Rock core internal optical fiber monitoring system and method

Abstract

The invention provides a core internal optical fiber monitoring system and method, and relates to the technical field of optical fiber sensing. The system comprises a rock core internally provided with a cross channel network, wherein the cross channel network is formed by cross arrangement of axial channels and radial channels; an optical fiber sensing assembly implanted in the cross channel network, wherein the optical fiber sensing assembly is integrated with a distributed acoustic sensing unit and an optical fiber grating sensing unit; the nano modified coupling medium is filled between the optical fiber sensing assembly and the inner wall of the cross channel network and is used for realizing stress transfer between the rock core and the optical fiber sensing assembly; and the monitoring equipment is in communication connection with the optical fiber sensing assembly and is used for receiving and processing the sensing signal so as to obtain an internal strain field, a temperature gradient field and a pressure wave field of the rock core. According to the invention, deep, long-term and multi-dimensional in-situ monitoring of the internal state of the rock core is realized, and the method is suitable for geological exploration and rock core experimental analysis.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a fiber optic monitoring system and method for the interior of rock cores. Background Technology

[0002] Geological storage of carbon dioxide is one of the key technological approaches, but CO2 stored underground poses a risk of leakage, making the development of reliable safety monitoring technologies crucial.

[0003] Currently, monitoring is mainly conducted using surface-wound optical fibers, where fiber optic sensing elements are directly wound around the outer wall of the core or well casing, and changes in the fiber optic signal are detected to infer the internal state. However, this method has limited detection depth and a single monitoring dimension, making it difficult to effectively identify the hidden fracture networks developed within the rock mass. It can only indirectly infer the internal state through surface strain and cannot fully reflect the true mechanical response of the sealed body. Furthermore, the existing fiber optic coating is prone to chemical corrosion in acidic environments with high concentrations of CO2, leading to deterioration in sensing performance and making it difficult to meet the long-term monitoring needs of sealed sites.

[0004] Therefore, there is an urgent need to design an effective solution that can achieve deep, long-term, and multi-dimensional in-situ monitoring. Summary of the Invention

[0005] This invention provides a fiber optic monitoring system and method for the interior of rock cores, which solves the problems of shallow detection depth, poor environmental tolerance and single monitoring dimension of existing surface-wound fiber optic monitoring technology.

[0006] This invention provides a core internal fiber optic monitoring system. The system includes a core drilled with an internal cross-channel network, formed by the interlacing of axial and radial channels. Fiber optic sensing components are embedded within the cross-channel network, integrating distributed acoustic sensing units and fiber Bragg grating sensing units. The gap between the fiber optic sensing components and the inner wall of the cross-channel network is filled with a nano-modified coupling medium, which facilitates stress transfer between the core and the fiber optic sensing components. The system is also equipped with a monitoring device that is communicatively connected to the fiber optic sensing components, capable of receiving and processing sensing signals to acquire the core's internal strain field, temperature gradient field, and pressure wave field.

[0007] According to the present invention, the fiber optic monitoring system for the interior of a rock core adopts a spiral trajectory for the layout of the cross-channel network.

[0008] According to the present invention, the pitch of the helical trajectory of the optical fiber monitoring system inside the rock core can be adjusted according to the actual monitoring requirements.

[0009] According to the present invention, the fiber optic monitoring system for the interior of a rock core can be selected from at least one of dual-mode optical cable, gas-sensitive film optical fiber, or quantum dot-doped optical fiber.

[0010] According to the present invention, a core internal optical fiber monitoring system is provided, wherein the surface of the gas-sensitive film optical fiber is provided with a sensitive film layer capable of responding to the target monitoring medium; and the quantum dot-doped optical fiber is used to improve the detection sensitivity to the target monitoring medium.

[0011] According to the present invention, a core internal optical fiber monitoring system is provided in which a nano-modified coupling medium is filled into a cross-channel network by vacuum negative pressure injection.

[0012] According to the present invention, a core internal fiber optic monitoring system comprises multiple functional units. A fracture identification unit performs rainflow counting analysis on the strain signals acquired by the fiber Bragg grating sensing unit to obtain the internal strain field. A temperature demodulation unit demodulates the wavelength shift signal of the fiber Bragg grating sensing unit to obtain the temperature gradient field. A noise suppression unit performs time-frequency analysis on the acoustic signals acquired by the distributed acoustic sensing unit using a wavelet transform algorithm to suppress noise and ultimately obtain the pressure wave field.

