True triaxial fracturing test magnetofluid monitoring system and method

Abstract

The invention relates to a true triaxial fracturing test magnetofluid monitoring system and method, and belongs to the field of coal bed gas exploration and development. The system comprises a magnetic fluid mixing device, an injection device, a 4 * 4 magnetic induction sensor array, a data acquisition device, a high-pressure water pump device and a true triaxial experiment device. The method comprises the following steps: mixing magnetic fluid and adjusting viscosity; injecting a test piece; applying three-dimensional stress; monitoring magnetic field change in real time; millimeter-level positioning and three-dimensional visualization of crack expansion are achieved through magnetofluid dynamic response and data collected by a magnetic induction sensor array in combination with differential analysis, edge detection and a finite element inversion algorithm, and the magnetofluid recovery rate is larger than 95%. The method solves the problems that traditional monitoring is low in precision and poor in real-time performance, and is suitable for dynamic analysis of coal bed gas and shale gas fracturing cracks.

Description

Technical Field

[0001] The present invention belongs to the field of coalbed methane exploration and development, and relates to a true triaxial fracturing test magneto-fluid monitoring system and method. Background Art

[0002] In the field of coalbed methane exploration and development, the true triaxial fracturing test is a core technical means for studying the formation mechanism and propagation law of underground rock fractures. By simulating the rock fracture process under a real three-dimensional stress state, this test can provide key data support for coalbed methane reservoir stimulation, fracture network prediction, and production plan optimization. However, there are still significant bottlenecks in the monitoring of the dynamic behavior of fractures during the fracturing process in the existing technologies, which restricts the accuracy of test results and the engineering guiding value.

[0003] Currently, the mainstream fracture monitoring methods mainly include visual observation and acoustic emission technology. Visual observation records the generation and propagation of fractures on the rock surface through direct visual inspection or optical equipment. Although it has intuitiveness, it has the following defects:

[0004] (1) It can only capture surface fractures and lacks the ability to monitor internal and micro fractures of the rock;

[0005] (2) It relies on manual interpretation and is difficult to achieve real-time and continuous data acquisition;

[0006] (3) Limited by the observation angle and lighting conditions, it is easy to miss key fracture information. Acoustic emission technology indirectly monitors by capturing the elastic wave signals released during rock fracture. Although it can cover internal fractures, its limitations are as follows:

[0007] (1) The density of sensor layout is negatively correlated with the positioning accuracy. High-precision monitoring requires dense layout of points, resulting in high costs;

[0008] (2) The signals are easily interfered by environmental noise, leading to misjudgment or missed detection;

[0009] (3) The data processing has strong hysteresis and is difficult to provide real-time feedback on the dynamic propagation trajectory of fractures. In addition, the existing technologies generally cannot quantify the three-dimensional morphological parameters of fractures (such as fracture width, branch angle), and lack the ability to dynamically characterize the fluid-rock interaction during fracture propagation.

[0010] In recent years, the application of magnetic sensing technology in the field of industrial inspection has provided new ideas for fracture monitoring. As a functional fluid with magnetic response characteristics, the distribution and movement of magnetic particles inside the magneto-fluid can reflect the changes in the external stress field in real time.

[0011] However, there is no scheme in the existing technologies that combines magneto-fluid with the true triaxial fracturing test. The main reasons are as follows:

[0012] (1) Traditional magneto - fluid detection is mostly used in static or low - stress environments and lacks adaptability design for complex working conditions such as high pressure and multiphase flow.

[0013] (2) The problem of magneto - fluid retention in cracks has not been solved, resulting in high test costs and difficulty in reusing.

[0014] (3) There is a lack of a coordinated control method with the true triaxial loading system, and multi - physical - field coupling monitoring of the stress field - magnetic field - flow field cannot be achieved. Therefore, it is urgent to develop a new monitoring system and method that can overcome the above defects to achieve high - precision, real - time and repeatable monitoring of the spatial morphology, dynamic propagation behavior of hydraulic fractures and fluid migration laws, so as to enhance the scientific value and engineering applicability of true triaxial tests. Therefore, it is extremely urgent to develop a technology that can monitor the crack propagation position and morphology in real time and dynamically. Summary of the Invention

[0015] In view of this, the purpose of the present invention is to provide a magneto - fluid monitoring system and method for true triaxial fracturing tests. The magneto - fluid monitoring system uses a fluid containing magnetic particles as a sensing medium, applies a magnetic field at the crack or deformation site, and monitors the stress state of the rock and the crack propagation by detecting the movement changes of the particles in the magneto - fluid. This system can achieve highly sensitive monitoring of the internal changes of cracks and is helpful for understanding the crack propagation of rock cracks during the fracturing process.

