Tracer agent for monitoring multi-layer fracturing flowback, preparation method and application

Abstract

The invention belongs to the technical field of oil and gas field development reservoir transformation, and discloses a tracer agent for multilayer fracturing flowback monitoring, a preparation method and application, according to the preparation method, the effectiveness of the tracer agent is evaluated through an experimental means, and it is guaranteed that the effective tracer agent can be used for monitoring a flowback curve of fracturing fluid flowback volume along with time; according to the application method, a tracer agent is used for marking fracturing fluid, and the initial fracturing fluid dosage and the initial tracer agent dosage are recorded; after flowback is started, accumulated flowback volume is recorded at a fixed time interval, the real-time concentration of the tracer agent is measured, and the flowback volume of each section is calculated; according to the return volume of each section and the accumulated return volume of each section, the return curve and the accumulated return rate of each section are calculated, the liquid production contribution rate and the production sequence of each layer section can be accurately obtained, and the liquid production capacity can be calculated in a semi-quantitative mode. Through flow-back liquid sample detection and data analysis, the liquid drainage sequence, the flow-back contribution rate and the like of each layer section can be determined, and data support is provided for subsequent construction.

Description

Technical Field

[0001] This invention belongs to the field of reservoir stimulation technology in oil and gas field development, specifically relating to a tracer for monitoring flowback from multi-layer fracturing, its preparation method, and its application. Background Technology

[0002] To improve the recovery rate, utilization level, reservoir permeability, and drainage area of ​​low-pressure, low-permeability oil and gas reservoirs, vertical well stratified fracturing and horizontal well staged fracturing technologies have been widely adopted. The fracturing flowback rate plays a crucial role in evaluating fracturing effectiveness and is an important reference for subsequent engineering operations. In multi-stage fracturing, the measured flowback volume is the sum of the flowback volumes of each individual stage; the flowback volume of each individual fracturing fluid stage cannot be determined, making it difficult to evaluate the fracturing effect. Currently, a staged fracturing effect monitoring method, "A Method for Monitoring the Effect of Staged Fracturing in Oil Wells (CN201410222097.0)," utilizes an oil-soluble solid material monitoring agent injected into the formation during fracturing. After flowback, crude oil flows through the fractures, gradually dissolving the oil-soluble material in the monitoring agent. The monitoring material in the agent gradually disperses into the oil and water phases of the crude oil. The oil production of each stage is calculated based on the distribution of the monitoring agent in the oil and water. This method requires a large investment and indirectly monitors the fracturing effect by observing the oil production situation. It cannot directly reflect the fracturing fluid flowback situation, and it is especially ineffective for sections where no oil is produced.

[0003] Additionally, the search revealed "A method for evaluating the water production of multiple stages in a multi-stage fracturing well of tight oil using tracers, CN202010478361.2," which involves adding tracers to the pre-flush fluid to monitor the water production of each stage, but cannot quantitatively describe the flowback of fracturing fluid in each stage. The search also found "A tracer method for multi-stage fracturing natural gas horizontal wells based on artificial intelligence, CN201910853493.6," which monitors the natural gas production of each layer by adding gas-sensitive and water-sensitive tracers, but cannot directly reflect the flowback of fracturing fluid, especially ineffective for stages where no gas is produced.

[0004] With the in-depth development of oil and gas fields, large-scale and multi-layer construction, including multi-stage fracturing of horizontal wells, often encounters situations where it is impossible to determine the flowback volume of fracturing fluid for each stage of fracturing or to know the flowback layer. Summary of the Invention

[0005] The purpose of this invention is to provide a tracer, preparation method and application for monitoring flowback in multi-stage fracturing, so as to solve the technical problem in the prior art that it is impossible to effectively judge the flowback volume and flowback curve of each stage of segmented fracturing.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A method for preparing a tracer for monitoring flowback in multi-layer fracturing includes: Solution A is obtained by dissolving chlorides of transition elements and lanthanides in water; Add a complexing agent to solution A to complex the element ions, adjust the pH to 7-9, and after the complexation is complete, make up the volume to prepare a trace element-EDTA complex tracer stock solution for later use. The tracer stock solution was added to the fracturing fluid, and the compatibility of the tracer with the fracturing fluid, the colloidal stability of the tracer, and the formation adsorption stability of the tracer were evaluated by comparing it with the fracturing fluid without the tracer. An effective tracer is obtained when the compatibility of the tracer with fracturing fluid, the tracer's gel breaking stability, and the tracer's formation adsorption stability all meet the specified requirements.

