Device and method for preparing hypotonic hydrate sample and testing mechanical properties

Abstract

The invention provides a low-permeability hydrate sample preparation and mechanical property testing device and method. The device comprises a base; the two top caps are arranged above the base at an interval; the two ends of the latex film are arranged on the two top caps in a sleeving mode respectively, and a sample preparation cavity can be defined by the two top caps and the latex film; the hollow pipe penetrates through the base, the two top caps and the sample, and the hollow pipe and the prepared cylindrical sample are coaxially arranged. According to the invention, through the hollow pipe coaxially inserted in the sample preparation cavity, water and / or gas are introduced into and discharged from different height positions of the cylindrical sample in the sample preparation cavity, especially water and / or gas can be introduced into different height positions of the cylindrical sample, so that water and / or gas can permeate into the cylindrical sample from different height positions; the permeation efficiency is greatly improved, the preparation efficiency of the low-permeability hydrate sample is further improved, and meanwhile, the distribution uniformity of the low-permeability hydrate sample is further improved by permeating into the cylindrical sample from the high position.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrate sediment sample preparation, and in particular to a device and method for preparing a low-permeability hydrate sample and testing its mechanical properties. Background Art

[0002] Natural gas hydrate is an ice-like cage formed by water and gas (mainly methane) under high pressure and low temperature conditions. It is mainly distributed in deep sea and permafrost areas. Due to its huge reserves, it is considered a potential energy source. However, the exploitation of natural gas hydrate faces many problems. The uncontrollable decomposition of hydrates during the exploitation process will lead to a decrease in the shear strength of hydrate sediments, which may lead to submarine landslides. Therefore, in-depth research on the engineering mechanical properties of hydrate-containing soils is a prerequisite for the safe exploitation of natural gas hydrates.

[0003] The in-situ test method is expensive and difficult to test mechanical properties. The current internationally accepted method is to conduct research by synthesizing hydrates indoors. The most commonly used equipment is the high-pressure and low-temperature triaxial instrument. This type of equipment has been used to conduct a lot of research on the mechanical properties of sandy hydrates at home and abroad, and many valuable results have been achieved. However, more than 90% of the total amount of hydrates in the world is stored in clay silt or silt sediments on the seabed. It is more valuable to study the mechanical properties of muddy silt hydrates. However, the biggest difficulty faced in the mechanical property test of muddy silt hydrates is the preparation of hydrate samples. Low permeability is one of the main differences between muddy silt hydrates and sandy hydrates. It is very difficult to prepare muddy silt high-pressure and low-temperature triaxial samples using traditional hydrate preparation methods such as saturated water and saturated gas. The efficiency of methane gas ventilation preparation during the traditional saturated water and saturated gas sample preparation is low, and the saturation of the prepared samples is not high. It is easy to cause hydrates to form at the inlet end, while no hydrates are generated at the far end. This is mainly due to the low permeability of muddy silt hydrate and the large aspect ratio of the existing high-pressure and low-temperature triaxial apparatus. Tetrahydrofuran (THF) hydrate has also been used to study the mechanical properties of hydrates. Since THF is a water-soluble liquid, it is easier to prepare hydrates with it, and the mechanical properties of the prepared hydrates are consistent with the in-situ hydrate properties. However, because THF is a liquid, it is impossible to conduct mechanical property research on hydrate decomposition. The high-pressure and low-temperature hydrate triaxial apparatus is currently a relatively mature hydrate preparation and testing equipment. In order to improve its effectiveness in the preparation and testing of low-permeability hydrates, it is very necessary to improve the triaxial equipment or develop new test equipment, and establish effective preparation methods and testing processes. Summary of the invention

[0004] In view of this, the present invention proposes a low-permeability hydrate sample preparation and mechanical property testing device and method, aiming to solve the problems of low efficiency in the existing traditional high-pressure and low-temperature triaxial preparation of low-permeability hydrate samples such as muddy silt and uneven distribution of the generated hydrates.

[0005] On the one hand, the present invention proposes a hypotonic hydrate sample preparation and mechanical properties testing device, which includes: a base; two top caps, which are arranged above the base at intervals; a latex membrane, wherein the two ends of the latex membrane are respectively mounted on the two top caps, and the two ends of the latex membrane are respectively connected to the two top caps in a detachable manner, and a sample preparation cavity can be formed between the two top caps and the latex membrane for preparing a cylindrical sample in the sample preparation cavity; a hollow tube, which is passed through the base, the two top caps, and the cylindrical sample, and the hollow tube is coaxially arranged with the cylindrical sample so that the hollow tube is inserted at the axial position of the cylindrical sample; the two ends of the hollow tube are extended on both sides of the heat-insulating and pressure-insulating cover and the base, and are used to introduce and discharge gas and / or water into and out of the sample preparation cavity at different heights.

[0006] Furthermore, in the above-mentioned hypotonic hydrate sample preparation and mechanical property testing device, the hollow tube includes: two through-flow sections and a solid isolation section arranged between the two through-flow sections.

[0007] Furthermore, in the above-mentioned hypo-permeability hydrate sample preparation and mechanical property testing device, a metal permeable stone is provided on the wall surface of the top cap facing the sample preparation cavity, and an insulating plate is also provided between the metal permeable stone and the wall surface of the top cap facing the sample preparation cavity.

[0008] Furthermore, in the above-mentioned hypotonic hydrate sample preparation and mechanical property testing device, one of the top caps is provided with an exhaust pipe and a drain pipe, and the other top cap is provided with an exhaust pipe and a water outlet pipe.

[0009] Furthermore, the above-mentioned hypotonic hydrate sample preparation and mechanical property testing device also includes: a three-petal mold, the length of the three-petal mold is higher than 1 / 4 of the preset sample height, and the three-petal mold is detachably arranged on the periphery of the latex membrane, and is used to play a supporting role when the sample is compacted to the preset sample height at the initial stage of sample installation, and at the same time, the latex membrane is close to the inner wall of the three-petal mold.

[0010] Furthermore, the above-mentioned hypotonic hydrate sample preparation and mechanical property testing device also includes: a heat-insulating and pressure-insulating cover, which is arranged on the outside of the base, the top cap and the latex film, and the heat-insulating and pressure-insulating cover is filled with silicone oil.

