CN113008700A - Method for testing mechanical properties of natural gas hydrate - Google Patents

Method for testing mechanical properties of natural gas hydrate Download PDF

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CN113008700A
CN113008700A CN202110168144.8A CN202110168144A CN113008700A CN 113008700 A CN113008700 A CN 113008700A CN 202110168144 A CN202110168144 A CN 202110168144A CN 113008700 A CN113008700 A CN 113008700A
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natural gas
test piece
gas hydrate
rubber column
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CN113008700B (en
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蒋宇静
颜鹏
栾恒杰
李萌
王冬
王长盛
马先壮
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Shandong University of Science and Technology
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    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/24—Investigating strength properties of solid materials by application of mechanical stress by applying steady shearing forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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Abstract

The invention provides a method for testing mechanical properties of a natural gas hydrate, relates to the technical field of natural gas hydrate exploitation and testing, and solves the technical problem of determining mechanical property parameters of the natural gas hydrate. The method comprises the following steps: processing a plurality of cored rubber column test pieces and grouping the cored rubber column test pieces, wherein one group is used for testing the mechanical properties of the cored rubber column test piece containing natural gas hydrate, and the other group is used for testing the mechanical properties of the cored rubber column test piece containing isobaric gas; and (3) calculating mechanical characteristic parameters of the natural gas hydrate, such as elastic modulus, peak shear strength, cohesion, friction angle and the like, by combining a linear superposition principle, Hooke's law and Mokolun's law. The mechanical characteristic parameters obtained by the method can be applied to a particle flow program to simulate the mechanical characteristics of sediments containing natural gas hydrates, so that the accuracy of a simulation result is ensured; in addition, the method has the advantages of simplicity, low test cost, high result accuracy and the like.

Description

Method for testing mechanical properties of natural gas hydrate
Technical Field
The invention relates to the technical field of natural gas hydrate exploitation and testing, in particular to a mechanical property testing method of a natural gas hydrate.
Background
The natural gas hydrate is also called as combustible ice, is an ice-like crystalline substance formed by natural gas and water under high pressure and low temperature conditions, and is mainly distributed in deep sea sediments or permafrost in land areas. Because of its advantages of wide distribution, large reserve, shallow burial, high energy efficiency, low pollution and the like, it is one of the best alternative energy sources in the post-petroleum era, and has been generally regarded by various countries, and the research on natural gas hydrate is gradually paid attention by various scholars. The mechanical response characteristic of a reservoir stratum in the natural gas hydrate exploitation process is deeply researched, and the method has great significance for realizing economic and efficient exploitation of the natural gas hydrate and effective control of geological disasters such as geological collapse, seabed landslide and the like.
In geotechnical engineering, in-situ testing or indoor testing is generally adopted to obtain geotechnical parameters under relevant engineering geological conditions, wherein the indoor testing needs to firstly obtain geotechnical materials on an engineering site. The method has strong applicability to common geotechnical engineering, but for deep sea natural gas hydrate, because the method exists hundreds of meters under sea, the method is difficult to carry out in-situ test and obtain a natural gas hydrate reservoir under the original temperature and pressure condition to carry out indoor test. At present, the test of the mechanical response characteristic of the natural gas hydrate reservoir is faced with the problems of serious low efficiency, high cost and the like. Numerical simulation is an important means for solving the above problems, and is also increasingly widely used. At the present stage, the numerical simulation of the mechanical response of the natural gas hydrate reservoir is mostly realized by a particle flow method, the whole model comprises two types of particles, namely natural gas hydrate particles and skeleton particles, and the two types of particles need to be respectively referred in the simulation. However, most of the researches of the related scholars focus on the mechanical properties of sediments containing the natural gas hydrate, the mechanical properties of the natural gas hydrate are rarely researched, and the accurate reference of natural gas hydrate particles is difficult to realize. Therefore, it is highly desirable to provide a method for testing the mechanical properties of natural gas hydrates to provide the necessary parameters for numerical simulation or other related studies.
Disclosure of Invention
In order to accurately determine the mechanical property parameters of the natural gas hydrate, the invention provides a method for testing the mechanical property of the natural gas hydrate, which is based on the linear superposition principle and respectively adopts Hooke's law and Mokolun's law to analyze the test result to obtain the mechanical property parameters of the natural gas hydrate including elastic modulus, peak shear strength, cohesion, friction angle and the like.
