A carbon dioxide hydrate tensile test device and system and method thereof

Abstract

The present invention discloses a carbon dioxide hydrate tensile test device and its system and method, which belong to the field of natural gas hydrate engineering mechanical property testing and evaluation. The device of the present invention includes a pressure autoclave, a flange and a hydraulic cylinder that are sealed and connected and assembled in sequence from bottom to top, and the lower part of the loading piston in the hydraulic cylinder passes through the flange and extends into the pressure chamber of the pressure autoclave. The system includes a methane injection pressure volume controller, a carbon dioxide injection pressure volume controller, an axial pressure controller, a methane cylinder, a carbon dioxide cylinder, an intermediate container, a water bath control box, a data acquisition device, a confining pressure controller and a carbon dioxide hydrate tensile test device. The present invention can better meet the carbon dioxide replacement experiment and the tensile strength destruction test of carbon dioxide hydrate under low temperature and high pressure conditions, and provide more complete technical parameters for studying the stability of natural gas hydrates during reservoir exploration and production.

Description

Technical Field

[0001] The invention relates to the field of testing and evaluating the engineering mechanical properties of natural gas hydrates, and in particular to a testing method and device for measuring the tensile strength of a hydrate sediment system formed in a reactor by a Brazilian splitting method. Background Art

[0002] Natural gas hydrate (NGH) is an ice-like crystalline substance formed by the combination of water and natural gas under high pressure and high temperature conditions. It has the characteristics of high reserves, high energy efficiency, and wide distribution. In recent years, the exploitation and utilization of NGH has become one of the international research hotspots, and its research focuses include formation mechanism, hydraulic fracturing, hydrate phase change, cementation stability, etc. Since NGH is an emerging form of energy, its exploitation and utilization still have many technical difficulties and challenges, and further in-depth research is still needed.

[0003] Normally, NGH is found in deep seabed and terrestrial permafrost zones, and has high stability in cold and high-pressure environments. However, during reservoir exploitation and transformation, hydrate pressure changes and temperature rises, and the reservoir inevitably undergoes brittle deformation, compression shear failure, and tensile failure. Understanding the changes in mechanical properties of hydrate-bearing sediments (HBS) plays a vital role in studying the stability of reservoir exploration. At present, the main hydrate mechanical testing equipment includes resonance columns, triaxial and direct shear instruments, and ring shear instruments, which correspond to the study of elastic deformation, elastoplastic deformation, and residual deformation stage characteristics in hydrate deformation. There are few studies and experimental equipment on the tensile mechanical behavior of hydrates, and most of the tensile characteristic parameters of experimental studies at this stage are estimated according to the empirical formulas in the field of conventional rocks, and there is no accurate experimental reference data. Therefore, it is urgent to strengthen the research on the tensile characteristics of natural gas hydrate reservoirs and test methods.

[0004] The Brazilian Test is an experimental method widely used to evaluate the mechanical properties of mineral materials. As for its experimental method, the direct tensile specimen is directly bonded to the tensile force application device, which cannot guarantee the stability of HBS under high pressure and low temperature conditions. Therefore, this method can effectively avoid the problem of uniaxial tensile loading difficulties, and when the environmental conditions of the thermodynamic characteristics of hydrates are met, the tensile strength characteristic parameters can be better obtained. Therefore, when studying the tensile mechanical properties of HBS, the Brazilian splitting method can be used as an effective test method to evaluate the tensile properties of natural gas hydrate reservoirs.

[0005] In summary, the existing equipment is unable to measure the tensile strength of hydrate sediments, and the direct tensile test is not suitable for hydrate test loading measurement. Therefore, it is necessary to develop an effective device and method for evaluating the tensile properties of natural gas hydrate reservoirs. This device can better meet the tensile strength destruction test of carbon dioxide hydrate under low temperature and high pressure conditions, and provide more complete technical parameters for studying the stability of natural gas hydrates during reservoir exploration and production. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a carbon dioxide hydrate tensile test device and its system and method on the basis of overcoming the defects of the prior art, so as to provide technical support for the testing and evaluation of the engineering mechanical properties of natural gas hydrates.