[0013] This invention provides a method for fiber optic monitoring inside rock cores. The method, applied to the aforementioned fiber optic monitoring system inside rock cores, includes the following steps: constructing a cross-channel network formed by the intersection of axial and radial channels inside the rock core; implanting a fiber optic sensing component integrating a distributed acoustic sensing unit and a fiber optic grating sensing unit into the constructed cross-channel network; filling the cross-channel network with a nano-modified coupling medium via vacuum injection; and finally processing the sensing signals collected by the fiber optic sensing component using a monitoring device to simultaneously analyze the internal strain field, temperature gradient field, and pressure wave field of the rock core.

[0014] According to the present invention, a method for monitoring the interior of a rock core using optical fiber includes the following steps for processing the sensing signals: performing wavelet transform time-frequency analysis on the acoustic signals collected by the distributed acoustic sensing unit to obtain the pressure wave field while suppressing noise; performing rainflow counting analysis on the strain signals collected by the fiber grating sensing unit to obtain the internal strain field by identifying the micro-pressure wave variation characteristics; and simultaneously demodulating the wavelength shift signal of the fiber grating sensing unit to obtain the temperature gradient field.

[0015] According to the present invention, a method for monitoring the interior of a rock core using optical fiber involves applying pre-tension to the optical fiber sensing component during the process of embedding the optical fiber sensing component into a cross-channel network to ensure that it remains in close contact with the interior of the cross-channel network.

[0016] The fiber optic monitoring system and method for rock cores provided by this invention achieves a fundamental transformation of the sensing structure from the surface to the interior of the rock mass by deploying a cross-channel network inside and embedding fiber optic sensing components therein. This increases the monitoring depth from the millimeter level to the centimeter level, effectively overcoming the shortcomings of traditional methods in detecting hidden fracture networks inside the rock mass.

[0017] Furthermore, by employing a spiral trajectory to deploy the channel network and making its pitch adjustable, a more comprehensive three-dimensional coverage of the core and flexible optimization of monitoring resolution were achieved. By using a synergistic process of functional optical fiber and vacuum negative pressure injection of nano-modified coupling medium, high-fidelity stress transmission was achieved while enhancing long-term durability in corrosive environments and high-sensitivity detection capability of target media.

[0018] The rainflow counting, wavelength demodulation, and wavelet transform algorithms integrated in the monitoring equipment respectively achieve accurate capture of internal crack propagation characteristics, stable calculation of temperature field, and effective suppression of environmental noise in acoustic signals. Ultimately, it simultaneously acquires multi-dimensional data of strain field, temperature field, and pressure wave field with high signal-to-noise ratio, solving the technical bottlenecks of shallow detection depth, poor environmental tolerance, and single monitoring dimension in existing technologies. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the structure of a fiber optic monitoring system for the interior of a rock core provided by the present invention.

[0021] Figure 2 This is a schematic flowchart of a fiber optic monitoring method for the interior of a rock core provided by the present invention.

[0022] Figure label: 100. Core; 110. Cross-channel network; 111. Axial channel; 112. Radial channel; 200. Fiber optic sensing component; 210. Distributed acoustic sensing unit; 220. Fiber Bragg grating sensing unit; 300, nano-modified coupling medium; 400. Monitoring equipment; 410. Crack identification unit; 420. Temperature demodulation unit; 430. Noise suppression unit. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0024] In oil and gas field exploration and development, geological scientific research, and other fields, core sampling and analysis are key methods for obtaining the physical properties of underground reservoirs. Traditional core analysis mainly relies on laboratory testing, which involves monitoring the mechanical behavior of the core by attaching strain gauges or placing point sensors on the core surface.

[0025] However, this type of method has the following limitations: on the one hand, surface monitoring cannot reflect the complex stress and strain distribution inside the core 100; on the other hand, point sensors have a limited monitoring range and it is difficult to capture full-scale field distribution information.

[0026] Furthermore, the existing technologies struggle to achieve simultaneous monitoring of multiple physical fields within the rock core. In core displacement experiments and hydraulic fracturing simulations, the coupled evolution of the internal strain field, temperature gradient field, and pressure wave field is crucial for understanding the rock mass failure mechanism. Currently, separate sensor systems are required for measurement, which is not only complex to operate but also results in poor data synchronization, failing to accurately reflect the multi-field coupling effects.

[0027] In addition, although existing fiber optic monitoring technology has the advantage of distributed measurement, it usually adopts a single sensing principle (such as using only distributed acoustic sensing), which makes it difficult to meet the high-precision monitoring requirements of multiple parameters such as strain, temperature, and pressure at the same time, thus affecting the accuracy of monitoring data.