[0011] On the other hand, the present invention proposes a method for preparing hypotonic hydrate samples and testing their mechanical properties. The method uses the above-mentioned hypotonic hydrate sample preparation and mechanical properties testing device to prepare the sample and test its mechanical properties. The sample is arranged on the central axis of the sample through a hollow tube, and the hollow tube is used as an air inlet pipe or a water pipe during the sample preparation process to pass water or gas to different height positions of the sample.

[0012] Furthermore, the above-mentioned method for preparing a hypo-permeability hydrate sample and testing its mechanical properties adopts a hollow tube triaxial method. During sample preparation, air is ventilated into a soil sample placed in a sample preparation cavity through a hollow tube.

[0013] Furthermore, the method for preparing a hypopermeability hydrate sample and testing its mechanical properties comprises the following steps: Step S221, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample. sd , the volume of water V w1 Step S222, according to the standard method of geotechnical testing, according to the mass m of dry soil required in the hydrate sediment to be prepared sd , the volume of water V w1 , mix and stir the corresponding dry soil and water to prepare a soil sample, and put the prepared soil sample into a fresh-keeping bag and let it stand for a preset soil sample standing time period; step S223, after lubricating the outer wall of the hollow tube, insert the lower top cap on the base, put the latex film on the lower top cap and install the three-petal mold, tighten the clamp, and adjust the latex film to fit the inner wall of the three-petal mold; step S224, put the soil sample in the fresh-keeping bag into the three-petal mold, install the upper top cap on the top, and make the upper top cap close to the soil sample Tightly fit, adjust the axial actuator to displacement control to move the upper cap downward to a preset height. After stabilization, remove the three-petal mold and put the latex film on the upper cap and tighten it, cover it with a pressure-keeping and heat-insulating cover, and then introduce silicone oil; step S225, according to the preset temperature and pressure conditions, apply confining pressure to the hydrate sample and cool it down, and stabilize it for a preset temperature and pressure time period after reaching the preset temperature and pressure conditions; step S226, use the exhaust pipe on the device to introduce methane gas into the sample, and measure and record the amount of gas introduced V gin And the exhaust volume V gout , according to the gas volume V gin And the exhaust volume V gout The methane hydrate saturation in the sample is calculated until a preset saturation is reached, and the introduction of methane gas is stopped; step S227, after the methane hydrate saturation in the sample reaches a preset saturation and stabilizes for a preset time period, a triaxial shear test is performed.

[0014] Furthermore, in the above-mentioned method for preparing and testing the mechanical properties of hypotonic hydrate samples, when conducting the mechanical properties test of the samples, the interior of the hollow tube is pressurized, and the internal air pressure is adapted to the sample confining pressure, so that when the mechanical properties of the samples are tested, the stress state of the samples is equivalent to the stress state of the triaxial samples.

[0015] Furthermore, the above-mentioned method for preparing low-permeability hydrate samples and testing mechanical properties adopts a temperature-pressure phase equilibrium critical method when preparing samples when the hollow tube is removed. The hydrate sample is prepared according to the temperature-pressure phase equilibrium relationship. The hydrate sample is at the critical point of decomposition and synthesis. The sample is ventilated, and the structural morphology of the sediment and the hydrate is controlled by controlling the phase equilibrium, so that the hydrate and the sediment in the sample are converted from the original discrete state to the cemented state.

[0016] Furthermore, the method for preparing a hypopermeability hydrate sample and testing its mechanical properties comprises the following steps: Step S211, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample. sd , the volume of water V w1 and the volume of methane V m1 ; Step S212, the volume V m1 Pure methane hydrate was prepared by mixing methane gas and water, and the volume of water consumed in the preparation process was recorded. w2 ; and the mass is m sd The dry soil is placed in a preset freezing environment and frozen for a preset freezing period of time to obtain freeze-dried soil; step S213, under low temperature conditions, the prepared pure methane hydrate and the mass m i The ice is beaten into powder and mixed evenly with freeze-dried soil to obtain a mixed soil sample; step S214, a latex film is put on the lower top cap and the three-petal mold is installed. After tightening the clamp, the latex film is adjusted to be close to the inner wall of the three-petal membrane; step S215, the mixed soil sample after mixing is loaded into the three-petal mold by the sand rain method for preparation. After preparation, the upper top cap located above is installed, and the upper top cap is tightly fitted with the soil sample, the latex film is put on the upper top cap and the three-petal mold is removed, and the silicone oil is introduced after the pressure-keeping and heat-insulating cover is covered; step S216, according to the preset temperature and pressure conditions, confining pressure is applied to the hydrate sample and the temperature is reduced, and the preset temperature and pressure time period is stabilized after reaching the preset temperature and preset pressure; step S217, the preset temperature is maintained unchanged, the confining pressure is reduced according to the temperature-pressure phase equilibrium relationship of methane hydrate, so that the hydrate is at the decomposition critical point, and the gas flowmeter at the exhaust pipe on the device is observed. When gas is discharged, the exhaust pipe is closed, and the exhaust gas volume V is measured and recorded. hout1 , and pass a volume of V hout1methane gas to keep the methane content in the sample unchanged, increase the confining pressure to a preset pressure and then stabilize it for a period of time; step S218, repeatedly perform the process of reducing the confining pressure, venting and increasing the pressure multiple times, so that the hydrate and soil sample in the sample are converted from the initial discrete state to the cemented state of the original muddy hydrate; step S219, adjust the temperature and confining pressure to the preset conditions required for the test, and carry out the triaxial shear test after stabilizing the preset temperature and pressure time period.

