A device and method for evaluating the stability of deep shale fractures

Abstract

This invention discloses a device and method for evaluating the stability of deep shale fractures. The device includes: a clamp, a confining pressure tube, a loading rod, a magnetic coil, a magnet, a receiving device, and a display terminal. The clamp has a receiving cavity; the loading rod is located at the bottom of the receiving cavity and can abut against the lower end of the rock sample; the confining pressure tube is located inside the receiving cavity and can be arranged around the outside of the rock sample, and high-pressure liquid can be introduced into the confining pressure tube; the magnet is arranged around the outside of the clamp; the magnetic coil is located inside the receiving cavity and is arranged around the outside of the confining pressure tube; the receiving device is connected to the display terminal to receive nuclear magnetic resonance (NMR) signals and convert them into electrical signals, and transmit the electrical signals to the display terminal; the display terminal collects the electrical signals transmitted by the receiving device and displays the NMR image of the rock sample. This allows for the visualization of deep shale fractures and fracture water distribution, enabling a direct evaluation of the influence mechanism of deep shale fractures and fracture water-bearing states on the frictional sliding stability of shale faults.

Description

Technical Field

[0001] This invention relates to the field of shale fracture stability evaluation technology, and particularly to a device and method for evaluating the fracture stability of deep shale. Background Technology

[0002] Currently, the most fundamental and important research method for understanding the basic mechanical properties, deformation and fracture mechanisms, and failure modes of deep shale is physical simulation testing, specifically true triaxial stress testing. Rock physical simulation testing involves using one or more artificial materials in a laboratory setting to simulate the properties of a prototype. Based on the properties of the prototype, similarity principles are applied to create a model. By analyzing the stress-strain characteristics of the similar model, the stress-strain characteristics of the simulated prototype and the fracture stability and water content changes caused by the mechanical phenomena are determined. This provides technical support for the extraction and development of deep shale oil and gas. Summary of the Invention

[0003] To visualize the distribution of fractures and fissure water in deep shale and to intuitively evaluate the influence mechanism of the water-bearing state of deep shale fractures and fissures on the frictional sliding stability of shale faults, this invention provides a device and method for evaluating the stability of deep shale fractures.

[0004] In a first aspect, embodiments of the present invention provide a deep shale fracture stability evaluation device, comprising: a clamp, a confining pressure pipe, a loading rod, a magnetic induction coil, a magnet, a receiving device, and a display terminal;

[0005] The clamp is provided with a receiving cavity, which is capable of accommodating the rock sample;

[0006] The loading rod is located at the bottom of the receiving cavity and can abut against the lower end of the rock sample, and is used to apply a preset axial stress to the rock sample;

[0007] The confining pressure tube is disposed within the receiving cavity and can be arranged around the outside of the rock sample. High-pressure liquid can be introduced into the confining pressure tube to apply a preset confining pressure to the rock sample.

[0008] The magnet ring is disposed on the outside of the clamp;

[0009] The magnetic induction coil is located inside the receiving cavity and is arranged around the outside of the confining pressure tube. It is used to excite radio frequency pulses under the action of the magnetic field generated by the magnet and act on the rock sample to form a nuclear magnetic resonance signal.

[0010] The receiving device is connected to the display terminal and is used to receive the nuclear magnetic resonance signal, convert it into an electrical signal, and transmit the electrical signal to the display terminal.

[0011] The display terminal collects the electrical signals transmitted by the receiving device and displays the nuclear magnetic resonance image of the rock sample.

[0012] Optionally, the deep shale fracture stability evaluation equipment may also include a base, a fluid pressure control device, and a fluid storage tank;

[0013] The base is connected to the lower ends of the display terminal, the receiving device, the magnet, and the clamp;

[0014] The inner cavity of the base is equipped with the fluid pressure control device and the fluid storage tank;

[0015] The fluid storage tank contains the high-pressure liquid, and the fluid storage tank is connected to the confining pressure pipe and the bottom of the receiving cavity;

[0016] The fluid pressure control device is connected to the fluid storage tank and is used to pressurize the high-pressure liquid to a preset pressure value and drive the high-pressure liquid into the confining pipe and the bottom of the receiving cavity.

[0017] Optionally, the deep shale fracture stability evaluation equipment also includes a control console connected to the base;

[0018] The control console is connected to the fluid pressure control device and the receiving device, respectively, and is used to control the pressure value of the high-pressure fluid and the frequency of the nuclear magnetic resonance signal.

[0019] Optionally, the deep shale fracture stability evaluation equipment may further include a confining pressure connection pipe and a pressure supply pipe;

[0020] The confining pressure connecting pipe is located in the inner cavity of the confining pressure pipe and is connected to the fluid storage tank;

[0021] The pressure supply pipe is located inside the loading rod and is connected to the fluid storage tank.

[0022] Optionally, the deep shale fracture stability evaluation equipment may also include a support rod and a limiting component;

[0023] The lower end of the support rod abuts against the upper end of the rock sample, and the upper end of the support rod extends outside the clamp.

[0024] The limiting member is connected to the upper end of the clamp and abuts against the upper end of the support rod.

[0025] Optionally, the deep shale fracture stability evaluation equipment may also include a tungsten carbide pad;

[0026] The tungsten carbide pads are provided between the support rod and the rock sample, and between the loading rod and the rock sample.

[0027] Optionally, the high-pressure liquid is a fluorinated liquid.

