Apparatus and method for strain-controlled continuous loading consolidation test of hydrate-bearing sediments

Abstract

The present application relates to natural gas hydrate mechanics test technical field, especially to a kind of hydrate-bearing sediment strain control continuous loading consolidation experiment device and method.The present application includes constant-temperature gas bath cabinet, consolidation test machine, consolidation reactor, counter-pressure module and gas recovery module, the consolidation test machine includes counter-force frame and loading mechanism, loading mechanism is installed at the bottom of counter-force frame, consolidation reactor is arranged on loading mechanism, consolidation test machine and consolidation reactor are located in constant-temperature gas bath cabinet, counter-pressure module and gas recovery module are connected with consolidation reactor respectively.By strain control continuous loading, the consolidation experiment period of hydrate-bearing sediment is shortened, by controlling sample bottom completely not draining water, only draining water from top single side, and cooperating with appropriate loading rate control bottom hole pressure variation, ensure that no excessive hydraulic gradient is generated in sample to cause sample damage, realize the purpose of quickly obtaining reliable hydrate-bearing sediment compressibility index.

Description

Technical Field

[0001] This invention relates to the field of mechanical testing technology for natural gas hydrates, and in particular to a strain-controlled continuous loading consolidation experimental apparatus and method for hydrate-containing sediments. Background Technology

[0002] Natural gas hydrates possess advantages such as wide distribution, high energy density, and cleanliness, and are considered one of the most promising new energy resources. The successful two trial extractions in the South my country Sea have demonstrated that depressurization is the most promising commercial extraction method. During depressurization extraction, the hydrates decompose, increasing the effective stress in the reservoir and causing compression deformation, which can affect reservoir stability. Long-term extraction may induce geological disasters such as land subsidence and submarine landslides. Therefore, determining the compressibility index of natural gas hydrate-bearing sediments is essential for assessing reservoir stability during depressurization extraction and predicting potential engineering geological risks.

[0003] Currently, consolidation experiments are widely used to test the compressibility of conventional soils. However, due to the difficulty and high cost of obtaining undisturbed samples of natural gas hydrate sediments, current experimental studies on the compressibility of natural gas hydrate sediments mainly rely on laboratory-synthesized samples. Existing conventional consolidation experimental devices cannot simulate the actual temperature and pressure conditions of seafloor hydrate reservoirs, and therefore lack the capability to prepare samples of natural gas hydrate sediments.

[0004] Patent publication number CN101846605B discloses an apparatus and method for testing the compressibility index of hydrate-containing sediments using standard consolidation experiments. However, this apparatus and method still suffer from the inherent drawbacks of standard consolidation experiments, mainly including the following three points:

[0005] (1) Long experimental cycle. Each loading stage in the experiment requires 24 hours, resulting in a complete loading process that takes several days or even tens of days. The long experimental cycle places higher demands on the temperature and pressure control and sealing of the equipment; otherwise, it will increase the risk of hydrate decomposition leading to experimental failure. In addition, this will greatly increase the complexity of the equipment, which is not conducive to commercial transformation and promotion.

[0006] (2) The large pressure gradient of pore water at the drainage surface of the sample may damage the original structure of the sample, especially the cemented structure of hydrate deposits, leading to inaccurate experimental results.

[0007] (3) The effect of loading rate on the compressibility of hydrate-bearing sediments cannot be tested. Different depressurization rates are used during hydrate depressurization mining, and hydrate-bearing sediments exhibit different compressibility characteristics under different loading rates. Therefore, testing the effect of loading rate on the compressibility of hydrate-bearing sediments would make the experimental results more applicable to practical engineering. Existing standard consolidation uses graded loading, and the results obtained from graded loading cannot be used to calculate indicators related to the loading rate. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned defects in the prior art and to propose a strain-controlled continuous loading consolidation experimental device and method for hydrate-bearing sediments. By controlling continuous loading with strain control, the consolidation experimental cycle of hydrate-bearing sediments is shortened. By controlling the bottom of the sample to be completely dewatered and only dewatering from the top, and by controlling the bottom pore pressure change with an appropriate loading rate, it is ensured that an excessive hydraulic gradient is not generated in the sample, which would lead to sample failure. This achieves the goal of rapidly obtaining reliable compressibility indicators of hydrate-bearing sediments.

