Natural gas hydrate sample preparation-triaxial mechanical experiment device and method

Abstract

The application belongs to the field of petroleum drilling engineering, and particularly relates to a natural gas hydrate sample preparation-triaxial mechanical experiment device and a preparation method. The natural gas hydrate sample preparation-triaxial mechanical experiment device comprises a support, and further comprises a hydrate synthesis unit, an acoustic emission signal processing unit, an axial displacement signal processing unit, a radial displacement signal processing unit, an axial pressure servo unit, a confining pressure servo unit, a gas supply unit, an exhaust and liquid discharge unit, a temperature controller and a computer system, wherein each unit is connected to the computer system and controls the operation of each unit by the computer system. The device can provide a suitable temperature and pressure environment for hydrate generation, can synthesize natural gas hydrate samples with different saturations and different shale contents indoors, and can perform mechanical experiments. The data acquisition system is complete, the test precision is high, the device has a moderate size, does not need to be additionally installed in a cold storage, is convenient for single-person operation, and has the characteristics of simple principle and convenient operation.

Description

Technical Field

[0001] This invention belongs to the field of oil drilling engineering, specifically relating to a triaxial mechanical experimental apparatus and method for the preparation of natural gas hydrate samples. Background Technology

[0002] Natural gas hydrates are cage-like solid crystalline compounds formed by the combination of methane and free water under high pressure and low temperature conditions. They are widely distributed in seafloor strata and permafrost zones. Studies have shown that natural gas hydrate resources are abundant and widely distributed, with conservative estimates suggesting that reservoir gas content exceeds 100 trillion cubic meters, more than twice the carbon content of conventional fossil fuels globally. Furthermore, natural gas hydrates are less polluting than conventional fossil fuels and are considered one of the most promising energy sources for the future. Therefore, researchers are dedicated to the exploration and development of natural gas hydrates. Due to the demanding phase equilibrium conditions and extremely high requirements for the occurrence environment of hydrates, obtaining hydrate sediment samples is extremely difficult. Given the insufficient number of in-situ hydrate samples, the use of large quantities of artificially synthesized hydrate sediment samples for mechanical experiments is particularly important.

[0003] In recent years, numerous methods for artificially preparing hydrate samples have emerged, primarily two: the mixed preparation method and the in-situ generation method. The mixed preparation method involves mixing and pressing pure natural gas hydrates with a sedimentary framework. This method has the advantages of short preparation time and uniform hydrate distribution, but it requires a high level of environmental control. The in-situ generation method involves preparing hydrate samples in a triaxial pressure chamber. This method has the advantages of shortening the experimental cycle and eliminating the need for an additional cold storage facility to provide ambient temperature, but it requires high-quality experimental equipment, and most existing equipment cannot simultaneously meet the needs of both methods. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a triaxial mechanical experimental apparatus for the preparation of natural gas hydrate samples, and to provide a sample preparation method and a triaxial mechanical experimental method, which can provide a suitable temperature and pressure environment for hydrate formation, and can synthesize natural gas hydrate sediment samples with different saturation and different mud content indoors, and is easy to operate.

[0005] The technical solution adopted is as follows:

[0006] A triaxial mechanical experimental apparatus for the preparation of natural gas hydrate samples includes a support frame, and further includes a hydrate synthesis unit, an acoustic emission signal processing unit, an axial displacement signal processing unit, a radial displacement signal processing unit, an axial pressure servo unit, a confining pressure servo unit, a gas supply unit, an exhaust and liquid drainage unit, a temperature controller, and a computer system.

[0007] The hydrate synthesis unit is mounted on a support and includes a reactor, an upper pressure head module, a lower pressure head module, and a copper tube temperature control system. The upper and lower pressure head modules are respectively installed at the top and bottom of the reactor via sealing threads. A capsule is installed in the center of the reactor, between the upper and lower pressure head modules. A confining pressure outlet and a confining pressure inlet are provided on the upper side wall of the reactor. The confining pressure inlet and outlet are connected to a confining pressure servo unit. The copper tube temperature control system is installed on the outside of the reactor and is connected to the temperature controller.

[0008] The upper pressure head module includes a top cover, a top piston for applying axial pressure, and a pressure chamber. The top cover is located at the top of the pressure chamber. The top piston passes through the top cover and the pressure chamber and extends downward, connecting with the capsule. The top piston has drainage and venting channels and acoustic emission signal wiring channels inside. The drainage and venting channels are connected to a drainage and venting unit, and an automatic valve is installed on the pipe between the drainage and venting channels and the drainage and venting unit. The acoustic emission signal wiring channels are connected to an acoustic emission signal processing unit. The upper end of the side wall of the pressure chamber is provided with an axial pressure interface, and the lower end is provided with an axial pressure circulation port. The axial pressure interface and the axial pressure circulation port are connected to the axial pressure servo unit.

