Supercritical carbon dioxide fracturing crack real-time visual monitoring system and method

Abstract

The invention discloses a supercritical carbon dioxide fracturing crack real-time visual monitoring system and method. The system comprises a transparent box body, an axial pressure mechanism, a multi-mode monitoring system, a supercritical carbon dioxide injection mechanism and a data processing module. The axial pressure mechanism is used for applying preset axial pressure to the upper end face of the rock sample and dynamically adjusting the load of the axial pressure mechanism according to crack propagation on the surface of the rock sample; the multi-mode monitoring system comprises a high-speed camera, an acoustic emission probe and an optical fiber demodulator; the supercritical carbon dioxide injection mechanism is used for injecting supercritical carbon dioxide into the containing cavity. According to the invention, through multi-mode monitoring systems such as the high-speed camera, the acoustic emission probe and the distributed optical fiber, real-time and visual monitoring of the propagation process of the supercritical carbon dioxide fracturing crack is realized, the problem of lack of real-time and visual monitoring means in the prior art is solved, and researchers can directly observe the dynamic evolution process of the crack.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas and deep geothermal reservoir stimulation, and particularly relates to a real-time visual monitoring system and method for supercritical carbon dioxide fracturing cracks. Background Art

[0002] In the development of unconventional oil and gas reservoirs, such as shale gas and tight oil, traditional extraction technologies face many challenges. Supercritical carbon dioxide (SCCO2), with its unique physical and chemical properties, brings new solutions to this field.

[0003] SCCO2 has characteristics such as low viscosity, high diffusivity, and strong corrosion, which make it show significant advantages in the development of unconventional oil and gas reservoirs. In terms of enhancing the complexity of the fracturing crack network, SCCO2 performs better than traditional fracturing fluids. It can reduce the rock fracture pressure, penetrate deep into the mineral pores and fine microfractures with its high diffusivity, and induce a complex crack network under a certain fluid pressure, thereby effectively increasing the reservoir stimulation volume and improving the oil and gas extraction efficiency.

[0004] From an environmental protection perspective, using SCCO2 as a fracturing medium has good benefits. It can be recycled, greatly reducing the consumption of fresh water, which is particularly important for oil and gas development in areas with relatively scarce water resources. At the same time, SCCO2 also has the potential for carbon sequestration, helping to reduce carbon emissions, which is in line with the current trend of environmental protection development.

[0005] In addition, SCCO2 has extremely strong adaptability. It can not only achieve good results in the stimulation of conventional oil and gas reservoirs, but also still be able to fracture hard rocks and open cracks in high-temperature and high-pressure reservoirs; in water-sensitive reservoirs, it can effectively avoid the swelling problem of clay minerals, avoiding problems such as the decrease in reservoir permeability caused by clay swelling.

[0006] However, despite the many advantages of SCCO2 as a fracturing medium, there are still bottlenecks in the current research on its crack propagation mechanism. The main problem lies in the lack of real-time and visual monitoring means, making it difficult to directly observe the dynamic evolution process of cracks. This prevents researchers from accurately grasping the laws and characteristics of crack propagation, and further restricts the optimization of fracturing design, unable to fully exert the potential of SCCO2 fracturing technology. Summary of the Invention

[0007] The purpose of the present invention is to overcome the above technical deficiencies, and propose a real-time visual monitoring system and method for supercritical carbon dioxide fracturing cracks, to solve the technical problem that there is a lack of real-time and visual monitoring means for supercritical carbon dioxide (SCCO2) fracturing cracks in the prior art, making it difficult to directly observe the dynamic evolution process of cracks, and further restricting the optimization of fracturing design.

[0008] To achieve the above technical objectives, the present invention adopts the following technical solutions:

[0009] The present invention provides a real-time visualization monitoring system for supercritical carbon dioxide fracturing cracks, including:

[0010] A transparent box body, which has a sealed accommodation cavity for placing a rock sample. A pressure sensor is arranged in the accommodation cavity. Wherein, the surface of the rock sample is sprayed with matte speckles, a distributed optical fiber is pasted on the surface of the rock sample, and a preset crack is pre-opened;

[0011] An axial compression mechanism, which is connected to the upper end surface of the rock sample in the accommodation cavity and is used to apply a preset axial pressure to the upper end surface of the rock sample, and dynamically adjusts the load of the axial compression mechanism according to the crack propagation on the surface of the rock sample;

