CO2 fracturing crack monitoring device and method based on axial optical fiber array

Abstract

The invention relates to the technical field of unconventional oil and gas resource development, and discloses a CO2 fracturing crack monitoring device and method based on an axial optical fiber array, and the device comprises a sample provided with a dual-mode optical fiber array, a CO2 fracturing system, a triaxial dynamic loading module, a multi-mode signal processing unit and a self-checking hydraulic system. According to the method, distributed temperature and strain signals in the fracturing process are synchronously collected through the dual-mode optical fiber, and a crack event is accurately recognized by establishing a triggering mechanism of temperature sudden drop, strain gradient sudden change and spatial correlation cooperation of the temperature sudden drop, the strain gradient sudden change and the spatial correlation cooperation. In combination with Gaussian kernel temperature mapping and an axial strain-crack width model, dynamic, accurate and quantitative characterization of crack initiation positions, expansion azimuth angles and widths is realized. The problems that a traditional point type sensor is poor in monitoring timeliness and a single mode is prone to misjudgment are solved, the temporal-spatial resolution and the monitoring efficiency are remarkably improved, meanwhile, the cost is reduced, and a reliable real-time monitoring and evaluation means is provided for CO2 anhydrous fracturing.

Description

Technical Field

[0001] This invention relates to the field of unconventional oil and gas resource development technology, specifically to a CO2 fracturing fracture monitoring device and method based on an axial fiber array. Background Technology

[0002] With the continuous growth of global energy demand, the effective development of unconventional oil and gas resources has become an important strategic direction. Hydraulic fracturing, as a core technology for improving reservoir permeability and production capacity, induces a network of fractures in rocks by injecting high-pressure fluids underground, thus creating new opportunities for oil and gas extraction. However, the effectiveness of this technology largely depends on the ability to accurately and in real-time monitor the initiation and propagation of fractures during the fracturing process.

[0003] Traditional monitoring methods, such as microseismic monitoring and acoustic emission detection, are fundamentally limited in their effectiveness when faced with complex underground environments. These methods retrieve fracture information by capturing weak acoustic or seismic wave signals generated when rocks fracture, but these signals are inevitably subject to interference and attenuation during propagation, significantly reducing the timeliness of monitoring results. Often, by the time data analysis is completed, the fracture morphology has already changed significantly, failing to provide timely guidance for operational decisions. Furthermore, these methods have relatively limited spatial resolution, making it difficult to clearly characterize the distribution and geometry of micro-fractures, thus limiting the detailed understanding of complex fracture networks. These inherent limitations make it difficult for traditional technologies to meet the urgent need for dynamic and precise tracking of fracture propagation processes.

[0004] To overcome the bottlenecks of traditional fracturing monitoring technologies, distributed fiber optic sensing technology has been introduced into the fracturing field due to its unique advantage of continuous spatial monitoring along the fracturing line. This technology is based on the principle that the characteristics of scattered light (such as intensity, frequency, and phase) change with ambient temperature or strain when light propagates in an optical fiber. It achieves high-precision distributed measurement of physical quantities by demodulating the returned optical signal. However, most existing applications are limited to a single sensing dimension. For example, some rely solely on temperature field monitoring to infer fracture propagation, but under complex geological conditions, factors such as geothermal gradients and fluid heat exchange can severely interfere with temperature signal analysis. Others rely solely on strain field analysis, but heterogeneous rock deformation and multi-source stress disturbances lead to mixed strain signals, greatly limiting the accuracy of fracture identification. This technological shortcoming directly restricts the accuracy and reliability of fracturing diagnosis.

[0005] Of particular note is a unique and highly identifiable physical phenomenon in the specific process of anhydrous fracturing using carbon dioxide (CO2) that has not yet been fully utilized. When high-pressure CO2 fluid enters a newly formed low-pressure fracture, it undergoes dramatic expansion and phase change due to the throttling effect, absorbing a large amount of heat and thus generating a significant localized low-temperature zone around the fracture. Existing technologies have failed to effectively integrate this clear temperature characteristic signal with the mechanical responses (such as strain abrupt changes) that inevitably accompany rock fracturing.

