A ring shear test device and method for hydrate sediments

Abstract

The present invention discloses a ring shear test device and method for hydrate sediments, belonging to the field of marine natural gas hydrate research. The device includes a ring shear shear box, a torsion and axial loading mainframe, a nitrogen gas source, a methane gas source, a pressure-volume controller, a water bath device, a back pressure valve, a gas-liquid separator, a gas flowmeter, and a data acquisition system. The ring shear shear box includes an upper pressing plate, an upper adapter, an upper heat insulation plate, a lower top cover, a base, a lower heat insulation plate, a lower adapter, and a lower pressing plate, which are coaxially connected in sequence from top to bottom. On the premise that the present invention can in-situ generate hydrate sediments in the shear box and obtain the mechanical strength of hydrate sediments under large deformations, it controls the formation of a specific shear plane, effectively controls the undrained state of the experiment, reduces gas consumption, accurately measures the hydrate saturation, and accurately controls the temperature of the hydrate sediment specimen, providing strong hardware support for exploring the landslide deformation mechanism of hydrate sediments.

Description

Technical Field

[0001] The present invention belongs to the field of marine natural gas hydrate research, and particularly relates to a ring shear test device and method for hydrate sediments. Background Art

[0002] Natural gas hydrate has the characteristics of high energy density, wide distribution, large reserves, etc., and is a clean energy source with good development prospects. Due to its formation conditions of low temperature and high pressure, hydrates are usually distributed in terrestrial permafrost zones, deep sea floors, and sediments at the edges of continents and islands. Global warming and ocean warming may make methane hydrates in slope sediments along the continental margin unstable, leading to slope failure. Therefore, there is an urgent need to simulate a hydrate sediment landslide test device in the laboratory to provide guarantee for the safe development of marine resources.

[0003] Due to the high-pressure and low-temperature conditions required for hydrate formation, existing conventional soil mechanics test devices cannot reflect the failure characteristics of hydrate sediments under large deformations. Before the present invention, the Qingdao Institute of Marine Geology proposed a ring shear failure simulation device and method for hydrate sediments in 2021 (application number: 202111439480.8); there are some problems and deficiencies in this device and method: 1. The experimental period is long. Since the inside of the reaction kettle is filled with gas and the thermal conductivity of the gas is low, a long time is required for temperature control to ensure temperature stability during the experiment; 2. Since hydrate formation in the experiment only occurs inside the shear box, a large amount of methane gas filling the reaction kettle is not used, resulting in excessive gas consumption and inaccurate measurement of hydrate saturation; 3. Since this device adds a reaction kettle outside the shear box and the ring shear experiment only occurs in the shear box, due to the insufficient sealing of the shear box, the experiment cannot control the undrained state.

[0004] In view of the deficiencies in the Chinese invention patent with the invention name of "a ring shear failure simulation device and method for hydrate sediments" and application number of 202111439480.8, the applicant redesigned and optimized the ring shear test device for hydrate sediments, overcame the defects of the above device, and provided strong hardware support for exploring the failure mechanism of large deformations of hydrate sediments. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects in the prior art and provide a ring shear test device and method for hydrate sediments. The present invention can accurately measure the mechanical properties of hydrate sediments under large deformations, display them using a data acquisition system, and is simple to operate and reliable in structure.

[0006] The specific technical solutions adopted by the present invention are as follows:

[0007] In a first aspect, the present invention provides a ring shear test device for hydrate sediments, comprising a ring shear box, a torsion and axial loading mainframe, a nitrogen gas source, a methane gas source, a pressure volume controller, a water bath device, a back pressure valve, a gas-liquid separator, a gas flowmeter and a data acquisition system;

[0008] The ring shear box includes an upper pressing plate, an upper adapter, an upper heat insulation plate, a lower top cover, a base, a lower heat insulation plate, a lower adapter and a lower pressing plate which are coaxially connected in sequence from top to bottom; a circular groove for placing a specimen is formed at the top of the base and serves as a pressure chamber, and an air inlet channel communicating with the pressure chamber is formed on the base; an auxiliary inner ring and an auxiliary outer ring are respectively placed on the inner wall and the outer wall of the circular groove in the upper half of the specimen, O-rings are respectively arranged at the upper and lower ends of the auxiliary inner ring and the auxiliary outer ring, and a base inner pressing plate and a base outer pressing ring for axial limiting are respectively arranged on the top of the base above the auxiliary inner ring and the auxiliary outer ring; a temperature control cylinder is arranged on the outer side of the base along the circumferential direction; a groove is formed at the top of the lower top cover, and a serpentine channel is arranged at the bottom of the groove. The notch at the upper part of the serpentine channel is sealed by the upper top cover; both the temperature control cylinder and the serpentine channel of the lower top cover are connected with the water bath device for adjusting the temperature of the pressure chamber; a circular protrusion is arranged at the bottom of the lower top cover, and the circular protrusion extends into the pressure chamber and is hermetically and slidably connected with the circular groove to make the pressure chamber in a sealed state; an air outlet channel communicating with the pressure chamber is formed on the lower top cover, and a temperature sensor for measuring the temperature of the specimen is arranged on the lower top cover; the axial actuator at the upper end of the torsion and axial loading mainframe is fixedly connected with the top of the upper pressing plate, and the torsion actuator at the lower end is fixedly connected with the bottom of the lower pressing plate. The axial actuator, the ring shear box and the torsion actuator are coaxially arranged;

