CT-fiber optic combined testing system and testing method for CO2 infiltration process in fractured caprock

Abstract

The application discloses a CT-fiber combined testing system and testing method for CO2 imbibition process in fractured caprock, and belongs to the technical field of caprock sealing property evaluation for CO2 geological storage. The method uses a fractured core as a sample, and a distributed fiber ring and a spiral are arranged on the outer surface of the core and are subjected to vacuum water saturation treatment. The method synchronously carries out a high-temperature and high-pressure seepage test and CT scanning under a plurality of confining pressures, establishes a calibration relationship among the confining pressure, the fiber strain and the crack opening degree and the permeability, calculates the permeability change by combining the flow and the pressure difference, and quantitatively evaluates the leakage process by monitoring and characterizing CO2 migration and crack deformation through the CT and the fiber. The method obtains multi-source coupling information such as the confining pressure, the crack opening degree, the strain response, the permeability evolution and CO2 breakthrough and leakage behavior under the same core and unified loading path, has high time and space resolution, complete process characterization, and can directly support fine evaluation of the sealing property and leakage risk of the CO2 storage caprock, and provides an experimental basis for caprock integrity evaluation and field fiber monitoring calibration for CO2 storage.

Description

Technical Field

[0001] This invention belongs to the field of carbon dioxide (CO2) geological storage and geotechnical engineering testing technology. Specifically, it relates to a test method for CO2 seepage and leakage process in fractured caprock based on the combined application of X-ray computed tomography (CT) and distributed fiber optic strain monitoring. This method is used to perform fine characterization and quantitative analysis of the evolution of caprock fracture aperture, permeability changes and CO2 seepage-leakage behavior under in-situ temperature and pressure and confining pressure conditions. Background Technology

[0002] With the advancement of CO2 geological storage projects, accurately evaluating the seepage-mechanical response of fractured caprock under near-in-situ temperature and pressure conditions and multi-field coupling has become a crucial prerequisite for ensuring storage safety. The development of testing methods such as high-temperature, high-pressure core seepage devices, X-ray CT imaging, and distributed fiber optic strain monitoring has provided the necessary technical foundation for reconstructing confining pressure, pore pressure, and CO2 multiphase flow conditions in the laboratory, and for real-time observation of fracture aperture changes and seepage channel evolution. However, most existing equipment and methods are used in a decentralized manner, lacking an integrated testing method for collaboratively acquiring multi-source information on the same core and loading path.

[0003] Regarding the characterization of the overall hydraulic sealing performance of fracture caprock and the fracture-seepage-leakage process, current engineering and scientific research practices primarily employ conventional triaxial permeability tests, breakthrough tests, and core displacement tests. Equivalent permeability or a single breakthrough pressure index is obtained through inversion of the inlet-outlet pressure difference and steady-state flow rate. This is supplemented by pre- and post-construction CT / μCT scans under various operating conditions, as well as point-based monitoring methods such as strain gauges and acoustic emission to qualitatively observe fracture morphology and damage evolution. Simultaneously, in studies of CO2 absorption and leakage behavior, the changes in inlet / outlet pressure, flow rate, and concentration over time are often relied upon to determine whether a breakthrough has occurred, the breakthrough time, and the total leakage amount. While the aforementioned methods can macroscopically evaluate the caprock's impermeability and leakage risk, they generally suffer from limited spatiotemporal resolution and fragmented information. On the one hand, CT imaging is mostly a before-and-after comparison of a few static conditions, making it difficult to continuously capture the dynamic evolution of fracture aperture and CO2 phase distribution throughout the entire process of confining pressure adjustment and CO2 injection. Furthermore, there is a lack of clear quantitative calibration relationship between fracture geometric parameters and inverted permeability. On the other hand, there is a lack of precise spatial registration between surface strain or acoustic emission signals and the internal fracture network. Although distributed fiber optic monitoring has been applied in engineering structures such as slopes, tunnels, and well shafts, it has not yet achieved synchronous and coaxial joint characterization with three-dimensional CT fracture morphology and CO2 migration process at the core scale. This makes it difficult to answer key scientific questions such as "where does leakage preferentially occur, under what confining pressure and fracture conditions does it form a dominant channel, and how does the time-varying fracture aperture feed back to leakage flux and equivalent permeability?" Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a test method based on a CT-fiber optic joint testing system for CO2 seepage and absorption processes in fractured caprock. Under a unified core and loading path, this joint test method organically couples high-resolution CT fracture aperture field, distributed fiber optic circumferential strain field, and CO2 seepage-leakage observation. The system establishes quantitative calibration and process correlation between confining pressure, strain, fracture aperture, equivalent permeability, breakthrough time, and leakage flux. This method breaks through the traditional coarse-grained characterization that only considers overall permeability and breakthrough pressure, and achieves high spatiotemporal resolution and strong coupling characterization of CO2 seepage and leakage behavior in fractured caprock, which has significant scientific value and engineering application prospects.

[0005] The technical solution adopted in this invention is: a test method for a CT-fiber optic joint testing system for CO2 infiltration and adsorption processes in fractured caprock, comprising the following steps:

[0006] S1, CT-fiber optic data coupling method for crack aperture identification:

[0007] S1.1: Record the geometric morphology of the crack in the sample as the initial input for subsequent CT image and fiber optic data registration;

[0008] S1.2: To bring the sample to a state of deionized water saturation;

[0009] S1.3: Circular fiber rings are arranged along the axial direction at a spacing Δz on the outer surface of the core, and spiral fiber rings are arranged with a pitch P, so that a fiber layout is formed within the working section length L of the sample, where the circular rings and spiral rings are covered together.

[0010] S1.4: The deionized water-saturated fractured core, wound with distributed optical fibers, is loaded into a high-temperature, high-pressure core clamping device suitable for CT scanning, and a sealed connection is completed. The inlet pressure P in Export pressure P out and confining pressure P c ;

[0011] S1.5: Inject confining pressure medium into the confining pressure chamber using a confining pressure pump, gradually increasing the confining pressure P in multiple levels. c (i) When the confining pressure P c (i) After reaching steady state, the position z along the test axis is acquired using a distributed fiber optic demodulator. f Circumferential strain field ε θ (z f ;P c (i)) thus obtaining the confining pressure-circumferential strain response dataset ε θ (i)={ε θ (z f ; P c (i))}; where z fLet ε be the axial coordinate of the fiber optic measuring point along the sample axis. θ For circumferential strain;

[0012] S1.6: Under confining pressure P c (i) Perform CT scans on the core under steady-state conditions to obtain the voxel grayscale field G(x, y, z). CT ; P c (i)), where (x, y, z) CT Let ) represent the spatial coordinates in the CT reconstructed volumetric data coordinate system O-xyz, and then perform crack image segmentation on it to extract the crack voxel set C(i)={(x j , y j , z j )}, where (x j , y j , z j Let be the three-dimensional coordinates of the j-th fracture voxel, and then calculate the fracture aperture field w(x, y, z; P) based on the fracture voxel set. c (i)), and convert the set of crack voxels C(i) into the surface crack location C according to the coordinates of the outer surface of the sample cylinder. s (i)={(θ j , z j )}, used to determine the position Z of the circumferential fiber optic measuring point. f ={z k Establish spatial registration relationships, where z k Let k be the axial coordinate of the k-th fiber optic measuring point;

[0013] S1.7: Based on confining pressure variation P c (i) Fiber strain distribution ε θ (z ; P c (i) and CT-identified crack aperture w(x,y,z; P c (i) Construct a three-element coupled calibration relationship of confining pressure-fiber strain-crack aperture, and further establish an equivalent permeability-crack aperture calibration relationship by combining crack channel changes, so as to provide the real crack morphology input for subsequent CO2 permeation and leakage assessment.