[0028] Secondly, embodiments of the present invention provide a method for evaluating the stability of deep shale fractures, employing the deep shale fracture stability evaluation equipment described in Embodiment 1, including:

[0029] The dried sample was placed in a holder for nuclear magnetic resonance imaging to obtain the original nuclear magnetic resonance image of the sample.

[0030] The sample is subjected to absorption of the target fluid in a vacuum environment to obtain a rock sample;

[0031] The rock sample is placed in the holder, and axial stress is applied to the rock sample step by step using the loading rod until the preset axial stress is reached. Confining pressure is applied to the rock sample step by step using the confining pressure tube until the preset confining pressure is reached. Experimental nuclear magnetic resonance images are obtained in real time using the display terminal.

[0032] Based on the original NMR image and the real-time experimental NMR image, a real-time NMR image of the rock sample is obtained.

[0033] Based on the axial stress and the confining pressure, the friction coefficient is calculated, and combined with the real-time nuclear magnetic resonance image of the rock sample, the stability analysis results of the rock sample are obtained.

[0034] Optionally, before placing the dried sample into the holder for nuclear magnetic resonance imaging to obtain the original nuclear magnetic resonance image of the sample, the method further includes:

[0035] Based on the core sampling operation, a rock core is obtained, and the rock core is cut to obtain a plunger;

[0036] A sample is obtained by creating a slit in the plunger at a preset angle.

[0037] Optionally, the friction coefficient is calculated based on the axial stress and the confining pressure, and the stability analysis results of the rock sample are obtained by combining the real-time nuclear magnetic resonance image of the rock sample, including:

[0038] Based on the axial displacement of the loading rod from the first moment to the second moment, the sliding rate at the second moment is calculated, and based on the axial stress, confining pressure, preset angle and obtained pore pressure at the second moment, the friction coefficient at the second moment is calculated.

[0039] Based on the axial displacement of the loading rod from the third to the fourth moment, the sliding rate at the fourth moment is calculated, and based on the axial stress, confining pressure, preset angle and the obtained pore pressure at the fourth moment, the friction coefficient at the fourth moment is calculated.

[0040] Based on the friction coefficient and sliding rate at the second time and the friction coefficient and sliding rate at the fourth time, the stability analysis results of the rock sample are obtained.

[0041] Optionally, obtaining the stability analysis results of the rock sample based on the friction coefficient and sliding rate at the second time point and the friction coefficient and sliding rate at the fourth time point includes:

[0042] The difference between the first parameter and the second parameter can be calculated using the following formula:

[0043]

[0044] in, The first parameter; For the second Parameter; is the friction coefficient at the fourth time step; Let be the sliding speed at the fourth moment; Let be the coefficient of friction at the second moment; Let be the sliding speed at the second moment;

[0045] Determine whether the difference between the first parameter and the second parameter is greater than 0. If yes, the rock sample is in a stable state; otherwise, the rock sample is in an unstable state.

[0046] The beneficial effects of the above-mentioned technical solutions provided in the embodiments of the present invention include at least the following:

[0047] This invention provides a device for evaluating the stability of deep shale fractures. A magnet is placed on the outside of a clamp to provide a stable magnetic field. A magnetic coil is placed inside the clamp's cavity. Under the influence of the magnetic field generated by the magnet, radio frequency pulses are excited and applied to the rock sample, generating a nuclear magnetic resonance (NMR) signal. A receiving device receives the NMR signal, converts it into an electrical signal, and transmits it to a display terminal to display the NMR image of the rock sample inside the clamp. This device can characterize the stability of deep shale fractures under changes in external pressure during actual geological activities without damaging the rock sample structure. It can detect the pore structure, water distribution, or fracture development degree within the shale, enabling visualization of fracture water distribution. In particular, it can visualize the changing trends of water distribution around shale structural fractures during fracture creep, providing a direct evaluation of the influence mechanism of fractures and water-bearing conditions on the frictional sliding stability of shale faults and offering data support for research on the stability of deep shale fractures.

[0048] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings.

[0049] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0050] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0051] Figure 1 This is a schematic diagram of the structure of the deep shale fracture stability evaluation device provided in this embodiment of the invention;

[0052] Figure 2 This is a flowchart of the deep shale fracture stability evaluation method provided in the embodiments of the present invention;

[0053] Figure 3 This is a graph showing the stability evaluation results provided in an embodiment of the present invention;

[0054] Explanation of reference numerals in the attached figures:

[0055] 1. Fluid pressure control equipment; 2. Base; 3. Control console; 4. Display terminal; 5. Receiving device; 6. Fluid storage tank; 7. Magnet; 8. Clamp; 9. Limiting component; 10. Magnetic coil; 11. Confining pressure pipe; 12. Loading rod; 13. Tungsten carbide pad; 14. Pressure supply pipe; 15. Confining pressure connecting pipe; 16. Rock sample; 17. Support rod. Detailed Implementation

[0056] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0057] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," "outer," "far," "near," "front," and "rear," etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0058] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0059] The inventors discovered that while existing rock physics simulation experiments can characterize the stability of shale under water-bearing conditions, they cannot characterize the water content during shale instability. Since shale generally contains water, the water content is an important attribute and evaluation indicator, and therefore cannot be well adapted to the evaluation of shale fracture stability.