[0009] The air inlet channel is respectively communicated with the nitrogen gas source and the methane gas source through a pipeline provided with a second valve, and this pipeline is also communicated with the pressure volume controller through a branch provided with a first valve; the air outlet channel is sequentially connected with a third valve, a back pressure valve, a gas-liquid separator and a gas flowmeter through a pipeline; the data acquisition system is connected with the pressure volume controller, the water bath device and the torsion and axial loading mainframe.

[0010] Preferably, the materials of the auxiliary inner ring and the auxiliary outer ring are both 316L stainless steel, the materials of the lower top cover and the base are both 316L stainless steel, and the material of the O-ring is Teflon.

[0011] Preferably, the air inlet channel is located below the pressure chamber, and the air outlet channel is located above the pressure chamber.

[0012] Preferably, the pressure chamber is a stepped structure in the vertical direction, and the cross section of the circular groove in the upper half of the specimen is larger than the cross section of the circular groove in the lower half of the specimen for placing the auxiliary inner ring and the auxiliary outer ring.

[0013] Preferably, a water-permeable plate is detachably fixed to the bottom of the annular groove of the base and the bottom of the annular protrusion of the lower top cover, respectively, and ribs for transmitting torque are provided on one side of the water-permeable plate adjacent to the specimen.

[0014] Preferably, a serpentine channel for the passage of the water bath liquid is provided inside the temperature control cylinder along the circumference of the base.

[0015] Preferably, a sealing ring and a guide ring are sequentially sleeved from bottom to top on both the inner and outer sides of the annular protrusion at the bottom of the lower top cover for sealing and sliding connection with the inner and outer walls of the pressure chamber.

[0016] Preferably, a detachable connection is adopted between the inner pressing plate of the base and the outer pressing ring of the base and the top of the base.

[0017] Preferably, sealing rings are provided between the auxiliary outer ring and the outer wall of the base pressure chamber and between the auxiliary inner ring and the inner wall of the base pressure chamber.

[0018] In a second aspect, the present invention provides a test method using the hydrate sediment ring shear test device according to any one of the first aspect, specifically as follows:

[0019] S1: Control the axial displacement loading of the torsion and axial loading host to make the lower top cover move downward into the base to seal the pressure chamber; close the first valve, the second valve, the third valve and the back pressure valve, open the nitrogen gas source, and then sequentially open the first valve, the second valve and the third valve, and use a leak detection liquid to detect leaks at the pipeline connection to confirm that there is no leakage in the pipeline. After the inspection, close the nitrogen gas source;

[0020] S2: Perform axial loading longitudinal movement wear calibration and torsion loading friction calibration on the torsion and axial loading host to determine the normal pressure loss P0 and the torsion loss torque M0 during axial loading;

[0021] S3: Uniformly mix deionized water and the soil sample, put the mixed soil sample into a sample preparation mold, use a compaction tool to compact it layer by layer, and take out the prepared specimen after freezing it at minus temperature for 24 hours;

[0022] S4: Open the back pressure valve to exhaust the gas in the pipeline; control the axial displacement loading of the torsion and axial loading host to make the lower top cover move upward, open the lower top cover, and respectively fix the ribbed water-permeable plates at the bottom of the annular groove of the base and the bottom of the annular protrusion of the lower top cover; place filter papers at the upper and lower ends of the specimen respectively, and then put the whole into the annular groove of the base so that it is located between the two water-permeable plates. Then, insert the annular protrusion of the lower top cover into the pressure chamber to keep the pressure chamber sealed;

