significant impact on their stability. By functionalizing the surface of ZnO@SiO₂NPs through the co-condensation of different silanes, the formed nanoparticles with different properties such as partially hydrophobic (pH NPs, S2), hydrophobic (HP NPs, S3), and hydrophilic (H-NPs, S4) show different stabilities, among which pH NPs (S2) exhibits the highest emission stability under harsh reservoir conditions. However, current research on silicon-based nano-tracers has many limitations. Ion impurities may be introduced during the synthesis process, affecting stability. The principle of surface grafting modification and how it affects the fluorescence characteristics of encapsulated fluorescent substances remaons to be further studied. Most of the research is in the laboratory stage, and there is a lack of industrial test data, making it difficult to comprehensively evaluate their performance and long-term effects in actual reservoirs. Future challenges include optimizing fluorescence performance, developing surface grafting modification methods that better meet reservoir requirements, deeply understanding their behavior mechanisms in complex reservoir environments, and exploring large-scale preparation technologies for industrial application.

4. Polymeric Nano-fluorescent Tracers

Polymer fluorescent microspheres are nanoparticles in size, with fluorescent substances on the surface or inside of the microspheres, which can emit fluorescence under external energy stimulation. The preparation techniques of fluorescent microspheres can be divided into physical methods, such as adsorption, embedding, and self-assembly, and chemical methods like grafting and copolymerization [80].

Minjie Li et al. [81] extracted nanocrystals (NCs) with a large amount of negative charge on their surface into chloroform using cationic surfactant octadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC), then, it was swollen into the formed polystyrene (PS) microspheres to prepare high fluorescence polymer composite microspheres with waterborne CdTe nanocrystals (see Figure 15). This composite microsphere inherits the strong photoluminescence properties of water-soluble nano-carbons. Additionally, the polymer exhibits good stability and can resist both polar and non-polar solvents.

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Figure 15.

Preparation method of fluorescent tellurium-polystyrene (PS) Composite microspheres (Experimental conditions: Extracting nanocrystals with a large amount of negative charge on their surface into chloroform using a cationic surfactant and swelling them into formed polystyrene microspheres to prepare composite microspheres).

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From the University of Technology Malaysia A. Salim et al. [82] prepared novel pink dye encapsulated cinnamon nanoparticles (RB CNPs) as shown in Figure 16 using liquid-phase laser ablation (PLAL) method, its diameter is about 16-32 nm, the fluorescence lifetime is 6.12-6.911 ns, and the quantum yield is 0.341-0.61%. The color purity, fluorescence, and absorption properties of synthesized RB CNPs are highly sensitive to changes in fluorescence.

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Figure 16.

Synthesis Mechanism of rose bengal dye-encapsulated cinnamon nanoparticles (RB CNPs) (Experimental conditions: Preparing RB CNPs by the liquid-phase laser ablation (PLAL) method to show the reaction mechanism of the synthesis process).

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Sehoon Chang et al. [83] designed and synthesized two types of superparamagnetic surface - enhanced raman scattering (SERS) nanoparticles, magnetic surface - enhanced raman scattering (mSERS-s), and mSERS-np (see Figure 17), with each magnetic core particle measuring ∼5-10 nm in size and each modified silver nanoparticle measuring ∼10-30 nm in size. The average size of the synthesized composite SERS particles was approximately 60-100 nm. Among them, mERS-s nano-particles are an efficient substrate, and the target tracer molecules will adsorb on the surface of mERS-s composite nano-particles, thereby assisting in the detection of traditional tracers in reservoir production fluids; SERS np involves embedding SERS active organic dyes into nanoparticles during the synthesis process, resulting in each composite nanoparticle possessing SERS activity, and magnetic control can be used to improve detect ability, this type of particle can be injected from the injection well, pass through the underground, and collect and detect SERS signals of nano-particles with different concentrations at the production well. Both Ag nanoparticles and dye molecules are sealed within the SiO₂ shell of mSERS nanoparticles, preventing direct exposure to harsh fluids and enhancing their durability.

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Figure 17.