[0060] To address the aforementioned issues, the inventors developed a device and method for evaluating the stability of deep shale fractures. This device enables visualization of deep shale fractures and fissure water distribution, providing an intuitive assessment of the impact mechanism of deep shale fractures and fissure water content on the frictional sliding stability of shale faults.

[0061] Example 1

[0062] See Figure 1This embodiment proposes a device for evaluating the fracture stability of deep shale formations, applicable to various types of deep rock formations, especially for the study of deep shale. The device includes: a clamp 8, a confining pressure tube 11, a loading rod 12, a magnetic coil 10, a magnet 7, a receiving device 5, and a display terminal 4. The clamp 8 has a receiving cavity (not shown in the figure) capable of accommodating a rock sample 16, forming a closed space. The loading rod 12 is located at the bottom of the receiving cavity and abuts against the lower end of the rock sample 16, applying a preset axial stress to the rock sample 16. The confining pressure tube 11 is located inside the receiving cavity and surrounds the outside of the rock sample 16. High-pressure liquid can be introduced into the confining pressure tube 11 to apply a preset confining pressure to the rock sample 16. The magnet 7, surrounding the clamp 8, generates a stable magnetic field, providing the necessary magnetic field environment for nuclear magnetic resonance (NMR). A magnetic induction coil 10 is disposed within the receiving cavity and encircles the outside of the confining pressure tube 11. It is used to excite radio frequency pulses under the influence of the magnetic field generated by the magnet 7, which then act on the rock sample 16 to form a nuclear magnetic resonance (NMR) signal. A receiving device 5 is connected to a display terminal 4 to receive the NMR signal, convert it into an electrical signal, and transmit the electrical signal to the display terminal 4 for display. Simultaneously, the receiving device 5 can control the magnet 7 to ensure that the rock sample 16 is in a stable and uniform magnetic field environment, and to ensure the accuracy of the NMR signal measurement. The display terminal 4 collects the electrical signal transmitted by the receiving device 5, analyzes and processes it, and displays the NMR image of the rock sample 16.

[0063] Nuclear magnetic resonance (NMR) analyzes the structure of matter by applying a magnetic field and radio frequency pulses to cause atomic nuclei (such as hydrogen nuclei) to resonate, and then receiving the emitted signals. In the device of this embodiment, magnet 7 generates a stable magnetic field, and magnetic coil 10 excites radio frequency pulses under the action of the magnetic field and applies them to rock sample 16. The atomic nuclei in rock sample 16 resonate under the action of the radio frequency pulses, releasing NMR signals. Receiving device 5 captures these signals and converts them into electrical signals, which are then transmitted to display terminal 4 for analysis and display. By analyzing the NMR images, the internal structure and fracture stability of rock sample 16 can be evaluated.

[0064] Taking shale as an example, given that the development and utilization of deep shale gas requires hydraulic fracturing of shale reservoirs, and current research on shale stability is limited to triaxial stress experiments, which cannot reflect the influence trend of fractures and fracture water distribution in shale formations on fault stability, the equipment in this embodiment can characterize the stability of shale reservoir fractures under actual geological activities using nuclear magnetic resonance without damaging the structure of rock sample 16. It can detect the pore structure, water distribution, or fracture development degree inside shale, and realize the visualization of deep shale fractures and fracture water distribution. In particular, it can visualize the trend of water distribution changes around shale formation structural fractures during fracture creep, which can be used to intuitively evaluate the influence mechanism of deep shale fractures and fracture water state on the frictional sliding stability of shale faults, providing data support for the study of deep shale reservoir fracture stability.

[0065] Furthermore, the clamp 8 forms a closed space to prevent high-pressure fluid leakage, ensuring that the rock sample 16 can reach the preset confining pressure and axial pressure. Through the synergistic action of the clamp 8, the confining pressure tube 11, and the loading rod 12, a composite loading of axial stress and confining pressure is achieved to accurately simulate the real stress state of deep underground rock strata (such as shale), making the obtained display images more consistent with the data of real strata and effectively improving the accuracy of stability evaluation.

[0066] In one specific embodiment, the device further includes a base 2, a fluid pressure control device 1, and a fluid storage tank 6. The base 2 is connected to the lower ends of the display terminal 4, receiving device 5, magnet 7, and clamp 8, serving as the supporting foundation for the entire device. Here, the base 2 can be frustum-shaped, resulting in a large bottom weight and a large contact area, ensuring stability during experiments and improving reliability. The inner cavity of the base 2 houses the fluid pressure control device 1 and the fluid storage tank 6, achieving a compact design and efficient integration. The fluid storage tank 6 contains high-pressure liquid, serving as the source of confining pressure and axial pressure. The fluid storage tank 6 is connected to the confining pressure pipe 11 and the bottom of the receiving cavity. The fluid pressure control device 1 is connected to the fluid storage tank 6, driving the high-pressure fluid within the tank and pressurizing it to a preset pressure value to meet the needs of different rock samples 16 and experimental conditions. It also drives the high-pressure liquid into the confining pressure pipe 11 and the bottom of the receiving cavity, providing confining pressure and axial pressure to the rock sample 16.