[0012] A multimodal monitoring system, which includes a high-speed camera, an acoustic emission probe and an optical fiber demodulator. The high-speed camera is used to collect the displacement of the matte speckles on the surface of the rock sample, the acoustic emission probe is used to receive the acoustic signals generated during the crack propagation of the rock sample, and the optical fiber demodulator is electrically connected to the distributed optical fiber and is used to receive the strain data of the distributed optical fiber;

[0013] A supercritical carbon dioxide injection mechanism, which is used to inject supercritical carbon dioxide into the accommodation cavity and keep the pressure of the supercritical carbon dioxide in the accommodation cavity at a preset pressure; and

[0014] A data processing module, which is communicatively connected to the high-speed camera, the acoustic emission probe, the optical fiber demodulator and the supercritical carbon dioxide injection mechanism, and is used to receive and process multimodal data, and generate a crack strain field and a pressure-load dynamic evolution map of the rock sample.

[0015] In some embodiments, the supercritical carbon dioxide injection mechanism includes a liquid carbon dioxide tank, a low-temperature water bath device, a booster pump, an injection pipe and a heating coil. The liquid carbon dioxide tank is arranged in the low-temperature water bath device. The inlet of the booster pump is communicated with the outlet of the liquid carbon dioxide tank. The outlet of the booster pump is communicated with one end of the injection pipe. The other end of the injection pipe is communicated with the accommodation cavity. The heating coil is wound around the injection pipe.

[0016] In some embodiments, the axial compression mechanism includes a hydraulic cylinder, a hydraulic source and a servo valve. The cylinder body of the hydraulic cylinder is fixed to the inner top surface of the transparent box body. The output shaft of the hydraulic cylinder abuts against the top surface of the rock sample. The output end of the hydraulic source is communicated with one end of the servo valve. The other end of the servo valve is communicated with the liquid inlet end of the hydraulic cylinder.

[0017] In some embodiments, the transparent box includes a steel frame and a high-transparency Pam board, the high-transparency Pam board is sealed to the steel frame, the optical transparency of the high-transparency Pam board is ≥90%, the compressive strength is ≥200MPa, and the temperature resistance range is -50°C to 150°C. The sealing gap between the high-transparency Pam board and the steel frame is filled with high-temperature resistant sealant.

[0018] In some embodiments, the distributed optical fiber is pasted along the crack direction of the rock sample and connected to a fiber optic demodulator through a jumper. The monitoring frequency of the distributed optical fiber is ≥100 Hz. The acoustic emission probe is a three-probe array arranged around the rock sample. The signal threshold is 45 dB. The two-dimensional spatial positioning of the crack extension is solved by iterative nonlinear equations in combination with the sound velocity model.

[0019] In some embodiments, the real-time visualization monitoring system for supercritical carbon dioxide fracturing cracks further includes a circulation mechanism, the inlet and outlet of which are respectively connected to the high-pressure transparent cavity, and the flow rate of the supercritical carbon dioxide is controlled by adjusting the circulation flow rate to analyze the effect of the flow rate on crack expansion.

[0020] The present invention also provides a method for real-time visualization monitoring of supercritical carbon dioxide fracturing cracks, which is applicable to the real-time visualization monitoring system for supercritical carbon dioxide fracturing cracks and includes the following steps:

[0021] Sample preparation: Shale or hot dry rock is processed into a flat sample as a rock sample. Cracks are pre-set on the surface of the rock sample and matte speckle is sprayed on it. Distributed optical fibers are attached around the cracks.

[0022] Device installation: Fix the rock sample in the holding chamber, install a high-speed camera, acoustic emission probe and optical fiber demodulator;

[0023] Pressurization and loading: starting the axial pressure mechanism to apply an initial load, and the supercritical carbon dioxide injection mechanism to inject supercritical carbon dioxide into the containing chamber and maintain the pressure of the supercritical carbon dioxide in the containing chamber at a preset pressure;

[0024] Data acquisition: Synchronous acquisition of high-speed cameras, acoustic emission probes and fiber optic demodulators to collect crack expansion data in real time;

[0025] Data processing: The crack strain field is calculated based on the digital speckle displacement. The formula is:

[0026] εxx(i,j)=2Δx(u(i+1,j)-u(i-1,j))

[0027] εyy(i,j)=2Δy(v(i,j+1)-v(i,j-1))

[0028] γxy(i,j) = 2Δy(u(i,j + 1) - u(i,j - 1)) + 2Δx(v(i + 1,j) - v(i - 1,j))

