A fluorescent dye tracer method for tracking hydraulic fracture cracks and applications thereof

Abstract

The application discloses a fluorescent dye tracer method for tracking hydraulic fracturing cracks and application, and the method comprises the following steps: S1, configuring a fluorescent dye solution: using a water-soluble fluorescent dye rhodamine B to prepare a mass-volume ratio aqueous solution with a concentration of 0.01% to 0.05%; S2, pre-treating and cleaning the cracks of a sample: using a hydrochloric acid solution with a concentration of 1% to 3% to acid-wash the cracks of the sample, and then rinsing the sample with clean water; S3, pre-saturating the crack channel: pre-saturating the crack network of the sample; S4, injecting the fluorescent dye solution under low pressure pulse, so that the crack channel is filled with the fluorescent dye solution; S5, soaking and permeating: after the crack channel is filled with the fluorescent dye solution, stopping the injection and placing the sample, and soaking the sample to make the fluorescent dye solution penetrate into fine cracks and pores; S6, observing the dyeing: after the surface of the sample is dried, cutting the sample along the main crack direction to observe the internal crack direction, and performing quantitative analysis by using image processing software.

Description

Technical Field

[0001] This invention relates to the fields of geotechnical engineering, coal / oil and gas resource development and hydraulic fracturing testing, and specifically to a fluorescent dye tracing method and its application for tracking hydraulic fracturing fractures. Background Technology

[0002] In existing technologies, the traditional dye tracing method involves dissolving the dye in water or a solvent and injecting it into the fracture. Color contrast is then used to visualize the fracture path on the exposed surface. This method is commonly used in laboratory core analysis and is suitable for preliminary fracture morphology assessment. It uses common dyes (such as methylene blue or red ink) as a contrast medium, enhancing the color of the rock sample's cut surface to identify fracture distribution and orientation. This method is simple, low-cost, and suitable for rapid qualitative observation. However, this method has the following drawbacks: First, limited identification of micro-fractures: In fine-grained rocks rich in clay minerals, water-soluble dyes are easily adsorbed, leading to a high adsorption rate and reduced tracing sensitivity and uniformity. Fracture boundaries are blurred, contrast is poor, and small branch fractures and terminal fractures are difficult to reliably identify. Second, injection disturbance leads to geometric changes: When high-pressure injection is used to overcome penetration resistance (e.g., pressures exceeding 1 MPa), it easily causes unnatural fracture expansion or "secondary fracture creation," especially noticeable in low-strength rocks (such as siltstone and shale). Fracture width may increase by 30%-50%, making it difficult to maintain the original morphology. Third, the quantitative analysis capability is insufficient: the stained layers mostly remain on open surfaces or relatively coarse channels, providing limited support for quantitative indicators such as crack connectivity, width distribution, and branching degree, resulting in significant errors during image processing. Therefore, although this method is simple to operate, it has low resolution and poor anti-interference ability, and is only suitable for rough qualitative analysis, failing to meet the requirements for high-precision quantitative characterization.

[0003] In the existing technology, physical monitoring methods are based on physical signals (such as X-rays, sound waves, and fiber optic responses) to monitor fractures, including: (1) X-ray CT scanning: Voxel reconstruction is performed through density / attenuation differences to obtain three-dimensional information of the fracture network. The common voxel resolution can reach about 100 μm. However, it has the following drawbacks: the equipment cost is high (a single unit is often in the millions of yuan), and the field application is poor; the ability to identify fine fractures is limited; it is usually difficult to observe the dynamics of the fracturing process in real time. (2) Acoustic emission (AE) monitoring: Based on the elastic wave radiation of the micro-fracture process, it can be used to locate and reflect the initiation and propagation activities. However, it has the following drawbacks: the positioning accuracy is generally in the millimeter to centimeter range; the monitoring results are significantly affected by lithological heterogeneity and noise conditions; it is difficult to directly quantify geometric parameters (width, conductivity, connectivity, etc.) and is generally only used for fracturing event distribution and time series interpretation. (3) Distributed fiber optic monitoring (DAS / DTS): The response of optical fiber to strain / temperature is used to realize quasi-real-time monitoring of strain changes induced by fracture propagation, which can cover long well sections. However, it has the following drawbacks: it requires the pre-laying of optical fibers, and construction is complicated in confined spaces such as underground coal mines; it cannot directly image the spatial morphology of cracks, and the results are greatly affected by large deformation of the rock mass and disturbance of boundary conditions.

[0004] In existing technologies, fluorescent resin and dye impregnation imaging methods involve introducing thermosetting resins (or fluorescent dyes) containing fluorescent substances into the fractures and pores of rock samples under vacuum and pressure. After the resin cures, observation under ultraviolet light yields high-contrast images of fractures and permeable zones on the slices. Another method involves injecting fluorescent resin directly as fracturing fluid, allowing it to cure after fracturing before cutting and observation, which can clearly distinguish areas reached and unreached by the dye. However, this method has the following drawbacks: after sample curing, only one-time destructive analysis is typically possible, making it difficult to perform subsequent repeated tests or rapid on-site assessments; the diffusion behavior of the resin dye before curing, along with changes in temperature and viscosity, affects the characterization of the true fracture width, making quantitative calibration difficult under non-constant operating conditions; furthermore, in confined downhole spaces or conventional fracturing sites, implementation is challenging, with high safety and environmental requirements, and a lack of standardized operating procedures.

[0005] In summary, aqueous fluorescent dye tracing and fluorescent resin impregnation imaging still have significant shortcomings in terms of microcrack identification, sensitivity to lithology and working conditions, disturbance to undisturbed cracks, and field adaptability. Summary of the Invention

[0006] The purpose of this invention is to provide a fluorescent dye tracing method and its application for tracking hydraulic fracturing fractures. This method provides a fluorescent tracing-visualization-quantification standard method that is applicable to media such as siltstone, has a certain degree of anti-adsorption capacity, and adapts to environmental sensitivity, without relying on high-cost equipment and minimizing injection disturbance. This method enables accurate and repeatable characterization of the spatial distribution and connectivity of hydraulic fracturing fractures (including micro-fractures), allows for effective comparison of laboratory data, field monitoring results, and numerical simulations, and achieves high-resolution, low-interference tracing and monitoring of micro-fracture networks.

[0007] To achieve the above objectives, the present invention provides a fluorescent dye tracing method for tracking hydraulic fracturing fractures, comprising the following steps:

[0008] S1. Preparation of fluorescent dye solution: Use water-soluble fluorescent dye Rhodamine B and deionized water or distilled water as solvent to prepare an aqueous solution with a concentration of 0.01% to 0.05 wt% by mass-volume ratio.

[0009] S2. Pre-treatment and cleaning of sample cracks: Use a 1% to 3% hydrochloric acid solution to pickle the sample cracks, dissolve and remove the carbonate cement and mud film on the crack surface to enhance the connectivity of the cracks. After pickling, rinse the sample with clean water to neutralize the residual acid and perform ion replacement.

