Hydrate sediment fracture toughness testing system

Abstract

The invention discloses a hydrate sediment fracture toughness testing system, and belongs to the technical field of hydrate sediment fracture toughness testing, the system comprises a sample synthesis unit, a three-point bending test unit and a direct shear test unit, the three-point bending test unit is detachably connected to the sample synthesis unit, and the direct shear test unit is detachably connected to the sample synthesis unit. The method is used for measuring the I-type fracture toughness of the hydrate sediment; the direct shear test unit can be detached to replace the three-point bending test unit, is connected to the sample synthesis unit and is used for measuring the II-type fracture toughness of the hydrate sediment. In the testing process, a horizontal loading mode is adopted, so that the influence of gravity on fracture toughness is eliminated. In order to reduce the influence of friction force on fracture toughness, the system adopts a design scheme of a three-point bending bracket, a sliding base and a guide rail. In addition, the hydrate has the characteristic of being easy to decompose at normal temperature and normal pressure, the sample synthesis unit can realize synthesis of the hydrate, and it is ensured that the hydrate is not decomposed in the testing process.

Description

Technical Field

[0001] The invention discloses a hydrate sediment fracture toughness testing system, belonging to the technical field of hydrate sediment fracture toughness testing. Background Art

[0002] Natural gas hydrates, with their high energy density, low carbon footprint, and clean character, are considered a promising alternative to traditional fossil fuels. Improving gas production rates and ensuring stable production over the long term are key challenges facing the commercialization of natural gas hydrates. Improving the permeability of hydrate sediment reservoirs is crucial for increasing hydrate decomposition capacity, gas production efficiency, and gas production rates. Hydraulic fracturing is a key technology for improving reservoir permeability in unconventional gas reservoirs, such as ultra-low permeability shale gas and tight gas reservoirs, and is a highly promising technology for increasing production in hydrate reservoirs.

[0003] The fracture mechanics properties of hydrate sediments are a key foundation for the theory and technology of hydrate reservoir fracturing. They play a crucial role in studying the initiation and propagation behavior of hydrate reservoir fractures, fracture propagation morphology, and production stimulation. Due to the unique properties of hydrate sediments, such as low mechanical strength and easy decomposition at room temperature and pressure, there is currently no testing system capable of measuring the Type I and Type II fracture toughness of hydrate sediments. Therefore, a new technical solution is urgently needed to address this issue. Summary of the Invention

[0004] To address the problems of the prior art, the present invention aims to provide a hydrate sediment fracture toughness testing system. During the test, horizontal loading is employed to eliminate the influence of gravity on fracture toughness. To reduce the effects of friction on fracture toughness, the hydrate sediment fracture toughness testing system utilizes a three-point bending bracket, a sliding base, and guide rails. Furthermore, hydrates readily decompose at room temperature and pressure. The sample synthesis unit is capable of synthesizing hydrates and ensuring that they do not decompose during testing, thus avoiding the impact of hydrate decomposition on the mechanical properties of the hydrate sediment.

[0005] To achieve the above object, the present invention adopts the following technical solution: A fracture toughness test system for hydrate sediments, comprising a specimen synthesis unit, the specimen synthesis unit including a reaction kettle, and the system further including a three-point bending test unit and a direct shear test unit that are interchangeably connected to the specimen synthesis unit; the three-point bending test unit includes a three-point bending bracket, a triangular bottom plate, a steel rod, a first top cover, a first horizontal loading mechanism, a first data acquisition instrument, and a screw; the bottom of the three-point bending bracket is provided with a ball socket connection groove, and a ball roller is installed in the ball socket connection groove. The number of three-point bending brackets is two and they are symmetrically arranged. The two three-point bending brackets form an integral structure. The side surface of the three-point bending bracket corresponds to the position of the fixed bracket to achieve positioning; the triangular bottom plate is arranged between the two three-point bending brackets, and it cooperates with the bottom plates of the three-point bending brackets to form a rectangular bottom plate; the steel rod penetrates through the side wall of the reaction kettle and locks the triangular bottom plate and the three-point bending bracket; the screw penetrates through the top of the reaction kettle and the first top cover and then cooperates with the bearing at the top of the three-point bending bracket; the first horizontal loading mechanism is horizontally arranged and abuts against the side baffle of the first top cover; the first data acquisition instrument is communicatively connected to the first horizontal loading mechanism; the direct shear test unit includes a second base, a cubic shear box, a second top cover, a second horizontal loading mechanism, and a second data acquisition instrument. The cubic shear box is formed by splicing a fixed base and a sliding base. The fixed base is fixedly connected to the second base, and the sliding base is slidably connected to the second base through a guide rail; the second top cover is a split structure and is respectively fixed above the fixed base and the sliding base; the second horizontal loading mechanism is horizontally arranged and abuts against the sliding base; the second data acquisition instrument is communicatively connected to the second horizontal loading mechanism.

