Test system and method for determining migration front based on pulse pressure enhanced distributed strain

Abstract

The invention discloses a test system and method for determining migration front based on pulse pressure enhanced distributed strain, and belongs to the technical field of CO2 gas geological storage and oil and gas resource development. The system comprises a CO2 displacement saline water unit, an optical fiber sensor, a pulse pressure injection unit, a data acquisition and processing system and the like. High-pressure gas is injected into the other side of the rock core, and a large pressure difference is instantly generated at a two-phase interface, so that obvious strain change is caused. The optical fiber sensor monitors the strain changes in real time, and the migration leading edge position of CO2 / saline water is analyzed and determined through the data collecting and processing system. The method has the advantages of being high in monitoring precision, rapid in response, wide in applicability and the like, the migration condition of CO2 in the underground storage process can be dynamically tracked, and storage safety and effectiveness are ensured.

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 displacement experiment using pulse pressure and optical fiber sensing technology.

[0002] Global climate change is becoming increasingly severe, and the increase in greenhouse gas emissions, especially carbon dioxide (CO2), is considered to be one of the main reasons. In order to meet this challenge, countries are actively seeking solutions to reduce CO2 emissions. As an effective means of emission reduction, CO2 geological storage technology can achieve long-term storage by injecting CO2 into deep underground geological structures (such as saline aquifers, depleted oil and gas fields, and coal seams) to prevent 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 potential, it still faces many challenges in practical applications, especially ensuring the safety and effectiveness of the storage process. Monitoring the migration behavior of CO2 underground, judging whether it is safely stored, and evaluating the long-term effects of storage are key steps. Effective monitoring technology can provide important information about CO2 diffusion paths, storage stability, and possible leakage risks, thereby 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 high sensitivity, strong anti-interference ability, and long-distance distributed monitoring. Common 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 reasons 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] In order to solve the above problems, the present invention proposes a testing system and method for determining the migration front based on pulse pressure enhanced distributed strain. This method generates a large pressure difference at the CO2 / brine interface instantly by injecting high-pressure gas on the other side of the core, thereby causing obvious strain changes. This method amplifies the stress change signal during the storage process, allowing the fiber optic sensor to monitor these changes more accurately and determine the position of the CO2 / brine migration front.

[0005] The technical solution of the present invention is: a test 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; The CO2-displaced brine unit includes an experimental core, a core holder, and a CO2 / brine injection pump. The core is installed in a dedicated core holder to ensure sealing and stability. CO2 and brine are injected into one side of the core through the injection system to simulate the fluid migration process under underground storage conditions. At the same time, the injection pressure and flow rate of CO2 and brine are controlled to ensure the controllability of the experimental conditions.

[0006] In the CO2 displacement brine unit, the brine pool is connected to the cross-way 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-way through the first injection pump and the first pressure sensor; the cross-way 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 responsible for transmitting the optical signal to the data processing module, and is spirally wound and fixed on the core; ensuring that the optical fiber is evenly distributed and firmly fixed. At the same time, one end of the optical fiber is connected to the optical fiber demodulator to monitor the stress changes on the surface of the core during the displacement process. The optical fiber demodulator receives and converts the optical signal of the optical fiber sensor, converts the optical signal into an electrical signal, and amplifies, filters, spectrum analyzes and demodulates it to extract strain data and monitor the strain changes along the optical fiber in real time. The analog-to-digital converter converts the analog electrical signal into a digital signal and transmits it to the data processing and analysis system. The computer module further analyzes the digital signal to obtain the strain distribution along the optical fiber.

[0007] 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.

[0008] 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 carrying information of amplitude, frequency and phase, analyzes and processes it through the computer module, and displays the excitation intensity corresponding to different positions on the optical fiber on the computer in real time.

[0009] A testing system and method for determining migration front based on pulse pressure enhanced distributed strain, comprising the following steps: S1. Select a suitable experimental core sample, spirally wrap the optical fiber and fix it on the outer surface of the core. After packaging, install the core in the core holder.