(a) Fe₃O₄@SiO₂/Ag composite surface-enhanced raman scattering (SERS) nanoparticles (mSERS - s); (b) Fe₃O₄/Ag@SiO₂ composite SERS nanoparticle tracer (mSERS - np); (c) Transmission electron microscopy images of superparamagnetic Fe₃O₄ nanoparticles and mSERS - s and mSERS - np composite SERS nanoparticles (Experimental conditions: designing and synthesizing two types of superparamagnetic SERS nanoparticles to show their structures and micro-morphologies).

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Xiong Jinping et al. [84-88] from the Beijing University of Chemical Technology have invented a series of fluorescent nanomaterials with core-shell structures that can be used as oil field tracers. The materials are made of polyvinylpyrrolidone (PVP) or sodium tartrate as the outer shell, NaYF₄: Yb/Tm, LiYF₄: Yb³⁺, Tb³⁺, LiBiF₄: Yb³⁺, Pr³⁺, LiBiF₄: Yb³⁺, Ce³⁺ and other fluorescent luminescent cores with rare earth elements as the core (as shown in Figure 18). The detection accuracy of fluorescent nanomaterials with core-shell structure is high, with a minimum detection concentration of 0.1 ppm. The surface features hydrophilic groups and good hydrophilicity and dispersibility, does not affect mineralization, and has good compatibility with polymers.

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Figure 18.

Structure schematic diagram of BEFC tracer (Experimental Conditions: Preparing BEFC tracer with polyvinylpyrrolidone or sodium tartrate as the shell and a fluorescent luminescent core containing rare earth elements as the core to show its structure).

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Polymer microspheres are generally made from acrylamide as the main raw material. Due to their strong hydrophilicity, good biocompatibility, good swelling, and viscoelasticity [89], they have been widely used in various large oil fields as a new type of reservoir development water control and plugging technology. However, when using polymer microspheres for displacement control, if the polymer microspheres have the same functional groups (usually amid groups) as the polymer solution, it is difficult to distinguish and accurately calculate the concentration of polyacrylamide microspheres from the extracted liquid and use it to guide the laboratory evaluation and field testing of polymer microsphere displacement control systems. To address this issue, researchers introduced monomers with fluorescent properties into the microspheres, enabling them to exhibit luminescent properties under ultraviolet light. This property enables accurate detection of the concentration of polyacrylamide microspheres in the extracted solution.

Kang Wanli et al. [90] used the fluorescent monomer allyl rhodamine B as the fluorescent monomer, and through reverse phase lotion polymerization with 2-acrylamido-2-methylpropane sulfonic acid (AMPS), a fluorescent acrylamide microsphere P (AM-AMPS RhB) was prepared. As shown in Figure 19, the average particle size of this microsphere is ∼200 µm, and it has a complete spherical shape with a smooth surface. The fluorescent monomers are evenly distributed throughout the microsphere structure, and under UV light irradiation, they can emit light red fluorescence. The experiment found that the prepared microspheres have good fluorescence stability in an aqueous solution containing metal ions with a pH of 3.0-10.0 and do not affect their function as a water-blocking agent.

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Figure 19.

(a-b) Fluorescence photos of fluorescent microspheres (Experimental Conditions: Using allyl rhodamine B as the fluorescent monomer and reacting with 2-acrylamido-2-methylpropane sulfonic acid through inverse emulsion polymerization to prepare fluorescent acrylamide microspheres, and taking fluorescence photos under ultraviolet light irradiation). (a) Ultraviolet radiation, (b) exposure to natural.

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Wan Li Kang et al. [91] synthesized polyacrylamide microspheres P (AM-BA-AMCO) that fluoresced under ultraviolet light using reverse phase suspension polymerization to detect polyacrylamide microspheres in reservoir produced fluids (see Figure 20). Indoor evaluation tests demonstrated that fluorescent polymer microspheres exhibit good water absorption and swelling ability, thus possessing the ability to block and transport in sand-filled tubes. The sealing rate of microspheres after treatment in sand-filled tubes is 99.8%, and the residual resistance coefficient is 800. Additionaly, the surface morphology of these microspheres was observed under ultraviolet light irradiation using a fluorescence microscope. The fluorescent microspheres were spherical in shape, smooth on the surface, and had an average diameter of about 200 microns. As shown in Figure 21, even if the fluorescent microspheres underwent compression and deformation in the sandbag, they could be accurately detected under ultraviolet light irradiation in the produced liquid.