[0023] S5: Open the first valve and the second valve, open the back pressure valve and adjust it to the maximum opening; turn on the methane gas source, and the methane gas is injected into the pressure - volume controller and the pressure chamber through pipelines respectively; when the pressure in the pressure - volume controller rises to 0.5 MPa, turn off the methane gas source, adjust the back pressure valve to the minimum opening, and close the third valve; adjust the pore pressure of the pressure - volume controller until the pore pressure in the pressure - volume controller reaches 12 Mpa; adjust the pressure - volume controller to enter the constant - pressure mode, and use the pressure - volume controller to introduce methane gas into the pressure chamber; use the water - bath device to adjust the temperature in the pressure chamber to rise to 15 °C to melt the ice in the specimen; when the volume of methane gas in the pressure - volume controller no longer decreases, it is considered that the specimen reaches complete gas saturation, and proceed to S6 to generate hydrate in - situ;

[0024] S6: Use the water - bath device to adjust the temperature in the pressure chamber to 1 °C, so that the temperature of the specimen is lower than the hydrate phase - equilibrium temperature; when the volume of methane gas in the pressure - volume controller no longer changes significantly again, it indicates that the water in the pores of the specimen has completely reacted with methane gas to form methane hydrate; calculate the saturation of methane hydrate according to the volume change of methane gas in the pressure - volume controller when it is stable in S5 and S6;

[0025] S7: According to the actual working conditions, determine the control mode of the pressure - volume controller, and use the torsion and axial loading mainframe to select stress control and strain control at different loading rates to shear the specimen, so that a shear band is formed at the bottom cross - section of the auxiliary inner ring and the auxiliary outer ring; the stress control is a control method for controlling the stress increment change rate, and the strain control is a control method for controlling the displacement change rate; during the test, collect and record the readings of the torsion - angle sensor, stress sensor, torque sensor, displacement sensor and pore - pressure sensor through the data - acquisition system, and analyze the mechanical properties of large - displacement shear of hydrate according to the collected data and the calibration parameters obtained in S2.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The present invention innovatively breaks the drawback that the existing ring - shear experimental device in soil mechanics cannot conduct experimental research on the mechanical properties of hydrate sediments under large deformations under low - temperature and high - pressure conditions. By using an auxiliary ring, a shear band is induced to form in a specific area of the specimen.

[0028] (2) The present invention can effectively control the undrained state of the experiment, accurately measure the pore pressure and temperature of the specimen, and can simulate the mechanical properties of hydrate under large deformations during submarine landslides, making the mechanical property indexes of hydrate obtained from this experiment more applicable to practical engineering.

[0029] (3) The test method of the present invention is overall simple, the instrument operation is convenient, and the results are accurate. It is of great significance in studying the mechanism of large deformation and failure of hydrate sediments, and provides data support for subsequent experimental simulation of hydrate sediment landslides. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is an overall structural diagram of a ring shear test device for hydrate sediments;

[0031] Figure 2 is a structural diagram of the ring shear box of a ring shear test device for hydrate sediments;

[0032] In the figure: 1 ring shear box; 2 torsion and axial loading mainframe; 3 nitrogen gas source; 4 methane gas source; 5 pressure - volume controller; 6 water bath device; 7 back - pressure valve; 8 gas - water separator; 9 gas flowmeter; 10 data acquisition system; 11a first valve; 11b second valve; 11c third valve; 12 upper pressure plate; 13 upper adapter; 14 upper heat insulation plate; 15 upper top cover; 16 temperature sensor; 17 lower top cover; 18 air inlet channel; 19 temperature control cylinder; 20 lower heat insulation plate; 21 lower adapter; 22 lower pressure plate; 23 inner pressure plate of the base; 24 outer pressure ring of the base; 25 auxiliary inner ring; 26 auxiliary outer ring; 27 specimen; 28 permeable plate; 29 air outlet channel; 30 base. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The present invention will be further described and explained below in conjunction with the drawings and specific embodiments. The technical features of each embodiment in the present invention can be combined correspondingly without conflict.

[0034] As Figure 1 shown, a ring shear test device for hydrate sediments provided by the present invention mainly includes a ring shear box 1, a torsion and axial loading mainframe 2, a nitrogen gas source 3, a methane gas source 4, a pressure - volume controller 5, a water bath device 6, a back - pressure valve 7, a gas - water separator 8, a gas flowmeter 9 and a data acquisition system 10.

[0035] The structures and connection methods of each component will be specifically described below.

[0036] In the present invention, as Figure 2 shown, the ring shear box 1 includes an upper pressure plate 12, an upper adapter 13, an upper heat insulation plate 14, a lower top cover 17, a base 30, a lower heat insulation plate 20, a lower adapter 21 and a lower pressure plate 22 which are coaxially connected in sequence from top to bottom. A circular groove is opened at the top of the base 30 and used as a pressure chamber. A hollow cylindrical specimen 27 is placed in the circular groove, that is, the cross - section of the specimen 27 is a circular structure matching the circular groove. An air inlet channel 18 communicating with the pressure chamber is opened on the base 30, and an air outlet channel 29 communicating with the pressure chamber is opened on the lower top cover 17.