[0006] Further, the first horizontal loading mechanism includes a first hydraulic cylinder, a first force sensor, and a pressure head that are sequentially connected. The pressure head is horizontally arranged and abuts against the side baffle of the first top cover; the first force sensor is communicatively connected to the first data acquisition instrument.

[0007] Further, the number of fixed brackets is the same as the number of three-point bending brackets and they correspond one by one. The fixed brackets and the first base are welded to form a rigid support structure; the first base is detachably fixed to the bottom of the reaction kettle; the side baffle of the three-point bending bracket and the fixed bracket form a positioning mating surface; the triangular bottom plate and the bottom plates of the three-point bending brackets on both sides are spliced into a complete bearing surface; the first top cover realizes vertical displacement control through a screw with a threaded fit.

[0008] Further, both ends of the triangular bottom plate are provided with threaded holes, and the bottom plate of the three-point bending bracket is provided with corresponding round holes. The steel rod sequentially passes through the side wall of the reaction kettle, the threaded holes of the triangular bottom plate, and the round holes on the bottom plate of the three-point bending bracket to achieve locking.

[0009] Further, the second horizontal loading mechanism includes a second force sensor and a second hydraulic cylinder connected in sequence. The second force sensor is horizontally arranged and abuts against the sliding base, and the second force sensor is communicatively connected to a second data acquisition instrument.

[0010] Further, a thermometer and a pressure gauge are connected to the top of the reaction kettle.

[0011] Further, the second base is detachably fixed to the bottom of the reaction kettle.

[0012] Further, the sample synthesis unit further includes a gas supply module, a vacuum pumping module, a constant temperature control module, and an exhaust gas metering module: The gas supply module includes a high-pressure gas cylinder, a gas booster pump, a first gas flowmeter, a first valve, a second valve, and an intermediate storage tank. The high-pressure gas cylinder, the gas booster pump, the first gas flowmeter, the first valve, and the intermediate storage tank are connected in sequence through pipelines, and the intermediate storage tank communicates with the reaction kettle through the second valve; The vacuum pumping module includes a third valve and a vacuum pump, and the third valve is installed on the connecting pipeline between the reaction kettle and the vacuum pump; The constant temperature control module includes a constant temperature water bath tank covering the intermediate storage tank and the reaction kettle; The exhaust gas metering module includes a fourth valve, a pneumatic back pressure valve, a gas-water separator, a second gas flowmeter, and a gas collecting bottle. The fourth valve is arranged at the gas outlet end of the reaction kettle, and the fourth valve, the pneumatic back pressure valve, the gas-water separator, the second gas flowmeter, and the gas collecting bottle are connected in sequence through pipelines.

[0013] Further, the first hydraulic cylinder is connected to a first hydraulic system.

[0014] Further, the second hydraulic cylinder is connected to a second hydraulic system.