[0010] S3, Pre-saturated crack channels: The crack network of the sample is pre-saturated.

[0011] S4. Low-pressure pulse injection of fluorescent dye solution to fill the crack channel with fluorescent dye solution;

[0012] S5. Immersion and Penetration: After the fluorescent dye solution fills the cracks, stop the injection and let the sample stand. Immerse the sample to allow the fluorescent dye solution to penetrate into the fine cracks and pores.

[0013] S6. Observe the staining: After the sample surface is dry, cut the sample along the direction of the main crack to observe the direction of the internal crack, and use image processing software for quantitative analysis.

[0014] In a preferred embodiment, in step S1, the fluorescent dye solution further includes an auxiliary additive, which is 0.1 wt% of a nonionic surfactant or 0.2 wt% of a thickener.

[0015] In a preferred embodiment, step S3 involves pre-saturating the fracture network of the sample by slowly injecting deionized water through the same injection port as the fracturing process until the fracture is filled with deionized water or deionized water is observed flowing out at the outlet end, in order to expel air from the fracture and wash away dust and debris in the fracture, so that the subsequently injected fluorescent dye solution can penetrate the entire fracture network.

[0016] In a preferred embodiment, step S4 involves injecting a fluorescent dye solution using a low-pressure pulse to fill the fracture channel with the fluorescent dye solution. This includes connecting a container containing the fluorescent dye solution to the injection port of the sample via a hose or syringe. The injection port of the sample is either the wellbore of the fracturing experiment or a pre-reserved inlet channel. The injection rate is controlled to maintain a pressure below 0.5 MPa. The fluorescent dye solution is slowly and uniformly injected into the fracture channel. The fluorescent dye solution is continuously injected until it is observed to seep out at the sample outlet. The sample outlet is either the exposed fracture surface or a pre-set outlet location. If the sample is a closed system without a clear outlet, the required solution volume is calculated based on the estimated fracture volume, and the corresponding volume of fluorescent dye solution is injected to ensure that the fracture is adequately wetted.

[0017] In a preferred embodiment, in step S4, the injection method is vacuum-assisted injection. The sample is placed in a vacuum chamber, and a vacuum is drawn to a degree not lower than -0.08 MPa. This vacuum is maintained for 30 to 90 minutes. While maintaining the vacuum state, the fluorescent dye solution is slowly introduced from the bottom of the sample or through a pre-drilled hole until the liquid level covers the top of the sample. After restoring normal pressure, a low-pressure pulse injection is performed on the sample using an injection pump or a pressure bottle. The injection pressure is 0.15 to 0.30 MPa, and the injection is performed 3 to 5 times. The injection time is 30 to 60 seconds per injection, with an interval of 1 to 2 minutes.

[0018] In a preferred embodiment, step S4 involves cyclic pressurized injection under vacuum conditions, including:

[0019] Connection equipment: Connect the pre-made injection port of the sample to a 50-100 mL syringe, and use a tubing and sealing connector to ensure a tight connection;

[0020] Cyclic injection: Inject the fluorescent dye solution, slowly advance the syringe plunger for 10-20 seconds to force the fluorescent dye solution into the crack, then maintain the pressure for 30-60 seconds, and then depressurize for 30 seconds to complete one injection cycle;

[0021] Repeatedly: Repeat the injection cycle 5 to 10 times;

[0022] Static soaking: The sample is kept in the dark for 6 to 24 hours. During this period, 1 to 2 injection cycles are performed every 1 to 2 hours to compensate for the attenuation of the driving force of the fluorescent dye solution caused by long-term static soaking.

[0023] In a preferred embodiment, in step S5, the soaking time is 12 to 24 hours for low-permeability rocks and 1 to 6 hours for high-permeability rocks. During the soaking process, dye molecules diffuse into the micropores of the rock matrix, thereby marking the microcrack network around the main crack.

[0024] In a preferred embodiment, in step S6, after the sample surface is dried, the sample is cut along the main crack direction to observe the internal crack orientation and perform quantitative analysis using image processing software. This includes: after the sample surface is dried, the sample is cut along the main crack direction, and the core surface is irradiated under natural light or ultraviolet light with a wavelength of 365nm. The location, length, and width distribution of the crack are recorded by visually observing the fluorescent traces of the crack. Subsequently, a high-resolution digital camera is used to take a fluorescent photograph of the crack under ultraviolet light, and the fluorescent photograph is imported into a computer for image processing and quantitative analysis of the crack geometry using image processing software.

[0025] In a preferred embodiment, image processing and quantitative analysis of crack geometry are performed using image processing software, including:

[0026] Image processing: Image processing software is used to binarize and extract contours from the fluorescence photograph, and the digital morphology of the crack is obtained after eliminating background noise;

[0027] Quantitative analysis: Record the length, width, number of branches, and intersection locations of each crack;

[0028] Calculate the average aperture: Measure the area of ​​the fluorescent region of the crack and divide it by the length of the crack skeleton centerline to obtain the average width of the crack;

[0029] Crack direction analysis: Fit the crack skeleton to a straight line or determine its major axis direction to generate a crack direction rose diagram;

[0030] Calculate the area ratio and penetration rate: the area ratio is the proportion of the area of ​​the fluorescent region of the crack to the total cross-sectional area, and the penetration rate is whether the crack penetrates the sample. If the crack reaches the boundary of the sample, it is considered to be penetrated.

[0031] The present invention also provides an application of the above-mentioned fluorescent dye tracing method for tracking hydraulic fracturing fractures in field hydraulic fracturing. Rhodamine B is added to the fracturing fluid at a concentration of 0.02% to 0.05 wt%, and injected into the formation in the later stage or throughout the fracturing process. Fracture tracking is performed by fluorescence analysis of the flowback fluid or by core sampling from adjacent wells.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] 1. High-resolution crack visualization: This invention utilizes a fluorescent staining solution to significantly enhance crack contrast under ultraviolet light excitation, enabling clear identification of even minute cracks. After fracturing siltstone samples, they are impregnated with the water-soluble fluorescent dye Rhodamine B. Under a fluorescence microscope, the spatial distribution of the main crack and other branch cracks can be clearly observed. Compared to the traditional methylene blue staining method, this invention significantly improves the identification of microcracks.

[0034] 2. Low-interference fracture tracking: This invention employs a low-pressure infiltration process for staining, avoiding the mechanical expansion interference caused by high-pressure injection. Experimental results show that after treating shale samples using the vacuum impregnation method, the width of the stained fractures remains essentially consistent with that of the unstained original sample. In contrast, the traditional high-pressure injection method (approximately 1 MPa) leads to an increase in fracture width, demonstrating that this invention better preserves the original morphology of the fractures.

