A test system and method for determining a travel front based on pulse pressure enhanced distributed strain

Abstract

A test system and method for determining migration front based on pulse pressure enhanced distributed strain belong to the technical field of CO2 geological storage and oil and gas resource development. The system includes a CO2 displacement brine unit, an optical fiber sensor, a pulse pressure injection unit, a data acquisition and processing system, etc. By injecting high-pressure gas on the other side of the core, a large pressure difference is generated at the two-phase interface, which in turn causes significant strain changes. The optical fiber sensor monitors these strain changes in real time, and the data acquisition and processing system analyzes and determines the CO2 / saline migration front position. The method has the advantages of high monitoring accuracy, fast response, wide applicability, etc., and can dynamically track the migration of CO2 during the underground storage process, ensuring the safety and effectiveness of the storage.

Description

Technical Field

[0001] The present invention relates to the field of CO2 gas geological storage and oil and gas resource development, and in particular to a method and system for determining the CO2 / brine migration front in a core flooding experiment using pulse pressure and fiber optic sensing technology.

[0002] Global climate change is intensifying, and the increase in greenhouse gas emissions, particularly carbon dioxide (CO2), is considered one of the main drivers. To address this challenge, countries are actively seeking solutions to reduce CO2 emissions. CO2 geological storage technology, as an effective emission reduction measure, injects CO2 into deep underground geological formations (such as saline aquifers, depleted oil and gas fields, and coal seams) for long-term storage, preventing its release into the atmosphere. This technology has shown great potential in mitigating global warming. Although CO2 geological storage technology is theoretically feasible and has great potential, its practical application still faces many challenges, particularly ensuring the safety and effectiveness of the storage process. Monitoring the underground migration behavior of CO2, determining its safe storage, and evaluating its long-term effectiveness are key steps. Effective monitoring technology can provide important information on CO2 diffusion pathways, storage stability, and potential leakage risks, providing scientific basis and technical support for the storage process.

[0003] Fiber optic sensing technology has been widely used in CO2 geological storage monitoring due to its advantages such as high sensitivity, strong anti-interference ability, and long-distance distributed monitoring. Commonly used fiber optic sensing technologies include fiber Brillouin scattering, fiber Raman scattering, and fiber Bragg grating technology, which can monitor changes in temperature, strain, and pressure. However, during the CO2 storage process, due to factors such as the low injection rate, the stress changes in the storage area may be small. Traditional fiber optic sensors are difficult to effectively capture these subtle changes, and the monitoring effect is limited. In order to solve the above problems, the present invention proposes a method and system for determining the CO2 / brine migration front based on pulse pressure. Summary of the Invention

[0004] To address these issues, the present invention proposes a testing system and method for determining the migration front based on pulsed pressure-enhanced distributed strain. This method injects high-pressure gas across the core, instantly generating a large pressure differential at the CO2 / brine interface, which induces significant strain changes. This method amplifies the stress change signals during the storage process, enabling fiber optic sensors to more accurately monitor these changes and determine the location of the CO2 / brine migration front.

[0005] The technical solution of the present invention is a testing system for determining fluid migration front based on pulse pressure enhanced distributed strain, the system comprising a CO2 displacement brine unit, a pulse pressure injection unit, a confining pressure control unit, and a distributed optical fiber strain monitoring unit;

[0006] The CO2-displacement brine unit consists of a test core, a core holder, and a CO2 / brine injection pump. The core is mounted in a dedicated core holder to ensure tightness and stability. The injection system injects CO2 and brine into one side of the core, simulating fluid migration under underground storage conditions. The injection pressure and flow rate of the CO2 and brine are simultaneously controlled to ensure controllable experimental conditions.