[0009] A radial displacement sensor is installed on the outside of the capsule, and an axial displacement sensor is placed on top of the top piston. The radial displacement sensor is connected to the radial displacement processing unit, and the axial displacement sensor is connected to the axial displacement processing unit.

[0010] The pressure head module includes a rotating piston and a bottom cover. The rotating piston is installed in the center of the bottom cover and extends upward to connect to the bottom of the capsule. The rotating piston has an air inlet channel and an acoustic emission signal cable channel inside. The air inlet channel is connected to the air supply unit, and the acoustic emission signal cable channel is connected to the acoustic emission signal processing unit.

[0011] The exhaust and liquid discharge unit includes a waste liquid treatment unit and a waste gas treatment unit. The acoustic emission signal processing unit, axial pressure servo unit, confining pressure servo unit, gas supply unit, waste liquid treatment unit, waste gas treatment unit and temperature controller are connected to the computer system via wireless data signals or connecting lines, and the computer system controls the operation of each unit.

[0012] Preferably, the upper pressure head module is connected to the reactor via a sealing thread, and a sealing ring is provided at the connection.

[0013] Preferably, the top cover of the upper pressure head module is detachable and is connected to the upper pressure head module by screws. The top cover has a hole in the center that matches the top piston, and a sealing ring is provided at the contact point between the hole in the center of the top cover and the top piston.

[0014] Preferably, the top piston is provided with a disc in the inner section of the pressure chamber. The disc is integrally formed with the top piston. The size of the disc is adapted to the inner diameter of the pressure chamber and divides the pressure chamber into upper and lower chambers. The axial pressure interface and axial pressure circulation port are located on the side walls of the upper and lower chambers, respectively. A sealing ring is installed at the part of the disc that contacts the interior of the pressure chamber.

[0015] Preferably, the bottom cover of the pressure head module is adapted to the bottom of the reactor, and is screwed to the bottom of the reactor by a sealing thread, and a sealing ring is provided at the connection. The rotating piston is connected to the bottom cover by a sealing thread, and a sealing ring is provided at the connection. The rotating piston can be moved up and down by rotating.

[0016] Preferably, the top piston is adapted to the top opening of the capsule, and the rotating piston is adapted to the bottom opening of the capsule; the bottom surface of the top piston and the top surface of the rotating piston are respectively provided with grooves, and each groove is equipped with an acoustic probe, and the signal line of each acoustic probe is connected to the acoustic emission signal processing unit through an acoustic emission signal cable channel; a sealing ring is installed at the contact point between each acoustic probe and the groove.

[0017] Preferably, several radial displacement sensors are installed on the outside of the capsule, and the radial displacement sensors are evenly distributed and connected to the radial displacement signal processing unit via connecting lines; an axial displacement sensor is installed on the top of the top piston via a rotatable bracket, and the axial displacement sensor is connected to the axial displacement signal processing unit via connecting lines; the axial displacement signal processing unit and the radial displacement signal processing unit are connected to a computer system.

[0018] Preferably, the copper-tube temperature control system is composed of copper tubes wound around copper plates and connected to a temperature controller.

[0019] Preferably, the gas supply unit includes a gas flow meter and a gas cylinder. One end of the gas flow meter is connected to the air inlet channel, and the gas cylinder and computer system are also connected. An automatic valve is installed between the gas cylinder and the gas flow meter.

[0020] A method for preparing a triaxial mechanical experimental apparatus for natural gas hydrate sample preparation, using the aforementioned apparatus, includes the following steps:

[0021] Step 1. Determine the ratio of kaolin and quartz sand, weigh the required amount of both using an electronic balance, and place the weighed mixture in a container;

[0022] Step 2. Based on the required burial depth of the simulated strata, determine the compressive force and the porosity of the saturated core under the corresponding compressive force using the following formula.

[0023] F n =(ρ0·h0+ρ1·h1)·g·π·r2 (1)

[0024]

[0025] In the formula, F n The pressure required for sample preparation is kN; ρ0 is the density of seawater, g·cm³. -3 h0 is the hydrate burial depth, in meters; ρ1 is the density of the hydrate cap layer, in g·cm³. -3 h1 is the hydrate burial depth, in meters; g is the acceleration due to gravity, in meters per second. -2 ; r is the sample preparation radius, mm. This represents the porosity of marine soil, dimensionless; m3 is the mass of the undried marine soil sample, in g; m4 is the mass of the dried marine soil sample, in g; ρ w This is the density of pure water at standard atmospheric pressure and 3.98℃, in g·cm³. -3 h1 is the height of the marine soil sample, mm; d1 is the diameter of the marine soil sample, mm.