[0028] To address the aforementioned issues, this invention provides a fiber optic monitoring system and method for core samples. Through innovative structural design and multi-sensor fusion technology, it solves the technical challenges existing in current core monitoring methods.

[0029] The core internal fiber optic monitoring system provided by this invention constructs a special cross-channel network inside the core, implants fiber optic sensing components integrating distributed acoustic sensing units and fiber optic grating sensing units, and uses a nano-modified coupling medium to realize stress transmission between the core and the sensor.

[0030] The fiber optic monitoring system for the interior of the core provided by this invention achieves synchronous and distributed monitoring of multiple physical fields within the core through architectural innovation. Specifically, the cross-channel network is formed by the cross-arrangement of axial and radial channels, providing optimized layout space for the fiber optic sensing components; the fiber optic sensing components adopt a multi-sensor unit integrated design, capable of simultaneously acquiring acoustic, strain, and temperature signals; and the nano-modified coupling medium ensures that the stress inside the core can be accurately transmitted to the sensing components.

[0031] The following is combined with Figure 1 The fiber optic monitoring system for the interior of rock cores provided by this invention will be described in detail.

[0032] Figure 1 This is a schematic diagram of the structure of a fiber optic monitoring system for the interior of a rock core provided by the present invention, as shown below. Figure 1 As shown, the core internal fiber optic monitoring system includes: core 100, cross-channel network 110, fiber optic sensing component 200, nano-modified coupling medium 300, and monitoring equipment 400.

[0033] In some embodiments, the core 100 is a rock sample used to reflect the characteristics of underground engineering rock masses, and its size can be adjusted according to experimental requirements.

[0034] For example, the core 100 can be a cube with a side length of 500 mm or a cylinder with a diameter of 500 mm.

[0035] It should be noted that the material of Core 100 can be selected according to the specific application scenario. For example, saline aquifer sandstone is selected in carbon dioxide geological storage research, while shale samples are selected in shale gas development research.

[0036] Specifically, the cross-channel network 110 is formed by the cross-arrangement of axial channels 111 and radial channels 112. The axial channels 111 extend along the axial direction of the core 100, while the radial channels 112 are arranged perpendicular to the axial direction. The two cross to form a three-dimensional monitoring network.

[0037] Thus, this invention can effectively capture the changes in the physical field in different directions inside the core through this cross-layout method.

[0038] In a preferred embodiment of the present invention, the deployment trajectory of the cross-channel network 110 adopts a spiral trajectory.

[0039] In this embodiment of the invention, the spiral layout has better spatial coverage characteristics than the traditional straight intersection, and can achieve monitoring of a larger area inside the core with a single continuous path.

[0040] Optionally, the pitch of the spiral trajectory can be adjusted according to monitoring requirements.

[0041] For example, a small pitch of 3-5 cm is used in areas sensitive to the development of microcracks, while a large pitch of 6-8 cm is used in areas where the physical field changes gently.

[0042] In this embodiment of the invention, the cross-channel network 110 is constructed using a high-precision drilling process.

[0043] For example, a diamond drill bit can be used to drill holes with a diameter controlled in the range of 0.5-2 mm, preferably 0.8 mm ± 0.1 mm.

[0044] It should be noted that the trajectory deviation during the drilling process must be strictly controlled within 1% to ensure that the optical fiber can be successfully implanted and match the preset trajectory. After drilling is completed, residual rock powder in the channel is removed by ultrasonic cleaning or high-pressure nitrogen purging to ensure that the channel is unobstructed.

[0045] In this embodiment of the invention, the fiber optic sensing component 200 is embedded inside the cross-channel network 110.

[0046] Optionally, the fiber optic sensing assembly 200 adopts an integrated design, encapsulating the distributed acoustic sensing unit 210 and the fiber optic grating sensing unit 220 in the same specially designed optical cable.

[0047] In practical use, the distributed acoustic sensing unit 210 is based on the backscattering Rayleigh principle and can continuously sense sound waves or vibration signals distributed along the optical fiber; the fiber optic grating sensing unit 220 is a series of Bragg grating arrays etched on the optical fiber. Each grating reflects light of a specific wavelength, and its reflected wavelength will drift linearly with the strain and temperature changes at its location.

[0048] Specifically, the distributed acoustic sensing unit 210 can be a distributed acoustic sensing (DAS); the fiber grating sensing unit 220 can be a fiber bragg grating (FBG).

[0049] In some embodiments, the fiber optic sensing component 200 may select different fiber optic types according to the specific monitoring target.