[0007] Preferably, the evaluation of the compatibility between the tracer and the fracturing fluid specifically includes: Using the fracturing fluid base fluid formula available at the work site, tracer stock solution was added to the clean water used to prepare the fracturing fluid to the specified concentration. After the fracturing fluid containing tracer is prepared, its viscosity before and after gel breaking, viscosity after gel breaking, pH value, gel forming time, and temperature shear resistance parameters are tested and compared with the fracturing fluid without tracer. If the parameters still meet the specified requirements, the compatibility evaluation between tracer and fracturing fluid is passed.

[0008] Preferably, the evaluation of the tracer's gel breaking stability specifically includes: Take fracturing fluid with a specified concentration of tracer stock solution added on site, add breaker until completely broken, and measure the concentration 'a' of tracer in fracturing fluid after digestion; Take fracturing fluid with the same concentration of tracer mother liquor, digest it directly without breaking the gel, and then measure the concentration b of the tracer in the fracturing fluid; If the tracer loss rate at concentration a is less than the preset value compared to concentration b, it is considered to have passed the tracer breakage stability evaluation.

[0009] Preferably, the preset value for the tracer loss rate is 10%.

[0010] Preferably, the fracturing fluid containing a tracer mother liquor of a specified concentration is added to a breaker and the mixture is broken down completely at 90°C.

[0011] Preferably, the evaluation of the formation adsorption stability of the tracer specifically includes: After the fracturing fluid containing a mother liquor of tracer with a specified concentration was broken up, fracturing sand and rock cuttings from the target formation were added. After adsorption, the concentration c of the tracer in the supernatant was measured. Take fracturing fluid containing mother liquor of the same tracer concentration, digest it directly without treatment, and then measure the concentration d of the tracer in the fracturing fluid; If the tracer loss rate at concentration c is less than the preset value compared to concentration d, it is considered to have passed the tracer formation adsorption stability evaluation.

[0012] Preferably, after the fracturing fluid containing a specified concentration of tracer mother liquor is broken up, 10g of fracturing sand and 10g of target formation rock cuttings are added. After adsorption for 24 hours, the concentration c of the tracer in the supernatant is measured; the preset value for the tracer loss rate is 10%.

[0013] A tracer for monitoring flowback in multi-layer fracturing is prepared using any one of the preparation methods described above.

[0014] This application also discloses the application of the above-mentioned tracer in multilayer fracturing flowback monitoring, including: The fracturing fluid was labeled with a tracer and the initial fracturing fluid volume and initial tracer volume were recorded. After the backflow begins, the cumulative backflow amount is recorded at fixed time intervals, the real-time concentration of the tracer is measured, and the backflow amount for each segment is calculated. The return curve and cumulative return rate for each segment are calculated based on the return volume of each segment and the cumulative return volume of each segment.

[0015] Preferably, the calculation of the backflow amount for each segment is first performed by calculating the total mass of tracer returned from each segment using the following formula:

[0016] The return flow rate for each section is then calculated using the following formula:

[0017] in, This represents the total mass of tracer discharged in the nth segment; To accumulate the backflow volume; This represents the real-time concentration of the tracer in the reflux solution; This represents the actual volume of liquid discharged in the nth segment; This represents the total mass of tracer used in the nth segment; This represents the total volume of fracturing fluid injected in the nth segment.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention uses chlorides of transition elements and lanthanides as the main components of the tracer. These elements possess unique chemical and physical properties, making them easy to detect and track in underground environments, thereby improving monitoring accuracy. The addition of a complexing agent and adjustment of the pH value allow the element ions to form stable complexes with the complexing agent, improving the stability and traceability of the tracer in fracturing fluid. By evaluating the compatibility, gel breaking stability, and formation adsorption stability of the tracer with fracturing fluid, it can be ensured that the tracer will not negatively affect the performance of the fracturing fluid, thus guaranteeing the optimization of fracturing effect.

