A device and method for preparing and testing the mechanical properties of a low-permeability hydrate sample

Abstract

The application provides a low-osmotic hydrate sample preparation and mechanical property testing device and method. The device comprises: a base; two top caps arranged above the base; two ends of a latex film are sleeved on the two top caps, and a sample preparation cavity can be formed between the two top caps and the latex film; and a hollow tube is arranged in the base, the two top caps and the sample, and the hollow tube is coaxially arranged with the prepared cylindrical sample. The hollow tube coaxially arranged in the sample preparation cavity introduces and discharges water and / or gas to different height positions of the cylindrical sample in the sample preparation cavity, especially water and / or gas can be introduced to different height positions of the cylindrical sample, so that water and / or gas can penetrate into the cylindrical sample from different height positions, greatly improving the penetration efficiency, and further improving the efficiency of low-osmotic hydrate sample preparation. Meanwhile, the penetration into the cylindrical sample at the height position also improves the uniformity of low-osmotic hydrate sample distribution.

Description

Technical Field

[0001] This invention relates to the field of hydrate sediment sample preparation technology, and more specifically, to a device and method for preparing low-permeability hydrate samples and testing their mechanical properties. Background Technology

[0002] Natural gas hydrates are ice-like cage-like substances formed by water and gas (mainly methane) under high pressure and low temperature conditions. They are mainly distributed in the deep sea and permafrost regions and are considered a potential energy source due to their huge reserves. However, the extraction of hydrates faces many problems. Uncontrollable decomposition of hydrates during the extraction process can lead to a decrease in the shear strength of hydrate sediments, which may trigger submarine landslides. Therefore, in-depth research on the engineering mechanical properties of hydrate-bearing soils is a prerequisite for the safe extraction of natural gas hydrates.

[0003] In-situ testing of mechanical properties is expensive and difficult. Currently, the internationally accepted method is to conduct research through indoor synthesis of hydrates. The most commonly used equipment is the high-pressure cryogenic triaxial apparatus, which has been widely used both domestically and internationally for studying the mechanical properties of sandy hydrates, yielding many valuable results. However, over 90% of the world's total hydrates are found in seafloor clayey silt or muddy sediments, making the study of the mechanical properties of silty mud hydrates even more valuable. The biggest challenge in testing the mechanical properties of silty mud hydrates is sample preparation. Low permeability is one of the main differences between silty mud hydrates and sandy hydrates. Preparing high-pressure cryogenic triaxial samples of silty mud hydrates using traditional saturated water and saturated gas methods is extremely difficult. The methane gas aeration process in traditional saturated water and saturated gas preparation methods is inefficient, resulting in low sample saturation and a tendency for hydrates to form at the inlet end while failing to form at the outlet. This is mainly due to the low permeability of silty clay hydrates and the large aspect ratio of existing high-pressure cryogenic triaxial apparatuses. Tetrahydrofuran (THF) hydrates have also been used to study the mechanical properties of hydrates. Since THF is a water-soluble liquid, it is relatively easy to prepare hydrates using it, and the mechanical properties of the prepared hydrates are consistent with those of in-situ hydrates. However, because THF is a liquid, it is also impossible to study the mechanical properties of hydrate decomposition. High-pressure cryogenic triaxial apparatuses are currently a relatively mature device for hydrate preparation and testing. To improve its effectiveness in the preparation and testing of low-permeability hydrates, it is essential to improve the triaxial apparatus or develop new experimental equipment, and to establish effective preparation methods and testing procedures. Summary of the Invention

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

[0005] On one hand, the present invention proposes a device for preparing low-permeability hydrate samples and testing their mechanical properties. The device includes: a base; two caps spaced apart above the base; a latex membrane, the two ends of which are respectively fitted onto the two caps, and the two ends of the latex membrane are detachably connected to the two caps, forming a sample preparation cavity between the two caps and the latex membrane for preparing cylindrical samples within the sample preparation cavity; a hollow tube passing through the base, the two caps, and the cylindrical sample, the hollow tube being coaxially arranged with the cylindrical sample so that it is inserted at the axial position of the cylindrical sample; the two ends of the hollow tube extending to both sides of the heat-insulating and pressure-maintaining cover and the base, for introducing and discharging gas and / or water to different height positions within the sample preparation cavity.

[0006] Furthermore, in the aforementioned apparatus for preparing low-permeability hydrate samples and testing their mechanical properties, the hollow tube comprises: two drainage sections and a solid isolation section disposed between the two drainage sections.

[0007] Furthermore, in the aforementioned apparatus for preparing and testing the mechanical properties of low-permeability hydrate samples, a permeable metal stone is provided on the wall surface of the top cap facing the sample preparation cavity, and an insulating plate is provided between the permeable metal stone and the wall surface of the top cap facing the sample preparation cavity.

[0008] Furthermore, in the aforementioned apparatus for preparing low-permeability hydrate samples and testing their mechanical properties, 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.

[0009] Furthermore, the aforementioned apparatus for preparing low-permeability hydrate samples and testing their mechanical properties further includes a three-lobed mold. The length of the three-lobed mold is greater than 1 / 4 of the preset sample height. The mold is detachably disposed on the outer periphery of the latex film. It serves to support the sample when it is compacted to the preset sample height during the initial sample installation, while simultaneously ensuring that the latex film adheres tightly to the inner wall of the three-lobed mold.