A method for testing mechanical properties of natural gas hydrate comprises the following steps:
step one, processing a plurality of cored rubber column test pieces and dividing the test pieces into S1And S2Two groups are respectively numbered with S1iAnd S2iWherein i is a positive integer, and the inner diameter of the cored rubber column test piece is d;
step two, taking S1The test comprises a triaxial shear test, wherein water is injected into the middle of the natural gas hydrate coring rubber column test piece, the water surface is flush with the upper end surface of the test piece, the test piece is placed into a triaxial confining pressure chamber after being sealed and fixed, and the temperature T after the natural gas hydrate is generated in the test process is recorded1Internal pressure P of cored rubber column test piece1iMagnitude of confining pressure P during testC1iTime t of sudden drop of axle pressure after axle pressure is loaded1iAt time t1iAxial pressure F ofN1iAnd axial strain epsilontiAnd at time t1iAxial stress F at/21iAnd axial strain epsiloni；
Step three, taking S2The method comprises the steps of testing the mechanical properties of a rubber column test piece containing isobaric gas and coring, wherein the test comprises a triaxial shear test, and recording the test processMiddle, temperature T after formation of natural gas hydrate2Internal pressure P of cored rubber column test piece2iMagnitude of confining pressure P during testC2iLoading axial pressure and recording axial strain value as epsiloniAnd εtiAxial stress F of2iAnd FN2i(ii) a Wherein T is1＝T2，P2i＝P1i，PC2i＝PC1i；
Step four, calculating S1Three-dimensional stress sigma of natural gas hydrate in test piece with core removed rubber columni1、σi2、σi3：
Figure BDA0002938210900000021
σi2＝Pci＝P1i＝P2i
σi3＝Pci＝P1i＝P2i
And step five, calculating the elastic modulus E of the natural gas hydrate synthesized at the middle position of the cored rubber column test piece according to Hooke's law, wherein the elastic modulus is calculated respectively:
Figure BDA0002938210900000022
the elastic modulus of the natural gas hydrate is obtained by averaging the calculation results
Figure BDA0002938210900000023
Step six, according to Moore's Coulomb's law, utilizing the first principal stress sigmai1And third principal stress σi3Drawing a molar stress circle in a shear stress-normal stress coordinate system, and drawing a shear strength envelope curve of the natural gas hydrate, wherein the envelope curve equation is
Figure BDA0002938210900000024
Calculating to obtain the peak shear strength taumaxInner angle of friction
Figure BDA0002938210900000025
And cohesion force c.
Preferably, the size of the cored rubber column test piece is phi 50 multiplied by d multiplied by 100mm, wherein the value range of the inner diameter d is that d is more than or equal to 10mm and less than or equal to 40 mm.
Preferably, the process of the triaxial shear test in the step two comprises:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face of the cored rubber column test piece;
c, injecting water into the middle of the natural gas hydrate coring rubber column test piece, enabling the water surface to be flush with the upper end face of the test piece, and sealing the upper end face of the coring rubber column test piece;
d, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece;
and E, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, starting loading, and recording test data.
Preferably, a back pressure pump is used for controlling a back pressure valve in the triaxial shear test to maintain the internal pressure of the coring rubber column to be stable in time.
Preferably, the process of the triaxial shear test in step three comprises:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face and the lower end face of the cored rubber column test piece;
step C, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece;
and D, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, starting loading, and recording test data.
Preferably, the internal pressure P of the cored rubber column test piece2iControlled by the pressure of the injected methane gas.
Preferably, the modulus of elasticity, peak shear strength, cohesion and angle of friction are used in a particle flow procedure to simulate the mechanical properties of natural gas hydrate containing deposits.
Further preferably, the triaxial shear test uses a device for testing the mechanical properties of the natural gas hydrate, and the device for testing the mechanical properties of the natural gas hydrate comprises a counter-force frame, a data acquisition and control system, a confining pressure pump, a pressure sensor, a temperature sensor, a confining pressure sensor, an axial pressure pump, a back pressure valve, a beaker, a triaxial confining pressure chamber and a hydraulic jack.