[0014] S2. Permeability calculation of fractured caprock core:

[0015] S2.1: Under confining pressure P c (i) Apply inlet pressure P to the core. in (i) and the outlet pressure P out (i) Establish a stable unidirectional flow condition so that deionized water flows unidirectionally along the axial direction inside the core.

[0016] S2.2: When the flow rate reaches steady state, record the volumetric flow rate Q(i) and inlet pressure P.in (i) Export pressure P out (i) and the corresponding confining pressure P c (i), the pressure difference ΔP(i) is calculated using equation (1), which is:

[0017] (1)

[0018] S2.3: Under confining pressure P c (i) Under the action of the circumferential strain field ε collected along the core axis, θ (z ; P c (i)), the average circumferential strain ε of the test working section under this confining pressure level is calculated by equation (2). θ (P c (i)):

[0019] (2)

[0020] In the formula: ͞͞ε θ (P c (i) represents the confining pressure P c (i) The average circumferential strain of the lower test working section; ε θ (z ; P c (i) represents the confining pressure P c (i) The circumferential strain value is obtained by distributed optical fiber at position z along the axial direction of the specimen; z is the axial coordinate along the specimen axis; z0 is the starting position of the test working section in the axial coordinate; L is the effective length of the test working section (the integral interval [z0, z0+L] corresponds to the effective measurement section of the optical fiber, which is used to avoid the influence of the end); dz is the infinitesimal element of the axial coordinate.

[0021] S2.4: For the specimen under confining pressure P c (i) The effective flow cross-sectional area under the action is corrected in real time, and the corrected cross-sectional area A(P) is calculated by equation (3). c (i)):

[0022] (3)

[0023] In equation (3): A0 is the geometric cross-sectional area of ​​the specimen when no confining pressure is applied;

[0024] S2.5: Based on the modified cross-sectional area A(P) c (i) ), sample length l, steady-state flow rate Q(i), pressure difference ΔP(i) and fluid viscosity μ(i) under test temperature and pressure conditions, the equivalent permeability k(i) under confining pressure conditions is calculated using equation (4):

[0025] (4)

[0026] S2.6: Repeat steps S2.1-S2.5 at each confining pressure P c (i) The permeability sequence {k(i)} is obtained and used to construct a pressure-sensitive permeability curve, thereby reflecting the permeability characteristics of the fracture cap rock under different pressure levels;

[0027] S3. Characterization of CO2 infiltration process based on CT-fiber optic joint monitoring:

[0028] S3.1: Close the deionized water loop and maintain the core under confining pressure P. c (i), under temperature T(i), it is in a state of deionized water saturation;

[0029] S3.2: Connect the core inlet to the CO2 supply system and establish the inlet pressure P. in (i) and the outlet pressure P out (i) forms a stable pressure difference ΔP(i), causing CO2 to be contained within the confining pressure P. c (i) Core of fractured cap rock saturated with deionized water that has been unidirectionally absorbed along the lower axial direction;

[0030] S3.3: During the CO2 infiltration process, the core was subjected to multi-time CT scans at preset time intervals to obtain the t values ​​at different times. k The voxel grayscale field GCO2(x, y, z; t) is shown below. k , P c (i) By using phase segmentation and threshold segmentation methods to distinguish between the CO2 phase and the water phase, the three-dimensional distribution of the CO2-occupied area is extracted to obtain a spatial evolution image sequence of the CO2 infiltration process;

[0031] S3.4: During the entire CO2 permeation process, the circumferential strain field ε along the axial position z of the sample is continuously acquired using a distributed fiber optic demodulator. θ ' (z, t; P c (i)), and according to the CT scan time t k The fiber optic strain data were synchronously processed to obtain the circumferential strain field ε corresponding to each CT working condition. θ ' (z, t; P c (i));

[0032] S3.5: Based on the established crack surface location C s (i)={(θ j , z j )} and the fiber optic measuring point position Z f ={z kThe spatial registration relationship is used to project the CO2-occupied area identified by CT at each time point onto the outer surface of the sample, obtaining the position of the surface CO2 leading edge and the distribution of preferential CO2 channels within the crack, and comparing it with the fiber optic circumferential strain distribution ε at the same time point. θ ' (z, t; P c (i) Perform corresponding analysis;

[0033] S3.6: By comparing different times t k The evolution of the CO2 front position, CO2 connectivity region within the fracture, and the peak position and amplitude of fiber optic circumferential strain in the CT images characterizes the CO2 absorption path, propagation speed, dominant channels, and fracture aperture variation characteristics in the fracture caprock core.

[0034] S3.7: Repeat steps S3.1-S3.6 under different confining pressures P c (i) CO2 infiltration tests were carried out under different inlet pressure differences ΔP(i) or different crack geometry conditions. The spatial distribution pattern of CO2 identified by CT and the circumferential strain response characteristics of optical fiber under different working conditions were compared. A CT-fiber joint identification spectrum of the influence of confining pressure and crack structure on CO2 infiltration behavior was constructed.

[0035] S4. Monitoring and Calculation of CO2 Leakage Process and Leakage Flux:

[0036] S4.1: Maintain inlet CO2 pressure P in (i) Back pressure at the outlet end P out (i) and confining pressure P c (i) Once the core is basically stable, continue injecting CO2 into the core inlet. When a CO2 signal is detected at the outlet or downstream monitoring unit, record that moment as the breakthrough time t. b (i) is the leakage initiation time under the confining pressure condition;

[0037] S4.2: Before and after the CO2 breakthrough, continuously record the outlet CO2 flow rate q using the outlet gas flow meter. out (t; P c (i)), calculate the CO2 displacement pressure difference under this operating condition using equation (1);

[0038] S4.3: During the CO2 leakage process, according to the preset time series t k Multi-time CT scans were performed on the core samples to obtain the voxel grayscale field GCO2(x, y, z; t). k , P c (i) By phase segmentation and threshold segmentation, the CO2 phase is separated from the water phase, and the three-dimensional distribution of CO2 enrichment area, through crack and leakage channel is extracted.