[0035] 3. Breakthrough in Quantitative Analysis Capabilities: This invention combines the calculation and analysis of fracture fractal dimension and connectivity to achieve a quantitative assessment of fracture network complexity. For example, in a shale fracturing experiment, fluorescence imaging analysis showed a fracture connectivity of approximately 75% and a fractal dimension of 1.62. These indicators suggest the formation of a complex fracture network structure within the rock sample. Furthermore, the results showed a greater than 90% agreement with concurrent CT scans. This fully validates the high reliability of this invention in quantitatively characterizing fracture networks.

[0036] 4. Significant Cost-Effectiveness: This invention offers a significant cost advantage. The cost of consumables (including dyes and equipment wear and tear) for a single experiment is approximately 500 yuan, only 5% of the cost of a traditional CT scan experiment (which typically exceeds 10,000 yuan per experiment). In field applications, the investment cost of a fluorescence imaging system (ultraviolet excitation source and high-resolution camera) is less than 20,000 yuan, far lower than that of a fiber optic monitoring system (which typically exceeds 500,000 yuan). Therefore, this invention demonstrates extremely high cost-effectiveness in both laboratory and field applications.

[0037] 5. Wide applicability to various rock types: This invention has broad applicability to different rock types and has been validated in various lithologies such as siltstone, shale, sandstone, and carbonate rocks. This invention is suitable for tracing and visualizing fractures in small rock samples at the laboratory scale. Without fundamentally altering the method, it can also be extended to field-scale formation fracture monitoring. By adjusting the dye dosage and injection method, the above method can be used for tracing and colorimetric visualization of actual reservoir fracture networks. It can also be combined with fracture image recognition algorithms or numerical simulation processes such as COMSOL to achieve automatic identification and quantitative analysis of fracture geometry and conductivity characteristics. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0039] Figure 2 This is a schematic diagram of the simplified cylindrical rock core injection device of the present invention;

[0040] Figure 3 The fluorescence performance of different cut surfaces of stained siltstone samples under ultraviolet light according to the present invention;

[0041] Figure 4 The fluorescence characteristics of different locations on the cut surface of the same siltstone sample after static treatment under ultraviolet light according to the present invention.

[0042] Figure 5 This is a sample image of the same sample of the present invention, after being stained and observed directly with the naked eye from a half-cut surface.

[0043] Figure 6 The crack distribution under fluorescence is shown by white thin lines on the half-cut surface of the same sample of the present invention. Detailed Implementation

[0044] The technical solutions in the embodiments of the present invention will be clearly and completely described below. All other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0045] like Figures 1 to 6 As shown, the preferred embodiment of the fluorescent dye tracing method for tracking hydraulic fracturing fractures of the present invention includes the following steps:

[0046] Step S1: Prepare the fluorescent dye solution: Use water-soluble fluorescent dye Rhodamine B and deionized water or distilled water as solvent to prepare an aqueous solution with a concentration of 0.01% to 0.05% by mass volume, that is, add about 0.1 to 0.5 g of fluorescent dye Rhodamine B per liter of water.

[0047] Step S2: Pre-treatment and cleaning of sample cracks: Use a 1% to 3% hydrochloric acid solution to pickle the sample cracks, dissolve and remove the carbonate cement and mud film on the crack surface to enhance the connectivity of the cracks. After pickling, immediately rinse the sample with clean water to neutralize the residual acid and perform ion replacement.

[0048] Step S3, Pre-saturate crack channels: Pre-saturate the crack network of the sample.

[0049] Step S4: Inject fluorescent dye solution with a low-pressure pulse to fill the crack channel with fluorescent dye solution.

[0050] Step S5, Immersion and Penetration: After the fluorescent dye solution fills the cracks, stop the injection and let the sample stand. Immerse the sample to allow the fluorescent dye solution to penetrate into the fine cracks and pores.

[0051] Step S6, Observe the staining: After the sample surface is dry, cut the sample along the direction of the main crack to observe the direction of the internal crack, and perform quantitative analysis using image processing software.

[0052] Furthermore, in step S1, the fluorescent dye solution also includes auxiliary additives, which are 0.1 wt% nonionic surfactants or 0.2 wt% thickeners.

[0053] Furthermore, in step S3, the fracture network of the sample is pre-saturated, including: slowly injecting deionized water from the same injection port as the fracturing process until the fracture is filled with deionized water or deionized water is observed flowing out at the outlet end, so as to remove the air in the fracture and wash away the dust and debris in the fracture, so that the subsequently injected fluorescent dye solution can penetrate the entire fracture network.

[0054] Furthermore, in step S4, the fluorescent dye solution is injected with a low-pressure pulse to fill the fracture channel with the fluorescent dye solution. This includes: connecting a container containing the fluorescent dye solution to the injection port of the sample via a hose or syringe. The injection port of the sample is the wellbore of the fracturing experiment or a reserved inlet channel. The injection rate is controlled to maintain the pressure below 0.5 MPa. The fluorescent dye solution is slowly and uniformly injected into the fracture channel. The fluorescent dye solution is continuously injected until the fluorescent dye solution is observed to seep out at the sample outlet end. The sample outlet end is the exposed fracture surface or a preset outlet position. If the sample is a closed system without a clear outlet, the required solution volume is calculated based on the estimated fracture volume, and the corresponding volume of fluorescent dye solution is injected to ensure that the fracture is fully wetted.

[0055] Furthermore, in step S4, the injection method is vacuum-assisted injection or cyclic pressurized injection under vacuum conditions.

[0056] Furthermore, in step S5, the soaking time is 12 to 24 hours for low-permeability rocks and 1 to 6 hours for high-permeability rocks. During the soaking process, dye molecules diffuse into the micropores of the rock matrix, thereby marking the microcrack network around the main fracture.

[0057] Furthermore, in step S6, after the sample surface is dry, the sample is cut along the direction of the main crack to observe the direction of the internal cracks and perform quantitative analysis using image processing software. This includes: after the sample surface is dry, the sample is cut along the direction of the main crack, and the core surface is irradiated under natural light or ultraviolet light with a wavelength of 365nm. The location, length, and width distribution of the cracks are recorded by visually observing the fluorescent traces of the cracks. Subsequently, a high-resolution digital camera is used to take fluorescent photos of the cracks under ultraviolet light, and the fluorescent photos are imported into a computer for image processing and quantitative analysis of the crack geometry using image processing software.

[0058] Furthermore, image processing software is used for image processing and quantitative analysis of crack geometry, including:

[0059] Image processing: Image processing software is used to binarize and extract contours from the fluorescence photograph, and the digital morphology of the crack is obtained after eliminating background noise;

[0060] Quantitative analysis: Record the length, width, number of branches, and intersection locations of each crack;

[0061] Calculate the average aperture: Measure the area of ​​the fluorescent region of the crack and divide it by the length of the crack skeleton centerline to obtain the average width of the crack;

[0062] Crack direction analysis: Fit the crack skeleton to a straight line or determine its major axis direction to generate a crack direction rose diagram;

[0063] Calculate the area ratio and penetration rate: the area ratio is the proportion of the area of ​​the fluorescent region of the crack to the total cross-sectional area, and the penetration rate is whether the crack penetrates the sample. If the crack reaches the boundary of the sample, it is considered to be penetrated.

