Abstract
Oil displacement agent technology is an attractive technique for improving oil recovery efficiency, as it not only enhances oil recovery efficiency and extends oilfield lifespan, but also helps reduce environmental impact and improve the precision of oilfield management. However, the application of traditional oil displacement agents in oilfield development is limited by poor local fluidity and difficulty in monitoring their effectiveness. To improve oil displacement efficiency and achieve more accurate process monitoring, this paper designs an oil displacement agent that has both oil displacement and tracer functions. In this work, we first synthesized a fluorescent nano displacement agent with an average particle size of 80 nm using an improved Stöber synthesis method and silane coupling agent surface modification theory and characterized it using characterization techniques. Finally, we evaluated the migration characteristics, oil displacement performance, and tracer performance of the fluorescent nano displacement agent in the reservoir through core flow experiments. The results showed that the fluorescent nano displacement agent has good migration ability, with an improved recovery efficiency of 14.09%. The error between the fitted permeability and the actual permeability of the core in the experiment was about 10%. The simulation results once again proved that the fluorescent nano displacement agent can be used for tracer testing.
Keywords: Fluorescent silica nanoparticles, Core flooding, Enhanced oil recovery (EOR), Reservoir heterogeneity, Wettability alteration, Interfacial tension reduction, Thermogravimetric analysis (TGA)
Subject terms: Engineering, Materials science, Nanoscience and technology
Introduction
With the reduction of recoverable reserves in medium and high-permeability reservoirs, the development of low-permeability reservoirs is increasingly attracting people’s attention1–5. Taking China’s oil and gas industry as an example, in the past five years, low-permeability oil reservoirs have accounted for more than 70% of the proven oil and gas reserves, and the development of low-permeability oil reservoirs has gradually become the pillar of China’s oil and gas industry6–11. Among them, the chemical agent system can reduce the interfacial tension and injection pressure between oil and water, and can emulsify, disperse, and strip crude oil, thereby improving oil recovery. However, for low-permeability reservoirs, chemical flooding systems suffer from severe adsorption losses in the formation, which affect the migration distance and effectiveness of surfactants in the formation, leading to a decrease in the effectiveness of improving oil recovery and causing serious high cost problems12–14.
In recent years, with the continuous development of nanotechnology and its successful application in multiple fields, relevant researchers and field engineers at home and abroad have found that nano oil displacement agents can solve engineering problems in traditional oil reservoir development processes, such as poor injectability and environmental adaptability in low-permeability reservoirs. They have broad application prospects in improving oil and gas recovery rates15–18.The Roman Shkin oilfield conducted field trials of injecting hydrophobic nanoparticles, resulting in a significant increase in oil recovery rate. This indicates that hydrophobic nanoparticles can potentially improve the development efficiency of low-permeability oil reservoirs19–21. After injecting nano displacement agents into Shengli Oilfield, Jiangsu Oilfield, Xinjiang Oilfield, and Daqing Oilfield in China, the permeability of rock cores significantly increased, and the effect of reservoir pressure reduction and injection enhancement was significant22–25. Among them, nano silica particles have the advantages of non-toxicity and non-pollution, and have demonstrated excellent performance and good application prospects in the evaluation of tertiary oil recovery. Dai prepared a novel self-dispersing silica nanoparticle and applied it to the spontaneous imbibition experiment of low-permeability rock cores. The results showed that the nano silica fluid could significantly improve crude oil recovery26–29. Zargartalebi et al. investigated the adsorption of nano silica at the oil-water interface and its ability to reduce interfacial tension under the combined action of surfactants. Research has found that nanoparticles’ adsorption capacity is greater than surfactants30–33.
However, with the gradual deepening of oilfield development, especially in complex oil reservoirs, how to accurately monitor the distribution and flow of nanodisplacement agents in the reservoir has become the key to optimizing the oil displacement process. The application of traditional oil displacement agents lacks real-time and visual tracking methods, which limits the adjustment and optimization in the development process. Therefore, introducing nano oil displacement agents with tracer function has become an important direction for improving oil displacement efficiency and precise management of oil fields34–36. In recent years, the combination of fluorescence and nanoparticles has developed into an emerging research field- fluorescent nanoparticles. Compared with traditional tracers, nanoparticles have adjustable fluorescence color, good photostability, and modifiable surface ligands (surface modifiability). Fluorescent tracers have very low detection limits, low cost, and are easy to operate. These unique properties make fluorescent nanoparticles superior and indispensable as fluorescent tracers37–39.
Clude Q proposed the patent “Evaluation of Chemical Agent System Effectiveness Using Single Well Tracer Method” in 1978 based on the basic principles of single well chemical tracer testing40. On this basis, he also proposed the patent “Evaluation of Surfactant Drive Effect Using Single Well Surfactant Test Method with Multiple Tracers” in 1979, which was verified in single well surfactant field tests in Wyoming and Big Muddy oil fields in the United States40, proving that it is consistent with the results of multi well field tests. This technology has also been applied in evaluating the oil displacement effect of surfactant flooding or ternary composite flooding in Oseberg oilfield in the North Sea region of Norway41, Gulfaks oilfield in Norway42, and C4 reservoir in Lake Maracaibo, Venezuela43. The United States evaluated the feasibility of using low salinity water flooding to improve oil recovery in four blocks on the northern slope of Alaska44 .
This article is based on the improved synthesis method of Stöber and the theory of surface modification of silane coupling agents, and develops a functional nano oil displacement agent with smaller size, good stability, and fluorescence. Subsequently, the fluorescence intensity and oil displacement performance of fluorescent nanodisplacement agents were studied, exploring the influence of different influencing factors, and obtaining the detection standard curve of nanodisplacement agents under specific conditions. Finally, through indoor physical oil displacement experiments, the mechanism of nano oil displacement agents in improving oil recovery was observed, analyzed, and summarized, providing new technical support for the efficient development of low-permeability reservoirs in the future.
Reagents and equipment
Reagents
Tetraethyl orthosilicate, anhydrous ethanol, ammonia water, 3-aminopropyltriethoxysilane, and hexadecyltrimethoxysilane are all analytical pure, and the fluorescent dye is fluorescein isothiocyanate. Deionized water is self-made in the laboratory and used to prepare the solution required for the experiment.