[0016] (1) Existing crack monitoring methods have problems such as low accuracy and poor real - time performance.

[0017] (2) The magneto - fluid detection system can accurately grasp the crack propagation behavior by using highly sensitive magnetic field sensors for the crack propagation position and morphology.

[0018] To achieve the above - mentioned purpose, the present invention provides the following technical solutions:

[0019] A magneto - fluid monitoring system and method for true triaxial fracturing tests, comprising:

[0020] A magneto - fluid liquid mixing device 2, used for mixing the magneto - fluid with a liquid and adjusting the viscosity of the mixed liquid;

[0021] A magneto - fluid injection device 5, connected to the magneto - fluid liquid mixing device 2, used for injecting the mixed magneto - fluid liquid into the specimen 6;

[0022] A magnetic induction sensor array 4, arranged around the specimen 6, used for real - time monitoring of the magnetic field change of the magneto - fluid inside the specimen;

[0023] A data acquisition device 1, connected to the magnetic induction sensor array 4, used for collecting magnetic field data;

[0024] A high-pressure water pump device 3, which is connected to the magnetorheological fluid mixing device 2 and the specimen 6, is used to inject magnetorheological fluid mixture and clear water into the specimen.

[0025] A true triaxial experimental device 7, configured to apply three-dimensional stress to the specimen and control the fracturing process.

[0026] Furthermore, the magnetic induction sensor array 4 is a 4×4 array, and each sensor is configured to collect magnetic field intensity and direction data.

[0027] Furthermore, the magnetorheological fluid mixing device 2 includes a magnetic field loader, which adjusts the viscosity of the magnetorheological fluid mixture by changing the magnetic field intensity.

[0028] A method for monitoring magnetorheological fluid in true triaxial fracturing tests includes the following steps:

[0029] S1: Prepare a standard specimen 6, drill a fracturing hole in the center of the specimen and perform hole sealing treatment.

[0030] S2: Mix the magnetorheological fluid with a liquid and adjust the viscosity of the mixed liquid.

[0031] S3: Start the data acquisition device 1 and debug the magnetic induction sensor array 4.

[0032] S4: Inject the magnetorheological fluid mixture into the specimen 6 through the high-pressure water pump device 3.

[0033] S5: Apply the target axial pressure and confining pressure to the specimen through the true triaxial experimental device 7.

[0034] S6: Monitor the magnetic field change of the magnetorheological fluid in the specimen in real time and invert the fracture propagation pattern.

[0035] S7: Inject clear water and change the external magnetic field to discharge the magnetorheological fluid mixture.

[0036] Furthermore, in S6, inverting the fracture propagation pattern includes:

[0037] Collect the baseline data and real-time data of the magnetic induction sensor array 4.

[0038] Perform low-pass filtering and baseline correction on the data.

[0039] Locate the fracture position and pattern through differential analysis, edge detection algorithm and finite element simulation.

[0040] Furthermore, the differential analysis includes calculating the difference between the real-time data and the baseline data of the sensor, and generating a magnetic field differential map to identify abnormal areas.

[0041] Furthermore, inverting the fracture propagation pattern further includes:

[0042] Generate a continuous magnetic field distribution map by bilinear interpolation and reconstruct the fracture morphology through 3D visualization technology.

[0043] Further, in step S7, by adjusting the magnetic field intensity, the viscosity of the ferrofluid is reduced, and clear water is used to drain the ferrofluid mixture from the specimen.

[0044] Further, in step S2, the adjustment of the viscosity of the mixture includes:

[0045] Changing the magnetic field intensity in the mixing device through a magnetic field loader until a preset viscosity threshold is reached.

[0046] Further, the data acquisition device 1 is also configured to collect the pumping range and pressure curve data of the high-pressure water pump device 3 in real time and perform correlation analysis with the magnetic field change data.