[0029] Among them, εxx(i,j) is the transverse normal strain, εyy(i,j) is the longitudinal normal strain, γxy(i,j) is the shear strain, Δx is the transverse pixel pitch, Δy is the longitudinal pixel pitch, u(i,j) is the transverse displacement component at the pixel point (i,j), and v(i,j) is the longitudinal displacement component at the pixel point (i,j);

[0030] Based on the time difference of acoustic emission signals to locate the crack propagation path, the formula is:

[0031]

[0032] Among them, the number of the acoustic emission probes is three, namely the first acoustic emission probe, the second acoustic emission probe and the third acoustic emission probe, with coordinates (x1,y1), (x2,y2), (x3,y3) respectively. The position of the sound source to be determined is (x,y), c is the sound speed, and Δt 12 is the time difference between the first probe and the second probe, and Δt 13 is the time difference between the first probe and the third probe;

[0033] Perform Taylor expansion linearization on the above equations, and use the least squares method or the iterative method to iteratively approximate the sound source position (x,y);

[0034] After the data processing is completed, comprehensively consider the crack propagation results in multiple directions to obtain the dynamic evolution process of multi-modal cracks.

[0035] In some embodiments, the preset crack is in a cross shape, one side of which is parallel to the pressing direction of the axial compression mechanism and the other side is perpendicular. By comparing the extension lengths of the vertical side and the horizontal side, analyze the influence of the overlying pressure on the crack propagation process.

[0036] In some embodiments, the lithologies at both ends of the preset crack are different, one end is a high clay content area and the other end is a low clay content area. By comparing the extension lengths at both ends, analyze the influence of the clay content on the crack propagation process.

[0037] In some embodiments, adjust the supercritical carbon dioxide flow rate through a circulation mechanism, and conduct multiple groups of parallel experiments under the same pressure to analyze the influence of the flow rate of critical carbon dioxide on the crack propagation process.

[0038] Compared with the prior art, the beneficial effects of the real-time visual monitoring system and method for supercritical carbon dioxide fracturing cracks provided by the present invention are as follows: Through a multi-modal monitoring system such as a high-speed camera, an acoustic emission probe, and a distributed optical fiber, real-time and visual monitoring of the supercritical carbon dioxide fracturing crack propagation process is achieved, solving the problem of the lack of real-time and visual monitoring means in the prior art, enabling researchers to directly observe the dynamic evolution process of cracks; at the same time, based on the generated crack strain field and the dynamic evolution map of pressure-load, researchers can accurately master the laws and characteristics of crack propagation, providing a scientific basis for fracturing design, thereby optimizing the fracturing design and giving full play to the potential of supercritical carbon dioxide fracturing technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a schematic structural diagram of a real-time visual monitoring system for supercritical carbon dioxide fracturing cracks provided by an embodiment of the present invention;

[0040] Figure 2 is Figure 1 a top view of the transparent box body in

[0041] Figure 3 is a schematic structural diagram of the circulation mechanism;

[0042] Figure 4 is a schematic structural diagram of a rock sample with a cross-shaped prefabricated crack;

[0043] Description of the reference numerals: 1 - transparent box body, 11 - steel frame, 12 - high-transparency PAM board, 2 - axial compression mechanism, 21 - hydraulic cylinder, 22 - hydraulic source, 23 - servo valve, 24 - axial compression detector, 3 - multi-modal monitoring system, 31 - distributed optical fiber, 32 - high-speed camera, 33 - acoustic emission probe, 34 - optical fiber demodulator, 4 - supercritical carbon dioxide injection mechanism, 41 - liquid carbon dioxide tank, 42 - low-temperature water bath device, 43 - booster pump, 44 - injection pipe, 45 - heating coil, 46 - internal heating element, 5 - data processing module, 6 - rock sample, 61 - prefabricated crack, 7 - circulation mechanism. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0045] In order to solve the technical problem in the prior art that there is a lack of real-time and visual monitoring means for supercritical carbon dioxide (SCCO2) fracturing cracks, it is difficult to directly observe the dynamic evolution process of cracks, and thus the optimization of fracturing design is limited, the present invention provides a real-time visual monitoring system and method for supercritical carbon dioxide fracturing cracks.

[0046] Please refer to Figure 1 and Figure 2 , Figure 1 FIG. is a schematic structural diagram of a real-time visual monitoring system for supercritical carbon dioxide fracturing cracks according to an embodiment of the present invention. The real-time visual monitoring system and method for supercritical carbon dioxide fracturing cracks include a transparent box body 1, an axial pressure mechanism 2, a multimodal monitoring system 3, a supercritical carbon dioxide injection mechanism 4, and a data processing module 5.