[0019] This invention utilizes tracer-labeled fracturing fluids and detects, analyzes, and calculates the produced fluid to accurately determine the production contribution rate and sequence of each segment, and can semi-quantitatively calculate the production volume. Through flowback fluid sample testing and data analysis, the flowback sequence and contribution rate of each segment can be determined, providing data support for subsequent operations. This effectively solves the problem of unclear and untraceable flowback data after fracturing in vertical wells and horizontal wells. This application can accurately monitor fracturing fluid flowback during multi-layer fracturing, ultimately generating a curve of fracturing fluid flowback volume versus flowback time. Engineering researchers can better evaluate the fracturing effect and more easily predict the production capacity of different layers using this curve. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of the preparation method of the present invention; Figure 2 This is a flowchart of the application method of the present invention; Figure 3 This is a schematic diagram illustrating the addition of the tracer in this invention; Figure 4 This is a schematic diagram of the tracer backflow of the present invention; Figure 5 This is a schematic diagram of the first segment of the airflow curve and airflow rate in an embodiment of the present invention; Figure 6 This is a schematic diagram of the second stage of the airflow curve and airflow rate in an embodiment of the present invention; Figure 7 This is a schematic diagram of the third segment of the return curve and return rate in an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0023] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0024] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0025] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0026] Furthermore, the use of the term "horizontal" does not imply that the component must be absolutely horizontal, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0027] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0028] The present invention will now be described in further detail with reference to the accompanying drawings: See Figure 1 This application discloses a method for preparing a tracer for monitoring flowback in multi-layer fracturing, comprising: S1: Solution A is obtained by dissolving chlorides of transition elements and lanthanides in water; these elements have unique chemical and physical properties that make them easy to detect and track in underground environments, thereby improving the accuracy of monitoring.

[0029] S2: Add a complexing agent to solution A to complex the element ions, and adjust the pH value to 7-9. After the complexation is complete, make up the volume to prepare a trace element-EDTA complex tracer stock solution for later use. Adding a complexing agent and adjusting the pH value allows the element ions to form a stable complex with the complexing agent, which improves the stability and traceability of the tracer in fracturing fluid. S3: Add the tracer stock solution to the fracturing fluid and compare it with the fracturing fluid without tracer to evaluate the compatibility of the tracer with the fracturing fluid, the colloidal stability of the tracer, and the formation adsorption stability of the tracer. S4: The tracer is obtained after the compatibility of the tracer with the fracturing fluid, the gel breaking stability of the tracer, and the formation adsorption stability of the tracer all meet the specified requirements. By evaluating the compatibility, gel breaking stability, and formation adsorption stability of the tracer with the fracturing fluid, it can be ensured that the tracer will not have a negative impact on the performance of the fracturing fluid, thereby ensuring the optimization of fracturing effect.

[0030] In some embodiments, a method for preparing a tracer for monitoring flowback in multilayer fracturing includes: S1: Use the corresponding mass of chlorides (or oxides) of transition elements (such as Co, Zn, Cd) and lanthanides (such as La, Ce) dissolved in hydrochloric acid in water.

[0031] S2: Add disodium ethylenediaminetetraacetate or other complexing agents by molar ratio to complex the element ions. Adjust the pH to 7-9 with NaOH. Check the completeness of complexation with Chrome Black T. Make up the volume of the above solution to prepare a trace element-EDTA complex stock solution of a certain concentration for later use.