[0009] The technical solution of the present invention is: a strain-controlled continuous loading consolidation experimental device for hydrate-containing sediments, comprising a constant temperature air bath and a consolidation experimental machine, wherein the device further comprises a consolidation reaction vessel, a back pressure module and a gas recovery module. The consolidation experimental machine comprises a reaction frame and a loading mechanism. The loading mechanism is installed at the bottom of the reaction frame, and the consolidation reaction vessel is disposed on the loading mechanism. Both the consolidation experimental machine and the consolidation reaction vessel are located inside the constant temperature air bath. The back pressure module and the gas recovery module are respectively connected to the consolidation reaction vessel.

[0010] The consolidation reactor includes a base, a pressure chamber, a piston, and a fixed flange. The pressure chamber is fixedly installed on the base, with its bottom sleeved on the top outside of the base. A piston is located in the upper part of the inner cavity of the pressure chamber and is slidably disposed within the pressure chamber. The top of the pressure chamber is fixedly connected to the fixed flange. A sample is placed in the pressure chamber cavity between the piston and the base. Metal permeable plates are provided between the top surface of the sample and the piston, and between the bottom surface of the sample and the base. The base has a second channel and a third channel. The base is connected to a pore pressure sensor through the second channel and to a gas recovery module through the third channel.

[0011] The gas recovery module includes a gas recovery device, which is connected to a third channel in the base via a connecting pipe. A first valve is provided on the connecting pipe between the gas recovery device and the base.

[0012] The back pressure module includes a methane cylinder, an air compressor, and a back pressure pump. The piston has a first channel connected to the top surface of the sample. The methane cylinder, air compressor, and back pressure pump are respectively connected to the first channel.

[0013] In this invention, a sealing ring is provided between the inner surface of the pressure chamber and the outer surface of the base, and a sealing ring is provided between the outer surface of the piston and the inner surface of the pressure chamber.

[0014] The reaction frame is also equipped with a pressure sensor and a displacement sensor.

[0015] The methane cylinder is connected to the first channel via a fourth valve and a second valve. The air compressor is connected to the first channel via a second valve, and the back pressure pump is connected to the first channel via a third valve.

[0016] The device also includes a data acquisition module, which is connected to the pressure sensor, displacement sensor, and pore pressure sensor. The data acquisition module automatically collects and records the data detected by each sensor.

[0017] The present invention also includes a method for conducting experiments using the above-mentioned strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments, comprising the following steps:

[0018] S1. Fill the pressure chamber with sediment;

[0019] S2. Preparation of hydrate-containing sediment samples:

[0020] First, open the fourth valve, then open the second valve to allow methane gas from the methane cylinder to enter the pressure chamber, gradually increasing the pressure in the pressure chamber to 6MPa-15MPa. While the pressure is increasing, activate the loading mechanism to control the axial pressure and keep the piston in its initial position. After the pressure stabilizes, close the second and fourth valves in sequence, start the constant temperature gas bath, and cool down to 0-2℃. Hydrates are generated in the pressure chamber. During the hydrate formation process, methane gas is continuously consumed, thereby reducing the gas pressure in the pressure chamber. When the pressure stabilizes, the sample preparation is complete.

[0021] S3. Sample saturation:

[0022] Start the air compressor, open the second valve, and cycle the first valve to displace the residual methane gas in the connecting pipeline and sample, while maintaining a constant pressure in the pressure chamber. Then close the second valve and open the third valve, using a back pressure pump to inject deionized water into the sample to saturate it and reach the set back pressure, simulating the actual deep-sea water pressure conditions of 10-15 MPa. During the saturation process, maintain a constant temperature and control the axial pressure to keep the piston in its initial position. Finally, with the first and third valves closed, apply an axial pressure increment of 20-50 kPa to the sample through the loading mechanism. When the ratio of the pore pressure increment to the axial pressure increment is greater than 0.95, saturation is complete; otherwise, open the third valve to continue saturation.

[0023] S4. Strain-controlled consolidation experiment:

[0024] The first valve is kept closed and the third valve is kept open to induce pore pressure changes at the bottom of the sample. The top of the sample is drained from one side only, and the pore pressure at the top of the sample is always equal to the back pressure applied by the back pressure pump. The loading mechanism is started and a fixed strain rate is set to make the loading mechanism move upward at a constant speed. During the uniform upward movement of the loading mechanism, a strain-controlled continuous loading consolidation experiment is conducted. The values ​​of the pressure sensor, displacement sensor and pore pressure sensor are continuously recorded by the data acquisition module to calculate the compressibility index of the hydrate-bearing sediment.