[0067] In operation, the fluid pressure control device 1 drives the high-pressure liquid in the fluid storage tank 6 according to preset parameters, causing it to enter the confining pressure pipe 11 and the bottom of the receiving cavity. The high-pressure liquid in the confining pressure pipe 11 applies confining pressure to the rock sample 16, simulating the geostress environment experienced by deep shale; simultaneously, the high-pressure liquid at the bottom of the receiving cavity applies axial pressure to the rock sample 16, further replicating the actual stress state of deep shale. By setting up the fluid pressure control device 1 and the fluid storage tank 6, the confining pressure and axial pressure experienced by the rock sample 16 are precisely controlled, improving the accuracy and reliability of the experiment.

[0068] It is worth noting that the aforementioned clamp 8 is detachably connected to the base 2, for example, by means of a snap-fit ​​connection or a threaded connection, to facilitate disassembly for taking out or placing the rock sample 16.

[0069] In one specific embodiment, the high-pressure liquid, as a crucial medium for transmitting confining pressure and axial pressure, significantly impacts the experimental results. In this embodiment, a fluorinated liquid can be used. Fluorinated liquids possess excellent chemical inertness, preventing chemical reactions with the various components of the equipment (such as the loading rod 12 and the clamp 8), thus ensuring the purity of the experimental environment and the long-term stability of the equipment, guaranteeing experimental reliability. The fluorinated liquid has extremely low conductivity, avoiding conductive interference signals during nuclear magnetic resonance imaging. Simultaneously, its non-ferromagnetic properties ensure no interference with the magnetic field, thereby ensuring the acquisition of clear and accurate images of the internal structure of rock sample 16, improving the accuracy and reliability of the experiment. Furthermore, the fluorinated liquid has low viscosity and good fluidity, enabling rapid response to pressure changes and uniform transmission of confining and axial pressure to rock sample 16, accurately simulating the geostress environment of deep shale.

[0070] In one specific embodiment, the device further includes a console 3 connected to the base 2, which serves as the core control unit of the entire device. The console 3 is connected to the fluid pressure control device 1 to control the pressure value of the high-pressure fluid and achieve precise regulation. The console 3 is also connected to the receiving device 5 to control the frequency of the nuclear magnetic resonance (NMR) signal to adapt to different rock samples 16 and experimental requirements. The console 3 can monitor the fluid pressure in real time, and the user can input the confining pressure and axial pressure to achieve dynamic adjustment according to experimental needs, ensuring the accuracy and stability of experimental conditions. Simultaneously, the console 3 allows the user to precisely set the frequency of the NMR signal according to the characteristics of the rock sample 16 and the experimental objectives to optimize signal reception and analysis. By adjusting the signal frequency, the resolution and contrast of the NMR image can be improved, thereby more accurately assessing the internal structure and fracture stability of the rock sample 16.

[0071] In one specific embodiment, the display terminal 4 is connected to the console 3. The console 3 transmits the current high-pressure fluid pressure value to the display terminal 4 in real time. Users can intuitively view the pressure value changes on the display terminal 4. Users can easily set experimental parameters and monitor the experimental process, ensuring the accuracy and reliability of experimental data.

[0072] In one specific embodiment, the device further includes a confining pressure connecting pipe 15 and a pressure supply pipe 14. The confining pressure connecting pipe 15 is located within the inner cavity of the confining pressure pipe 11 and communicates with the fluid storage tank 6, serving as a fluid transmission channel to deliver high-pressure liquid from the fluid storage tank 6 to the confining pressure pipe 11, providing a stable confining pressure environment for the rock sample 16. The pressure supply pipe 14 is located within the inner cavity of the loading rod 12 and communicates with the fluid storage tank 6. Through the pressure supply pipe 14, the high-pressure liquid in the fluid storage tank 6 can act on the loading rod 12, thereby applying a preset axial stress to the rock sample 16, simulating the geostress conditions experienced by deep shale. By setting up the confining pressure connecting pipe 15 and the pressure supply pipe 14, it is ensured that the high-pressure liquid can be smoothly transmitted to the confining pressure pipe 11 and the loading rod 12, improving the operating efficiency of the device.

[0073] In one specific embodiment, the device further includes a support rod 17 and a limiting member 9. The lower end of the support rod 17 abuts against the upper end of the rock sample 16, and the upper end of the support rod 17 extends outside the clamp 8. The limiting member 9 is connected to the upper end of the clamp 8 and abuts against the upper end of the support rod 17 to form a closed experimental space, ensuring the airtightness of the pressure chamber during the experiment and preventing leakage of high-pressure liquid or gas. When the loading rod 12 applies axial stress to the rock sample 16, the support rod 17 provides a reaction force to the rock sample 16, ensuring that the rock sample 16 remains stable during the compression process, which is beneficial for accurately assessing the fracture stability of the rock sample 16.

[0074] In one specific embodiment, both the support rod 17 and the loading rod 12 can be made of corundum rods, which have excellent compressive strength. However, corundum rods can generate signal feedback in a magnetic field, potentially affecting the accurate imaging of the internal structure of the rock sample 16 and thus reducing the reliability of the experimental data. To address this issue, tungsten carbide pads 13 are provided between the support rod 17 and the rock sample 16, and between the loading rod 12 and the rock sample 16. Specifically, tungsten carbide pads 13 are placed on the upper and lower surfaces of the rock sample 16. Tungsten carbide has 86 protons, which do not generate a signal during nuclear magnetic resonance (NMR) measurements and do not affect the magnetic field, ensuring the accuracy and clarity of the NMR imaging of the rock sample 16. The tungsten carbide pads 13 act as a signal shielding layer, effectively isolating the signal interference generated by the corundum rod, protecting the purity of the NMR signal from the rock sample 16, and improving imaging quality. Simultaneously, by setting the tungsten carbide pads 13, the stress applied by the loading rod 12 can be uniformly transferred to the rock sample 16, avoiding stress concentration and improving the stability and reliability of the experiment.