[0026] Step 3. In this patent, the formation of natural gas hydrates uses the excess gas method to control hydrate saturation. A certain mass of pure water is added during the preparation of the natural gas hydrate-containing sample to achieve the target hydrate saturation. The method for calculating the mass of pure water is as follows:

[0027]

[0028] Step 4. Mix the well-mixed sand with a certain proportion of water to form unsaturated wet sand. Then fill the wet sand into the capsule and compact it. Install several radial displacement sensors on the outside of the capsule.

[0029] Step 5. Place the capsule onto the lower pressure head module and place a sand-proof mesh on the retractable piston to prevent sand from entering the channel;

[0030] Step 6. Place the capsule from Step 5 and the pressure head module together into the reactor, and seal it using the sealing threads and sealing ring of the pressure head module;

[0031] Step 7. Place another sandproof mesh on top of the wet sand, and then screw the upper pressure head module into the upper part of the reactor. The sealing is completed by the sealing threads and sealing ring of the upper pressure head.

[0032] Step 8. Adjust the position of the axial displacement sensor so that it contacts the top piston. Then check the readings and records of the axial displacement sensor and radial displacement sensor using a computer to see if they are normal. Connect the acoustic emission signal processing unit to the computer and check if the readings and records are normal.

[0033] Step 9. Connect the air supply unit to the air inlet of the lower pressure head module, connect the exhaust and drainage unit to the exhaust and drainage channel of the upper pressure head module, and ensure that the screws at the connection are tightened.

[0034] Step 10. Use a computer-controlled confining pressure servo unit to raise the confining pressure to the set value at a fixed rate, and at the same time raise the axial pressure to the set value at the same rate. During this period, observe the changes in the readings of the axial displacement sensor and the radial displacement sensor. After the rock sample has solidified for the predetermined time, remove the axial pressure and confining pressure.

[0035] Step 11. Open the automatic valve for venting and draining liquid, use a computer-controlled gas flow meter to inject methane gas into the core to displace the air in the core, and then close the automatic valve for venting and draining liquid.

[0036] Step 12. Based on the required hydrate saturation, continuously inject methane gas into the core using a computer-controlled gas flow meter;

[0037] Step 13. Control the axial pressure servo unit to restrict the movement of the top piston, and then increase the confining pressure to the set value at a certain rate, while increasing the hole pressure to the set value at the same rate, to ensure that the final confining pressure is greater than the hole pressure.

[0038] Step 14. Turn on the copper tube temperature control system to lower the temperature inside the reactor to the set value and maintain it for a certain period of time. During this period, the formation of hydrates can be observed through sound wave signals, gas consumption and pressure changes inside the reactor.

[0039] Step 15. After the hydrate is formed, conduct relevant triaxial mechanical experiments on the hydrate according to the confining pressure and axial pressure set in the experiment.

[0040] Step 16. After all tests are completed, turn off the gas flow meter and copper tube temperature control system, move the top piston up to remove the axial pressure, open the exhaust and liquid discharge valve to remove the orifice pressure, and finally remove the confining pressure through the confining pressure servo unit. Disconnect the pipeline at the connection point, take out the upper pressure head module, and then take out the capsule together with the lower pressure head module. Finally, take out the core from the capsule for observation and photography.

[0041] The triaxial mechanical experiment in step 15 involves determining the hydrate formation based on acoustic signals, gas consumption, and pressure changes within the reactor. Simultaneously, based on the natural gas hydrate phase equilibrium curve, the experiment uses conditions of 0-0.5℃ and 4-6MPa pore pressure to generate hydrates. Therefore, the sample filled into the capsule is subjected to controlled temperature and pressure in the reactor to generate hydrates. The hydrated sample is then consolidated under effective confining pressure for 8-10 hours. Next, the hydrated sample is sheared at a strain rate of 0.1% / min. During the experiment, axial and radial displacement sensors installed around the capsule are used to characterize the effect of the shearing process on the sample deformation until the desired results are achieved, at which point the experiment ends.

[0042] Compared with the prior art, the advantages of the present invention are:

[0043] This invention provides a triaxial mechanical experimental apparatus and preparation method for natural gas hydrate sample preparation, which can provide a suitable temperature and pressure environment for hydrate formation, synthesize natural gas hydrate sediment samples with different saturations and different clay contents indoors, and conduct mechanical experiments. It has a complete data acquisition system and high testing accuracy.

[0044] The preparation device provided by this invention has a moderate size, does not require additional cold storage, is easy for single-person operation, and has the characteristics of simple principle and convenient operation. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the overall structure of a triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to the present invention;

[0046] Figure 2 This is a cross-sectional view of the upper pressure head module described in this invention;

[0047] Figure 3 This is a cross-sectional view of the pressure head module described in this invention;

[0048] Figure 4 These are cross-sectional and top views of the copper tube winding described in this invention;

[0049] Figure 5 This is a cross-sectional view of the capsule described in this invention;

[0050] Figure 6 This is the phase equilibrium curve of natural gas hydrate (5MPa, 0.5℃) of the present invention.