[0039] S4.4: During the entire CO2 leakage process, a distributed fiber optic demodulator was used to synchronously acquire the circumferential strain field ε at position z along the core axis. θ ' (z, t; P c (i)), and the set of crack surface locations C s (i) and the set of fiber optic measurement point locations Z f Spatial registration is performed to map the strain anomaly zone during the leakage stage to the leakage channel identified by CT scan, in order to identify the main channel of CO2 leakage and its dynamic evolution.

[0040] S4.5: Breakthrough moment t b (i) Starting from this point, the confining pressure P is obtained by integrating the outlet flow rate curve over time using equation (5). c (i) Cumulative CO2 leakage M under the given conditions leak (t; P c (i)), Formula (5) is:

[0041] (5)

[0042] In the formula: q out (τ;P c (i) is under confining pressure P c (i) Under operating conditions, the outlet CO2 flow rate at time τ is measured by the gas flow meter at the outlet end; t b (i) represents the time when the downstream monitoring unit first detects the breakthrough under this confining pressure condition; is the integral variable, and t is any observation time;

[0043] S4.6: Select the observation end time t under this working condition end At this point, the cumulative leakage is M. leak (t end ; P c (i)); combined with confining pressure P c (i) Corrected cross-sectional area A(P) under the condition c (i)), calculate the time interval [t] using equation (6). b (i),t end The average leakage flux on the surface is given by formula (6):

[0044] (6)

[0045] S4.7: Under different confining pressures P c (i), under different CO2 inlet pressure differences ΔP(i) and different fracture geometries, repeat steps S4.1-S4.6 to obtain the breakthrough time t under each condition. b (i) Cumulative leakage M leak (t; P c (i) and average leakage flux qleak (P c (i)).

[0046] By combining the leakage channel morphology identified by CT and the abnormal distribution of circumferential strain in the optical fiber, the influence of confining pressure, pressure difference and crack structure on CO2 leakage behavior was compared and analyzed.

[0047] Furthermore, the crack geometry in step S1.1 includes the number of cracks, their location, orientation, and initial aperture range.

[0048] Furthermore, in step S1.2, the method for achieving deionized water saturation of the sample is as follows: the sample is placed in a vacuum saturation device, and negative pressure is drawn to expel the air from the pores; deionized water is slowly introduced under negative pressure and the sample is submerged; after restoring normal pressure, the sample is soaked for more than 24 hours; the negative pressure is -5 to -10 kPa.

[0049] Furthermore, heat- and pressure-resistant epoxy adhesive or high-temperature curing adhesive is used to bond the optical fiber in dots or lines along the spiral path to fix it. After curing, a reliable mechanical coupling is formed between the optical fiber and the outer surface of the core to ensure that the deformation of the core surface is effectively transmitted to the optical fiber.

[0050] An experimental method based on a CT-fiber optic joint testing system for CO2 seepage in fractured caprock. This method uses fractured caprock cores as samples. After vacuum saturation, distributed fiber optic circumferential and helical loops are deployed on the outer surface of the core. High-temperature, high-pressure seepage tests and CT scans are conducted simultaneously under multi-level confining pressure conditions to obtain fracture aperture distribution, pore structure changes, and the circumferential strain response of the core surface. A calibration relationship is established between confining pressure, fiber optic strain, fracture aperture, and equivalent permeability. Furthermore, the effective flow cross-sectional area of ​​the core under confining pressure is corrected using the average circumferential strain measured by the fiber optics. Permeability changes under different confining pressure levels are calculated by combining steady-state flow rate and pressure difference. During the CO2 seepage stage, multi-time CT imaging and continuous fiber optic monitoring are used to characterize the CO2 front advancement, dominant seepage channel formation, and fracture deformation response. During the CO2 leakage stage, the cumulative leakage and average leakage intensity are calculated using the outlet flow rate versus time curve, achieving a quantitative evaluation of the leakage process. This invention can acquire multi-source coupled information such as confining pressure, fracture aperture, strain response, permeability evolution, and carbon dioxide breakthrough and leakage behavior under the same core and unified loading path. Compared with traditional experimental methods that only provide overall permeability or breakthrough pressure, it has the technical advantages of high spatiotemporal resolution, complete process characterization, and can directly support the detailed evaluation of the sealing performance and leakage risk of carbon dioxide storage caprock. It provides experimental basis for the assessment of caprock integrity and the calibration of on-site fiber optic monitoring in CO2 storage sites.

[0051] An experimental method for a CT-fiber optic joint testing system based on the CO2 infiltration process in fractured caprock is provided.

[0052] Beneficial Effects: The advantages of this invention lie in its testing method. By constructing a joint CT imaging and distributed fiber optic strain testing platform on the same fractured caprock core, it simultaneously acquires multi-source coupled data such as confining pressure, fracture aperture, circumferential strain, permeability, and CO2 breakthrough and leakage processes under a unified loading path. Unlike existing testing methods that only provide a single permeability or breakthrough pressure, this method can output a complete dataset of "confining pressure-fracture aperture-strain-permeability-leakage behavior." Under multiple confining pressures, different fracture geometries, and various injection pressure differentials, this method can form a set of test index systems with good comparability and repeatability, such as equivalent permeability curves, fracture aperture evolution, breakthrough time, etc. The cumulative leakage rate and average leakage intensity provide quantitative basis for screening the optimal caprock lithology, fracture state, and injection strategy. Simultaneously, by utilizing CT to identify CO2 enrichment zones and interconnected channels, and fiber optics for high-resolution monitoring of local strain anomalies, this method can detect early leakage and localized dominant channels before significant changes in outlet flow, significantly improving the ability to identify caprock sealing degradation. Based on the above multi-index test data, a dynamic evaluation index system comprehensively considering fracture geometry, confining pressure, and leakage process can be further constructed. This provides reliable experimental support for establishing new caprock sealing evaluation standards and optimizing CO2 storage design, possessing significant scientific value and promising engineering applications. Attached Figure Description

[0053] To more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0054] Figure 1 This is a schematic diagram of the CT-fiber optic joint testing system for CO2 infiltration and absorption processes in fractured caprock of the present invention.

[0055] Figure 2 This is a schematic diagram illustrating the effect of layer-by-layer porosity on the fracture cap rock of the present invention.

[0056] Figure 3 This is a schematic diagram illustrating the layer-by-layer porosity effect of CO2 infiltration at different stages in the fracture cap rock according to the present invention.

[0057] Figure 4 This is a three-dimensional CT scan illustration of different displacement stages of CO2 infiltration and adsorption in the fractured cap rock according to the present invention.