[0064] Example 1:

[0065] The complete indoor experimental procedure is described in detail below:

[0066] Scope of application: Silty sandstone cubes or cylindrical cores after hydraulic fracturing.

[0067] Tracing targets: Clearly visualizes the main fracture and permeable zone, and can quantify fracture length, aperture, and area ratio.

[0068] Key steps of the process: Vacuum degassing → Low-pressure pulse injection → Isotonic light rinsing → Darkroom ultraviolet imaging (reference calibration) → Quantitative analysis.

[0069] Reagents and equipment:

[0070] Fluorescent staining solution: water-soluble fluorescent dye Rhodamine B (store protected from light). Optional additives: 0.1 mol / L NaCl to adjust ionic strength, phosphate buffer to adjust pH (6.8–7.4).

[0071] Equipment: Vacuum tank or vacuum drying oven (ultimate vacuum up to -0.08 MPa), pressure-resistant injection bottle or syringe pump (pressure range 0 to 0.5 MPa), sealed tubing and connectors, handheld or tabletop UV lamp with wavelength 365 nm, dark box or darkroom.

[0072] Pretreatment: Surface cleaning / light acid washing. Spray or inject 1-2% HCl solution onto the surface of the siltstone sample for 1-3 minutes to remove mud and carbonate cement. Rinse thoroughly with clean water immediately. Allow to air dry until the surface is dry (do not dry completely to avoid secondary changes in cracks due to excessive drying).

[0073] Vacuum-Injection-Pulse: Place the core sample in a vacuum chamber, evacuate to -0.08 MPa and maintain this vacuum for 30–90 minutes (to promote the expulsion of gas from the fractures). While maintaining the vacuum, slowly introduce a fluorescent dye solution (containing 0.1 mol / L NaCl) from the bottom of the sample or a pre-drilled hole, ensuring the liquid level covers the top of the sample. After restoring atmospheric pressure, use a syringe pump or pressurized bottle to perform low-pressure pulse injection on the sample (injection pressure 0.15–0.30 MPa, 3–5 injections in total, injection time 30–60 s / injection, interval 1–2 min) to promote more thorough penetration of the fluorescent dye solution into the micro-fractures.

[0074] Settling: Place the rock sample filled with fluorescent dye solution in the dark at room temperature for 12–24 h (adjustment can be made according to sample size and permeability; the lower limit can be used for higher permeability, but at least 2 h should be allowed to ensure uniform penetration of the fluorescent dye solution). If the ambient temperature fluctuates significantly, the temperature change curve can be recorded for subsequent correction of the fluorescence imaging results.

[0075] Isotonic rinsing and surface treatment: Slowly spray or wipe the surface of the rock sample with 0.1 mol / L NaCl solution for 10-30 seconds to remove the free fluorescent dye solution on the surface (avoid excessive rinsing to prevent dye "back diffusion" or bringing the dye out of the cracks). After treatment, place the rock sample in a cool place to air dry for 10-30 minutes until the surface is dry.

[0076] Imaging and Observation: In dim lighting conditions, a portable 365 nm UV lamp was used to illuminate and observe the core surface. Results showed that the main fracture path on the core surface was clearly colored by the injected water-soluble fluorescent dye Rhodamine B. Under natural light, the fracture appeared pale red and was directly identifiable to the naked eye. Under UV light, the fracture emitted a bright orange-red fluorescence, contrasting sharply with the surrounding unstained rock, and the fracture direction and branches were clearly visible. Direct visual observation allowed for recording the location, length, and approximate width distribution of the fractures. Subsequently, a high-resolution digital camera was used to capture fluorescent photographs of the core fractures under UV light conditions, and the images were imported into a computer for processing. Image analysis software was used to binarize and extract contours from the fluorescent photographs, eliminating background noise to obtain the digital morphology of the fractures.

[0077] Measurement results show that the main fracture extends approximately 78.8 mm outward from the injection port without interruption, proving that the fluorescent dye solution has penetrated the entire fracture. Furthermore, image analysis yielded data on the fracture width distribution. Pixel calibration showed that the width of the fluorescent band corresponds to an actual fracture width of approximately 0.2 mm. This data also provides a basis for assessing the fracture's conductivity. This embodiment demonstrates that injecting Rhodamine B fluorescent dye solution into small-scale cores using a manual syringe can mark fine fractures with low disturbance. This method is simple and intuitive, yielding clear fracture fluorescence images that can be directly used for quantitative analysis of fracture geometry.

[0078] Quantitative analysis: Record the length and width of each crack, as well as the number of branches and the location of intersections.

[0079] Calculate the average aperture: Measure the area of ​​the fluorescent region of the crack and divide it by the length of the crack skeleton centerline to obtain the average width of the crack.

[0080] Crack direction analysis: Fit the crack skeleton to a straight line or determine its major axis direction to generate a crack direction rose diagram.

[0081] Area ratio and penetration rate: Area ratio refers to the proportion of the fluorescent area of ​​the crack to the total cross-sectional area; penetration rate refers to whether the crack penetrates the sample (the crack is considered to be penetrated when it reaches the boundary of the sample).

[0082] Fluorescence intensity correction: The gray value of the reference fluorescent strip is used to normalize and correct the fluorescence gray field of the sample to compensate for the influence of environmental factors such as temperature and turbidity on the fluorescence intensity.

[0083] Example 2:

[0084] Cyclic pressurized injection under vacuum conditions:

[0085] Scope of application: When a good crack staining and imaging effect is desired using a simple device without vacuum equipment.

[0086] Key points: Overcoming capillary resistance and gas resistance through multiple rounds of "push-stop" cyclic pressurized injection, and compensating for insufficient injection driving force by pushing the syringe for a long time.

[0087] Specific steps:

[0088] (1) Connecting equipment: Connect the pre-made injection port of the rock sample to a 50-100 mL syringe, and use a hose and sealing connector to ensure a tight connection;

[0089] (2) Cyclic injection: Inject the fluorescent dye solution (water-soluble fluorescent dye Rhodamine B), slowly advance the syringe plunger for 10-20 s to force the fluorescent dye solution into the crack, then maintain the pressure for 30-60 s, and then depressurize for 30 s to complete one injection cycle;

[0090] (3) Repeatedly: Repeat the injection cycle 5 to 10 times;

[0091] (4) Standing soaking: The sample is kept in the dark for 6 to 24 hours. During this period, 1 to 2 injection cycles are performed every 1 to 2 hours to compensate for the decrease in driving force of the fluorescent dye solution caused by long-term standing.

[0092] (5) Rinsing and Imaging: The sample was rinsed lightly under isotonic conditions according to the method of Example 1, and then ultraviolet fluorescence imaging was performed in a dark room and quantitative analysis of cracks was performed.