[0051] The components include: 1. Support frame; 2. Computer system; 3. Acoustic emission signal cable channel; 4. Top piston; 5. Screw; 6. Pressure chamber; 7. Sealing thread; 8. Confining pressure outlet; 9. Sealing ring; 10. Capsule; 11. Acoustic probe; 12. Copper tube temperature control system; 13. Reactor; 14. Radial displacement sensor; 15. Rock sample; 16. Sandproof mesh; 17. Automatic valve; 18. Upper pressure head module; 19. Acoustic emission signal processing unit; 20. Waste liquid. Processing unit; 21. Axial pressure servo unit; 22. Confining pressure servo unit; 23. Temperature controller; 24. Radial displacement signal processing unit; 25. Gas cylinder; 26. Gas flow meter; 27. Air inlet channel; 28. Lower pressure head module; 29. ​​Axial pressure interface; 30. Exhaust and liquid drainage channel; 31. Axial displacement sensor; 32. Axial pressure circulation port; 33. Confining pressure inlet; 34. Axial displacement signal processing unit; 35. Waste gas treatment unit; 36. Top cover; 37. Rotating piston; 38. Copper tube coil. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention, and that the described embodiments are only a part of the embodiments of the invention, not all of them. Some components in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product; for those skilled in the art, some well-known structures and their descriptions in the drawings may be omitted; the terms "upper," "lower," "top," "bottom," "side," "end," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention.

[0053] like Figure 1 As shown, a triaxial mechanical experimental apparatus for the preparation of natural gas hydrate samples includes a support 1, a hydrate synthesis unit, an acoustic emission signal processing unit 19, an axial displacement signal processing unit 34, a radial displacement signal processing unit 24, an axial pressure servo unit 21, a confining pressure servo unit 22, a gas supply unit, an exhaust and liquid discharge unit, a temperature controller 23, and a computer system 2. The exhaust and liquid discharge unit further includes a waste liquid treatment unit 20 and a waste gas treatment unit 35. The acoustic emission signal processing unit 19, the axial pressure servo unit 21, the confining pressure servo unit 22, the gas supply unit, the waste liquid treatment unit 20, the waste gas treatment unit 35, and the temperature controller 23 are connected to the computer system 2 via wireless data signals or connecting cables, and the computer system 2 controls the operation of each unit.

[0054] The hydrate synthesis unit is mounted on a support 1 and includes a reactor 13, an upper pressure head module, a lower pressure head module 28, and a copper tube temperature control system 12. The upper pressure head module and the lower pressure head module 28 are respectively installed at the top and bottom of the reactor 13 via sealing threads 7. A capsule 10 is installed in the center of the reactor 13, between the upper pressure head module and the lower pressure head module 28. A confining pressure outlet 8 and a confining pressure inlet 33 are provided on the upper side wall of the reactor 10. The confining pressure inlet 33 and the confining pressure outlet 8 are connected to the confining pressure servo unit 22. The copper tube temperature control system 12 is installed on the outside of the reactor 13 and is connected to the temperature controller 23.

[0055] like Figure 2 As shown, the upper pressure head module 18 includes a top cover 36, a top piston 4 for applying axial pressure, and a pressure chamber 6. The top cover 36 and the top piston 4 are connected by screws 5 to form the pressure chamber 6. The top cover covers the top of the pressure chamber 6. The top piston 4 passes through the top cover 36 and the pressure chamber 6 and extends downward, connecting with the capsule. The top piston 4 has a drain and exhaust channel 30 and an acoustic emission signal cable channel 3 inside. The waste liquid treatment unit 20 and the waste gas treatment unit 35 are connected to the drain and exhaust channel 30. The axial pressure servo unit 21 is connected to the pressure chamber 6. An automatic valve 17 is installed on the pipe between the exhaust and drain channel 30 and the exhaust and drain unit. The acoustic emission signal cable channel is connected to the acoustic emission signal processing unit. The upper end of the side wall of the pressure chamber is provided with an axial pressure interface 29, and the lower end is provided with an axial pressure circulation port 32. The axial pressure interface 29 and the axial pressure circulation port 32 are connected to the axial pressure servo unit 21.

[0056] like Figure 3 As shown, the pressure head module 28 includes a rotating piston 37 and a bottom cover. The rotating piston 37 is installed in the center of the bottom cover and extends upward to connect to the bottom of the capsule 10. The rotating piston 37 has an air inlet channel 27 and an acoustic emission signal cable channel inside. The air inlet channel 27 is connected to the air supply unit. One acoustic probe 11 in the acoustic emission signal processing unit 19 is placed in the groove on the bottom end face of the top piston 4, and another acoustic probe 11 is placed in the groove on the top end face of the rotating piston 37 of the pressure head module 28. Each acoustic probe 11 is exposed or at least flush with the end face of its groove, and is used to monitor the formation of hydrates in the rock sample. The connecting wires of each acoustic probe 11 are connected to the acoustic emission signal processing unit 19 through the acoustic emission signal cable channel 3.