[0050] For example, the fiber optic sensing component 200 can be at least one of a dual-mode optical cable, a gas-sensitive film optical fiber, or a quantum dot-doped optical fiber.

[0051] Specifically, dual-mode optical cable serves as a basic configuration, possessing both distributed acoustic sensing and fiber Bragg grating sensing capabilities.

[0052] Specifically, the surface of the gas-sensitive optical fiber is provided with a sensitive film layer for responding to the target monitoring medium, which is obtained by coating the surface of the optical fiber with a sensitive polymer film layer that has a specific response to the target monitoring medium.

[0053] In practical applications, such as carbon dioxide monitoring, a polyetherimide membrane with high selectivity for CO2 is used. When CO2 molecules come into contact with the membrane, it causes a change in the membrane volume or refractive index, which in turn modulates the phase or intensity of the light signal, thus achieving highly sensitive detection of gas concentration.

[0054] Specifically, quantum dot-doped optical fibers are used to improve the detection sensitivity of the target monitoring medium, and are obtained by doping the fiber core with specific quantum dot materials.

[0055] In practical applications, lead sulfide quantum dots are used to detect changes in their fluorescence properties caused by environmental factors, thereby raising the detection sensitivity of the target medium to a higher level.

[0056] In some embodiments, the nano-modified coupling medium 300 is filled in the micron-level gap between the fiber optic sensing component 200 and the inner wall of the cross-channel network 110 to realize stress transmission between the core 100 and the fiber optic sensing component 200.

[0057] For example, the nano-modified coupling medium 300 can be an epoxy resin matrix modified by adding nano-scale inorganic fillers. Specifically, silica nanoparticles can be added.

[0058] In one specific embodiment of the present invention, the nano-modified coupling medium 300 is filled between the fiber optic sensing component 200 and the inner wall of the cross-channel network 110 by vacuum negative pressure injection.

[0059] In the actual filling process, the core 100 with embedded optical fibers is first placed in a sealed chamber and evacuated to below 10 Pa. Then, under negative pressure, the premixed nano-modified epoxy resin is injected into the channel network. After injection, the core is moved to a constant temperature oven and cured using a stepped heating process: first, preheating at 50°C for 2 hours, then curing at 85°C for 10 hours, and finally naturally cooling to room temperature.

[0060] In some embodiments, the monitoring device 400 is connected to the fiber optic sensing component 200 via a communication optical cable to receive and process sensing signals in order to obtain the internal strain field, temperature gradient field and pressure wave field of the core 100.

[0061] For example, the monitoring device 400 integrates the following three core functional units at the software level, each specifically designed for different physical fields.

[0062] 1) The crack identification unit 410 is configured to perform rainflow counting analysis on the strain signal acquired by the fiber Bragg grating sensing unit 220 to obtain the internal strain field.

[0063] It should be noted that the initiation and propagation of fractures within the rock mass generate a series of transient strain pulses with small amplitudes. The rainflow counting algorithm can accurately count these cyclic loads and extract their amplitude and frequency characteristics, thereby identifying the micropressure wave variation characteristics related to fracture activity, and thus accurately reconstructing the dynamic evolution of the internal strain field.

[0064] 2) Temperature demodulation unit 420 is configured to demodulate the wavelength offset signal of fiber optic grating sensing unit 220 in real time to obtain temperature gradient field.

[0065] In this embodiment of the invention, since the fiber grating is sensitive to both strain and temperature, a pure strain event is first identified by a distributed acoustic sensing unit or other reference signal. Then, the remaining wavelength drift is calculated as a temperature change using the built-in temperature-wavelength calibration coefficient, thereby independently obtaining the temperature gradient field.

[0066] 3) The noise suppression unit 430 is configured to perform time-frequency analysis on the acoustic signal acquired by the distributed acoustic sensing unit 210 through wavelet transform algorithm to suppress noise and obtain the pressure wave field.

[0067] It should be noted that noise from drilling vibrations, equipment operation, and other sources in the downhole environment typically has specific frequency band characteristics. Wavelet transform can effectively separate and filter out these noises from the useful pressure wave signal, improving the signal-to-noise ratio and thus accurately acquiring the pressure wave field.

[0068] Thus, through innovative internal embedded architecture design and multi-sensor information fusion processing, this invention achieves high-precision, distributed, synchronous monitoring of multiple physical fields inside the core, effectively overcoming the limitations of traditional surface monitoring methods.