[0058] Explanation of reference numerals in the attached figures:

[0059] 1. High-temperature confining pressure loading system; 11. Confining pressure pump; 12. Silicone oil intermediate container; 13. Ball valve; 14. High-temperature oil bath;

[0060] 2. Fluid injection system; 21. Fluid pressure pump; 22. Six-way valve; 23. CO2 intermediate container; 24. Deionized water intermediate container;

[0061] 3. CT scanning system; 31. X-ray source;

[0062] 4. Fiber optic monitoring system; 41. Rock holder; 42. Fiber optic sensor; 43. Fiber optic demodulator; 44. Fiber optic data acquisition computer;

[0063] 5. Temperature and pressure monitoring system; 51. Temperature sensor; 52. Pressure sensor;

[0064] 6. Back pressure valve; 7. Vacuum pump;

[0065] 8. Gas-liquid separation system; 81. Weighing balance; 82. Liquid collection bottle; 83. Gas-liquid separation container; 84. Gas flow meter. Detailed Implementation

[0066] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0067] The CT-fiber optic joint testing system based on the CO2 infiltration process in fractured caprock includes a high-temperature confining pressure loading system 1, a fluid injection system 2, a CT scanning system 3, a fiber optic monitoring system 4, a temperature and pressure monitoring system 5, and a gas-liquid separation system 8. The entire experimental system is centered around a rock holder 41, with all subsystems forming a complete closed loop around it.

[0068] In the high-temperature confining pressure loading system 1, the confining pressure pump 11 drives the pressure transmission medium in the silicone oil intermediate container 12, which is heated by the ball valve 13 and the high-temperature oil bath 14 and then injected into the rock holder 41 to provide a high-temperature and high-pressure environment for the core. The temperature sensor 51 and the pressure sensor 52 monitor the temperature and pressure of this branch in real time.

[0069] In the fluid injection system 2, the fluid pressure pump 21 can be switched to connect to the CO2 intermediate container 23 or the deionized water intermediate container 24 through the six-way valve 22, so as to pump the experimental fluid into the core of the rock holder 41 and realize the injection of different displacement media.

[0070] The X-ray source 31 of the CT scanning system 3 emits rays that penetrate the rock holder 41 to acquire CT images of the internal structure of the rock core in real time, so as to observe the porosity changes during the displacement process.

[0071] The fiber optic sensor 42 of the fiber optic monitoring system 4 is embedded inside the rock core. The collected signals such as strain and temperature are transmitted to the fiber optic demodulator 43 via fiber optic cable, and finally analyzed and stored in the fiber optic data acquisition computer 44. The outlet of the rock holder 41 is connected to the back pressure valve 6 to maintain the outlet pressure required for the experiment, and the vacuum pump 7 is used to evacuate the entire pipeline before the experiment.

[0072] The mixed fluid flowing out of the core enters the gas-liquid separation system 8, where gas-liquid separation is achieved in the gas-liquid separation container 83. The liquid is collected in the collection bottle 82 and measured by the weighing balance 81, while the gas flow is monitored by the gas flow meter 84.

[0073] This invention provides a test method for a CT-fiber optic joint testing system based on the CO2 seepage process in fractured caprock. The method uses fractured caprock cores as samples, deploying distributed fiber optic circumferential and helical loops on the outer surface of the core and subjecting it to vacuum saturation. High-temperature and high-pressure seepage tests and CT scans are simultaneously conducted under multi-level confining pressure conditions to obtain fracture aperture distribution, pore structure changes, and the circumferential strain response of the core surface, establishing a calibration relationship between confining pressure, fiber optic strain, fracture aperture, and equivalent permeability. Furthermore, the effective flow cross-sectional area of ​​the core under confining pressure is corrected using the average circumferential strain measured by the fiber optics, and permeability changes under different confining pressure levels are calculated by combining steady-state flow rate and pressure difference. During the CO2 seepage stage, multi-time CT imaging and continuous fiber optic monitoring are used to characterize the CO2 front advancement, dominant seepage channel formation, and fracture deformation response process. During the CO2 leakage stage, the cumulative leakage and average leakage intensity are calculated using the outlet flow rate versus time curve, achieving a quantitative evaluation of the leakage process. The specific process is as follows:

[0074] Crack Aperture Identification Method Based on CT-Fiber Optic Data Coupling:

[0075] Step 1: Select representative caprock materials, such as hard mudstone, shale or silty mudstone, and process them into cylindrical core samples, preferably with a diameter of 25 mm and a length of 50 mm.

[0076] Step 2: Prepare fracture structures on core samples: Through-through or partially through-through main fractures can be formed on the core by mechanical cutting, with the preferred inclination angle of the fractures relative to the core axis being 30° to 60°; or a near-natural fracture network can be induced by preloading.

[0077] Step 3: Record the geometry of the cracks, including the number, location, orientation, and initial aperture range of the cracks, as the initial input for subsequent CT image and fiber optic data registration;

[0078] Step 4: Place the sample in a vacuum saturation device and evacuate it to a negative pressure of 5-10 kPa for a certain period of time to expel the air from the pores.

[0079] Step 5: Slowly introduce deionized water under negative pressure and immerse the sample, maintaining negative pressure to promote water entry into pores and fissures;

[0080] Step 6: After restoring normal pressure, continue soaking for more than 24 hours to allow the sample to reach deionized water saturation.

[0081] Step 7: Arrange circumferential fiber rings along the axial direction on the outer surface of the core at a spacing of Δz (3-5mm), and arrange helical fiber rings with a pitch of P (3-5mm) to form a fiber layout in which circumferential and helical rings are covered within the working section length L (40-45mm) of the sample.

[0082] Step 8: Use heat- and pressure-resistant epoxy adhesive or high-temperature curing adhesive to bond the optical fiber in dots or lines along the spiral path to fix it. After curing, a reliable mechanical coupling is formed between the optical fiber and the outer surface of the core to ensure that deformation of the core surface can be effectively transmitted to the optical fiber.

[0083] Step 9: Load the deionized water-saturated fractured core, which is wrapped with distributed optical fibers, into a high-temperature, high-pressure core clamping device suitable for CT scanning, and complete the inlet end pressure P. in Export pressure P out and confining pressure P c Sealed connection;

[0084] Step 10: Inject confining pressure medium into the confining pressure chamber using a confining pressure pump, gradually increasing the confining pressure P in multiple levels. c (i) (e.g., 2MPa, 5MPa, 10MPa, etc.), when the confining pressure P c (i) After reaching steady state, the circumferential strain distribution ε along the experimental axis at position z is acquired using a distributed fiber optic demodulator. θ (z ; P c (i)) thus obtaining the confining pressure-circumferential strain response dataset ε θ (i)={ε θ (z; P c (i))};

[0085] Step 11: Under confining pressure P c (i) Perform CT scans on the core under steady-state conditions to obtain the voxel grayscale field G(x, y, z; P c(i)) and perform crack image segmentation to extract the crack voxel set C(i)={(x j , y j , z j Furthermore, the crack aperture field w(x, y, z; P) is calculated based on the crack voxel set. c (i)), and convert the set of crack voxels C(i) into the surface crack location C according to the coordinates of the outer surface of the sample cylinder. s (i)={(θ j , z j )}, used to determine the position Z of the circumferential fiber optic measuring point. f ={z k Establish spatial registration relationships;

[0086] Step 12: Based on the confining pressure change P c (i) Fiber strain distribution ε θ (z ; P c (i) and CT-identified crack aperture w(x, y, z; P c (i) Construct a three-element coupled calibration relationship of confining pressure-fiber strain-crack aperture, and further establish an equivalent permeability-crack aperture calibration relationship by combining crack channel changes, so as to provide the real crack morphology input for subsequent CO2 permeation and leakage assessment.