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Figure 20.

Synthetic route of fluorescent polyacrylamide microspheres P (AM - BA - AMCO) (Experimental conditions: Synthesizing P (AM - BA - AMCO) by inverse suspension polymerization to show the reaction route of the synthesis process).

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Figure 21.

Fluorescence microscope images of P(AM - BA - AMCO) microspheres; (a) Phase contrast image of microspheres under natural light after swelling in salt water for 24h; (b) Fluorescence micrograph of figure as under ultraviolet irradiation; (c) Phase contrast image of the effluent under natural light after the subsequent water flooding experiment; (d) Fluorescence microscopy of figure^© under ultraviolet irradiation (Experimental conditions: Preparing P(AM - BA - AMCO) microspheres, conducting salt water swelling and water flooding experiments, and observing the morphologies of microspheres at different stages using a fluorescence microscope).

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ChuMi [92] of Shaanxi University of Science and Technology used inverse micro lotion polymerization to use acrylamide (AM) as the main monomer, 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and allyl fluorescein as the functional monomer, as shown in Figure 22. Nano-scale fluorescent polyacrylamide microspheres with uniform microsphere size and excellent fluorescence properties were prepared, with a high conversion rate, average particle size of 50.75 nm, and good thermal stability. In the performance evaluation experiment, the fluorescent polymer microspheres exhibited good temperature and salt resistance, acid and alkali resistance, suspension stability, and compatibility within the temperature range <120°C, mineralization degree <10g/L, and pH range of 3-12. The fluorescent polyacrylamide nanoparticles prepared by the authors showed good temperature and salinity resistance, acid-base resistance and suspension stability when the temperature was lower than 120°C, the salinity was lower than 10g/L, and the pH value was within the range of 3-12. However, when the environmental conditions exceed these ranges, the stability of microspheres may be affected. In high temperature and high salt environments, polymer microspheres may swell, degrade or aggregate, resulting in the decline of fluorescence performance and poor tracing effect. This is because high temperatures and high salt concentrations will destroy the molecular structure and chemical bonds of the polymer and affect its physical and chemical properties.

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Figure 22.

Synthetic route of fluorescent polyacrylamide microspheres (Experimental conditions: Using acrylamide as the main monomer, 2-acrylamido-2-methylpropane sulfonic acid and allyl fluorescein as functional monomers, and preparing fluorescent polyacrylamide microspheres by inverse microemulsion polymerization to show the synthetic route).

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However, introducing organic fluorescent dyes into polymer microspheres also has problems, such as limited dye types, poor thermal stability, poor resistance to pH, and interference from metal ions in groundwater. In response, researchers have attempted to introduce CQDs with better stability into polymer microspheres. Liu Yao [55] emulsified CQDs prepared from triamine citrate and trisodium phosphate as raw materials with microspheres to obtain composite microspheres with an average particle size of 955 nm. The composite microspheres have high stability, emit blue light under ultraviolet light irradiation, and can effectively reduce interfacial tension. Wang Nan [93] used the gel embedding method to introduce CQDs into ordinary nano-spheres to synthesize fluorescent microspheres that emit yellow light under ultraviolet light irradiation. The microspheres have good fluorescence properties, and in the aging experiment, the fluorescence intensity of the microspheres decreases slowly. This decrease is relatively low. In large-scale mining experiments, fluorescent microspheres can still achieve a sealing rate of ∼60%, which is not much different from the injectability and sealing performance of ordinary microspheres and can meet the requirements for on-site use. Ions may be introduced by monomer impurities, initiators, or catalysts during the preparation process. In high-temperature and high-salinity environments, changes in the structure of microspheres may cause internal ions to leach out. When organic fluorescent dyes or CQDs are introduced, ion leaching may also occur due to environmental factors.