[0017] The low-permeability hydrate sample preparation and mechanical property testing device and method provided by the present invention, through a hollow tube coaxially inserted in the sample preparation cavity, introduces and discharges water and / or gas at different height positions of the sample in the sample preparation cavity, especially can introduce water and / or gas at different height positions of the sample, can make water and / or gas penetrate into the sample from different height positions, compared with the introduction and discharge at both ends, the introduction of water or gas at different height positions allows it to penetrate to different height positions of the sample, and can also continue to penetrate from different height positions, thereby increasing its further penetration efficiency in the sample, thereby improving the efficiency of low-permeability hydrate sample preparation, at the same time, introducing water or gas at different height positions also increases its distribution uniformity, can effectively improve the saturation of the hydrate sample, and improve the uniformity of hydrate distribution in the sediment, solving the problems of low efficiency of sample preparation of low-permeability hydrate samples such as muddy silt sand and uneven distribution of generated hydrates in the existing traditional high-pressure and low-temperature triaxial method. The stress state of the hydrate sample during shear test using this device is closer to that of the triaxial sample during shear test. In addition, the repeated pressure drop and rise sampling method based on the proposed temperature-pressure phase equilibrium critical method can construct the cementation relationship between hydrate and sediment. It is more practical to use this device to conduct strength and stress-strain tests on hydrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0019] Figure 1 A schematic diagram of the structure of a hypotonic hydrate sample preparation and mechanical property testing device provided in an embodiment of the present invention;

[0020] Figure 2 A schematic diagram of the internal structure of a hypotonic hydrate sample preparation and mechanical property testing device provided in an embodiment of the present invention;

[0021] Figure 3 A schematic diagram of the structure of a three-petal mold provided by an embodiment of the present invention;

[0022] Figure 4Another structural schematic diagram of a three-petal mold provided by an embodiment of the present invention;

[0023] Figure 5 A schematic diagram of the structure of a hollow tube provided in an embodiment of the present invention;

[0024] Figure 6 A schematic structural diagram of a hollow tube exhaust section provided in an embodiment of the present invention;

[0025] Figure 7 A flowchart of a method for preparing a hypotonic hydrate sample and testing mechanical properties using a temperature-pressure phase equilibrium critical method provided in an embodiment of the present invention;

[0026] Figure 8 A flowchart of a method for preparing a hypotonic hydrate sample and testing its mechanical properties using a hollow tube triaxial method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to be able to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0028] Device Example:

[0029] See also Figure 1 to Figure 2 , which shows the preferred structure of the hypotonic hydrate sample preparation and mechanical property testing device provided by the embodiment of the present invention. As shown in the figure, the device 100 includes: a base 1, two top caps 2, a latex film 3, a three-petal mold 4, a heat-insulating and pressure-insulating cover 5 and a hollow tube 6; wherein,

[0030] Two top caps 2 are arranged above the base 1 at intervals. Specifically, the two top caps 2 can be a lower top cap 201 and an upper top cap 202, respectively. The lower top cap 201 is arranged on the base 1 and can be supported on the base 1. The lower top cap 201 is used to support the bottom of the cylindrical sample 7; the upper top cap 202 is arranged above the lower top cap 201 at intervals and can be supported on the top of the cylindrical sample 7; of course, the upper top cap 202 can also be locked on the top of the lower top cap 201 by a locking member, and can also be unlocked and supported on the top of the cylindrical sample 7 to achieve compaction of the cylindrical sample 7.

[0031] The two ends of the latex film 3 (such as Figure 2The upper and lower ends shown in the figure are respectively mounted on the two top caps 2, and the two ends of the latex membrane 3 are respectively connected to the two top caps 2 in a detachable manner, and a sample preparation cavity can be formed between the two top caps 2 and the latex membrane 3 for preparing the sample in the sample preparation cavity. Specifically, the latex membrane 3 can be arranged between the upper top cap 202 and the lower top cap 201, and the two ends of the latex membrane 3 are respectively mounted on the upper top cap 202 and the lower top cap 201, and the two ends of the latex membrane 3 are respectively connected to the upper top cap 202 and the lower top cap 201 in a detachable manner, and a sample preparation cavity can be formed between the upper top cap 202, the lower top cap 201 and the latex membrane 3 for preparing and compacting the sample.

[0032] The three-flap mold 4 is detachably arranged on the periphery of the latex film 3, and is used to support the cylindrical sample 7 when it is compacted to a preset sample height at the initial stage of installation, and at the same time, the latex film 3 is closely attached to the inner wall of the three-flap mold 4. Specifically, the structure of the three-flap mold 4 can be referred to Figure 3 and Figure 4 During the compaction of the cylindrical sample 7, a three-petal mold 4 can be installed on the lower top cap 201. The length of the three-petal mold 4 should be higher than 1 / 4 of the height of the cylindrical sample 7. The installation sequence is: first put the latex film 3 on the lower top cap 201, and then install the three-petal mold 4. The clamp can be tightened, and the latex film 3 can be pressed against the inner wall of the three-petal mold 4. The upper end of the latex film 3 can be pulled up and turned down onto the three-petal mold 4 to keep the latex film 3 pressed against the inner wall of the three-petal mold 4. Then, the cylindrical sample 7 can be compacted to a specified height, i.e., a preset height, by moving the axial actuator downward, and then the three-petal mold 4 can be removed. Negative pressure suction can also be used to make the latex film 3 pressed against the inner wall of the three-petal membrane 4, so as to prevent the latex film 3 from being damaged during the sample preparation process or causing the cylindrical sample 7 to generate additional stress that affects the experimental results.

[0033] The heat-insulating and pressure-maintaining cover 5 is arranged outside the base 1, the top cap 2 and the latex film 3, and the heat-insulating and pressure-maintaining cover 5 is filled with silicone oil 51. Specifically, the bottom of the heat-insulating and pressure-maintaining cover 5 is supported on the base 1, and the base 1, the top cap 2 and the latex film 3 can be covered inside, forming a heat-insulating and pressure-maintaining cavity outside the base 1, the top cap 2 and the latex film 3, and the heat-insulating and pressure-maintaining cavity can be filled with silicone oil to achieve heat preservation and pressure maintenance, so as to test the mechanical properties of the sample.