[0006] In summary, given the shortcomings of traditional monitoring methods in terms of timeliness and resolution, as well as the high misjudgment rate and poor positioning accuracy of existing distributed optical fiber technology due to its reliance on a single sensing mode, and its failure to fully utilize the unique temperature change characteristics during CO2 fracturing, this invention proposes a CO2 fracturing crack monitoring device and method based on an axial optical fiber array. Summary of the Invention

[0007] The technical problem to be solved by this invention is that existing CO2 fracturing crack monitoring technologies suffer from low timeliness and are prone to misjudgment due to a single sensing mode.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0009] The first aspect of the present invention provides a CO2 fracturing fracture monitoring device based on an axial fiber array, the device comprising: a CO2 fracturing system, a triaxial dynamic loading module, a multimode signal processing unit, and a self-calibrating hydraulic system.

[0010] The CO2 fracturing system is used to inject CO2 into a sample made of a two-mode optical fiber in a pre-defined axial fiber array. The CO2 fracturing system includes a CO2 cylinder connected to a CO2 fracturing system pipeline. The CO2 fracturing system pipeline is sequentially equipped with a cryogenic booster pump, a temperature sensor, and a nozzle. A safety pressure relief valve is also installed on the pipeline.

[0011] The triaxial dynamic loading module is used to apply three-dimensional stress to the sample. This module includes a three-dimensional independent hydraulic servo pressure plate and a stress transmission column, which transmits pressure to the sample. The module also includes a thermostatic pressure chamber with a heat transfer oil injection port. A strain transfer pad is installed on the thermostatic pressure chamber; this pad is connected to the control system and transmits deformation data to the control system.

[0012] The multimode signal processing unit is connected to the dual-mode optical fiber. This unit includes a dual-mode optical fiber demodulator connected to a data storage device. The unit also includes an analysis computer with a built-in algorithm for crack propagation inversion based on a temperature-strain coupled field model, to identify the temperature drop region and strain gradient change region induced by the CO2 phase transition.

[0013] The self-calibrating hydraulic system provides injection power to the CO2 fracturing system. This system includes a high-pressure gear pump supplied with hydraulic oil from a storage tank. The outlet of the high-pressure gear pump is connected in series with a dual-turbine flow meter, and a magnetorheological damper is also installed on the pipeline to adjust the damping coefficient based on pressure feedback.

[0014] A second aspect of the present invention provides a method for monitoring CO2 fracturing fractures based on an axial fiber array. This method is executed using the aforementioned apparatus and includes the following steps:

[0015] a) Apply preset stress and temperature to the sample using the triaxial dynamic loading module;

[0016] b) Inject CO2 into the sample using the CO2 fracturing system until it fractures;

[0017] c) The temperature and strain data of the dual-mode optical fiber are collected in real time using the multimode signal processing unit.

[0018] d) Based on the coordinated changes in the temperature and strain data, determine the triggering of the crack and locate its propagation.

[0019] In one embodiment, the determination of crack triggering in step d) is based on whether the following set of co-triggered conditions are simultaneously met:

[0020] A temperature drop ΔT ≤ -2.5℃ was detected along the dual-mode optical fiber and lasted for 3 seconds;

[0021] The strain gradient change rate is not less than 200 με / s;

[0022] The temperature-strain spatial correlation coefficient ρ ≥ 0.7.

[0023] In one embodiment, after the cooperative triggering condition is met, the propagation of the location crack includes the following steps:

[0024] The analysis computer was used to establish a temperature mapping model for the rock sample using a Gaussian kernel summation model, and the center coordinates of the temperature drop zone were extracted based on the model.

[0025] And the crack azimuth angle is calculated based on the center coordinates and the initial perforation position.

[0026] The Gaussian kernel summation model is calculated using the following formula:

[0027]

[0028] In the formula, T(x,y) is the mapping temperature, T 光纤i Let x be the measured temperature of the i-th two-mode fiber (1-1), and (x) be the measured temperature of the i-th two-mode fiber (1-1). i ,y i ) represents the coordinates of the i-th dual-mode fiber, and σ represents the bandwidth parameter.