[0037] In the present invention, as Figure 2 shown, an auxiliary inner ring 25 and an auxiliary outer ring 26 are respectively fixed to the inner wall and the outer wall of the annular groove in the upper half of the specimen 27. O-rings are respectively provided at the upper and lower ends of the auxiliary inner ring 25 and the auxiliary outer ring 26. Inner base pressing plates 23 and outer base pressing rings 24 for axial limiting are respectively provided at the top of the base 30 above the auxiliary inner ring 25 and the auxiliary outer ring 26. The inner base pressing plates 23 and the outer base pressing rings 24 are detachably connected to the top of the base 30 to prevent the upper and lower sliding of the auxiliary rings caused by the upper and lower sliding of the lower top cover 17 during the experiment.

[0038] In actual use, the materials of the auxiliary inner ring 25 and the auxiliary outer ring 26 can both be made of stainless steel. The O-rings provided at the upper and lower ends can be made of a corrosion-resistant organic material with an extremely low coefficient of friction (such as Teflon). The auxiliary inner ring 25 and the auxiliary outer ring 26 can induce the specimen to form shear bands in a specific area (i.e., at the O-ring at the bottom) during the shearing process.

[0039] In actual use, in order to ensure a better installation effect, the pressure chamber can be set as a stepped structure in the vertical direction. The cross-section of the annular groove in the upper half of the specimen 27 is larger than the cross-section of the annular groove in the lower half of the specimen 27, so as to facilitate the installation of the auxiliary inner ring 25 and the auxiliary outer ring 26 at the annular groove in the upper half. A water-permeable plate 28 with ribs is fixed to the bottom of the annular groove of the base 30, and a water-permeable plate 28 with ribs is fixed to the bottom of the annular protrusion of the lower top cover 17. And ribs are provided on one side of both water-permeable plates 28 adjacent to the specimen 27, and the ribs are used to embed into the specimen to transmit torque.

[0040] In actual use, a sealing ring can be provided between the auxiliary outer ring 26 and the outer wall of the pressure chamber of the base 30, and a sealing ring can be provided between the auxiliary inner ring 25 and the inner wall of the pressure chamber of the base 30. A sealing ring and a guide ring are sleeved on the annular protrusion at the bottom of the lower top cover 17 for sealed sliding connection with the inner wall and the outer wall of the pressure chamber. Among them, the diameter of the guide ring in contact with the outer wall is slightly larger than the outer diameter of the part where the lower annular protrusion of the lower top cover 17 enters the pressure chamber of the base 30, and the diameter of the guide ring in contact with the inner wall is slightly smaller than the inner diameter of the part where the lower annular protrusion of the lower top cover 17 enters the pressure chamber of the base 30, so as to avoid the eccentricity of the lower top cover 17 during the experiment and realize the high-pressure sealing of the experimental device.

[0041] In the present invention, as Figure 2As shown in the figure, a temperature control cylinder 19 is provided circumferentially along the outside of the base 30. A slot is opened at the top of the lower top cover 17, and a serpentine channel is provided at the bottom of the slot. The notch at the upper part of the serpentine channel is sealed by the upper top cover 15. During actual use, a serpentine channel for the passage of the water bath liquid can also be provided circumferentially along the base 30 inside the temperature control cylinder 19. The serpentine channel can supply the liquid of the water bath device 6 to pass through, which can increase the heat exchange area and better and accurately control the temperature inside the pressure chamber. The serpentine channels of the temperature control cylinder 19 and the lower top cover 17 are both connected to the water bath device 6 for adjusting the temperature of the pressure chamber. A circular protrusion is provided at the bottom of the lower top cover 17, and the circular protrusion extends into the pressure chamber and is hermetically and slidably connected to the circular groove to make the pressure chamber in a sealed state. A temperature sensor 16 for measuring the temperature of the specimen 27 is provided on the lower top cover 17. Here, the hermetic sliding means that the lower top cover 17 can move up and down hermetically along the vertical direction and the base 30 can rotate hermetically along the circumferential direction under the limitation of the circular groove by the circular protrusion.

[0042] During actual use, the lower top cover 17 and the base 30 are made of corrosion-resistant and high-pressure-resistant alloy (such as 316L stainless steel), which can prevent acidic substances from corroding the ring shear box 1.

[0043] During actual use, in order to facilitate the installation of the temperature sensor 16, a through hole for placing the temperature sensor 16 can be opened on the lower top cover 17, a channel for placing the temperature sensor 16 can be provided on the upper adapter 13, and a through hole for the temperature sensor 16 to pass through can be provided on the upper heat insulation plate 14.