[0034] The hollow tube 6 is detachably arranged through the base 1, the two top caps 2, and the cylindrical sample 7. The hollow tube 6 is coaxially arranged with the cylindrical sample 7 so that the hollow tube 6 is inserted in the axial position of the cylindrical sample 7; and the two ends of the hollow tube 6 are extended on both sides of the heat preservation and pressure preservation cover 5 and the base 1 (such as Figure 1The upper and lower sides shown in the figure are used to introduce and exhaust gas and / or water into and out of the sample preparation chamber at different heights. Specifically, the hollow tube 6 is coaxially arranged with the latex membrane 3, that is, a hollow tube 6 can be installed at the center axis position of the cylindrical sample 7. The hollow tube 6 can be a thin hollow metal tube or other materials, which is not limited in this embodiment. In this embodiment, the hollow tube 6 is detachably installed on the base 1, the two top caps 2, the latex membrane 3 and the heat-insulating and pressure-maintaining cover 5. For example, the base 1, the two top caps 2, the latex membrane 3 and the heat-insulating and pressure-maintaining cover 5 may be provided with a mounting hole at the axial position, and a detachable sealing plate may be provided at the mounting hole. When the hollow tube 6 is not installed, sealing is achieved by the detachable sealing plate. At the same time, the hollow tube 6 can be inserted into the mounting hole of the base 1, the two top caps 2, the latex membrane 3 and the heat-insulating and pressure-maintaining cover 5 to introduce and exhaust gas and / or Water can not only realize the introduction and discharge of gas and / or water, accelerate the drainage efficiency, but also make the gas and / or water evenly distributed in the sample, improve the saturation of the hydrate sample, and improve the uniformity of the hydrate distribution in the sediment, overcoming the problems of the traditional high-pressure and low-temperature hydrate triaxial equipment due to the low permeability of muddy silt sand hydrate, the low saturation of the prepared hydrate sample, the uneven distribution of hydrates and the long sample preparation time. At the same time, when the shear test of the hydrate sample is carried out, the stress state of the sample is closer to the state of the hydrate in the actual formation, and the use of the device to carry out the strength and stress strain test of the hydrate is more in line with reality. In this embodiment, before inserting the hollow tube 6, lubricants can be applied to both ends of the hollow tube 6 to reduce the friction generated by the compression of the sample during the shearing process. When the hollow tube and the sample are sufficiently smooth, the stress state of the hydrate sample during the shearing process is basically consistent with the stress state of the conventional triaxial sample. Of course, water can also be passed through the hollow tube 6, and water can be added and drained from the hydrate.

[0035] In this embodiment, the low-permeability sample may be a muddy silt sand type, a silt soil type, a clay hydrate, or may be other low-permeability samples.

[0036] In this embodiment, a metal permeable stone 21 is provided on the wall of the top cap 2 facing the sample preparation chamber, that is, on the bottom wall of the upper top cap 202 facing the sample preparation chamber, and an insulating plate 22 is also provided between the metal permeable stone 21 and the bottom wall of the upper top cap 202; and / or, a metal permeable stone 21 is provided on the top wall of the lower top cap 201, and an insulating plate 22 is also provided between the metal permeable stone 21 and the top wall of the lower top cap 201. Specifically, the top cap 2, the insulating plate 22, and the metal permeable stone 21 can form an integral structure by bonding or other means, so that the upper top cap 202, the insulating plate 22, and the metal permeable stone 21 can be installed as a whole. Among them, the metal permeable stone 21 can improve thermal conductivity, and the insulating plate 22 can isolate static electricity to prevent electroosmosis.

[0037] In this embodiment, one of the top caps 2 is provided with an air inlet pipe 23 and a water inlet pipe 24, and the other top cap 2 is provided with an exhaust pipe 25 and a drain pipe 26. Figure 1 and Figure 2 As shown, in this embodiment, the air inlet pipe 23 and the water inlet pipe 24 are arranged on the lower top cap 201, and the exhaust pipe 25 and the drain pipe 26 are arranged on the upper top cap 202. Of course, the pipe arranged on the upper top cap 202 can also be used as a water ventilation pipe, and the pipe arranged on the lower top cap 201 can also be used as a drainage and exhaust pipe, that is, it can be in from the bottom and out from the top or in from the top and out from the bottom, and this embodiment does not make any restrictions on it. One end of the air inlet pipe 23, the water inlet pipe 24, the exhaust pipe 25 and the drain pipe 26 all extend into the sample preparation chamber to vent and exhaust water into the sample.

[0038] See also Figure 5 , which is a schematic diagram of the structure of the hollow tube provided in an embodiment of the present invention. As shown in the figure, the hollow tube 6 includes: two drain sections 61, and a solid isolation section 62 arranged between the two drain sections 61. Specifically, the hollow tube 6 can be divided into three sections, for example, the upper 1 / 3 and the lower 1 / 3 are both drain sections 61, and the middle part is blocked and not ventilated. The middle part can be an insulating section or an isolation section to avoid the connection between the upper and lower sections, that is, the upper and lower ends are not connected, so that the gas can be discharged from the pores of the drain section 61 into the sample, or enter the drain section 61 from the sample. In this embodiment, the length of the two drain sections 61 is longer, and the solid isolation section 62 is shorter. As Figure 6 As shown, the exhaust section 61 is provided with air holes 611 to allow air or water to enter or exit.

[0039] In summary, the low-permeability hydrate sample preparation and mechanical property testing device provided in this embodiment, through a hollow tube coaxially inserted in the sample preparation cavity, introduces and discharges water and / or gas at different heights of the sample in the sample preparation cavity. In particular, water and / or gas can be introduced into different heights of the sample, so that water and / or gas can penetrate into the sample from different heights. Compared with the introduction and discharge at both ends, the water or gas at different heights can penetrate into the sample at different heights, and can continue to penetrate from different heights, thereby increasing its further penetration efficiency in the sample, thereby improving the efficiency of low-permeability hydrate sample preparation. At the same time, the introduction of water or gas at different heights also increases its distribution uniformity, which can effectively improve the saturation of the hydrate sample and improve the uniformity of hydrate distribution in the sediment, solving the problems of low efficiency of sample preparation of low-permeability hydrate samples such as muddy silt sand and uneven distribution of generated hydrates in the existing traditional high-pressure and low-temperature triaxial method. The stress state of the hydrate sample during shear test using this device is closer to that of the triaxial sample during shear test. In addition, the repeated pressure drop and rise sampling method based on the proposed temperature-pressure phase equilibrium critical method can construct the cementation relationship between hydrate and sediment. It is more practical to use this device to conduct strength and stress-strain tests on hydrate.