[0047] The transparent box body 1 has a sealed accommodation cavity for placing a rock sample 6, and a pressure sensor is arranged in the accommodation cavity. Wherein, a matte speckle is sprayed on the surface of the rock sample 6, a distributed optical fiber 31 is pasted on the surface of the rock sample 6, and a preset crack 61 is pre-opened.

[0048] The axial pressure mechanism 2 is connected to the upper end surface of the rock sample 6 in the accommodation cavity and is used to apply a preset axial pressure to the upper end surface of the rock sample 6, and adjust the load of the axial pressure mechanism 2 according to the dynamic crack propagation on the surface of the rock sample 6.

[0049] The multimodal monitoring system 3 includes a high-speed camera 32, an acoustic emission probe 33, and an optical fiber demodulator 34. The high-speed camera 32 is used to collect the displacement of the matte speckle on the surface of the rock sample 6, the acoustic emission probe 33 is used to receive the acoustic signal generated when the crack of the rock sample 6 propagates, and the optical fiber demodulator 34 is electrically connected to the distributed optical fiber 31 and is used to receive the strain data of the distributed optical fiber 31.

[0050] The supercritical carbon dioxide injection mechanism 4 is used to inject supercritical carbon dioxide into the accommodation cavity and keep the pressure of the supercritical carbon dioxide in the accommodation cavity at a preset pressure.

[0051] The data processing module 5 is communicatively connected to the high-speed camera 32, the acoustic emission probe 33, the optical fiber demodulator 34, and the supercritical carbon dioxide injection mechanism 4, and is used to receive and process multimodal data, and generate a crack strain field and a pressure-load dynamic evolution map of the rock sample 6.

[0052] The working process of the above technical solution is as follows:

[0053] (1) Preparation stage

[0054] Rock sample preparation: Process the rock sample 6, spray a matte speckle on its surface, paste the distributed optical fiber 31, and pre-open the preset crack 61, and then place the rock sample 6 in the accommodation cavity of the transparent box body 1.

[0055] Equipment installation and debugging: Install the axial compression mechanism 2 and connect it to the upper end face of the rock sample 6; install the high-speed camera 32, acoustic emission probe 33 and fiber optic demodulator 34 of the multimodal monitoring system 3; connect the supercritical carbon dioxide injection mechanism 4; communicatively connect the data processing module 5 with the high-speed camera 32, acoustic emission probe 33, fiber optic demodulator 34 and supercritical carbon dioxide injection mechanism 4.

[0056] (2) Experimental stage

[0057] Apply axial compression: The axial compression mechanism 2 applies a preset axial compression to the upper end face of the rock sample 6 and dynamically adjusts the load according to the crack propagation on the surface of the rock sample 6.

[0058] Inject supercritical carbon dioxide: The supercritical carbon dioxide injection mechanism 4 injects supercritical carbon dioxide into the accommodation chamber and keeps the pressure of the supercritical carbon dioxide in the chamber at the preset pressure. Under the action of the supercritical carbon dioxide, the pre-set cracks 61 of the rock sample 6 begin to expand.

[0059] Data acquisition: The high-speed camera 32 acquires the displacement of the matte speckle on the surface of the rock sample 6. The acoustic emission probe 33 receives the acoustic signals generated during the crack propagation of the rock sample 6. The fiber optic demodulator 34 receives the strain data transmitted by the distributed optical fiber 31. The pressure sensor monitors the pressure in the accommodation chamber.

[0060] (3) Data processing stage

[0061] The data processing module 5 receives the multimodal data transmitted from the high-speed camera 32, acoustic emission probe 33, fiber optic demodulator 34 and supercritical carbon dioxide injection mechanism 4, and generates a crack strain field and a pressure-load dynamic evolution map of the rock sample 6 based on these data.

[0062] The technical effects of the above technical solution are as follows: Through the multimodal monitoring system 3 such as the high-speed camera 32, acoustic emission probe 33 and distributed optical fiber 31, the real-time and visual monitoring of the crack propagation process of supercritical carbon dioxide fracturing is realized, solving the problem of the lack of real-time and visual monitoring means in the prior art, enabling researchers to directly observe the dynamic evolution process of cracks; at the same time, based on the generated crack strain field and pressure-load dynamic evolution map, researchers can accurately master the laws and characteristics of crack propagation, provide a scientific basis for fracturing design, thereby optimizing the fracturing design and giving full play to the potential of supercritical carbon dioxide fracturing technology.