[0007] In the CO2 displacement brine unit, the brine pool is connected to the cross-connection through the second valve, the second injection pump and the third valve in sequence, and the first CO2 gas cylinder is connected to the cross-connection through the first injection pump and the first pressure sensor; the cross-connection is then connected to the pore pressure injection hole of the core holder through the fourth valve and the second pressure sensor, and is connected to the core;

[0008] In the pulse pressure injection unit, the second CO2 cylinder is connected to the four-way valve through the third injection pump, the third pressure sensor and the fifth valve in sequence;

[0009] In the confining pressure control unit, the deionized water bottle is connected to the confining pressure injection hole of the core holder through the fourth injection pump, the seventh valve, and the fourth pressure sensor in sequence, and is connected to the confining pressure interlayer;

[0010] The distributed fiber optic strain monitoring unit comprises a distributed optical fiber, a fiber optic demodulator, and a computer module. The distributed optical fiber transmits the optical signal to the data processing module and is spirally wound and fixed to the rock core, ensuring that the optical fiber is evenly distributed and securely fixed. One end of the optical fiber is connected to the fiber optic demodulator to monitor stress changes on the core surface during displacement. The fiber optic demodulator receives and converts the optical signal from the fiber optic sensor, converts the optical signal into an electrical signal, and performs amplification, filtering, spectrum analysis, and demodulation to extract strain data and monitor strain changes along the optical fiber in real time. An analog-to-digital converter converts the analog electrical signal into a digital signal, which is transmitted to the data processing and analysis system. The computer module further analyzes the digital signal to obtain the strain distribution along the optical fiber.

[0011] Furthermore, the system also includes a collection unit, and the core holder is connected to the gas-water collection tank through a pipeline passing through a sixth valve.

[0012] Furthermore, the computer module is electrically connected to the data demodulator, and the data demodulator is electrically connected to the distributed optical fiber; the distributed optical fiber is responsible for transmitting the detected optical signal to the optical fiber demodulator; the optical fiber demodulator modulates the collected signal and carries information on amplitude, frequency and phase, which is analyzed and processed by the computer module, and the excitation intensity corresponding to different locations on the optical fiber is displayed in real time on the computer.

[0013] A testing system and method for determining migration front based on pulse pressure enhanced distributed strain, comprising the following steps:

[0014] S1. Select a suitable experimental core sample, spirally wrap the optical fiber around the outer surface of the core, and install the core in the core holder after packaging.

[0015] S2. Core flooding experiment: CO2 and brine were injected into one side of the core through the injection system, and the injection pressure and flow rate of CO2 and brine were controlled to ensure the controllability of the experimental conditions.

[0016] S3. High-pressure gas pulse injection: Rapidly inject high-pressure gas on the other side of the core, using a fast response valve to achieve instantaneous injection of high-pressure gas, generating a significant pressure difference.

[0017] S4. Using optical fiber strain sensors arranged around the core to monitor and obtain corresponding relevant data information;

[0018] S5. The signal transmitted in the optical fiber cable enters the optical fiber demodulator for demodulation;

[0019] S6. The computer module analyzes and processes the data and outputs the strain change. The point where the strain peak suddenly changes is the CO2 / brine migration front.

[0020] Furthermore, the method comprises the following steps:

[0021] S1. Core preparation: The core is saturated with deionized water to ensure that there is no air inside the core. Distributed optical fibers are arranged on the core surface along the fluid migration path to ensure that the distributed optical fibers cover the entire displacement area. The distributed optical fibers are tightly wound in a spiral shape and fixed to the core.

[0022] S2. Confining pressure control: Use the fourth syringe pump to inject deionized water from the deionized water bottle into the confining pressure interlayer of the core holder through the confining pressure injection hole to generate the required confining pressure. This ensures that the core is always subjected to a uniform and stable confining pressure during the displacement experiment.

[0023] S3. CO2 displacement of brine: The second injection pump injects brine from the brine pool into the core through the pore pressure injection hole to ensure complete saturation of the core pores. Then, the first injection pump injects CO2 from the first CO2 cylinder into the core to displace brine, ensuring that the pore pressure does not exceed the confining pressure throughout the displacement process. The displaced brine is collected in a gas-water collection tank.

[0024] S4. Injection pulse pressure: With the fifth valve closed, the third injection pump is used to adjust the pulse pressure to a preset value that is higher than the original pore pressure but lower than the confining pressure. The pulse pressure is then injected into the core holder by controlling the start and stop of the fifth valve.