[0007] The specific technical solutions adopted by the present invention are as follows:

[0008] In a first aspect, the present invention provides a carbon dioxide hydrate tensile test device, comprising a pressure autoclave, a flange and a hydraulic cylinder which are assembled and sealed in sequence from bottom to top, wherein the lower portion of a loading piston in the hydraulic cylinder passes through the flange and extends into a pressure chamber of the pressure autoclave;

[0009] A lower support plate is installed at the bottom of the pressure chamber, and a sample assembly is placed on the lower support plate; the sample assembly includes an upper pressure head, an upper permeable stone, a sediment sample, a lower permeable stone and a lower pressure head which are placed in sequence from left to right, and the right half of the upper pressure head, the upper permeable stone, the sediment sample, the lower permeable stone and the left half of the lower pressure head are wrapped as a whole by a nitrile rubber film; a groove is opened on the lower support plate and an LVDT local strain sensor is placed thereon, and the upper surface of the LVDT local strain sensor is in contact with the nitrile rubber film wrapping the sediment sample;

[0010] The flange is provided with a thermocouple channel, a confining pressure control channel, a pore pressure control channel and a back pressure control channel, all of which are connected to the pressure chamber; the pore pressure control channel and the back pressure control channel are connected to the upper permeable stone and the lower permeable stone through pipelines passing through the upper pressure head and the lower pressure head, respectively, to form a connecting passage;

[0011] The top of the loading piston is exposed from the hydraulic cylinder, and the bottom is located above the sediment sample. The middle part divides the inner cavity of the hydraulic cylinder into an upper cavity and a lower cavity that are not connected to each other. The loading piston can slide in a sealed manner along the vertical direction under the limitation of the hydraulic cylinder and the flange; the upper cavity is connected to the outside through an axial pressure control channel; a baffle that is always in a horizontal state is fixedly connected to the top of the loading piston, and an axial displacement sensor for measuring the axial displacement of the loading piston is provided at the other end of the baffle.

[0012] Preferably, the lower part of the pressure vessel is a water bath jacket having a groove inside and serving as a pressure chamber; a cooling liquid channel inlet and a cooling liquid channel outlet are respectively provided on both sides of the water bath jacket, and the cooling liquid channel inlet and the cooling liquid channel outlet are connected to form a cooling liquid channel for cooling the pressure chamber.

[0013] Furthermore, the water bath jacket is made of heat-insulating material, a low-temperature cooling pipe is spirally arranged in the cooling liquid channel, and two ends of the low-temperature cooling pipe are respectively connected to the cooling liquid channel inlet and the cooling liquid channel outlet.

[0014] Preferably, a sealed sliding connection is achieved between the top of the loading piston and the hydraulic cylinder via a first sealing ring, a sealed sliding connection is achieved between the middle of the loading piston and the inner wall of the hydraulic cylinder via a second sealing ring, and a sealed sliding connection is achieved between the lower part of the loading piston and the flange via a third sealing ring; a sealed fixed connection is achieved between the pressure cooker, the flange and the hydraulic cylinder via a plurality of bolts and sealing rings.

[0015] Preferably, the lower support plate is fixed in the pressure vessel by threaded connection, and the LVDT local strain sensor is fixed in the lower support plate by threaded connection.

[0016] Preferably, the upper pressure head and the lower pressure head are made of carbon dioxide corrosion-resistant material, and the internal pipelines are made of flexible pressure-resistant and corrosion-resistant nylon tubes, and the inner wall of the nylon tube is provided with a silicon carbide wear-resistant and corrosion-resistant coating.

[0017] Preferably, both the upper pressure head and the lower pressure head are equipped with sound velocity sensors.

[0018] Preferably, the bottom width of the loading piston is consistent with the flat width of the sediment sample.

[0019] In a second aspect, the present invention provides a carbon dioxide hydrate tensile test system, including a methane injection pressure volume controller, a carbon dioxide injection pressure volume controller, an axial pressure controller, a methane cylinder, a carbon dioxide cylinder, an intermediate container, a water bath control box, a data acquisition device, a confining pressure controller, and a carbon dioxide hydrate tensile test device as described in any one of the first aspects;

[0020] A thermocouple is placed in the pressure chamber through the thermocouple channel, and a confining pressure controller is externally connected to the confining pressure control channel; the inlet end of the bore pressure control channel located outside the autoclave is divided into two branches through a three-way valve, the first branch is connected to the methane gas cylinder, and a methane injection pressure volume controller and a first ball valve are sequentially arranged on it along the fluid flow direction, and the second branch is connected to the carbon dioxide gas cylinder, and a carbon dioxide injection pressure volume controller and a second ball valve are sequentially arranged on it along the fluid flow direction; a pressure gauge, a gas flow meter, a third ball valve and an intermediate container are sequentially arranged on the back pressure control channel located outside the autoclave along the fluid flow direction; the axial pressure control channel is connected to the axial pressure controller through a pipeline provided with a fourth ball valve; the coolant channel inlet and the coolant channel outlet of the water bath jacket at the lower part of the autoclave are respectively connected to the water bath control box;

[0021] The data collector is respectively connected to the LVDT local strain sensor, the thermocouple, the axial displacement sensor, the gas flow meter, the pressure gauge, the methane injection pressure volume controller, the carbon dioxide injection pressure volume controller, the axial pressure controller, and the confining pressure controller to collect data changes during the tensile test of the sediment sample in real time.