Low-permeability core samples
The rock cores used in the experiment belong to natural low-permeability rock cores. After the core is saturated with saltwater, the crude oil sample is continuously injected into the core at a rate of 1 ml/min until only oil is produced at the outlet end.
Equipment
The main instruments include magnetic heating stirrer (Shanghai McLean Biochemical Technology Co., Ltd.); high-speed centrifuge (Shanghai Lichenbangxi Instrument Technology Co., Ltd.); KQ3200DE ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd.); Thermo Scientific Nicolet 6700 Fourier transform infrared spectrometer (American Thermoelectric Company); LS13320XR laser diffraction particle size analyzer (Northern Coulter Co., Ltd., United States); UV-2550PC UV-visible spectrophotometer (Shimadzu Company); RF-5301PC fluorescence spectrometer (Shimadzu company).
Experiment design and procedures
Preparation of fluorescent nano oil displacement agent
The selection of APTES (3-aminopropyltriethoxysilane) and HDTMS (hexadecyltrimethoxysilane) was primarily based on the following considerations and their unique advantages: As a classic amino-silane, APTES efficiently introduces reactive amine groups (-NH2) onto the surface of silica nanoparticles. Its reaction conditions are mild, easily controllable, and the formed siloxane bonds (Si-O-Si) exhibit good chemical stability. The main purpose of choosing HDTMS was to introduce long-chain alkyl groups (hexadecyl) onto the nanoparticle surface to impart hydrophobicity. This hydrophobic modification helps to adjust the nanoparticle’s hydrophilic-lipophilic balance, and in oil displacement applications, it can reduce the interfacial tension between the nanoparticles and the oil-water interface. This may enhance their migration ability and oil displacement efficiency in reservoirs by modulating their partitioning behavior in different fluid phases.
Functionalizing fluorescent dyes using silane coupling agent 3-aminopropyltrimethoxysilane (APTES) to synthesize APTES-FITC precursor solution. Dissolve 5 mg (12.8 µmol) of Fluorescein Isothiocyanate (FITC) in 10 mL of ethanol. Then, 28.43 mg of APTES (12.8 µmol) was added to the above solution, and the solution system was stirred at 50 ℃ in the dark and dry conditions for 24 h at a stirring speed of 1200 r/min. The APTES conjugated precursor solution of the fluorescent dye was obtained and used for the next step of synthesizing fluorescent silica nanoparticles.
Add 4 mL of ammonia water and 1mL of water to 100mL of ethanol solution, stir for 5 min, and then add 2.5 mL of Tetraethyl Orthosilicate (TEOS) to the solution. Continue stirring for 10 min, then measure 0.2 mL of FITC-APTES precursor solution and add it to the mixed solution. Continue stirring for 6 h, and then add 0.5 mL of TEOS again. Continue stirring for 2 h until the reaction is complete. The reaction temperature for this step is 50℃, and the stirring speed is 1000 r/min.
After the reaction, place the reactants in a centrifuge and centrifuge at 12,000 r/min for 10 min to collect the fluorescent silica nanoparticles after the reaction. Wash the centrifuged product three times with anhydrous ethanol and pure water to remove unreacted chemicals. Add 10 mg of the synthesized fluorescent silica nanoparticles to 15 mL ethanol solution and sonicate for 40 min. Then add 0.2 mL of hexadecyltrimethoxysilane (HDTMS) and mix thoroughly. Stir the solution at room temperature for several hours, centrifuge the reaction solution at 12,000 r/min for 10 min, and wash it three times with ethanol to remove unreacted substances. Dry the washed product in an oven for 4 h to obtain a fluorescent nano oil displacement agent.
Ftir tests
Take a small amount of sample powder and place it together with potassium bromide in a mortar, grinding it to a very fine powder. Put it into a dedicated mold for compression, obtain a flat and smooth disc, and characterize the structure of the tested sample.
Tem tests
The morphology of different prepared samples was observed using FEI Quanta 650 FEG environmental scanning electron microscopy (as shown in Fig. 1). Dilute different samples with anhydrous ethanol to a certain concentration, take a small amount of solution and drop it onto the carrier, let it dry naturally, then perform vacuum pumping, and finally perform gold spray coating testing and observe its appearance, size, etc.
Fig. 1.
FEI quanta 650 FEG environmental scanning electron microscope.
Particle size distribution and dispersibility tests
The Zetasizer NanoZS3600 laser particle size analyzer produced by the British company Malvern was used to measure the particle size of the synthesized fluorescent nanodisplacement agent. The particle size measurement range is 0.3 nm (diameter) to 10 μm, and the sample concentration range is 0.1–40 mg/L. Refer to GB/T32668-2016 “General rules for Zeta potential analysis of colloidal particles by electrophoresis”.
Fluorescence property measurement
Using a fluorescence spectrometer to test the fluorescence spectra of fluorescent nanodisplacement agents and fluorescent monomers (FITC) at a certain excitation wavelength, in order to characterize their fluorescence performance. Simultaneously test the fluorescence emission spectra of fluorescent oil displacement agents at different concentrations and explore their impact on fluorescence performance.
Interfacial tension
Interface tension is an important parameter for evaluating the oil displacement performance of oil displacement agents. According to the method in SY/T 5370 − 2018 “Determination of Surface and Interface Tension”, the interface tension between the sample and simulated crude oil is measured using the rotating droplet method.
Wettability testing method
(1) Wettability treatment of rock cores.
Put the rock core into a container filled with crude oil and seal it. Place the container in a 45 ℃ constant temperature box. This process is called a one-time treatment, and the aging time is adjusted according to the expected oil moisture strength. Generally speaking, the longer the aging time, the stronger the oil affinity of the rock core.
(2) Contact angle measurement.
Polish the top of the rock core smooth, measure the contact angle of distilled water on the rock core and record it; Weigh a quantitative amount of fluorescent nanodisplacement agent and prepare it into a solution with a concentration of 0.3%, which is used as the permeate solution. Soak the rock core in the leachate for 48 h, which is called secondary treatment, and finally measure the contact angle of distilled water on the rock core after secondary treatment.