[0075] In one specific embodiment, the device of this embodiment images the internal structure of rock sample 16 based on the principle of nuclear magnetic resonance (NMR). However, ferromagnetic materials can significantly interfere with NMR signals, affecting the accuracy and reliability of the signals. Therefore, in the device of this embodiment, key components such as the confining pressure tube 11, the confining pressure connecting tube 15, the clamp 8, and the pressure supply tube 14 are all made of non-ferromagnetic materials, such as carbon tetrafluoride, to ensure that they do not interfere with the NMR signals, thereby ensuring the smooth operation of the NMR experiment and improving the accuracy and reliability of the experiment.

[0076] In one specific embodiment, see [reference] Figure 1 The device in this embodiment also includes a displacement sensor (not shown in the figure) located on the loading rod 12. This displacement sensor can be positioned at the bottom end of the loading rod 12, which helps to more accurately measure the axial displacement of the loading rod 12. The axial displacement of the loading rod 12 reflects the change in axial displacement of the rock sample during the stress process. This displacement sensor is connected to the control console 3, and can record the displacement of the rock sample in the axial direction in real time and accurately during the experiment. The control console 3 transmits the current axial displacement to the display terminal 4 in real time, and the user can intuitively view the change in displacement on the display terminal 4. This data recording function is of great significance for the evaluation of the fracture stability of deep shale. By obtaining the key parameter of the axial displacement of the rock sample, the parameter system required for evaluation can be further enriched and improved, providing strong data support for a more comprehensive and in-depth assessment of the fracture stability of deep shale.

[0077] In one specific embodiment, see [reference] Figure 1 The device in this embodiment also includes pressure sensors (not shown in the figure) respectively located at the bottom of the pressure supply pipe 14 and the confining pressure pipe 11 to detect pore pressure and confining pressure. Both pressure sensors are connected to the control console 3, enabling real-time and accurate recording of the rock sample pore pressure and confining pressure during the experiment. The control console 3 transmits the current pore pressure and confining pressure to the display terminal 4 in real time, allowing the user to visually view the changes in pore pressure and confining pressure on the display terminal 4, providing strong data support for a more comprehensive and in-depth assessment of the fracture stability of deep shale.

[0078] Example 2

[0079] Based on the same inventive concept, see [reference] Figure 2 This embodiment proposes a method for evaluating the stability of deep shale fractures, using the deep shale fracture stability evaluation equipment described in Embodiment 1. Specifically, it may include the following steps:

[0080] Step S1: Place the dried sample into the holder for nuclear magnetic resonance imaging to obtain the original nuclear magnetic resonance image of the sample;

[0081] Before step S1 above, there is also a sample preparation process, which may include the following steps: obtaining a core based on coring operations, cutting the core to obtain a plunger (the cutting method can be wire cutting or water drilling, as is available in existing technologies); creating a fracture in the plunger at a preset angle to obtain the sample. It is worth noting that if multiple plungers are used for fracture creation, a single variable should be set in principle. That is, if comparing the impact of fracture size on the stability of deep shale, microfractures or fractures should be created at the same angle, at the same location, and with different fracture sizes.

[0082] The prepared sample was placed in a drying oven (the operating parameters of the drying oven can be set to: temperature 100℃, working time 48 hours) to remove the moisture inside the sample, and the dry weight m of the sample was measured. 干 .

[0083] The dried sample is placed in the holder and nuclear magnetic resonance imaging is performed using the equipment in Example 1. The mechanism of nuclear magnetic resonance imaging can be referred to in Example 1, and will not be repeated here. The original nuclear magnetic resonance image of the sample is obtained.

[0084] Step S2: In a vacuum environment, the sample absorbs the target fluid to obtain a rock sample.

[0085] In step S2 above, the sample is evacuated, and then immersed in the target fluid under vacuum to allow the sample to absorb the target fluid. After absorption, the wet weight m of the rock sample is measured. 湿 The moisture content of the sample is calculated based on the dry weight and wet weight.

[0086] It is worth noting that this embodiment is not limited to evaluating the impact of fractures on shale stability. It can also evaluate the impact of water content on the stability of shale fractures or fissures by artificially creating microfractures or fissures of uniform size and altering the water content. If evaluating the impact of fractures or fissures on the stability of deep shale from multiple rock samples, it is necessary to control the volume of the target fluid absorbed by different samples to ensure that the final water content of the multiple rock samples is the same; if evaluating the impact of water content on shale stability from multiple rock samples, then the water content of the multiple rock samples should be different.

[0087] Step S3: Place the rock sample into the holder, apply axial stress to the rock sample step by step using the loading rod until the preset axial stress is reached, apply confining pressure to the rock sample step by step using the confining pressure tube until the preset confining pressure is reached, and obtain the experimental nuclear magnetic resonance image in real time using the display terminal.

[0088] Step S4: Based on the original NMR image and the real-time experimental NMR image, obtain the real-time NMR image of the rock sample.

[0089] Step S5: Based on axial stress and confining pressure, the friction coefficient is calculated, and combined with the real-time nuclear magnetic resonance image of the rock sample, the stability analysis results of the rock sample are obtained.