[0022] In a third aspect, the present invention provides a tensile test method using the carbon dioxide hydrate tensile test system according to the second aspect, which is as follows:

[0023] S1: Prepare a sample assembly using the prepared cylindrical sediment sample, freeze the sample assembly and place it on the lower support plate;

[0024] S2: Assemble all components and pipelines to achieve a sealed state of the pressure chamber; open the first ball valve, close the second ball valve, introduce high-pressure methane gas into the pore pressure control channel through the methane injection pressure volume controller, use a water bath control box to keep the pressure chamber at a low temperature, and the sediment sample reacts with the methane gas under a high-pressure and low-temperature environment to generate natural gas hydrates. During the hydrate generation process, the temperature and pressure changes in the pressure chamber and the volume changes of the gas in the methane injection pressure volume controller are collected in real time through a data acquisition device; when the volume of the gas in the methane injection pressure volume controller remains stable, it indicates that the natural gas hydrate has been generated and is stable, and then the carbon dioxide replacement test of S3 is carried out;

[0025] S3: Open the second ball valve, close the first ball valve, and introduce carbon dioxide gas into the pore pressure control channel through the carbon dioxide injection pressure volume controller to react and generate carbon dioxide hydrate in the sediment sample; open the third ball valve, discharge the unreacted carbon dioxide into the intermediate container through the back pressure control channel for collection, and obtain the gas flow parameter through the gas flow meter to detect the change in carbon dioxide volume; when the carbon dioxide gas volume is stable, it indicates that the replacement is completed and carbon dioxide hydrate is generated;

[0026] S4: After carbon dioxide hydrate is generated, the fourth ball valve is opened, and the hydraulic oil is pressurized into the upper chamber of the hydraulic cylinder by using the axial pressure controller to drive the loading piston to move downward. The sediment sample is subjected to downward pressure by the loading piston until the applied vertical load destroys the sediment sample, completing the Brazilian splitting test. The stress-strain curve is obtained based on the LVDT local strain sensor and the axial displacement sensor, and then the indirect tensile strength of the sediment sample is obtained.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) It can accurately simulate the fracturing pressure of hydrate reservoirs at different burial depths (different mining conditions), so as to further analyze the mechanical properties of hydrates under different mining conditions, and add ultrasonic pressure heads to study the stress-strain relationship during the hydrate fracturing process, thus achieving synchronized measurement of permeability and different fracturing stages.

[0029] (2) Both the upper and lower pressure heads are made of materials resistant to carbon dioxide corrosion. Flexible pressure-resistant and corrosion-resistant nylon pipes are used to connect the bore pressure control channel and the back pressure control channel to the upper and lower pressure heads. Silicon carbide wear-resistant and corrosion-resistant coatings are installed inside the nylon pipes to prevent possible corrosion problems caused by carbon dioxide flowing through the pipes and pressure heads.

[0030] (3) The water bath jacket is made of heat-insulating material, and the inner wall is spirally arranged with a low-temperature cooling pipe, which can increase the heat exchange intensity and better maintain the low-temperature environment required for hydrate formation.

[0031] (4) After the sample preparation is completed, the hydrate sample can be directly put into the rubber sleeve, which is simpler than manually burying the sample in sand. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The present invention is a structural schematic diagram of a carbon dioxide hydrate tensile test device.

[0033] Figure 2 The schematic diagram is a structural diagram of a carbon dioxide hydrate tensile test system.

[0034] Figure 3 yes Figure 1 Cross-sectional view of the device.

[0035] Figure 4 These are several preferred structural cross-sectional views of the contact between the bottom of the loading piston, the specimen assembly and the lower support plate during the tensile test.

[0036] In the figure: 1 pressure chamber; 2 water bath jacket; 3 coolant channel; 3-a coolant channel inlet, 3-b coolant channel outlet; 4 sediment sample; 5 upper pressure head; 6 upper permeable stone; 7 LVDT local strain sensor; 8 nitrile rubber membrane; 9 lower permeable stone; 10 lower support plate; 11 lower pressure head; 12 flange; 13 thermocouple channel; 14 confining pressure control channel; 15 pore pressure control channel; 16 back pressure control channel; 17 thermocouple; 18 axial displacement sensor; 19 baffle; 20 loading piston; 21 hydraulic cylinder; 22 axial pressure control Channel; 23-a first sealing ring, 23-b second sealing ring, 23-c third sealing ring; 24 methane injection pressure volume controller; 25 carbon dioxide injection pressure volume controller; 26 axial pressure controller; 27 methane cylinder; 28 carbon dioxide cylinder; 29a first ball valve, 29b second ball valve, 29c third ball valve, 29d fourth ball valve; 30 gas flow meter; 31 intermediate container; 32 pressure gauge; 33 water bath control box; 34 data acquisition device; 35 confining pressure controller; 36 carbon dioxide hydrate tensile test device. DETAILED DESCRIPTION

[0037] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0038] In the description of the present invention, it is to be understood that when an element is considered to be "connected" to another element, it may be directly connected to the other element or indirectly connected, that is, there are intermediate elements. On the contrary, when an element is said to be "directly" connected to another element, there are no intermediate elements.