[0044] That is to say, for the ring shear box 1, the bottom of the upper pressure plate 12 is connected to the top of the upper adapter 13, the bottom of the upper adapter 13 is connected to the top of the upper heat insulation plate 14, the bottom of the upper heat insulation plate 14 is connected to the top of the lower top cover 17, the upper top cover 15 is fixed in the groove at the upper end of the lower top cover 17, the lower top cover 17 is slidably arranged up and down in the annular pressure chamber of the base 30, the temperature control cylinder 19 is wrapped outside the base 30, the outer pressure ring 24 of the base and the inner pressure ring 23 of the base are respectively fixed on the outer ring and the inner ring at the top of the base 30, the outer O-ring is placed at the upper and lower ends of the auxiliary outer ring 26 respectively, the inner O-ring is placed at the upper and lower ends of the auxiliary inner ring 25 respectively, and the auxiliary outer ring 26 and the auxiliary inner ring 25 are respectively placed on the outer wall and the inner wall inside the pressure cavity of the base 30. A water permeable plate 28 is fixed at the inner bottom of the annular groove of the base 30, and another water permeable plate 28 is fixed at the top of the protrusion of the lower top cover 17. The specimen 27 is placed inside the pressure cavity of the base 30, the bottom of the base 30 is connected to the top of the lower heat insulation plate 20, the bottom of the lower heat insulation plate 20 is connected to the top of the lower adapter 21, and the bottom of the lower adapter 21 is connected to the top of the lower pressure plate 22.

[0045] It should be noted that in the present invention, unless otherwise specified, the "upper adapter" and the "lower adapter" both refer to the names of connecting components and belong to mechanical structures. Among them, the upper adapter 13 is used to connect the upper pressure plate 12 and the upper heat insulation plate 14, and the lower adapter 21 is used to connect the lower heat insulation plate 20 and the lower pressure plate 22.

[0046] In the present invention, as Figure 1 shown, the torsional and axial loading main machine 2 adopts the device structure in the prior art, which includes an axial actuator at the upper end, a torsional actuator at the lower end, and force and torque sensors (for recording stress sensor readings and torque sensor readings), displacement sensors, torsional angle sensors, etc. The specific structure will not be elaborated here. Among them, the axial actuator at the upper end is fixedly connected to the top of the upper pressure plate 12, the torsional actuator at the lower end is fixedly connected to the bottom of the lower pressure plate 22, and the axial actuator, the ring shear box 1 and the torsional actuator are coaxially arranged.

[0047] In the present invention, as Figure 1 shown, the intake channel 18 is respectively communicated with the nitrogen gas source 3 and the methane gas source 4 through pipelines provided with the second valve 11b, and valves for controlling opening and closing are respectively provided on the nitrogen gas source 3 and the methane gas source 4. The intake channel 18 is also communicated with the pressure volume controller 5 through a branch provided with the first valve 11a. Along the medium flow direction, the outlet channel 29 is sequentially connected to the third valve 11c, the back pressure valve 7, the gas-water separator 8 and the gas flowmeter 9 through pipelines. The data acquisition system 10 is connected to the pressure volume controller 5, the water bath device 6 and the torsional and axial loading main machine 2.

[0048] Using the above-mentioned ring shear experimental device for hydrate sediments, the present invention also provides a test method, which is specifically as follows:

[0049] Before the experiment, first confirm whether the pipelines of the device are correctly connected according to Figure 1 shown, which is specifically as follows:

[0050] Connect the water bath channel on the temperature control cylinder 19 to the water bath device 6 using a flexible pipeline (such as a rubber tube) to achieve temperature control of the ring shear box 1. Connect the air inlet channel 18 to the pressure-volume controller 5, methane gas source 4, and nitrogen gas source 3 respectively through the first valve 11a and the second valve 11b using a pressure-resistant pipeline (such as a steel pipe) to achieve pore pressure control of the sediment sample. Among them, the methane gas source 4 provides the gas supply required for in-situ hydrate formation of the sediment sample, and the nitrogen gas source 3 is mainly used for pipeline leak detection. Connect the gas outlet channel 29 of the hydrate sediment ring shear experimental device to the back pressure valve 7, gas-water separator 8, and gas flow meter 9 respectively through the third valve 11c using a pressure-resistant pipeline to achieve back pressure control of the sediment sample. Among them, the gas flow meter 9 is used to calculate the change in hydrate saturation of the sediment sample during the decomposition experiment. Connect the temperature sensor 16, pressure-volume controller 5, torsion and axial loading mainframe 2, and water bath device 6 to the data acquisition system 10 to achieve data acquisition during the experiment of the device of the present invention.