[0032] S3: Add the tracer stock solution to the fracturing fluid and compare it with the fracturing fluid without tracer to evaluate the compatibility of the tracer with the fracturing fluid, the colloidal stability of the tracer, and the formation adsorption stability of the tracer. S4: The tracer is obtained after the compatibility of the tracer with the fracturing fluid, the tracer's gel breaking stability, and the tracer's formation adsorption stability all meet the specified requirements.

[0033] In some embodiments, a method for preparing a tracer for monitoring flowback in multilayer fracturing includes: S1: Use the corresponding mass of chlorides (or oxides) of transition elements (such as Co, Zn, Cd) and lanthanides (such as La, Ce) dissolved in hydrochloric acid in water.

[0034] S2: Add disodium ethylenediaminetetraacetate or other complexing agents at a molar ratio to complex the element ions. Adjust the pH to 7-9 with NaOH. Check the completeness of complexation with Chrome Black T. Make up the volume of the above solution to prepare a trace element-EDTA complex stock solution of a certain concentration for later use.

[0035] S3: Add the tracer stock solution to the fracturing fluid and compare it with the fracturing fluid without tracer to evaluate the compatibility of the tracer with the fracturing fluid, the colloidal stability of the tracer, and the formation adsorption stability of the tracer. 1) Compatibility evaluation of tracer and fracturing fluid: Using the fracturing fluid base fluid formula at the operation site, the tracer to be evaluated was added to the water in the fracturing fluid preparation to the desired concentration. After the fracturing fluid containing tracer was prepared, its viscosity before and after gel breaking, viscosity after gel breaking, pH value, gel forming time, and temperature shear resistance were tested and compared with the fracturing fluid without tracer. If the parameters still meet the design requirements, it is considered to have passed the compatibility evaluation of tracer and fracturing fluid.

[0036] 2) Tracer breaking stability evaluation: a) Take fracturing fluid with tracer added at the field concentration, add breaker and break it completely at 90℃, then measure the concentration of tracer in the fracturing fluid after digestion; b) Take fracturing fluid with tracer of the same concentration, digest it directly without breaking it, and then measure the concentration of tracer in the fracturing fluid; if the tracer loss rate is less than 10% compared with b, it is considered to have passed the tracer breaking stability evaluation.

[0037] 3) Tracer formation adsorption stability evaluation: c) Take fracturing fluid with field-added tracer concentration, break it up, add 10g of fracturing sand and 10g of target formation rock cuttings, and after adsorption for 24 hours, measure the concentration of tracer in the supernatant; d) Take fracturing fluid with field-added tracer concentration, digest it directly without treatment, and measure the concentration of tracer in the fracturing fluid; c) Compared with d, the tracer loss rate is less than 10%, which is considered to pass the tracer formation adsorption stability evaluation.

[0038] S4: The tracer is obtained after the compatibility of the tracer with the fracturing fluid, the tracer's gel breaking stability, and the tracer's formation adsorption stability all meet the specified requirements.

[0039] This application also discloses a tracer for monitoring flowback in multi-layer fracturing, prepared using any of the methods described above. This method evaluates the effectiveness of the fracturing fluid tracer experimentally, ensuring that an effective tracer can be used to monitor the flowback curve of fracturing fluid flowback over time.

[0040] See Figure 2 This application also discloses the application of the above-mentioned tracer in multi-layer fracturing flowback monitoring, including: S5: Use a tracer to label the fracturing fluid and record the initial fracturing fluid volume and the initial tracer volume; S6: After the return begins, record the cumulative return amount at fixed time intervals, measure the real-time concentration of the tracer, and calculate the return amount for each segment; S7: Calculate the return curve and cumulative return rate for each segment based on the return volume of each segment and the cumulative return volume of each segment.

[0041] In some embodiments, the calculation of the backflow amount for each segment first uses the following formula to calculate the total mass of tracer returned from each segment:

[0042] The return flow rate for each section is then calculated using the following formula:

[0043] in, This represents the total mass of tracer discharged in the nth segment; To accumulate the backflow volume; This represents the real-time concentration of the tracer in the reflux solution; This represents the actual volume of liquid discharged in the nth segment; This represents the total mass of tracer used in the nth segment; This represents the total volume of fracturing fluid injected in the nth segment.