[0010] Furthermore, the aforementioned apparatus for preparing low-permeability hydrate samples and testing their mechanical properties further includes: a heat-insulating and pressure-insulating cover, which covers 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 low-permeability hydrate samples and testing their mechanical properties. The method uses the aforementioned low-permeability hydrate sample preparation and mechanical property testing device to prepare the sample and test its mechanical properties. A hollow tube is set on the central axis of the sample and serves as an air inlet or water inlet during the sample preparation process to pass water or gas to different height positions of the sample.

[0012] Furthermore, in the above-mentioned method for preparing low-permeability hydrate samples and testing their mechanical properties, the method employs the hollow tube triaxial method, in which air is introduced into the soil sample placed in the sample preparation cavity through the hollow tube during sample preparation.

[0013] Furthermore, the above-mentioned method for preparing low-permeability hydrate samples and testing their mechanical properties includes the following steps: Step S221, calculating the required mass m of dry soil in the hydrate sediment to be prepared based on the required preset saturation and preset particle component content percentage of the sample. sd The volume of water, V w1 Step S222: According to the standard method of geotechnical testing, calculate the required mass m of dry soil in the hydrate sediment to be prepared. sd The volume of water, V w1 In step S223, the corresponding dry soil and water are mixed and stirred to prepare a soil sample, and the prepared soil sample is placed in a plastic bag and left to stand for a preset soil sample standing time. In step S224, after lubricating the outer wall of the hollow tube, the lower cap located on the base is inserted, the latex film is put on the lower cap and the three-lobed mold is installed. After tightening the clamps, the latex film is adjusted to fit tightly against the inner wall of the three-lobed mold. In step S225, the soil sample in the plastic bag is put into the three-lobed mold, the upper cap located on top is installed, and the upper cap is aligned with the soil sample. After ensuring a tight fit, adjust the axial actuator to displacement control to move the top cap downwards to a preset height. Once stable, remove the three-lobed mold and secure the latex film over the top cap. After covering with a pressure-holding and heat-insulating cover, introduce silicone oil. In step S225, apply confining pressure to the hydrate sample and cool it down according to the preset temperature and pressure conditions. Once the preset temperature and pressure conditions are reached, stabilize for a preset temperature and pressure time period. In step S226, use the exhaust pipe on the device to introduce methane gas into the sample and record the amount of gas introduced, V. gin and the volume of gas discharged V gout According to the amount of gas introduced, V gin and the volume of gas discharged V gout Calculate the methane hydrate saturation in the sample until the preset saturation is reached, then stop the introduction of methane gas; in step S227, after the methane hydrate saturation in the sample reaches the preset saturation and stabilizes for a preset time period, conduct a triaxial shear test.

[0014] Furthermore, in the above-mentioned method for preparing and testing the mechanical properties of low-permeability hydrate samples, the hollow tube is pressurized during the mechanical property testing, and the internal air pressure is matched with the confining pressure of the sample so that the stress state of the sample during the mechanical property testing is equivalent to that of a triaxial sample.

[0015] Furthermore, in the above-mentioned method for preparing low-permeability hydrate samples and testing their mechanical properties, the method employs a critical temperature-pressure phase equilibrium method when preparing samples after removing the hollow tube. The hydrate sample is prepared based on 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 hydrate is controlled by controlling the phase equilibrium, so that the hydrate and sediment in the sample are transformed from the original discrete state to a cemented state.

[0016] Furthermore, the above-mentioned method for preparing low-permeability hydrate samples and testing their mechanical properties includes the following steps: Step S211, calculating the required mass m of dry soil in the hydrate sediment to be prepared based on the required preset saturation and preset particle component content percentage of the sample. sd The volume of water, V w1 and methane volume V m1 Step S212, change volume V m1 Pure methane hydrate was prepared from methane gas and water, and the volume of water consumed, V, was recorded during the preparation process. w2 ; and the mass is m sd The dry soil is placed in a preset freezing environment and frozen for a preset freezing time to obtain freeze-dried soil; in step S213, the prepared pure methane hydrate and a mass of m are respectively placed under low temperature conditions. i Ice is crushed into powder and mixed evenly with freeze-dried soil to obtain a mixed soil sample; Step S214: A latex film is placed on the lower top cap and the three-lobed mold is installed. After tightening the clamps, the latex film is adjusted to fit tightly against the inner wall of the three-lobed mold; Step S215: The mixed soil sample is prepared by filling the three-lobed mold using the sand rain method. After preparation, the upper top cap is installed and made to fit tightly against the soil sample. The latex film is placed on the upper top cap and the three-lobed mold is removed. After covering with the pressure-insulating and heat-insulating cover, silicone oil is introduced; Step S216: According to the preset temperature and pressure conditions, the confining pressure is applied to the hydrate sample and the temperature is lowered. After reaching the preset temperature and pressure, the temperature and pressure are stabilized for a preset time period; Step S217: The preset temperature is kept 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 gas flow meter at the exhaust pipe of the device is observed. When gas is discharged, the exhaust pipe is closed and the volume of discharged gas V is recorded. hout1 And introduce a volume of V hout1Methane gas is introduced to maintain a constant methane content in the sample. The confining pressure is increased to a preset pressure and then stabilized for a period of time. In step S218, the process of decreasing the confining pressure, venting, and increasing the pressure is repeated multiple times to change the hydrates and soil samples in the sample from the initial discrete state to the cemented state of the original clay-type hydrates. In step S219, the temperature and confining pressure are adjusted to the preset conditions required for the test. After stabilizing the preset temperature and pressure time period, the triaxial shear test is carried out.