[0069] In the core internal fiber optic monitoring system provided by this invention, a structural design that constructs an internal cross-channel network extends the sensing carrier from the core surface to the interior, establishing a physical channel for deep in-situ monitoring. By embedding fiber optic components integrating distributed acoustic sensing and fiber optic grating sensing, the different response characteristics of acoustic signals to dynamic pressure waves and strain signals to static deformation and temperature lay the sensing foundation for multi-dimensional synchronous monitoring. By filling with a nano-modified coupling medium, efficient transmission of rock mass stress to the optical fiber is achieved while providing physical protection for the sensing elements against harsh geological and chemical environments, ensuring the long-term stability of monitoring performance. Finally, the monitoring equipment fuses and processes multi-source sensor signals, enabling synchronous analysis and visualization of the internal strain field, temperature gradient field, and pressure wave field, thereby achieving deep, long-term, and multi-dimensional in-situ monitoring of the core's internal state.

[0070] The following is combined with Figure 2 The present invention describes a method for fiber optic monitoring inside rock cores.

[0071] Figure 2 This is a flowchart illustrating a method for fiber optic monitoring inside a rock core provided by the present invention, as shown below. Figure 2 As shown, the method includes the following: S201. Construct a cross-channel network inside the core, formed by the intersection of axial and radial channels.

[0072] In some embodiments, a specific deployment scheme for the cross-channel network 110 can be designed according to monitoring requirements.

[0073] For example, for shale cores with significant anisotropy, a denser cross-layout can be used; for relatively homogeneous sandstone cores, a sparser layout can be used.

[0074] Specifically, the drilling process is carried out using a high-precision CNC drilling machine with a diamond drill bit. The drill bit diameter is selected according to the fiber size. For example, it can be 0.1-0.3 mm larger than the fiber diameter to allow space for the coupling medium to fill.

[0075] Optionally, the drill bit's feed rate and rotation speed can be monitored in real time during drilling to ensure a smooth and flat inner wall of the channel. After drilling is completed, a high-pressure nitrogen spray gun is used to purge the channel from different directions to ensure no rock powder remains.

[0076] S202, The fiber optic sensing component integrating a distributed acoustic sensing unit and a fiber optic grating sensing unit is embedded into the cross-channel network.

[0077] Optionally, during the process of embedding the fiber optic sensing component 200 into the cross-channel network 110, a pre-tension can be applied to the fiber optic sensing component 200 to make the fiber optic sensing component 200 fit into the interior of the cross-channel network 110.

[0078] In some embodiments, the pretension is controlled within the range of 0.5-1N, and can be adjusted specifically according to the fiber type and channel size.

[0079] For example, for a dual-mode optical cable with a diameter of 0.9 mm, the pretension is preferably 0.8 N; for a thinner gas-sensitive film optical fiber, the pretension is appropriately reduced to 0.5 N.

[0080] Specifically, a precise tension control system can be used during the implantation process to monitor and adjust the tension in real time.

[0081] Optionally, a small amount of UV-curable adhesive can be used to temporarily fix the optical fiber at the channel port to prevent it from sliding during subsequent processes. For longer channels, a segmented implantation strategy is adopted, pausing after implanting a certain length to check the fiber status and make timely adjustments.

[0082] S203. Vacuum injection of nano-modified coupling medium into the cross-channel network.

[0083] In some embodiments, the preparation of the nano-modified coupling medium 300 requires strict control of the proportions and mixing order of each component.

[0084] For example, the epoxy resin matrix can be heated to 40°C to reduce its viscosity, and nano-silica filler can be slowly added while stirring at 2000 rpm for 30 minutes to ensure uniform dispersion of the filler. Finally, the curing agent can be added and mixed evenly.

[0085] Specifically, the vacuum negative pressure injection process is divided into the following three stages: 1) Inject slowly under a low negative pressure (e.g., -0.05MPa) to expel most of the air.

[0086] 2) Inject rapidly under high negative pressure (e.g., -0.1MPa) to ensure that the medium fully fills the micron-level gaps.

[0087] 3) After injection, maintain negative pressure for 30 minutes to allow the medium to further penetrate and stabilize in the channel.

[0088] S204. The sensing signals of the fiber optic sensing components are processed by the monitoring equipment to simultaneously acquire the internal strain field, temperature gradient field and pressure wave field of the rock core.

[0089] In one alternative implementation, wavelet transform can be used to perform time-frequency analysis on the acoustic signal acquired by the distributed acoustic sensing unit 210 to suppress noise and obtain the pressure wave field.