Static adsorption testing method
In the process of oil displacement, due to the contact between the oil displacement agent and the core, physical and chemical adsorption will occur on the surface of the core, causing the concentration of the oil displacement agent to gradually decrease and forming an adsorption layer on the surface of the core, resulting in a decrease in the oil displacement effect. Before using fluorescent nanoparticles as oil displacement agents for oil displacement experiments, adsorption experiments need to be conducted to calculate the adsorption amount of the core on the oil displacement agent.
Reservoir adaptability evaluation experiment of fluorescent nanodisplacement agent
The feasibility of using fluorescent nano oil displacement agents for both oil displacement and monitoring in reservoirs was verified through core flow experiments, which involved parallel three core flow experiments. In the parallel three core flow experiment, the injection layer simulation was divided into three sub layers, with three different permeability parameters of high, medium, and low set in each sub layer. The reliability of the fluorescent nano tracer interpretation was judged by the error between the permeability obtained by inverting and fitting the tracer production concentration distribution and the true permeability. The parallel three core flow experiment included three groups, simulating low, medium, and strong heterogeneity conditions, and the permeability variation coefficients of the three groups were 0.085, 0.11, and 0.48, respectively. The experimental cores used are all sandstone cores with a permeability range of 300 ~ 3000 mD. The reagent used in the experiment is the fluorescent nanodisplacement agent synthesized in this article, with an initial concentration of 10 mg/L and an injection rate of 0.05 mL/min. The injection medium selected is simulated injection water used in the X oilfield injection well, with a mineralization degree of 6100 mg/L. The experimental parameters are shown in Table 1.
Table 1.
Tracer core flow experiment design.
| Experi-ment number | Core number | Core permeability (md) | Porosity (%) | Core length (cm) | Initial concentration | Injection speed |
|---|---|---|---|---|---|---|
| 1 | B1 | 726.31 | 15.42 | 6.10 | 10 mg/L | 0.05 mL/min |
| B2 | 617.34 | 13.21 | 6.20 | |||
| B3 | 547.28 | 11.74 | 6.10 | |||
| 2 | B4 | 1358.12 | 18.81 | 5.90 | ||
| B5 | 822.31 | 12.66 | 6.01 | |||
| B6 | 361.32 | 10.24 | 6.10 | |||
| 3 | B7 | 823.11 | 12.87 | 6.11 | ||
| B8 | 789.23 | 13.33 | 6.04 | |||
| B9 | 771.91 | 13.58 | 5.92 |
Figure 2 is a schematic diagram of the core flow experiment device, which is connected to three core holders for installing cores with different permeabilities. The inlet end is equipped with a valve to control fluid entry, and the holders are connected in parallel with each other. The inlet end is connected in series with two intermediate containers, each containing fluorescent nano tracer solution and simulated formation water. Three confining pressure pumps are respectively connected to the core clamp for fixing the core. Collect samples at the fluid outflow end of the core holder and place the collected samples into a fluorescence spectrometer for quantitative detection. The entire device is placed in a constant temperature chamber to maintain consistent ambient temperature.
Fig. 2.
Core flow experimental device.
The method for the core flow experiment of fluorescent nano oil displacement agent is as follows: (1) Dry and weigh the experimental core, then place it in a vacuum dish saturated with formation water for 24 h; (2) Weigh the saturated experimental core again to calculate its porosity; (3) The prepared fluorescent nano oil displacement agent solution is ultrasonic dispersed for 2 h to make it fully dispersed in the simulated injection water. After the ultrasonic is completed, it is loaded into the intermediate container. At the same time, another intermediate container is filled with simulated formation water, and the saturated experimental core is loaded into the core holder, and the pressure of the confining pump is adjusted to 4 MPa; (4) Conduct a water flooding experiment, turn on the constant speed and flow pump, set the displacement speed to 0.05 mL/min, and use simulated formation water for displacement until no oil is produced and record the volume of produced fluid,; (5) Perform oil displacement with a constant speed and flow pump at a displacement rate of 0.05 mL/min to continue the fluorescence nano oil displacement experiment, and take samples at the outlet with a sampling interval of 5–10 s; (6) Place the collected samples into a fluorescence spectrometer to detect the concentration of fluorescent nanodisplacement agents at each sampling point.
Results and discussion
Reaction mechanism analysis
The reaction equation between FITC and APTES is shown in Fig. 3. The active functional group isothiocyanate (-N = C = S) in FITC molecule undergoes addition reaction with the amino group in APTES molecule to form Schiff base, thus forming the precursor of FITC-APTES. In the process of synthesizing fluorescent nanoparticle tracer for EOR, TEOS and HDTMS are co hydrolyzed under the catalytic action of ammonia water to generate silanol (-Si-OH), and the silanol groups generated by hydrolysis form silicon oxygen bonds (-Si-O-Si-) through condensation reaction.
Fig. 3.
Reaction process of fluorescent nano oil displacement agent.
Infrared spectroscopy characterization
In Fourier transform infrared spectroscopy (FTIR), the vibration frequency of molecules determines the position of the absorption peak of the substance, and the peak is determined by the amount of sample content. Different forms of molecular vibration will produce different infrared absorption peaks, and the position of the absorption peak of the same molecular group will also change left and right due to different chemical environments. Using the above principle, we can effectively use an infrared spectrometer to identify the structure of fluorescent nano displacement agents preliminarily.
Figure 4 shows the infrared spectrum of the synthesized fluorescent nano displacement agent. The analysis of the infrared spectrum of the product shows that the synthesized nanoparticles exhibit typical stretching vibration absorption peaks of Si-O-Si at 480 cm− 1, 800 cm− 1, and 1100 cm− 1, stretching vibration absorption peaks of Si-OH at 960 cm− 1, stretching vibration absorption peaks of -OH at 3478 cm− 1, and bending vibration absorption peaks of H-O-H at 1634 cm− 1 due to the incomplete removal of water adsorbed by the nano-silica. In addition, compared to the original silica nanoparticles, the fluorescent nano displacement agent exhibits stretching vibration absorption bands of -CH2 bonds at 2919 cm− 1 and 2846 cm− 1, indicating successful grafting of functionalized groups in the silane coupling agent onto the surface of silica nanoparticles. Based on the above analysis of functional groups in infrared spectroscopy, it can be proven that the synthesized fluorescent nano oil displacement agent has been successfully synthesized.
Fig. 4.
Infrared spectral characterization diagram.