[0010] S2. Core displacement experiment: CO2 and brine are injected into one side of the core through the injection system, and the injection pressure and flow rate of CO2 and brine are controlled to ensure the controllability of the experimental conditions.

[0011] 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.

[0012] S4. Using optical fiber strain sensors arranged around the core to monitor and obtain corresponding relevant data information; S5. The signal transmitted in the optical fiber cable enters the optical fiber demodulator for demodulation; S6. The computer module outputs the strain change after analysis and processing. The peak of the strain is the CO2 / brine migration front.

[0013] Furthermore, the method comprises the following steps: S1. Core preparation stage: The core is saturated with deionized water to ensure that there is no air inside the core; distributed optical fibers are arranged on the surface of the core 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 on the core; S2. Confining pressure control: using the fourth injection pump to inject the deionized water in 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; and ensuring that the core is always subjected to uniform and stable confining pressure during the displacement experiment; S3. CO2 displacing brine stage: The brine in the brine pool is injected into the core through the pore pressure injection hole by the second injection pump to ensure that the pores inside the core are fully saturated; then, the CO2 in the first CO2 cylinder is injected into the core using the first injection pump to perform a brine displacing experiment to ensure that the pore pressure does not exceed the confining pressure during the entire displacing process; the displaced brine will be collected in the gas-water collection tank; S4. Injection pulse pressure: When the fifth valve is closed, the third injection pump is used to adjust the preset pulse pressure to a pressure higher than the original pore pressure but lower than the confining pressure, and the pulse pressure is injected into the core holder by starting and stopping 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 the strain changes at the CO2 / brine interface; the fiber optic demodulator 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-over-time curves to identify strain changes caused by pulse pressure and determine the position of the CO2 / brine migration front. Through the above technical solution, the present invention has the following beneficial effects: The present invention combines pulse pressure and distributed optical fiber sensing technology to effectively solve the problem that the strain changes are subtle and it is difficult to determine the fluid migration front due to the low injection speed and other reasons in the core displacement experiment. By injecting high-pressure gas on the other side of the core, a large pressure difference is instantly generated at the CO2 / brine interface, which in turn causes significant strain changes. The optical fiber sensor monitors these strain changes in real time, and analyzes and determines the position of the CO2 / brine migration front through the data acquisition and processing system. This technology significantly improves the accuracy and sensitivity of CO2 / brine migration front monitoring, can provide high-resolution strain data in real time, ensure the accuracy and reliability of experimental data, and provide strong technical support for CO2 geological storage research. This method has the advantages of high monitoring accuracy, fast response, and wide applicability. It can dynamically track the migration of CO2 during underground storage and ensure the safety and effectiveness of storage. At the same time, the present invention 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 THE DRAWINGS

[0014] The accompanying drawings described herein are for explanation purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the various components in the drawings are only schematic, used to help understand the present invention, and are not specifically limited to the shapes and proportional dimensions of the various components of the present invention. Those skilled in the art can select various possible shapes and proportional dimensions to implement the present invention according to the teachings of the present invention.

[0015] Figure 1 Schematic diagram of a testing system for determining migration front based on pulse pressure-enhanced distributed strain.

[0016] Figure 2 is a top view of the core holder.

[0017] In the figure: 1. first injection pump, 2. first CO2 gas 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 gas 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

[0018] 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.

[0019] Figure 1 A test system for determining migration front based on pulse pressure enhanced distributed strain is shown, including: a CO2 displacement brine unit, a confining pressure control unit, a pulse pressure injection unit, and a distributed optical fiber strain monitoring unit. The distributed optical fiber strain monitoring unit includes a signal transmission module, an optical fiber demodulator, and a computer module.