[0025] In step S1 above, the sediment is layered and filled into the pressure chamber according to the required water content. After the sediment is filled, the piston is first placed into the pressure chamber so that the bottom of the piston just contacts the sediment. The fixing flange is fixed to the upper surface of the pressure chamber. Finally, the entire consolidation reactor is moved onto the loading mechanism. The loading mechanism is started and the loading stops after the pressure sensor contacts the piston. The displacement sensor always contacts the fixing flange. The consolidation reactor is connected to the back pressure module, the gas recovery module, and the pore pressure sensor through connecting pipes, and the airtightness is checked.

[0026] In step S4 above, the strain rate of the loading mechanism can be 0.0001 mm / min to 3 mm / min.

[0027] After step S4 is completed, the temperature is increased to decompose the hydrates in the sample. The first valve is opened, and the volume of gas produced by the decomposition of hydrates is collected using a gas recovery instrument to correct the calculation of hydrate saturation.

[0028] The beneficial effects of this invention are:

[0029] (1) This application can effectively shorten the consolidation test cycle of hydrate-bearing sediments by controlling continuous loading with strain control. By controlling the bottom of the sample to not drain at all and draining only from the top, and coordinating with an appropriate loading rate to control the bottom pore pressure change, it ensures that no excessive hydraulic gradient is generated in the sample, which will cause the sample to break, and achieves rapid and reliable compressibility index of hydrate-bearing sediments.

[0030] (2) It can simulate the effect of depressurization rate on the compressibility of hydrate sediments during mining, making the compressibility index obtained by the experiment more applicable to actual engineering.

[0031] (3) The experimental device of the present invention has a simple structure, the experimental method is easy to operate and easy to promote, and can provide theoretical and technical support for the commercial exploitation of natural gas hydrates. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of the present invention;

[0033] Figure 2This is a schematic diagram of the solidification reactor;

[0034] Figure 3 yes Figure 2 Sectional view along direction AA.

[0035] In the diagram: 1. Constant temperature gas bath; 2. Consolidation reactor; 3. Gas recovery instrument; 4. Data acquisition module; 5. Base; 6. Pressure chamber; 7. Piston; 8. Fixed flange; 9. Reaction frame; 10. Loading mechanism; 11. Methane cylinder; 12. Air compressor; 13. Back pressure pump; 14. Pressure sensor; 15. Displacement sensor; 16. Pore pressure sensor; 17. Sample; 18. Sealing ring; 19. Metal permeable plate; C1. Third channel; C2. Second channel; C3. Third channel; V1. First valve; V2. Second valve; V3. Third valve; V4. Fourth valve. Detailed Implementation

[0036] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0037] Specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many ways other than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0038] like Figures 1 to 3 As shown, the strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments described in this application includes a constant-temperature air bath 1, a consolidation reactor 2, a consolidation testing machine, a back pressure module, and a gas recovery module. Both the consolidation testing machine and the consolidation reactor 2 are located within the constant-temperature air bath 1, with the consolidation reactor 2 mounted on the consolidation testing machine. The back pressure module and the gas recovery module are respectively connected to the consolidation reactor 2. The back pressure module and the constant-temperature air bath 1 simulate the actual temperature and pressure conditions at the seabed, thereby preparing hydrate-bearing sediment samples within the consolidation reactor 2.

[0039] The consolidation testing machine includes a reaction frame 9 and a loading mechanism 10. The loading mechanism 10 is installed at the bottom of the reaction frame 9, and the consolidation reactor 2 is mounted on the loading mechanism 10. Figure 2As shown, the consolidation reactor 2 includes a base 5, a pressure chamber 6, a piston 7, and a fixed flange 8. The pressure chamber 6 is fixedly mounted on the base 5, with its bottom fitted over the top of the base 5. A sealing ring 18 is provided between the inner surface of the pressure chamber 6 and the outer surface of the base 5. The piston 7 is located at the upper part of the inner cavity of the pressure chamber 6, and can move vertically up and down within the pressure chamber 6. A sealing ring 18 is provided between the outer surface of the piston 7 and the inner surface of the pressure chamber 6. The top of the pressure chamber 6 is fixedly connected to the fixed flange 8 by bolts. The fixed flange 8 is used to limit the piston 7, preventing it from coming out of the pressure chamber during the experiment. A sample 17 is placed in the pressure chamber cavity between the piston 7 and the base 5. Metal permeable plates 17 are provided between the top surface of the sample 17 and the piston 7, and between the bottom surface of the sample 17 and the base 5. The base 5 has a second channel C2 and a third channel C3. The base 5 is connected to the pore pressure sensor 16 through the second channel C2. The pore pressure sensor 16 is used to monitor the pore pressure on the bottom surface of the sample in real time. The base 5 is connected to the gas recovery module through the third channel C3. The reaction frame 9 is also equipped with a pressure sensor 14 and a displacement sensor 15. The pressure sensor 14 is in contact with the top surface of the piston located outside the pressure chamber. The upper end of the displacement sensor 15 is fixedly connected to the reaction frame 9, and the lower end of the displacement sensor 15 is in contact with the top surface of the fixed flange 8. The pressure and deformation of the sample are collected by the above sensors. In this embodiment, the consolidation reactor 2 is made of stainless steel and can withstand pressures of more than 30 MPa. The sidewall of the pressure chamber 6 is completely rigid, thereby ensuring that the sample deforms only in the vertical direction during the pressure process and does not produce lateral deformation.