Scanning electron microscopy and particle size distribution testing analysis
Observe the appearance, size, and dispersion of the fluorescent nano displacement agent using electron microscopy (Fig. 5), and analyze the sample distribution using a laser particle size analyzer, as shown in Fig. 6.
Fig. 5.
SEM image of fluorescent nanodisplacement agent.
Fig. 6.
Laser particle size distribution of fluorescent nano oil displacement agent.
The Scanning Electron Microscope (SEM) image of the fluorescent nanoparticle tracer for EOR is shown in Fig. 5. From the SEM image, it can be seen that the particle shape of the fluorescent nanoparticle tracer for EOR is spherical and the morphology is uniform, indicating that the prepared fluorescent nanoparticle tracer for EOR meets good dispersibility and can be further studied. The laser particle size distribution diagram (Fig. 6) shows that the particle size of the fluorescent nano oil displacement agent is 80 nm, and the particle size distribution is relatively uniform, indicating that the prepared oil displacement agent has uniform particle size and stable size.
Fluorescence performance analysis
Figure 7(1) shows the original morphology of the synthesized fluorescent nano oil displacement agent under natural light, and Fig. 7(2) shows the oil displacement agent’s fluorescence color under a 270 eV ultraviolet lamp. The fluorescent nano oil displacement agent has good fluorescence performance. Measure the UV visible and fluorescence emission spectra of the fluorescent nano displacement agent separately, as shown in Figs. 8, 9 and 10.
Fig. 7.
Macroscopic states of samples under different lighting conditions.
Fig. 8.
UV-visible absorption spectrum of fluorescent nano oil displacement agent.
Fig. 9.
Fluorescence spectra of different concentrations of fluorescent nano displacement agents.
Fig. 10.
Relationship between fluorescence intensity and concentration.
The UV-visible absorption of fluorescent nano displacement agents is mainly related to the electronic transition from the ground state to the excited state of the molecule. According to Fig. 8, the maximum absorption wavelength of the fluorescent nano displacement agent is 450 nm, indicating that the energy of its electronic transition corresponds to approximately 2.75 eV (i.e. the energy of 450 nm light), which is consistent with the transition characteristics of its electronic structure.
From Figs. 9 and 10, it can be seen that as the concentration increases, the relative fluorescence intensity of the fluorescent nano displacement agent at 520 nm also continuously increases; And even at concentrations as low as 50 mg/L, fluorescence intensity can still be detected, indicating that the fluorescence performance of the oil displacement agent is good. From Fig. 10, it can be seen that after linear fitting, the relative fluorescence intensity of the fluorescent oil displacement agent shows a good linear relationship with the mass concentration, with a linear correlation coefficient of 0.99. Therefore, the concentration of fluorescent oil displacement agents in the solution can be quantitatively calculated by measuring the relative fluorescence intensity of the solution, which provides a basis for the detection of fluorescence concentration in oilfield-produced fluids.
Stability analysis of fluorescent nanofluid
This study systematically evaluated the fluorescence stability of fluorescent nano-tracers, investigating the effects of temperature, salinity, and pH on their fluorescence intensity. As shown in Fig. 11a, at lower temperatures (25 ℃ and 40 ℃), the fluorescence intensity of the tracer remained largely stable over 72 h, demonstrating good fluorescence stability. However, with increasing temperature, the rate of fluorescence intensity decay accelerated, and the time required to reach stabilization shortened. At 80 ℃, the fluorescence performance remained relatively good; yet, at 100 ℃, the fluorescence intensity significantly decreased, indicating a clear detrimental effect of high temperature on fluorescence performance. Under different salinity conditions, as depicted in Fig. 11b, the fluorescence intensity showed no significant change over time, suggesting good tolerance of the tracer to salinity variations. As illustrated in Fig. 11c, the fluorescence intensity of the fluorescent nano-tracer exhibited a trend of initial enhancement followed by reduction across different pH values. The fluorescence intensity was lower in acidic environments. Given that the pH of formation water in typical oil reservoirs generally ranges between 6 and 8, the fluorescent tracer demonstrates good fluorescence stability under common reservoir conditions, providing crucial support for its practical application.
Fig. 11.
Fluorescence intensity variation of fluorescent nano-tracer under different factors (a: temperature; b: salinity; c: pH).
Experimental study on adsorption kinetics
The experiment was conducted in four sets. First, sandstone fragments were pulverized into uniform 60-mesh fine sand particles. For each set, 30 g of fine sandstone and 30 mL of fluorescent nanofluid (initial concentration: 10 mg/L) were placed in a 100 mL beaker. Subsequently, 50 mL of sodium chloride electrolyte solutions with concentrations of 0.001, 0.01, 0.1, and 0.2 mol/L, respectively, were added to each beaker. The mixtures were thoroughly shaken and then continuously stirred using a magnetic stirrer at 120 rpm and 25 °C for 4 h. During this period, 2 mL samples were collected from each experimental set at 0, 5, 10, 30, 60, 90, and 120 min. These samples were filtered through a 0.45 μm microporous membrane to remove any entrained sandstone particles, and the concentration of the fluorescent nanofluid was measured. The adsorption capacity of the fine sandstone for the fluorescent nanofluid at each time point was calculated using Eq. (1), where 
(mg/L) is the Initial concentration of fluorescent nanofluid, 
(mg/L) is the Concentration of fluorescent nanofluid at different time intervals, 
(mL) is the solution volume, 
(g) is the Mass of fine sandstone.
 |
1 |
In the aforementioned adsorption experiments, when the contact time between the fluorescent nanofluid and fine sandstone equals or exceeds the equilibrium time, the equilibrium adsorption capacity of the fluorescent nanofluid on the fine sandstone is calculated using Eq. (2). The adsorption efficiency of the fluorescent nanofluid is determined by Eq. (3), where 
(mg/L) is the Equilibrium concentration of fluorescent nanofluid in solution
 |
2 |
 |
3 |
After 4 h of stirring, the remaining solution in the beaker was used for adsorption reversibility experiments of the fluorescent nanofluid. In this step, the residual fluorescent nanofluid was removed and replaced with an electrolyte solution of identical chemical composition but devoid of the tracer. This tracer-free electrolyte solution was then added to the beaker, and the mixture was stirred for an additional 2 h. The initial and final concentrations of the tracer were measured before and after this stirring period.