[0087] Methods for calculating the permeability of fractured caprock cores:

[0088] Step A1: Under confining pressure P c (i) Apply inlet pressure P to the core. in (i) and the outlet pressure P out (i) Establish a stable unidirectional flow condition so that deionized water flows unidirectionally along the axial direction inside the core.

[0089] Step A2: When the flow rate reaches steady state, record the volumetric flow rate Q(i) and inlet pressure P. in (i), Export pressure P out (i), and the corresponding confining pressure P c (i), the pressure difference ΔP(i) is calculated using equation (1), which is:

[0090] (1)

[0091] Step A3: Under confining pressure P c (i) Under the action of the circumferential strain field ε distributed along the core axis as collected in step 10. θ (z ; P c (i)), the average circumferential strain ε of the test working section under this confining pressure level is calculated by equation (2). θ (Pc (i)):

[0092] (2)

[0093] Step A4: Apply confining pressure P to the sample. c (i) The effective flow cross-sectional area under the action is corrected in real time, and the corrected cross-sectional area A(P) is calculated by equation (3). c (i)):

[0094] (3)

[0095] In equation (3): A0 is the geometric cross-sectional area of ​​the specimen when no confining pressure is applied;

[0096] Step A5: Based on the corrected cross-sectional area A(P) c (i)), sample length L, steady-state flow rate Q(i), pressure difference ΔP(i) and fluid viscosity μ(i) under test temperature and pressure conditions, the equivalent permeability k(i) under confining pressure conditions is calculated using equation (4):

[0097] (4)

[0098] Step A6: Repeat steps A1 to A5 at each confining pressure P c (i) The permeability sequence {k(i)} is obtained and used to construct a pressure-sensitive permeability curve, thereby reflecting the permeability characteristics of the fracture cap rock under different pressure levels;

[0099] A method for characterizing CO2 adsorption processes based on CT-fiber optic joint monitoring:

[0100] Step B1: Complete the deionized water permeability test in sub-method two and maintain the confining pressure P. c (i) After stabilization, close the deionized water circuit and maintain the core under confining pressure P. c (i), under temperature T(i), it is in a state of deionized water saturation;

[0101] Step B2: Connect the core inlet to the CO2 supply system and establish the inlet pressure P. in (i), with the outlet pressure P out (i) forms a stable pressure difference ΔP(i), causing CO2 to be contained within the confining pressure P. c (i) Core of fractured cap rock saturated with deionized water that has been unidirectionally absorbed along the lower axial direction;

[0102] Step B3: During the CO2 infiltration process, perform multi-time CT scans on the core at preset time intervals to obtain the t values ​​at different times. k The voxel grayscale field GCO2(x, y, z; t) is shown below. k , Pc (i) By using phase segmentation and threshold segmentation methods to distinguish between the CO2 phase and the water phase, the three-dimensional distribution of the CO2-occupied area is extracted to obtain a spatial evolution image sequence of the CO2 infiltration process;

[0103] Step B4: During the entire CO2 permeation process, the circumferential strain distribution ε along the axial position z of the sample is continuously acquired using a distributed fiber optic demodulator. θ (z, t; P c (i)), and according to the CT scan time t k The fiber optic strain data were synchronously processed to obtain the circumferential strain field ε corresponding to each CT working condition. θ (z, t; P c (i));

[0104] Step B5: Based on the crack surface location C established in step 11 of sub-method one s (i)={(θ j , z j )} and the fiber optic measuring point position Z f ={z k The spatial registration relationship is used to project the CO2-occupied area identified by CT at each time point onto the outer surface of the sample, obtaining the position of the surface CO2 leading edge and the distribution of preferential CO2 channels within the crack, and comparing it with the fiber optic circumferential strain distribution ε at the same time point. θ (z, t;P c (i) Perform corresponding analysis;

[0105] Step B6: By comparing different times t k The evolution of the CO2 front position, CO2 connectivity region within the fracture, and the peak position and amplitude of fiber optic circumferential strain in CT images can be used to characterize the CO2 infiltration path, propagation speed, dominant channels, and fracture aperture variation characteristics in the fracture caprock core, thereby achieving CT-fiber joint monitoring and qualitative and semi-quantitative characterization of the CO2 infiltration process.

[0106] Step B7: Repeat steps B1 to B6 under different confining pressures P c (i) CO2 infiltration tests were conducted under different inlet pressure differences ΔP(i) or different crack geometry conditions. The spatial distribution patterns of CO2 identified by CT and the circumferential strain response characteristics of optical fiber under different working conditions were compared. A CT-optical fiber joint identification spectrum of the influence of confining pressure and crack structure on CO2 infiltration behavior was constructed to provide process monitoring basis for subsequent CO2 leakage risk assessment.

[0107] Methods for monitoring and calculating CO2 leakage flux:

[0108] Step C1: After completing the CO2 percolation monitoring in sub-method three, maintain the inlet CO2 pressure P.in (i) Back pressure P at the outlet end out (i) and confining pressure P c (i) Once the core is basically stable, continue injecting CO2 into the core inlet. When a significant increase in CO2 concentration is detected at the outlet or downstream monitoring unit, record that moment as the breakthrough time t. b (i) is the leakage initiation time under the confining pressure condition;

[0109] Step C2: Before and after the CO2 breakthrough, continuously record the outlet CO2 flow rate q using the outlet gas flow meter. out (t; P c (i)), calculate the CO2 displacement pressure difference under this operating condition using equation (1);

[0110] Step C3: During the CO2 leakage process, according to the preset time series t k Multi-time CT scans were performed on the core samples to obtain the voxel grayscale field GCO2(x, y, z; t). k , P c (i) By phase segmentation and threshold segmentation, the CO2 phase is separated from the water phase, and the three-dimensional distribution of CO2 enrichment area, through cracks and leakage channels is extracted, and their expansion and connectivity evolution characteristics over time are observed.

[0111] Step C4: During the entire CO2 leakage process, a distributed fiber optic demodulator is used to synchronously acquire the circumferential strain distribution ε at position z along the core axis. θ (z, t; P c (i)), and combined with the set of crack surface locations C established in sub-method one. s (i) and the set of fiber optic measurement point locations Z f Spatial registration is performed to map the strain anomaly zone during the leakage stage to the leakage channel identified by CT scan, in order to identify the main channel of CO2 leakage and its dynamic evolution.