Currently, research on nanofluorescent tracers are mostly in the laboratory stage, and the industrial test data is relatively scarce. This leads to an insufficient understanding of the performance and stability of tracers in large-scale applications in actual reservoirs, making it difficult to comprehensively evaluate its long-term effect and potential risks. Under laboratory conditions, environmental factors are relatively simple and controllable, whereas the actual reservoir environment is complex and changeable, including a variety of minerals, fluids, and microorganisms that may have unknown effects on the stability and performance of the tracer. Therefore, there are many uncertainties in the transformation process from the laboratory to practical application. There are some limitations in the synthesis of carbon-based nano-fluorescent tracers. Although hydrothermal synthesis is a simple, safe, and cost-effective method, the yield is low. Although the microwave method has fast heating speed and short synthesis time, it presents some challenges, including difficulties in controlling particle size, uneven distribution, difficulties in separation and purification, and unclear synthesis mechanisms. The defects of these synthetic methods limit the high-quality and large-scale preparation of tracers and, in turn, their industrial application process. In industrial production, a stable and efficient synthesis method is needed to ensure the quality and yield of tracer. The current synthesis method cannot meet this demand.

In summary, introducing fluorescent materials and utilizing their fluorescent properties is a new approach for detecting and tracking polymer microspheres. Moreover, compared to traditional fluorescent dyes, CQDs have better luminescent properties and stability, which has attracted increasing attention from researchers [94]. However, the stability and dispersibility of polymer nano-fluorescent tracers in special reservoir conditions, such as high temperature and high salinity still needs to be researched.

From the above, it can be seen that as shown in Table 2, nano-fluorescent tracers have the following characteristics.

5. Field Application of Nano-fluorescent Tracers

Currently, most tracers used in oil fields are traditional chemical and trace element tracers [95], while nano-fluorescent tracers are still in the experimental laboratory stage. However, some oil fields have also conducted on-site experiments based on carbon-based nano-fluorescent tracers.

Mazen Y. Kanj et al. [96] conducted single well testing on carbon-based nano-fluorescent particles A-Dots with an average diameter of nearly 8 nm in a vertical observation well in Ghawar oil field. The well (Well-X) continued to produce at a relatively high rate of 3300 barrels per day, with a water content of 99%, an open hole porosity of 10% -20%, a permeability of 10∼20mD, and a bottom hole temperature of 90°C. The formation was mainly composed of calcite and dolomite; over a period of 2 days, continuous collection and analysis of fluid samples from the well confirmed a high cumulative recovery rate of nanoparticles, reaching 86%.

Dmitry Kosynkin et al. [97] from Saudi Aramco conducted an on-site well group experiment on carbon-based nano-fluorescent particles A-Dots in Arabia. As shown in Figure 17, I1-I4 wells are seawater injection wells, while P1-P4 wells are production wells. The oil field has a water content of up to 95%, a total salt content of up to 22%, and a pore pressure of 3200 psi. A-Dots particles were injected from well I3, and sampling and testing were conducted at a frequency of 2 times per week in four production wells was. As shown in Figure 18, after injecting the tracer for about 50 days, A-Dots were detected in the produced water of Well P3, which is consistent with the results of previous chemical tracer tests. The concentration of A-Dots gradually increased, indicating that A-Dots successfully penetrated the reservoir pores in the area. In this 26-month field experiment, A-Dots were successfully detected in the production well, proving that they have successfully crossed the carbonate formation of the oil field. During the testing process, they showed good stability and excellent fluorescence performance, bringing new directions for the development of nano-fluorescent tracers for oil fields.