[0017] The present invention provides a device and method for preparing low-permeability hydrate samples and testing their mechanical properties. Through a hollow tube coaxially inserted into the sample preparation chamber, water and / or gas are introduced and discharged at different heights of the sample within the chamber. Specifically, the ability to introduce water and / or gas at different heights allows them to permeate into the sample from these different heights. Compared to a two-end inlet / outlet method, introducing water or gas at different heights allows it to permeate to and continue permeating from these heights, increasing the further permeation efficiency within the sample and thus improving the efficiency of low-permeability hydrate sample preparation. Simultaneously, introducing water or gas at different heights also increases the uniformity of hydrate distribution, effectively improving the saturation of the hydrate sample and the uniformity of hydrate distribution in sediments. This solves the problems of low sample preparation efficiency and uneven hydrate distribution in existing traditional high-pressure, low-temperature triaxial preparation methods for low-permeability hydrate samples such as silt and mudstone. The stress state of hydrate samples during shear tests using this device is close to that of triaxial samples during shear tests. Furthermore, the repeated lifting and lowering compression method based on the proposed temperature-pressure phase equilibrium critical method can establish the cementation relationship between hydrates and sediments. Therefore, using this device to test the strength and stress-strain of hydrates is more in line with reality. Attached Figure Description

[0018] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0019] Figure 1 A schematic diagram of the apparatus for preparing low-permeability hydrate samples and testing their mechanical properties provided in an embodiment of the present invention;

[0020] Figure 2 This is a schematic diagram of the internal structure of the device for preparing low-permeability hydrate samples and testing mechanical properties provided in an embodiment of the present invention.

[0021] Figure 3 This is a schematic diagram of the structure of the three-lobed mold provided in an embodiment of the present invention;

[0022] Figure 4This is another structural schematic diagram of the three-lobed mold provided in an embodiment of the present invention;

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

[0024] Figure 6 This is a schematic diagram of the structure of the through section of the hollow tube provided in an embodiment of the present invention;

[0025] Figure 7 A flowchart illustrating the method for preparing low-permeability hydrate samples and testing their mechanical properties using the temperature-pressure phase equilibrium critical method, as provided in this embodiment of the invention.

[0026] Figure 8 The flowchart illustrates the method for preparing low-permeability hydrate samples and testing their mechanical properties using the hollow tube triaxial method, as provided in this embodiment of the invention. Detailed Implementation

[0027] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] Device Example:

[0029] See Figures 1 to 2 This figure illustrates a preferred structure of the apparatus for preparing low-permeability hydrate samples and testing their mechanical properties provided in an embodiment of the present invention. As shown, the apparatus 100 includes: a base 1, two top caps 2, a latex membrane 3, a three-lobed mold 4, a heat-insulating and pressure-maintaining cover 5, and a hollow tube 6; wherein,

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

[0031] The two ends of the latex film 3 (e.g.) Figure 2The upper and lower ends of the latex film 3 are respectively fitted onto the two top caps 2, and the two ends of the latex film 3 are detachably connected to the two top caps 2. A sample preparation cavity can be formed between the two top caps 2 and the latex film 3 for sample preparation. Specifically, the latex film 3 can be disposed between the upper top cap 202 and the lower top cap 201, with both ends fitted onto the upper top cap 202 and the lower top cap 201, and the two ends of the latex film 3 are detachably connected to the upper top cap 202 and the lower top cap 201. A sample preparation cavity can be formed between the upper top cap 202, the lower top cap 201, and the latex film 3 for sample preparation and compaction.

[0032] The three-lobed mold 4 is detachably disposed on the outer periphery of the latex film 3. It serves to support the cylindrical sample 7 during the initial installation phase when the sample is compacted to a preset height, while simultaneously ensuring the latex film 3 adheres tightly to the inner wall of the three-lobed mold 4. Specifically, the structure of the three-lobed mold 4 can be referred to... Figure 3 and Figure 4 During the compaction of the cylindrical specimen 7, a three-lobed mold 4 can be installed on the lower cap 201. The length of the three-lobed mold 4 should be more than 1 / 4 higher than the height of the cylindrical specimen 7. The installation sequence is as follows: first, put the latex film 3 on the lower cap 201, and then install the three-lobed mold 4. Tighten the clamps and make the latex film 3 tightly adhere to the inner wall of the three-lobed mold 4. The upper end of the latex film 3 can be lifted upward and flipped down onto the three-lobed mold 4 to keep the latex film 3 tightly adhered to the inner wall of the three-lobed mold 4. Then, the cylindrical specimen 7 is compacted to the specified height, i.e., the preset height, by moving it down using the axial actuator. After that, the three-lobed mold 4 is removed. Alternatively, the latex film 3 can be tightly adhered to the inner wall of the three-lobed mold 4 by negative pressure suction to prevent damage to the latex film 3 during the sample preparation process or to generate additional stress on the cylindrical specimen 7 that would affect the experimental results.

[0033] A thermal insulation and pressure-holding cover 5 is installed over the base 1, the top cap 2, and the latex film 3. The thermal insulation and pressure-holding cover 5 is filled with silicone oil 51. Specifically, the bottom of the thermal insulation and pressure-holding 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. A thermal insulation and pressure-holding cavity is formed outside the base 1, the top cap 2, and the latex film 3. The interior of the thermal insulation and pressure-holding cavity can be filled with silicone oil to achieve thermal insulation and pressure holding for mechanical property testing of the sample.