[0029] In one embodiment, the calculation of the crack azimuth angle adopts a moving weighted method, which calculates the weighting coefficient based on the temperature drop value of the corresponding region of each dual-mode fiber, and dynamically updates the center coordinates.

[0030] In one embodiment, the method further includes establishing a relationship model between axial strain and crack width before determining crack triggering, for calculating the equivalent crack width after crack propagation.

[0031] This invention provides a CO2 fracturing fracture monitoring device and method based on an axial fiber array. It has the following beneficial effects:

[0032] 1. This invention integrates a distributed acoustic sensor (DAS) and a distributed temperature sensor (DTS) onto the same optical fiber, achieving "one fiber, multiple sensors". The long-cycle thermal recovery characteristics of the DTS can clearly reveal fluid distribution patterns and the most promising fracture networks, while the DAS captures the acoustic characteristics during the dynamic propagation of fractures to accurately identify key events such as pump start-up and shutdown times, pore elasticity effects, and fracture closure.

[0033] 2. This invention utilizes dual-mode optical fibers and multi-mode signal processing units deployed within the sample to simultaneously acquire temperature and strain data. By setting coordinated triggering conditions for temperature drop, strain gradient change rate, and their spatial correlation coefficient, the sudden temperature drop caused by CO2 phase transition endothermic reaction and the abrupt strain change caused by rock fracture are cross-validated. This effectively eliminates spurious signals generated by environmental interference or rock heterogeneity under single-sensor mode, thereby achieving reliable identification of fracture events.

[0034] 3. The multi-mode signal processing unit of this invention can perform high-frequency data acquisition. Through the DTS and DAS collaborative monitoring system, it achieves synchronous sensing and dynamic fusion of temperature and strain fields. This not only realizes coupled diagnosis of temperature-strain-acoustic multi-physics fields, but also effectively filters out interference signals from single-mode monitoring through a cross-validation mechanism. For example, during the pumping stage, DAS can accurately identify transient acoustic waves generated by fracturing impact, while the temperature change region recorded synchronously by DTS can verify the actual fracture propagation path. Furthermore, by combining algorithms such as the moving weighted method to dynamically update the fracture azimuth, this design enables the device to capture the instantaneous process of micro-crack formation and propagation, and output the fracture propagation path in real time, solving the technical problems of poor timeliness and inability to effectively characterize the dynamic behavior of fractures in traditional monitoring methods. This multi-parameter collaborative monitoring mode significantly improves the analytical accuracy of fracture geometric parameters (depth, azimuth, propagation velocity), providing a scientific basis for real-time feedback correction of fracturing models and optimization of fluid distribution strategies.

[0035] 4. This invention features a self-calibrating hydraulic system. This system performs cross-calibration of flow rate using a dual-turbine redundant flow meter and actively compensates for pressure fluctuations using a magnetorheological damper. This closed-loop self-calibration mechanism ensures the accuracy and stability of CO2 injection parameters, avoids interference with fiber optic signals caused by equipment parameter fluctuations, and guarantees the authenticity of the collected temperature and strain data. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the overall structure of a device according to an embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of optical fiber deployment according to an embodiment of the present invention.

[0038] The components include: 1. Rock sample; 1-1. Dual-mode optical fiber; 1-2. Simulated wellbore; 2. CO2 fracturing system; 2-1. CO2 cylinder; 2-2. Safety relief valve; 2-3. Temperature sensor; 2-4. Cryogenic booster pump; 2-5. Nozzle; 3. Hydraulic servo pressure plate; 4. Stress transmission column; 5. Constant temperature pressure chamber; 5-1. Heat transfer oil injection port; 5-2. Strain transfer gasket; 5-3. Control system; 5-4. Hydraulic oil storage tank; 6. Dual-mode optical fiber demodulator; 6-1. Data storage instrument; 6-2. Analysis computer; 7. High-pressure gear pump; 7-1. Dual turbine flow meter; 7-2. Magnetorheological damper. Detailed Implementation

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Please refer to the appendix. Figure 1 , Figure 1 This is a schematic diagram of the overall structure of a device according to an embodiment of the present invention. The present invention provides a CO2 fracturing fracture monitoring device based on an axial fiber array, which may include: a CO2 fracturing system 2, a triaxial dynamic loading module, a multimode signal processing unit, and a self-calibrating hydraulic system.