[0040] Method Example:

[0041] In this embodiment, the present invention also proposes a method for preparing a hypotonic hydrate sample and testing its mechanical properties. The hypotonic hydrate sample preparation and mechanical properties testing method 200 uses the above-mentioned device 100 to prepare the sample and test its mechanical properties. The hollow tube 6 is arranged on the central axis of the cylindrical sample 7, and the hollow tube 6 is used as an air inlet pipe or a water pipe during the sample preparation process to pass water or gas to different height positions of the sample.

[0042] In this embodiment, the method 200 can have two implementation modes, namely, the two methods are respectively a temperature-pressure phase equilibrium critical method 210 for preparing hypotonic hydrate samples and testing mechanical properties, and a hollow tube triaxial method 220 for preparing hypotonic hydrate samples and testing mechanical properties.

[0043] In one implementation of the present embodiment, the method 200 adopts the temperature-pressure phase equilibrium critical method 210, and prepares the hydrate sample according to the temperature-pressure phase equilibrium relationship. The hydrate sample is at the critical point of decomposition and synthesis, and the sample is ventilated. By controlling the phase equilibrium, the structural morphology of the sediment and the hydrate is controlled, so that the hydrate and the sediment in the sample are converted from the original discrete state to the cemented state. Specifically, the temperature-pressure phase equilibrium critical method 210 mainly makes the hydrate in the sample at the critical point of decomposition and synthesis according to the temperature-pressure phase equilibrium relationship, so that the hydrate and the sediment in the sample are converted from the original discrete state, that is, the uncemented state, to the cemented state. Since the solid hydrate powder and the sediment are mixed before the sample is prepared, the structural morphology of the sediment and the hydrate is controlled by controlling the phase equilibrium in the later stage, so that it is closer to the original characteristics.

[0044] See also Figure 7 , which is a flowchart of a method for preparing a hypotonic hydrate sample and testing mechanical properties using a temperature-pressure phase equilibrium critical method provided by an embodiment of the present invention. As shown in the figure, the method for preparing a hypotonic hydrate sample and testing mechanical properties using a temperature-pressure phase equilibrium critical method 210 includes the following steps:

[0045] Step S211, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample sd , the volume of water V w1 and the volume of methane V m1 .

[0046] Specifically, the mass m of dry soil required in the hydrate sediment to be prepared is calculated according to the required saturation of the sample and the percentage of particle component content. sd , the volume of water V w1 and the volume of methane V m1 .

[0047] Step S212: Volume V m1 Pure methane hydrate was prepared by mixing methane gas and water, and the volume of water consumed in the preparation process was recorded. w2 ; and the mass is m sd The dry soil is placed in a preset freezing environment and frozen for a preset freezing period to obtain freeze-dried soil.

[0048] Specifically, with volume V m1 Pure methane hydrate can be prepared by mixing methane gas and water. The preparation method can be a hydrate reactor, or by spraying water mist into a hydrate reactor with a volume of V under low temperature and high pressure conditions. m1 The prepared hydrate mass m h2 , record the volume of water V consumed in the preparation w2 and the volume of methane V m2 , V m2=V m1 . It is also possible to prepare a mass of m with water at the same time. i The frozen soil is placed in an environment below -20°C and frozen for a preset freezing period to obtain frozen dried soil. The preset freezing period can be 30 minutes.

[0049] Step S213, under low temperature conditions, the prepared pure methane hydrate and the mass m i The ice was ground into powder and mixed evenly with freeze-dried soil to obtain a mixed soil sample.

[0050] Specifically, the prepared hydrate and ice are respectively powdered under low temperature conditions, and quickly mixed with freeze-dried soil to obtain a mixed soil sample. That is, the volume corresponding to the sum of the weights of the water used to prepare pure methane hydrate in step S212 and the ice used in step S213 is equal to the volume of water required in the hydrate sediment to be prepared calculated in step S211. That is, the volume of ice required to be added when preparing the mixed soil sample is calculated based on the volume of water required in the hydrate sediment to be prepared calculated in step S211 and the weight of water used to prepare pure methane hydrate in step S212. That is, the weight of the ice prepared above can be calculated based on the volume of water required in the hydrate sediment to be prepared calculated in step S211 and the weight of water used to prepare pure methane hydrate in step S212.

[0051] Step S214, put the latex film on the lower top cap and install the three-petal mold, tighten the clamp, and adjust the latex film to fit tightly against the inner wall of the three-petal membrane.

[0052] Specifically, the latex film 3 is put on the lower top cap 201, and then the three-valve mold 4 is installed. After tightening the clamp, the latex film 3 is pulled up and the latex film 3 exposed at the upper end is turned outward to the outer wall of the three-valve membrane 4. It is necessary to ensure that the inner latex film 3 is in close contact with the inner wall of the three-valve membrane 4. Negative pressure suction can also be used to ensure that the latex film 3 is in close contact with the inner wall of the three-valve membrane 4 to prevent the latex film from being damaged during the sample preparation process or causing additional stress on the sample that affects the experimental results.

[0053] Step S215, the mixed soil sample is loaded into the three-petal mold by the sand-rain method. After preparation, the upper cap is installed and made to fit tightly with the soil sample. The latex film is put on the upper cap and the three-petal mold is removed. After covering with a pressure-keeping and heat-insulating cover, silicone oil is introduced.

[0054] Specifically, first, the mixed soil sample is loaded into the three-petal mold by the sand rain method. The preparation process can refer to the geotechnical test standard. After the preparation is completed, the upper cap 202 is quickly installed to make the bottom wall of the upper cap 202 fit tightly with the top wall of the cylindrical sample 7, that is, contact. After the latex film 3 is put on the upper cap 202, the three-petal mold 4 is removed. Note that before preparation, a small amount of methane gas is introduced into the air inlet pipe 23, the exhaust pipe 25 and the drain pipe 26, and a small amount of water is introduced into the water inlet pipe 24 to remove the impurity gas in each circuit. Finally, after covering the pressure and heat preservation cover 5, silicone oil 51 is introduced, and the air inlet pipe 23, the water inlet pipe 24, the exhaust pipe 25 and the drain pipe 26 are closed. Of course, in this embodiment, the hollow tube 6 can also be installed, that is, the outer wall of the hollow tube is first lubricated, and then the lower cap on the base is inserted, and the latex film is put on the lower cap and the three-petal mold is installed. After adjusting the latex film to be close to the inner wall of the three-petal membrane, the mixed soil sample after mixing is loaded into the three-petal mold by the sand rain method for preparation, and the upper cap located above is installed, so that the upper end of the hollow tube 6 is inserted into the upper cap 202. Among them, whether to install the hollow tube 6 can be determined according to actual conditions, and the installation of the hollow tube 6 can further improve the efficiency and uniformity of sample preparation.