[0040] The gas recovery module includes a gas recovery instrument 3, which is connected to the third channel C3 inside the base 5 via a connecting pipe. A first valve V1 is provided on the connecting pipe between the gas recovery instrument 3 and the base 5. During the consolidation experiment, the first valve V1 is in a closed state, thereby preventing water from draining from the bottom of the sample.

[0041] The backpressure module includes a methane cylinder 11, an air compressor 12, and a backpressure pump 13. The backpressure module is connected to the first channel C1 inside the piston 7, which communicates with the top surface of the sample. The backpressure module applies pore pressure and water pressure to the sample 17. A fourth valve V4 and a second valve V2 are installed on the connecting pipe between the methane cylinder 11 and the first channel C1, allowing methane gas to be injected into the pressure chamber 6 through the methane cylinder 11. A second valve V2 is installed on the connecting pipe between the air compressor 12 and the first channel C1. A third valve V3 is installed on the connecting pipe between the backpressure pump 13 and the first channel C1. When the third valve V3 is open, the pressure at the top of the sample is always equal to the pressure applied by the backpressure pump, achieving single-sided drainage from the top of the sample. Therefore, the pore pressure increment at the top of the drained sample is always zero; only the bottom surface of the sample experiences a pore pressure increment.

[0042] The device also includes a data acquisition module 4, which is connected to the pressure sensor 14, the displacement sensor 5 and the pore pressure sensor 16 respectively. The data acquisition module 4 automatically acquires and records the data detected by each sensor.

[0043] During its movement, the loading mechanism 10 moves at a constant speed, continuously loading the sample in the pressure chamber 6 and enabling the sample to undergo compressive deformation at a constant strain rate. Furthermore, the loading mechanism can be set to different strain rates, i.e., its movement speed, to control the change in pore pressure at the bottom of the sample and to simulate the depressurization rate during actual hydrate extraction.

[0044] The present invention also includes a method for conducting experiments using the above-mentioned strain-controlled continuous loading consolidation experimental apparatus for hydrate deposits, the method comprising the following steps.

[0045] The first step is to fill the container with sediment.

[0046] The sediment is layered and filled into pressure chamber 6 according to the required moisture content. In this embodiment, the inner diameter of pressure chamber 6 is 6.18 cm, so the height of the sample is controlled at 4 cm. After the sediment is filled, piston 7 is first placed into pressure chamber 6 so that the bottom of piston 7 just contacts the sediment sample. Then, fixing flange 8 to the upper surface of pressure chamber 6 is fixed with bolts. Finally, the entire consolidation reactor 2 is moved onto loading mechanism 10, and loading mechanism 10 is started. Loading is stopped after pressure sensor 14 slightly contacts piston 7, ensuring that displacement sensor 15 always contacts fixing flange 8. The consolidation reactor is connected to back pressure module, gas recovery module, pore pressure sensor 16, etc. through connecting pipelines, and the airtightness is checked.

[0047] The second step is to prepare hydrate-containing sediment samples.

[0048] First, open the fourth valve V4, then open the second valve V2, allowing methane gas from methane cylinder 11 to enter pressure chamber 6. Gradually increase the pressure in pressure chamber 6 to 6 MPa-15 MPa. Simultaneously, activate the loading mechanism 10 to control the axial pressure, keeping piston 7 in its initial position and preventing upward or downward movement. After the pressure stabilizes, first close the second valve V2, then close the fourth valve V4, and start the constant temperature gas bath 1 to cool to 0-2℃. Hydrates will form in pressure chamber 6. The formation of hydrates continuously consumes methane gas, causing the gas pressure in pressure chamber 6 to decrease. Once the pressure stabilizes, sample preparation is complete.

