Hydrate sediment creep seepage testing system and method based on pulse attenuation method

Abstract

The invention discloses a hydrate sediment creep seepage testing system and method based on a pulse attenuation method.The hydrate sediment creep seepage testing system comprises a true triaxial stress loading system and a permeability measuring system, and the true triaxial stress loading system comprises a first principal stress loading system, a second principal stress loading system and a third principal stress loading system; the first principal stress loading system is used for vertically applying a first principal stress to the sample, the second principal stress loading system is used for horizontally applying a second principal stress to the sample, and the third principal stress loading system is used for applying a third principal stress; the permeability measuring system comprises a gas supply system, and the gas supply system is connected with the second gas conveying pipe and the third gas conveying pipe through a first gas conveying pipe. According to the method, the rigid loading system considering the sigma 2 direction is introduced, and the intermediate principal stress is accurately applied and regulated, so that the influence of the second principal stress on the creep behavior of the hydrate sediment is obtained; meanwhile, a permeability measurement system based on a pulse attenuation method is matched, so that the test error is effectively reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrate sediment testing, and in particular to a hydrate sediment creep seepage testing system and method based on a pulse decay method. Background Art

[0002] Currently, most tests of the mechanical and seepage properties of hydrate deposits rely on separate, step-by-step testing with independent devices, which cannot simultaneously measure the dynamic coupling between creep and seepage parameters. For example, creep properties are typically measured using a low-temperature triaxial instrument by monitoring axial displacement under constant load. However, such devices lack a seepage channel design and cannot measure pore pressure or permeability changes in real time. Conversely, while seepage testing devices can obtain permeability using steady-state or transient methods, they cannot simulate the synergistic effects of stress and seepage during actual mining.

[0003] There are some existing studies on integrated triaxial shear and seepage testing methods for hydrate sediments. However, most of these studies involve conventional triaxial testing equipment and ignore the impact of intermediate principal stresses, a key factor in mechanical properties. Furthermore, existing technologies lack stability when simulating high-pressure and low-temperature environments, which affects the stability of the hydrate phase. Furthermore, traditional seepage testing methods, such as steady-state methods, are often time-consuming and subject to significant measurement errors, particularly when working with low-permeability samples. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the purpose of the present invention is to provide a hydrate sediment creep seepage test system and method based on the pulse decay method. By introducing a rigid loading system that considers the σ2 direction, the intermediate principal stress is accurately applied and regulated to obtain the influence of the second principal stress on the creep behavior of hydrate sediments; at the same time, in conjunction with a permeability measurement system based on the pulse decay method, the test error is effectively reduced.

[0005] In order to achieve the above object, the present invention is implemented through the following technical solutions: In a first aspect, an embodiment of the present invention provides a hydrate sediment creep seepage testing system based on a pulse decay method, comprising: A true triaxial stress loading system is provided in the pressure chamber and includes a first principal stress loading system, a second principal stress loading system, and a third principal stress loading system. The first principal stress loading system is used to apply a first principal stress vertically to the specimen, the second principal stress loading system is used to apply a second principal stress horizontally to the specimen, and the third principal stress loading system is used to inject a medium into the pressure chamber through a hydraulic pump to apply the third principal stress. The permeability measurement system includes an air supply system, which is connected to a second air supply pipe and a third air supply pipe through a first air supply pipe. The second air supply pipe is used to transport the seepage medium to the top of the sample, and the third air supply pipe is used to transport the seepage medium to the bottom of the sample. Valves are installed on the first air supply pipe, the second air supply pipe and the third air supply pipe to control the gas flow and pressure gradient.

[0006] As a further implementation, the second principal stress loading system has two groups of horizontal loading modules, and the two groups of horizontal loading modules are symmetrically arranged on both sides of the base, and the base is used to place the sample.

[0007] As a further implementation, the first main stress loading system and the second main stress loading system respectively include a piston cylinder, one end of which is connected to a flange cover.

[0008] As a further implementation, the true triaxial stress loading system further includes a displacement monitoring module, which is used to detect real-time deformation.

[0009] As a further implementation, the second gas pipe is installed with a first pore pressure sensor, and the third gas pipe is installed with a second pore pressure sensor. The first pore pressure sensor and the second pore pressure sensor can monitor the pressure difference at both ends of the sample.