[0034] A hollow tube 6 is detachably inserted 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 at the axial position of the cylindrical sample 7. Furthermore, both ends of the hollow tube 6 extend to the sides of the heat insulation and pressure-holding cover 5 and the base 1 (e.g., Figure 1The upper and lower sides (shown) are used to introduce and discharge gas and / or water into different height positions of the sample preparation chamber. Specifically, the hollow tube 6 is coaxially arranged with the latex film 3, that is, a hollow tube 6 can be installed at the central axis position of the cylindrical sample 7. The hollow tube 6 can be a thin hollow metal tube or other materials, and no limitation is made on it in this embodiment. In this embodiment, the hollow tube 6 is detachably inserted into the base 1, the two top caps 2, the latex film 3, and the heat insulation and pressure-maintaining cover 5. For example, mounting through holes can be provided at the axial positions of the base 1, the two top caps 2, the latex film 3, and the heat insulation and pressure-maintaining cover 5. A detachable sealing plate can be provided at the mounting through hole. When the hollow tube 6 is not installed, the detachable sealing plate achieves sealing. At the same time, the hollow tube 6 can be inserted into the mounting through holes of the base 1, the two top caps 2, the latex film 3, and the heat insulation and pressure-maintaining cover 5 to allow gas to be introduced and discharged to different height positions of the sample in the sample preparation chamber and / or Water not only facilitates the introduction and discharge of gas and / or water, accelerating the process, but also ensures the uniform distribution of gas and / or water within the sample, increasing the saturation of the hydrate sample and improving the uniformity of hydrate distribution in the sediment. This overcomes the problems of traditional high-pressure, low-temperature triaxial hydrate testing equipment, such as low permeability of silty clay hydrates, low saturation of prepared hydrate samples, uneven hydrate distribution, and long sample preparation time. Furthermore, during shear tests on hydrate samples, the stress state of the sample more closely resembles that of hydrates in actual formations, making strength and stress-strain testing of hydrates more realistic. In this embodiment, lubricant can be applied to both ends of the hollow tube 6 before insertion to reduce friction caused by sample compression during shearing. When the space between the hollow tube and the sample is sufficiently smooth, the stress state of the hydrate sample during shearing is essentially consistent with that of a conventional triaxial sample. Of course, water can also be circulated within the hollow tube 6 for adding and draining hydrates.

[0035] In this embodiment, the low-permeability sample can be a silty clay hydrate, a silty soil hydrate, or a clay hydrate, or other low-permeability samples.

[0036] In this embodiment, a permeable metal stone 21 is provided on the wall surface of the top cap 2 facing the sample preparation chamber, i.e., on the bottom wall of the upper top cap 202 facing the sample preparation chamber. An insulating plate 22 is also provided between the permeable metal stone 21 and the bottom wall of the upper top cap 202. Alternatively, a permeable metal stone 21 is provided on the top wall of the lower top cap 201, and an insulating plate 22 is also provided between the permeable metal stone 21 and the top wall of the lower top cap 201. Specifically, the top cap 2, the insulating plate 22, and the permeable metal stone 21 can be formed into an integral structure by bonding or other means, so that the upper top cap 202, the insulating plate 22, and the permeable metal stone 21 can be installed as a whole. The permeable metal stone 21 improves thermal conductivity, and the insulating plate 22 isolates static electricity, preventing 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. Specifically, as Figure 1 and Figure 2 As shown, in this embodiment, the air inlet pipe 23 and water inlet pipe 24 are installed on the lower cap 201, and the exhaust pipe 25 and drain pipe 26 are installed on the upper cap 202. Of course, the pipe installed on the upper cap 202 can also be used as a water and air passage pipe, and the pipe installed on the lower cap 201 can also be used as a drain and exhaust pipe. That is to say, it can be either bottom inlet and top outlet or top inlet and bottom outlet. This embodiment does not impose any limitations on it. One end of the air inlet pipe 23, water inlet pipe 24, exhaust pipe 25, and drain pipe 26 all extend into the sample preparation chamber to pass air and water into the sample.

[0038] See Figure 5 This is a schematic diagram of the hollow tube structure provided in an embodiment of the present invention. As shown in the figure, the hollow tube 6 includes: two through-flow sections 61 and a solid isolation section 62 disposed between the two through-flow 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 through-flow sections 61, and the middle part is sealed off from airflow. The middle part can be an insulating section or an isolation section to prevent the upper and lower sections from connecting, that is, the upper and lower ends are not connected, so that gas can be discharged from the vent of the through-flow section 61 into the sample, or enter the through-flow section 61 from the sample. In this embodiment, the two through-flow sections 61 are relatively long, and the solid isolation section 62 is relatively short. Figure 6 As shown, the ventilation 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 chamber, allows water and / or gas to be introduced and discharged at different height positions of the sample within the chamber. Specifically, the ability to introduce water and / or gas at different height positions allows them to permeate into the sample from these different heights. Compared to end-to-end inlet and outlet methods, introducing water or gas at different height positions allows it to permeate to and continue permeating from these positions, increasing the further permeation efficiency within the sample and thus improving the efficiency of low-permeability hydrate sample preparation. Furthermore, introducing water or gas at different height positions increases the uniformity of hydrate distribution, effectively improving the saturation of the hydrate sample and the uniformity of hydrate distribution in the sediment. This solves the problems of low sample preparation efficiency and uneven hydrate distribution in existing traditional high-pressure, low-temperature triaxial preparation methods for low-permeability hydrate samples such as silt and mudstone. The stress state of hydrate samples during shear tests using this device is close to that of triaxial samples during shear tests. Furthermore, the repeated lifting and lowering compression method based on the proposed temperature-pressure phase equilibrium critical method can establish the cementation relationship between hydrates and sediments. Therefore, using this device to test the strength and stress-strain of hydrates is more in line with reality.