[0090] In step S5 above, before calculating the friction coefficient, the contact area needs to be corrected. During the experiment, after the loading rod applies axial pressure, the rock sample undergoes frictional sliding, causing the actual contact area of ​​the friction surface to decrease with the increase of axial displacement. Therefore, the axial stress recorded in the experiment... It will be less than the actual axial stress. Therefore, various data corrections are required. First, the axial stress needs to be corrected by area, which can be done using the following formula (1):

[0091]

[0092] In the above formula (1), θ is the angle between the axial direction and the weak surface of the fracture, that is, the angle between the fracture and the horizontal plane when the fracture is created in step S1 above; r is the radius of the rock sample; △L is the axial displacement recorded in the experiment, which can be measured by the displacement sensor in Example 1.

[0093] Then, the shear stress is calculated using the following formula (2):

[0094]

[0095] In the above formula (2), Represents shear stress; The actual axial stress is calculated using formula (1); The confining pressure can be directly read from the experimental equipment; θ is the angle between the axial direction and the weak surface of the fracture, which is preset and measured in the experiment.

[0096] Then, the effective normal stress is corrected, that is, the corresponding axial stress is calculated based on the corrected shear stress. Normal stress on the friction surface axial stress It can be expressed by the following formula (3):

[0097]

[0098] In the above formula (3), The shear stress is represented by formula (2); The meanings of θ are the same as those mentioned above.

[0099] The normal stress on the friction surface is calculated using the following formula (4). :

[0100]

[0101] Effective normal stress This is the difference between the calculated value of the normal stress and the pore pressure observed in the experiment, i.e.:

[0102]

[0103] In the above formula (5), It can be calculated according to formula (4); The pore pressure can be measured using the pressure sensor described in Example 1.

[0104] The specific process of obtaining the stability analysis results of rock samples may include the following steps:

[0105] Step S501: Based on the axial displacement of the loading rod from the first moment to the second moment, calculate the sliding rate at the second moment, and based on the axial stress, confining pressure, preset angle and obtained pore pressure at the second moment, calculate the friction coefficient at the second moment.

[0106] In step S501 above, the displacement sensor in Embodiment 1 is used to obtain the axial displacement of the loading rod from the first moment to the second moment. The axial displacement of the loading rod from the first moment to the second moment is divided by the difference between the second moment and the first moment, which is the sliding rate at the second moment. The friction coefficient is calculated using the following formula (6). :

[0107]

[0108] Step S502: Based on the axial displacement of the loading rod from the third to the fourth moment, calculate the sliding rate at the fourth moment, and based on the axial stress, confining pressure, preset angle and pore pressure at the fourth moment, calculate the friction coefficient at the fourth moment.

[0109] The displacement sensor in Example 1 is used to obtain the axial displacement of the loading rod from the third time to the fourth time. The axial displacement of the loading rod from the third time to the fourth time is divided by the difference between the fourth time and the third time, which is the sliding rate at the fourth time. The friction coefficient at the fourth time is calculated according to the above formula (6).

[0110] See Figure 3 The friction coefficients under different shear displacements are calculated according to the above formula and plotted into a graph, which makes it easier to observe the change process of the friction coefficient of the rock sample. At the same time, according to the nuclear magnetic resonance image, the corresponding nuclear magnetic resonance image can be found at the location where the friction coefficient changes abruptly, which is convenient for observing the sliding process of the rock sample, especially for exploring the occurrence state and migration trend of water in the sliding process of the rock sample.

[0111] It is worth noting that the first, second, third, and fourth moments mentioned in this embodiment are time sampling points randomly selected based on experimental observation needs, and their specific time parameter values ​​do not have any specific technical limitation significance.

[0112] Step S503: Based on the friction coefficient and sliding rate at the second time and the friction coefficient and sliding rate at the fourth time, the stability analysis results of the rock sample are obtained.

[0113] In step S503 above, the sliding stability can be described using state variables, that is, the sliding characteristics of the rock sample can be used to characterize the friction coefficient:

[0114]

[0115] In the above formula (7), This is the steady-state value of the friction coefficient at the sliding speed V, i.e., the friction coefficient at the fourth moment; For a certain reference sliding speed The steady-state value of the friction coefficient at the second moment is the friction coefficient at the second moment. The first parameter is a constitutive parameter that characterizes the magnitude of the direct rate response of the friction intensity during rate switching. The second parameter is a constitutive parameter that characterizes the evolution of friction intensity with displacement; Let be the sliding speed at the fourth moment; Let be the sliding speed at the second moment; θ is the characteristic sliding distance, representing the distance the friction intensity slides to reach a new steady-state value when the rate changes; θ is a state variable characterizing the evolution of the cross-sectional area of ​​the friction sliding surface as displacement changes.

[0116] Equation (7) above can be transformed into:

[0117]

[0118] Furthermore, the difference between the friction coefficient at the fourth moment and the friction coefficient at the second moment can be expressed as:

[0119]

[0120] The stability analysis of rock samples can be evaluated using the above formula (8) or (9). The basic principle is based on... The mathematical meaning of represents the parameters related to the steady-state rate and is crucial for determining sliding stability. When When the speed increases, the frictional strength increases, i.e., velocity intensification; when At this point, the friction intensity decreases as the speed increases, i.e., speed weakening occurs. The difference in the steady-state friction coefficient is the value of the difference. As the sliding speed increases, if the friction coefficient increases, the frictional sliding is in a stable state, that is, the speed is enhanced; if the friction coefficient decreases, the frictional sliding is in an unstable state, that is, the speed is weakened.

