CO2 geological storage fluid monitoring system and method based on nuclear magnetic resonance and distributed optical fiber sensing

Abstract

The invention discloses a CO2 geological sequestration fluid migration monitoring system and method based on nuclear magnetic resonance and distributed optical fiber sensing combined monitoring, and belongs to the technical field of CO2 gas geological sequestration and oil and gas resource development. Aiming at core displacement and seepage processes thereof, the method utilizes an NMR technology to accurately characterize fluid saturation distribution and displacement front evolution, and meanwhile, adopts DOFS to monitor stress response and pore compression behaviors of rocks in real time, so that high-precision monitoring of CO2-brine two-phase flow in a reservoir is realized. Through combined analysis of the two, high-precision fluid migration frontier identification can be realized, and a fluid-solid coupling mechanism of a displacement frontier region is disclosed. Compared with a traditional method, a CO2 geological storage process monitoring scheme which is higher in resolution and more real-time is provided, the migration condition of CO2 in the underground storage process can be dynamically tracked, and technical support is provided for safety assessment and optimization of CO2 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 CO2 geological storage fluid migration detection method based on combined monitoring of nuclear magnetic resonance and distributed optical fiber sensing.

[0002] As global climate change intensifies, carbon dioxide (CO2) geological storage (CCS) is considered an effective greenhouse gas emission reduction technology. By injecting CO2 into deep geological reservoirs, it can be stored long-term, thereby reducing its impact on the atmospheric environment. During the storage process, CO2 undergoes multiphase flow in the reservoir and interacts complexly with the reservoir rocks and pore fluids. Its main storage mechanisms include structural storage, dissolution storage, residual storage, and mineralization storage. To ensure the safety and long-term stability of storage, the migration behavior of CO2 in the reservoir must be accurately monitored to prevent potential leakage risks and optimize storage strategies.

[0003] Currently, monitoring methods for CO2 geological storage primarily include seismic, electrical, geochemical, and gravity monitoring. These methods provide effective information on the migration of CO2 plumes over large scales. However, due to the complexity of the geological environment and the highly dynamic nature of fluid migration within reservoirs, these traditional monitoring methods still have limitations in terms of spatial resolution, temporal accuracy, and the ability to analyze local fluid-rock interactions. For example, while seismic monitoring can provide information on reservoir structural changes, its low resolution makes it difficult to accurately characterize the displacement process of CO2-brine two-phase flow. Electrical monitoring is significantly affected by salinity changes, making it difficult to achieve real-time quantitative measurement of fluids. Geochemical methods require sampling and analysis of reservoir fluids, resulting in long response times and difficulties in achieving long-term continuous monitoring of deep reservoirs. Therefore, developing high-resolution, real-time, and long-term CO2 storage monitoring methods has become a current research focus and challenge. Summary of the Invention

[0004] To address the above-mentioned issues, the present invention provides a combined monitoring method based on NMR and DOFS, aiming to achieve high-precision, real-time dynamic monitoring of the CO2 displacement of brine. NMR quantitatively measures the water volume distribution and accurately identifies the advancement of the displacement front. Simultaneously, DOFS is used to obtain information on rock strain changes and analyze the impact of fluid migration on reservoir structure. Experimental results show that the displacement front positions measured by the two technologies are highly consistent, and the DOFS strain data can further reveal local permeability paths and pressure transmission characteristics. This method not only improves the monitoring accuracy of the CO2 storage process but also provides important technical support for optimizing storage strategies and improving storage efficiency.

[0005] The technical solution of the present invention is a CO2 geological storage fluid migration monitoring system based on nuclear magnetic resonance and distributed optical fiber sensing. The system includes a CO2 displacement brine unit, a nuclear magnetic resonance monitoring unit, a confining pressure control unit, a back pressure control unit, and a distributed optical fiber strain monitoring unit. The core flooding unit consists of an experimental core sample, 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 then 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.

[0006] In the core flooding unit, the brine bottle is connected to the tee through the third injection pump and the third valve in sequence, and the CO2 gas cylinder is connected to the tee through the second injection pump and the second valve 6; the tee is then connected to the core holder through the fourth valve and the first pressure sensor; In the confining pressure control unit, the N2 gas cylinder passes through the first injection pump, the first valve, and is connected to the core holder; The NMR monitoring unit is a low-field NMR instrument, which mainly includes a magnet system, a radio frequency system, a sample holder, and a data acquisition and processing system, which together constitute a NMR monitoring system; The back pressure control system includes a sixth valve, a piston container, a seventh valve, a fourth injection pump, and a deionized water bottle to simulate the fluid pressure of a deep reservoir and prevent premature gasification of CO2.