Preferably, the confining pressure pump is connected with a three-shaft confining pressure chamber, the hydraulic jack is connected with the shaft pressure pump, and the confining pressure chamber is arranged in the counter-force frame; the back pressure valve is respectively connected with the back pressure pump and the beaker; pressure sensor and temperature sensor set up in the test piece, and confined pressure sensor sets up in the triaxial confined pressure indoor.
The method for testing the mechanical properties of the natural gas hydrate has the advantages that the method realizes indoor measurement of the mechanical properties of the natural gas hydrate, and can measure mechanical property parameters such as elastic modulus, peak shear strength, cohesion, friction angle and the like of the natural gas hydrate under different conditions; the obtained mechanical characteristic parameters can be used for simulating the mechanical characteristic behavior of the sediments containing the natural gas hydrate by using a particle flow program and can also be used for making reference to natural gas hydrate particles in the mechanical response process of a natural gas hydrate reservoir, so that the simulation result is more accurate.
Drawings
FIG. 1 is a schematic flow chart of a method for testing mechanical properties of natural gas hydrates;
FIG. 2 is a schematic structural diagram of a device for testing mechanical properties of natural gas hydrate;
FIG. 3 is a schematic illustration of a natural gas hydrate cored rubber column test piece;
FIG. 4 is a front view of the loading principle of a natural gas hydrate cored rubber column test piece;
FIG. 5 is a schematic top view of the loading of a natural gas hydrate cored rubber column test piece;
FIG. 6 is a schematic diagram of a Moore stress circle and its shear strength envelope;
in the figure: 1. a counter-force frame; 2. a data acquisition and control system; 3. a confining pressure pump; 4. a latex film; 5. removing a core rubber column test piece; 6. a natural gas hydrate; 7. a pressure sensor; 8. a temperature sensor; 9. a confining pressure sensor; 10. an axial compression pump; 11. a back pressure pump; 12. a back pressure valve; 13. a beaker; 14. a triaxial confining pressure chamber; 15. and a hydraulic jack.
Detailed Description
A specific embodiment of the method for testing mechanical properties of a natural gas hydrate according to the present invention is described with reference to fig. 1 to 6.
Example 1
The method for testing the mechanical properties of the natural gas hydrate is based on a linear superposition principle, and analyzes a test result by respectively adopting Hooke's law and Mokolun's law to obtain the mechanical property parameters of the natural gas hydrate including elastic modulus, peak shear strength, cohesion, friction angle and the like, and comprises the following specific steps of:
step one, processing a plurality of cored rubber column test pieces and dividing the test pieces into S1And S2Two groups are respectively numbered with S1iAnd S2iAnd i is a positive integer, and the inner diameter of the cored rubber column test piece is d. The size of the cored rubber column test piece is phi 50 multiplied by d multiplied by 100mm, wherein the value range of the inner diameter d is that d is more than or equal to 10mm and less than or equal to 40 mm. The number of the core-removed rubber column test pieces is determined according to the test requirements.
Step two, taking S1The test comprises a triaxial shear test, wherein water is injected into the middle of the natural gas hydrate coring rubber column test piece, the water surface is flush with the upper end surface of the test piece, the test piece is placed into a triaxial confining pressure chamber after being sealed and fixed, and the natural gas hydrate hydration rubber column test piece records the mechanical property of the natural gas hydrate coring rubber column test piece in the test processTemperature T after product formation1The measurement can be made with a temperature sensor. Internal pressure P of cored rubber column test piece1iAnd in the triaxial shear test, a back pressure pump is utilized to control a back pressure valve to maintain the internal pressure stability of the coring rubber column in time. Confining pressure P in test processC1iAnd the tracking mode of the data acquisition and control system is utilized to ensure that the confining pressure and the internal pressure difference in the test process are not changed to be 0.1MPa, so that the external confining pressure is not changed. Time t of sudden drop of axle pressure after loading axle pressure1iI.e. a significant decrease in axial pressure, at time t1iAxial pressure F ofN1iAnd axial strain epsilontiAnd at time t1iAxial stress F at/21iAnd axial strain epsiloni。
Wherein the triaxial shear test process comprises:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face of the cored rubber column test piece;
c, injecting water into the middle of the natural gas hydrate coring rubber column test piece, enabling the water surface to be flush with the upper end face of the test piece, and sealing the upper end face of the coring rubber column test piece;
d, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece; when all the water in the cored rubber column test piece is used for synthesizing the natural gas hydrate, residual methane gas is filled between the natural gas hydrate and the inner wall of the cored rubber column test piece, so that the natural gas hydrate is not extruded with the inner wall of the cored rubber column test piece when being pressed;
and E, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, starting loading, and recording test data.