[0040] Method Implementation Examples:

[0041] In this embodiment, the present invention also proposes a method for preparing low-permeability hydrate samples and testing their mechanical properties. The method 200 for preparing low-permeability hydrate samples and testing their mechanical properties uses the above-mentioned device 100 to prepare the samples and test their mechanical properties. A hollow tube 6 is set on the central axis of the cylindrical sample 7 and serves as an air inlet or water inlet 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 implementations, namely, the temperature and pressure phase equilibrium critical method 210 for preparing low-permeability hydrate samples and testing their mechanical properties, and the hollow tube triaxial method 220 for preparing low-permeability hydrate samples and testing their mechanical properties.

[0043] In one embodiment of this example, method 200 employs a temperature-pressure phase equilibrium critical method 210. A hydrate sample is prepared based on the temperature-pressure phase equilibrium relationship. The hydrate sample is at the critical point of decomposition and synthesis. The sample is aerated, and the structural morphology of the sediment and hydrate is controlled by adjusting the phase equilibrium, transforming the hydrate and sediment in the sample from a discrete state to a cemented state. Specifically, the temperature-pressure phase equilibrium critical method 210 mainly uses the temperature-pressure phase equilibrium relationship to bring the hydrate in the sample to the critical point of decomposition and synthesis, thereby transforming the hydrate and sediment in the sample from a discrete state (i.e., uncemented) to a cemented state. Since the solid hydrate powder and sediment are mixed before sample preparation, the structural morphology of the sediment and hydrate is controlled by adjusting the phase equilibrium in the later stages, making it closer to its original characteristics.

[0044] See Figure 7 The figure shows a flowchart of a method for preparing and testing the mechanical properties of low-permeability hydrate samples using the thermo-pressure phase equilibrium critical method, as provided in an embodiment of the present invention. As shown, the method for preparing and testing the mechanical properties of low-permeability hydrate samples using the thermo-pressure phase equilibrium critical method 210 includes the following steps:

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

[0046] Specifically, based on the required saturation and percentage of particulate components in the sample, the required mass of dry soil (m) in the hydrate sediment to be prepared is calculated. sd The volume of water, V w1 and methane volume V m1 .

[0047] Step S212, change volume V m1 Pure methane hydrate was prepared from methane gas and water, and the volume of water consumed, V, was recorded during the preparation process. w2 ; and the mass is m sd Dry soil is placed in a preset freezing environment and frozen for a preset freezing time to obtain frozen dry soil.

[0048] Specifically, using a volume of V m1 Pure methane hydrate can be prepared from methane gas and water. The preparation method can be a hydrate reaction vessel, or by spraying water mist into a volume of V under low temperature and high pressure conditions. m1 The hydrate was prepared in methane gas, and the mass m of the prepared hydrate was weighed. h2 Record the volume of water consumed in the preparation, V. w2 and methane volume V m2 V m2=V m1 It can also simultaneously prepare a mass of m using water. i The process involves freezing the dry soil in an environment below -20°C for a preset freezing time period to obtain frozen dry soil. The preset freezing time period can be 30 minutes.

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

[0050] Specifically, the prepared hydrate and ice are ground into powder under low-temperature conditions and quickly mixed with freeze-dried soil to obtain a mixed soil sample. That is, the sum of the weights of the water used in step S212 to prepare pure methane hydrate and the ice used in step S213 corresponds to the volume of water required in the hydrate sediment to be prepared, as calculated in step S211. In other words, the volume of ice required to be added during the preparation of the mixed soil sample is calculated based on the volume of water required in the hydrate sediment to be prepared, as calculated in step S211, and the weight of the water used in preparing pure methane hydrate in step S212. In other words, the weight of ice used in the preparation can be calculated based on the volume of water required in the hydrate sediment to be prepared, as calculated in step S211, and the weight of the water used in preparing pure methane hydrate in step S212.

[0051] Step S214: Place the latex film onto the lower cap and install the three-part mold. After tightening the clamps, adjust the latex film to fit tightly against the inner wall of the three-part mold.

[0052] Specifically, the latex membrane 3 is fitted onto the lower cap 201, then the three-lobed mold 4 is installed. After tightening the clamps, the latex membrane 3 is lifted and the exposed upper end of the latex membrane 3 is turned outward onto the outer wall of the three-lobed mold 4. It is necessary to ensure that the inner latex membrane 3 is tightly attached to the inner wall of the three-lobed mold 4. Alternatively, the latex membrane 3 can be tightly attached to the inner wall of the three-lobed mold 4 by negative pressure suction to prevent damage to the latex membrane during sample preparation or to generate additional stress on the sample that may affect the experimental results.

[0053] Step S215: The mixed soil sample is filled into a three-lobed mold using the sand rain method. After preparation, the top cap is installed and made to fit tightly with the soil sample. The latex film is then placed over the top cap and the three-lobed mold is removed. After covering with a pressure-insulating and heat-insulating cover, silicone oil is introduced.