[0093] For high-permeability siltstone, multiple cycles of pressure injection are significantly more effective than a single rapid injection, allowing the fluorescent dye solution to penetrate the micro-cracks more fully.

[0094] Example 3:

[0095] Manual injection tracer experiment with small cylindrical cores:

[0096] Sample preparation: Select a homogeneous and dense siltstone cylindrical core. Drill a 5mm diameter through-hole in the axial center of the core as a simulated injection channel. Dry the core to constant weight, remove surface dust, and set aside for use.

[0097] Preparation of Fluorescent Dye Solution: A fluorescent dye solution was prepared using Rhodamine B powder. In this example, the concentration of Rhodamine B dye was 0.05 wt% (approximately 500 mg / L), and the fluorescent dye solution exhibited a distinct rose-red color. To improve the wetting and penetration effect of the solution on the rock surface, 0.1 wt% of a nonionic wetting agent (e.g., Tween-80) was added to the fluorescent dye solution to reduce the surface tension of the liquid. The solution was gently stirred to ensure complete dissolution and uniform dispersion of the Rhodamine B powder. After standing to confirm that there was no sediment or bubbles, the solution was poured into a 50 mL plastic syringe for later use.

[0098] Injection Procedure: Insert the syringe needle into the pre-drilled hole, ensuring it is in communication with the fracture. Slowly advance the plunger manually at a rate of approximately 5 mL / min to inject the fluorescent dye solution into the core fracture.

[0099] (1) When a small amount of fluorescent dye solution was initially injected, it was observed that the fluorescent dye solution seeped out of the crack at the other end of the core and dyed the rock surface red, which proved that the prefabricated crack had been connected.

[0100] (2) Continue to inject slowly until about 30 mL of fluorescent dye solution has entered the core. Maintain low pressure and steady progress throughout the injection process to avoid causing new fractures or erosion of the fracture walls due to excessively rapid injection;

[0101] (3) After injection, stop pressurizing and let stand for 5 minutes to allow the fluorescent dye solution to fully penetrate the inner surface of the crack. Then, gently wipe away any excess fluorescent dye solution that seeps out from the core surface with a damp cotton cloth to avoid residual dye affecting subsequent observation.

[0102] Imaging and Observation: In dim lighting conditions, a portable UV lamp with a wavelength of 365nm was used to illuminate and observe the core surface. The pre-existing main fracture path on the core surface was clearly marked by a fluorescent dye solution: under natural light, the fracture appeared pale red and was visible to the naked eye; under UV light, the fracture emitted a bright orange-red fluorescence, contrasting sharply with the surrounding undyed rock, clearly showing the fracture's direction and fine branches. The location, length, and approximate width distribution of the fracture were recorded visually. Subsequently, a high-resolution digital camera was used to capture fluorescent photographs of the fracture under UV light, and the images were imported into a computer for processing.

[0103] Image processing: Image analysis software was used to binarize and extract contours from the fluorescence photographs, and the digital morphology of the cracks was obtained after eliminating background noise.

[0104] This embodiment demonstrates that injecting fluorescent dye solution into small-scale core samples using a manual syringe can mark fine cracks with minimal disturbance. The method is simple and intuitive, yielding clear fluorescent images of the cracks, which can be used for quantitative analysis of crack geometry.

[0105] Example 4:

[0106] Hydraulic fracturing and tracing of large true triaxial sandstone specimens:

[0107] This embodiment describes the hydraulic fracturing of a large sandstone sample under true triaxial stress conditions, and the crack propagation morphology is traced by injecting a Rhodamine B fluorescent dye solution using a plunger pump. This experiment simulates the visualization process of large-scale cracks in a near-field stress environment, demonstrating the applicability of the method of this invention to large samples and complex stress conditions.

[0108] Samples and Equipment: A cubic siltstone sample with a side length of 150 mm (uniform and dense grains, high mechanical strength) was selected and placed in a true triaxial loading device to apply triaxial confining pressure to simulate underground stress (σ_v = 22.5 MPa, σ_H = 27 MPa, σ_h = 24 MPa). A perforation hole with a diameter of 10 mm and a depth half the sample thickness (75 mm) was drilled in the center of the sample to serve as a simulated wellbore. After installing a packer, fluid was injected into the sample through this hole to perform fracturing. A small laboratory plunger pump was used for injection, providing stable, constant-speed, high-pressure fluid injection.

[0109] Preparation of fracturing fluid and fluorescent dye solution: In this embodiment, Rhodamine B dye is directly mixed into the fracturing fluid to complete the tracer labeling simultaneously with fracture formation. The fracturing working fluid formulation is as follows: using water as the base fluid, 0.2 wt% guar gum thickener (to increase viscosity and more closely resemble the performance of the field fracturing fluid), 0.05 wt% Rhodamine B dye (approximately 500 mg / L), and a small amount of preservative and pH buffer (such as 0.01 wt% borate, used to stabilize the performance of the fluorescent dye solution). After thorough mixing of all components, the resulting tracer fracturing fluid has a viscosity of approximately 30 mPa·s at room temperature, is light red and transparent, and has no visible suspended impurities. Rhodamine B dye is chemically stable in this system and does not affect the thickening effect of guar gum.

[0110] Hydraulic fracturing and injection process: Hydraulic fracturing and injection process:

[0111] (1) The above-mentioned tracer fracturing fluid was injected into the reservoir of the plunger pump. After removing air bubbles from the pipeline, pumping began into the sample well at a constant flow rate of 60 mL / min, and the injection pressure was monitored over time. When the pressure rose to approximately 32.7 MPa, a typical pressure drop signal appeared, indicating that hydraulic fractures had initiated inside the sample. Pumping continued at the same flow rate to further extend the main fracture. The entire fracturing process lasted approximately 10 minutes, with a cumulative injection volume of approximately 2 L and a maximum pressure of approximately 35 MPa.

[0112] (2) During the fracturing process, the liquid containing Rhodamine B dye seeps into the newly formed fracture as the fracture expands. After the fracturing is completed, the pressure is slowly released and the injection is stopped. At the same time, the sample is kept under confining pressure for 10 minutes to allow the fluorescent dye solution to fully contact the fracture wall and remain there.

[0113] (3) Then remove the confining pressure and take out the sandstone sample. To prevent the dye from flowing out of the crack during transportation, immediately perform a rapid sealing treatment on the exposed surface of the sample (wrap it with plastic wrap or spray a thin layer of glue) to preserve the in-situ state of the fluorescent dye solution in the crack.

[0114] Crack visualization and measurement: The extracted sample was subjected to X-ray CT scanning (industrial CT, resolution 0.5 mm) to obtain a three-dimensional image of the cracks inside the sample filled with fluorescent dye solution. Subsequently, the sandstone sample was split along the main crack surface to expose the internal crack surface. Under ultraviolet light, orange-red fluorescent traces (Rhodamine B deposited on the crack wall) were visible on the crack wall, clearly depicting the crack morphology and its fine branch details. Visual observation revealed that the main crack extends roughly perpendicular to the direction of the minimum principal stress, with a width of about 1 mm near the wellbore, gradually decreasing to about 0.2 mm towards the distal end; secondary cracks branched out in the middle of the main crack, with lengths of about 40 mm and 30 mm respectively, some of which penetrated to the sample surface, while others did not.