[0010] As a further implementation, the third gas pipeline is further installed with a CH4 sensor for monitoring whether methane is generated during the seepage process.

[0011] As a further implementation, the pressure chamber is arranged in a temperature box, and the temperature box is connected to a temperature control system.

[0012] As a further implementation, the temperature control system includes a PT compensation module, which is used to dynamically adjust temperature and pressure parameters.

[0013] In a second aspect, embodiments of the present invention further provide a method for testing creep seepage of hydrate sediments based on a pulse decay method, comprising: The hydrate sediment sample was wrapped with a rubber film and placed on a base; The first, second and third principal stresses are applied synchronously, and the creep strain-time curve is continuously recorded through the displacement monitoring module; Open the gas supply system and the valves on each gas pipeline. After a set time, close the valve of the third gas pipeline. When there is a significant pressure difference between the first pore pressure sensor and the second pore pressure sensor, close the valve of the first gas pipeline and the gas supply system. Open the valve of the third gas pipe and let the fluid flow from the top to the bottom of the sample. When the data of the first pore pressure sensor and the second pore pressure sensor are the same, record the pore pressure decay data.

[0014] As a further implementation method, the temperature and pressure are dynamically adjusted based on the PT compensation theory.

[0015] The beneficial effects of the present invention are as follows: (1) The present invention is based on a true triaxial stress loading system. By introducing a rigid loading system that considers the σ2 direction, the intermediate principal stress is accurately applied and regulated, thereby obtaining the influence of the second principal stress on the creep behavior of hydrate sediments. At the same time, combined with a permeability measurement system based on the pulse decay method, the dynamic coupling data of creep strain and seepage parameters can be obtained synchronously, effectively reducing the test error.

[0016] (2) The seepage test system of the present invention includes a gas supply system, a gas pipe connected to the top and bottom ends of the sample respectively, and a pore pressure sensor installed on the gas pipe to monitor the pressure difference between the two ends of the sample; by cooperating with the valve on the gas pipe, the gas on and off and the pressure gradient can be controlled to achieve non-steady-state permeability measurement; a CH4 sensor, a pore pressure sensor and a displacement monitoring module are set up to monitor the hydrate phase stability and experimental parameter anomalies in real time, and the temperature and pressure boundary conditions are actively adjusted in combination with the PT curve compensation method to ensure that the experimental process is safe and controllable.

[0017] (3) The present invention adds a temperature box outside the pressure chamber. During the seepage test, the temperature and pressure can be dynamically adjusted according to the actual working conditions, while ensuring that the hydrate is stable and does not decompose, so that the experimental conditions are more in line with the complex actual mining environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0019] Figure 1 is a schematic diagram of the overall structure of a test system according to one or more embodiments of the present invention; Figure 2 is a front view of a true triaxial stress loading system according to one or more embodiments of the present invention; Figure 3 FIG. 1 is a top view of a true triaxial stress loading system according to one or more embodiments of the present invention.

[0020] Among them, 1. gas supply system, 2. first gas pipe, 3. filter, 4. first valve, 5. second valve, 6. third valve, 7. second gas pipe, 8. third gas pipe, 9. first pore pressure sensor, 10. second pore pressure sensor, 11. CH4 sensor, 12. temperature box, 13. pressure chamber, 14. first principal stress loading system, 15. second principal stress loading system, 16. third displacement monitoring module, 17. third principal stress control system, 18. temperature control system, 19. computer, 20. first displacement monitoring module, 21. second displacement monitoring module, 22. base, 23. second flange cover, 24. first flange cover, 25. third flange cover, 26. specimen, 27. rubber membrane, 28. air inlet, 29. air outlet, 30. first piston cylinder, 31. second piston cylinder, 32. third piston cylinder. DETAILED DESCRIPTION