[0112] Step C5: From the breakout moment t b (i) Starting from this point, the confining pressure P is obtained by integrating the outlet flow rate curve over time using equation (5). c (i) Cumulative CO2 leakage M under the given conditions leak (t; P c (i)), Formula (5) is:

[0113] (5)

[0114] Step C6: Select the observation end time t under this working condition. end At this point, the cumulative leakage is M. leak (t end ; P c(i)). Combining the confining pressure P obtained in sub-method two c (i) Corrected cross-sectional area A(P) under the condition c (i)), calculate the time interval [t] using equation (6). b (i),t end The average leakage flux on the surface is given by formula (6):

[0115] (6)

[0116] Step C7: Under different confining pressures P c (i) Repeat steps C1 to C6 under different CO2 inlet pressure differences ΔP(i) and different crack geometries to obtain the breakthrough time t under each condition. b (i) Cumulative leakage M leak (t; P c (i)), average leakage flux q leak (P c (i)) and combined with the leakage channel morphology identified by CT and the abnormal distribution of fiber optic circumferential strain, we compared and analyzed the influence of confining pressure, pressure difference and crack structure on CO2 leakage behavior, providing a basis for the risk assessment of CO2 leakage in the cap layer and the analysis of sealing degradation.

[0117] like Figure 2 As shown, the surface porosity n of the fracture cap rock in its original state exhibits an overall increase with the slice number, accompanied by local fluctuations. This indicates that the distribution of fractures / pores in the axial direction of the sample is significantly heterogeneous: the local peak porosity in several slice segments corresponds to areas with larger fracture apertures or enhanced fracture bifurcation / connectivity. The typical CT cross-section inserted in the figure also shows that the fracture characteristics are more obvious at the corresponding slice positions, thus achieving a direct correspondence between "curve peak value - cross-sectional fracture morphology".

[0118] like Figure 3As shown, the CO2 infiltration process was divided into Stage 1–Stage 4, and the CT data for each stage were segmented into fractures / pores and statistically analyzed layer by layer to obtain the layer-by-layer porosity distribution at different stages. As the infiltration stage progresses, the layer-by-layer porosity curves show an overall upward trend, increased peak value, and a continuous expansion of the high-porosity range from local to continuous: Stage 1 shows a low porosity response only within a local slice; after entering Stages 2–3, the high-porosity segment significantly expands and its fluctuations intensify; Stage 4 reaches its highest porosity and remains within a larger slice range, indicating that the occupancy / connectivity of CO2 in the fractures and adjacent pores continuously increases, leading to an increase in effective seepage channels or a larger channel size. The repeated appearance of high-value segments at similar slice locations in the curves of different stages indicates that the spatial location of the three-dimensional dominant channels is stable and mainly controlled by the geometry of the pre-fabricated fractures and the core structure.

[0119] like Figure 4 As shown, the CT 3D reconstruction results further validated the above quantitative statistical regularities: in Stage 1, the occupied volume is mainly limited to the vicinity of the entrance or the tip of the crack; in Stage 2, it begins to extend along the direction of the main crack; in Stage 3, a more continuous through-path is formed; and in Stage 4, it continues to expand and enhance connectivity based on the main channel. The corresponding CT cross-sections also show that the crack pixel area gradually increases and connectivity is enhanced, consistent with... Figure 3 The patterns of "expansion of high porosity sections and increase in peak value" corroborate each other.

[0120] In conclusion, Figure 2 Used to characterize the axial heterogeneous distribution of original fracture cores and provide a baseline reference; Figure 3 To achieve a layer-by-layer quantitative characterization of the staged evolution of CO2 percolation; Figure 4 This study verifies the formation and expansion process of the seepage channels at the three-dimensional morphological level. Combining these three aspects establishes a consistent chain of evidence: "layer-by-layer porosity - seepage stage - three-dimensional channel evolution," providing reliable input for subsequent crack aperture / connectivity determination, breakthrough / leakage process identification, and permeability and leakage calculation.

[0121] The technical features of this invention are: (1) integrating CT imaging, distributed fiber optic strain monitoring, and high-temperature and high-pressure seepage loading on the same fracture cap rock core to achieve integrated acquisition of multi-field information; (2) achieving spatial registration and mechanical coupling between the surface circumferential strain field and the internal fracture network through the fiber optic deployment and bonding process of circumferential loops and spiral loops; (3) simultaneously acquiring the fracture aperture field, fiber optic strain field, and seepage response under multi-level confining pressure conditions, and establishing the calibration relationship of confining pressure-strain-fracture aperture-equivalent permeability; (4) proposing to use the average circumferential strain measured by optical fiber to correct the confining pressure under the action of confining pressure. (5) The effective flow cross-sectional area of ​​the core is increased to improve the accuracy of permeability inversion; (6) During the CO2 infiltration process, multi-time CT scanning and continuous fiber optic monitoring are used to characterize the spatiotemporal evolution of CO2 front advancement and dominant seepage channels; (7) By coupling the outlet flow curve with CT-fiber data, the cumulative leakage amount and average leakage flux of CO2 are quantitatively calculated, and the main leakage channel and its evolution characteristics are identified; (8) It can output permeability, breakthrough time and leakage intensity with good comparability under different confining pressure, fracture geometry and injection conditions, and provide a unified data framework for the evaluation of caprock sealing.