[0055] Step S216, according to the preset temperature and pressure conditions, applying confining pressure to the hydrate sample and cooling it down, and stabilizing it for a preset temperature and pressure time period after reaching the preset temperature and preset pressure.

[0056] Specifically, according to the preset temperature and pressure conditions, a confining pressure is applied to the hydrate sample and the temperature is lowered, and after reaching the preset temperature and the preset pressure, the sample is stabilized for a preset temperature and pressure time period, wherein the preset temperature and pressure time period may be 30 minutes.

[0057] Step S217, maintaining the preset temperature unchanged, reducing the confining pressure according to the temperature-pressure phase equilibrium relationship of methane hydrate, so that the hydrate is at the decomposition critical point, observing the gas flow meter at the exhaust pipe on the device, when gas is discharged, closing the exhaust pipe, and measuring and recording the exhaust gas volume V hout1 , and pass a volume of V hout1 methane gas to keep the methane content in the sample unchanged, increase the confining pressure to the preset pressure and then stabilize it for the preset temperature and pressure time period.

[0058] Specifically, the temperature is maintained constant, and the confining pressure is reduced according to the temperature-pressure phase equilibrium relationship of methane hydrate, so that the hydrate is at the decomposition critical point, the exhaust pipe 25 and the drain pipe 26 are opened, and the gas flow meter is observed. When a small amount of gas is discharged, the exhaust pipe 25 and the drain pipe 26 are closed, and the discharged gas volume V is measured and recorded. hout1 , open the lower air inlet pipe 23 and let in a volume of V hout1methane gas to keep the methane content in the sample unchanged, and then increase the confining pressure to a preset pressure and stabilize the preset temperature and pressure time period. Of course, when opening the lower air inlet pipe 23, the hollow tube can also be opened to pass the gas. Among them, the preset temperature and pressure time period can be determined according to actual conditions, and there is no limitation on it in this embodiment.

[0059] Step S218, repeatedly performing the process of reducing the confining pressure, venting and increasing the pressure multiple times, so that the hydrate and soil sample in the sample are transformed from the initial discrete state to the cemented state of the original muddy hydrate.

[0060] Specifically, step S217 is repeated multiple times, at least 4 times, so that the hydrate and soil sample in the sample can be transformed from an initial discrete state to a cemented state of the original muddy hydrate.

[0061] Step S219, adjusting the temperature and confining pressure to the preset conditions required for the test, and conducting a triaxial shear test after stabilizing the preset temperature and pressure time period.

[0062] Specifically, the confining pressure is maintained unchanged, the temperature is lowered to the temperature conditions required for the test, and after the preset temperature period is stabilized, the triaxial shear test is carried out. During the test, the opening and closing of the exhaust pipe 25 and the drain pipe 26 can be adjusted according to the test plan; after the shearing is completed, the recorded stress and strain are processed according to the conventional triaxial test data processing method. The preset temperature period can be 30 minutes.

[0063] In another implementation of the present embodiment, method 200 adopts a hollow tube triaxial method 220. When preparing the sample, air is ventilated into the soil sample placed in the sample preparation cavity through the hollow tube. Specifically, the hollow tube triaxial method 220 mainly improves the sample preparation efficiency by adding a hollow tube as an air inlet pipe on the center axis of the sample, and overcomes the problem of low sample preparation efficiency of the traditional combustible ice triaxial method due to the large aspect ratio. When the diameter of the hollow tube is small and smooth enough, as long as the internal pressure is controlled to be consistent with the confining pressure, its stress state during the experiment can be approximated to the stress state of the equivalent triaxial sample. These two methods can also be applied simultaneously, and the sample preparation efficiency will be higher, that is, in the temperature-pressure phase equilibrium critical method 210, a hollow tube is combined to ventilate different positions of the center axis of the sample.

[0064] See also Figure 8 , which is a flowchart of a method for preparing a hypotonic hydrate sample and testing mechanical properties using a hollow tube triaxial method provided by an embodiment of the present invention. As shown in the figure, the method for preparing a hypotonic hydrate sample and testing mechanical properties using a hollow tube triaxial method 220 includes the following steps:

[0065] Step S221, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample sd , the volume of water V w1.

[0066] Specifically, the mass m of dry soil required in the hydrate sediment to be prepared is calculated according to the required saturation of the sample and the percentage of particle component content. sd , the volume of water V w1 and the volume of methane V m1 .

[0067] Step S222, according to the geotechnical test standard method, according to the calculated mass m of dry soil required in the hydrate sediment to be prepared sd , the volume of water V w1 , the corresponding dry soil and water are mixed and stirred to prepare a soil sample, and the prepared soil sample is put into a fresh-keeping bag and left to stand for more than a preset soil sample standing time period.

[0068] Specifically, the dry soil calculated in step 1 is mixed with water according to the method recommended by the geotechnical test standard, and then placed in a fresh-keeping bag and left to stand for more than a preset soil sample standing time period, wherein the preset soil sample standing time period may be 1 hour.

[0069] Step S223, after lubricating the outer wall of the hollow tube, insert the lower cap located on the base, put the latex film on the lower cap and install the three-petal mold, tighten the clamp, and adjust the latex film to fit tightly against the inner wall of the three-petal membrane.