[0021] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0022] For ease of description, the words "upper," "lower," "left," and "right" appearing in this disclosure merely indicate the same orientation as in the accompanying drawings and do not limit the structure. These are intended solely to facilitate and simplify the description of the present invention and are not intended to indicate or imply that the device or component referred to must have a specific orientation, be constructed, or operate in a specific manner. Therefore, they should not be construed as limitations on the present invention. In the description of the present invention, the terms "first," "second," and the like are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0023] Example 1: This embodiment provides a hydrate sediment creep seepage test system based on the pulse decay method, such as Figure 1 As shown, the system includes a pressure control system, a temperature control system 18, and a permeability measurement system. The pressure control system includes a pressure chamber 13, a first principal stress loading system 14, a second principal stress loading system 15, and a third principal stress loading system. The first principal stress loading system 14 is used to vertically apply a first principal stress σ1 to the specimen 26, the second principal stress loading system 15 is used to horizontally apply a second principal stress σ2 to the specimen 26, and the third principal stress loading system is connected to the third principal stress control system 18 and is used to inject a medium (such as silicone oil) into the pressure chamber 13 via a hydraulic pump to apply a uniformly distributed third principal stress σ3. The permeability measurement system includes an air supply system 1, a filter 3, a pore pressure sensor, a CH4 sensor 11, and other components.

[0024] Specifically, the pressure chamber 13 is housed within a temperature box 12, which is connected to a temperature control system 18. This system includes a temperature control module and a PT compensation module. The temperature control module integrates a semiconductor refrigeration element and a heating film, and controls the temperature within the temperature box 12 in a closed-loop manner via a computer 19. In this embodiment, the temperature control range is -20°C to 25°C. The PT compensation module, based on the PT curve compensation theory, dynamically adjusts temperature and pressure parameters to ensure hydrate phase stability.

[0025] The first principal stress loading system 14, the second principal stress loading system 15 and the loading parts of the third principal stress loading system are all arranged in the pressure chamber 13, forming a true triaxial stress loading system; Figure 1-Figure 3 As shown, a base 22 for placing a sample 26 is provided in the pressure chamber 13, and a first main stress loading system 14 is installed on the top of the pressure chamber 13. The first main stress loading system 14 includes a first piston cylinder 30 and a first flange cover 24. The first piston cylinder 30 is vertically arranged in the middle position of the pressure chamber 13, and one end of the first piston cylinder 30 is connected to the first flange cover 24. Under the pushing action of the first piston cylinder 30, the first flange cover 24 contacts the top of the sample 26, thereby realizing vertical loading of the sample 26.

[0026] The second principal stress loading system 15 includes two groups of horizontal loading modules, and the two groups of horizontal loading modules are symmetrically distributed relative to the base 22. The left horizontal loading module includes a second piston cylinder 31 and a second flange cover 23 connected to one end of the second piston cylinder 31. The right horizontal loading module includes a third piston cylinder 32 and a third flange cover 25 connected to one end of the third piston cylinder 32. Under the pushing action of the second piston cylinder 31 and the third piston cylinder 32, the second flange cover 23 and the third flange cover 25 contact the side of the sample 26 to achieve horizontal loading of the sample 26.

[0027] During the triaxial stress loading process, the displacement monitoring module is used to measure the loading deformation in real time. Therefore, a first displacement monitoring module 20, a second displacement monitoring module 21 and a third displacement monitoring module 16 are set to measure the real-time deformation in the directions of σ1, σ2 and σ3 respectively. In this embodiment, the displacement monitoring module uses a high-precision LVDT sensor. Figure 2 As shown, the first displacement monitoring module 20, the second displacement monitoring module 21 and the third displacement monitoring module 16 are arranged on the outside of the temperature box 12, and the power part of the piston column, that is, the hydraulic cylinder body, is also located outside the temperature box 12; of course, in other embodiments, for ease of installation, the hydraulic cylinder body of the piston column can also be installed on the outside of the pressure chamber 13.

[0028] The gas supply system 1 is used to deliver inert seepage medium to the top and bottom of the sample 26; Figure 1As shown, the gas supply system 1 is connected to the first gas pipe 2, on which a filter 3 and a first valve 4 are installed in sequence. The end of the first gas pipe 2 on which the first valve 4 is installed is connected to two branches, one of which is the second gas pipe 7, which is used to supply gas to the top of the sample 26. The second gas pipe 7 is installed with a second valve 5 and a first pore pressure sensor 9 in sequence according to the gas transmission direction; the other branch is the third gas pipe 8, which is used to supply gas to the bottom of the sample 26. The third gas pipe 8 is installed with a third valve 6, a CH4 sensor 11 and a second pore pressure sensor 10 from one end close to the filter 3 to the other end.