[0039] In the description of the present invention, it should be understood that the terms "first", "second", "third", and "fourth" are only used for distinguishing description purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined as "first", "second", "third", and "fourth" may explicitly or implicitly include at least one of the features.

[0040] like Figure 1 and 3As shown, a carbon dioxide hydrate tensile test device provided by the present invention mainly comprises a pressure vessel, a flange 12 and a hydraulic cylinder 21. The pressure vessel, the flange 12 and the hydraulic cylinder 21 are sealed and connected in sequence from bottom to top to form an integrated structure, and the lower part of the loading piston 20 in the hydraulic cylinder 21 passes through the flange 12 and extends into the pressure chamber 1 of the pressure vessel.

[0041] The structure and connection method of each component will be described in detail below.

[0042] In the device of the present invention, a lower support plate 10 is installed at the bottom of the pressure chamber 1, and a sample assembly is placed on the lower support plate 10. The sample assembly includes an upper pressure head 5, an upper permeable stone 6, a sediment sample 4, a lower permeable stone 9 and a lower pressure head 11, which are placed in sequence from left to right, and the right half of the upper pressure head 5, the upper permeable stone 6, the sediment sample 4, the lower permeable stone 9 and the left half of the lower pressure head 11 are wrapped as a whole by a nitrile rubber film 8 to isolate the hydraulic oil from the sediment sample. The lower support plate 10 is grooved and an LVDT local strain sensor 7 is placed, and the upper surface of the LVDT local strain sensor 7 is just in contact with the nitrile rubber film 8 that wraps the sediment sample 4, so as to measure the strain of the sediment sample 4 in real time during the test.

[0043] As a preferred embodiment of the present invention, the lower support plate 10 is fixed in the autoclave by threaded connection, and the LVDT local strain sensor 7 is fixed in the lower support plate 10 by threaded connection. The nitrile rubber membrane 8 is made of nitrile material resistant to carbon dioxide corrosion.

[0044] As a preferred embodiment of the present invention, the lower part of the autoclave is a water bath jacket 2, and the interior of the autoclave has a groove and serves as a pressure chamber 1. The two sides of the water bath jacket 2 are respectively provided with a coolant channel inlet 3-a and a coolant channel outlet 3-b, and the coolant channel inlet 3-a and the coolant channel outlet 3-b are connected by a pipeline inside the water bath jacket 2, and the pipeline serves as a coolant channel 3 for cooling the pressure chamber 1. In actual use, the water bath jacket 2 should be made of heat-insulating material, and a low-temperature cooling pipe is spirally arranged in the coolant channel 3, and the two ends of the low-temperature cooling pipe are respectively connected to the coolant channel inlet 3-a and the coolant channel outlet 3-b. This arrangement can increase the heat exchange intensity and better maintain the low-temperature environment required for hydrate formation.

[0045] It should be understood that the expression "low temperature" in the above-mentioned "low temperature cooling pipe" is only used for distinguishing description purposes and refers to the low temperature relative to the same pipeline, and cannot be understood as indicating or implying relative importance or implicitly indicating the temperature limit of the indicated technical features.

[0046] In the device of the present invention, the flange 12 is provided with a thermocouple channel 13, a confining pressure control channel 14, a pore pressure control channel 15 and a back pressure control channel 16, and one end of each of the four channels is connected to the outside and the other end is connected to the pressure chamber 1. Among them, the thermocouple channel 13 is used to insert a thermocouple to measure the temperature inside the pressure chamber 1, and the confining pressure control channel 14 can be injected with hydraulic oil or inert gas as needed to control the confining pressure of the sediment sample 4. The pore pressure control channel 15 and the back pressure control channel 16 are respectively connected to the upper permeable stone 6 and the lower permeable stone 9 through the pipelines that pass through the upper pressure head 5 and the lower pressure head 11 to form a connecting passage.