[0015] The beneficial effects of the hydrate sediment fracture toughness test system provided by the present invention are as follows: Different from traditional fracture toughness test systems, hydrate sediments have the characteristics of low strength, resulting in the non-negligible influence of gravity on fracture toughness. By adopting a horizontal loading method, the influence of gravity is eliminated. To reduce friction, the three-point bending test unit is designed with ball pulleys. After the hydrate sediment specimen is synthesized, the triangular bottom plate can be separated from the bottom plate of the three-point bending bracket by pulling the steel rod, and then a horizontal load is applied through the first horizontal loading mechanism. The fixed bracket and the three-point bending bracket are only in close contact without connection, and the fixed bracket can act as a reaction force. Due to the integrated structure formed by symmetrically arranging two three-point bending brackets on the left and right, under the action of force, the specimen bends, and the three-point bending bracket rotates at both ends with the contact point between the vertex of the triangular bottom plate and the bottom plate of the three-point bending bracket as the center of the circle, playing a role in reducing friction. The direct shear test unit adopts a sliding base and guide rail scheme. In addition, hydrates are prone to decomposition under normal pressure and temperature conditions, and hydrates have a significant impact on the mechanical properties of hydrate sediments. The specimen synthesis unit can realize the synthesis of hydrates and ensure that hydrates do not decompose during the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the application of the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention to the type I fracture toughness test; Figure 2 Schematic diagram when the three-point bending test unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention is combined with the reaction kettle; Figure 3 Schematic diagram of the three-point bending test unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention; Figure 4 Partial assembly drawing of the three-point bending test unit; Figure 5 Is Figure 4 Partial schematic diagram of; Figure 6 Schematic diagram when the direct shear test unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention is combined with the reaction kettle; Figure 7 Schematic diagram of the direct shear test unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention from one perspective; Figure 8 Schematic diagram of the direct shear test unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention from another perspective; Figure 9 Assembly schematic diagram of the specimen synthesis unit in the hydrate sediment fracture toughness test system proposed in the embodiment of the present invention; Figure 10 It is a schematic diagram of the mechanical model for the three-point bending test of the fracture toughness of hydrate sediments of type I; Figure 11 It is a schematic diagram of the mechanical model for the direct shear test of the fracture toughness of hydrate sediments of type II.

[0017] The markings in the figure are as follows: 1 - fixed support, 2 - first base, 3 - three-point bending support, 4 - indenter, 5 - first force sensor, 6 - first hydraulic cylinder, 7 - first data acquisition instrument, 8 - first hydraulic system, 9 - triangular bottom plate, 10 - steel rod, 11 - first top cover, 12 - screw rod, 13 - ball pulley, 14 - second top cover, 15 - fixed base, 16 - second base, 17 - sliding base, 18 - second force sensor, 19 - second hydraulic cylinder, 20 - second data acquisition instrument, 21 - second hydraulic system, 22 - guide rail, 23 - high-pressure gas cylinder, 24 - gas booster pump, 25 - first gas flowmeter, 26 - first valve, 27 - second valve, 28 - third valve, 29 - vacuum pump, 30 - thermometer, 31 - fourth valve, 32 - pneumatic back pressure valve, 33 - gas-liquid separator, 34 - second gas flowmeter, 35 - gas collecting cylinder, 36 - constant temperature water bath, 37 - intermediate storage tank, 38 - pressure gauge, 39 - reaction kettle. Detailed implementation manners

[0018] To more clearly illustrate the present invention, the present invention will be further described below with reference to the accompanying drawings. Those skilled in the art should understand that the content specifically described below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should have the ordinary meaning understood by those of ordinary skill in the field to which the present invention belongs.

[0019] To avoid confusing the essence of the present invention, well-known methods, processes, procedures, components and circuits are not described in detail.

[0020] It should be understood that the terms "first", "second", "third", and "fourth" are only used for descriptive purposes, and the features defined with "first", "second", "third", and "fourth" do not represent any order, quantity or importance, but are only used to distinguish different components.

[0021] The present invention provides a fracture toughness test system for hydrate sediments, as Figures 1 to 11As shown, the system includes a specimen synthesis unit, a three-point bending test unit, and a direct shear test unit. The three-point bending test unit is detachably connected to the specimen synthesis unit and is used to measure the mode I fracture toughness of hydrate sediments. The direct shear test unit can be detachably replaced with the three-point bending test unit and connected to the specimen synthesis unit, and is used to measure the mode II fracture toughness of hydrate sediments. The specimen synthesis unit is used to synthesize hydrate sediment specimens and control their decomposition process.