[0025] S5. Optical fiber strain signal acquisition stage: Real-time monitoring and recording of distributed optical fiber strain data during pulse pressure injection, focusing on the strain change of the CO2 / saline interface; the optical fiber demodulator extracts high-resolution strain data and analyzes and processes them through the computer module; visualize the data to generate strain distribution maps and strain-time curves to identify strain changes caused by pulse pressure and determine the position of the CO2 / saline migration front.

[0026] Through the above technical solutions, the present application has the following beneficial effects:

[0027] The present application combines pulse pressure and distributed optical fiber sensing technology, effectively solving the problem of subtle strain changes and difficulty in determining the fluid migration front in core displacement experiments due to small injection speed and other reasons. By injecting high-pressure gas on the other side of the core, a large pressure difference is instantly generated at the CO2 / saline interface, causing significant strain changes. The optical fiber sensor monitors these strain changes in real time, and the data acquisition and processing system analyzes and determines the position of the CO2 / saline migration front. This technology significantly improves the accuracy and sensitivity of CO2 / saline migration front monitoring, providing high-resolution strain data in real time to ensure the accuracy and reliability of experimental data, and providing strong technical support for CO2 geological storage research. This method has the advantages of high monitoring accuracy, fast response, wide applicability, etc., and can dynamically track the migration of CO2 in the underground storage process, ensuring the safety and effectiveness of the storage. At the same time, the present application can also be applied to the monitoring of other geological fluid migration processes, such as groundwater resource management and oil and gas field development. BRIEF DESCRIPTION OF DRAWINGS

[0028] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes and proportions of the components in the figures are only illustrative and are used to help understand the present application, and are not specific limitations on the shapes and proportions of the components of the present application. Those skilled in the art can choose various possible shapes and proportions to implement the present application according to specific circumstances under the guidance of the present application.

[0029] Figure 1 is a schematic diagram of a test system for determining the migration front based on pulse pressure enhanced distributed strain.

[0030] Figure 2 is a top view of a core holder.

[0031] In the figure: 1. first injection pump, 2. first CO2 cylinder, 3. first valve, 4. first pressure sensor, 5. brine pool, 6. second valve, 7. third valve, 8. four-way valve, 9. fourth valve, 10. second injection pump, 11. fifth valve, 12. second CO2 cylinder, 13. third pressure sensor, 14. third injection pump, 15. second pressure sensor, 16. distributed optical fiber, 17. core holder, 18. gas-water collecting tank, 19. sixth valve, 20. fourth pressure sensor, 21. seventh valve, 22. fourth injection pump, 23. deionized water bottle, 24. optical fiber demodulator, 25. computer module, 26. pore pressure injection hole, 27. confining pressure interlayer, 28. confining pressure injection hole, 29. core. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0033] Figure 1 A testing system for determining migration fronts based on pulse pressure-enhanced distributed strain is shown. The system includes a CO2-displaced brine unit, a confining pressure control unit, a pulse pressure injection unit, and a distributed fiber-optic strain monitoring unit. The distributed fiber-optic strain monitoring unit includes a signal transmission module, a fiber demodulator, and a computer module.

[0034] The CO2-displacement brine unit includes a first injection pump 1, a CO2 cylinder 2, a first valve 3, a first pressure sensor 4, a brine reservoir 5, a second valve 6, a third valve 7, a four-way pipe fitting 8, a second injection pump 10, and a core holder 17. The first injection pump 1 is used to inject CO2 gas, while the second injection pump is used to inject brine. Both are connected to the pore pressure injection port of the core holder. The pulse pressure injection unit includes a fifth valve 11, a second CO2 cylinder 12, a third pressure sensor 13, and a third injection pump 14. The third injection pump 14 achieves a preset high pulse pressure. To minimize the impact on the displacement experiment, the pulse pressure medium is also CO2. The confining pressure control unit includes a fourth pressure sensor 20, a seventh valve 21, a fourth injection pump 22, and a deionized water bottle 23. The distributed optical fiber strain monitoring unit includes a distributed optical fiber 16, an optical fiber interrogator 24, and a computer module 25. The distributed optical fiber 16 is responsible for transmitting the detected optical signal to the optical fiber demodulator; after the optical fiber demodulator 24 modulates the collected signal, it carries key information such as amplitude, frequency and phase, and is analyzed and processed by the computer module 25, and the excitation intensity corresponding to different locations on the optical fiber is displayed in real time on the computer; the computer module 25 is connected to the data demodulator 24, and the data demodulator is connected to the distributed optical fiber 16.