[0115] Image Acquisition and Processing: High-definition cameras were used to photograph the surface of the exposed cracks segment by segment, acquiring multiple fluorescence images covering the entire crack. These images were imported into professional crack analysis software, and after image stitching, distortion correction, and threshold segmentation, binary images of the fluorescent regions of the cracks were extracted. Parameters such as the area, length, and width distribution of the cracks were calculated. Then, a three-dimensional model of the cracks was further constructed using CT scan data. The crack contours extracted from the photographs were superimposed onto the spatial coordinate system of the CT images to obtain a spatial distribution map of the cracks within the sample. This process, combining optical fluorescence imaging with CT imaging and supplemented by image processing analysis, allows qualitative results to corroborate quantitative data: the morphology and location of the cracks were not only observed intuitively in the experiment, but crack parameters were also accurately measured. This embodiment demonstrates that the method of the present invention can be implemented on large-sized rock samples under near-field conditions. By injecting tracer dye through a plunger pump, the formation and marking of cracks were completed simultaneously; combined with ultraviolet fluorescence observation and digital image processing, complex crack networks were effectively characterized, providing a reliable basis for evaluating fracturing effects.

[0116] Example 5:

[0117] Practical procedure for fluorescent tracing of fractures in on-site hydraulic fracturing:

[0118] This embodiment describes the application process of a fluorescent dye tracing method for tracking hydraulic fracturing fractures in a coal mine, based on actual field conditions. A comparison with a laboratory approach illustrates the applicability and precautions of this invention in engineering practice. The illustration uses a horizontal well fracturing operation in a sandstone coal mine as a case study (this method is also applicable to shale gas well stimulation, carbonate reservoir fracturing, and other scenarios).

[0119] On-site tracer design: A fracturing section in the target well is selected, and a fluorescent tracer solution is added during the fracturing operation of this section. Rhodamine B dye is used as the tracer solution; it is chemically stable, poses no radioactive hazard to the environment or personnel, and its damage to the reservoir is negligible at low concentrations. The dosage concentration is determined based on laboratory results and the scale of on-site fracturing: Considering the large total volume of fracturing fluid and the possibility of dye adsorption and dilution by the formation, this embodiment uses a 0.02wt% Rhodamine B aqueous solution (approximately 200 mg / L) added to the fracturing fluid. At this concentration, the fluorescent dye solution is pale and difficult to detect with the naked eye, but a low concentration of fluorescence signal can be detected by instruments subsequently. Simultaneously, as needed, no more than 0.1wt% of a nonionic wetting agent (to improve the distribution and penetration of the fluorescent dye solution in the reservoir), as well as appropriate amounts of preservatives and anti-sticking agents, can be added to the fracturing fluid to ensure that the dye remains stable for a long time under the high temperature and pressure environment of the formation and will not be degraded by microorganisms or react adversely with other fracturing additives.

[0120] Fracturing Operation and Tracer Fluid Injection: On-site fracturing operation is carried out according to conventional procedures, the difference being the injection of fracturing fluid mixed with Rhodamine B dye at a predetermined time for tracer purposes. Two implementation methods are available:

[0121] Method 1 (Post-fracturing clear fluid tracing): After the last sand-addition stage of fracturing is completed, switch to pumping in clear fluid mixed with dye (without proppant) and continue for a period of time, so that Rhodamine B enters the newly generated fracture along with the liquid in this stage and washes the fracture surface.

[0122] Method 2 (Full-process tracing): The target concentration of Rhodamine B in the fracturing fluid is maintained throughout the entire fracturing process, so that all fluids entering the formation have tracing capabilities.

[0123] This embodiment uses Method 1. Near the end of fracturing in each perforated section, approximately 5 m³ of Rhodamine B fluorescent tracer fluid (concentration approximately 200 mg / L) is metered and added to replace the original base fluid, pumped into the wellbore at a rate of approximately 2 m³ / min. During the operation, the peak pressure stabilizes at approximately 60 MPa as the fracture expands. After the tracer fluid enters the formation, it forms a dye-containing liquid film within the fracture, adhering to the fracture wall. After completing the fracturing of this section, the remaining sections are fracturing sequentially according to the standard procedure (if necessary, tracer fluid can be added to specific key sections). After the entire fracturing operation is completed, some dye-containing fracturing fluid remains in the wellbore that has not yet been flushed back. By appropriately extending the well closure time (e.g., approximately 2 hours), the contact time between the dye and the fracture wall can be increased, thereby enhancing the fluorescent tracer effect.

[0124] On-site testing and results acquisition: Immediately after fracturing, the wellhead was opened to vent the flowback fluid, which was then directed to the flow channel. Flowback fluid samples were collected at the wellhead at regular intervals. The concentration change of Rhodamine B in the flowback fluid was detected on-site using a portable fluorescence spectrophotometer. Results typically showed a significant fluorescence signal peak in the initial flowback fluid, indicating that the tracer dye was flowed out along with the fluid in the fracture. By analyzing the dye concentration decay curves of samples collected at different times, the volume and conductivity of the fracture network could be estimated (e.g., the fracture surface area could be roughly estimated based on the total dye recovery rate).

[0125] In addition, direct evidence can be obtained by combining physical sampling and observation. For example, a monitoring well can be placed near the fractured well and sealed before fracturing to prevent fracturing fluid from entering. After fracturing, core samples can be taken from the monitoring well to obtain core specimens near the fractured section. The cores are placed in a darkroom and scanned along their axis using a portable ultraviolet lamp. If an orange-red fluorescent band appears on the core at a certain depth, it indicates that the area has been stained by fracturing fluid containing Rhodamine B, suggesting that the main fracture extends to that location. For areas where fluorescent fractures are found on the core, samples can be sent to the laboratory for further microscopic observation and verification.

[0126] In open-hole fracturing operations, imaging logging techniques can be used to assist in assessing fracture tracing results. Conventional resistivity imaging or acoustic imaging methods struggle to directly detect chemical dyes. If conditions permit, using downhole cameras in conjunction with ultraviolet light sources to scan the open-hole wall may capture faint fluorescence emanating from fractures penetrating the wellbore. While this downhole fluorescence imaging method currently faces technical challenges in field applications, it holds significant exploratory value as a supplementary approach.