[0047] As a preferred embodiment of the present invention, one end of the pipeline of the upper pressure head 5 and the lower pressure head 11 is located at the top of the pressure head, and the other end is located inside the pressure head. The upper pressure head 5 and the lower pressure head 11 can both be made of carbon dioxide corrosion-resistant materials, and the internal pipelines can all be made of flexible pressure-resistant and corrosion-resistant nylon tubes. The inner wall of the nylon tube is provided with a silicon carbide wear-resistant and corrosion-resistant coating to prevent the pipeline corrosion problem that may be caused when carbon dioxide flows through the pipeline and the pressure head. The upper pressure head 5 and the lower pressure head 11 are both equipped with a sound velocity sensor.

[0048] In the device of the present invention, the loading piston 20 is a structure with both ends vertical and the middle part convex laterally, the top of which is exposed to the hydraulic cylinder 21, the bottom is located above the sediment sample 4, and the middle part divides the inner cavity of the hydraulic cylinder 21 into an upper cavity and a lower cavity which are not connected to each other. The loading piston 20 can slide in a sealed manner in the vertical direction under the limit of the hydraulic cylinder 21 and the flange 12. The upper cavity is connected to the outside through the axial pressure control channel 22, and the axial pressure control channel 22 is used to inject hydraulic oil to control the axial loading of the loading piston 20.

[0049] As a preferred embodiment of the present invention, a sealed sliding connection is achieved between the top of the loading piston 20 and the hydraulic cylinder 21 through a first sealing ring 23-a, a sealed sliding connection is achieved between the middle of the loading piston 20 and the inner wall of the hydraulic cylinder 21 through a second sealing ring 23-b, and a sealed sliding connection is achieved between the lower part of the loading piston 20 and the flange 12 through a third sealing ring 23-c.

[0050] As a preferred embodiment of the present invention, the bottom width of the loading piston 20 is consistent with the horizontal width of the sediment pattern 4.

[0051] In the device of the present invention, a baffle 19 which is always in a horizontal state is fixedly connected to the top of the loading piston 20 , and an axial displacement sensor 18 for measuring the axial displacement of the loading piston 20 is provided at the other end of the baffle 19 .

[0052] As a preferred embodiment of the present invention, the upper portion of the loading piston 20 is connected to the baffle 19, and the baffle 19 and the loading piston 20 are fixedly connected by bolts.

[0053] As a preferred embodiment of the present invention, the pressure autoclave, the flange 12 and the hydraulic cylinder 21 are sealed and fixedly connected by a plurality of bolts and sealing rings. Specifically, the upper part of the pressure autoclave contacts the flange 12 and is sealed by a sealing ring, and a plurality of bolts reinforce the tight connection between the pressure autoclave and the flange 12. An O-ring is used to seal the flange 12 and the hydraulic cylinder 21, and a plurality of bolts reinforce the tight connection between the flange 12 and the hydraulic cylinder 21. In other words, the upper part of the pressure autoclave and the lower part of the hydraulic cylinder are connected by a plurality of bolts on the flange 12 to reinforce the three parts, and are sealed by two layers of sealing rings.

[0054] like Figure 2 As shown, the present invention provides a carbon dioxide hydrate tensile test system, which mainly includes a methane injection pressure volume controller 24, a carbon dioxide injection pressure volume controller 25, an axial pressure controller 26, a methane gas cylinder 27, a carbon dioxide gas cylinder 28, an intermediate container 31, a water bath control box 33, a data acquisition device 34, a confining pressure controller 35 and as shown in FIG. Figure 1 The carbon dioxide hydrate tensile test device 36 is shown. The structure and connection method of each component will be described in detail below.

[0055] In the system of the present invention, a thermocouple 17 is inserted into the pressure chamber 1 through the thermocouple channel 13, and a confining pressure controller 35 is connected to the confining pressure control channel 14. The inlet end of the bore pressure control channel 15 located outside the autoclave is divided into two branches through a three-way valve. The first branch is connected to the methane gas cylinder 27 and a methane injection pressure volume controller 24 and a first ball valve 29a are sequentially arranged on it along the fluid flow direction. The second branch is connected to the carbon dioxide gas cylinder 28 and a carbon dioxide injection pressure volume controller 25 and a second ball valve 29b are sequentially arranged on it along the fluid flow direction. The back pressure control channel 16 located outside the autoclave is sequentially provided with a pressure gauge 32, a gas flow meter 30, a third ball valve 29c and an intermediate container 31 along the fluid flow direction. The axial pressure control channel 22 is connected to the axial pressure controller 26 through a pipeline provided with a fourth ball valve 29d. The coolant channel inlet 3-a and the coolant channel outlet 3-b of the water bath jacket 2 at the lower part of the autoclave are respectively connected to the water bath control box 33.