[0044] In some embodiments, using tracers to label fracturing fluids and detecting, analyzing, and calculating the produced fluid can accurately determine the production contribution rate and production sequence of each segment, and can semi-quantitatively calculate the production volume. Through flowback fluid sample testing and data analysis, the flowback sequence and contribution rate of each segment can be determined, providing data support for subsequent operations. This effectively solves the problem of unclear and untraceable flowback data after fracturing in vertical wells and horizontal wells. See also Figure 3 , Figure 4 Assume that the total mass of tracer added in the nth fracturing stage is M. n By detecting the concentration C of the tracer in the reflux solution n With the cumulative backflow volume V, the backflow volume of each segment can be quantitatively calculated. The quantitative calculation method for the backflow volume of each segment is as follows: calculate the total mass of tracer backflow used in each segment using formula (1), and then calculate the actual backflow volume using formula (2).

[0045] ...(1) The total mass of tracer discharged in the nth segment; : Cumulative volume of backflow; The concentration of tracer in the reflux solution; ...(2) The actual volume of liquid discharged in the nth segment; The total mass of tracer used in the nth segment; : The total volume of fracturing fluid injected in the nth segment.

[0046]

Example

[0047] 2) Tracer breaking stability evaluation: Take 200 mL of fracturing fluid with 10 mg / L tracer, add 1000 ppm ammonium persulfate, and break the gel completely at 90℃. After digestion, measure the concentration of tracer in the fracturing fluid and compare it with the unbroken fracturing fluid:

[0048] 3) Tracer formation adsorption stability evaluation: Take the fracturing fluid after gel breaking, add 10g of fracturing sand and 10g of target well cuttings, mix them separately and contact them for 24 hours, then measure the concentration of the tracer and compare it with the original fracturing fluid:

[0049] 4) M1, M2 and M3, which all passed the evaluation experiments in 1), 2) and 3) above, can be used as tracers for effectively monitoring the flowback curve of the fracturing fluid flowback of the target well over time.

[0050] Note: Due to differences in fracturing fluid formulation, type of breaker, and target well rock type, a tracer that is effective in other wells may not be applicable to all wells. Evaluation experiments 1), 2), and 3) above should be conducted each time based on the actual situation.

[0051] Using tracers to monitor fracturing fluid flowback 1) A well used three-stage fracturing and employed three tracers to indicate flowback. The fracturing fluid and tracer dosages are shown in the table below (all three tracers used have passed evaluation in 8.1).

[0052] 2) After the backflow begins, record the cumulative backflow amount every 0.5 hours, and take samples from the backflow pipeline to determine the real-time concentration of the tracer and calculate the backflow amount for each section.

[0053]

[0054] 3) Output results: Calculate the return curve for each segment according to the parameters input in 1) and 2).

[0055] 3)-1: First section (400m) 3 For the calculation of the return curve and final return rate, please refer to [link / reference]. Figure 5 :

[0056] 3)-2: Second section (500m) 3 For the calculation of the return curve and final return rate, please refer to [link / reference]. Figure 6 :

[0057] 3)-3: Third section (300m) 3 For the calculation of the return curve and final return rate, please refer to [link / reference]. Figure 7 :

[0058] 4) The final calculation results obtained using this invention are as follows: The first phase, lasting 8 hours, discharged a total of 370m. 3 The return rate was 92.5%. The second phase, lasting 8 hours, discharged a total of 405m. 3 The return rate was 81%. The third phase, lasting 8 hours, discharged a total of 210m. 3 The return rate is 70%. Total backflow volume: 1500m³ 3 In addition to fracturing fluid, there is also 515m 3 Other sources of liquid (such as formation water).