[0063] In one of the embodiments, please refer to Figure 1 and Figure 2, the supercritical carbon dioxide injection mechanism 4 includes a liquid carbon dioxide tank 41, a low-temperature water bath device 42, a booster pump 43, an injection pipe 44, and a heating coil 45. The liquid carbon dioxide tank 41 is disposed inside the low-temperature water bath device 42. The inlet of the booster pump 43 is communicated with the outlet of the liquid carbon dioxide tank 41. The outlet of the booster pump 43 is communicated with one end of the injection pipe 44. The other end of the injection pipe 44 is communicated with the accommodation cavity. The heating coil 45 is wound around the injection pipe 44. Preferably, an internal heating element 46 is further disposed in the accommodation cavity, and the internal heating element 46 is used to heat the supercritical carbon dioxide in the accommodation cavity to keep it at a preset temperature.

[0064] In one embodiment, please refer to Figure 1 and Figure 2 , the axial pressure mechanism 2 includes a hydraulic cylinder 21, a hydraulic source 22, a servo valve 23, and an axial pressure detection member 24. The cylinder body of the hydraulic cylinder 21 is fixed to the inner top surface of the transparent box body 1. The output shaft of the hydraulic cylinder 21 abuts against the top surface of the rock sample 6. The output end of the hydraulic source 22 is communicated with one end of the servo valve 23. The other end of the servo valve 23 is communicated with the liquid inlet end of the hydraulic cylinder 21. The axial pressure detection member 24 is used for placing the rock sample 6.

[0065] In this embodiment, the liquid carbon dioxide tank 41 is placed inside the low-temperature water bath device 42, and the liquid carbon dioxide can be maintained in a suitable low-temperature state. The booster pump 43 pumps out and pressurizes the liquid carbon dioxide from the liquid carbon dioxide tank 41, and the heating coil 45 wound around the injection pipe 44 can heat the carbon dioxide during the injection process. Through the synergistic effect of the low-temperature water bath device 42 and the heating coil 45, the temperature and pressure conditions required for the carbon dioxide to reach the supercritical state can be accurately controlled, ensuring that the supercritical carbon dioxide injected into the accommodation cavity is stable, providing a basis for accurately simulating the supercritical carbon dioxide fracturing process.

[0066] In one embodiment, please refer to Figure 1 and Figure 2 , the transparent box body 1 includes a steel frame 11 and a high-transparency PAM plate 12. The high-transparency PAM plate 12 is sealed to the steel frame 11. The optical transparency of the high-transparency PAM plate 12 is ≥90%, the compressive strength is ≥200 MPa, and the temperature tolerance range is -50°C to 150°C. The sealing gap between the high-transparency PAM plate 12 and the steel frame 11 is filled with a high-temperature resistant sealant.

[0067] In one embodiment, please refer to Figure 1 and Figure 2, the distributed optical fiber 31 is pasted along the crack direction of the rock sample 6 and is connected to the optical fiber demodulator 34 through a jumper wire. The monitoring frequency of the distributed optical fiber 31 is ≥100 Hz; the acoustic emission probe 33 is a three-probe array arranged around the rock sample 6, with a signal threshold of 45 dB, and the two-dimensional spatial positioning of crack propagation is iteratively solved through a nonlinear equation in combination with the sound velocity model.

[0068] In this embodiment, the distributed optical fiber 31 is pasted along the crack direction of the rock sample 6 and can closely fit the crack area. Since its monitoring frequency is ≥100 Hz, it can quickly and accurately capture extremely subtle strain changes during the crack propagation of the rock sample 6. During the supercritical carbon dioxide fracturing process, the crack propagation may be instantaneous and tiny. High-frequency monitoring can record these changes in real time, providing high-precision data support for subsequent analysis of the dynamic process of crack propagation. The acoustic emission probe 33 is arranged around the rock sample 6 in a three-probe array. When the crack of the rock sample 6 propagates, acoustic signals will be generated. The acoustic emission probe 33 can receive these signals, and the two-dimensional spatial positioning of crack propagation is iteratively solved through a nonlinear equation in combination with the sound velocity model. This positioning method can relatively accurately determine the position and direction of crack propagation, helping researchers clearly understand the development of cracks inside the rock sample 6.