[0122] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the scope of the technology disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. All should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0123] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A CT-fiber optic combined testing system for CO2 infiltration and adsorption processes in fractured caprock, characterized in that, The device includes: a high-temperature confining pressure loading system (1), a fluid injection system (2), a CT scanning system (3), a fiber optic monitoring system (4), a temperature and pressure monitoring system (5), a back pressure valve (6), a vacuum pump (7), and a gas-liquid separation system (8). The high-temperature confining pressure loading system (1) includes a confining pressure pump (11), a silicone oil intermediate container (12), a ball valve (13), and a high-temperature oil bath (14); the confining pressure injection pump (11) is connected to the silicone oil intermediate container (12) through a pipeline, and the silicone oil intermediate container (12) is connected to the inlet end of the rock clamp (41) after being connected to the high-temperature oil bath (14) via the ball valve (13); the high-temperature confining pressure loading system (1) is used to provide confining pressure to the rock clamp (41) and realize pressure regulation and temperature control; The fluid supply system (2) consists of a fluid pressure pump (21), a six-way valve (22), a CO2 intermediate container (23), and a deionized water intermediate container (24). The fluid pressure pump (21) is connected to the six-way valve (22), which is connected to the CO2 intermediate container (23) and the deionized water intermediate container (24) respectively. Both the CO2 intermediate container (23) and the deionized water intermediate container (24) are connected to the inlet end of the rock holder (41). The fluid injection system (2) is used to selectively inject different media into the rock holder (41) according to the experimental requirements to achieve multi-fluid displacement, adsorption, or control testing. The CT scanning system (3) is used to perform in-situ imaging monitoring of interface migration, front advancement, local deformation or damage processes of the sample during the absorption, displacement and phase transition process. The fiber optic monitoring system (4) consists of a fiber optic demodulator (43) and a fiber optic data acquisition computer (44). The fiber optic demodulator (43) is connected to the rock holder (41), and the fiber optic data acquisition computer (44) is connected to the fiber optic demodulator (43) to realize the synchronous acquisition, transmission, display, storage and analysis of fiber optic signals. The temperature and pressure monitoring system (5) consists of a temperature sensor (51) and a pressure sensor (52). The temperature sensor (51) and the pressure sensor (52) are respectively installed on the inlet and outlet pipes of the sample testing system (3) to monitor the temperature and pressure changes of the fluid at both ends of the sample in real time. The back pressure valve (6) is installed on the outlet side pipeline of the rock holder (41) to adjust the pressure boundary conditions at the outlet end of the sample, stabilize the pressure difference between the inside and outside of the sample, and control the back pressure state during fluid breakthrough, discharge and airtightness test. The vacuum pump (7) is connected to the rock holder (41) and the external connecting pipeline, and is used to evacuate and degas the sample and pipeline before the test; The gas-liquid separation system (8) consists of an electronic balance (81), a collection bottle (82), a gas-liquid separation container (83), and a gas flow meter (84). The outlet end of the rock holder (41) is connected to the gas-liquid separation container (83) via a back pressure valve (6). The lower part of the gas-liquid separation container (83) is connected to the collection bottle (82). The collection bottle (82) is mounted on the electronic balance (81). The gas flow meter (84) is connected to the electronic balance (81) and is used to measure and display the flow rate of the gas discharged from the sample in real time. The high-temperature confining pressure loading system (1) is connected to the confining pressure chamber of the rock holder (41) through the silicone oil intermediate container (12) and connecting pipes. The fluid supply system (2) is connected to the sample inlet end of the rock holder (41) through the inlet pipe. The temperature and pressure monitoring system (5) is installed on the inlet and outlet pipes of the rock holder (41). The back pressure valve (6) is installed at the outlet end of the rock holder (41) and connected in sequence to the gas-liquid separation system (8). The vacuum pump (7) is connected to the rock holder (41) and the... Its external connecting pipeline is connected; the CT scanning system (3) is used to perform in-situ scanning of the sample inside the rock holder (41), and the fiber optic monitoring system (4) is connected to the fiber optic cable laid on the outer surface of the sample to synchronously collect the strain response signal of the sample during the confining pressure loading and fluid injection process; thus forming a CT-fiber optic joint testing system that integrates confining pressure loading, CO2 / deionized water injection, CT in-situ imaging, fiber optic strain monitoring, temperature and pressure monitoring, back pressure control, vacuum degassing and gas-liquid separation and metering.