[0007] The distributed fiber optic strain monitoring unit (DFOS) consists of a distributed optical fiber, a fiber optic interrogator, and a computer module. It is used to monitor stress changes on the core surface in real time during displacement. First, the distributed optical fiber is spirally wound and fixed to the core surface to ensure uniform distribution and a secure fit. This fiber not only senses strain changes in the core but also transmits the optical signal to the subsequent data processing unit. Next, a fiber optic interrogator is connected to one end of the fiber, receiving the transmitted optical signal and converting it into an electrical signal. The interrogator integrates amplification, filtering, spectrum analysis, and signal demodulation functions to accurately extract strain data distributed along the fiber, enabling high-resolution continuous measurement. Finally, an analog-to-digital converter (ADC) converts the demodulated analog electrical signal into a digital signal and transmits it to the computer module. The computer module further processes this data, analyzes the strain distribution along the fiber, and generates visualizations that reveal the impact of fluid migration on core stress.

[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 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.

[0009] A CO2-brine displacement monitoring system and method based on nuclear magnetic resonance and distributed optical fiber sensing includes the following steps: S1. Select a suitable experimental core sample, spirally wrap the optical fiber around the outer surface of the core, and install the core in a nuclear magnetic holder after packaging.

[0010] 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.

[0011] S3. Perform T2 / saturation measurements at regular injection volume (PV) intervals during the displacement process.

[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; the computer module outputs the strain change after analysis and processing.

[0013] S6. Process and analyze strain and NMR data, and compare the migration fronts obtained from the two.

[0014] Furthermore, the method comprises the following steps: S1. Core preparation: Select suitable core samples, clean them, and place them in a drying oven for complete drying. Distributed fiber optic sensors are placed on the outer surface of the core along the fluid migration path, tightly fitting them using a spiral winding method and fixed with two-component epoxy resin to ensure sensor stability and measurement accuracy. After curing for 24 hours, heat shrink tubing is used to seal the cylindrical surface of the core and both ends of the plunger to isolate the confining pressure medium and improve system stability.

[0015] S2. Confining Pressure Control: Install the encapsulated core into the core holder and ensure proper connection with the injection system and fiber optic interrogator. Connect the confining pressure control system and inject 15 MPa of nitrogen into the confining pressure interlayer of the core holder using the first injection pump to establish a stable confining pressure and ensure that the confining pressure remains constant during the experiment.

[0016] S3. Back pressure control: Inject CO2 into the core inside the nuclear magnetic holder through the fourth injection pump in a constant pressure mode of 8 MPa. After maintaining the pressure at a stable level for 8 h, perform a T2 / saturation measurement as the background measurement of the entire experiment.

[0017] S3. Core flooding stage: Use a second injection pump to inject brine into the core at a constant flow rate of 0.1 ml / min to displace the CO2 in the core pores. Throughout the flooding process, carefully monitor the inlet and outlet pressure sensors to ensure they do not exceed the confining pressure to ensure system stability.

[0018] S4. Distributed fiber optic strain and NMR monitoring: During the displacement experiments, NMR saturation measurements were performed at regular injection volume (PV) intervals using distributed fiber optic sensors placed on the outer surface of the core and a low-field nuclear magnetic resonance (NMR) testing system. Distributed fiber optic strain was continuously recorded in real time throughout the displacement process.

[0019] S5. Fiber optic strain and NMR signal analysis: High-resolution strain data is extracted through a fiber optic demodulator and analyzed and processed by a computer module to obtain the strain distribution and strain-time curve to identify the position of the CO2 / brine migration front during the displacement process. At the same time, NMR is used to obtain fluid distribution data inside the core and compare it with DOFS strain data to improve the accuracy of displacement front identification.

[0020] S6. Migration Front Identification: Since the sample is dry core, brine continuously displaces CO2 during the displacement process and gradually infiltrates the core. Therefore, the location where the water volume gradually decreases and eventually approaches zero in the NMR saturation measurement results can be considered the displacement front. As the injection volume increases, the strain response gradually propagates deeper into the core, forming a series of decreasing strain curves along the length of the core. When the strain curve decays to a minimum and stabilizes, the corresponding location can be considered the migration front of deionized water, indicating that the area has reached the displacement boundary.