[0090] For example, wavelet transform can use the db4 wavelet basis function to perform 5-level decomposition, decompose the signal into different frequency bands, and then use a threshold denoising method to suppress noise and retain the useful pressure wave signal based on the difference in energy distribution of noise and useful signal in different frequency bands.

[0091] Specifically, the threshold denoising method can be implemented according to the following steps: 1) Perform wavelet decomposition on the original acoustic signal to obtain a series of wavelet coefficients at different resolution levels. These coefficients reflect the energy intensity of the signal in different frequency bands.

[0092] 2) Calculate the threshold for the wavelet coefficients of each frequency band. For example, the threshold can be calculated by multiplying the absolute value of the median of the wavelet coefficients at that level by a scaling factor.

[0093] 3) Execute the threshold processing function. For each wavelet coefficient, compare its absolute value with the calculated threshold: if the absolute value of the wavelet coefficient is lower than the threshold, it is determined that the coefficient is mainly contributed by noise and is set to zero; if the absolute value of the wavelet coefficient is higher than the threshold, it is determined that the coefficient contains useful pressure wave signals and is retained.

[0094] Furthermore, the retained coefficients can be processed using a soft thresholding function, which shrinks them towards zero to smooth the signal.

[0095] 4) By using the wavelet coefficients of each level after thresholding, wavelet reconstruction can be performed to obtain the denoised pressure wave signal.

[0096] In practical monitoring, such as carbon dioxide sequestration monitoring, the analysis of pressure wave fields focuses on capturing the characteristic signals generated when the CO2 front breaks through the capillary barrier. These characteristic signals are typically concentrated in the 100-500Hz frequency band, which is significantly different from the main frequency band of environmental noise such as drilling vibration (i.e., below 50Hz). Through wavelet decomposition, the noise-dominated low-frequency band (such as the first and second layer decompositions) will obtain a higher threshold and thus be largely suppressed; while the signal-dominated high-frequency band (such as the third and fourth layer decompositions) will obtain a lower threshold, allowing the micro-pressure wave characteristic signals representing CO2 transport to be fully preserved.

[0097] In another alternative implementation, the strain signal acquired by the fiber Bragg grating sensing unit 220 can be analyzed by rainflow counting to identify micro-pressure wave variation characteristics and obtain the internal strain field.

[0098] For example, the rainflow counting algorithm can be configured with an amplitude threshold of 1 microstrain, and strain cycles exceeding the threshold can be counted and classified. Based on the statistical characteristics of the cycle amplitude and frequency, micropressure wave events related to fracture activity can be identified.

[0099] Specifically, rainflow counting analysis can be performed according to the following procedure: 1) Connect the collected time-domain strain signal data points sequentially to form a time-strain sequence.

[0100] 2) The four-peak point method is used to extract closed stress-strain cycles from the sequence, that is, starting from a peak (or valley) and ending at the next peak (or valley) in the opposite direction, forming a complete cycle.

[0101] 3) For each extracted cycle, calculate its strain range (i.e., the amplitude of the cycle) and mean value. Count all cycles whose amplitude exceeds the preset threshold of 1 microstrain and classify them statistically according to their amplitude and frequency of occurrence to form an amplitude-frequency matrix.

[0102] 4) Feature identification based on the matrix: The expansion of tensile fractures inside the rock mass usually corresponds to strain cycles with large amplitude (e.g., more than 5 microstrains) but low frequency; while the slip of shear fractures often produces strain cycles with small amplitude (e.g., 1-3 microstrains) but high frequency.

[0103] In practical monitoring, such as shale fracturing monitoring, rainflow counting can distinguish the characteristic signals of tensile fractures and shear fractures, enabling accurate judgment of fracture type and propagation mechanism.

[0104] In another alternative implementation, the temperature gradient field can be obtained by demodulating the wavelength shift signal of the fiber grating sensing unit 220.

[0105] For example, temperature demodulation can be performed using a reference grating method, in which a grating unaffected by stress is placed inside the core as a temperature reference. By comparing the wavelength offset between the measured grating and the reference grating, the wavelength drift caused by temperature changes can be separated.

[0106] Specifically, the reference grating method can be implemented by the following steps: 1) Select a region inside the core where the stress can be considered zero (e.g., a small, freely suspended cavity) and encapsulate a fiber optic grating as a temperature reference grating, whose wavelength change is caused only by temperature.

[0107] 2) The measurement grating, which serves as the sensing element, is tightly coupled to the rock mass, and its wavelength change is simultaneously affected by both temperature and strain.