Step three, taking S2Testing the mechanical properties of the rubber column test piece containing isobaric gas by a triaxial shear test and recordingRecording the temperature T after formation of the gas hydrate during the test2The measurement can be made with a temperature sensor. Internal pressure P of cored rubber column test piece2iInternal pressure P of cored rubber column test piece2iThe pressure may be measured by a pressure sensor, controlled by the gas pressure of the injected methane gas. Confining pressure P in test processC2iAnd can be measured by a confining pressure sensor. Load the axial pressure and record the axial strain as εiAnd εtiAxial stress F of2iAnd FN2i(ii) a Wherein T is1＝T2，P2i＝P1i，PC2i＝PC1i。
Wherein, the triaxial shear test's process includes:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face and the lower end face of the cored rubber column test piece;
step C, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece; when all the water in the cored rubber column test piece is used for synthesizing the natural gas hydrate, residual methane gas is filled between the natural gas hydrate and the inner wall of the cored rubber column test piece, so that the natural gas hydrate is not extruded with the inner wall of the cored rubber column test piece when being pressed;
and D, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, ensuring that the temperature is higher than zero so as to avoid the formation of ice from influencing a test result, starting loading, and recording test data.
Step four, calculating S1Three-dimensional stress sigma of natural gas hydrate in test piece with core removed rubber columni1、σi2、σi3：
Figure BDA0002938210900000051
σi2＝Pci＝P1i＝P2i
σi3＝Pci＝P1i＝P2i
And step five, calculating the elastic modulus E of the natural gas hydrate synthesized at the middle position of the cored rubber column test piece according to Hooke's law, wherein the elastic modulus is calculated respectively:
Figure BDA0002938210900000052
the elastic modulus of the natural gas hydrate is obtained by averaging the calculation results
Figure BDA0002938210900000053
Step six, according to Moore's Coulomb's law, utilizing the first principal stress sigmai1And third principal stress σi3Drawing a molar stress circle in a shear stress-normal stress coordinate system, and drawing a shear strength envelope curve of the natural gas hydrate, wherein the envelope curve equation is
Figure BDA0002938210900000061
Calculating to obtain the peak shear strength taumaxInner angle of friction
Figure BDA0002938210900000062
And cohesion force c.
The modulus of elasticity, peak shear strength, cohesion and friction angle were used in a particle flow program to simulate the mechanical properties of natural gas hydrate-containing deposits. The obtained mechanical characteristic parameters can be used for simulating the mechanical characteristic behavior of the sediments containing the natural gas hydrate by using a particle flow program and can also be used for making reference to natural gas hydrate particles in the mechanical response process of a natural gas hydrate reservoir, so that the simulation result is more accurate.
In addition, the triaxial shear test uses a mechanical property testing device of the natural gas hydrate, and the mechanical property testing device of the natural gas hydrate comprises a counter-force frame, a data acquisition and control system, a confining pressure pump, a pressure sensor, a temperature sensor, a confining pressure sensor, an axial pressure pump, a back pressure valve, a beaker, a triaxial confining pressure chamber and a hydraulic jack. Wherein the confining pressure pump links to each other with triaxial confining pressure room, and the triaxial confining pressure room can exert confining pressure to the test piece, and hydraulic jack links to each other with the axle pressure pump, can exert axial pressure to the time. The confining pressure chamber is arranged in the counter-force frame, and the test piece is placed in the confining pressure chamber during the test. The back pressure valve is connected with the back pressure pump and the beaker respectively, and the back pressure pump controls the back pressure valve to maintain the internal pressure of the coring rubber column to be stable in time. Pressure sensor and temperature sensor set up in the test piece, monitor pressure and temperature, and confined pressure sensor sets up in the triaxial confined pressure room, can monitor confined pressure. The testing device is simple in structure and convenient to operate, realizes indoor measurement of mechanical properties of the natural gas hydrate, and can measure mechanical property parameters such as elastic modulus, peak shear strength, cohesion, friction angle and the like of the natural gas hydrate under different conditions.