[0035] The core holder 17 has a confining pressure interlayer 27 disposed outside the core 29 , a pore pressure injection hole 26 disposed on the core 29 , and a confining pressure injection hole 28 disposed on the confining pressure interlayer 27 ; the distributed optical fiber 16 is wound around the outside of the core 29 .

[0036] In actual operation, brine is first drawn from the brine pool 5 using the second injection pump 10 and injected into the core 29 in the core holder 17 through the second valve 6, fully saturating the pores within the core 29. During the injection process, the pressure changes are monitored using the first pressure sensor 4, and the brine is observed to flow out from the other side to ensure that the brine completely fills the core 29.

[0037] Next, a first injection pump 1 was used to extract CO2 from a CO2 cylinder and inject it into the core 29 to displace the brine. The first and third valves 7 and 9 were opened to ensure smooth CO2 entry into the core holder 17. To control the pressure during injection, a second pressure sensor 15 was used to monitor the injection rate and adjust it appropriately to ensure the pore pressure did not exceed the confining pressure.

[0038] To enhance the strain signal during the CO2 displacement of brine, a high pulse pressure is pre-stored via the third injection pump 14. This process requires that, with the fifth valve 11 closed, the third injection pump 14 is adjusted to a preset pulse pressure that is higher than the pore pressure but lower than the confining pressure. The injection of the pulse pressure is controlled by the rapid opening and closing of the fifth valve 11.

[0039] Throughout this process, distributed optical fiber monitoring of stress distribution is performed in real time, with a particular focus on recording optical fiber strain data during pulse pressure injection to observe strain changes at the CO2 / saltwater interface. A fiber optic interrogator 24 extracts high-resolution strain data, which is analyzed and visualized by a computer module 25 to generate a strain distribution diagram and a graph of strain change over time.

[0040] When the above technical solution is used, the following steps are included:

[0041] Core preparation: Saturate the core 29 with deionized water to ensure that it is free of air and other impurities. Distributed optical fibers 16 are placed along the core surface along the fluid migration path, ensuring coverage of the entire displacement area. Distributed optical fibers 16 are tightly wound in a spiral and secured to the core 29 to maximize strain capture. Fiber connections are inspected to ensure stable and reliable signal transmission.

[0042] Confining pressure control experiment: Deionized water is injected into the confining pressure layer 27 of the core holder 17 using the fourth syringe pump 22 to generate the desired confining pressure. This step ensures that the core 29 is subjected to a uniform and stable confining pressure throughout the displacement experiment. By uniformly applying external pressure, the confining pressure can reproduce the three-dimensional stress state of the subsurface formation, prevent core fracture or deformation, and ensure the accuracy and repeatability of the measured data.

[0043] CO2 displacement of brine: Brine is injected via the first injection pump 1 to ensure complete saturation of the pores within the core 29. Then, CO2 is injected via the second injection pump 10 to displace the brine, ensuring that the pore pressure does not exceed the confining pressure throughout the displacement process. The displaced brine is collected in the gas-water collection tank 18.

[0044] To enhance the strain signal generated during the CO2 displacement of brine, pulse pressure is injected into the system. While valve 11 is closed, a third injection pump 14 is used to adjust the pressure to a preset pulse pressure that is higher than the original pore pressure but lower than the confining pressure. The injection of pulse pressure is controlled by turning valve 11 on and off.