[0021] Through the above technical solution, the present invention has the following beneficial effects: NMR can non-destructively and accurately measure the distribution and dynamic changes of pore fluids, making it particularly suitable for capturing the evolution of the displacement front. DOFS, on the other hand, can continuously measure strain, pressure, and temperature along the entire length of the optical fiber, sensitively reflecting the stress response of rocks and changes in pore structure. The combined application of these two techniques not only overcomes the shortcomings of individual methods but also provides multi-parameter coupled information, offering a new technical approach for studying CO2-brine two-phase fluid migration, displacement mechanisms, and reservoir deformation characteristics.

[0022] This invention combines nuclear magnetic resonance (NMR) and distributed fiber optic sensing (DOFS) technologies to effectively address the difficulty of simultaneously capturing fluid saturation distribution and rock strain changes in core flooding experiments using a single monitoring method. NMR accurately measures fluid saturation and the dynamic evolution of the flooding front, while DOFS provides high-resolution strain monitoring, enabling real-time capture of rock stress responses caused by fluid migration. Through combined analysis of these two technologies, the invention can accurately identify the CO2 / brine migration front and reveal key features of the fluid-solid coupling effect during the flooding process. This technology significantly improves the accuracy and sensitivity of fluid migration front monitoring in CO2 geological storage experiments, enabling simultaneous acquisition of fluid distribution and mechanical response information, providing more comprehensive data support for flooding mechanism research. Compared to traditional monitoring methods, this invention offers high spatial resolution, real-time monitoring capabilities, and precise data fusion analysis. It effectively overcomes the shortcomings of single technologies in analyzing CO2 migration pathways and reservoir deformation, providing strong technical support for CO2 storage safety assessment and optimization design. In addition, this method is suitable for monitoring other geological fluid migration processes, such as groundwater seepage, oil and gas development, and reservoir mechanical response research, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings described herein are for illustrative 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 merely illustrative and are used to facilitate understanding of the present invention. They are not intended to specifically limit 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 specific circumstances under the guidance of the present invention.

[0024] Figure 1 Schematic diagram of the CO2-brine displacement monitoring system based on nuclear magnetic resonance and distributed optical fiber sensing.

[0025] Figure 2 This is the displacement boundary result map.

[0026] In the figure: 1. N2 gas cylinder, 2. first valve, 3. first injection pump, 4. CO2 gas cylinder, 5. second injection pump, 6. second valve, 7. saline bottle, 8. third valve, 9. third injection pump, 10. tee, 11. fourth valve, 12. first pressure sensor, 13. distributed optical fiber sensor, 14. core sample, 15. core holder, 16. low-field nuclear magnetic resonance test system, 17. fifth valve, 18. vacuum pump, 19. sixth valve, 20. piston container, 21. seventh valve, 22. fourth injection pump, 23. deionized water bottle, 24. optical fiber demodulator, 25. computer module, 26. second pressure sensor. DETAILED DESCRIPTION

[0027] In order to enable people 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.

[0028] Figure 1 The system, based on nuclear magnetic resonance and distributed fiber optic sensing, includes a core displacement unit, a confining pressure control unit, a back pressure control unit, a nuclear magnetic resonance monitoring unit, and a distributed fiber optic strain monitoring unit. The distributed fiber optic strain monitoring unit includes a signal transmission module, a fiber optic demodulator, and a computer module.

[0029] The core flooding unit includes a second injection pump 5, a CO2 cylinder 4, a second valve 6, a first pressure sensor 6, a brine bottle 7, a third valve 8, a fourth valve 11, a three-way fitting 10, a third injection pump 9, a core sample 14, and a core holder 15. The second injection pump 5 is used to inject CO2 gas, and the third injection pump 9 is used to inject brine. Both are connected to the pore pressure injection port of the core holder. The nuclear magnetic resonance monitoring unit includes a low-field nuclear magnetic resonance test system 16, which can monitor changes in fluid distribution during the flooding process in real time. The confining pressure control unit includes an N2 cylinder 1, a second valve 2, and a first injection pump 3. The distributed optical fiber strain monitoring unit includes a distributed optical fiber 13, an optical fiber interrogator 24, and a computer module 25. The distributed optical fiber transmits the detected optical signal to the optical fiber interrogator 24. The optical fiber interrogator 24 modulates the received signal, extracts key information such as amplitude, frequency, and phase, and processes it through the computer module 25. The computer module 25 can display the excitation intensity corresponding to different locations on the optical fiber in real time. In the entire system, the computer module 25 is connected to the fiber optic interrogator 24, and the fiber optic interrogator 24 is connected to the distributed optical fiber.