[0069] In one of the embodiments, please refer to Figure 3 , the supercritical carbon dioxide fracturing crack real-time visualization monitoring system further includes a circulation mechanism 7. The circulation mechanism 7 is a circulation pump. The inlet and outlet of the circulation pump are respectively connected to the high-pressure transparent cavity, and the flow rate of supercritical carbon dioxide is controlled by adjusting the circulation flow rate, which is used to analyze the influence of the flow rate on crack propagation. Specifically, by adjusting the flow rate of supercritical carbon dioxide through the circulation mechanism 7, multiple groups of parallel experiments are carried out under the same pressure to analyze the influence of the flow rate of critical carbon dioxide on the crack propagation process.

[0070] In this embodiment, during the supercritical carbon dioxide fracturing process, crack propagation is affected by various factors. The circulation mechanism 7 can adjust the flow rate of supercritical carbon dioxide, enabling researchers to use the flow rate as a single variable to carry out multiple groups of parallel experiments while keeping other conditions (such as pressure) the same. By comparing the crack propagation situations at different flow rates, the specific influence of the flow rate on the crack propagation process can be clearly analyzed. This helps to deeply reveal the crack propagation mechanism of supercritical carbon dioxide fracturing and fills the research gap in the influence of flow rate in this field.

[0071] The present invention also provides a method for real-time visualization monitoring of supercritical carbon dioxide fracturing cracks, which is applicable to the supercritical carbon dioxide fracturing crack real-time visualization monitoring system and includes the following steps:

[0072] Specimen preparation: Process shale or hot dry rock into flat specimens as rock specimens 6, preset cracks 61 on the surface of the rock specimen 6 and spray matte speckles, and paste distributed optical fibers 31 around the cracks;

[0073] Device installation: Fix the rock specimen 6 in the accommodation cavity, and install a high-speed camera 32, an acoustic emission probe 33 and an optical fiber demodulator 34;

[0074] Pressurization and loading: Start the axial pressure mechanism 2 to apply an initial load, and the supercritical carbon dioxide injection mechanism 4 injects supercritical carbon dioxide into the accommodation cavity and keeps the pressure of the supercritical carbon dioxide in the accommodation cavity at a preset pressure;

[0075] Data acquisition: Synchronously collect the high-speed camera 32, the acoustic emission probe 33 and the optical fiber demodulator 34, and collect crack propagation data in real time;

[0076] Data processing: Calculate the crack strain field based on digital speckle displacement, and the formula is:

[0077] εxx(i,j) = 2Δx(u(i + 1,j) - u(i - 1,j))

[0078] εyy(i,j) = 2Δy(v(i,j + 1) - v(i,j - 1))

[0079] γxy(i,j) = 2Δy(u(i,j + 1) - u(i,j - 1)) + 2Δx(v(i + 1,j) - v(i - 1,j))

[0080] Among them, εxx(i,j) is the transverse normal strain, εyy(i,j) is the longitudinal normal strain, γxy(i,j) is the shear strain, Δx is the transverse pixel pitch, Δy is the longitudinal pixel pitch, u(i,j) is the transverse displacement component at the pixel point (i,j), and v(i,j) is the longitudinal displacement component at the pixel point (i,j);

[0081] Locate the crack propagation path based on the time difference of acoustic emission signals, and the formula is:

[0082]

[0083] Among them, the number of the acoustic emission probes 33 is three, which are the first acoustic emission probe, the second acoustic emission probe and the third acoustic emission probe, and the coordinates are (x1,y1), (x2,y2), (x3,y3) respectively. The position of the sound source to be determined is (x,y), c is the sound speed, and Δt 12 is the time difference between the first probe and the second probe, and Δt 13 is the time difference between the first probe and the third probe;

[0084] Perform Taylor expansion linearization on the above equations, and use the least squares method or the iterative method to iteratively approximate the sound source position (x, y);

[0085] After the data processing is completed, comprehensively combine the crack propagation results from multiple directions to obtain the dynamic evolution process of multi-modal cracks.

[0086] In one embodiment, please refer to Figure 4 , the pre-set crack 61 is in a cross shape, one side of which is parallel to the pressing direction of the axial compression mechanism 2, and the other side is perpendicular. By comparing the extension lengths of the vertical side and the horizontal side, the influence of the overlying pressure on the crack propagation process is analyzed.

[0087] In this embodiment, the pre-set crack 61 is designed in a cross shape, and one side of it is parallel to the pressing direction of the axial compression mechanism 2, and the other side is perpendicular. In this way, the connection between the overlying pressure (applied by the axial compression mechanism 2) and the crack propagation direction is clearly constructed. By accurately measuring and comparing the extension lengths of the vertical side and the horizontal side, the influence of the overlying pressure on the crack propagation process in different directions can be intuitively and accurately analyzed. For example, if the extension length of the vertical side is greater than that of the horizontal side, it may indicate that cracks are more likely to propagate in the axial compression direction, providing clear data support for studying the relationship between crack propagation and overlying pressure.