[0108] 3) Synchronously acquire the center wavelength offset data of the reference grating and the measurement grating. For each sampling time, subtract the wavelength offset of the reference grating at the same time from the total wavelength offset of the measurement grating to obtain the wavelength offset component purely caused by strain.

[0109] 4) Using the known temperature sensitivity coefficient of the fiber optic grating, the wavelength offset of the reference grating is converted into an accurate temperature change value, thereby obtaining the absolute temperature at that location inside the core.

[0110] 5) By deploying multiple such sensing units in the three-dimensional space inside the core and repeating the above process, the temperature gradient field of the entire core can be plotted.

[0111] In practical experiments, such as CO2 sequestration experiments in saline water layers, temperature demodulation can be used to capture the local low-temperature zone caused by CO2 dissolving in saline water with high precision.

[0112] Thus, through systematic process design and advanced signal processing algorithms, this invention achieves high-precision synchronous monitoring of the strain field, temperature gradient field, and pressure wave field inside the rock core, providing a reliable technical means for geological engineering safety assessment.

[0113] The following provides a detailed description of typical application examples of the fiber optic monitoring method for the interior of rock cores provided by this invention.

[0114] Example 1: Safety monitoring of carbon dioxide saline aquifer storage.

[0115] In one specific application embodiment of the present invention, it can be used for safety monitoring of carbon dioxide geological sequestration in saline aquifers.

[0116] ① Select a 100mm diameter, 150mm length saline sandstone core 100 and construct a spiral cross channel network 110 inside it, including 3 sets of axial channels 111 and radial channels 112, with a channel spacing of 6cm, an inclination angle of 60° and a hole diameter of 0.8mm.

[0117] ② A 0.9mm diameter dual-mode optical cable was selected as the fiber optic sensing component 200 for implantation, and a pre-tension of 0.8N was applied during the implantation process. A nano-modified epoxy resin coupling medium 300 was injected under vacuum negative pressure and cured at 85℃ for 12 hours.

[0118] ③ The sealed core 100 was placed in a high-pressure fluid injection device, and a confining pressure of 15 MPa was applied to simulate the actual underground stress environment. CO2 was injected into one end of the core using an injection pump, and the injection flow rate and pressure gradient were controlled. The monitoring device 400 collected sensor signals in real time at a sampling rate of 100 Hz.

[0119] ④ The noise suppression unit 410 performs wavelet transform processing on the distributed acoustic sensing signal to effectively remove drilling vibration noise and clearly capture the pressure wave signal generated when the CO2 front breaks through the micro-fracture; the fracture identification unit 420 performs rainflow counting analysis on the fiber optic grating strain signal and successfully identifies multiple micro-pressure wave events with amplitudes lower than 1 micro-strain; the temperature demodulation unit 430 demodulates the fiber optic grating wavelength and plots the temperature field evolution diagram of the core during the CO2 injection process, observing the local low temperature zone caused by CO2 dissolution endothermic heat.

[0120] In this embodiment of the invention, after the experiment, the system was verified by computed tomography (CT) to have a microcrack detection rate of 85% and high positioning accuracy, providing a reliable experimental basis for on-site engineering.

[0121] Example 2: Evaluation of fracturing effect in shale reservoirs.

[0122] In another specific application embodiment of the present invention, it can be used for evaluating the fracturing effect of shale reservoirs.

[0123] ① A shale sample with a diameter of 100 mm was selected to construct a cross-channel network 110, and a micro-electro-mechanical system (MEMS) fiber array with a compressive strength of 1.2 GPa was selected as the fiber optic sensing component 200.

[0124] ② During the fracturing process, acoustic emission events are captured by the distributed acoustic sensing unit 210, and the fracture propagation trajectory is inverted in real time by combining the strain data from the fiber optic grating sensing unit 220. Wavelet transform is used to remove drilling vibration noise, and rainflow counting is used to identify shear and tensile fracture characteristics.

[0125] In this embodiment of the invention, the above method enables a quantitative assessment of the complexity of the fracturing fracture network, with a fracture location accuracy of 5 cm and a strain monitoring accuracy of ±1 microstrain, providing key data support for optimizing fracturing schemes.