[0029] Sample 26 is provided with an air inlet 28 at the top and an air outlet 29 at the bottom. In this embodiment, N₂ is used as the inert seepage medium. After being purified by a filter 3, it is delivered to the air inlet 28 of sample 26 via the first and second air pipes 2 and 7. After passing through sample 26, the inert seepage medium flows out of the air outlet 29. During this process, the first and second pore pressure sensors 9 and 10 monitor the pressure differential across sample 26 and transmit the data in real time to a computer 19. A CH₄ sensor 11 monitors the formation of methane during the seepage process to ensure that hydrates are not decomposing. The first, second, and third valves 4, 5, and 6 work together to control the flow of gas and the pressure gradient, enabling unsteady-state permeability measurements.

[0030] This embodiment incorporates airtight gas pipes at the upper and lower ends of the chamber of specimen 26, ensuring tightness. The use of inert nitrogen prevents the fluid from reacting with hydrates. Furthermore, a temperature chamber 12 is incorporated into the true triaxial system, utilizing the PT compensation theory to prevent hydrate decomposition. This integrates the permeability measurement system with the true triaxial stress loading system, enabling simultaneous acquisition of dynamic coupled data on creep strain and seepage parameters. The incorporation of a CH4 sensor 11, a pore pressure sensor, and a displacement monitoring module enables real-time monitoring of hydrate phase stability and experimental parameter anomalies. A dynamic compensation algorithm (PT curve compensation) is then used to proactively adjust temperature and pressure boundary conditions, ensuring a safe and controllable experimental process.

[0031] Example 2: This embodiment provides a method for testing creep seepage of hydrate sediments based on a pulse decay method, using the measuring device described in Example 1, and comprising the following steps: Step 1: Wrap the hydrate sediment sample 26 with a rubber film 27 and then place it on the base 22 .

[0032] Step 2: According to the actual formation stress conditions, σ1, σ2, and σ3 are applied synchronously, and the creep strain-time curve is continuously recorded through the displacement monitoring module.

[0033] Step 3: Open the air supply system 1 and the three valves (1, 4, 5, and 6). After a while, close the third valve 6 to increase the air supply pressure. Read the data from the first and second pore pressure sensors 9 and 10. Once a significant pressure differential is established, close the first valve 4 and stop the air supply. Open the third valve 6, and the fluid will flow from the top of the specimen 26 through the specimen 26 to the bottom. When the readings from the two pore pressure sensors are identical, record the pore pressure decay data.

[0034] Calculation based on the unsteady-state permeability formula: (1) in, A represents the cross-sectional area of ​​sample 26, t represents the seepage time, μ represents the fluid viscosity, β represents the compressibility of the fluid, L Indicates the length of the sample, p i represents the applied pore pressure, p f It indicates the pressure after the pressure difference between the two ends of the sample is eliminated and becomes stable.

[0035] In this embodiment, μ =1.8×10 -5 Pa·s, =4.5×10 -10 Pa -1 .

[0036] Step 4: PT dynamic compensation verification: According to the phase equilibrium curve, the changes in temperature and pressure must satisfy: (2) According to PT compensation theory: (3) in, Indicates the temperature adjustment amount, Indicates the pressure change.

[0037] Substituting the above expression into formula (1), we get: (4) (5) Thereafter, the CH4 sensor 11 confirms that no methane is released.

[0038] Step 5: Repeat steps 2 and 3, and dynamically adjust the temperature and pressure based on the PT compensation theory. This allows for the measurement of pre-creep seepage, creep seepage, and post-creep seepage under conditions that ensure the stability of the natural gas hydrate phase, taking into account pressure disturbances.

[0039] Example 3: The hydrate sediment creep seepage test method based on the pulse decay method in this embodiment uses the hydrate sediment in the Shenhu area of ​​the South China Sea as a sample, and the stress loading parameters are: σ 1=18 MPa (simulated vertical stress), σ 2=12 MPa (simulated horizontal tectonic stress), σ 3 = 10 MPa (confining pressure); the temperature parameters are initially T = 5°C and P = 15 MPa (based on the phase equilibrium conditions of the South China Sea reservoir).