[0056] In the system of the present invention, the data collector 34 is respectively connected to the LVDT local strain sensor 7, the thermocouple 17, the axial displacement sensor 18, the gas flow meter 30, the pressure gauge 32, the methane injection pressure volume controller 24, the carbon dioxide injection pressure volume controller 25, the axial pressure controller 26, and the confining pressure controller 35 to collect real-time data changes during the tensile test of the sediment sample 4. Among them, the LVDT local strain sensor 7 is used to measure the stress and strain parameters of the hydrate sediment sample 4 when the axial load applies Newton force, and the thermocouple 17 is used to monitor the temperature of hydrate formation in the reactor.

[0057] Using the above carbon dioxide hydrate tensile test system, the present invention also provides a tensile test method, which is specifically as follows:

[0058] S1: A sample assembly is prepared using the prepared cylindrical sediment sample 4 , and the sample assembly is frozen and placed on the lower support plate 10 .

[0059] In actual use, the steps are as follows:

[0060] S11: Prepare the sediment sample to be tested:

[0061] Calculate the required amount of soil and determine the amount of water to be added according to the required porosity of the sample. After mixing evenly in the sample bag, compact it with a compacting hammer in the sample tube. Fix the number of compactions and draw a "well" after each compaction to mix different soil layers to prevent stratification of soil blocks caused by impact and affect the experimental results. After making a cylindrical sample of the mixture, freeze it for 4 to 8 hours and take out the sample after freezing.

[0062] S12: Sample installation:

[0063] Remove the bolts between the flange 12 and the pressure chamber 1, remove the pressure chamber 1, and place the upper pressure head 5, the upper permeable stone 6, the sediment sample 4, the lower permeable stone 9 and the lower pressure head 11 together from left to right; wrap the right half of the upper pressure head 5, the upper permeable stone 6, the sediment sample 4, the lower permeable stone 9 and the left half of the lower pressure head 11 with the nitrile rubber film 8, then fit the pressure chamber 1 with the flange 12, and tighten the bolts between the flange 12 and the pressure chamber 1.

[0064] S2: Hydrate formation:

[0065] Assemble all components and pipelines to achieve the sealed state of the pressure chamber 1. Open the first ball valve 29a, close the second ball valve 29b, and introduce high-pressure methane gas into the pore pressure control channel 15 through the methane injection pressure volume controller 24. Use the water bath control box 33 to keep the pressure chamber 1 at a low temperature. The sediment sample 4 reacts with the methane gas under high pressure and low temperature to generate natural gas hydrate. During the hydrate generation process, the temperature and pressure changes in the pressure chamber 1 and the volume changes of the gas in the methane injection pressure volume controller 24 are collected in real time through the data acquisition device 34. When the volume of the gas in the methane injection pressure volume controller 24 is stable, it indicates that the natural gas hydrate has been generated and stabilized, and then the carbon dioxide replacement test of S3 is carried out.

[0066] It should be understood that the "high pressure" and "low temperature" conditions in the above "high pressure and low temperature" can be numerically adjusted according to actual conditions, based on the specific conditions under which the sediment sample 4 can react with methane gas to generate natural gas hydrates.

[0067] S3: Carbon dioxide replacement:

[0068] The second ball valve 29b is opened, the first ball valve 29a is closed, the air source gas is exhausted, and carbon dioxide gas is introduced into the pore pressure control channel 15 through the carbon dioxide injection pressure volume controller 25, so that carbon dioxide hydrate is generated by reaction in the sediment sample 4. The third ball valve 29c is opened, and the unreacted carbon dioxide is discharged into the intermediate container 31 through the back pressure control channel 16 for collection, so as to keep the pressure of the sediment sample 4 stable. In this process, the gas flow parameter is obtained by the gas flow meter 30 to detect the change in the volume of carbon dioxide. When the volume of carbon dioxide gas is stable (or when the volume change is negligible), it indicates that the replacement is completed and carbon dioxide hydrate is generated.

[0069] In actual use, after carbon dioxide hydrate is generated, the confining pressure of the sediment sample 4 can be controlled by injecting hydraulic oil or inert gas through the confining pressure control channel 14 using the confining pressure controller 35 as needed.

[0070] S4: Carbon dioxide hydrate tensile test:

[0071] After the carbon dioxide hydrate is generated, the fourth ball valve 29d is opened, and the axial pressure controller 26 is used to pass hydraulic oil into the upper chamber of the hydraulic cylinder 21 to pressurize it to drive the loading piston 20 to move downward, and the sediment sample 4 is subjected to downward pressure by the loading piston 20 until the applied vertical load destroys the sediment sample 4, completing the Brazilian splitting test. The stress-strain curve is obtained according to the LVDT local strain sensor 7 and the axial displacement sensor 18, and then the indirect tensile strength of the sediment sample 4 is obtained.