[0045] Fiber-optic strain signal acquisition: Fiber-optic strain data during pulse pressure injection is monitored and recorded in real time, with particular attention paid to strain changes at the CO2 / brine interface. A fiber-optic interrogator extracts high-resolution strain data, which is analyzed and processed by a computer module. Data visualization generates strain distribution maps and strain-versus-time graphs to identify significant strain changes induced by the pulse pressure and determine the location of the CO2 / brine migration front.

Claims

1. A testing method for determining fluid migration front based on pulse pressure enhanced distributed strain, characterized by: The test system used in the test method includes a CO2 displacement brine unit, a pulse pressure injection unit, a confining pressure control unit, and a distributed optical fiber strain monitoring unit; In the CO2 displacement brine unit, the brine pool is connected to the cross-connection through the second valve, the second injection pump and the third valve in sequence, and the first CO2 gas cylinder is connected to the cross-connection through the first injection pump and the first pressure sensor; the cross-connection is then connected to the pore pressure injection hole of the core holder through the fourth valve and the second pressure sensor, and is connected to the core; In the pulse pressure injection unit, the second CO2 cylinder is connected to the four-way valve through the third injection pump, the third pressure sensor and the fifth valve in sequence; In the confining pressure control unit, the deionized water bottle is connected to the confining pressure injection hole of the core holder through the fourth injection pump, the seventh valve, and the fourth pressure sensor in sequence, and is connected to the confining pressure interlayer; The distributed optical fiber strain monitoring unit includes a distributed optical fiber, an optical fiber demodulator and a computer module; The distributed optical fiber is wound in a spiral shape and fixed on the core; The test method comprises the following steps: S1. Core preparation stage; S2. Confining pressure control; S3. CO2 displacement of brine: The second injection pump injects brine from the brine pool into the core through the pore pressure injection hole to ensure complete saturation of the core pores. Then, the first injection pump injects CO2 from the first CO2 cylinder into the core to displace brine, ensuring that the pore pressure does not exceed the confining pressure throughout the displacement process. The displaced brine is collected in a gas-water collection tank. S4. Injection pulse pressure: With the fifth valve closed, the third injection pump is used to adjust the pulse pressure to a preset value that is higher than the original pore pressure but lower than the confining pressure. The pulse pressure is then injected into the core of the core holder by controlling the start and stop of the fifth valve. S5. Fiber-optic strain signal acquisition phase: Real-time monitoring and recording of distributed fiber-optic strain data during pulse pressure injection, focusing on strain changes at the CO2 / brine interface; a fiber-optic interrogator extracts high-resolution strain data and analyzes and processes it through a computer module; the data is visualized to generate strain distribution maps and strain-versus-time graphs to identify strain changes caused by pulse pressure and determine the location of the CO2 / brine migration front.

2. The testing method according to claim 1, wherein: The system also includes a collection unit, and the core holder is connected to the gas-water collection tank through a pipeline passing through a sixth valve.

3. The testing method according to claim 1, wherein: The computer module is electrically connected to the optical fiber demodulator, and the optical fiber demodulator is electrically connected to the distributed optical fiber; the distributed optical fiber is responsible for transmitting the detected optical signal to the optical fiber demodulator; The optical fiber demodulator modulates the collected signal to carry information on amplitude, frequency and phase, analyzes and processes it through a computer module, and displays the excitation intensity corresponding to different locations on the optical fiber on the computer in real time.

4. The testing method according to claim 1, wherein: In step S1, the core preparation stage specifically includes: saturating the core with deionized water to ensure that there is no air inside the core; arranging distributed optical fibers on the surface of the core along the fluid migration path to ensure that the distributed optical fibers cover the entire displacement area; and tightly winding the distributed optical fibers in a spiral shape and fixing them on the core.

5. The testing method according to claim 1, wherein: In step S2, the confining pressure control is specifically as follows: using the fourth injection pump to inject deionized water in the deionized water bottle into the confining pressure interlayer through the confining pressure injection hole to generate the required confining pressure; and ensuring that the core is always subjected to a uniform and stable confining pressure during the displacement experiment.

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