Carlos A Franco et al. [54] injected CQDs synthesized from biomass into an oil well in Colombia at a concentration of 500000 mg ∙ L^-1, and took samples twice a day at 9 production wells located approximately 150 to 750 meters away from the injection well, the tracer exploded 3 days after injection, with a concentration of 3.1 mg ∙ L^-1, continuously increasing for 15 days, and reaching a maximum concentration of 41.3 mg ∙ L^-1. After 25 days, it decreased to 2.5mg ∙ L^-1. In Figure 17 and Figure 18 in Dmitry Kosynkin et al. [97] with Figure 23 and Figure 24, respectively. Figure 25, the tracer exploded at a concentration of 4.2 mg ∙ L^-1 after 3 days, continued to increase to a maximum of 15.4 mg ∙ L^-1 after 3 days, and decreased to 1.4mg ∙ L^-1 after 18 days, demonstrating the excellent performance of this nanoparticle as a green tracer. Compared with the tracer used on site before, the cost of this experiment and operation has been reduced by more than 70%, mainly due to the lower cost of CQDs (about $50/kg).

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Figure 23.

Well location map (Experimental conditions: Conducting a field well group experiment of carbon-based nano-fluorescent particles A-Dots in Saudi Arabia, showing the location distribution of experimental wells).

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Figure 24.

Tracer output curve (Experimental Conditions: Injecting A-Dots particles in an oil field in Saudi Arabia and sampling and testing at four production wells with a frequency of 2 times per week to plot the curve of tracer concentration changes over time).

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Figure 25.

CQDs tracer test in conventional water flooding of an oil field in colombia (Experimental Conditions: Injecting CQDs synthesized from biomass into an oil well in Colombia at a concentration of 500000 mg∙L⁻¹ and sampling twice a day at 9 production wells about 150 - 750 meters away from the injection well to monitor the change of tracer concentration).

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Rigu Su et al. [45] from China National Petroleum Corporation successfully applied the synthesized dual emission wavelength red quantum dots to identify high permeability channels in oilfield injection wells. The author transported the carbon quantum dot powder along with the injected water to a designated layer. After a period, samples were taken from the oil well, and the content of fluorescent CQDs was determined using a spectrometer. By measuring and analyzing the time and content of quantum dots appearing in injection wells at different layers, it is determined that the 1704-meter location is a high-permeability channel.

In addition to directly injecting quantum dots into oil wells for tracking and monitoring, quantum dots can also be labeled in polymers and then adsorbed onto the surface of supporting agents such as ceramic particles or quartz sand [98] or prepared as tracer strips fixed to base pipes (as shown in Figure 26). During oil well operation, these proppants and bands can release corresponding quantum dots based on the encountered oil/water phase, facilitating sampling and analysis at the production well. This emerging intelligent slow-release quantum dot production logging technology has wide applicability, low cost, and can monitor the situation for an extended period. Geosplit, a Russian company, has developed two intelligent tracer monitoring technologies based on CQDs: quantum coating proppants and quantum tracer tapes. In 2017, they successfully applied intelligent tracers for inter-well monitoring in 55 wells [99].

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Figure 26.

Polymer-coated proppant marked with quantum dots (Experimental Conditions: Labeling quantum dots on polymers and then adsorbing them on the surface of proppants to show the structure of the labeled proppant).

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When the volume fracturing and sand addition of ZPX well [100] in the Ansari Oilfield, China reached the 85% stage, a nano-crystal tracer labeled with quantum dots synthesized by colloids was introduced. After analyzing 1206 effective samples collected 420 times, the contribution rates of each layer were accurately obtained, and the dynamic production of oil, gas, and water in each horizontal section was quantitatively analyzed, providing important data support for the comprehensive evaluation of fracturing effects in layered sections and subsequent work.

From October 2017 to July 2018, samples were collected from a horizontal well [98] in the Tyumen oilfield in Russia, and samples were studied using quantum dot encapsulation tracking technology. From the results of sampling interpretation (Figure 27), it can be seen that

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Figure 27.

Oil and water production of each fracturing section (Experimental Conditions: Conducting a quantum dot encapsulation tracking technology experiment in a horizontal well in the Tyumen Oilfield, Russia, sampling and studying from October 2017 to July 2018 to analyze the changes in oil and water production of each fracturing section).