[0030] In practice, taking a water-displacement experiment as an example, after evacuating all air from the pipeline using vacuum pump 18, the confining pressure control unit first uses the first syringe pump 3 to draw gas from an N2 cylinder and inject 15 MPa of confining pressure into the confining pressure layer of the core holder. This pressure must be greater than the maximum pore pressure during the experiment, and the confining pressure medium should be a hydrogen-free medium, such as nitrogen or floating oil. The fourth syringe pump 22 in the backpressure control system then pumps the core to the planned backpressure, such as 8 MPa. The backpressure medium is consistent with the displaced medium.

[0031] After the above preparations are completed, the low-field nuclear magnetic resonance monitoring system 16 and the optical fiber strain signal acquisition system are turned on to start data acquisition, and then the water-displacement gas experiment is started through the core displacement unit.

[0032] Ensure that the second valve 6 is closed, draw brine from the brine bottle 7 using the third syringe pump, open the third valve 8 and the fourth valve 11, and inject the brine into the dry core sample 14 in the core holder 15 to conduct the displacement experiment. During the injection process, monitor the injection pressure using the first pressure sensor 12 to ensure that the pore pressure does not exceed the set confining pressure.

[0033] During the entire displacement process, the low-field nuclear magnetic resonance (LNFNMR) performs a T2\ saturation measurement every time a certain volume of brine is injected, or by setting an acquisition plan, multiple measurement plans are used for continuous data acquisition.

[0034] Simultaneously, throughout this entire process, distributed optical fiber monitors stress distribution in real time, focusing on recording optical fiber strain data from the start of displacement to breakthrough, observing strain changes at the CO2 / brine interface. An optical fiber 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.

[0035] When the above technical solution is used, the following steps are included: 1. Core Preparation First, a suitable core sample 14 is selected, cleaned, and thoroughly dried in a drying oven. Distributed fiber optic sensors 13 are then placed on the outer surface of the core along the fluid migration path. A spiral winding method is used to ensure a tight fit between the fibers, and two-component epoxy resin is used to secure the core to improve measurement stability and accuracy. After securing, the core is left to rest for 24 hours to ensure the epoxy resin is fully cured. Finally, heat shrink tubing is used to seal the cylindrical surface of the core and both ends of the plunger to isolate the confining pressure medium and enhance system stability.

[0036] 2. Application of confining pressure The encapsulated core sample was installed in the core holder 15 and confirmed to be properly connected to the injection system and fiber optic interrogator. Subsequently, 15 MPa of nitrogen was injected into the confining pressure layer of the core holder via the first syringe pump to establish a stable confining pressure. This pressure was maintained constant throughout the experiment to ensure controllable experimental conditions.

[0037] 3. Backpressure Setting and Baseline Measurement Before the formal displacement experiment, CO2 was first injected into the core inside the NMR holder at a constant pressure of 8 MPa using the fourth syringe pump 22. This pressure was maintained for 8 hours to ensure that the fluid in the core reached an initial equilibrium state. Subsequently, a T2 relaxation time and saturation measurement were performed to obtain baseline data for the experiment.

[0038] 4. CO2 flooding experiment A third syringe pump (9) was used to inject brine into the core bottom at a constant flow rate of 0.1 ml / min, gradually displacing the CO2 in the core pores. Throughout the displacement process, the inlet and outlet pressures were monitored in real time to ensure that the pore pressure remained below the confining pressure, avoiding system overpressure and maintaining the stability of the displacement process.

[0039] 5. Real-time monitoring During the displacement experiment, the distributed optical fiber sensor 13 and the low-field nuclear magnetic resonance system 16 were used for synchronous monitoring: (1) Distributed fiber optic sensor (DOFS): During the entire displacement process, the strain distribution on the core surface is recorded in real time to reflect the stress impact of fluid migration on the core.

[0040] (2) Low-field nuclear magnetic resonance (NMR): NMR saturation measurements are performed every certain injection volume (PV) to obtain the dynamic changes in fluid distribution inside the core.

[0041] 6. Data Analysis Strain data from the fiber optic interrogator 24 is processed by the computer module 25 to analyze the strain distribution and its temporal changes, identifying the locations of sudden strain changes during the CO2 / brine displacement process. Water volume distribution data measured by NMR is combined with the DOFS strain signal for comparative analysis, improving the accuracy of identifying the migration front.