[0127] Results Analysis and Engineering Adaptability: Through the aforementioned monitoring methods, this embodiment successfully obtained tracer information of fracturing fractures in the field. Fluorescence analysis of the flowback fluid confirmed the flow path of the fracturing fluid in the formation and the volume retained in the fractures; fluorescence observation of adjacent well cores provided spatial location information for fracture orientation. The main difference between the laboratory approach and the field procedure lies in the observation methods and controllability: In the laboratory, the morphology of fluorescent fractures on rock samples can be observed directly with the naked eye or with the aid of instruments, and the complete fracture geometry can be obtained through methods such as cutting rock samples and CT scanning; however, in the field, since underground fractures cannot be directly seen, fracture distribution can only be indirectly inferred through tracer fluid recovery data and sampling analysis. Nevertheless, the core idea of ​​the method of this invention remains consistent in both scenarios: using fluorescent dyes to mark fractures and tracking and characterizing fractures through fluorescence signal detection.

[0128] Field application results show that Rhodamine B fluorescent tracer fluid has good compatibility with conventional fracturing fluids and does not disrupt normal construction processes. This tracer fluid has low addition costs but provides valuable fracture distribution information after fracturing, helping to evaluate the fracturing effect and guide subsequent production enhancement measures. Especially in horizontal well fracturing with multiple sections, this method can help determine the relative production contribution of each fracture section (e.g., by using different types of fluorescent dyes in different sections, or by sequentially monitoring the flowback fluid characteristics of each section), and determine whether the fractures formed by fracturing are connected to adjacent wells, among other key engineering issues. In summary, the fluorescent dye tracer method of this invention has the advantages of being intuitive and having high resolution in the laboratory. After formulation optimization and monitoring scheme improvement, it is also applicable in the field, showing good engineering application prospects and serving as a useful supplement to conventional fracturing monitoring methods.

[0129] Example 6:

[0130] The effects of fluorescent staining agent solution ratio and excipient selection on staining effect:

[0131] This embodiment compares the tracing and imaging effects of different Rhodamine B staining tracer solution formulations under similar fracture conditions, summarizes the influence of dye concentration and auxiliary additives on staining quality, and optimizes the dye solution composition to meet different lithologies and experimental requirements.

[0132] Experimental Design: Three sandstone cores of similar lithology and size (25 mm in diameter, 50 mm in height) were prepared. Each core was pre-fabricated with a through fracture using the same method (a straight fracture approximately 0.2 mm wide was created using a splitting method). The three cores with similar fracture morphologies were divided into three groups, each injected with a different formulation of Rhodamine B staining tracer solution. Except for the fluorescent staining solution formulation, all other operating conditions (injection method, environmental conditions, soaking time, etc.) were kept consistent to individually evaluate the effect of differences in fluorescent staining solution ratios on the tracer effect.

[0133] Fluorescent staining agent solution formulation settings:

[0134] Formula A (standard solution): Rhodamine B dye concentration 0.01 wt% (approximately 100 mg / L), without other additives. At this concentration, the fluorescent dye solution is pale pink and has a viscosity close to that of water.

[0135] Formula B (Wetting Enhancement): Add 0.1 wt% nonionic surfactant (such as nonylphenol polyoxyethylene ether NP-10) to Formula A. After adding the wetting agent, the surface tension of the fluorescent dye solution decreases by about 30%, which is beneficial for the liquid to wet the rock surface and penetrate into the fine cracks.

[0136] Formula C (thickening and stabilizing): Add 0.2 wt% thickening agent (such as hydroxypropyl methylcellulose HPMC) to Formula A. This addition increases the viscosity of the fluorescent dye solution to approximately 5 mPa·s, which helps the liquid stay on the crack wall for a longer time and is less likely to flow away too quickly; at the same time, HPMC has a certain stabilizing effect, which can reduce dye sedimentation.

[0137] Injection and Imaging: Using the same manual injection method as in Example 1, the three formulations of fluorescent dye solutions were injected into the pre-fabricated fractures of the corresponding core samples. The injection volume for each sample was approximately 20 mL, and the injection process was kept uniform and slow (approximately 5 mL / min) to ensure that each fracture was filled with the fluorescent dye solution and immersed for approximately 10 minutes. Subsequently, each core sample was split along the fracture to expose the complete fracture surface, and the differences in the staining and imaging effects of the three formulations of fluorescent dye solutions were observed under the same ultraviolet light irradiation conditions.

[0138] Observation results:

[0139] Formula A: The fluorescent dye solution can penetrate the main fracture, but the fluorescence is not uniform. Strong orange-red fluorescence is observed at the fracture initiation (near the injection port), while the fluorescence weakens at the fracture distally; some small secondary fractures are only locally stained. The presumed reason is that the dye concentration is low, some dye is adsorbed by the rock during seepage, and the lack of added additives results in insufficient wetting of extremely fine fractures.

[0140] Formula B: Due to the addition of a wetting agent, the staining of the cracks is more comprehensive and continuous. The main crack exhibits bright fluorescence from start to finish without significant intensity decay; simultaneously, multiple fine branch cracks that are difficult to detect with the naked eye also show continuous fluorescent lines under ultraviolet light. This indicates that the wetting agent enhances the ability of the fluorescent dye solution to penetrate into small pores and cracks, allowing the dye to penetrate deeper into the crack tips.

[0141] Formula C: The fluorescence on the crack surface is brighter and more persistent. Because the thickener increases the viscosity of the fluorescent dye solution, the liquid flow rate in the crack decreases, leading to increased dye retention and deposition on the crack surface. Therefore, the entire crack segment exhibits higher fluorescence brightness than Formulas A and B. Especially in the vertically downward crack segment, the higher viscosity effectively hinders the gravity-induced leakage of the fluorescent dye solution, and the dye adheres to the crack top as well. However, it was observed that Formula C's penetration into extremely narrow branch cracks is slightly inferior to Formula B: some fine cracks with a width less than 0.1 mm were not completely filled by the fluorescent dye solution under Formula C conditions, possibly because the high viscosity makes it difficult to enter these micro-channels.

[0142] Quantitative image analysis: Images were captured of the staining results of the three formulations, and image analysis software was used to calculate indicators such as fluorescence coverage and brightness uniformity of the cracks. The results showed that: formulation A had a fluorescence coverage of approximately 80% for the cracks, with the brightness of the crack tip region being approximately 50% of that of the proximal region; formulation B increased the coverage to over 95%, with the fluorescence brightness at the tip reaching over 80% of that of the proximal region, and significantly improved staining continuity; formulation C slightly reduced the coverage of the finest crack to approximately 90%, but had the highest overall fluorescence intensity, and the brightness difference between different segments of the crack was less than 20%.

[0143] A comparison of formulations A and B reveals that the addition of a wetting agent significantly reduces the adsorption loss of Rhodamine B on the rock surface, thereby enhancing the fluorescence intensity at the distal end of the fracture. This effect is even more pronounced in rocks with abundant clay minerals and poor wettability (such as argillaceous sandstone and organic-rich carbonate rocks). The viscosity-increasing measure in formulation C is suitable for staining larger fractures or vertical fractures, preventing excessively rapid leakage of the fluorescent dye solution.