[0047] The beneficial effects of the present invention are as follows:

[0048] (1) By using ferrofluid as the sensing medium and utilizing the dynamic response characteristics of its magnetic particles during the fracture propagation process, combined with a high-density magnetic induction sensor array, millimeter-level positioning monitoring of microfractures and three-dimensional propagation trajectories inside the rock is achieved. Compared with traditional acoustic emission technology, the magnetic field signal is less affected by environmental noise, and through differential analysis and edge detection algorithms, the fracture positioning accuracy is improved to ±2 mm, and the spatial resolution is increased by more than 3 times, effectively solving the defects of insensitive internal fracture monitoring and fuzzy positioning in the prior art.

[0049] (2) The coordinated operation of the magnetic induction sensor array and the data acquisition device can achieve a data sampling frequency of thousands of times per second. Combined with real-time inversion algorithms (such as Fourier frequency domain analysis and finite element simulation), a three-dimensional thermal map of the fracture morphology can be generated synchronously during the fracturing process, dynamically displaying the change trends of key parameters such as fracture length, branch angle, and aperture, making up for the deficiencies of visual observation and acoustic emission technology in terms of data lag and single dimension.

[0050] (3) By dynamically adjusting the viscosity of the ferrofluid through a magnetic field loader and cooperating with the injection of clear water, efficient recovery of the ferrofluid is achieved (recovery rate > 95%), and the loss of ferrofluid in a single test is reduced to less than 1 / 10 of that of traditional tracers. This technical breakthrough solves the problems of high consumable costs and high environmental pollution risks in traditional monitoring, and is especially suitable for large-scale repetitive test scenarios.

[0051] (4) The system deeply integrates true triaxial stress loading, ferrofluid migration, and magnetic field response, and can synchronously obtain the correlation between stress-strain curves, fluid pressure pulsations, and magnetic field distortion data, providing a new tool for studying the interaction mechanism between fracture propagation, in-situ stress field, and fluid seepage, filling the gap in multi-field coupling analysis in the prior art.

[0052] (5) The modular-designed magnetic induction sensor array can be adapted to specimens of different sizes (covering specimens from 100 mm × 100 mm to 500 mm × 500 mm), and can adapt to various magnetic fluid properties by adjusting the magnetic field strength (adjustable in the range of 0.1 - 2 T), significantly improving the applicability of the system in the simulation of complex geological conditions, and providing a general solution for fracture monitoring in the development of unconventional energy sources such as shale gas and hot dry rock.

[0053] Other advantages, objectives, and features of the present invention will, to some extent, be described in the subsequent specification, and to some extent, will be obvious to those skilled in the art based on the study of the following text, or can be learned from the practice of the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the following specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be described in detail preferably with reference to the accompanying drawings, where:

[0055] Figure 1 is a flowchart of the present invention.

[0056] Reference numerals: 1, data acquisition device; 2, magnetic fluid liquid mixing device; 3, high-pressure water pump device; 4, magnetic induction sensor array; 5, magnetic fluid injection device; 6, specimen; 7, true triaxial test device. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0057] The following specific examples illustrate the embodiments of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the drawings provided in the following embodiments only illustrate the basic concept of the present invention schematically. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0058] Among them, the drawings are only for illustrative purposes, showing only schematic diagrams, not physical diagrams, and should not be construed as limiting the present invention; in order to better illustrate the embodiments of the present invention, some components in the drawings will be omitted, enlarged, or reduced, which do not represent the dimensions of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0059] In the drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc. indicating the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings. This is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0060] As Figure 1 shown, it is the flowchart of the present invention.

[0061] Example 1: Magnetic fluid viscosity adjustment and crack dynamic monitoring

[0062] Workflow:

[0063] Specimen preparation: Prepare a 200mm×200mm×200mm concrete specimen, drill a fracturing hole with a diameter of 10mm and a height of 150mm in the center, and seal the hole with epoxy resin.