[0042] 7. Determination of displacement front During the experiment, since the core was initially dry, the injected brine gradually displaced the CO2 and infiltrated the core. In the NMR saturation measurements, the point where the water volume gradually decreased and eventually approached zero can be considered the displacement front. Simultaneously, the strain response measured by DOFS gradually propagated deeper into the core, forming a decreasing strain curve along the length of the core. When the strain curve decayed to a minimum and stabilized, the corresponding position can be considered the migration front of deionized water, indicating that the region had reached the displacement boundary. This experimental method accurately captures the fluid migration characteristics during the CO2 / brine displacement process and quantifies the fluid-solid coupling effect, providing reliable data support for CO2 geological storage and multiphase fluid displacement research.

[0043] The above embodiments are only used to illustrate the present invention. Any equivalent transformations and improvements based on the technical solution of the present invention should not be excluded from the protection scope of the present invention.

Claims

1. A CO2 geological storage fluid migration monitoring system based on combined nuclear magnetic resonance and distributed fiber optic sensing is characterized by: The system includes a core displacement unit, a confining pressure control unit, a back pressure control unit, a low-field nuclear magnetic resonance monitoring unit, and a distributed optical fiber strain monitoring unit; In the core displacement unit, the brine bottle (7) is connected to the tee (10) through the third valve (8), the third injection pump (9) and the fourth valve (11) in sequence, and the CO2 gas cylinder (4) is connected to the tee (10) through the second injection pump (5) and the second valve (6); the tee (10) is then connected to the core holder (15) through the fourth valve (11) and the first pressure sensor (12); In the back pressure control unit, the deionized water bottle (23) is connected to the outlet of the core holder through the fourth injection pump (22), the seventh valve (21), the piston container (20), the sixth valve (19), and the second pressure sensor (26) in sequence; In the confining pressure control unit, the N2 gas cylinder (1) passes through the first injection pump (3), the first valve (2), and is connected to the core holder (17); The distributed optical fiber strain monitoring unit comprises a distributed optical fiber (13), an optical fiber demodulator (24), and a computer module (25); the distributed optical fiber (13) is spirally wound and fixed on a rock core.

2. The operating method of the CO2 geological storage fluid migration monitoring system based on nuclear magnetic resonance and distributed optical fiber sensing combined monitoring according to claim 1 is characterized in that: The following steps are involved: S1. Core Preparation Distributed optical fiber sensors (13) are arranged on the outer surface of the core along the fluid migration path; S2. Application of confining pressure The encapsulated core sample is mounted in the core holder (15), and nitrogen gas at 15 MPa is injected into the confining pressure interlayer of the core holder (15) through the first injection pump (3), and is kept constant during the entire experiment; S3. Backpressure Setting and Baseline Measurement Before the formal displacement experiment, CO2 was first injected into the core inside the nuclear magnetic holder through the fourth injection pump (22) at a constant pressure of 8 MPa, and the pressure was kept stable to allow the fluid in the core to reach an initial equilibrium state; a T2 relaxation time measurement and saturation measurement were performed to obtain the baseline data of the experiment; S4. CO2 flooding experiment A third injection pump (9) is used to inject brine into the bottom of the core at a constant flow rate to gradually displace the CO2 in the core pores; during the entire displacement process, the core inlet and outlet pressures are monitored in real time; S5. Synchronous Monitoring During the displacement experiment, a distributed fiber optic sensor (13) and a low-field nuclear magnetic resonance system (16) were used for synchronous monitoring: Distributed fiber optic sensor: During the entire displacement process, it records the strain distribution on the core surface in real time to reflect the stress impact of fluid migration on the core; Low-field nuclear magnetic resonance: NMR saturation measurements are performed every certain injection volume to obtain the dynamic changes in fluid distribution inside the core; S6. Data Analysis The strain data from the fiber optic demodulator (24) is processed by a computer module (25) to analyze the strain distribution and its changes over time, and determine the location of the strain mutation during the CO2 / brine displacement process; the water volume distribution data measured by NMR is combined with the DOFS strain signal for comparison and analysis to identify the migration front.

3. The application of the CO2 geological storage fluid migration monitoring system based on combined nuclear magnetic resonance and distributed optical fiber sensing according to claim 1, characterized in that: The system is used in CO2 displacement of brine, CO2 leakage monitoring, mineral storage, and fluid migration analysis in aquiferous reservoirs.

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