[0054] Specifically, first, the mixed soil sample is placed into a three-lobed mold using the sand rain method. The preparation process can refer to the geotechnical testing standards. After preparation, the top cap 202 is quickly installed, ensuring that the bottom wall of the top cap 202 is in close contact with the top wall of the cylindrical sample 7. The latex film 3 is then placed on the top cap 202, and the three-lobed 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 impurities from each circuit. Finally, the pressure-insulating and heat-preserving cover 5 is covered, and silicone oil 51 is introduced. The air inlet pipe 23, the water inlet pipe 24, the exhaust pipe 25, and the drain pipe 26 are then closed. Of course, in this embodiment, a hollow tube 6 can also be installed. First, the outer wall of the hollow tube is lubricated, then inserted into the lower cap located on the base. A latex membrane is then fitted onto the lower cap, and a three-lobed mold is installed. After adjusting the latex membrane to fit tightly against the inner wall of the three-lobed mold, the mixed soil sample is filled into the three-lobed mold using the sand-rain method for preparation. The upper cap is then installed, so that the upper end of the hollow tube 6 is inserted into the upper cap 202. Whether or not to install the hollow tube 6 can be determined based on actual conditions. Installing the hollow tube 6 can further improve the sample preparation efficiency and uniformity.

[0055] Step S216: Apply confining pressure to the hydrate sample and cool it down according to the preset temperature and pressure conditions, and stabilize the preset temperature and pressure for a preset time period after reaching the preset temperature and pressure.

[0056] Specifically, according to preset temperature and pressure conditions, a confining pressure is applied to the hydrate sample and the temperature is lowered until the preset temperature and pressure are reached, at which point the temperature and pressure are stabilized for a preset time period. The preset temperature and pressure time period can be 30 minutes.

[0057] Step S217: Maintaining the preset temperature, reduce the confining pressure according to the temperature-pressure phase equilibrium relationship of methane hydrate to bring the hydrate to the decomposition critical point. Observe the gas flow meter at the exhaust pipe of the device. When gas is discharged, close the exhaust pipe and record the volume V of the discharged gas. hout1 And introduce a volume of V hout1 Methane gas is introduced to maintain a constant methane content in the sample, and the confining pressure is increased to a preset pressure and then stabilized for a preset temperature and pressure period.

[0058] Specifically, while maintaining a constant temperature, the confining pressure is reduced according to the temperature-pressure phase equilibrium relationship of methane hydrate, bringing the hydrate to the decomposition critical point. The exhaust pipe 25 and 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 drain pipe 26 are closed, and the volume V of the discharged gas is recorded. hout1 Open the lower air intake pipe 23 and introduce a volume of V. hout1Methane gas is introduced to maintain a constant methane content within the sample, and the confining pressure is increased to a preset pressure and then stabilized for a preset temperature and pressure period. Alternatively, when the lower inlet pipe 23 is opened, the hollow tube can also be opened to allow gas to enter. The preset temperature and pressure period can be determined based on actual conditions; this embodiment does not impose any limitations on it.

[0059] Step S218 involves repeatedly performing the processes of reducing confining pressure, ventilating, and increasing pressure to transform the hydrates and soil samples in the specimen from their initial discrete state to the cemented state of the original clayey hydrates.

[0060] Specifically, by repeatedly performing step S217 at least four times, the hydrates and soil samples in the specimen can be transformed from their initial discrete state to a cemented state of undisturbed clay-type hydrates.

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

[0062] Specifically, while maintaining a constant confining pressure, the temperature is lowered to the required experimental temperature. After stabilizing for a preset temperature period, a triaxial shear test is conducted. 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 shear test 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 embodiment of this example, method 200 employs the hollow tube triaxial method 220. During sample preparation, air is introduced into the soil sample placed in the sample preparation chamber through a hollow tube. Specifically, the hollow tube triaxial method 220 mainly improves sample preparation efficiency by adding a hollow tube as an air inlet pipe along the central axis of the sample. This overcomes the problem of low sample preparation efficiency in traditional combustible ice triaxial methods due to excessive length-to-diameter ratio. When the diameter of the hollow tube is small and sufficiently smooth, as long as the internal pressure is controlled to be consistent with the confining pressure, the stress state during the experiment can be approximately equivalent to the stress state of a triaxial sample. These two methods can also be used simultaneously for even higher sample preparation efficiency, i.e., by combining the hollow tube with the critical temperature-pressure phase equilibrium method 210 to introduce air at different positions along the central axis of the sample.

[0064] See Figure 8 The flowchart illustrates the method for preparing and testing the mechanical properties of low-permeability hydrate samples using the hollow tube triaxial method, as provided in this embodiment of the invention. As shown, the method for preparing and testing the mechanical properties of low-permeability hydrate samples using the hollow tube triaxial method 220 includes the following steps:

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

[0066] Specifically, based on the required saturation and percentage of particulate components in the sample, the required mass of dry soil (m) in the hydrate sediment to be prepared is calculated. sd The volume of water, V w1 and methane volume V m1 .

[0067] Step S222: According to the standard method for geotechnical testing, calculate the required mass m of dry soil 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 placed in a plastic bag and left to stand for a preset soil sample standing time period.

[0068] Specifically, according to the method recommended in the geotechnical testing standards, the dry soil and water calculated in step 1 are mixed and stirred, then placed in a plastic bag and left to stand for a preset set time period. The preset set time period can 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-lobed mold, tighten the clamps, and adjust the latex film to fit tightly against the inner wall of the three-lobed mold.

[0070] Specifically, after applying Vaseline to the hollow tube 6 for lubrication, insert it into the lower cap 201. Then, put the latex membrane 3 onto the lower cap 201, install the three-lobed mold 4, tighten the clamps, lift the latex membrane 3, and fold the exposed upper end of the latex membrane 3 onto the outer wall of the three-lobed mold 4. It is essential to ensure that the inner latex membrane 3 is tightly attached to the inner wall of the three-lobed mold 4. Alternatively, the latex membrane 3 can be tightly attached to the inner wall of the three-lobed mold 4 by negative pressure suction. This prevents damage to the latex membrane during sample preparation or additional stress on the sample that could affect the experimental results.