[0020] The CO2 displacement brine unit includes a first injection pump 1, a CO2 gas cylinder 2, a first valve 3, a first pressure sensor 4, a brine pool 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, and the second injection pump is used to inject brine, both of which 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 gas cylinder 12, a third pressure sensor 13, and a third injection pump 14. The preset high pulse pressure is achieved by the third injection pump 14. In order to minimize the impact on the displacement experiment, the pulse pressure medium also uses 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 demodulator 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.

[0021] The core holder 17 has a confining pressure interlayer 27 disposed on the outer side of 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 outer side of the core 29 .

[0022] In actual operation, firstly, brine is extracted from the brine pool 5 by the second injection pump 10, and the brine is injected into the core 29 of the core holder 17 through the second valve 6, so that the pores inside the core 29 are fully saturated. During the injection process, the pressure change is monitored by 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.

[0023] Then, the first injection pump 1 is used to extract CO2 through the CO2 cylinder and inject CO2 into the core 29 to perform the brine displacement experiment. The first valve 7 and the third valve 9 are opened to ensure that CO2 smoothly enters the core holder 17. In order to control the pressure during the injection process, the second pressure sensor 15 is used to monitor and adjust the injection rate in time to ensure that the pore pressure does not exceed the confining pressure.

[0024] In order to enhance the strain signal during the CO2 displacement of brine, a high pulse pressure is pre-reserved through the third injection pump 14. This process requires that when the fifth valve 11 is 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, and the injection of the pulse pressure is controlled by the rapid opening and closing of the fifth valve 11.

[0025] During the whole process, the stress distribution is monitored in real time by distributed optical fiber, with the focus on recording the optical fiber strain data during the pulse pressure injection period and observing the strain change at the CO2 / salt water interface. The optical fiber demodulator 24 extracts high-resolution strain data, and analyzes and visualizes it through the computer module 25 to generate a strain distribution diagram and a curve diagram of strain change over time.

[0026] When the above technical solution is used, the following steps are included: Core preparation stage: Saturate the core 29 with deionized water to ensure that there is no air or other impurities inside the core 29. Distributed optical fiber 16 is arranged on the core surface along the fluid migration path to ensure that the optical fiber covers the entire displacement area. The distributed optical fiber 16 is tightly wound and fixed on the core 29 in a spiral shape to capture strain changes to the greatest extent. Check the optical fiber connection to ensure the stability and reliability of signal transmission.

[0027] Confining pressure control experiment: Deionized water is injected into the confining pressure interlayer 27 of the core holder 17 by the fourth injection pump 22 to generate the required confining pressure. This step ensures that the core 29 is always subjected to uniform and stable confining pressure during the displacement experiment. By uniformly applying external pressure, the confining pressure can reproduce the three-dimensional stress state of the underground rock formation, prevent the core from breaking or deforming, and ensure the accuracy and repeatability of the measurement data.

[0028] CO2 displacement brine stage: brine is injected through the first injection pump 1 to ensure that the pores inside the core 29 are fully saturated. Then, CO2 is injected using the second injection pump 10 to perform a brine displacement experiment to ensure that the pore pressure does not exceed the confining pressure during the entire displacement process. The displaced brine will be collected in the gas-water collection tank 18.

[0029] In order to enhance the strain signal generated during the CO2 displacement of brine, pulse pressure is injected into the system. When the valve 11 is closed, the third injection pump 14 is used to adjust the preset pulse pressure to a pressure higher than the original pore pressure but lower than the confining pressure, and the injection of the pulse pressure is controlled by starting and stopping the valve 11.

[0030] Fiber strain signal acquisition stage: Real-time monitoring and recording of fiber strain data during pulse pressure injection, with special attention paid to the strain changes at the CO2 / brine interface. The fiber demodulator extracts high-resolution strain data and analyzes and processes it through a computer module. The data is visualized to generate strain distribution graphs and strain-over-time curves to identify significant strain changes caused by pulse pressure and determine the location of the CO2 / brine migration front.