[0022] The three-point bending test unit includes a fixed bracket 1, a first base 2, a three-point bending bracket 3, a pressing head 4, a first force sensor 5, a first hydraulic cylinder 6, a first data collector 7, a first hydraulic system 8, a triangular base plate 9, a steel rod 10, a first top cover 11, a screw 12, and a ball pulley 13. The fixed bracket 1 is a cylinder, and the fixed bracket 1 is welded to the first base 2. The first base 2 is detachably fixed to the bottom of the reactor 39 by screws. There are two fixed brackets 1, which correspond to the positions of the baffles arranged on the sides of the two three-point bending brackets 3. The three-point bending bracket 3 is close to the fixed bracket 1. A ball socket connection groove is provided at the bottom of the three-point bending bracket 3, and a ball pulley 13 is provided in the ball socket connection groove. The three-point bending bracket 3 reduces friction through the ball pulley 13 to ensure uniform stress on the specimen when a horizontal load is applied. Bearings are provided at the top of the three-point bending bracket 3. The number of three-point bending brackets 3 is two, and the two three-point bending brackets 3 are symmetrically arranged and form an integral structure. As Figure 4 and Figure 5As shown in the figure, in this embodiment, a T-shaped baffle matching the concave-shaped structure formed by the sides of the two three-point bending brackets 3 is provided on the side of the first top cover 11. The T-shaped baffle on one side of the fixed bracket 1 is in contact with the indenter 4 arranged in the horizontal direction. The indenter 4 is connected to the first force sensor 5. The first force sensor 5 is connected to the first hydraulic cylinder 6. The first force sensor 5 is communicatively connected to the first data acquisition instrument 7 through a data line. The first hydraulic cylinder 6 is an actuator of the first hydraulic system 8. The first hydraulic system 8 is connected to the first hydraulic cylinder 6. The first hydraulic system 8 is used to drive the first hydraulic cylinder 6 to act; the screw 12 sequentially passes through the end cover of the top of the reaction kettle 39 provided with a through hole and the first top cover 11 provided with a threaded hole, and finally is connected to the bearing arranged on the top of the three-point bending bracket 3; the triangular bottom plate 9 is arranged between the two three-point bending brackets 3. The triangular bottom plate 9 can cooperate with the bottom plates of the three-point bending brackets 3 to form a complete rectangular bottom plate. Threaded holes are opened at both ends of the triangular bottom plate 9, and circular holes corresponding to the triangular bottom plate 9 are opened on the bottom plates of the three-point bending brackets 3. The steel rod 10 first passes through the side wall of the reaction kettle 39, and then is sequentially threadedly connected to the circular holes corresponding to the triangular bottom plate 9 and the bottom plates of the three-point bending brackets 3; the specimen to be tested is placed on the three-point bending bracket 3. The first hydraulic system 8 controls the first hydraulic cylinder 6 to slowly apply a horizontal load at a constant speed and constant pressure. The load is sequentially transmitted to the specimen through the first force sensor 5 and the indenter 4 until the specimen undergoes tensile failure. The first data acquisition instrument 7 records the load value when the specimen fails, that is, the maximum load value, and calculates the mode I fracture toughness value of the specimen through the expression.

[0023] The direct shear test unit includes a second top cover 14, a fixed base 15, a second base 16, a sliding base 17, a second force sensor 18, a second hydraulic cylinder 19, a second data acquisition instrument 20, a second hydraulic system 21 and a guide rail 22. The second top cover 14 is a split structure, divided into a first split and a second split. The first split and the second split are respectively fixed to the fixed base 15 and the sliding base 17 with screws. The fixed base 15 is fixed to the second base 16 and forms a cubic shear box with the sliding base 17. The fixed base 15 and the sliding base 17 are two symmetrical parts. When used, they are only pressed against each other without being connected. They are used to place the sample. The second base 16 is detachably fixed to the bottom of the reactor 39 by screws. One side of the sliding base 17 It contacts the second force sensor 18, the sliding base 17 slides with the guide rail 22, the guide rail 22 is welded to the second base 16, the second force sensor 18 is connected to the second data acquisition instrument 20 through a data line, the second force sensor 18 is connected to the second hydraulic cylinder 19, and the second hydraulic system 21 is connected to the second hydraulic cylinder 19; the test sample is placed on the cubic shear box composed of the fixed base 15 and the sliding base 17, and the second hydraulic system 21 controls the second hydraulic cylinder 19 on one side of the sliding base 17 to slowly apply a horizontal load at a constant pressure and speed. The fixed base 15 acts as a reaction force and is fixed, and the sliding base 17 slides along the guide rail 22. Finally, the sample is sheared and broken. The second data acquisition instrument 20 can record the maximum load value, that is, the shear failure load. According to The sliding base 17 cooperates with the guide rail 22 through a slideway to ensure that the friction force is minimized during the shearing process.