[0059] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a tracer for monitoring flowback in multi-layer fracturing, characterized in that, include: Solution A is obtained by dissolving chlorides of transition elements and lanthanides in water; Add a complexing agent to solution A to complex the element ions, adjust the pH to 7-9, and after the complexation is complete, make up the volume to prepare a trace element-EDTA complex tracer stock solution for later use. The tracer stock solution was added to the fracturing fluid, and the compatibility of the tracer with the fracturing fluid, the colloidal stability of the tracer, and the formation adsorption stability of the tracer were evaluated by comparing it with the fracturing fluid without the tracer. An effective tracer is obtained when the compatibility of the tracer with fracturing fluid, the tracer's gel breaking stability, and the tracer's formation adsorption stability all meet the specified requirements.

2. The method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 1, characterized in that, The evaluation of the compatibility between the tracer and the fracturing fluid specifically includes: Using the fracturing fluid base fluid formula available at the work site, tracer stock solution was added to the clean water used to prepare the fracturing fluid to the specified concentration. After the fracturing fluid containing tracer is prepared, its viscosity before and after gel breaking, viscosity after gel breaking, pH value, gel forming time, and temperature shear resistance parameters are tested and compared with the fracturing fluid without tracer. If the parameters still meet the specified requirements, the compatibility evaluation between tracer and fracturing fluid is passed.

3. The method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 1, characterized in that, The evaluation of the tracer's gel breaking stability specifically includes: Take fracturing fluid with a specified concentration of tracer stock solution added on site, add breaker until completely broken, and measure the concentration 'a' of tracer in fracturing fluid after digestion; Take fracturing fluid containing tracer mother liquor of the same concentration, digest it directly without breaking the gel, and then measure the concentration b of tracer in the fracturing fluid; If the tracer loss rate at concentration a is less than the preset value compared to concentration b, it is considered to have passed the tracer breakage stability evaluation.

4. The method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 3, characterized in that, The preset value for the tracer loss rate is 10%.

5. A method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 3, characterized in that, The fracturing fluid, to which a specified concentration of tracer mother liquor is added on-site, is then mixed with a breaker and broken down completely at 90°C.

6. The method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 1, characterized in that, The evaluation of the formation adsorption stability of the tracer specifically includes: After the fracturing fluid containing a mother liquor of tracer with a specified concentration was broken up, fracturing sand and rock cuttings from the target formation were added. After adsorption, the concentration c of the tracer in the supernatant was measured. Take fracturing fluid containing mother liquor of the same tracer concentration, digest it directly without treatment, and then measure the concentration d of the tracer in the fracturing fluid; If the tracer loss rate at concentration c is less than the preset value compared to concentration d, it is considered to have passed the tracer formation adsorption stability evaluation.

7. A method for preparing a tracer for monitoring flowback in multi-layer fracturing according to claim 6, characterized in that, After the fracturing fluid containing a specified concentration of tracer mother liquor is broken up, 10g of fracturing sand and 10g of target formation rock cuttings are added. After adsorption for 24 hours, the concentration c of the tracer in the supernatant is measured. The preset value for the tracer loss rate is 10%.

8. A tracer for monitoring flowback in multi-layer fracturing, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. The application of the tracer according to claim 8 in multilayer fracturing flowback monitoring, characterized in that, include: The fracturing fluid was labeled with a tracer and the initial fracturing fluid volume and initial tracer volume were recorded. After the backflow begins, the cumulative backflow amount is recorded at fixed time intervals, the real-time concentration of the tracer is measured, and the backflow amount for each segment is calculated. The return curve and cumulative return rate for each segment are calculated based on the return volume of each segment and the cumulative return volume of each segment.

10. The application of the tracer according to claim 9 in multilayer fracturing flowback monitoring, characterized in that, To calculate the backflow amount in each segment, the total mass of tracer returned in each segment is first calculated using the following formula: The return flow rate for each section is then calculated using the following formula: in, This represents the total mass of tracer discharged in the nth segment; To accumulate the backflow volume; This represents the real-time concentration of the tracer in the reflux solution; This represents the actual volume of liquid discharged in the nth segment; This represents the total mass of tracers used in the nth segment; This represents the total volume of fracturing fluid injected in the nth segment.

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