[0040] The specific steps are: Step 1: Stress loading and creep monitoring.

[0041] After applying triaxial stress, the specimen deformation is monitored in real time through the displacement monitoring module.

[0042] The initial creep rate and the creep rate after 24 hours were measured to obtain the typical creep characteristics test results of the sample under moderate stress.

[0043] Step 2: Pulse decay method seepage test.

[0044] Open the gas supply system 1 to establish the initial pressure difference , and record the pore pressure decay curve. The permeability calculation is completed based on the non-steady-state permeability formula, that is, formula (1).

[0045] Step 3: PT dynamic compensation verification.

[0046] To simulate the pressure disturbance caused by operating errors or instrument errors in the laboratory (e.g., P drops from 15 MPa to 14.5 MPa), the temperature control system 18 automatically adjusts the temperature to 4.96°C ( =0.04°C).

[0047] Observe the CH4 sensor data. If the methane concentration is <5 ppm throughout the process, it verifies that the hydrate has not decomposed.

[0048] Step 4: Repeat steps 2 and 3 to complete the measurement of seepage before creep, seepage during creep, and seepage after creep.

[0049] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. Hydrate sediment creep seepage test system based on pulse decay method, characterized by: include: A true triaxial stress loading system is provided in the pressure chamber and includes a first principal stress loading system, a second principal stress loading system, and a third principal stress loading system. The first principal stress loading system is used to apply a first principal stress vertically to the specimen, the second principal stress loading system is used to apply a second principal stress horizontally to the specimen, and the third principal stress loading system is used to inject a medium into the pressure chamber through a hydraulic pump to apply the third principal stress. The permeability measurement system includes an air supply system, which is connected to a second air supply pipe and a third air supply pipe through a first air supply pipe. The second air supply pipe is used to transport the seepage medium to the top of the sample, and the third air supply pipe is used to transport the seepage medium to the bottom of the sample. Valves are installed on the first air supply pipe, the second air supply pipe and the third air supply pipe to control the gas flow and pressure gradient.

2. The hydrate sediment creep seepage test system based on the pulse decay method according to claim 1 is characterized in that: The second principal stress loading system comprises two sets of horizontal loading modules, which are symmetrically arranged on both sides of a base, and the base is used to place the sample.

3. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 1 or 2, characterized in that: The first main stress loading system and the second main stress loading system respectively include a piston cylinder, and one end of the piston cylinder is connected to a flange cover.

4. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 1 is characterized in that: The true triaxial stress loading system further includes a displacement monitoring module, which is used to detect real-time deformation.

5. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 1 is characterized in that: The second gas pipe is installed with a first pore pressure sensor, and the third gas pipe is installed with a second pore pressure sensor. The pressure difference between the two ends of the sample can be monitored through the first pore pressure sensor and the second pore pressure sensor.

6. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 1 or 5, characterized in that: The third gas pipeline is also equipped with a CH4 sensor for monitoring whether methane is generated during the seepage process.

7. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 1 is characterized in that: The pressure chamber is arranged in a temperature box, and the temperature box is connected to a temperature control system.

8. The hydrate sediment creep seepage testing system based on the pulse decay method according to claim 7 is characterized in that: The temperature control system includes a PT compensation module, which is used to dynamically adjust temperature and pressure parameters.

9. A hydrate sediment creep seepage test method based on the pulse decay method, characterized in that: include: The hydrate sediment sample was wrapped with a rubber film and placed on a base; The first, second and third principal stresses are applied synchronously, and the creep strain-time curve is continuously recorded through the displacement monitoring module; Open the gas supply system and the valves on each gas pipeline. After a set time, close the valve of the third gas pipeline. When there is a significant pressure difference between the first pore pressure sensor and the second pore pressure sensor, close the valve of the first gas pipeline and the gas supply system. Open the valve of the third gas pipe and let the fluid flow from the top to the bottom of the sample. When the data of the first pore pressure sensor and the second pore pressure sensor are the same, record the pore pressure decay data.

10. The hydrate sediment creep seepage test method based on the pulse decay method according to claim 9, characterized in that: Dynamically adjust temperature and pressure based on PT compensation theory.

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