[0024] The sample synthesis unit includes a high-pressure gas cylinder 23, a gas booster pump 24, a first gas flowmeter 25, a first valve 26, a second valve 27, a third valve 28, a vacuum pump 29, a thermometer 30, a fourth valve 31, a pneumatic back-pressure valve 32, a gas-water separator 33, a second gas flowmeter 34, a gas collecting cylinder 35, a constant temperature water bath 36, an intermediate storage tank 37, a pressure gauge 38, and a reaction kettle 39. The high-pressure gas cylinder 23 is connected to the gas booster pump 24 through a pipeline, the gas booster pump 24 is connected to the first gas flowmeter 25 through a pipeline, the first gas flowmeter 25 is connected to the first valve 26 through a pipeline, the first valve 26 is connected to the intermediate storage tank 37 through a pipeline, the intermediate storage tank 37 is connected to the second valve 27 through a pipeline, and the second valve 27 is connected to the reaction kettle 39 through a pipeline; the reaction kettle 39 is connected to the third valve 28 through a pipeline, the third valve 28 is connected to the vacuum pump 29 through a pipeline, and the third valve 28 and the second valve 27 are connected to the reaction kettle 39 through the same pipeline; a thermometer 30 and a pressure gauge 38 are connected to the top of the reaction kettle 39; the reaction kettle 39 is connected to the fourth valve 31 through a pipeline, the fourth valve 31 is connected to the pneumatic back-pressure valve 32 through a pipeline, the pneumatic back-pressure valve 32 is connected to the gas-water separator 33 through a pipeline, the gas-water separator 33 is connected to the second gas flowmeter 34 through a pipeline, and the second gas flowmeter 34 is connected to the gas collecting cylinder 35 through a pipeline; water is stored in the constant temperature water bath 36, which can be used to reduce the temperature of the reaction kettle 39, and the intermediate storage tank 37 and the reaction kettle 39 are located in the constant temperature water bath 36; a mixture of water and muddy silt is added into the reaction kettle 39. After the reaction kettle 39 is sealed, the third valve 28 is opened, and the air in the reaction kettle 39 is evacuated by the vacuum pump 29. Then the third valve 28 is closed, the water bath temperature in the constant temperature water bath 36 is adjusted to the designed temperature, the first valve 26 is opened, and methane gas in the high-pressure gas cylinder 23 is pumped into the intermediate storage tank 37 through the gas booster pump 24. Let the methane gas stand in the intermediate storage tank 37 for half an hour to avoid the influence of the gas temperature entering the kettle on the hydrate synthesis. The first valve 26 and the second valve 27 are closed, and the gas in the intermediate storage tank 37 is steadily injected into the reaction kettle 39 until the pressure in the kettle is higher than the designed value. Then the second valve 27 is closed, and the pressure value on the pressure gauge 38 is observed. When the pressure value finally drops to and maintains at the designed pressure value, it indicates that the hydrate is completely formed. Subsequently, the hydrate is decompressed and decomposed. The water bath temperature in the constant temperature water bath 36 remains unchanged. A series of pressures lower than the designed pressure value are set through the pneumatic back-pressure valve 32. The fourth valve 31 is opened, the pressure in the reaction kettle 39 starts to drop, and the hydrate starts to decompose. The decomposition products of the hydrate pass through the gas-water separator 33, and finally the gas reaches the gas collecting cylinder 35. When the pressure value on the pressure gauge 38 maintains at the back-pressure value and the second gas flowmeter 34 is 0, it indicates that the hydrate decomposition stops.