[0071] Step S224: Put the soil sample from the plastic bag into the three-part mold, install the top cap located at the top, and make the top cap fit tightly with the soil sample. Adjust the axial actuator to displacement control to move the top cap down to the preset sample height. After stabilization, remove the three-part mold and put the latex film on the top cap and tie it tightly. After covering with the pressure-insulating and heat-insulating cover, introduce silicone oil.

[0072] Specifically, first, the soil sample prepared in step S222 is placed into the three-lobed mold 4, and the top cap 202 is installed, ensuring that the top cap 202 fits tightly against the sample. At the same time, it is important to ensure that the upper end of the hollow tube 6 is inserted into the top cap 202. The initial position L1 of the top cap is recorded. The distance L2 that the top cap moves down can be calculated based on the preset sample height Hs, where L2 = L1 - Hs. Based on the distance L2 that the top cap moves down, the axial actuator is adjusted to displacement control to move the top cap 202 down by a distance L2. After stabilizing for 10 minutes, the three-lobed mold 4 is removed, and the latex film 3 is placed on the top cap 202 and tightened. 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 impurities from each circuit. After the removal is completed, the water injection and air injection circuits and the drain and exhaust circuits are connected to the base and the corresponding air inlet pipe 23, water inlet pipe 24, exhaust pipe 25 and drain pipe 26, respectively. After covering with the pressure-insulating and heat-insulating cover 5, silicone oil is introduced.

[0073] Step S225: Apply confining pressure to the hydrate sample and cool it down according to the preset temperature and pressure conditions, and stabilize the preset temperature and pressure for a preset time period after reaching the preset temperature and pressure conditions.

[0074] Specifically, according to the temperature and pressure conditions required by the test, a confining pressure is applied to the hydrate sample and the temperature is lowered. After the temperature and pressure conditions are reached, the sample is stabilized for a preset temperature and pressure time period. The preset temperature and pressure time period can be 30 minutes.

[0075] Step S226: Using the exhaust pipe on the device, methane gas is introduced into the sample, and the amount of gas introduced, V, is measured and recorded. gin and the volume of gas discharged V gout According to the amount of gas introduced, V gin and the volume of gas discharged V gout Calculate the methane hydrate saturation in the sample until the preset saturation is reached, then stop introducing methane gas; or, measure and record the amount of gas introduced, V. gin and the volume of gas discharged V gout Until the gas flow rate reaches V m1 Stop the flow of methane gas to allow the methane hydrate saturation in the sample to reach the preset saturation level.

[0076] Specifically, open the valves connected to the air inlet pipe 23, water inlet pipe 24, exhaust pipe 25, drain pipe 26, and the two ventilation sections 61 of the hollow pipe 6, and introduce methane gas into the air inlet pipe 23, water inlet pipe 24, and the two ventilation sections 61 respectively, while simultaneously measuring and recording the gas volume V introduced. gin And the volume of gas 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; according to V gin -Vgout The methane hydrate saturation in the sample can be calculated. Once the required saturation is reached, the inlet pipe 23, water inlet pipe 24, exhaust pipe 25, and drain pipe 26, as well as the valves connected to the two ventilation sections 61 of the hollow tube 6, are closed. Alternatively, the amount of gas introduced, V, is measured and recorded. gin and the volume of gas discharged V gout Until the gas flow rate reaches V m1 Stop the flow of methane gas to allow the methane hydrate saturation in the sample to reach the preset saturation level.

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

[0078] Specifically, after stabilizing for 4 hours, a triaxial shear test is initiated; and after the shear test is completed, the recorded stress and strain are processed according to conventional triaxial test data processing methods. In this embodiment, during the mechanical property test of the specimen, the interior of the hollow tube 6 is pressurized, and its internal air pressure is matched with the confining pressure of the specimen, so that the stress state of the specimen during the mechanical property test is equivalent to the stress state of the triaxial specimen. That is, when the diameter of the hollow tube is small and it is sufficiently smooth, 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 specimen.

[0079] In summary, the method for preparing and testing the mechanical properties of low-permeability hydrate samples provided in this embodiment, using the aforementioned apparatus, can be used to test the mechanical properties of low-permeability hydrates. It can also be applied to the mechanical property testing of hydrate decomposition and slope stability evaluation studies. This method overcomes the shortcomings of traditional high-pressure, low-temperature triaxial preparation methods for low-permeability hydrate samples such as silty clay, which suffers from low sample preparation efficiency and uneven hydrate distribution. The apparatus and method effectively improve the saturation of hydrate samples and enhance the uniformity of hydrate distribution in sediments. Furthermore, the apparatus makes the stress state of hydrate samples during shear tests more closely resemble the state of hydrates in actual formations, making the method for testing hydrate strength and stress-strain more realistic.