[0049] The third step is to saturate the sample.

[0050] Start the air compressor 12, open the second valve V2, and cycle the first valve V1 to displace the residual methane gas in the connecting pipeline and sample 17. During this process, maintain a constant pressure in the pressure chamber 6 to prevent hydrate decomposition. Then, close the second valve V2 and open the third valve V3. Use the back pressure pump 13 to inject deionized water into the sample 17 to saturate the sample and reach the set back pressure, simulating the actual deep-sea water pressure conditions of 10-15 MPa. In the initial stage of back pressure loading, the first valve V1 can be opened appropriately to remove residual air in the connecting pipeline and sample 17, reducing the saturation time. Throughout the saturation process, the loading mechanism 10 maintains a constant temperature and controls the axial pressure to keep the piston 7 in its initial position, preventing upward or downward movement. Finally, with the first valve V1 and the third valve V3 closed, apply a small axial pressure increment of 20-50 kPa to the sample through the loading mechanism 10. Saturation is complete when the ratio of the pore pressure increment to the axial pressure increment is greater than 0.95; otherwise, open the third valve V3 to continue saturation.

[0051] The fourth step is the strain-controlled consolidation experiment.

[0052] The first valve V1 is always kept closed and the third valve V3 is always open, so that pore pressure changes occur at the bottom of the sample 17 during the experiment, and the top of the sample 17 is drained from one side. Therefore, the pore pressure at the top of the sample 17 is always equal to the back pressure applied by the back pressure pump 13. The loading mechanism 10 is started and a fixed strain rate is set to make the loading mechanism move upward at a constant speed. The strain rate of the loading mechanism is usually between 0.004 mm / min and 1.6 mm / min. During the uniform upward movement of the loading mechanism, a strain-controlled continuous loading consolidation experiment is carried out. The data acquisition module 4 continuously records the values ​​of the pressure sensor 14, displacement sensor 15 and pore pressure sensor 16 to calculate the compressibility index of the hydrate-bearing sediment.

[0053] After the experiment, the temperature was increased to decompose the hydrate in sample 17. The first valve V1 was opened, and the gas volume generated by the decomposition of hydrate was collected by the gas recovery instrument 3 to correct the calculation of hydrate saturation.

[0054] The above provides a detailed description of the experimental apparatus and method for strain-controlled continuous loading consolidation of hydrate-bearing sediments provided by this invention. Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of this invention. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from the principles of this invention, and these improvements and modifications also fall within the protection scope of the claims of this invention. The above description of the disclosed embodiments enables those skilled in the art to implement or use this invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of this invention. Therefore, this invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments, comprising a constant-temperature air bath (1) and a consolidation experimental machine, characterized in that, It also includes a consolidation reactor (2), a back pressure module and a gas recovery module. The consolidation test machine includes a reaction frame (9) and a loading mechanism (10). The loading mechanism (10) is installed at the bottom of the reaction frame (9). The consolidation reactor (2) is set on the loading mechanism (10). The consolidation test machine and the consolidation reactor (2) are both located in a constant temperature gas bath (1). The back pressure module and the gas recovery module are respectively connected to the consolidation reactor (2). The solidification reactor (2) includes a base (5), a pressure chamber (6), a piston (7), and a fixed flange (8). The pressure chamber (6) is fixedly installed on the base (5). The bottom of the pressure chamber (6) is fitted on the top outside of the base (5). The upper part of the inner cavity of the pressure chamber (6) is provided with a piston (7). The piston (7) is slidably disposed in the pressure chamber (6). The top of the pressure chamber (6) is fixedly connected to the fixed flange (8). A sample (17) is provided in the pressure chamber cavity between the piston (7) and the base (5). A metal permeable plate (19) is provided between the top surface of the sample (17) and the piston (7), and between the bottom surface of the sample (17) and the base (5). The base (5) is provided with a second channel (C2) and a third channel (C3). The base (5) is connected to the pore pressure sensor (16) through the second channel (C2), and the base (5) is connected to the gas recovery module through the third channel (C3). The gas recovery module includes a gas recovery device (3), which is connected to the third channel (C3) in the base (5) through a connecting pipe. A first valve (V1) is provided on the connecting pipe between the gas recovery device (3) and the base (5). During the consolidation experiment, the first valve (V1) is in the closed state. The back pressure module includes a methane cylinder (11), an air compressor (12), and a back pressure pump (13). The piston (7) is provided with a first channel (C1) for connection. The first channel (C1) is connected to the top surface of the sample. The methane cylinder (11), the air compressor (12), and the back pressure pump (13) are respectively connected to the first channel (C1). The methane cylinder (11) is connected to the first channel (C1) by a fourth valve (V4) and a second valve (V2), the air compressor (12) is connected to the first channel (C1) by a second valve (V2), and the back pressure pump (13) is connected to the first channel (C1) by a third valve (V3).