[0025] The specific working process is as follows: I. Test on the fracture toughness of type I hydrate sediments The test of the mode-I fracture toughness of hydrate sediments requires the combined use of a three-point bending test unit and a specimen synthesis unit. After the specimen synthesis unit and the three-point bending test unit are assembled respectively, push the steel rod 10 to drive the triangular bottom plate 9 to be in close contact with the bottom plate of the three-point bending bracket 3. Add a mixture of water and muddy silt into the three-point bending bracket 3 and compact it. The crack width of the specimen will be adjusted automatically. Open a through crack according to the test requirements. Rotate the screw 12 to drive the first top cover 11 to descend, so that the first top cover 11 is tightly combined with the three-point bending bracket 3. After the reactor 39 is sealed, open the third valve 28, evacuate the air in the reactor 39 with the vacuum pump 29, close the third valve 28, adjust the water bath temperature in the constant temperature water bath 36 to the designed temperature, open the first valve 26, pump the methane gas in the high-pressure gas cylinder 23 into the intermediate storage tank 37 through the gas booster pump 24, let the methane gas stand in the intermediate storage tank 37 for half an hour, close the first valve 26, open the second valve 27, and inject the gas in the intermediate storage tank 37 into the reactor 39 smoothly until the pressure in the reactor is higher than the designed value. Then close the second valve 27 and observe the pressure value of the pressure gauge 38. When the pressure value finally drops to and maintains at the designed pressure value, it indicates that the hydrate is completely formed. Subsequently, use the constant temperature water bath 36 to cool down and use the pneumatic back pressure valve 32 to reduce the pressure to ensure that the hydrate does not decompose under atmospheric pressure conditions. Rotate the screw 12 to raise the first top cover 11 above the top of the specimen, pull the steel rod 10 to drive the triangular bottom plate 9, so that the triangular bottom plate 9 is separated from the bottom plate of the three-point bending bracket 3. The first hydraulic system 8 controls the first hydraulic cylinder 6 to apply a slow load at a constant speed and constant pressure. Under the action of the force, the specimen bends. The three-point bending bracket 3 takes the contact point between the vertex of the triangular bottom plate 9 and the bottom plate of the three-point bending bracket 3 as the center of the circle. Under the rolling of the ball pulley 13, the two ends of the three-point bending bracket 3 move in an arc until the specimen undergoes tensile failure. The first data acquisition instrument 7 can record the load when the specimen fails, that is, the maximum load value, and substitute it into the expression to calculate the mode-I fracture toughness value. Subsequently, carry out the pressure reduction and decomposition of the hydrate. The water bath temperature in the constant temperature water bath 36 remains unchanged. Set a series of pressure values lower than atmospheric pressure through the pneumatic back pressure valve 32. Open the fourth valve 31, the pressure in the reactor 39 starts to drop, and the hydrate starts to decompose. The decomposition products of the hydrate pass through the gas-liquid separator 33, and finally the gas reaches the gas collecting bottle 35. When the pressure value of the pressure gauge 38 maintains at the back pressure value and the second gas flowmeter 34 is 0, it indicates that the decomposition of the hydrate stops. At this time, the reactor 39 can be opened, the three-point bending test unit can be taken out, and the test of the mode-I fracture toughness of hydrate sediments is completed.

[0026] ; ; where Denote the mode-I fracture toughness of hydrate sediments; Denote the correlation coefficient of mode-I fracture toughness, which is related to the parameter and related; Denote the critical failure load; Denote the specimen span; Denote the specimen thickness; Denote the specimen width; Denote the initial crack depth; the specific measurement methods of the corresponding letters in the formula are the same as those shown in Figure 10 shown in.