[0041] CO2 fracturing system 2 functions to inject CO2 into sample 1 via a pre-distributed optical fiber to perform fracturing operations. This system provides the source and phase control for the fluid injection.

[0042] The triaxial dynamic loading module applies three-dimensional stress to sample 1 and loads it under set temperature conditions to establish a specific temperature-stress environment. This module is in direct contact with sample 1, forming the mechanical boundary of the experiment.

[0043] The multimode signal processing unit, connected to the dual-mode optical fiber 1-1 embedded within sample 1, functions to synchronously acquire and process temperature and strain signals distributed along the optical fiber. This unit is the data processing component for acquiring crack response data.

[0044] The self-calibrating hydraulic system provides stable hydraulic power for the injection process of CO2 fracturing system 2 and calibrates and compensates for the flow rate and pressure of the injected fluid. This system works in conjunction with CO2 fracturing system 2. The self-calibrating hydraulic system works in conjunction with parts of the piping of CO2 fracturing system 2 to achieve precise control of the injected fluid. Specifically, the hydraulic oil storage tank 5-4 and the high-pressure gear pump 7 constitute the hydraulic source providing power to the hydraulic servo pressurizing plate 3. To control the flow rate and pressure of the injected CO2, a dual-turbine flow meter 7-1 and a magnetorheological damper 7-2 are connected in series on the outlet pipe of the cryogenic booster pump 2-4. In addition, a piezoelectric pressure oscillation sensor is also installed on this pipe. The dual-turbine flow meter 7-1 is used to monitor the injected CO2 flow rate in real time, while the magnetorheological damper 7-2 adjusts the pipe damping according to the feedback signal to compensate for pressure fluctuations. The piezoelectric pressure oscillation sensor works in conjunction with the dual-turbine flow meter 7-1 to feed back the pressure pulsation signal to the control system, which is used to adjust the output of the cryogenic booster pump 2-4 or the response of the magnetorheological damper 7-2, thereby achieving closed-loop self-calibration of the injection parameters.

[0045] See attached document Figure 2 , Figure 2 This is a schematic diagram of optical fiber layout according to an embodiment of the present invention. Sample 1 is a physical carrier used to simulate the fracturing process.

[0046] In one embodiment, Sample 1 is made of artificial materials. Specifically, cement, quartz sand, and epoxy resin can be mixed in a mass ratio of 5:3:2 and cast into a cube of 300mm × 300mm × 300mm. After casting, the cube is placed in an environment with a temperature of 25℃ ± 1℃ and a humidity of not less than 90% for 28 days.

[0047] In another embodiment, shale or sandstone or other geological rocks can be directly selected, processed into cubes of the same size, and their surfaces can be polished.

[0048] At the center of sample 1, a simulated wellbore 1-2 is pre-embedded. The simulated wellbore 1-2 is a stainless steel pipe with a diameter of 3 mm, and its surface is uniformly provided with 3 clusters of perforations along the axial direction. Each cluster contains 4 perforations with a diameter of 1 mm.

[0049] Four BOTDA / DTS dual-mode optical fibers 1-1 are arranged in a cross-symmetrical manner along the axis of the simulated wellbore 1-2. The spacing between these four optical fibers 1-1 is 1 / 4 of the side length of sample 1. To fix the optical fibers, grooves can be pre-cut in sample 1, and after the optical fibers are placed in the grooves, they are fixed with epoxy resin.

[0050] To protect the structural integrity of the dual-mode fiber 1-1 during the experiment, its outer layer is covered with a 0.3 mm thick polyimide protective layer. The tensile strength of this protective layer is not less than 5 N.