[0072] The above method is based on the carbon dioxide hydrate tensile test system, which can include different pressure head forms of the loading piston 20, and complete the measurement of the tensile strength of carbon dioxide hydrate through a variety of pressure head loading methods, such as Figure 4 As shown, the specific details can be adjusted according to the actual situation.

[0073] The present invention can better meet the requirements of carbon dioxide replacement experiments and carbon dioxide hydrate tensile strength destruction tests under low temperature and high pressure conditions, and provide more complete technical parameters for studying the stability of natural gas hydrates during reservoir exploration and production.

[0074] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A carbon dioxide hydrate tensile test device, characterized in that: The invention comprises a pressure vessel, a flange (12) and a hydraulic cylinder (21) which are assembled and sealed in sequence from bottom to top, wherein the lower part of a loading piston (20) in the hydraulic cylinder (21) passes through the flange (12) and extends into a pressure chamber (1) of the pressure vessel; A lower support plate (10) is installed at the bottom of the pressure chamber (1), and a sample assembly is placed on the lower support plate (10); the sample assembly comprises an upper pressure head (5), an upper permeable stone (6), a sediment sample (4), a lower permeable stone (9) and a lower pressure head (11) which are placed in sequence from left to right, and the right half of the upper pressure head (5), the upper permeable stone (6), the sediment sample (4), the lower permeable stone (9) and the left half of the lower pressure head (11) are wrapped as a whole by a nitrile rubber film (8); the lower support plate (10) is grooved and an LVDT local strain sensor (7) is placed on it, and the upper surface of the LVDT local strain sensor (7) is in contact with the nitrile rubber film (8) wrapping the sediment sample (4); The flange (12) is provided with a thermocouple channel (13), a confining pressure control channel (14), a pore pressure control channel (15) and a back pressure control channel (16) all of which are connected to the pressure chamber (1); the pore pressure control channel (15) and the back pressure control channel (16) are connected to the upper permeable stone (6) and the lower permeable stone (9) respectively through pipelines penetrating the upper pressure head (5) and the lower pressure head (11), so as to form a connecting passage; The top of the loading piston (20) is exposed from the hydraulic cylinder (21), the bottom is located above the sediment sample (4), and the middle portion divides the inner cavity of the hydraulic cylinder (21) into an upper cavity and a lower cavity which are not connected to each other. The loading piston (20) can slide in a sealed manner in a vertical direction under the limit of the hydraulic cylinder (21) and the flange (12); the upper cavity is connected to the outside through an axial pressure control channel (22); a baffle (19) which is always in a horizontal state is fixedly connected to the top of the loading piston (20), and an axial displacement sensor (18) for measuring the axial displacement of the loading piston (20) is provided at the other end of the baffle (19).

2. A carbon dioxide hydrate tensile test device according to claim 1, characterized in that: The lower part of the autoclave is a water bath jacket (2) having a groove inside and serving as a pressure chamber (1); a cooling liquid channel inlet (3-a) and a cooling liquid channel outlet (3-b) are respectively provided on both sides of the water bath jacket (2); the cooling liquid channel inlet (3-a) and the cooling liquid channel outlet (3-b) are connected to form a cooling liquid channel (3) for cooling the pressure chamber (1).

3. A carbon dioxide hydrate tensile test device according to claim 2, characterized in that: The water bath jacket (2) is made of heat-insulating material, a low-temperature cooling pipe is spirally arranged in the cooling liquid channel (3), and two ends of the low-temperature cooling pipe are respectively connected to the cooling liquid channel inlet (3-a) and the cooling liquid channel outlet (3-b).

4. A carbon dioxide hydrate tensile test device according to claim 1, characterized in that: A first sealing ring (23-a) is used to achieve a sealed sliding connection between the top of the loading piston (20) and the hydraulic cylinder (21); a second sealing ring (23-b) is used to achieve a sealed sliding connection between the middle of the loading piston (20) and the inner wall of the hydraulic cylinder (21); and a third sealing ring (23-c) is used to achieve a sealed sliding connection between the lower part of the loading piston (20) and the flange (12); and a plurality of bolts and sealing rings are used to achieve a sealed fixed connection between the pressure autoclave, the flange (12) and the hydraulic cylinder (21).

5. The carbon dioxide hydrate tensile test device according to claim 1, characterized in that: The lower support plate (10) is fixed in the pressure vessel via a threaded connection, and the LVDT local strain sensor (7) is fixed in the lower support plate (10) via a threaded connection.

6. A carbon dioxide hydrate tensile test device according to claim 1, characterized in that: The upper pressure head (5) and the lower pressure head (11) are made of carbon dioxide corrosion resistant material, and the internal pipelines are made of flexible pressure-resistant and corrosion-resistant nylon tubes. The inner wall of the nylon tube is provided with a silicon carbide wear-resistant and corrosion-resistant coating.