[0121] Therefore, in this embodiment, the specific process of obtaining the stability analysis results of the rock sample based on the friction coefficient and sliding rate at the second time and the friction coefficient and sliding rate at the fourth time may include the following steps:

[0122] Step S5031: Calculate the difference between the first parameter and the second parameter using the above formula (8);

[0123] Step S5032: Determine whether the difference between the first parameter and the second parameter is greater than 0; if yes, the rock sample is in a stable state; if no, the rock sample is in an unstable state.

[0124] The equipment and evaluation method provided by this invention, based on the mechanism of nuclear magnetic resonance, determine the hydrogen ion nuclear magnetic resonance signal response characteristics of deep shale fractures under varying external pressure, simulate and analyze the water content distribution and tectonic activity of deep shale fractures, and, combined with the stress characteristics of deep shale, establish an in-situ stress-effect model of deep shale fractures (i.e., Figure 3 This invention clarifies the pressure threshold for instability in deep shale, providing guidance for deep shale exploration and development. It employs a high-resolution "atom-displacement" analysis method to quantify the changing trend of shale pore space under external pressure, enabling the evaluation and analysis of the impact of internal shale fractures on stability, thus providing support for in-situ reservoir stimulation. This evaluation method is convenient to operate and provides accurate results. It focuses on analyzing the impact of shale fractures, fissures, and changes in geostress on reservoir stability, and delves into the influencing factors of reservoir instability during actual mining, to a certain extent supporting and addressing the adverse effects of reservoir fracture instability during fracturing. This invention is unaffected by external characterization and human factors, resulting in more scientific and objective evaluation results. The invention is simple to operate, has a clear principle, accurate results, is easy to implement, and can be widely promoted. Compared with existing shale triaxial testing methods, the evaluation parameters of this invention are easier to obtain, the calculation process is relatively simple, and it can evaluate shale pore connectivity stimulation, providing more accurate and instructive results.

[0125] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. This disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is limited only by the appended claims. Thus, if these modifications and variations of the invention fall within the scope of the claims of the invention and their equivalents, the invention is also intended to include these modifications and variations.

Claims

1. A method for evaluating the stability of deep shale fractures, implemented using deep shale fracture stability evaluation equipment, characterized in that... The deep shale fracture stability evaluation device includes: a clamp, a confining pressure tube, a loading rod, a magnetic coil, a magnet, a receiving device, and a display terminal; the clamp has a receiving cavity capable of accommodating a rock sample; the loading rod is located at the bottom of the receiving cavity and abuts against the lower end of the rock sample, used to apply a preset axial stress to the rock sample; the confining pressure tube is located inside the receiving cavity and can be arranged around the outside of the rock sample, and high-pressure liquid can be introduced into the confining pressure tube to apply a preset confining pressure to the rock sample; the magnet is arranged around the outside of the clamp; the magnetic coil is located inside the receiving cavity and is arranged around the outside of the confining pressure tube, used to excite radio frequency pulses under the action of the magnetic field generated by the magnet and act on the rock sample to form a nuclear magnetic resonance signal; the receiving device is connected to the display terminal, used to receive the nuclear magnetic resonance signal and convert it into an electrical signal, and transmit the electrical signal to the display terminal; the display terminal collects the electrical signal transmitted by the receiving device and displays the nuclear magnetic resonance image of the rock sample; the method includes: The dried sample was placed in a holder for nuclear magnetic resonance imaging to obtain the original nuclear magnetic resonance image of the sample; the sample was then subjected to absorption of the target fluid in a vacuum environment to obtain a rock sample. The rock sample is placed in the holder, and axial stress is applied to the rock sample step by step using the loading rod until the preset axial stress is reached. Confining pressure is applied to the rock sample step by step using the confining pressure tube until the preset confining pressure is reached. Experimental nuclear magnetic resonance images are obtained in real time using the display terminal. Based on the original nuclear magnetic resonance image and the experimental nuclear magnetic resonance image obtained in real time, the real-time nuclear magnetic resonance image of the rock sample is obtained. Based on the axial stress and the confining pressure, the friction coefficient under different shear displacements is calculated. Combined with the real-time nuclear magnetic resonance image of the rock sample, the corresponding nuclear magnetic resonance image is found at the location where the friction coefficient changes abruptly, so as to observe the slip process of the rock sample. The trend of structural fractures and the distribution of water in the surrounding shale layer system during the fracture creep process is visualized and characterized, and the stability analysis results of the rock sample are obtained. Before calculating the friction coefficient, the axial stress can be corrected for area using the following formula (1): （1） In the above formula (1), θ represents the axial stress recorded in the experiment, θ is the angle between the axial direction and the weak surface of the fracture, i.e., the angle between the fracture and the horizontal plane when the fracture is created; r is the radius of the rock sample; ΔL is the axial displacement recorded in the experiment. Calculate the shear stress using the following formula (2): （2） In the above formula (2), Represents shear stress; The actual axial stress is calculated using formula (1); The confining pressure can be directly read from the experimental equipment display. The corresponding axial stress is calculated using the following formula (3) based on the corrected shear stress. The normal stress on the friction surface is calculated using the following formula (4). : （3） （4） The effective normal stress is calculated using the following formula (5). : （5） In the above formula (5), Pore ​​pressure; The coefficient of friction is calculated using the following formula (6). : （6）。 2. The method for evaluating the stability of deep shale fractures according to claim 1, characterized in that, Before placing the dried sample into the holder for nuclear magnetic resonance imaging to obtain the original nuclear magnetic resonance image of the sample, the procedure further includes: Based on the core sampling operation, a rock core is obtained, and the rock core is cut to obtain a plunger; A sample is obtained by creating a slit in the plunger at a preset angle.