[0051] Please refer to the appendix. Figure 1 , Figure 1 This is a schematic diagram of the overall structure of a device according to an embodiment of the present invention.

[0052] The CO2 fracturing system 2 consists of a CO2 cylinder 2-1, a cryogenic booster pump 2-4, a safety relief valve 2-2, a temperature sensor 2-3, and a nozzle 2-5. In terms of piping connections, the CO2 cylinder 2-1 is connected to the inlet end of the cryogenic booster pump 2-4. The temperature sensor 2-3 and the nozzle 2-5 are sequentially connected to the outlet end of the cryogenic booster pump 2-4. The safety relief valve 2-2 is located on the high-pressure pipeline of the CO2 fracturing system 2, in the connection pipeline between the CO2 cylinder 2-1 and the cryogenic booster pump 2-4, and is used to release pressure when the system pressure exceeds a preset threshold.

[0053] The triaxial dynamic loading module houses sample 1 within the thermostatic pressure chamber 5. A three-dimensional independent hydraulic servo pressure plate 3 contacts the surface of sample 1 via a tungsten carbide alloy stress transmission column 4. The thermostatic pressure chamber 5 is equipped with a heat transfer oil inlet 5-1 for injecting or discharging heat transfer oil to regulate the chamber temperature. A strain transfer pad 5-2 is positioned at the interface between the stress transmission column 4 and sample 1; this pad is electrically connected to the control system 5-3 to transmit the deformation data of sample 1.

[0054] The multimode signal processing unit is used for data acquisition and analysis. The end of the dual-mode fiber 1-1, which is installed inside sample 1, is connected to the dual-mode fiber demodulator 6. The output of the dual-mode fiber demodulator 6 is connected to the data storage unit 6-1 and the analysis computer 6-2, respectively, to realize real-time demodulation, storage, and calculation of data.

[0055] The self-calibrating hydraulic system and a portion of the piping in the CO2 fracturing system 2 work together to achieve precise control of the injected fluid. The hydraulic oil storage tank 5-4 and the high-pressure gear pump 7 constitute the hydraulic source providing power to the hydraulic servo pressurization plate 3. To control the flow rate and pressure of the injected CO2, a magnetorheological damper 7-2 is connected in series on the outlet pipe of the cryogenic booster pump 2-4. A dual-turbine flow meter 7-1 is connected in series on the outlet pipe of the high-pressure gear pump 7. The dual-turbine flow meter 7-1 monitors the CO2 flow rate through the pipeline, while the magnetorheological damper 7-2 adjusts the pipeline damping based on the feedback signal to compensate for pressure fluctuations.

[0056] Regarding the aforementioned CO2 fracturing fracture monitoring device based on axial fiber array, this invention also relates to an experimental embodiment, which demonstrates how the device operates and how the method is executed, as detailed below:

[0057] Before monitoring hydraulic fracturing fractures, a series of experimental preparations and system initialization operations are required for the device.

[0058] First, the sample 1, which has been prepared and equipped with dual-mode fiber 1-1, is placed into the isothermal pressurization chamber 5 of the triaxial dynamic loading module, and the chamber is sealed. Then, the port of the dual-mode fiber 1-1 leading out from the sample 1 is connected to the dual-mode fiber demodulator 6 in the multimode signal processing unit.

[0059] The triaxial dynamic loading module is activated, and the triaxial hydraulic servo pressure plate 3 and stress transmission column 4 are driven by the control system 5-3 to apply initial triaxial stress to sample 1 in order to establish a simulated formation stress environment.

[0060] In one specific embodiment, the vertical stress that can be applied to sample 1 is 30 MPa, the maximum horizontal principal stress is 25 MPa, and the minimum horizontal principal stress is 20 MPa.

[0061] At the same time, heat transfer oil is injected into the constant temperature and pressure chamber 5 through the heat transfer oil injection port 5-1, and its built-in temperature regulation system is activated to set and maintain the temperature inside the chamber at a preset value, such as 60°C.