[0051] After confirming that the pipelines in the device are correctly connected, perform the following operations in sequence:

[0052] S1, Experimental leak detection:

[0053] Control the axial displacement loading of the torsion and axial loading mainframe 2 to make the lower top cover 17 move downward into the base 30 to form a closed pressure chamber. Close the first valve 11a, second valve 11b, third valve 11c, and back pressure valve 7, open the nitrogen gas source 3, then open the first valve 11a, second valve 11b, and third valve 11c in sequence, and use a leak detection liquid to detect leaks at the pipeline connection points to confirm that there is no leakage in the pipeline. If there is air leakage, tighten the corresponding valve nuts again. After the inspection is completed, close the nitrogen gas source 3.

[0054] S2, Loss calibration:

[0055] Perform axial loading longitudinal movement wear calibration and torsional loading friction calibration on the torsion and axial loading mainframe 2 to determine the normal pressure loss P0 and torsional loss torque M0 during axial loading.

[0056] S3, Specimen preparation:

[0057] According to the working conditions required for the experiment, uniformly mix deionized water and soil samples, put the mixed soil samples into a sample preparation mold, use a compaction tool to compact them in layers (for example, 8 layers), put the prepared specimen 27 into the refrigerator, and take it out after freezing at minus temperature for 24 hours.

[0058] S4, Specimen installation:

[0059] Open the back pressure valve 7 to remove the gas in the pipeline; control the axial displacement loading of the torsion and axial loading mainframe 2 to make the lower top cover 17 move upward, open the lower top cover 17, and fix the ribbed water-permeable plates 28 at the bottom of the annular groove of the base 30 and at the bottom of the annular protrusion of the lower top cover 17 respectively; place filter papers at the upper and lower ends of the specimen 27 respectively, and then place the specimen 27 and the filter papers on both sides of it as a whole into the annular groove of the base 30, so that the whole is located between the two water-permeable plates 28. Then, insert the annular protrusion of the lower top cover 17 into the pressure chamber to keep the pressure chamber sealed, and make the two water-permeable plates 28 clamp the specimen 27. Among them, the ribs of the water-permeable plate need to be pressed into the specimen for torque transmission.

[0060] S5, specimen saturation:

[0061] Open the first valve 11a and the second valve 11b, open the back pressure valve 7 and adjust it to the maximum opening. Open the methane gas source 4, and methane gas is injected into the pressure volume controller 5 and the pressure chamber through the pipeline respectively. When the pressure of the pressure volume controller 5 rises to 0.5 MPa, close the methane gas source 4, adjust the back pressure valve 7 to the minimum opening, and close the third valve 11c. Adjust the pressure volume controller 5 to control the pore pressure until the pore pressure of the pressure volume controller 5 reaches 12 Mpa. Adjust the pressure volume controller 5 to enter the constant pressure mode (i.e., stabilize at 12 Mpa), and introduce methane gas into the pressure chamber by using the pressure volume controller 5. Use the water bath device 6 to adjust the temperature in the pressure chamber to rise to 15 °C to melt the ice in the specimen 27. When the volume of methane gas in the pressure volume controller 5 no longer decreases, it is considered that the specimen 27 reaches complete gas saturation, and proceed to S6 to generate hydrates in-situ.

[0062] S6, in-situ hydrate generation:

[0063] Use the water bath device 6 to adjust the temperature in the pressure chamber to 1 °C, so that the temperature of the sediment specimen 27 is lower than the hydrate phase equilibrium temperature. When the volume of methane gas in the pressure volume controller 5 no longer changes significantly again, it indicates that the water in the pores of the specimen 27 has completely reacted with methane gas to form methane hydrate. Calculate the saturation of methane hydrate according to the volume change of methane gas in the pressure volume controller 5 when it is stable in S5 and S6.

[0064] S7, specimen shear:

[0065] According to the actual working conditions, determine the control mode of the pressure-volume controller, and use the torsion and axial loading host 2. According to the actual working conditions, select different control loading methods, that is, select stress control and strain control to conduct a shear experiment on the specimen 27, so that a shear band is formed at the bottom cross-section of the auxiliary inner ring 25 and the auxiliary outer ring 26. Among them, stress control is a control method for controlling the stress increment change rate, and strain control is a control method for controlling the displacement change rate. Both control methods can determine different loading rates according to requirements. In the test, it can be manually stopped or stopped automatically by setting the failure criterion. At the same time, during the test process, the data acquisition system 10 collects and records the readings of the torsion angle sensor, stress sensor, torque sensor, displacement sensor, and pore pressure sensor. According to the collected data and the calibration parameters obtained from S2, analyze the mechanical properties of the large-displacement shear of hydrates.