[0057] like Figure 1 , 5The capsule 10 is tubular, with its top and bottom ends connected to the upper pressure head module 18 and lower pressure head module 28 respectively. The connecting parts are adapted to achieve a tight connection, separating the hydraulic oil from the rock sample 15. Four radial displacement sensors 14 are installed on its outer side, and an axial displacement sensor 31 is positioned at the top of the top piston 4. The capsule 10 is placed in the center of the reaction vessel 13, with its lower part connected to the rotating piston 37 of the lower pressure head module 28. The air inlet 27 is connected to the gas flow meter 26. Figure 4 As shown, the copper tube temperature control system 12 is placed outside the reactor 13. The copper tube temperature control system 12 is composed of a copper tube coil 38 wound around a copper plate and is connected to the temperature controller 23.

[0058] When axial pressure is applied, the computer system 2 controls the axial pressure servo unit 21 to inject hydraulic oil into the pressure chamber 6 through the axial pressure interface 29. The top piston 4 moves downward under the action of the hydraulic oil, thereby achieving the purpose of applying axial pressure. When hydraulic oil is injected through the axial pressure circulation port 32, the top piston 4 moves upward, thereby removing the axial pressure. This design can simplify operation to the greatest extent and save time in raising and lowering the top piston 4. The axial pressure servo unit 21 and the confining pressure servo unit 22 can realize the in-situ consolidation of the rock sample 15. Controlled by the gas flow meter 26, the gas required for hydrate generation is injected into the rock core 15. The copper tube temperature control system 12 can provide the temperature conditions required for hydrate generation.

[0059] Figure 2 and Figure 1 Correspondingly, the venting and draining channel 30 and the acoustic emission signal cable channel 3 are located in the top piston 4. The venting and draining channel 30 is used to remove gas and liquid from the capsule 10 after the experiment, and the acoustic emission signal cable channel 3 is used to store the connection cable of the acoustic probe. The sealing thread 7 tightly connects the upper pressure head module and the reactor. All the sealing rings 9 shown in the figure are used for pressure sealing to prevent pressure leakage of the device.

[0060] Figure 3 and Figure 1 Correspondingly, the air inlet channel 29 and the acoustic emission signal cable channel 3 are located in the rotating piston 37. The air inlet channel 27 is used to inject hydrate into the rock sample 15 to generate the required gas, and the acoustic emission signal cable channel 3 is used to store the data transmission line of the acoustic probe. The sealing thread 7 tightly connects the pressure head module and the reactor. All the sealing rings 9 shown in the figure are used for pressure sealing to prevent pressure leakage in the device.

[0061] Figure 4 This is a cross-sectional view of the copper tube winding described in this invention. This design is for more precise control of the temperature of the reactor. The copper tube temperature control system 12 is connected to the temperature controller 23, which can control the cooling of the reactor 13. The temperature controller 23 is connected to the computer system 2, which monitors the temperature of the reactor 13 in real time.

[0062] Furthermore, there are four radial displacement sensors 14, which are evenly installed around the capsule 10 along the axial direction. The axial displacement sensor 31 is placed on top of the top piston 4. The purpose is to monitor and record the deformation process of the rock sample 15 during consolidation and to collect the deformation of the rock sample 15 during mechanical experiments. This allows for more detailed and comprehensive collection of information on the deformation development of the rock sample. Preferably, the amount of liquid added to the dry rock sample is determined by the hydrate saturation required by the specific experimental design.

[0063] This invention also provides a method for preparing a natural gas hydrate sample, using the above-mentioned apparatus, and the steps include:

[0064] Step 1: Determine the ratio of kaolin and quartz sand, weigh the required amounts of both using an electronic balance, and mix them in a container;

[0065] Step 2: Based on the required burial depth of the simulated strata, determine the compressive force and the porosity of the saturated core under the corresponding compressive force using the following formula.

[0066] F n =(ρ0·h0+ρ1·h1)·g·π·r 2 (1)

[0067]

[0068] In the formula, F n The pressure required for sample preparation is kN; ρ0 is the density of seawater, g·cm³. -3 h0 is the hydrate burial depth, in meters; ρ1 is the density of the hydrate cap layer, in g·cm³. -3 h1 is the hydrate burial depth, in meters; g is the acceleration due to gravity, in meters per second. -2 ; r is the sample preparation radius, mm. This represents the porosity of marine soil, dimensionless; m3 is the mass of the undried marine soil sample, in g; m4 is the mass of the dried marine soil sample, in g; ρ w This is the density of pure water at standard atmospheric pressure and 3.98℃, in g·cm³. -3 h1 is the height of the marine soil sample, mm; d1 is the diameter of the marine soil sample, mm.