[0062] Initialize the multimode signal processing unit and set the data acquisition parameters of the dual-mode fiber optic demodulator 6.

[0063] In one embodiment, the sampling frequency can be set to 1000Hz and the spatial resolution to 5mm. Before the CO2 injection officially begins, the system collects environmental background data for a period of time for noise calibration in subsequent data processing.

[0064] After completing experimental preparations and system initialization, the fracturing experiment was carried out.

[0065] The CO2 fracturing system 2 is started, and the liquid CO2 in the CO2 cylinder 2-1 is extracted and pressurized by the cryogenic booster pump 2-4. The pressurized CO2 flows through the system pipeline, and its temperature is monitored in real time by the temperature sensor 2-3. Finally, it is injected into the simulated wellbore 1-2 in the center of sample 1 through the nozzle 2-5.

[0066] Throughout the injection process, the self-calibrating hydraulic system operates continuously to maintain stable injection parameters. A dual-turbine flow meter 7-1, connected in series in the pipeline, monitors the CO2 injection flow rate in real time. When the monitored flow rate deviates from the preset value, the control system sends an adjustment command to the magnetorheological damper 7-2. The magnetorheological damper 7-2 adjusts its internal fluid viscosity according to this command, thereby adjusting the pipeline damping, compensating for the injection pressure, and restoring the flow rate to the set value.

[0067] In one embodiment, the injection process can be divided into two stages. First, a steady-flow injection is performed at a constant rate of 20 mL / min to gradually increase the pore pressure inside sample 1. Subsequently, the injection mode can be switched to a pulse injection mode to impact the inner wall of the simulated wellbore 1-2 with a short-duration, high-pressure pulse jet to induce the formation of cracks.

[0068] This injection process continues until the data collected by the multi-mode signal processing unit meets the preset crack triggering criteria, indicating that sample 1 has ruptured. Throughout the process, safety relief valve 2-2 continuously monitors the pipeline pressure to ensure safe system operation.

[0069] During and after the fracturing experiment, the multimode signal processing unit processes and analyzes the synchronous temperature and strain data collected by the dual-mode fiber optic demodulator 6 to determine, locate, and characterize the fractures. This data processing and analysis is based on the fundamental physical principles of distributed fiber optic sensing.

[0070] Specifically, when light propagates in an optical fiber, it generates backscattered Rayleigh light, the spectral characteristics of which remain stable when the fiber's state remains unchanged. When the fiber undergoes deformation or a temperature change at a certain location, the wavelength of the backscattered Rayleigh light signal at that location will change accordingly. By comparing the backscattered Rayleigh light information before and after the change in the fiber, the location of the change in the fiber's state can be pinpointed.

[0071] In the initial stage of fiber optic measurement, it is necessary to first preserve a Rayleigh scattering signal in a reference state, because changes in local strain and temperature in the fiber will cause a local drift in the backscattered Rayleigh light reflection spectrum. This spectral drift can be represented as a function of strain ε and temperature T, allowing for the measurement of local strain or temperature changes in the fiber, resulting in:

[0072]

[0073] In the formula, λ is the average wavelength of light; ν is the frequency; Δλ is the wavelength shift; Δν is the frequency change; K T and K ε These are the constant coefficients for temperature and strain, respectively.

[0074] If temperature changes are ignored, the strain can be written as:

[0075]

[0076] In the formula, c is the speed of light in a vacuum. The wavelength is the center wavelength during the scanning of the fiber optic demodulator. This formula shows that by comparing the spectral shift of the fiber during the current measurement with that during the reference state, the measured strain value can be obtained.

[0077] Based on the aforementioned basic principle of distributed sensing, this invention obtains temperature and strain data continuously distributed along the fiber path by synchronously demodulating the back Brillouin scattering and Raman scattering signals in the dual-mode fiber 1-1, thus providing raw data for subsequent crack identification and characterization.