[0144] Therefore, the ratio of different fluorescent dye solutions and the selection of auxiliary materials directly affect the imaging effect of fracture tracing. Optimal combinations should be made based on the wettability of the rock and the characteristics of the fracture: for low-permeability, oleophilic rock formations, wetting agents should be added to improve the penetration of the fluorescent dye solution; in large-pore or vertical fracture systems, thickeners can be added to increase the residence time of the fluorescent dye solution within the fracture. The determination of dye concentration needs to balance fluorescence intensity and adsorption balance; excessively high concentrations do not necessarily result in linear enhancement. The comparative experiment in this embodiment verifies the influence of auxiliary component adjustments on the performance of the tracer dye solution, providing a reference for preparing the optimal dye solution in field applications.

[0145] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for tracking hydraulic fracturing fractures using fluorescent dyes, characterized in that: Includes the following steps: S1. Preparation of fluorescent dye solution: Using water-soluble fluorescent dye Rhodamine B, and using deionized water or distilled water as solvent, prepare an aqueous solution of Rhodamine B with a concentration of 0.01% to 0.05 wt%. S2. Pre-treatment and cleaning of sample cracks: Use a 1% to 3% hydrochloric acid solution to pickle the sample cracks, dissolve and remove the carbonate cement and mud film on the crack surface to enhance the connectivity of the cracks. After pickling, rinse the sample with clean water to neutralize the residual acid and perform ion replacement. S3, Pre-saturated crack channels: The crack network of the sample is pre-saturated. S4. Low-pressure pulse injection of fluorescent dye solution to fill the crack channel with fluorescent dye solution; S5. Immersion and Penetration: After the fluorescent dye solution fills the cracks, stop the injection and let the sample stand. Immerse the sample to allow the fluorescent dye solution to penetrate into the fine cracks and pores. During the immersion process, the dye molecules diffuse into the micropores of the rock matrix, thereby marking the microcrack network around the main crack. S6. Observation and staining: After the sample surface is dry, cut the sample along the direction of the main crack, use an ultraviolet lamp to irradiate along the core surface to observe the direction of the internal crack, and use image processing software for quantitative analysis. In step S3, the fracture network of the sample is pre-saturated, including: slowly injecting deionized water from the same injection port as the fracturing process until the fracture is filled with deionized water or deionized water is observed flowing out at the outlet end, so as to remove the air in the fracture and wash away the dust and debris in the fracture, so that the subsequently injected fluorescent dye solution can penetrate the entire fracture network. In step S4, a low-pressure pulse injection of fluorescent dye solution is performed to fill the fracture channel with the fluorescent dye solution. This includes: connecting a container containing the fluorescent dye solution to the injection port of the sample via a hose or syringe. The injection port of the sample is the wellbore of the fracturing experiment or a pre-reserved inlet channel. The injection rate is controlled to maintain the pressure below 0.5 MPa. The fluorescent dye solution is slowly and evenly injected into the fracture channel. The fluorescent dye solution is continuously injected until it is observed to seep out at the sample outlet. The sample outlet is the exposed fracture surface or a preset outlet position. If the sample is a closed system without a clear outlet, the required solution volume is calculated based on the estimated fracture volume, and the corresponding volume of fluorescent dye solution is injected to ensure that the fracture is fully wetted.

2. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 1, characterized in that: In step S1, the fluorescent dye solution further includes auxiliary additives, which are 0.1 wt% nonionic surfactants or 0.2 wt% thickeners.

3. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 1, characterized in that: In step S4, the injection method is vacuum-assisted injection. The sample is placed in a vacuum chamber, and a vacuum is drawn to a degree not lower than -0.08 MPa. This vacuum is maintained for 30 to 90 minutes. While maintaining the vacuum state, the fluorescent dye solution is slowly introduced from the bottom of the sample or through a pre-drilled hole until the liquid level covers the top of the sample. After restoring normal pressure, a low-pressure pulse injection is performed on the sample using an injection pump or a pressure bottle. The injection pressure is 0.15 to 0.30 MPa, and the injection is performed 3 to 5 times. The injection time is 30 to 60 seconds per injection, with an interval of 1 to 2 minutes.

4. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 1, characterized in that: In step S4, the injection method is cyclic pressurized injection under vacuum conditions, including: Connection equipment: Connect the pre-made injection port of the sample to a 50-100 mL syringe, and use a tubing and sealing connector to ensure a tight connection; Cyclic injection: Inject the fluorescent dye solution, slowly advance the syringe plunger for 10-20 seconds to force the fluorescent dye solution into the crack, then maintain the pressure for 30-60 seconds, and then depressurize for 30 seconds to complete one injection cycle; Repeatedly: Repeat the injection cycle 5 to 10 times; Static soaking: The sample is kept in the dark for 6 to 24 hours. During this period, 1 to 2 injection cycles are performed every 1 to 2 hours to compensate for the attenuation of the driving force of the fluorescent dye solution caused by long-term static soaking.

5. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 1, characterized in that: In step S5, the soaking time is 12 to 24 hours for low-permeability rocks and 1 to 6 hours for high-permeability rocks.

6. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 1, characterized in that: In step S6, after the sample surface is dry, the sample is cut along the direction of the main crack to observe the direction of the internal crack and perform quantitative analysis using image processing software. This includes: after the sample surface is dry, the sample is cut along the direction of the main crack, and the core surface is irradiated with ultraviolet light at a wavelength of 365nm. The location, length, and width distribution of the crack are recorded by visually observing the fluorescent traces of the crack. Then, a high-resolution digital camera is used to take fluorescent photos of the crack under ultraviolet light, and the fluorescent photos are imported into a computer for image processing and quantitative analysis of the crack geometry using image processing software.

7. The fluorescent dye tracing method for tracking hydraulic fracturing fractures according to claim 6, characterized in that: Image processing and quantitative analysis of crack geometry were performed using image processing software, including: Image processing: Image processing software is used to binarize and extract contours from the fluorescence photograph, and the digital morphology of the crack is obtained after eliminating background noise; Quantitative analysis: Record the length, width, number of branches, and intersection locations of each crack; Calculate the average aperture: Measure the area of ​​the fluorescent region of the crack and divide it by the length of the crack skeleton centerline to obtain the average width of the crack; Crack direction analysis: Fit the crack skeleton to a straight line or determine its major axis direction to generate a crack direction rose diagram; Calculate the area ratio and penetration rate: the area ratio is the proportion of the area of ​​the fluorescent region of the crack to the total cross-sectional area, and the penetration rate is whether the crack penetrates the sample. If the crack reaches the boundary of the sample, it is considered to be penetrated.

8. The application of the fluorescent dye tracing method for tracking hydraulic fracturing fractures according to any one of claims 1 to 7 in field hydraulic fracturing, characterized in that: Rhodamine B dye was added to the fracturing fluid at a concentration of 0.02% to 0.05 wt% and injected into the formation during the later stages or throughout the fracturing process. Fracture tracking was performed by fluorescence analysis of the flowback fluid or by core sampling from adjacent wells.

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