As depicted in Fig. 12, an increase in the solution’s ionic strength leads to a reduction in the time required to reach adsorption equilibrium. However, the equilibrium adsorption concentration remains unaffected. Specifically, the fluorescent nano solution reached adsorption equilibrium at 30 min for ionic strengths of 0.01 mol/L and 0.1 mol/L, respectively, and at 90 min for an ionic strength of 0.2 mol/L. The equilibrium adsorption capacity was consistently determined to be 0.0096 mg/g. Furthermore, it is evident that ionic strength is a predominant factor influencing adsorption. At a solution ionic strength of 0.001 mol/L, the fluorescent nano solution exhibited negligible adsorption on the sandstone particle surface.
Fig. 12.
Adsorption kinetics of fluorescent nano solution under varying ionic strengths.
In Fig. 13, the desorption of fluorescent nano solution adsorbed on sandstone particle surfaces was investigated at different ionic strengths. It is evident that the desorption amount of the tracer from the sandstone surface is very low across all tested ionic strengths. Moreover, a slight decreasing trend in the desorption amount of the tracer adsorbed on the sandstone particles is observed with increasing ionic strength. The strong adsorption between the solution and the sandstone surface suggests that the hydroxyl and amine bonds on its surface play a decisive role in controlling its transport in electrolyte solutions. At high ionic strengths, the desorption of the solution from the sandstone particle surface becomes negligible, which is attributed to the formation of cation bridging between the particles and the sandstone surface.
Fig. 13.
Desorption kinetics of fluorescent nano solutions at different ion intensities.
Thermogravimetric characterization results analysis
Figure 14 displays the thermogravimetric analysis (TGA) curve of the fluorescent nano-tracer, revealing its mass loss behavior as temperature increases under an inert atmosphere. The results show an initial mass loss of approximately 2–3% below 150 °C, which is primarily attributed to the volatilization of adsorbed water and residual solvents. As the temperature further increased, a significant mass loss (approximately 27–33%) was observed within the temperature range of approximately 200–600 °C. This stage of mass loss mainly corresponds to the thermal decomposition of the organic modification layers (such as APTES, HDTMS). Finally, when the temperature reached above 700 °C, the mass loss curve stabilized, with the final residual mass being approximately 65–70% of the total mass, indicating the excellent thermal stability of the inorganic silica nanostructure. The TGA results not only confirm the successful organic modification of the fluorescent nano-tracer but also provide quantitative information regarding its thermal stability and compositional content, offering an important reference for the tracer’s application under various reservoir temperature conditions.
Fig. 14.
Thermogravimetric analysis (TGA) curve of fluorescent nano-tracer.
Interface/surface tension test evaluation
Interface tension test evaluation
Take 2 portions of fluorescent nano oil displacement agent solid, each 0.3 g, and dissolve them in 100 mL of injection water from different blocks. First, inject the injection solution into the rotary drop interfacial tension meter. Heat the crude oil in each block of the oilfield using a boiling water bath to melt the crude oil and make it fluid. Then use a needle to suck a small amount of crude oil and inject one drop into the rotary drop interfacial tension meter. Adjust the speed to 6000 r/min and read the interfacial tension value, the results are shown in Table 2.
Table 2.
Interface tension between fluorescent nano displacement agent and crude oil.
| Oil reservoir block | Oil displacement agent type | Interface tension (mN/m) |
|---|---|---|
| T123 | Fluorescent nano oil displacement agent | 0.098 |
| H34 | 0.076 | |
| TH12W | 0.042 | |
| TY762 | 0.0089 |
According to the table, the interfacial tension between the fluorescent nano displacement agent solution and the crude oil in the four blocks of X oilfield is less than 0.1 mN/m. This indicates that the fluorescent nano displacement agent has good surface activity and excellent oil-water distribution ability, which can significantly reduce the interfacial tension between oil and water, thereby effectively promoting oil desorption, emulsification, and flow. Make it easier to disperse large oil droplets into small ones, thereby reducing the resistance of crude oil passing through small channels and achieving the effect of improving oil recovery.
Evaluation of surface tension testing
Dissolve a certain amount of fluorescent nano displacement agent in water to prepare solution samples with concentrations of 50 mg/L, 100 mg/L, 150 mg/L, 200 mg/L, 300 mg/L, and 400 mg/L. The surface tension of the polymer at different concentrations was measured using the hanging ring method in a fully automatic surface tension meter. After recording the data, the relationship curve between surface tension and concentration was plotted, and the critical micelle concentration corresponds to the concentration at the curve’s inflection point. The surface tension and critical micelle concentration of fluorescent nano displacement agent solution were measured, and the surface tension and critical micelle concentration of commonly used displacement agent (HY10S) solution in the oil field were compared, as shown in Fig. 15.
Fig. 15.
Surface tension of HY10S oil displacement agent solution.
According to Figs. 15 and 16, it can be seen that the surface tension of the HY10S oil displacement agent is 27.2 mN/m when the critical micelle solution concentration is 350 mg/L. When the critical micelle concentration of the fluorescent nano displacement agent is 350 mg/L, the surface tension is 21.8 mN/m. From this, it can be seen that fluorescent nano displacement agents exhibit lower surface tension (21.8 mN/m) at the same critical micelle concentration, indicating that fluorescent nano displacement agents have strong interfacial activity and can effectively reduce oil-water interfacial tension, promote oil-water separation and droplet rupture. Make it have significant advantages in improving oil recovery efficiency and water drive effectiveness.
Fig. 16.
Surface tension of fluorescent nano displacement agent solution.
Wetting performance evaluation
Core slices soaked in crude oil from various blocks of X oilfield were used to measure wettability. The contact angle was measured once before treatment with fluorescent nano displacement agent, and once after treatment with fluorescent nano displacement agent. The data results are shown in Table 3.
Table 3.