[0069] Step 3: In this patent, the formation of natural gas hydrates uses an excess gas method to control hydrate saturation. A certain mass of pure water is added during the preparation of the natural gas hydrate sample to achieve the target hydrate saturation. The method for calculating the mass of pure water is as follows:

[0070]

[0071] Step 4: Mix the well-mixed sand with a certain proportion of water to form unsaturated wet sand, then fill the capsule with the wet sand and compact it, and install the radial displacement sensor on the outside of the capsule.

[0072] Step 5: Place the capsule onto the lower pressure head module and place a sand-proof mesh on the retractable piston to prevent sand from entering the channel;

[0073] Step 6: Place the capsule from Step 5 and the pressure head module together into the reactor, and seal it using the threads and sealing ring of the pressure head module;

[0074] Step 7: Place another sandproof mesh on top of the wet sand, and then screw the upper pressure head module into the upper part of the reactor. The upper pressure head thread and sealing ring will complete the sealing.

[0075] Step 8: Adjust the position of the axial displacement sensor so that it contacts the top piston. Then check the readings and records of the axial displacement sensor and radial displacement sensor using a computer to see if they are normal. Connect the acoustic emission signal processing unit to the computer and check if the readings and records are normal.

[0076] Step 9: Connect the air supply unit to the air inlet of the lower pressure head module, connect the exhaust and drainage unit to the exhaust and drainage channel of the upper pressure head module, and ensure that the screws at the connection are tightened.

[0077] Step 10: Use a computer-controlled confining pressure servo unit to increase the confining pressure to the set value at a fixed rate, and at the same time increase the axial pressure to the set value at the same rate. During this period, observe the changes in the readings of the axial displacement sensor and the radial displacement sensor. After the rock sample has solidified for the predetermined time, remove the axial pressure and confining pressure.

[0078] Step 11: Open the automatic valve for venting and draining liquid, use a computer-controlled gas flow meter to inject methane gas into the core to displace the air in the core, and then close the automatic valve for venting and draining liquid.

[0079] Step 12: Based on the required hydrate saturation, continuously inject methane gas into the core using a computer-controlled gas flow meter;

[0080] Step 13: Control the axial pressure servo unit to restrict the movement of the top piston, and then increase the confining pressure to the set value at a certain rate, while increasing the hole pressure to the set value at the same rate (the hole pressure is generated when the axial pressure is applied, and the hole pressure will increase accordingly when the axial pressure is applied), to ensure that the final confining pressure is greater than the hole pressure.

[0081] Step 14: Turn on the copper tube temperature control system to lower the temperature inside the reactor to the set value and maintain it for a certain period of time. During this period, the formation of hydrates can be observed through sound wave signals, gas consumption and pressure changes inside the reactor.

[0082] Step 15: After the hydrate is formed, conduct relevant triaxial mechanical experiments on the hydrate according to the confining pressure and axial pressure set in the experiment.

[0083] Step 16: After all tests are completed, turn off the gas flow meter and copper tube temperature control system, move the top piston up to remove the axial pressure, open the exhaust and liquid discharge valve to remove the orifice pressure, and finally remove the confining pressure through the confining pressure servo unit. Disconnect the pipeline at the connection point, take out the upper pressure head module, and then take out the capsule together with the lower pressure head module. Finally, take out the core from the capsule for observation and photography.

[0084] like Figure 6 As shown, the triaxial mechanical experiment determines the hydrate formation based on the acoustic signal, gas consumption, and pressure changes within the reactor 13. Simultaneously, based on the natural gas hydrate phase equilibrium curve (a pressure phase equilibrium curve can be plotted in the system based on the corresponding temperature; inputting temperature and pressure into the system will mark the corresponding points), the experiment uses experimental conditions of 0-0.5℃ and pore pressure of 4-6MPa to generate hydrates. Therefore, the sample filled into capsule 10 is subjected to controlled temperature and pressure in reactor 13 to generate hydrates. The hydrate sample is consolidated under effective confining pressure for 8-10 hours, and then sheared at a strain rate of 0.1% / min. During the experiment, axial displacement sensors 31 and radial displacement sensors 14 installed around capsule 10 characterize the effect of the shearing process on the sample deformation until the corresponding requirements are met, at which point the experiment ends.

[0085] In the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," etc., should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections. Unless otherwise specified, the specific connection methods of each part adopt conventional methods that are mature in the prior art, such as bolts, rivets, and welding. In addition, the circuit connections also adopt conventional connection methods in the prior art, and will not be described in detail here.