[0027] II. Test on the mode-II fracture toughness of hydrate sediments The test on the mode-II fracture toughness of hydrate sediments needs to be carried out by the combined use of a direct shear test unit and a specimen synthesis unit. Assemble the components except the second top cover 14 respectively. Add a mixture of water and muddy silt into the cubic shear box assembled by the fixed base 15 and the sliding base 17, and compact it. The crack width of the specimen can be adjusted automatically. According to the test requirements, open a through crack. The second top cover 14 is divided into two symmetric parts on the left and right, namely the first split body and the second split body. The first split body and the second split body are respectively fixed on the fixed base 15 and the sliding base 17 with screws. After the reactor 39 is sealed, open the third valve 28, evacuate the air in the reactor 39 with a vacuum pump 29, close the third valve 28, adjust the water bath temperature in the constant temperature water bath 36 to the designed temperature, open the first valve 26, pump the methane gas in the high-pressure gas cylinder 23 into the intermediate storage tank 37 through the gas booster pump 24, let the methane gas stand in the intermediate storage tank 37 for half an hour, close the first valve 26, open the second valve 27, and inject the gas in the intermediate storage tank 37 into the reactor 39 smoothly until the pressure in the reactor 39 is higher than the designed value. Then close the second valve 27 and observe the pressure value of the pressure gauge 38. When the pressure value finally drops to and maintains at the designed pressure value, it means that the hydrate is completely formed. Subsequently, use the constant temperature water bath 36 to cool down and use the pneumatic back pressure valve 32 to reduce the pressure to ensure that the hydrate does not decompose under normal pressure conditions. The second hydraulic system 21 controls the second hydraulic cylinder 19 to apply a constant pressure and a constant speed of slow loading. The sliding base 17 moves along the guide rail 22, and the fixed base 15 remains stationary, causing the specimen to finally undergo shear failure. The second data acquisition instrument 20 can record the load when the specimen undergoes shear failure, that is, the maximum load, and substitute it into The expression can be used to calculate the Type II fracture toughness of the hydrate sediment. Hydrate decomposition is then performed by depressurizing the hydrate. The water bath temperature in the constant-temperature water bath 36 is maintained constant. A series of pressures below atmospheric pressure are set using the pneumatic backpressure valve 32. The fourth valve 31 is opened, and the pressure in the reactor 39 begins to drop, causing hydrate decomposition. The hydrate decomposition products pass through the gas-water separator 33, and the gas eventually reaches the gas collection bottle 35. When the pressure on the pressure gauge 38 remains at the backpressure value and the second gas flowmeter 34 reaches 0, hydrate decomposition has ceased. At this point, the reactor 39 can be opened, the direct shear test unit removed, and the Type II fracture toughness test of the hydrate sediment is complete.

[0028] ; ; ; in represents the type II fracture toughness of hydrate sediments; Represents the correlation coefficient of mode II fracture toughness, which is related to the parameter and Related; represents the critical shear load; Indicates the thickness of the specimen; Indicates the width of the specimen; represents the initial crack depth; Indicates the width of the specimen; the specific measurement method of the corresponding letters in the formula is as follows Figure 11 Same as shown in .

[0029] In summary, the advantages of the hydrate sediment fracture toughness testing system proposed in the present invention are as follows: 1. In the hydrate sediment fracture toughness testing system proposed by the present invention, the first hydraulic cylinder 6 and the second hydraulic cylinder 19 are placed horizontally, adopting a horizontal loading method to eliminate the influence of gravity on the fracture toughness of hydrate sediments. On the other hand, the system adopts a solution of a ball pulley 13, a sliding base 17 and a guide rail 22 to reduce the effect of friction and make the fracture toughness more accurate.

[0030] 2. The sample synthesis unit in the hydrate sediment fracture toughness testing system proposed in the present invention can realize the synthesis and decomposition of hydrates and ensure that the hydrates do not decompose during the testing process, thereby avoiding the problem of hydrate decomposition affecting the mechanical properties of hydrate sediments.

Claims

1. A fracture toughness test system for hydrate sediments, comprising a sample synthesis unit, the sample synthesis unit including a reaction kettle (39), characterized in that, The system further includes a three-point bending test unit and a direct shear test unit that can be interchangeably connected to the specimen synthesis unit; the three-point bending test unit includes a three-point bending support (3), a triangular bottom plate (9), a steel rod (10), a first top cover (11), a first horizontal loading mechanism, a first data collector (7), and a screw rod (12); the bottom of the three-point bending support (3) is provided with a ball socket connection groove, and a ball pulley (13) is installed in the ball socket connection groove. The number of three-point bending supports (3) is two and they are symmetrically arranged. The two three-point bending supports (3) form an integral structure. The side surface of the three-point bending support (3) corresponds to the position of the fixed support (1) to achieve positioning; the triangular bottom plate (9) is arranged between the two three-point bending supports (3), and it cooperates with the bottom plates of the three-point bending supports (3) to form a rectangular bottom plate; the steel rod (10) penetrates through the side wall of the reaction kettle (39) and locks the triangular bottom plate (9) and the three-point bending support (3); the screw rod (12) penetrates through the top of the reaction kettle (39) and the first top cover (11) and then cooperates with the bearing at the top of the three-point bending support (3); the first horizontal loading mechanism is horizontally arranged and abuts against the side baffle of the first top cover (11); the first data collector (7) is communicatively connected to the first horizontal loading mechanism; the direct shear test unit includes a second base (16), a cube shear box, a second top cover (14), a second horizontal loading mechanism, and a second data collector (20). The cube shear box is formed by splicing a fixed base (15) and a sliding base (17). The fixed base (15) is fixedly connected to the second base (16), and the sliding base (17) is slidably connected to the second base (16) through a guide rail (22); the second top cover (14) is a split structure and is respectively fixed above the fixed base (15) and the sliding base (17); the second horizontal loading mechanism is horizontally arranged and abuts against the sliding base (17); the second data collector (20) is communicatively connected to the second horizontal loading mechanism.