[0064] Magnetic fluid mixing: Add Fe3O4 nano magnetic powder (particle size 50nm) and silicone oil to the magnetic fluid liquid mixing device 2 at a volume ratio of 1:5, start the stirrer (rotation speed 300rpm), and at the same time apply an axial magnetic field of 0.5T for 10min to obtain a magnetic fluid mixture with a viscosity of 120mPa·s. The magnetic fluid injection device 5 is connected to the magnetic fluid liquid mixing device 2 and is used to inject the mixed magnetic fluid liquid into the specimen 6.

[0065] Sensor layout: Arrange a 4×4 magnetic induction sensor array 4 on the surface of the specimen 6, with a sensor spacing of 50mm, connect to the data acquisition device 1 and calibrate the baseline.

[0066] Fracturing injection: Inject the magnetic fluid into the specimen at a rate of 2mL / s through the high-pressure water pump device 3, and simultaneously start the true triaxial test device 7 to load the axial pressure (X / Y / Z directions: 10 / 8 / 6MPa).

[0067] Real-time monitoring: The magnetic induction sensor array collects the magnetic field intensity at a frequency of 1kHz (range ±50mT, resolution 0.1mT). After the data is low-pass filtered (cutoff frequency 100Hz), the difference value from the baseline is calculated. When the magnetic field change in a certain area exceeds the threshold (ΔB>5mT), the edge detection algorithm is triggered to generate a crack thermal map.

[0068] Inversion verification: After the fracturing is completed, the specimen is dissected to compare the actual cracks with the three-dimensional reconstruction model output by the system. The results show that the crack length error is <2 mm and the aperture error is <0.1 mm.

[0069] Example 2: Recyclable mechanism of magnetic fluid

[0070] Workflow:

[0071] Magnetic fluid recovery: After the fracturing is completed, clean water is injected into the magnetic fluid liquid mixing device 2, and at the same time, the external magnetic field strength is reduced to 0.1 T, and the viscosity of the magnetic fluid is reduced to 20 mPa·s. The high-pressure water pump device 3 is started to backwash the specimen 6 at 5 mL / s for 10 min, and the outflowing magnetic fluid mixture is collected.

[0072] Magnetic powder separation: The recovered liquid is introduced into a centrifuge (5000 rpm, 10 min) to separate the Fe3O4 magnetic powder, and the magnetic powder recovery rate is 98.2%.

[0073] Repeated test: The recovered magnetic powder is mixed with new silicone oil in a ratio of 1:5, and the process of Example 1 is repeated. By comparing the sensor data of the two tests, the magnetic field response consistency error is <3%, which proves that the performance of the magnetic fluid has not decayed.

[0074] Example 3: Modular magnetic induction sensor array and multi-field coupling analysis

[0075] Workflow:

[0076] Specimen adaptation: Replace the specimen with granite of 300 mm×300 mm×300 mm, adjust the magnetic induction sensor array 4 to a 6×6 layout, expand the spacing to 75 mm, and adjust the output of the magnetic field loader to 1.2 T.

[0077] Multi-physical field synchronous acquisition:

[0078] The true triaxial experimental device 7 applies an axial pressure of 15 / 12 / 10 MPa and a confining pressure of 8 MPa;

[0079] The injection pressure of the high-pressure water pump device 3 is 20 MPa and the flow rate is 3 mL / s;

[0080] The data acquisition device 1 synchronously records the stress-strain curve, fluid pressure pulsation and magnetic field distortion data.

[0081] Coupling analysis: A stress-magnetic field coupling model is established through finite element software, and the real-time data is input to invert the crack propagation rate (peak value 0.5 mm / s) and the main crack azimuth angle (deviation <3°), and an association map of the crack network and the in-situ stress direction is output.

[0082] Example 4: High-sensitivity microcrack detection

[0083] Workflow:

[0084] Microcrack simulation: An artificial crack with a width of 0.1 mm and a depth of 30 mm was prefabricated in specimen 6, and a magnetic fluid containing 1% fluorescent dye was injected.

[0085] Data acquisition: The magnetic induction sensor array 4 sampled at a high frequency of 10 kHz, combined with wavelet transform for denoising, and extracted weak signals of 0.1 - 1 mT.

[0086] Quantitative analysis: Differential analysis was used to identify cracks with a width ≥ 0.08 mm; Fourier spectrum analysis was used to determine the resonance frequency of the cracks (main frequency 85 Hz) and eliminate noise interference; 3D reconstruction showed that the surface area error of the cracks was < 5%.