[0080] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0081] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0082] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing low-permeability hydrate samples and testing their mechanical properties, characterized in that, The method uses a low-permeability hydrate sample preparation and mechanical property testing device to prepare the sample and test its mechanical properties. A hollow tube is set on the central axis of the sample and serves as an air inlet or water inlet during the sample preparation process to pass water or gas to different height positions of the sample. The method employs the hollow tube triaxial method, in which air is introduced into the soil sample placed in the sample preparation cavity through a hollow tube during sample preparation. The method includes the following steps: Step S221: Calculate the required mass m of dry soil in the hydrate sediment to be prepared based on the preset saturation and preset particle component content percentage of the sample. sd The volume of water, V w1 ; Step S222: According to the standard method for geotechnical testing, calculate the required mass m of dry soil 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 soil samples, and the prepared soil samples are placed in plastic bags and left to stand for a preset soil sample standing time. 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-lobed mold, tighten the clamps, and adjust the latex film to fit tightly against the inner wall of the three-lobed mold. Step S224: Put the soil sample from the plastic bag into the three-petal mold, install the top cap located at the top, and make the top cap fit tightly with the soil sample. Adjust the axial actuator to displacement control to move the top cap down to the preset sample height. After stabilization, remove the three-petal mold and put the latex film on the top cap and tie it tightly. After covering with the pressure-insulating and heat-insulating cover, introduce silicone oil. Step S225: Apply confining pressure to the hydrate sample and cool it down according to the preset temperature and pressure conditions, and stabilize the preset temperature and pressure for a preset time period after reaching the preset temperature and pressure conditions. Step S226: Using the air inlet pipe, water inlet pipe, and hollow tube on the device, methane gas is introduced into the sample, and the amount of gas introduced, V, is measured and recorded. gin and the volume of gas discharged V gout According to the amount of gas introduced, V gin and the volume of gas discharged V gout Calculate the methane hydrate saturation in the sample until the preset saturation is reached, then stop introducing methane gas. Step S227: After the methane hydrate saturation in the sample reaches the preset saturation and stabilizes for a preset time period, a triaxial shear test is performed. The apparatus for preparing low-permeability hydrate samples and testing their mechanical properties is characterized by comprising: Base; Two top caps are spaced apart above the base; A latex film, with its two ends respectively fitted onto two top caps, and the two top caps and the latex film forming a sample preparation cavity for preparing cylindrical samples within the sample preparation cavity; A hollow tube is inserted through the base, the two top caps, and the cylindrical sample. The hollow tube is coaxially arranged with the cylindrical sample so that it is inserted at the axial position of the cylindrical sample. The two ends of the hollow tube extend to both sides of the heat insulation and pressure-maintaining cover and the base, and are used to introduce and discharge gas and / or water to different height positions in the sample preparation cavity. The hollow tube includes: two passage sections and a solid isolation section disposed between the two passage sections. A thermal insulation and pressure-maintaining cover is installed over the base, the top cap, and the latex film, and the thermal insulation and pressure-maintaining cover is filled with silicone oil.

2. The method for preparing low-permeability hydrate samples and testing their mechanical properties according to claim 1, characterized in that, The method includes the following steps: Step S211: Calculate the required mass m of dry soil in the hydrate sediment to be prepared, based on the preset saturation and preset particle component content percentage of the sample. sd The volume of water, V w1 and methane volume V m1 ; Step S212, change volume V m1 Pure methane hydrate was prepared from methane gas and water, and the volume of water consumed, V, was recorded during the preparation process. w2 ; and the mass is m sd Dry soil is placed in a preset freezing environment and frozen for a preset freezing time to obtain freeze-dried soil; Step S213: Under low temperature conditions, the prepared pure methane hydrate and a mass of m are respectively... i The ice was crushed into powder and mixed evenly with the freeze-dried soil to obtain a mixed soil sample; Step S214: Place the latex film onto the lower cap and install the three-lobed mold. After tightening the clamps, adjust the latex film to fit tightly against the inner wall of the three-lobed mold. Step S215: The mixed soil sample is prepared by filling a three-lobed mold with sand rain method. After preparation, the top cap is installed and made to fit tightly with the soil sample. The latex film is put on the top cap and the three-lobed mold is removed. After covering with pressure and heat preservation cover, silicone oil is introduced. Step S216: Apply confining pressure to the hydrate sample and cool it down according to the preset temperature and pressure conditions, and stabilize the preset temperature and pressure for a preset time period after reaching the preset temperature and pressure. Step S217: Maintaining the preset temperature, reduce the confining pressure according to the temperature-pressure phase equilibrium relationship of methane hydrate to bring the hydrate to the decomposition critical point. Observe the gas flow meter at the exhaust pipe of the device. When gas is discharged, close the exhaust pipe and record the volume V of the discharged gas. hout1 And introduce a volume of V hout1 Methane gas is introduced to maintain a constant methane content in the sample, and the confining pressure is increased to a preset pressure and then stabilized for a period of time. Step S218 involves repeatedly performing multiple processes of reducing confining pressure, ventilating, and increasing pressure to transform the hydrates and soil samples in the specimen from their initial discrete state to the cemented state of the original clayey hydrates. Step S219: Adjust the temperature and confining pressure to the preset conditions required for the test, and after stabilizing the preset temperature and pressure time period, conduct the triaxial shear test.

3. The method for preparing low-permeability hydrate samples and testing their mechanical properties according to claim 1, characterized in that, The top cap has a permeable metal stone on its wall facing the sample preparation cavity, and an insulating plate is provided between the permeable metal stone and the wall facing the top cap.

4. The method for preparing low-permeability hydrate samples and testing their mechanical properties according to claim 1, 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 method for preparing low-permeability hydrate samples and testing their mechanical properties according to claim 1, characterized in that, Also includes: The three-lobed mold, whose length is greater than 1 / 4 of the preset sample height, is detachably set on the outer periphery of the latex film. It serves to support the sample when it is compacted to the preset sample height in the initial stage of sample installation, while keeping the latex film tightly attached to the inner wall of the three-lobed mold.

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