Claims

1. A test system for determining fluid migration front based on pulse pressure enhanced distributed strain, characterized by: The system includes a CO2-displaced 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 (5) is connected to the four-way valve (8) through the second valve (6), the second injection pump (10) and the third valve (7) in sequence; the first CO2 gas cylinder (2) is connected to the four-way valve (8) through the first injection pump (1) and the first pressure sensor (4); the four-way valve (8) is then connected to the pore pressure injection hole (16) of the core holder (17) through the fourth valve (9) and the second pressure sensor (15), and is connected to the core (29); In the pulse pressure injection unit, the second CO2 cylinder (12) is connected to the four-way valve (8) through the third injection pump (14), the third pressure sensor (13) and the fifth valve (11) in sequence; In the confining pressure control unit, the deionized water bottle (23) is connected to the confining pressure injection hole (28) of the core holder (17) through the fourth injection pump (22), the seventh valve (21), and the fourth pressure sensor (20) in sequence, and is connected to the confining pressure interlayer (27); The distributed optical fiber strain monitoring unit comprises a distributed optical fiber (16), an optical fiber demodulator (24) and a computer module (25); the distributed optical fiber (16) is wound in a spiral shape and fixed on a rock core.

2. A testing system for determining fluid migration front based on pulse pressure enhanced distributed strain according to claim 1, characterized in that: The system also includes a collection unit, wherein the core holder (17) is connected to a gas-water collection tank (18) via a pipeline passing through a sixth valve (19).

3. A testing system for determining fluid migration front based on pulse pressure enhanced distributed strain according to claim 1, characterized in that: The computer module (25) is electrically connected to the data demodulator (24), and the data demodulator (24) is electrically connected to the distributed optical fiber (16); the distributed optical fiber (16) is responsible for transmitting the detected optical signal to the optical fiber demodulator (24); the optical fiber demodulator (24) modulates the collected signal to carry information on amplitude, frequency and phase, analyzes and processes the signal through the computer module (25), and displays the excitation intensity corresponding to different positions on the optical fiber on the computer in real time.

4. The testing method of a testing system for determining fluid migration front based on pulse pressure enhanced distributed strain according to claim 1, characterized in that: The following steps are involved: S1. Core preparation stage: the core is saturated with deionized water to ensure that there is no air inside the core; a distributed optical fiber (16) is arranged on the surface of the core (29) along the fluid migration path to ensure that the distributed optical fiber (16) covers the entire displacement area; the distributed optical fiber (16) is tightly wound in a spiral shape and fixed on the core (29); S2. Confining pressure control: using the fourth injection pump (22) to inject deionized water in the deionized water bottle (23) into the confining pressure interlayer (27) through the confining pressure injection hole (28) 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; S3. CO2 displacement of brine stage: The brine in the brine pool (5) is injected into the core (29) through the pore pressure injection hole (26) by the second injection pump (10) to ensure that the pores inside the core are completely saturated; then, the CO2 in the first CO2 cylinder (2) is injected into the core (29) using the first injection pump (1) to perform a brine displacement experiment to ensure that the pore pressure does not exceed the confining pressure during the entire displacement process; the displaced brine is collected in the gas-water collection tank (18); S4. Injection pulse pressure: When the fifth valve (11) is closed, the third injection pump (14) is used to adjust the pulse pressure to a preset value higher than the original pore pressure but lower than the confining pressure, and the pulse pressure is injected into the core (29) of the core holder (17) by starting and stopping the fifth valve (11); S5. Fiber strain signal acquisition stage: real-time monitoring and recording of the distributed fiber (16) strain data during the pulse pressure injection period, focusing on the strain changes at the CO2 / salt water interface; the fiber demodulator extracts high-resolution strain data and analyzes and processes it through a computer module; Visualize the data to generate strain distribution maps and strain versus time plots to identify strain changes caused by pulse pressure and determine the location of the CO2 / brine migration front.

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