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There are significant differences in the produced fluids during each fracturing stage in the experimental process: In the first month after fracturing, the production of each oil well is relatively consistent. Overall, the production at the toe of the horizontal section is relatively low, accounting for only 7%, while the production at the heel of the horizontal section is relatively high, accounting for 35%; Between December 2017 and May 2018, the production of Block 4 increased rapidly, rising from 39% to 81%; In terms of water production, the fourth stage accounts for the largest proportion, and over time, the water production gradually increases. By using intelligent tracking monitoring, the production changes of each section of the test can be timely understood, thus achieving accurate measurement of the liquid production and oil production contribution rate of each stage of the horizontal well section. This provides a basis for optimizing oil well production and wellbore intervention, as well as new ideas for evaluating production and optimizing fracturing schemes at different stages of the horizontal well section.

Overall, due to their excellent performance, nano-fluorescent particles have shown good reservoir applicability in indoor core simulation evaluations. At the same time, carbon-based nano-fluorescent particles have been successfully applied on a large scale in the field, opening up new directions for the subsequent application of nano-fluorescent materials in oilfield tracing. However, most of this technology is still in the laboratory research stage, and there is no mature application system yet. The actual reservoir environment is extremely complex, and the geological conditions of different reservoirs vary greatly, including various complex minerals, fluid components, and different temperature, pressure, and pH conditions. In such an environment, complex physical and chemical changes may occur to the nano fluorescent tracer, which will affect its stability and performance. How to improve the tracer’s ability to adapt to various complex reservoir environments and accurately and stably play the role of tracer is an important challenge for future research. Researchers need to understand the behavior mechanism of tracers in complex environments and develop tracer materials and technologies with stronger adaptability.

6. Conclusions

Nano-fluorescent tracers have broad application prospects in the oilfield-domain. Carbon-based nano-fluorescent tracers are low-cost, stable, and have low formation adsorption. Their raw materials are widely available, like sugars and biomass waste. Preparation methods are mainly “top-down” and “bottom-up”. Hydrothermal and microwave synthesis are common in the “bottom-up” approach. Hydrothermal synthesis uses small organic carbon sources to form CQDs via complex processes in a high-temperature, high-pressure reactor. Microwave synthesis carbonizes carbon-containing organics instantly with microwave-generated high temperatures, which is efficient and eco-friendly, but results in uneven particle-size distribution, posing separation and application challenges.

Silica-based nano-fluorescent tracers typically have a core-shell structure, with cores like quantum dots, organic fluorescent dyes, or rare-earth luminescent materials, and silica as the shell material. For example, using a modified Stöber method to coat quantum dots with silica can enhance their stability in harsh reservoir conditions. ZnO quantum dots, when encapsulated into ZnO@SiO₂NPs, maintain high emission intensity at 0-100°C and 0-40g/L salinity.

The preparation of polymer-based nano-fluorescent tracers involves physical methods (e.g., adsorption, embedding, self-assembly) and chemical methods (e.g., grafting, copolymerization). For instance, inverse emulsion polymerization can incorporate fluorescent monomers into polymer microspheres, making the detection of polymer drive effluent concentrations more accurate and straightforward, overcoming traditional method limitations.

The stability of nano-fluorescent tracers is influenced by various factors. For carbon-based tracers, some suffer reduced fluorescence intensity at high temperatures, while silica-based ZnO@SiO₂NPs show enhanced stability. High salinity can degrade polymer-based tracers, but silica-based ones are more salt-resistant. Extreme pH levels can destabilize carbon and polymer-based tracers. Surface modification and functionalization can enhance stability, but interactions with reservoir substances may interfere. Despite many advantages, research is predominantly lab-based, with industrial applications hindered by high costs and immature large-scale preparation technologies. The complex actual reservoir environment poses a significant challenge for adaptation. Thus, in-depth research on preparation technology, performance optimization, and application expansion is crucial.

In the future, research on nano-fluorescent tracers will focus on enhancing stability, developing large-scale preparation technologies, expanding application domains, achieving intelligent and multifunctional capabilities, ensuring environmental friendliness and safety, and strengthening interdisciplinary collaboration. These efforts will drive the widespread adoption of nano-fluorescent tracers across diverse fields, offering more effective solutions to practical challenges.