Contact angles of core slices before and after treatment with fluorescent nano displacement agent.
| Oil reservoir block | Oil-displacing agent | Core contact angle (°) | |
|---|---|---|---|
| Before treatment | After treatment | ||
| T123 | Fluorescent nano oil displacement agent | 127.3 | 115.9 |
| H34 | 125.4 | 112.4 | |
| TH12W | 117.6 | 103.2 | |
| TY762 | 113.8 | 105.7 | |
Table 3 shows that the contact angles of the rock core slices soaked in crude oil from various blocks of X oilfield are all greater than 100° before being treated with fluorescent nano displacement agent solution. This indicates that when the rock core slices are soaked in crude oil, some hydrocarbon small molecules enter the pore structure of the rock surface, and the hydrocarbon molecules in the crude oil are adsorbed on the rock surface through van der Waals forces and other actions, causing a layer of oil film to adhere to the rock surface, thus making the rock surface oil wet.
After treatment with fluorescent nano oil displacement agent solution, the contact angle decreases to a certain extent, that is, the lipophilicity decreases. The reason is that the oil displacement agent can adsorb on the surface of the rock core or the oil film on the rock surface, so that the hydrophobic groups face the surface of the rock core, while the hydrophilic groups face the water solution of the oil displacement agent itself. To a certain extent, the wetting reversal occurred on the rock surface, and the hydrophilicity of the rock surface increased. Once the lipophilicity of the core thin section decreases, the crude oil adsorbed on the rock is more likely to detach from the rock surface, thereby increasing the oil recovery rate.
Performance evaluation of fluorescent nano oil displacement agent
Physical property analysis of natural sandstone core
In order to investigate the effect of fluorescent nano displacement agents on improving oil recovery in real sandstone environments, a total of 6 natural sandstone cores from different blocks of X oilfield were selected for this experiment. Figure 17 shows the physical images of the 6 cores in a dry state. The physical parameters of the 6 natural sandstone cores were measured using the SCMS-E high-temperature and high-pressure core multi parameter tester. The specific parameters are shown in Table 4. The selected natural cores have a permeability between 10 and 40 D and a porosity between 8% and 14%, belonging to low-permeability reservoirs.
Fig. 17.
Physical image of natural rock core used in the experiment (A1 ~ A6 from left to right).
Table 4.
Physical parameters of natural sandstone cores used in experiments.
| Number | Porosity (%) | Permeability (mD) | Core length (cm) | Pore volume (mL) |
|---|---|---|---|---|
| A1 | 12.30 | 17.32 | 6.39 | 3.64 |
| A2 | 8.10 | 20.63 | 6.04 | 3.33 |
| A3 | 12.10 | 36.27 | 6.15 | 3.76 |
| A4 | 10.50 | 22.35 | 5.97 | 3.35 |
| A5 | 13.10 | 14.91 | 6.27 | 3.48 |
| A6 | 9.50 | 10.32 | 6.33 | 3.81 |
Figure 18; Table 5 show the X-ray diffraction patterns and mineral species content statistics of the mineral composition of cores 1–6. It can be seen that the main mineral components of the six natural cores are quartz and albite, accounting for about 70–90%, followed by potassium feldspar and clay minerals. The sensitive clay minerals in the rock core include kaolinite, illite montmorillonite, chlorite, illite, and calcite. Among them, kaolinite accounts for a relatively high proportion, around 3% -13%, while the proportion of illite montmorillonite mixed layer is relatively low, around 0% -7%. The content of chlorite is around 0% -16%, second only to kaolinite. Based on the above analysis, the main clay mineral of the natural rock core selected in this experiment is kaolinite, followed by illite and illite montmorillonite mixed layer, which is mainly affected by velocity sensitivity and water sensitivity.
Fig. 18.
X-ray diffraction patterns of mineral composition in natural rock cores A1 ~ A6 (from left to right, they are rock cores A1 ~ A6).
Table 5.
Statistics of clay mineral types and contents in natural sandstone cores used in experiments.
| Number | Quartz (%) | Potassium feldspar (%) | Kaolinite (%) | Imon mixed layer (%) | Illite (%) | Chlorite (%) |
|---|---|---|---|---|---|---|
| A1 | 25.67 | 7.88 | 4.11 | 0.00 | 0.00 | 0.00 |
| A2 | 20.3 | 8.02 | 9.68 | 0.98 | 2.15 | 2.91 |
| A3 | 31.5 | 9.21 | 6.82 | 2.03 | 6.09 | 6.46 |
| A4 | 39.78 | 9.84 | 2.89 | 1.15 | 2.14 | 3.67 |
| A5 | 5.22 | 17.69 | 12.24 | 0.00 | 0.00 | 0.00 |
| A6 | 22.88 | 10.10 | 5.49 | 3.07 | 2.18 | 9.78 |
Migration characteristics of fluorescent nano displacement agents in sandstone
In injecting oil displacement agents, the injection methods can generally be divided into two types: slug injection and continuous injection. Under these two different injection methods, there will be significant differences in the migration and adsorption characteristics of fluorescent nano oil displacement agents. Therefore, the natural rock cores were divided into two groups for experimentation, with N1 ~ N3 being injected through plug injection and N4 ~ N6 being injected continuously.
(1) Slug injection.
Figures 19, 20 and 21 show the breakthrough curves and dynamic adsorption curves of the fluorescent nano-tracer injected into the middle section of cores N1, N2, and N3. The dimensionless concentration is defined as the ratio of the effluent concentration of the detected fluorescent nano-tracer to the initial injection concentration (Ci/C0). The dynamic adsorption amount represents the adsorption amount of fluorescent nano displacement agent in the rock core at the sampling time, and the injection plug amount of all experimental fluorescent nano displacement agents is 3 PV.
Fig. 19.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N1.
Fig. 20.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N2.
Fig. 21.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N3.
According to the analysis of experimental results, it can be concluded that in the initial stage (0–1 PV), the fluorescent nano displacement agent continues to enter the rock core and stagnates internally, with the dynamic adsorption capacity continuously increasing to its maximum. As the injection volume continues to increase, the concentration of tracer detected at the output end also increases continuously. When the concentration of the output liquid reaches its maximum value, it remains stable, and the breakthrough curve shows a shoulder characteristic at the top. The continuous progress of subsequent water flooding gradually reduces the output concentration and dynamic adsorption amount until they stabilize, indicating that there is still fluorescent nanodisplacement agent retained in the core. The final stable adsorption amount is the maximum irreversible adsorption amount of fluorescent nano displacement agent in the core Qmax1. Figure 22 shows the schematic diagram of the migration of fluorescent nano displacement agent in various stages of rock pores under slug injection, reflecting the migration process after tracer injection into the core.