[0066] Compared with the prior art (including the Chinese invention patent with the invention name of a ring shear failure simulation device and method for hydrate-containing sediments and the application number of 202111439480.8), on the premise that the present invention can in-situ generate hydrate sediments in the shear box and obtain the mechanical strength of hydrate sediments under large deformation, it can effectively control the undrained state of the experiment, reduce gas consumption, accurately measure the hydrate saturation, and accurately control the temperature of the hydrate sediment specimen, providing strong hardware support for exploring the landslide deformation mechanism of hydrate sediments.

[0067] The above-described embodiments are only a preferred solution of the present invention, but it is not intended to limit the present invention. Those of ordinary skill in the relevant technical field can still make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by adopting equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A ring shear test device for hydrate sediments, characterized in that It includes a ring shear box (1), a torsion and axial loading mainframe (2), a nitrogen gas source (3), a methane gas source (4), a pressure - volume controller (5), a water bath device (6), a back - pressure valve (7), a gas - water separator (8), a gas flowmeter (9) and a data acquisition system (10); The ring shear box (1) includes an upper pressure plate (12), an upper adapter (13), an upper heat insulation plate (14), a lower top cover (17), a base (30), a lower heat insulation plate (20), a lower adapter (21) and a lower pressure plate (22) which are coaxially connected in sequence from top to bottom; a circular groove for placing a specimen (27) is formed at the top of the base (30) and serves as a pressure chamber, and an air inlet channel (18) communicating with the pressure chamber is formed on the base (30); an auxiliary inner ring (25) and an auxiliary outer ring (26) are respectively placed on the inner wall and outer wall of the circular groove in the upper half of the specimen (27), and O - rings are respectively arranged at the upper and lower ends of the auxiliary inner ring (25) and the auxiliary outer ring (26). Inner pressure plates (23) and outer pressure rings (24) for axial limit are respectively arranged at the top of the base (30) above the auxiliary inner ring (25) and the auxiliary outer ring (26); a temperature control cylinder (19) is arranged along the circumference on the outside of the base (30); a groove is formed at the top of the lower top cover (17), and a serpentine channel is arranged at the bottom of the groove. The notch at the upper part of the serpentine channel is sealed by an upper top cover (15); the serpentine channels of the temperature control cylinder (19) and the lower top cover (17) are both connected to the water bath device (6) for adjusting the temperature of the pressure chamber; a circular protrusion is arranged at the bottom of the lower top cover (17), and the circular protrusion extends into the pressure chamber and is in sealed sliding connection with the circular groove to make the pressure chamber in a sealed state; an air outlet channel (29) communicating with the pressure chamber is formed on the lower top cover (17), and a temperature sensor (16) for measuring the temperature of the specimen (27) is arranged on the lower top cover (17); the axial actuator at the upper end of the torsion and axial loading mainframe (2) is fixedly connected to the top of the upper pressure plate (12), and the torsion actuator at the lower end is fixedly connected to the bottom of the lower pressure plate (22). The axial actuator, the ring shear box (1) and the torsion actuator are coaxially arranged; The air inlet channel (18) is respectively communicated with the nitrogen gas source (3) and the methane gas source (4) through a pipeline provided with a second valve (11b), and this pipeline is also communicated with the pressure - volume controller (5) through a branch provided with a first valve (11a); the air outlet channel (29) is sequentially connected to a third valve (11c), a back - pressure valve (7), a gas - water separator (8) and a gas flowmeter (9) through a pipeline; the data acquisition system (10) is connected to the pressure - volume controller (5), the water bath device (6) and the torsion and axial loading mainframe (2); The pressure chamber is a stepped structure in the vertical direction, and the cross - section of the circular groove in the upper half of the specimen (27) is larger than the cross - section of the circular groove in the lower half of the specimen (27) for placing the auxiliary inner ring (25) and the auxiliary outer ring (26); A serpentine channel for the passage of the water bath liquid is arranged along the circumference of the base (30) inside the temperature control cylinder (19); On the inner and outer sides of the annular protrusion at the bottom of the lower top cover (17), a sealing ring and a guide ring are sleeved from bottom to top in sequence for sealing and sliding connection with the inner wall and the outer wall of the pressure chamber; Sealing rings are provided between the auxiliary outer ring (26) and the outer wall of the pressure chamber of the base (30), and between the auxiliary inner ring (25) and the inner wall of the pressure chamber of the base (30).