[0126] In the fiber optic monitoring method for rock cores provided by this invention, by constructing a cross-channel network inside the rock core, the sensing structure is transformed from a traditional surface-based layout to an internal three-dimensional layout, fundamentally breaking through the limitation of detection depth and laying the physical foundation for deep monitoring. Subsequently, fiber optic components integrating distributed acoustic sensing and fiber optic grating sensing units are implanted. Utilizing the complementarity of the two sensing mechanisms, hardware support is provided for the simultaneous acquisition of strain, temperature, and pressure wave signals, laying the foundation for multi-dimensional monitoring. Then, by vacuum-injecting a nano-modified coupling medium, efficient and stable stress transfer between the rock mass and the fiber optics is achieved while isolating the harsh environment, ensuring signal fidelity and system durability, and guaranteeing the reliability of long-term monitoring. Finally, the monitoring equipment processes the signals to simultaneously acquire three-field data, completing the accurate interpretation from the original signal to the internal physical field, realizing deep, long-term, and multi-dimensional in-situ monitoring of the internal state of the rock core.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fiber optic monitoring system for the interior of a rock core, characterized in that, The system includes: A core (100) with an internal cross-channel network (110) formed by the cross-arrangement of axial channels (111) and radial channels (112); An optical fiber sensing component (200) is embedded in the cross-channel network (110), the optical fiber sensing component (200) integrating a distributed acoustic sensing unit (210) and a fiber optic grating sensing unit (220). A nano-modified coupling medium (300) is filled between the inner wall of the fiber optic sensing component (200) and the cross-channel network (110) to realize stress transfer between the core (100) and the fiber optic sensing component (200). The monitoring device (400) is communicatively connected to the fiber optic sensing component (200) to receive and process sensing signals to obtain the internal strain field, temperature gradient field and pressure wave field of the core (100).

2. The fiber optic monitoring system for the interior of a rock core according to claim 1, characterized in that, The layout trajectory of the cross-channel network (110) is a spiral trajectory.

3. The fiber optic monitoring system for the interior of a rock core according to claim 2, characterized in that, The pitch of the spiral trajectory is adjustable.

4. The fiber optic monitoring system for the interior of a rock core according to claim 1, characterized in that, The fiber optic sensing component (200) is at least one of a dual-mode optical cable, a gas-sensitive film optical fiber, or a quantum dot-doped optical fiber.

5. The fiber optic monitoring system for the interior of the core as described in claim 4, characterized in that, The surface of the gas-sensitive film optical fiber is provided with a sensitive film layer for responding to the target monitoring medium, and the quantum dot-doped optical fiber is used to improve the detection sensitivity of the target monitoring medium.

6. The fiber optic monitoring system for the interior of a rock core as described in claim 1, characterized in that, The nano-modified coupling medium (300) is filled by vacuum negative pressure injection.

7. The fiber optic monitoring system for the interior of a rock core as described in claim 1, characterized in that, The monitoring device (400) includes: The crack identification unit (410) is configured to perform rainflow counting analysis on the strain signal acquired by the fiber grating sensing unit (220) to obtain the internal strain field; The temperature demodulation unit (420) is configured to demodulate the wavelength offset signal of the fiber grating sensing unit (220) to obtain the temperature gradient field; The noise suppression unit (430) is configured to perform time-frequency analysis on the acoustic signal acquired by the distributed acoustic sensing unit (210) using a wavelet transform algorithm to suppress noise and acquire the pressure wave field.

8. A method for fiber optic monitoring inside a rock core, characterized in that, The method, applied to a fiber optic monitoring system inside a rock core, includes: A cross-channel network (110) formed by the intersection of axial channels (111) and radial channels (112) is constructed inside the core (100). An optical fiber sensing assembly (200) integrating a distributed acoustic sensing unit (210) and a fiber optic grating sensing unit (220) is embedded in the cross-channel network (110). Vacuum injection of nano-modified coupling medium (300) into the cross-channel network (110). The sensing signal of the fiber optic sensing component (200) is processed by the monitoring device (400) to simultaneously acquire the internal strain field, temperature gradient field and pressure wave field of the core (100).

9. The method for fiber optic monitoring inside a rock core according to claim 8, characterized in that, The process of processing the sensing signal of the fiber optic sensing component (200) by the monitoring device (400) includes: The acoustic signal acquired by the distributed acoustic sensing unit (210) is subjected to time-frequency analysis by wavelet transform to suppress noise and obtain the pressure wave field; The strain signal collected by the fiber optic grating sensing unit (220) is analyzed by rainflow counting to identify the micro-pressure wave change characteristics and obtain the internal strain field; The temperature gradient field is obtained by demodulating the wavelength offset signal of the fiber grating sensing unit (220).

10. The method for fiber optic monitoring inside a rock core according to claim 8, characterized in that, During the process of embedding the fiber optic sensing component (200) into the cross-channel network (110), a pre-tension is applied to the fiber optic sensing component (200) so that the fiber optic sensing component (200) fits into the interior of the cross-channel network (110).

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