2. The test method of the CT-fiber optic joint testing system for CO2 infiltration process in fractured caprock as described in claim 1, characterized in that, Includes the following steps: S1, CT-fiber optic data coupling method for crack aperture identification: S1.1: Record the geometric morphology of the crack in the sample as the initial input for subsequent CT image and fiber optic data registration; S1.2: To bring the sample to a state of deionized water saturation; S1.3: Circular fiber rings are arranged along the axial direction at a spacing Δz on the outer surface of the core, and spiral fiber rings are arranged with a pitch P, so that a fiber layout is formed within the working section length L of the sample, where the circular rings and spiral rings are covered together. S1.4: The deionized water-saturated fractured core, wound with distributed optical fibers, is loaded into a high-temperature, high-pressure core clamping device suitable for CT scanning, and a sealed connection is completed. The inlet pressure P in Export pressure P out and confining pressure P c ; S1.5: Inject confining pressure medium into the confining pressure chamber using a confining pressure pump, gradually increasing the confining pressure P in multiple levels. c (i) When the confining pressure P c (i) After reaching steady state, the position z along the test axis is acquired using a distributed fiber optic demodulator. f Circumferential strain field ε θ (z f ; P c (i)) thus obtaining the confining pressure-circumferential strain response dataset ε θ (i)={ε θ (z f ; P c (i))}; where z f Let ε be the axial coordinate of the fiber optic measuring point along the sample axis. θ For circumferential strain; S1.6: Under confining pressure P c (i) Perform CT scans on the core under steady-state conditions to obtain the voxel grayscale field G(x, y, z). CT ; P c (i)), where (x, y, z) CT Let ) represent the spatial coordinates in the CT reconstructed volumetric data coordinate system O-xyz, and then perform crack image segmentation on it to extract the crack voxel set C(i)={(x j , y j , z j )}, where (x j , y j , z j Let be the three-dimensional coordinates of the j-th fracture voxel, and then calculate the fracture aperture field w(x, y, z; P) based on the fracture voxel set. c (i)), and convert the set of crack voxels C(i) into the surface crack location C according to the coordinates of the outer surface of the sample cylinder. s (i)={(θ j , z j )}, used to determine the position Z of the circumferential fiber optic measuring point. f ={z k Establish spatial registration relationships, where z k Let k be the axial coordinate of the k-th fiber optic measuring point; S1.7: Based on confining pressure variation P c (i) Fiber strain distribution ε θ (z ; P c (i) and CT-identified crack aperture w(x, y, z;P) c (i) Construct a three-element coupled calibration relationship of confining pressure-fiber strain-crack aperture, and further establish an equivalent permeability-crack aperture calibration relationship by combining crack channel changes, so as to provide the real crack morphology input for subsequent CO2 permeation and leakage assessment. S2. Permeability calculation of fractured caprock core: S2.1: Under confining pressure P c (i) Apply inlet pressure P to the core. in (i) and the outlet pressure P out (i) Establish a stable unidirectional flow condition so that deionized water flows unidirectionally along the axial direction inside the core. S2.2: When the flow rate reaches steady state, record the volumetric flow rate Q(i) and inlet pressure P. in (i) Export pressure P out (i) and the corresponding confining pressure P c (i), the pressure difference ΔP(i) is calculated using equation (1), which is: （1）； S2.3: Under confining pressure P c (i) Under the action of the circumferential strain field ε collected along the core axis, θ (z ; P c (i)), the average circumferential strain ε of the test working section under this confining pressure level is calculated by equation (2). θ (P c (i)): （2）； In the formula: ͞͞ε θ (P c (i) represents the confining pressure P c (i) The average circumferential strain of the lower test working section; ε θ (z ; P c (i) represents the confining pressure P c (i) The circumferential strain value is acquired by distributed optical fiber at position z along the axial direction of the specimen; z is the axial coordinate along the axial direction of the specimen; z0 is the starting position of the test working section in the axial coordinate; L is the effective length of the test working section; dz is the infinitesimal element of the axial coordinate. S2.4: For the specimen under confining pressure P c (i) The effective flow cross-sectional area under the action is corrected in real time, and the corrected cross-sectional area A(P) is calculated by equation (3). c (i)): （3）； In equation (3): A0 is the geometric cross-sectional area of ​​the specimen when no confining pressure is applied; S2.5: Based on the modified cross-sectional area A(P) c (i) ), sample length l, steady-state flow rate Q(i), pressure difference ΔP(i) and fluid viscosity μ(i) under test temperature and pressure conditions, the equivalent permeability k(i) under confining pressure conditions is calculated using equation (4): （4）； S2.6: Repeat steps S2.1-S2.5 at each confining pressure P c (i) The permeability sequence {k(i)} is obtained and used to construct a pressure-sensitive permeability curve, thereby reflecting the permeability characteristics of the fracture cap rock under different pressure levels; S3. Characterization of CO2 infiltration process based on CT-fiber optic joint monitoring: S3.1: Close the deionized water loop and maintain the core under confining pressure P. c (i), under temperature T(i), it is in a state of deionized water saturation; S3.2: Connect the core inlet to the CO2 supply system and establish the inlet pressure P. in (i) and the outlet pressure P out (i) forms a stable pressure difference ΔP(i), causing CO2 to be contained within the confining pressure P. c (i) Core of fractured cap rock saturated with deionized water that has been unidirectionally absorbed along the lower axial direction; S3.3: During the CO2 infiltration process, the core was subjected to multi-time CT scans at preset time intervals to obtain the t values ​​at different times. k The voxel grayscale field GCO2(x, y, z; t) is shown below. k , P c (i) By using phase segmentation and threshold segmentation methods to distinguish between the CO2 phase and the water phase, the three-dimensional distribution of the CO2-occupied area is extracted to obtain a spatial evolution image sequence of the CO2 infiltration process; S3.4: During the entire CO2 permeation process, the circumferential strain field ε along the axial position z of the sample is continuously acquired using a distributed fiber optic demodulator. θ '(z, t; P c (i)), and according to the CT scan time t k The fiber optic strain data were synchronously processed to obtain the circumferential strain field ε corresponding to each CT working condition. θ '(z, t; P c (i)); S3.5: Based on the established crack surface location C s (i)={(θ j , z j )} and the fiber optic measuring point position Z f ={z k The spatial registration relationship is used to project the CO2-occupied area identified by CT at each time point onto the outer surface of the sample, obtaining the position of the surface CO2 leading edge and the distribution of preferential CO2 channels within the crack, and comparing it with the fiber optic circumferential strain distribution ε at the same time point. θ '(z, t; P c (i) Perform corresponding analysis; S3.6: By comparing different times t k The evolution of the CO2 front position, CO2 connectivity region within the fracture, and the peak position and amplitude of fiber optic circumferential strain in the CT images characterizes the CO2 absorption path, propagation speed, dominant channels, and fracture aperture variation characteristics in the fracture caprock core. S3.7: Repeat steps S3.1-S3.6 under different confining pressures P c (i) CO2 infiltration tests were carried out under different inlet pressure differences ΔP(i) or different crack geometry conditions. The spatial distribution pattern of CO2 identified by CT and the circumferential strain response characteristics of optical fiber under different working conditions were compared. A CT-fiber joint identification spectrum of the influence of confining pressure and crack structure on CO2 infiltration behavior was constructed. S4. Monitoring and Calculation of CO2 Leakage Process and Leakage Flux: S4.1: Maintain inlet CO2 pressure P in (i) Back pressure at the outlet end P out (i) and confining pressure P c (i) Once the core is basically stable, continue injecting CO2 into the core inlet. When a CO2 signal is detected at the outlet or downstream monitoring unit, record that moment as the breakthrough time t. b (i) is the leakage initiation time under the confining pressure condition; S4.2: Before and after the CO2 breakthrough, continuously record the outlet CO2 flow rate q using the outlet gas flow meter. out (t; P c (i)), calculate the CO2 displacement pressure difference under this operating condition using equation (1); S4.3: During the CO2 leakage process, according to the preset time series t k Multi-time CT scans were performed on the core samples to obtain the voxel grayscale field GCO2(x, y, z; t). k , P c (i) By phase segmentation and threshold segmentation, the CO2 phase is separated from the water phase, and the three-dimensional distribution of CO2 enrichment area, through crack and leakage channel is extracted. S4.4: During the entire CO2 leakage process, a distributed fiber optic demodulator was used to synchronously acquire the circumferential strain field ε at position z along the core axis. θ '(z, t; P c (i)), and the set of crack surface locations C s (i) and the set of fiber optic measurement point locations Z f Spatial registration is performed to map the strain anomaly zone during the leakage stage to the leakage channel identified by CT scan, in order to identify the main channel of CO2 leakage and its dynamic evolution. S4.5: Breakthrough moment t b (i) Starting from this point, the confining pressure P is obtained by integrating the outlet flow rate curve over time using equation (5). c (i) Cumulative CO2 leakage M under the given conditions leak (t; P c (i)), Formula (5) is: （5）； In the formula: q out (τ;P c (i) is under confining pressure P c (i) Under operating conditions, the outlet CO2 flow rate at time τ is measured by the gas flow meter at the outlet end; t b (i) represents the time when the downstream monitoring unit first detects the breakthrough under this confining pressure condition; is the integral variable, and t is any observation time; S4.6: Select the observation end time t under this working condition end At this point, the cumulative leakage is M. leak (t end ; P c (i)); combined with confining pressure P c (i) Corrected cross-sectional area A(P) under the condition c (i)), calculate the time interval [t] using equation (6). b (i),t end The average leakage flux on the surface is given by formula (6): （6）； S4.7: Under different confining pressures P c (i), under different CO2 inlet pressure differences ΔP(i) and different fracture geometries, repeat steps S4.1-S4.6 to obtain the breakthrough time t under each condition. b (i) Cumulative leakage M leak (t; P c (i) and average leakage flux q leak (P c (i)).

3. The test method of the CT-fiber optic joint testing system for CO2 infiltration process in fractured caprock as described in claim 2, characterized in that, The fracture geometry in step S1.1 includes the number of fractures, their location, orientation, and initial aperture range.

4. The test method of the CT-fiber optic joint testing system for CO2 infiltration process in fractured caprock as described in claim 2, characterized in that, The method for achieving deionized water saturation in step S1.2 is as follows: the sample is placed in a vacuum saturation device, and negative pressure is drawn to expel the air from the pores; deionized water is slowly introduced under negative pressure and the sample is submerged; after restoring normal pressure, the sample is soaked for more than 24 hours; the negative pressure is -5 to -10 kPa.

5. The test method of the CT-fiber optic joint testing system for CO2 infiltration process in fractured caprock as described in claim 2, characterized in that, The optical fiber is fixed by dot- or line bonding with heat- and pressure-resistant epoxy adhesive or high-temperature curing adhesive on the spiral path. After curing, a reliable mechanical coupling is formed between the optical fiber and the outer surface of the core to ensure that the deformation of the core surface is effectively transmitted to the optical fiber.

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