2. The ring shear test device for hydrate sediments according to claim 1, wherein The materials of the auxiliary inner ring (25) and the auxiliary outer ring (26) are both 316L stainless steel, the materials of the lower top cover (17) and the base (30) are both 316L stainless steel, and the material of the O-ring is Teflon.

3. The ring shear test device for hydrate sediments according to claim 1, characterized in that The intake passage (18) is located below the pressure chamber, and the outlet passage (29) is located above the pressure chamber.

4. The annular shear test device for hydrate sediments according to claim 1, characterized in that, A water-permeable plate (28) is detachably fixed to the bottom of the annular groove of the base (30) and the bottom of the annular protrusion of the lower top cover (17) respectively, and ribs for transmitting torque are provided on one side of the water-permeable plate (28) adjacent to the specimen (27).

5. The ring shear test device for hydrate sediment according to claim 1, characterized in that, Detachable connections are adopted between the inner pressure plate (23) of the base and the outer pressure ring (24) of the base and the top of the base (30).

6. A testing method using the hydrate sediment ring shear test device according to any one of claims 1 to 5, characterized in that, Specifically as follows: S1: Control the axial displacement loading of the torsional and axial loading main machine (2) to make the lower top cover (17) move downward into the base (30) to seal the pressure chamber; close the first valve (11a), the second valve (11b), the third valve (11c) and the back pressure valve (7), open the nitrogen gas source (3), then open the first valve (11a), the second valve (11b) and the third valve (11c) in sequence, and use a leak detection liquid to detect leaks at the pipeline connection to confirm that there is no leakage in the pipeline. After the inspection, close the nitrogen gas source (3); S2: Conduct axial loading longitudinal movement wear calibration and torsional loading friction calibration on the torsional and axial loading main machine (2) to determine the normal pressure loss P0 and the torsional loss torque M0 during axial loading; S3: Uniformly mix deionized water and the soil sample, put the mixed soil sample into a sample preparation mold, use a compaction tool to compact it layer by layer, and take out the prepared specimen (27) after freezing it at minus temperature for 24 hours; S4: Open the back pressure valve (7) to exhaust the gas in the pipeline; control the axial displacement loading of the torsional and axial loading main machine (2) to make the lower top cover (17) move upward, open the lower top cover (17), and respectively fix the water-permeable plates (28) with ribs at the bottom of the annular groove of the base (30) and the bottom of the annular protrusion of the lower top cover (17); place filter papers at the upper and lower ends of the specimen (27) respectively, and then put the whole into the annular groove of the base (30) so that it is located between the two water-permeable plates (28). Then, extend the annular protrusion of the lower top cover (17) into the pressure chamber to keep the pressure chamber sealed; S5: Open the first valve (11a) and the second valve (11b), open and adjust the back pressure valve (7) to the maximum opening; open the methane gas source (4), and inject methane gas into the pressure volume controller (5) and the pressure chamber through pipelines respectively; when the pressure in the pressure volume controller (5) rises to 0.5 MPa, close the methane gas source (4), adjust the back pressure valve (7) to the minimum opening, and close the third valve (11c); adjust the pore pressure of the pressure volume controller (5) until the pore pressure of the pressure volume controller (5) reaches 12 Mpa; adjust the pressure volume controller (5) to enter the constant pressure mode, and introduce methane gas into the pressure chamber by using the pressure volume controller (5); use the water bath device (6) to adjust the temperature in the pressure chamber to rise to 15 °C to melt the ice in the specimen (27); when the volume of methane gas in the pressure volume controller (5) no longer decreases, it is considered that the specimen (27) reaches complete gas saturation, and proceed to S6 to generate hydrates in-situ; S6: Use the water bath device (6) to adjust the temperature in the pressure chamber to 1 °C so that the temperature of the specimen (27) is lower than the hydrate phase equilibrium temperature; when the volume of methane gas in the pressure volume controller (5) no longer changes significantly, it indicates that the water in the pores of the specimen (27) has completely reacted with methane gas to form methane hydrate; calculate the saturation of methane hydrate according to the volume change of methane gas in the pressure volume controller (5) when it is stable in S5 and S6; S7: According to the actual working conditions, determine the control mode of the pressure volume controller (5), and use the torsion and axial loading host (2) to select stress control and strain control at different loading rates to shear the specimen (27) so that a shear band is formed at the bottom cross-section of the auxiliary inner ring (25) and the auxiliary outer ring (26); the stress control is a control method for controlling the stress increment change rate, and the strain control is a control method for controlling the displacement change rate; during the test, collect and record the readings of the torsion angle sensor, stress sensor, torque sensor, displacement sensor and pore pressure sensor through the data acquisition system (10), and analyze the mechanical properties of large displacement shear of hydrates according to the collected data and the calibration parameters obtained in S2.

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