[0088] In one embodiment, please refer to Figure 1 , the lithologies at both ends of the pre-set crack 61 are different, one end is a high clay content area, and the other end is a low clay content area. By comparing the extension lengths at both ends, the influence of the clay content on the crack propagation process is analyzed.

[0089] In this embodiment, directly comparing the differences in the extension lengths at both ends of the pre-set crack 61 due to different clay contents can clearly determine the qualitative and quantitative relationships between the clay content and crack propagation. For example, if the crack extension length at the high clay content end is shorter, it indicates that the clay content may have an inhibitory effect on crack propagation, thus providing a key clue for in-depth study of its internal influence mechanism. In practical applications, for reservoirs with different clay content areas, according to the relationship between the clay content and crack propagation obtained in the experiment, the fracturing strategy can be adjusted accordingly. For example, in areas with high clay content, appropriately increase the injection volume or pressure of supercritical carbon dioxide to overcome the hindrance of clay to crack propagation, thereby optimizing the crack network and improving the reservoir stimulation effect.

[0090] The specific embodiments of the present invention described above do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and deformations made according to the technical concept of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A real-time visual monitoring system for supercritical carbon dioxide fracturing cracks, characterized in that, Comprising: A transparent box body, which has a sealed accommodation cavity for placing rock samples, and a pressure sensor is arranged in the accommodation cavity. Wherein, a matte speckle is sprayed on the surface of the rock sample, a distributed optical fiber is pasted on the surface of the rock sample, and a preset crack is pre-opened; An axial compression mechanism, which is connected to the upper end surface of the rock sample in the accommodation cavity and is used to apply a preset axial compression to the upper end surface of the rock sample, and dynamically adjusts the load of the axial compression mechanism according to the crack propagation on the surface of the rock sample; A multi-modal monitoring system, which includes a high-speed camera, an acoustic emission probe and an optical fiber demodulator. The high-speed camera is used to collect the displacement of the matte speckle on the surface of the rock sample, the acoustic emission probe is used to receive the acoustic signal generated during the crack propagation of the rock sample, and the optical fiber demodulator is electrically connected to the distributed optical fiber and is used to receive the strain data of the distributed optical fiber; A supercritical carbon dioxide injection mechanism, which is used to inject supercritical carbon dioxide into the accommodation cavity and keep the pressure of the supercritical carbon dioxide in the accommodation cavity at a preset pressure; and A data processing module, which is communicatively connected to the high-speed camera, the acoustic emission probe, the optical fiber demodulator and the supercritical carbon dioxide injection mechanism, and is used to receive and process multi-modal data, and generate a crack strain field and a pressure-load dynamic evolution map of the rock sample.

2. The real-time visual monitoring system for supercritical carbon dioxide fracturing cracks according to claim 1, wherein The supercritical carbon dioxide injection mechanism includes a liquid carbon dioxide tank, a low-temperature water bath device, a booster pump, an injection pipe and a heating coil. The liquid carbon dioxide tank is arranged in the low-temperature water bath device. The inlet of the booster pump is communicated with the outlet of the liquid carbon dioxide tank. The outlet of the booster pump is communicated with one end of the injection pipe. The other end of the injection pipe is communicated with the accommodation cavity. The heating coil is wound around the injection pipe.

3. The real-time visual monitoring system for supercritical carbon dioxide fracturing fractures according to claim 1, wherein The axial compression mechanism includes a hydraulic cylinder, a hydraulic source and a servo valve. The cylinder body of the hydraulic cylinder is fixed on the inner top surface of the transparent box body. The output shaft of the hydraulic cylinder abuts against the top surface of the rock sample. The output end of the hydraulic source is communicated with one end of the servo valve. The other end of the servo valve is communicated with the liquid inlet end of the hydraulic cylinder.

4. The real-time visual monitoring system for supercritical carbon dioxide fracturing fractures according to claim 1, characterized in that, The transparent box body includes a steel frame and a highly transparent PAM plate. The highly transparent PAM plate is sealed in the steel frame. The optical transparency of the highly transparent PAM plate is ≥90%, the compressive strength is ≥200 MPa, and the temperature tolerance range is -50°C to 150°C. The sealing gap between the highly transparent PAM plate and the steel frame is filled with a high-temperature resistant sealant.