[0070] Specifically, the hollow tube 6 is lubricated with vaseline and then inserted into the lower top cap 201. The latex film 3 is then put on the lower top cap 201. The three-valve mold 4 is then installed. After the clamp is tightened, the latex film 3 is pulled up and the latex film 3 exposed at the upper end is turned outward onto the outer wall of the three-valve membrane 4. It is necessary to ensure that the inner latex film 3 is in close contact with the inner wall of the three-valve membrane 4. Negative pressure suction can also be used to ensure that the latex film 3 is in close contact with the inner wall of the three-valve membrane 4 to prevent the latex film from being damaged during the sample preparation process or causing additional stress on the sample that affects the experimental results.

[0071] Step S224, put the soil sample in the fresh-keeping bag into the three-petal mold, install the upper cap on top, and make the upper cap fit tightly with the soil sample, adjust the axial actuator to displacement control to move the upper cap downward to the preset sample height, after stabilization, remove the three-petal mold and put the latex film on the upper cap and tighten it, cover it with a pressure-keeping and heat-insulating cover and then pass silicone oil.

[0072] Specifically, first, load the soil sample prepared in step S222 into the three-petal mold 4, install the upper cap 202, make the upper cap 202 fit tightly with the sample, and note that the upper end of the hollow tube 6 must be inserted into the upper cap 202, record the initial position L1 of the upper cap, and calculate the downward distance L2 of the upper cap according to the preset sample height Hs, wherein L2=L1-Hs; according to the downward distance L2 of the upper cap, adjust the axial actuator to displacement control to move the upper cap 202 downward a distance L2. After stabilization for 10 minutes, remove the three-petal mold 4 and put the latex film 3 on the upper cap 202 and tighten it. Then, a small amount of methane gas is introduced into the hollow tube 6, the air inlet pipe 23, the exhaust pipe 25 and the drain pipe 26, and a small amount of water is introduced into the water inlet pipe 24 to remove the impurity gases in each circuit. After the removal is completed, the water and gas injection circuit and the drainage and exhaust circuit are respectively connected to the base and the corresponding air inlet pipe 23, water inlet pipe 24, exhaust pipe 25 and drain pipe 26, and then covered with the pressure-maintaining and heat-insulating cover 5 and silicone oil is introduced.

[0073] Step S225, according to the preset temperature and pressure conditions, applying confining pressure to the hydrate sample and cooling it down, and stabilizing it for a preset temperature and pressure time period after reaching the preset temperature and pressure conditions.

[0074] Specifically, according to the temperature and pressure conditions required by the test, confining pressure is applied to the hydrate sample and the temperature is reduced. After the temperature and pressure conditions are reached, the preset temperature and pressure time period is stabilized; wherein the preset temperature and pressure time period may be 30 minutes.

[0075] Step S226, using the exhaust pipe on the device, introduce methane gas into the sample, and measure and record the amount of gas introduced V gin And the exhaust volume V gout , according to the amount of gas V gin And the exhaust volume V gout Calculate the methane hydrate saturation in the sample until the preset saturation is reached and stop introducing methane gas; or, measure and record the introduced gas volume V gin And the exhaust volume V gout , until the amount of gas introduced reaches V m1 , stop introducing methane gas so that the methane hydrate saturation in the sample reaches the preset saturation.

[0076] Specifically, the valves connected to the air inlet pipe 23, the water inlet pipe 24, the exhaust pipe 25, the drain pipe 26 and the two exhaust sections 61 of the hollow pipe 6 are opened, and methane gas is introduced into the air inlet pipe 23, the water inlet pipe 24 and the two exhaust sections 61 respectively, and the amount of gas introduced V is measured and recorded at the same time. gin and the gas volume V discharged from the exhaust pipe 25 and the drain pipe 26 gout During the experiment, the state of the exhaust section 61 can be adjusted according to the exhaust volume. For example, when the exhaust volume is very large, the exhaust section 61 can be closed. gin -Vgout The methane hydrate saturation in the sample can be calculated. When the required saturation is reached, the air inlet pipe 23, the water inlet pipe 24, the exhaust pipe 25 and the drain pipe 26 as well as the valves connected to the two drain sections 61 of the hollow pipe 6 are closed. In other words, the amount of gas introduced V is measured and recorded. gin And the exhaust volume V gout , until the amount of gas introduced reaches V m1 , stop introducing methane gas so that the methane hydrate saturation in the sample reaches the preset saturation.

[0077] Step S227, after the methane hydrate saturation in the sample reaches a preset saturation and stabilizes for a preset period of time, a triaxial shear test is performed.

[0078] Specifically, after stabilization for 4 hours, the triaxial shear test is started; and after the shearing is completed, the recorded stress and strain are processed according to the conventional triaxial test data processing method. In this embodiment, when the mechanical properties of the sample are tested, the inside of the hollow tube 6 is pressurized, and its internal air pressure is adapted to the sample confining pressure, so that when the mechanical properties of the sample are tested, the stress state of the sample is equivalent to the stress state of the triaxial sample, that is, when the diameter of the hollow tube is small and smooth enough, as long as the internal pressure is controlled to be consistent with the confining pressure, its stress state during the experiment can be approximately equivalent to the stress state of the triaxial sample.

[0079] In summary, the method for preparing low-permeability hydrate samples and testing mechanical properties provided in this embodiment uses the above-mentioned device to prepare low-permeability hydrate samples and test mechanical properties, and can perform mechanical property tests on low-permeability hydrates, and can be used for mechanical property tests on hydrate decomposition and slope stability evaluation research, overcoming the shortcomings of low efficiency of preparing low-permeability hydrate samples such as muddy silt sand and uneven distribution of generated hydrates by traditional high-pressure and low-temperature triaxial testing. The device and method can effectively improve the saturation of hydrate samples and improve the uniformity of hydrate distribution in sediments. The device makes the stress state of the hydrate sample during the shear test closer to the state of the hydrate in the actual formation, and the method using the device to perform strength and stress-strain tests on hydrates is more practical.

[0080] It should be noted that, in the description of the present invention, terms such as "up", "down", "left", "right", "inside" and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings. This is merely for the convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation on the present invention.