[0086] Furthermore, it should be understood that although this specification describes the embodiments, the inventive device does not necessarily contain only one independent technical solution. This description method is merely for clarity. Those skilled in the art should consider the specification as a whole. The technical solutions in the embodiments can also be appropriately combined to form other embodiments that can be understood by those skilled in the art, which are also within the scope of protection of this invention.

Claims

1. A triaxial mechanical experimental apparatus for the preparation of natural gas hydrate samples, comprising a support frame, characterized in that, It also includes a hydrate synthesis unit, an acoustic emission signal processing unit, an axial displacement signal processing unit, a radial displacement signal processing unit, an axial pressure servo unit, a confining pressure servo unit, an air supply unit, an exhaust and liquid discharge unit, a temperature controller, and a computer system; The hydrate synthesis unit is mounted on a support and includes a reactor, an upper pressure head module, a lower pressure head module, and a copper tube temperature control system. The upper and lower pressure head modules are respectively installed at the top and bottom of the reactor via sealing threads. A capsule is installed in the center of the reactor, between the upper and lower pressure head modules. A confining pressure outlet and a confining pressure inlet are provided on the upper side wall of the reactor. The confining pressure inlet and outlet are connected to a confining pressure servo unit. The copper tube temperature control system is installed on the outside of the reactor and is connected to the temperature controller. The upper pressure head module includes a top cover, an exhaust and drainage channel, a top piston for applying axial pressure, and a pressure chamber. The top cover is located at the top of the pressure chamber. The top piston passes through the top cover and the pressure chamber and extends downward, connecting with the capsule. The top piston has drainage and exhaust channels and acoustic emission signal wiring channels inside. The exhaust and drainage channels are connected to an exhaust and drainage unit, and an automatic valve is installed on the pipe between the exhaust and drainage channels and the exhaust and drainage unit. The acoustic emission signal wiring channels are connected to an acoustic emission signal processing unit. An axial pressure interface is provided at the upper end of the side wall of the pressure chamber, and an axial pressure circulation port is provided at the lower end. The axial pressure interface and the axial pressure circulation port are connected to the axial pressure servo unit. A radial displacement sensor is installed on the outside of the capsule, and an axial displacement sensor is placed on top of the top piston. The radial displacement sensor is connected to the radial displacement processing unit, and the axial displacement sensor is connected to the axial displacement processing unit. The pressure head module includes a rotating piston and a bottom cover. The rotating piston is installed in the center of the bottom cover and extends upward to connect to the bottom of the capsule. The rotating piston has an air inlet channel and an acoustic emission signal cable channel inside. The air inlet channel is connected to the air supply unit, and the acoustic emission signal cable channel is connected to the acoustic emission signal processing unit. The acoustic emission signal processing unit, axial pressure servo unit, confining pressure servo unit, air supply unit, exhaust and liquid discharge unit, and temperature controller are connected to the computer system via wireless data signals or connecting cables, and the computer system controls the operation of each unit.

2. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The upper pressure head module is connected to the reactor via a sealing thread, and a sealing ring is provided at the connection.

3. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The top cover of the upper pressure head module is detachable and is connected to the upper pressure head module by screws. The top cover has a hole in the center that matches the top piston, and a sealing ring is provided at the contact point between the hole in the center of the top cover and the top piston.

4. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The top piston has a disc installed in the inner section of the pressure chamber. The disc is integrally formed with the top piston. The size of the disc is adapted to the inner diameter of the pressure chamber and divides the pressure chamber into upper and lower chambers. The axial pressure interface and axial pressure circulation port are located on the side walls of the upper and lower chambers, respectively. A sealing ring is installed at the part of the disc that contacts the interior of the pressure chamber.

5. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The bottom cover of the pressure head module is adapted to the bottom of the reactor and is screwed to the bottom of the reactor through a sealing thread. A sealing ring is provided at the connection. The rotating piston is connected to the bottom cover through a sealing thread and a sealing ring is provided at the connection. The rotating piston can be moved up and down by rotating.

6. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The top piston is adapted to the top opening of the capsule, and the rotating piston is adapted to the bottom opening of the capsule; the bottom surface of the top piston and the top surface of the rotating piston are respectively provided with grooves, and each groove is equipped with an acoustic wave probe. The signal line of each acoustic wave probe is connected to the acoustic emission signal processing unit through the acoustic emission signal cable channel; a sealing ring is installed at the part of each acoustic wave probe that contacts the groove.

7. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, Several radial displacement sensors are installed on the outside of the capsule, and the radial displacement sensors are evenly distributed and connected to the radial displacement signal processing unit through connecting lines; an axial displacement sensor is installed on the top of the top piston through a rotatable bracket, and the axial displacement sensor is connected to the axial displacement signal processing unit through connecting lines; the axial displacement signal processing unit and the radial displacement signal processing unit are connected to a computer system.

8. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 7, characterized in that, The copper tube temperature control system consists of copper tubes wound with copper plates and connected to a temperature controller.

9. The triaxial mechanical experimental apparatus for preparing natural gas hydrate samples according to claim 1, characterized in that, The gas supply unit includes a gas flow meter and a gas cylinder. One end of the gas flow meter is connected to the air inlet channel, and the other end is connected to the gas cylinder and a computer system. An automatic valve is installed between the gas cylinder and the gas flow meter.

10. A method for preparing a natural gas hydrate sample, using the natural gas hydrate sample preparation-triaxial mechanical experimental apparatus according to any one of claims 1 to 9, characterized in that, The method includes the following steps: Step 1. Determine the ratio of kaolin and quartz sand, weigh the required amount of both using an electronic balance, and place the weighed mixture in a container; Step 2. Based on the required burial depth of the simulated strata, determine the compressive force and the porosity of the saturated core under the corresponding compressive force using the following formula. (1)； (2)； In the formula, F n This is the pressing force required for sample preparation, in kN; ρ 0 is the density of seawater, in g·cm³. -3 ; h 0 represents the hydrate burial depth, in meters (m). ρ 1 represents the density of the hydrate cap layer, in g·cm³. -3 ; h 1 represents the hydrate burial depth, in meters (m). g It is the acceleration due to gravity, m·s -2 ; r The sample preparation radius is in mm. φ 1 represents the porosity of marine soil, which is dimensionless; m 3 represents the mass of the undried marine soil sample, in grams; m 4 represents the mass of the dried marine soil, in grams; ρ w This is the density of pure water at standard atmospheric pressure and 3.98℃, in g·cm³. -3 ; h 1 represents the height of the marine soil sample, in mm; d 1 is the diameter of the marine soil sample, in mm; Step 3. Natural gas hydrate formation: The excess gas method is used to control the hydrate saturation. When preparing the natural gas hydrate-containing sample, a certain mass of pure water is added to achieve the target hydrate saturation. The method for calculating the mass of pure water is as follows: (3)； Step 4. Mix the well-mixed sand with a certain proportion of water to form unsaturated wet sand. Then fill the wet sand into the capsule and compact it. Install several radial displacement sensors on the outside of the capsule. Step 5. Place the capsule onto the lower pressure head module and place a sandproof mesh on the retractable piston; Step 6. Place the capsule from Step 5 and the pressure head module together into the reactor, and seal it using the sealing threads and sealing ring of the pressure head module; Step 7. Place another sand-proof net on top of the wet sand, then screw the upper pressure head module into the upper part of the reactor; Step 8. Adjust the position of the axial displacement sensor so that it contacts the top piston. Then check the readings and records of the axial displacement sensor and radial displacement sensor through the computer to see if they are normal. Connect the acoustic emission signal processing unit to the computer. Step 9. Connect the air supply unit to the air inlet of the lower pressure head module and connect the exhaust and drainage unit to the exhaust and drainage channel of the upper pressure head module. Step 10. Use a computer-controlled confining pressure servo unit to increase the confining pressure to the set value at a fixed rate, and at the same time increase the axial pressure to the set value at the same rate. During this period, observe the changes in the readings of the axial displacement sensor and the radial displacement sensor. After the rock sample has solidified for the predetermined time, remove the axial pressure and confining pressure. Step 11. Open the automatic valve for venting and draining liquid, use a computer-controlled gas flow meter to inject methane gas into the core to displace the air in the core, and then close the automatic valve for venting and draining liquid. Step 12. Based on the required hydrate saturation, continuously inject methane gas into the core using a computer-controlled gas flow meter; Step 13. Control the axial pressure servo unit to restrict the movement of the top piston, and then increase the confining pressure to the set value at a certain rate, while increasing the hole pressure to the set value at the same rate, to ensure that the final confining pressure is greater than the hole pressure. Step 14. Turn on the copper tube temperature control system to lower the temperature inside the reactor to the set value and maintain it for a certain period of time. During this period, the formation of hydrates can be observed through sound wave signals, gas consumption and pressure changes inside the reactor. Step 15. After the hydrate is formed, conduct relevant triaxial mechanical experiments on the hydrate according to the confining pressure and axial pressure set in the experiment. Step 16. After all tests are completed, turn off the gas flow meter and copper tube temperature control system, move the top piston up to remove the axial pressure, open the exhaust and liquid discharge valve to remove the orifice pressure, and finally remove the confining pressure through the confining pressure servo unit. Disconnect the pipeline at the connection point, take out the upper pressure head module, and then take out the capsule together with the lower pressure head module. Finally, take out the core from the capsule for observation and photography.

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