7. A carbon dioxide hydrate tensile test device according to claim 1, characterized in that: The upper pressure head (5) and the lower pressure head (11) are both equipped with sound velocity sensors.

8. The carbon dioxide hydrate tensile test device according to claim 1, characterized in that: The bottom width of the loading piston (20) is consistent with the horizontal width of the sediment sample (4).

9. A carbon dioxide hydrate tensile test system, characterized in that: It comprises a methane injection pressure volume controller (24), a carbon dioxide injection pressure volume controller (25), an axial pressure controller (26), a methane gas cylinder (27), a carbon dioxide gas cylinder (28), an intermediate container (31), a water bath control box (33), a data acquisition device (34), a confining pressure controller (35), and a carbon dioxide hydrate tensile test device (36) as claimed in any one of claims 1 to 8; A thermocouple (17) is inserted into the pressure chamber (1) through the thermocouple channel (13), and the confining pressure control channel (14) is externally connected to a confining pressure controller (35); the inlet end of the pore pressure control channel (15) located outside the autoclave is divided into two branches through a three-way valve, the first branch being connected to a methane gas cylinder (27) and having a methane injection pressure volume controller (24) and a first ball valve (29a) arranged in sequence along the fluid flow direction, and the second branch being connected to a carbon dioxide gas cylinder (28) and having a carbon dioxide injection pressure volume controller (24) and a first ball valve (29a) arranged in sequence along the fluid flow direction. A volume controller (25) and a second ball valve (29b); a pressure gauge (32), a gas flow meter (30), a third ball valve (29c) and an intermediate container (31) are sequentially arranged on the back pressure control channel (16) outside the autoclave along the fluid flow direction; the axial pressure control channel (22) is connected to the axial pressure controller (26) via a pipeline provided with a fourth ball valve (29d); the coolant channel inlet (3-a) and the coolant channel outlet (3-b) of the water bath jacket (2) at the lower part of the autoclave are respectively connected to the water bath control box (33); The data collector (34) is respectively connected to the LVDT local strain sensor (7), the thermocouple (17), the axial displacement sensor (18), the gas flow meter (30), the pressure gauge (32), the methane injection pressure volume controller (24), the carbon dioxide injection pressure volume controller (25), the axial pressure controller (26), and the confining pressure controller (35) to collect data changes of the sediment sample (4) during the tensile test in real time.

10. A tensile test method using the carbon dioxide hydrate tensile test system according to claim 9, characterized in that: The details are as follows: S1: using the prepared cylindrical sediment sample (4) to prepare a sample assembly, freezing the sample assembly and placing it on a lower support plate (10); S2: Assemble all components and pipelines to achieve a sealed state of the pressure chamber (1); open the first ball valve (29a), close the second ball valve (29b), and introduce high-pressure methane gas into the pore pressure control channel (15) through the methane injection pressure volume controller (24); use a water bath control box (33) to keep the pressure chamber (1) at a low temperature; the sediment sample (4) reacts with the methane gas under a high-pressure and low-temperature environment to generate natural gas hydrates; during the hydrate generation process, the temperature and pressure changes in the pressure chamber (1) and the volume changes of the gas in the methane injection pressure volume controller (24) are collected in real time through a data acquisition device (34); when the volume of the gas in the methane injection pressure volume controller (24) remains stable, it indicates that the natural gas hydrate has been generated and is stable, and then a carbon dioxide replacement test in S3 is performed; S3: opening the second ball valve (29b), closing the first ball valve (29a), and introducing carbon dioxide gas into the pore pressure control channel (15) through the carbon dioxide injection pressure volume controller (25), so that carbon dioxide hydrate is generated by reaction in the sediment sample (4); opening the third ball valve (29c), discharging unreacted carbon dioxide into the intermediate container (31) through the back pressure control channel (16) for collection, and obtaining gas flow parameters through the gas flow meter (30) to detect the change in carbon dioxide volume; when the carbon dioxide gas volume is stable and unchanged, it indicates that the replacement is completed and carbon dioxide hydrate is generated; S4: After the carbon dioxide hydrate is generated, the fourth ball valve (29d) is opened, and the hydraulic oil is introduced into the upper chamber of the hydraulic cylinder (21) by using the axial pressure controller (26) to pressurize the hydraulic oil so as to drive the loading piston (20) to move downward, and the sediment sample (4) is subjected to downward pressure by the loading piston (20) until the applied vertical load destroys the sediment sample (4), thereby completing the Brazilian splitting test; a stress-strain curve is obtained according to the LVDT local strain sensor (7) and the axial displacement sensor (18), and then the indirect tensile strength of the sediment sample (4) is obtained.

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