[0081] In addition, it should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0082] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A device for preparing hypotonic hydrate samples and testing mechanical properties, characterized in that: include: Pedestal; Two top caps are spaced apart and arranged above the base; A latex film, wherein both ends of the latex film are respectively sleeved on the two top caps, and a sample preparation cavity can be formed between the two top caps and the latex film, for preparing a cylindrical sample in the sample preparation cavity; A hollow tube is passed through the base, the two top caps, and the cylindrical sample. The hollow tube and the cylindrical sample are coaxially arranged so that the hollow tube is inserted at the axial position of the cylindrical sample. Both ends of the hollow tube extend on both sides of the heat-insulating and pressure-insulating cover and the base, and are used to introduce and exhaust gas and / or water into and out of the sample preparation chamber at different heights.

2. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 1, characterized in that: The hollow tube comprises: two through-flow sections and a solid isolation section arranged between the two through-flow sections.

3. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 1 or 2, characterized in that: A metal permeable stone is arranged on the wall surface of the top cap facing the sample preparation cavity, and an insulating plate is arranged between the metal permeable stone and the wall surface of the top cap facing the sample preparation cavity.

4. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 1 or 2, characterized in that: One of the top caps is provided with an exhaust pipe and a drain pipe, and the other top cap is provided with an air outlet pipe and a water outlet pipe.

5. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 1 or 2, characterized in that: Also includes: A three-lobed mold, the length of which is higher than 1 / 4 of the preset sample height, is detachably arranged on the periphery of the latex membrane, and is used to provide support when the sample is compacted to the preset sample height at the initial stage of sample installation, while keeping the latex membrane close to the inner wall of the three-lobed mold.

6. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 1 or 2, characterized in that: Also includes: A heat-insulating and pressure-insulating cover is arranged outside the base, the top cap and the latex film, and the heat-insulating and pressure-insulating cover is filled with silicone oil.

7. A method for preparing a hypotonic hydrate sample and testing its mechanical properties, characterized in that: The method adopts the hypotonic hydrate sample preparation and mechanical property testing device as described in any one of claims 1 to 6 to prepare the sample and test its mechanical properties, and the hollow tube is arranged on the central axis of the sample, and the hollow tube is used as an air inlet pipe or a water pipe during the sample preparation process to pass water or gas to different height positions of the sample.

8. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 6, characterized in that: The method adopts the hollow tube triaxial method. When preparing the sample, air is ventilated into the soil sample placed in the sample preparation cavity through the hollow tube.

9. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 8, characterized in that: The method comprises the following steps: Step S221, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample sd , the volume of water V w1 ; Step S222, according to the geotechnical test standard method, according to the calculated mass m of dry soil required in the hydrate sediment to be prepared sd , the volume of water V w1 , mixing and stirring the corresponding dry soil and water to prepare a soil sample, and putting the prepared soil sample into a fresh-keeping bag and letting it stand for more than a preset soil sample standing time period; Step S223, after lubricating the outer wall of the hollow tube, insert the lower cap on the base, put the latex film on the lower cap and install the three-petal mold, tighten the clamp, and adjust the latex film to fit the inner wall of the three-petal membrane; Step S224, put the soil sample in the fresh-keeping bag into the three-flap mold, install the upper cap located on the top, and make the upper cap fit tightly with the soil sample, adjust the axial actuator to displacement control to move the upper cap downward to the preset sample height, after stabilization, remove the three-flap mold and put the latex film on the upper cap and tighten it, cover it with a pressure-keeping and heat-insulating cover, and then pass silicone oil; Step S225, applying confining pressure to the hydrate sample and reducing the temperature according to the preset temperature and pressure conditions, and stabilizing the preset temperature and pressure time period after reaching the preset temperature and pressure conditions; Step S226, using the exhaust pipe on the device, introduce methane gas into the sample, and measure and record the amount of gas introduced V gin And the exhaust volume V gout , according to the amount of gas V gin And the exhaust volume V gout Calculate the methane hydrate saturation in the sample until the preset saturation is reached and stop introducing methane gas; Step S227, after the methane hydrate saturation in the sample reaches a preset saturation and stabilizes for a preset period of time, a triaxial shear test is performed.

10. The hypotonic hydrate sample preparation and mechanical property testing device according to claim 7, characterized in that: The method comprises the following steps: Step S211, calculating the mass m of dry soil required in the hydrate sediment to be prepared according to the preset saturation and the preset particle component content percentage required for the sample sd , the volume of water V w1 and the volume of methane V m1 ; Step S212: Volume V m1 Pure methane hydrate was prepared by mixing methane gas and water, and the volume of water consumed in the preparation process was recorded. w2 ; and the mass is m sd The dried soil is placed in a preset freezing environment and frozen for a preset freezing period to obtain freeze-dried soil; Step S213, under low temperature conditions, the prepared pure methane hydrate and the mass m i The ice is crushed into powder and mixed evenly with freeze-dried soil to obtain a mixed soil sample; Step S214, inserting the latex film on the lower top cap and installing the three-petal mold, tightening the clamp, and adjusting the latex film to be close to the inner wall of the three-petal membrane; Step S215, the mixed soil sample after mixing is loaded into the three-petal mold by using the sand-rain method for preparation, after preparation, the upper cap located at the top is installed, and the upper cap is closely fitted with the soil sample, the latex film is put on the upper cap and the three-petal mold is removed, and the pressure-keeping and heat-insulating cover is covered and silicone oil is introduced; Step S216, applying confining pressure to the hydrate sample and cooling it down according to preset temperature and pressure conditions, and stabilizing it for a preset temperature and pressure time period after reaching the preset temperature and preset pressure; Step S217, maintaining the preset temperature unchanged, reducing the confining pressure according to the temperature-pressure phase equilibrium relationship of methane hydrate, so that the hydrate is at the decomposition critical point, observing the gas flow meter at the exhaust pipe on the device, when gas is discharged, closing the exhaust pipe, and measuring and recording the exhaust gas volume V hout1 , and pass a volume of V hout1 methane gas to keep the methane content in the sample constant, increase the confining pressure to the preset pressure and stabilize it for a period of time; Step S218, repeatedly performing the process of reducing the confining pressure, venting, and increasing the pressure multiple times, so that the hydrate and the soil sample in the sample are transformed from the initial discrete state to the cemented state of the original muddy hydrate; Step S219, adjusting the temperature and confining pressure to the preset conditions required for the test, and conducting a triaxial shear test after stabilizing the preset temperature and pressure time period.

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