[0078] The confirmation of a crack event employs a collaborative triggering mechanism, which requires the following three conditions to be met simultaneously:

[0079] 1) The temperature drop ΔT ≤ -2.5℃ and the duration of this state is not less than 3 seconds;

[0080] 2) The strain gradient change rate is not less than 200 με / s;

[0081] 3) The spatial correlation coefficient ρ between the temperature drop region and the strain gradient abrupt change region is ≥0.7. This synergistic triggering mechanism correlates the CO2 phase change cooling effect with the rock fracture mechanical response for reliable crack identification.

[0082] During real-time monitoring, when the strain change Δε monitored on any dual-mode fiber 1-1 satisfies Δε≥800με, the system initiates an additional analysis to determine the crack propagation morphology. This analysis first calculates the strain difference D, and its calculation formula is as follows:

[0083]

[0084] Where, Δε i Let be the strain change measured in the i-th dual-mode fiber 1-1, where i = 1, 2, 3, 4. If the calculated strain difference D is greater than 0.7 and the duration of this state is not less than 2 seconds, then the crack is determined to be a unilateral propagating crack.

[0085] After confirming the occurrence of a crack through a collaborative triggering mechanism or the aforementioned strain analysis, the system performs crack location and geometric parameter calculations. First, a two-dimensional temperature mapping model of the sample 1 cross-section is established using a Gaussian kernel summation model to reconstruct the continuous temperature field. The calculation formula is as follows:

[0086]

[0087] Where T(x,y) is the mapped temperature at coordinate (x,y) on the sample cross section; T 光纤i Let x be the measured temperature of the i-th two-mode fiber 1-1; i ,y i ) represents the known coordinates of the i-th two-mode fiber 1-1 at this cross-section; σ is the bandwidth parameter, the value of which is set according to the sample size and fiber spacing. The coordinates of the lowest point of the temperature field (x, y) are calculated using this model. c ,y c ).

[0088] Subsequently, based on the coordinates of the temperature drop center (x c ,y c Given the initial perforation position (x0, y0) and the known initial perforation position, calculate the azimuth angle θ of the crack using the following formula:

[0089] To achieve dynamic tracking of crack propagation, the azimuth angle is calculated and updated using a moving weighted method. Specifically, the coordinates (x, y, y) of the temperature drop center are updated every 0.5 seconds. c ,y c The weighting coefficient α for each fiber optic measurement point in the calculation. i Determined by the following formula:

[0090] Where, ΔT i Let be the temperature drop value of the region corresponding to the i-th optical fiber.

[0091] Furthermore, this method can also calculate the equivalent crack width w. This calculation utilizes strain changes measured by four optical fibers to establish an axial strain-crack width relationship model, and the calculation formula is as follows:

[0092] Where w is the equivalent crack width; |Δε i | represents the absolute value of the strain change measured in the i-th optical fiber; θ i Let be the angle between the axis of the i-th fiber and the main crack surface.

[0093] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A CO2 fracturing fracture monitoring device based on an axial fiber array, characterized in that, include: A CO2 fracturing system (2) is used to inject CO2 into a sample (1) of a dual-mode fiber (1-1) of a preset axial fiber array for fracturing. The CO2 fracturing system includes a CO2 cylinder (2-1) connected to the CO2 fracturing system pipeline. The CO2 fracturing system pipeline is equipped with a safety relief valve (2-2), a cryogenic booster pump (2-4), a temperature sensor (2-3), and a nozzle (2-5). The nozzle (2-5) is used to inject CO2. A triaxial dynamic loading module is used to apply three-dimensional stress to the sample (1) to simulate the formation stress environment; A multimode signal processing unit is connected to the dual-mode fiber (1-1) of the axial fiber array and is used to synchronously acquire and process the temperature and strain data of the axial fiber array. A self-calibrating hydraulic system is used to provide stable injection power for the CO2 fracturing system (2) and to calibrate the flow rate.

2. The CO2 fracturing fracture monitoring device based on axial fiber array according to claim 1, characterized in that, The triaxial dynamic loading module includes: Three-way independent hydraulic servo pressure plate (3) is used to apply the three-dimensional stress; Stress transmission column (4) is used to uniformly transmit pressure to the sample (1); The thermostatic pressurization chamber (5) constructs a temperature-stress coupling environment through a built-in temperature regulation system. The thermostatic pressurization chamber (5) is equipped with a heat transfer oil injection port (5-1) and a strain transfer pad (5-2). The strain transfer pad (5-2) transmits deformation data to the control system (5-3).