Fig. 22.
Schematic diagram of injection of fluorescent nano displacement agent plug.
(2) Continuous injection.
Figures 23, 24 and 25 show the breakthrough curves and dynamic adsorption curves of continuous injection of fluorescent nano oil displacement agent in cores N4, N5 and N6. In the three experiments, the fluorescent nano oil displacement agent enters the core and accumulates continuously in the initial injection stage, and the dynamic adsorption amount increases sharply. When the fluorescent nanoparticle tracer for EOR fills the entire pore volume, the tracer accumulated in the core begins to break through and flow out of the core. When the relative production concentration reaches 100% after injection of about 4 PV, the adsorption amount no longer increases. The experimental results prove that the synthesized fluorescent nanoparticle tracer for EOR has good migration ability in sandstone reservoirs with permeability higher than 300 mD.
Fig. 23.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N4.
Fig. 24.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N5.
Fig. 25.
Distribution of production concentration and dynamic adsorption capacity of fluorescent nano displacement agent in natural core N6.
Influence of clay minerals on the migration of fluorescent nano displacement agents
Test the Zeta potential between fluorescent nano displacement agents and clay minerals to evaluate the interaction between displacement agents and different clay minerals. Figure 26 shows the Zeta potential changes of sensitive clay minerals and fluorescent nano displacement agents at various pH values. It can be seen that all clay minerals and fluorescent nano displacement agents have negative Zeta potentials at experimental pH values 7–8, indicating that they have the same surface characteristics. According to the double-layer repulsion theory, negatively charged fluorescent nano displacement agents generate electrostatic attraction with positive ions in the fluid, forming a diffusion layer around particles, rock pore walls, and clay minerals, overlapping with diffusion layers with similar charges, and generating double-layer repulsion between the two surfaces. Therefore, the inherent charge property of clay minerals is not conducive to the adsorption of displacement agents. However, due to the special nature of sensitive clay minerals, after injection into the fluid, the exchangeable cations in the clay minerals dissociate in water, forming a diffusion double layer, which makes the surface of the sheet-like structure negatively charged. Due to the electrostatic repulsion, the negatively charged sheet-like structure separates itself, causing clay expansion and narrowing the pore channels for oil displacement agent migration, increasing the retention rate.
Fig. 26.
Zeta potential changes of various sensitive clay minerals and fluorescent nano displacement agents at different pH levels.
Evaluation of enhanced oil recovery performance of fluorescent nano displacement agent
By injecting different volumes of oil displacement agents, the enhanced oil recovery performance of fluorescent nano oil displacement agents and commonly used oil displacement agents (HY10S) in oil fields was studied, as shown in Tables 6 and 7, respectively. Among them, HY10S displacement agent is a nanoparticle-free displacement agent. The results are shown in Fig. 27. Fluorescent nano oil displacement agent shows a more significant effect in improving oil recovery, with a displacement efficiency exceeding that of HY10S oil displacement agent, reaching a maximum of 14.09%. Due to the large specific surface area and high surface activity of fluorescent nano displacement agents, they can effectively reduce the interfacial tension between oil and water, making oil droplets more prone to rupture, emulsification, or dispersion. The small size of fluorescent nano oil displacement agents allows them to penetrate into the micropores or fractures of rock cores, enabling deeper contact with the oil-water interface than traditional oil displacement agents (such as HY10S oil displacement agent), thereby achieving improved oil recovery efficiency.
Table 6.
Oil displacement efficiency of fluorescent nano-oil displacement agent solution.
| Number | Permeability (mD) | Pore volume | Total recovery | Water recovery | Oil displacement agent recovery rate |
|---|---|---|---|---|---|
| A1 | 17.32 | 0.1 PV | 48.98 | 37.92 | 11.06 |
| A2 | 20.63 | 0.2 PV | 54.32 | 40.23 | 14.09 |
| A3 | 36.27 | 0.2 PV | 52.76 | 41.76 | 11 |
| A4 | 22.35 | 0.3 PV | 53.15 | 42.43 | 10.72 |
| A5 | 14.91 | 0.3 PV | 52.78 | 41.14 | 11.64 |
| A6 | 10.32 | 0.4 PV | 49.21 | 37.06 | 12.15 |
Table 7.
Oil displacement efficiency of HY10S oil displacement agent solution.
| Number | Permeability (mD) | Pore volume | Total recovery | Water recovery | Oil displacement agent recovery rate |
|---|---|---|---|---|---|
| A1 | 17.32 | 0.1 PV | 56.23 | 49.76 | 6.47 |
| A2 | 20.63 | 0.2 PV | 56.35 | 48.41 | 7.94 |
| A3 | 36.27 | 0.2 PV | 57.21 | 54.32 | 2.89 |
| A4 | 22.35 | 0.3 PV | 56.91 | 48.26 | 8.65 |
| A5 | 14.91 | 0.3 PV | 58.43 | 51.23 | 7.2 |
| A6 | 10.32 | 0.4 PV | 55.18 | 45.98 | 9.2 |
Fig. 27.
Comparison of recovery rates of different oil displacement agents.
Evaluation of traceability performance of fluorescent nano oil displacement agent
After converting the concentration distribution curve of the fluorescent nano displacement agent into a dimensionless concentration curve, it was observed that the concentration distribution shapes in experiments 1 and 3 exhibited a unimodal pattern, while experiment 2 showed a bimodal curve. And fit it to further analyze the concentration distribution of fluorescent nano displacement agent. The detailed results are shown in Figs. 28, 29 and 30 and summarized in Table 8.
Fig. 28.
Fitting results of fluorescent nano oil displacing agent concentration distribution in No. 1 displacement experiment (experimental core: B1, B2, B3).
Fig. 29.
Fitting results of fluorescent nano oil displacing agent concentration distribution in No. 2 displacement experiment (experimental cores: B4, B5, B6).
Fig. 30.
Fitting results of fluorescent nano oil displacing agent concentration distribution in No. 3 displacement experiment (experimental cores: B7, B8, B9).
Table 8.