2. The hydrate sediment fracture toughness test system according to claim 1, wherein The first horizontal loading mechanism includes a first hydraulic cylinder (6), a first force sensor (5), and a pressure head (4) connected in sequence. The pressure head (4) is horizontally arranged and abuts against the side baffle of the first top cover (11); the first force sensor (5) is communicatively connected to the first data collector (7).

3. The hydrate sediment fracture toughness testing system according to claim 1, wherein The number of the fixed supports (1) is the same as that of the three-point bending supports (3) and they correspond one by one. The fixed supports (1) and the first base (2) are welded to form a rigid support structure; the first base (2) is detachably fixed to the bottom of the reaction kettle (39); the side baffle of the three-point bending support (3) and the fixed support (1) form a positioning mating surface; the triangular bottom plate (9) and the bottom plates of the two side three-point bending supports (3) are spliced into a complete bearing surface; the first top cover (11) realizes vertical displacement control through the screw rod (12) with a threaded fit.

4. The hydrate sediment fracture toughness test system according to claim 1 or 3, characterized in that, The two ends of the triangular bottom plate (9) are provided with threaded holes, and the bottom plate of the three-point bending bracket (3) is provided with corresponding round holes. The steel rod (10) passes through the side wall of the reaction kettle (39), the threaded holes of the triangular bottom plate (9), and the round holes on the bottom plate of the three-point bending bracket (3) in sequence to achieve locking.

5. The hydrate sediment fracture toughness test system according to claim 1, characterized in that, The second horizontal loading mechanism includes a second force sensor (18) and a second hydraulic cylinder (19) connected in sequence. The second force sensor (18) is horizontally arranged and abuts against the sliding base (17), and the second force sensor (18) is communicatively connected to the second data acquisition instrument (20).

6. The hydrate sediment fracture toughness test system according to claim 1, characterized in that A thermometer (30) and a pressure gauge (38) are connected to the top of the reaction kettle (39).

7. The hydrate sediment fracture toughness test system according to claim 1, characterized in that, The second base (16) is detachably fixed to the bottom of the reaction kettle (39).

8. The hydrate sediment fracture toughness test system according to claim 1, characterized in that, The sample synthesis unit further includes a gas supply module, a vacuum pumping module, a constant temperature control module, and an exhaust gas metering module: The gas supply module includes a high-pressure gas cylinder (23), a gas booster pump (24), a first gas flowmeter (25), a first valve (26), a second valve (27), and an intermediate storage tank (37). The high-pressure gas cylinder (23), the gas booster pump (24), the first gas flowmeter (25), the first valve (26), and the intermediate storage tank (37) are connected in sequence through pipelines, and the intermediate storage tank (37) is communicated with the reaction kettle (39) through the second valve (27); The vacuum pumping module includes a third valve (28) and a vacuum pump (29), and the third valve (28) is installed on the connecting pipeline between the reaction kettle (39) and the vacuum pump (29); The constant temperature control module includes a constant temperature water bath tank (36) covering the intermediate storage tank (37) and the reaction kettle (39); The exhaust gas metering module includes a fourth valve (31), a pneumatic back pressure valve (32), a gas-water separator (33), a second gas flowmeter (34), and a gas collecting bottle (35). The fourth valve (31) is arranged at the gas outlet end of the reaction kettle (39), and the fourth valve (31), the pneumatic back pressure valve (32), the gas-water separator (3), the second gas flowmeter (34), and the gas collecting bottle (35) are connected in sequence through pipelines.

9. The hydrate sediment fracture toughness test system according to claim 2, characterized in that The first hydraulic cylinder (6) is connected to the first hydraulic system (8).

10. The hydrate sediment fracture toughness test system according to claim 5, wherein The second hydraulic cylinder (19) is connected to the second hydraulic system (21).

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