5. The real-time visual monitoring system for supercritical carbon dioxide fracturing fractures according to claim 1, wherein The distributed optical fiber is pasted along the crack direction of the rock sample and is connected to the optical fiber demodulator through a jumper. The monitoring frequency of the distributed optical fiber is ≥100 Hz; the acoustic emission probe is a three-probe array, which is arranged around the rock sample, the signal threshold is 45 dB, and the two-dimensional spatial positioning of the crack propagation is iteratively solved through a nonlinear equation in combination with a sound velocity model.

6. The real-time visual monitoring system for supercritical carbon dioxide fracturing fractures according to claim 1, wherein It further includes a circulation mechanism, whose inlet and outlet are respectively connected to the high-pressure transparent cavity, and the flow rate of the supercritical carbon dioxide is controlled by adjusting the circulation flow rate, which is used to analyze the influence of the flow rate on the crack propagation.

7. A real-time visual monitoring method for supercritical carbon dioxide fracturing cracks, characterized in that, Applicable to the supercritical carbon dioxide fracturing crack real-time visualization monitoring system as described in claims 1-6, and includes the following steps: Specimen preparation: Shale or hot dry rock is processed into flat specimens as rock samples. Preset cracks are made on the surface of the rock samples and matte speckles are sprayed. Distributed optical fibers are pasted around the cracks. Device installation: The rock sample is fixed in the accommodating cavity, and a high-speed camera, an acoustic emission probe, and an optical fiber demodulator are installed. Pressurization and loading: Start the axial pressure mechanism to apply an initial load. The supercritical carbon dioxide injection mechanism injects supercritical carbon dioxide into the accommodating cavity and keeps the pressure of the supercritical carbon dioxide in the accommodating cavity at a preset pressure. Data acquisition: Synchronously collect data from the high-speed camera, the acoustic emission probe, and the optical fiber demodulator, and collect crack propagation data in real time. Data processing: Calculate the crack strain field based on digital speckle displacement. The formula is: εxx(i,j) = 2Δx(u(i + 1,j) - u(i - 1,j)) εyy(i,j) = 2Δy(v(i,j + 1) - v(i,j - 1)) γxy(i,j) = 2Δy(u(i,j + 1) - u(i,j - 1)) + 2Δx(v(i + 1,j) - v(i - 1,j)) where εxx(i,j) is the transverse normal strain, εyy(i,j) is the longitudinal normal strain, γxy(i,j) is the shear strain, Δx is the transverse pixel pitch, Δy is the longitudinal pixel pitch, u(i,j) is the transverse displacement component at the pixel point (i,j), and v(i,j) is the longitudinal displacement component at the pixel point (i,j). Locate the crack propagation path based on the time difference of acoustic emission signals. The formula is: Among them, the number of the acoustic emission probes is three, namely the first acoustic emission probe, the second acoustic emission probe and the third acoustic emission probe, with coordinates (x1, y1), (x2, y2), (x3, y3) respectively. The position of the sound source to be determined is (x, y), c is the speed of sound, and Δt 12 is the time difference between the first probe and the second probe, and Δt 13 is the time difference between the first probe and the third probe; Perform Taylor expansion linearization on the above equations, and use the least squares method or the iterative method to iteratively approximate the sound source position (x,y). After the data processing is completed, comprehensively analyze the crack propagation results from multiple directions to obtain the dynamic evolution process of multi-modal cracks.

8. The real-time visual monitoring method for fractures in supercritical carbon dioxide fracturing according to claim 7, characterized in that, The preset crack is in a cross shape, one side of which is parallel to the pressing direction of the axial pressure mechanism, and the other side is perpendicular. By comparing the propagation lengths of the vertical side and the horizontal side, analyze the influence of overlying pressure on the crack propagation process.

9. The real-time visual monitoring method for supercritical carbon dioxide fracturing fractures according to claim 7, wherein The lithologies at both ends of the preset crack are different, one end is a high clay content area, and the other end is a low clay content area. By comparing the propagation lengths of both ends, analyze the influence of clay content on the crack propagation process.

10. The real-time visual monitoring method for supercritical carbon dioxide fracturing fractures according to claim 7, characterized in that, Adjust the flow rate of supercritical carbon dioxide through the circulation mechanism, and conduct multiple groups of parallel experiments at the same pressure to analyze the influence of the flow rate of critical carbon dioxide on the crack propagation process.

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