Parallel multi-core fluorescent nano oil displacing agent flow experiment data fitting results.
| Number | Core number | Core permeability (mD) | Fitted permeability | Error |
|---|---|---|---|---|
| 1 | B1 | 726.31 | 689.62 | 9.41 |
| B2 | 617.34 | |||
| B3 | 547.28 | |||
| 2 | B4 | 1358.12 | 1257.03 | 48.37 |
| B5 | 822.31 | |||
| B6 | 361.32 | |||
| 3 | B7 | 823.11 | 887.18 | 11.63 |
| B8 | 789.23 | |||
| B9 | 771.91 |
The simulation results reveal that the coefficients of variation between the permeabilities of the three cores in Experiment No. 1 and Experiment No. 3 are small, measuring 0.11 and 0.085, respectively. This suggests relatively uniform flow distribution of the fluorescent nano oil displacing agent output concentration within the three cores, resulting in a unimodal concentration distribution. Further analysis of the fitting results indicates better agreement between the model results and experimental data. Specifically, the fitted permeability for Experiment No. 1 is 689.62 mD, and for Experiment No. 3, it is 887.18 mD. These interpreted permeabilities exceed the average permeability of the experimental cores, with errors of 9.41% and 11.63%, respectively.
In contrast, Experiment No.2 exhibits a higher permeability variation coefficient of 0.48, indicating a significant permeability range among the cores. The average permeability for this set of experiments is 847.25 mD, with the fluorescent nano oil displacing agent output concentration distribution revealing a “bimodal” shape, indicative of uneven the flow distribution from the cores. The permeability fitted by the experimental data is 1257.03 mD, exhibiting a considerable discrepancy from the true average permeability, with an error of 48.37%. This discrepancy is attributed to the strong heterogeneity of the experimental cores in Experiment No.2, with a majority of the fluorescent nano oil displacing agent entering the high-permeability core B4 post-injection. Notably, core B4 serves as the primary flow channel for the fluorescent nano oil displacing agent within the system, hence the permeability fitted by the experimental data closely aligns with the permeability of core B4, with an error of 7.44%.
In summary, based on the simulation results, it can be inferred that fluorescent nano oil displacing agent effectively explain experimental core permeability with an error margin of approximately 10%. They exhibit robust injection and migration capabilities, accurately delineating highly permeable channels within the reservoir.
Practical application potential and sustainability
The modified Stöber method used in this study offers mild conditions and operational simplicity, suggesting potential for scale-up through process optimization and continuous-flow techniques. While current work is at lab scale, industrial feasibility appears promising. Cost-wise, the use of non-toxic silica and low-dosage fluorescent tracers helps reduce operational expenses compared to traditional flooding agents. Environmentally, the system avoids hazardous solvents and was designed with ecological safety in mind. Further studies will evaluate long-term stability and biodegradability under reservoir conditions to support future large-scale applications.
Conclusion
This study investigated the development and application of a fluorescent nano-oil displacement agent, leading to the following conclusions:
A fluorescent nano-displacement agent with an average particle size of approximately 80 nm was successfully synthesized via an improved Stöber method, utilizing TEOS as the silicon source, ammonia water as the catalyst, and HDTMS and APTES for surface modification and FITC dye incorporation. Comprehensive characterization (including FTIR, SEM, particle size analysis, UV-Vis, fluorescence spectroscopy, and TGA/XRD) confirmed its successful synthesis, strong fluorescence intensity, and ability to effectively reduce interfacial tension to less than 0.1 mN/m. These findings collectively demonstrate its promising dual capabilities as both an enhanced oil displacement agent and a real-time tracer under laboratory conditions.
Core flow experiments were conducted to evaluate the migration characteristics of the synthesized fluorescent nano-tracer in natural sandstone. The experimental results indicated that the fluorescent nano-tracer exhibits good migration capability within sandstone reservoirs with permeability greater than 300 mD under the tested laboratory conditions. Further studies on its fluorescence stability under varied temperatures, salinities, and pH values have also confirmed its robustness, highlighting its potential for reliable tracing in complex reservoir environments.
Experiments on the oil displacement and tracer performance of the fluorescent nano-displacement agent demonstrated that, under equivalent concentration and injection volumes, it achieved a significant incremental oil recovery compared to the conventional nanoparticle-free HY10S oil displacement agent, reaching a maximum of 14.09%. In core flow experiments, particularly with heterogeneous systems (e.g., Experiment No. 2), the permeability fitted by tracer data, while showing a notable discrepancy from the average core permeability (up to 48.37% in specific cases), closely aligned with the permeability of the dominant high-permeability flow channel (e.g., core B4, with an error of 7.44%). This discrepancy, rather than merely being an error, highlights the fluorescent nano-tracer’s diagnostic capability to reveal preferential flow paths in heterogeneous media. Simulation results further corroborate that fluorescent nano-displacement agents can be effectively utilized for tracer testing, especially in diagnosing complex reservoir flow patterns.
While the present study focuses on the transport behavior and oil displacement performance of the fluorescent nano-tracer, further investigation into its structural and surface properties is planned. In future work, XRD and BET analyses will be carried out to provide supplementary information on the material’s crystalline structure and specific surface area, which may help refine the understanding of its behavior in porous media.
Acknowledgements
This paper is funded by CNPC Innovation Found (2022DQ02-0305) and General Project of Sichuan Science and Technology Education Joint Fund (2024 NSFSC1981).
Abbreviations
- APTES 3
aminopropyltriethoxysilane
- BET
Brunauer emmett teller
- EOR
Enhanced oil recovery
- FITC
Fluorescein isothiocyanate
- FT-IR
Fourier-transform infrared spectroscopy
- HDTMS
Hexadecyltrimethoxysilane
- SEM
Scanning electron microscope
- TEM
Transmission electron microscopy
- TEOS
Tetraethyl orthosilicate
- TGA
Thermogravimetric analysis
- UV-Vis
Ultraviolet-visible spectrophotometer
- XRD
X-ray diffraction
Author contributions
Qi Gong: Conceptualization, Data curation, Writing – original draft (experimental sections). Haitao Li: Supervision, Writing – review & editing. Xin Ma: Writing – original draft (main manuscript). Jiating Hao: Investigation (literature review). Fangjun Jia, Hongwen Luo and Sujuan Gao: Investigation (laboratory experiments). All authors reviewed the manuscript.
Data availability
The datasets used and analysed during the current study are available from the corresponding author on reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and analysed during the current study are available from the corresponding author on reasonable request.