3. The method for evaluating the stability of deep shale fractures according to claim 2, characterized in that, The specific process for obtaining the stability analysis results of the rock sample includes: Based on the axial displacement of the loading rod from the first moment to the second moment, the sliding rate at the second moment is calculated, and based on the axial stress, confining pressure, preset angle and obtained pore pressure at the second moment, the friction coefficient at the second moment is calculated. Based on the axial displacement of the loading rod from the third to the fourth moment, the sliding rate at the fourth moment is calculated, and based on the axial stress, confining pressure, preset angle and the obtained pore pressure at the fourth moment, the friction coefficient at the fourth moment is calculated. Based on the friction coefficient and sliding rate at the second time and the friction coefficient and sliding rate at the fourth time, the stability analysis results of the rock sample are obtained.

4. The method for evaluating the stability of deep shale fractures according to claim 3, characterized in that, The stability analysis results of the rock sample, based on the friction coefficient and sliding rate at the second time point and the friction coefficient and sliding rate at the fourth time point, include: The difference between the first parameter and the second parameter can be calculated using the following formula: ； in, The first parameter; This is the second parameter; Let be the coefficient of friction at the fourth moment; Let be the sliding speed at the fourth moment; Let be the coefficient of friction at the second moment; Let be the sliding speed at the second moment; Determine whether the difference between the first parameter and the second parameter is greater than 0. If yes, the rock sample is in a stable state; otherwise, the rock sample is in an unstable state.

5. A deep shale fracture stability evaluation device applied to the deep shale fracture stability evaluation method according to any one of claims 1-4, characterized in that, include: Clamping device, confining pressure tube, loading rod, magnetic induction coil, magnet, receiving device, display terminal, fluid pressure control equipment, fluid storage tank, support rod and limiting component; The clamp is provided with a receiving cavity, which can accommodate a rock sample; wherein, the rock sample is obtained by cutting a rock core to obtain a plunger, and making a slit in the plunger at a preset angle to obtain a solid plunger rock sample; The loading rod is located at the bottom of the receiving cavity and can abut against the lower end of the rock sample to apply a preset axial stress to the rock sample; a displacement sensor is provided at the bottom end of the loading rod to measure the axial displacement of the loading rod. The confining pressure tube is disposed within the receiving cavity and can be arranged around the outside of the rock sample. High-pressure liquid can be introduced into the confining pressure tube to apply a preset confining pressure to the rock sample. The fluid storage tank contains the high-pressure liquid, and the fluid storage tank is connected to the confining pressure pipe and the bottom of the receiving cavity; The fluid pressure control device is connected to the fluid storage tank and is used to pressurize the high-pressure liquid to a preset pressure value and drive the high-pressure liquid into the bottom of the confining pressure pipe and the receiving cavity; a pressure sensor is installed at the bottom of the confining pressure pipe to detect the confining pressure; a pressure sensor is installed at the bottom of the receiving cavity to detect the pore pressure of the rock sample. The lower end of the support rod abuts against the upper end of the rock sample, and the upper end of the support rod extends outside the clamp. The limiting member is connected to the upper end of the clamp and abuts against the upper end of the support rod; The loading rod and the support rod are made of corundum, and tungsten carbide pads are provided between the support rod and the rock sample and between the loading rod and the rock sample. The magnet ring is disposed on the outside of the clamp; The magnetic induction coil is located inside the receiving cavity and is arranged around the outside of the confining pressure tube. It is used to excite radio frequency pulses under the action of the magnetic field generated by the magnet and act on the rock sample to form a nuclear magnetic resonance signal. The receiving device is connected to the display terminal and is used to receive the nuclear magnetic resonance signal, convert it into an electrical signal, and transmit the electrical signal to the display terminal. The display terminal collects the electrical signals transmitted by the receiving device and displays the nuclear magnetic resonance image of the rock sample, thus visually characterizing the changes in the distribution of structural fractures in the shale strata and the surrounding water bodies during the fracture creep process.

6. The deep shale fracture stability evaluation equipment according to claim 5, characterized in that, It also includes a base; The base is connected to the lower ends of the display terminal, the receiving device, the magnet, and the clamp; The base has the fluid pressure control device and the fluid storage tank inside its cavity.

7. The deep shale fracture stability evaluation equipment according to claim 6, characterized in that, It also includes a console connected to the base; The control console is connected to the fluid pressure control device and the receiving device, respectively, and is used to control the pressure value of the high-pressure fluid and the frequency of the nuclear magnetic resonance signal.

8. The deep shale fracture stability evaluation equipment according to claim 6, characterized in that, It also includes confining pressure connection pipes and pressure supply pipes; The confining pressure connecting pipe is located in the inner cavity of the confining pressure pipe and is connected to the fluid storage tank; The pressure supply pipe is located inside the loading rod and is connected to the fluid storage tank.

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