2. The strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments according to claim 1, characterized in that, A sealing ring (18) is provided between the inner surface of the pressure chamber (6) and the outer surface of the base (5), and a sealing ring (18) is provided between the outer surface of the piston (7) and the inner surface of the pressure chamber (6).

3. The strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments according to claim 1, characterized in that, The reaction frame (9) is also equipped with a pressure sensor (14) and a displacement sensor (15).

4. The strain-controlled continuous loading consolidation experimental apparatus for hydrate-bearing sediments according to claim 3, characterized in that, It also includes a data acquisition module (4), which is connected to a pressure sensor (14), a displacement sensor (15), and a pore pressure sensor (16), respectively.

5. A method for conducting experiments using the apparatus according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Fill the pressure chamber with sediment; S2. Preparation of hydrate-containing sediment samples: First, open the fourth valve, then open the second valve to allow methane gas from the methane cylinder to enter the pressure chamber, gradually increasing the pressure in the pressure chamber to 6MPa-15MPa. While the pressure is increasing, activate the loading mechanism to control the axial pressure and keep the piston in its initial position. After the pressure stabilizes, close the second and fourth valves in sequence, start the constant temperature gas bath, and cool down to 0-2°C. Hydrates will form in the pressure chamber. During the hydrate formation process, methane gas is continuously consumed, thereby reducing the gas pressure in the pressure chamber. Once the pressure stabilizes, the sample preparation is complete. S3. Sample saturation: Start the air compressor, open the second valve, and cycle the first valve to displace the residual methane gas in the connecting pipeline and sample, while maintaining a constant pressure in the pressure chamber. Then close the second valve and open the third valve, using a back pressure pump to inject deionized water into the sample to saturate it and reach the set back pressure, simulating the actual deep-sea water pressure conditions of 10-15 MPa. During the saturation process, maintain a constant temperature and control the axial pressure to keep the piston in its initial position. Finally, with the first and third valves closed, apply an axial pressure increment of 20-50 kPa to the sample through the loading mechanism. When the ratio of the pore pressure increment to the axial pressure increment is greater than 0.95, saturation is complete; otherwise, open the third valve to continue saturation. S4. Strain-controlled consolidation experiment: The first valve is kept closed and the third valve is kept open to induce pore pressure changes at the bottom of the sample. The top of the sample is drained from one side only, and the pore pressure at the top of the sample is always equal to the back pressure applied by the back pressure pump. The loading mechanism is started and a fixed strain rate is set to make the loading mechanism move upward at a constant speed. During the uniform upward movement of the loading mechanism, a strain-controlled continuous loading consolidation experiment is conducted. The values ​​of the pressure sensor, displacement sensor and pore pressure sensor are continuously recorded by the data acquisition module to calculate the compressibility index of the hydrate-bearing sediment.

6. The method according to claim 5, characterized in that, In step S1 above, the sediment is layered and filled into the pressure chamber according to the required water content. After the sediment is filled, the piston is first placed into the pressure chamber so that the bottom of the piston just contacts the sediment. The fixing flange is fixed to the upper surface of the pressure chamber. Finally, the entire consolidation reactor is moved onto the loading mechanism. The loading mechanism is started and the loading stops after the pressure sensor contacts the piston. The displacement sensor always contacts the fixing flange. The consolidation reactor is connected to the back pressure module, the gas recovery module, and the pore pressure sensor through connecting pipes, and the airtightness is checked.

7. The method according to claim 5, characterized in that, In step S4 above, the strain rate of the loading mechanism is 0.0001 mm / min - 3 mm / min.

8. The method according to claim 5, characterized in that, After step S4 is completed, the temperature is increased to decompose the hydrates in the sample. The first valve is opened, and the volume of gas produced by the decomposition of hydrates is collected using a gas recovery instrument to correct the calculation of hydrate saturation.

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