[0087] Verification: The specimen was irradiated with ultraviolet light, and the coincidence degree between the fluorescent area and the system output was > 95%.

[0088] 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 preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the present technical solution, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A magneto-fluid monitoring system and method for true triaxial fracturing tests, characterized in that: Including: A magnetorheological fluid liquid mixing device (2) for mixing magnetorheological fluid with liquid and adjusting the viscosity of the mixed liquid; A magnetorheological fluid injection device (5) connected to the magnetorheological fluid liquid mixing device (2) for injecting the mixed magnetorheological fluid liquid into a specimen (6); A magnetic induction sensor array (4) arranged around the specimen (6) for real-time monitoring of the magnetic field change of the magnetorheological fluid inside the specimen; A data acquisition device (1) connected to the magnetic induction sensor array (4) for acquiring magnetic field data; A high-pressure water pump device (3) connecting the magnetorheological fluid liquid mixing device (2) and the specimen (6) for injecting magnetorheological fluid mixed liquid and clear water into the specimen; A true triaxial experimental device (7) configured to apply three-dimensional stress to the specimen and control the fracturing process.

2. The true triaxial fracturing test magneto-fluid monitoring system according to claim 1, wherein: The magnetic induction sensor array (4) is a 4×4 array, and each sensor is configured to acquire magnetic field intensity and direction data.

3. The true triaxial fracturing test magneto - fluid monitoring system according to claim 1, characterized in that: The magnetorheological fluid liquid mixing device (2) includes a magnetic field loader for adjusting the viscosity of the magnetorheological fluid mixed liquid by changing the magnetic field intensity.

4. A magnetic fluid monitoring method for true triaxial fracturing tests, characterized in that: Including the following steps: S1: Prepare a standard specimen (6), drill a fracturing hole in the center of the specimen and perform hole sealing treatment; S2: Mix the magnetorheological fluid with the liquid and adjust the viscosity of the mixed liquid; S3: Start the data acquisition device (1) and debug the magnetic induction sensor array (4); S4: Inject the magnetorheological fluid mixed liquid into the specimen (6) through the high-pressure water pump device (3); S5: Apply the target axial pressure and confining pressure to the specimen through the true triaxial experimental device (7); S6: Real-time monitor the magnetic field change of the magnetorheological fluid in the specimen and invert the crack propagation morphology; S7: Inject clear water and change the external magnetic field to discharge the magnetorheological fluid mixed liquid.

5. The true triaxial fracturing test magnetic fluid monitoring method according to claim 4, characterized in that: In the said S6, inverting the crack propagation morphology includes: Acquiring the baseline data and real-time data of the magnetic induction sensor array (4); Performing low-pass filtering and baseline correction on the data; Locating the crack position and morphology through differential analysis, edge detection algorithm and finite element simulation.

6. The magneto-fluid monitoring method for true triaxial fracturing test according to claim 5, characterized in that: The said differential analysis includes calculating the difference between the real-time data and the baseline data of the sensor to generate a magnetic field differential map to identify the abnormal area.

7. The magneto-fluid monitoring method for true triaxial fracturing test according to claim 5, characterized in that: The said inverting the crack propagation morphology further includes: Applying bilinear interpolation to generate a continuous magnetic field distribution map and reconstructing the crack morphology through three-dimensional visualization technology.

8. The true triaxial fracturing test magnetic fluid monitoring method according to claim 4, wherein: In the said S7, the viscosity of the magnetorheological fluid is reduced by adjusting the magnetic field intensity, so that the clear water discharges the magnetorheological fluid mixed liquid from the specimen.

9. The true triaxial fracturing test magnetic fluid monitoring method according to claim 4, characterized in that: In the said S2, the adjusting the viscosity of the mixed liquid includes: Changing the magnetic field intensity in the mixing device through the magnetic field loader until the preset viscosity threshold is reached.

10. The true triaxial fracturing test magneto-fluid monitoring system according to claim 1, characterized in that: The data acquisition device (1) is further configured to acquire the pumping range and pressure curve data of the high-pressure water pump device (3) in real time and perform correlation analysis with the magnetic field change data.

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