Example 2
The calculation of the elastic modulus, peak shear strength, cohesion and friction angle is explained on the basis of example 1.
Selecting the size of a cored rubber column test piece as phi 50 multiplied by 20 multiplied by 100mm, namely the inner diameter of the test piece is 20mm, each group of test pieces comprises 5 test pieces which are divided into S1Group sum S2Group S1The test piece in the group is denoted as S11、S12、S13、S14And S15，S2Set as the inner test piece is marked as S21、S22、S23、S24And S25。
Testing the mechanical properties of the cored rubber column test piece containing the natural gas hydrate S1The experimental data measured included: t is1＝2℃，P11＝0.11MPa，P12＝0.44MPa，P13＝0.76MPa，P14＝1.06MPa，P15＝1.31MPa，FN11＝120.06KN，FN12＝143.35KN，FN13＝161.76KN，FN14＝182.54KN，FN15＝203.68KN，F11＝61.50KN，F12＝62.62KN，F13＝62.03KN，F14＝61.03KN，F15＝61.60KN，ε1＝2.3×103，ε2＝2.5×103，ε3＝2.4×103，ε4＝2.2×103，ε5＝2.3×103。
Testing the mechanical properties of the rubber column test piece containing the isobaric gas coring S2The experimental data measured included: t is2＝2℃，P21＝0.11MPa，P22＝0.44MPa，P23＝0.76MPa，P24＝1.06MPa，P25＝1.31MPa，FN21＝119.32KN，FN22＝142.50KN，FN23＝160.80KN，FN24＝181.47KN，FN25＝202.52KN，F21＝52.90KN，F22＝53.19KN，F23＝53.04KN，F24＝52.75KN，F25＝52.89KN。
Five elastic modulus values of the natural gas hydrate are calculated:
E1＝11.90GPa E2＝12.00GPa E3＝11.92GPa E4＝11.98GPa E5＝12.04GPa
the elastic modulus E of the natural gas hydrate obtained by averaging is 11.97 GPa.
Three-dimensional stresses for 5 groups of natural gas hydrates were obtained:
σ11＝2.46MPa σ12＝0.11MPa σ13＝0.11MPa
σ21＝3.15MPa σ22＝0.44MPa σ23＝0.44MPa
σ31＝3.83MPa σ32＝0.76MPa σ33＝0.76MPa
σ41＝4.48MPa σ42＝1.06MPa σ43＝1.06MPa
σ51＝5.01MPa σ52＝1.31MPa σ53＝1.31MPa
drawing a mole circle through the five groups of three-dimensional stresses and obtaining the natural gas hydrate with the shear strength envelope line of taumaxσ tan22 ° +0.79, the internal content of natural gas hydrateAngle of friction
Figure BDA0002938210900000071
At 22 ℃ and a cohesion c of 0.79 MPa.
It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.

Claims (9)

1. A method for testing mechanical properties of natural gas hydrate is characterized by comprising the following steps:
step one, processing a plurality of cored rubber column test pieces and dividing the test pieces into S1And S2Two groups are respectively numbered with S1iAnd S2iWherein i is a positive integer, and the inner diameter of the cored rubber column test piece is d;
step two, taking S1The test comprises a triaxial shear test, wherein water is injected into the middle of the natural gas hydrate coring rubber column test piece, the water surface is flush with the upper end surface of the test piece, the test piece is placed into a triaxial confining pressure chamber after being sealed and fixed, and the temperature T after the natural gas hydrate is generated in the test process is recorded1Internal pressure P of cored rubber column test piece1iMagnitude of confining pressure P during testC1iTime t of sudden drop of axle pressure after axle pressure is loaded1iAt time t1iAxial pressure F ofN1iAnd axial strain epsilontiAnd at time t1iAxial stress F at/21iAnd axial strain epsiloni；
Step three, taking S2The method comprises the steps of testing the mechanical properties of a rubber column test piece containing isobaric gas and cored, wherein the test comprises a triaxial shear test, and recording the temperature T after the generation of the natural gas hydrate in the test process2Internal pressure P of cored rubber column test piece2iMagnitude of confining pressure P during testC2iLoading axial pressure and recording axial strain value as epsiloniAnd εtiAxial stress F of2iAnd FN2i(ii) a Wherein T is1＝T2，P2i＝P1i，PC2i＝PC1i；
Step four, calculating S1Three-dimensional stress sigma of natural gas hydrate in test piece with core removed rubber columni1、σi2、σi3：
Figure FDA0002938210890000011
σi2＝Pci＝P1i＝P2i
σi3＝Pci＝P1i＝P2i
And step five, calculating the elastic modulus E of the natural gas hydrate synthesized at the middle position of the cored rubber column test piece according to Hooke's law, wherein the elastic modulus is calculated respectively:
Figure FDA0002938210890000012
the elastic modulus of the natural gas hydrate is obtained by averaging the calculation results
Figure FDA0002938210890000013
Step six, according to Moore's Coulomb's law, utilizing the first principal stress sigmai1And third principal stress σi3Drawing a molar stress circle in a shear stress-normal stress coordinate system, and drawing a shear strength envelope curve of the natural gas hydrate, wherein the envelope curve equation is
Figure FDA0002938210890000014
Calculating to obtain the peak shear strength taumaxInner angle of friction
Figure FDA0002938210890000015
And cohesion force c.