3. The CO2 fracturing fracture monitoring device based on axial fiber array according to claim 1, characterized in that, The multi-mode signal processing unit includes: A dual-mode fiber optic demodulator (6) is used to analyze the signals acquired by the axial fiber optic array in order to synchronously acquire temperature and strain data. The dual-mode fiber optic demodulator (6) is connected to a data storage device (6-1). The analysis computer (6-2) has a built-in crack propagation inversion algorithm. Based on the temperature-strain coupled field model, it identifies the temperature drop zone and strain gradient change zone caused by the COC phase transition, thereby locating the crack and tracking its propagation path.

4. The CO2 fracturing fracture monitoring device based on axial fiber array according to claim 3, characterized in that, The self-calibrating hydraulic system includes: A high-pressure gear pump (7), supplied with hydraulic oil from a hydraulic oil reservoir (5-4), is used to provide injection pressure; A dual-turbine flow meter (7-1) is connected in series with a high-pressure gear pump (7) to monitor flow rate and detect deviations in real time; The magnetorheological damper (7-2) is installed on the pipeline of the CO2 fracturing system to dynamically adjust the damping coefficient according to the pressure feedback in order to compensate for pressure fluctuations.

5. A method for monitoring CO2 fracturing fractures based on axial fiber arrays, characterized in that, Using the apparatus as described in any one of claims 1-4, the method includes the following steps: The sample (1) is subjected to a preset stress and temperature using the triaxial dynamic loading module. CO2 is injected into the sample (1) using the CO2 fracturing system (2) until it ruptures; The temperature and strain data of the dual-mode fiber (1-1) of the axial fiber array are collected in real time using the multimode signal processing unit. Based on the coordinated changes in the temperature and strain data, the triggering of the crack and the location of its propagation are determined.

6. The CO2 fracturing fracture monitoring method based on axial fiber array according to claim 5, characterized in that, The determination of whether a crack is triggered requires the simultaneous fulfillment of the following coordinated triggering conditions: A temperature drop ΔT ≤ -2.5℃ was monitored in the dual-mode fiber (1-1) of the fiber array along the axial direction for 3 seconds; The strain gradient change rate is not less than 200 με / s; The temperature-strain spatial correlation coefficient ρ ≥ 0.

7.

7. The CO2 fracturing fracture monitoring method based on axial fiber array according to claim 5, characterized in that, After the coordinated triggering condition is met, the propagation of the location crack includes the following steps: Using the aforementioned analysis computer (6-2), a rock sample temperature mapping model was established through a Gaussian kernel summation model, and the center coordinates of the temperature drop zone were extracted based on the model. Calculate the crack azimuth angle based on the center coordinates and the initial perforation position.

8. The CO2 fracturing fracture monitoring method based on axial fiber array according to claim 5, characterized in that, The rock sample temperature mapping model established using the Gaussian kernel summation model is specifically calculated using the following formula: Where T(x,y) is the mapping temperature, T 光纤i Let x be the measured temperature of the i-th two-mode fiber (1-1), and (x) be the measured temperature of the i-th two-mode fiber (1-1). i ,y i ) represents the coordinates of the i-th dual-mode fiber (1-1), and σ is the bandwidth parameter.

9. The CO2 fracturing fracture monitoring method based on axial fiber array according to claim 7, characterized in that, The calculation of the crack azimuth angle adopts the moving weighted method, which calculates the weight coefficient based on the temperature drop value of the corresponding region of each dual-mode fiber (1-1) and dynamically updates the center coordinates.

10. The CO2 fracturing fracture monitoring method based on axial fiber array according to claim 5, characterized in that, The method also includes establishing a relationship model between axial strain and crack width before determining crack triggering, which is used to calculate the equivalent crack width after crack propagation.

Images