2. The method for testing the mechanical properties of the natural gas hydrate as claimed in claim 1, wherein the size of the cored rubber column test piece is phi 50 x d x 100mm, and the value range of the inner diameter d is that d is not less than 10mm and not more than 40 mm.
3. The method for testing the mechanical properties of the natural gas hydrate according to claim 1, wherein the process of the triaxial shear test in the second step comprises the following steps:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face of the cored rubber column test piece;
c, injecting water into the middle of the natural gas hydrate coring rubber column test piece, enabling the water surface to be flush with the upper end face of the test piece, and sealing the upper end face of the coring rubber column test piece;
d, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece;
and E, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, starting loading, and recording test data.
4. The method for testing the mechanical properties of the natural gas hydrate, according to claim 3, is characterized in that a back pressure pump is used for controlling a back pressure valve to maintain the internal pressure of the de-cored rubber column to be stable in time in the triaxial shear test.
5. The method for testing the mechanical properties of the natural gas hydrate, according to claim 1, wherein the process of the triaxial shear test in the third step comprises:
step A, a cored rubber column test piece is loaded into a latex film with the same outer diameter size by using a film bearing cylinder, wherein both ends of the latex film exceed the upper end surface and the lower end surface of the cored rubber column test piece;
b, placing the wrapped test piece on a natural gas hydrate sediment mechanical property test device and sealing the lower end face and the lower end face of the cored rubber column test piece;
step C, placing the sealed cored rubber column test piece into a triaxial confining pressure chamber, and injecting sufficient methane gas into the cored rubber column test piece;
and D, reducing the temperature in the triaxial confining pressure chamber by using a low-temperature water bath, starting loading, and recording test data.
6. The method for testing mechanical properties of natural gas hydrate as claimed in claim 5, wherein the internal pressure P of the cored rubber column test piece2iControlled by the pressure of the injected methane gas.
7. The method for testing the mechanical properties of the natural gas hydrate, according to claim 1, wherein the elastic modulus, the peak shear strength, the cohesion and the friction angle are used in a particle flow program to simulate the mechanical properties of a natural gas hydrate-containing sediment.
8. The method for testing the mechanical properties of the natural gas hydrate as claimed in claim 1, wherein the triaxial shear test uses a device for testing the mechanical properties of the natural gas hydrate, and the device for testing the mechanical properties of the natural gas hydrate comprises a counterforce frame, a data acquisition and control system, a confining pressure pump, a pressure sensor, a temperature sensor, a confining pressure sensor, an axial pressure pump, a back pressure valve, a beaker, a triaxial confining pressure chamber and a hydraulic jack.
9. The method for testing the mechanical properties of the natural gas hydrate as claimed in claim 8, wherein the confining pressure pump is connected with a three-axis confining pressure chamber, the hydraulic jack is connected with an axial pressure pump, and the confining pressure chamber is arranged in the counterforce frame; the back pressure valve is respectively connected with the back pressure pump and the beaker; pressure sensor and temperature sensor set up in the test piece, and confined pressure sensor sets up in the triaxial confined pressure indoor.
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