CN113340746A - Calculation method of hydrate deposit shear strength - Google Patents

Calculation method of hydrate deposit shear strength Download PDF

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CN113340746A
CN113340746A CN202110284018.9A CN202110284018A CN113340746A CN 113340746 A CN113340746 A CN 113340746A CN 202110284018 A CN202110284018 A CN 202110284018A CN 113340746 A CN113340746 A CN 113340746A
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hydrate
shear strength
saturation
deposit
fitting
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CN113340746B (en
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刘晓强
郭天魁
曲占庆
孙莹
齐宁
王志远
侯健
徐鸿志
贺甲元
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China University of Petroleum East China
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    • 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
    • G01N3/24—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
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001—Type of application of the stress
    • G01N2203/0003—Steady
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0014—Type of force applied
    • G01N2203/0025—Shearing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/0069—Fatigue, creep, strain-stress relations or elastic constants
    • G01N2203/0075—Strain-stress relations or elastic constants
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/025—Geometry of the test
    • G01N2203/0256—Triaxial, i.e. the forces being applied along three normal axes of the specimen

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Abstract

The embodiment of the invention relates to a method for calculating the shear strength of a hydrate deposit, which comprises the following steps: constructing a triaxial shear test of the hydrate deposit under preset experimental conditions, wherein the preset experimental conditions comprise preset effective confining pressure, hydrate saturation and clay content; acquiring mechanical parameters of the hydrate deposit under corresponding conditions based on the experimental conditions of the triaxial shear test of the hydrate deposit, wherein the mechanical parameters of the hydrate deposit comprise shear strength, cohesive force and internal friction angle; and establishing a corrected M-C criterion according to the mechanical parameters of the hydrate deposit.

Description

Calculation method of hydrate deposit shear strength
Technical Field
The embodiment of the invention relates to the technical field of oil and gas field development, in particular to a calculation method of the shear strength of hydrate sediments.
Background
The rock-soil failure mainly comprises tension failure and shear failure, wherein Mohr-Cou lomb failure criterion (M-C criterion) is a commonly used strength criterion for judging the occurrence of shear failure. The conventional M-C criterion considers the rock-soil shear strength as a function of cohesion, internal friction angle and effective confining pressure. The strength of the hydrate deposit is determined by the cementation of the primary filler (clay and the like) among reservoir particles to the particles, and the cementation of solid hydrate in the pores of the hydrate reservoir to the deposit particles. The cementation effect of the hydrate on the sediment particles can change along with the phase change of the hydrate, when the phase equilibrium of the hydrate is broken, the hydrate is decomposed, and the cementation effect of the hydrate on the sediment particles is weakened. The mechanical property of hydrate deposit is different from that of conventional oil-gas reservoir due to the cementing action of hydrate on deposit particles, and the basic mechanical property of hydrate deposit is a dynamic change process at different hydrate saturation degrees. Therefore, the shear strength of the hydrate deposit core is not only a function of cohesive force, an internal friction angle and effective confining pressure, but also related to the saturation of the hydrate and the content of clay; however, the conventional M-C criterion cannot judge whether the shear failure of the hydrate deposit occurs at present.
Disclosure of Invention
The embodiment of the invention aims to provide a calculation method for the shear strength of a hydrate deposit, and aims to solve the problem that whether the shear damage of the hydrate deposit cannot be judged according to the conventional M-C rule in the prior art.
In order to solve the technical problem, an embodiment of the present invention provides a method for calculating shear strength of a hydrate deposit, including:
constructing a triaxial shear test of the hydrate deposit under preset experimental conditions, wherein the preset experimental conditions comprise preset effective confining pressure, hydrate saturation and clay content;
acquiring mechanical parameters of the hydrate deposit under corresponding conditions based on the experimental conditions of the triaxial shear test of the hydrate deposit, wherein the mechanical parameters of the hydrate deposit comprise shear strength, cohesive force and internal friction angle;
and establishing a corrected M-C criterion according to the mechanical parameters of the hydrate deposit.
Preferably, the step of establishing a modified M-C criterion according to the mechanical parameters of the hydrate deposit specifically includes:
and taking the cohesive force of the hydrate sediment core as a hydrate saturation function, wherein the corrected M-C criterion is as follows:
Figure BDA0002979669500000021
wherein, the part of shear strength increase is apparent strength, namely the strengthening effect of hydrate glue to shear is:
Figure BDA0002979669500000022
the shear strength increase value and the hydrate saturation of the hydrate sediment with different clay contents satisfy the power function relationship:
△σ(sh)＝a·sh b； (3)
substituting (3) into (2) yields:
Figure BDA0002979669500000023
after finishing, the relation formula of the cohesive force of the hydrate core considering the influence of the saturation degree of the hydrate is as follows:
Figure BDA0002979669500000024
substituting (6) into (1) to obtain a corrected hydrate deposit shear strength expression considering hydrate saturation, effective confining pressure, cohesive force and internal friction angle:
Figure BDA0002979669500000025
fitting the constants a and b and the relation between the cohesive force value and the clay content when the hydrate saturation is 0;
according to the fitting result, obtaining a corrected hydrate deposit shear strength expression considering the clay content, the hydrate saturation, the effective confining pressure, the cohesion and the internal friction angle:
Figure BDA0002979669500000026
σ1(sh,σ3) Is a hydrate with a saturation of shEffective confining pressure of σ3Shear strength in time;
c(sh) Is a hydrate with a saturation of shCohesion in time;
c(sh＝0) The cohesive force is the cohesive force when the saturation of the hydrate is 0;
Figure BDA0002979669500000027
is an internal friction angle;
σ3effective confining pressure;
△σ(sh) Is a hydrate with a saturation of shThe shear strength is increased compared with the shear strength when the hydrate saturation is 0;
σ1(sh＝0,σ3) The saturation of hydrate is 0 and the effective confining pressure is sigma3Shear strength in time;
b, b is a fitting constant;
shis the hydrate saturation;
Sh＝0represents a hydrate saturation of 0;
cvis the clay content.
Preferably, the step of constructing a triaxial shear test of the hydrate deposit under the preset test conditions comprises:
constructing two cores of an unconsolidated clayey silt hydrate deposit and a weakly consolidated clayey silt hydrate deposit, and respectively simulating a sea area hydrate deposit and a land frozen soil hydrate deposit;
and under the preset experimental condition, carrying out in-situ generated hydrate triaxial shear test on the two hydrate sediments.
Preferably, the shear strength increase values and the hydrate saturation of the hydrate sediments with different clay contents satisfy a power function relationship, and the process is as follows:
based on the experimental conditions of the triaxial shear test of the hydrate deposits, measuring the shear strength of the two hydrate deposits at different clay contents and hydrate saturation degrees;
according to the shear strength of the two hydrate sediments with different clay contents and hydrate saturation, obtaining the corresponding shear strength increase value of the hydrate sediment core when the hydrate saturation is increased under different effective confining pressures;
fitting the hydrate sediment cores with different clay contents, and fitting the shear strength increase value and the hydrate saturation degree under different effective surrounding pressures and corresponding errors.
Preferably, the fitting of the hydrate deposit cores with different clay contents and the fitting relation between the shear strength increase value and the hydrate saturation and the corresponding error under different effective surrounding pressures comprises:
when the clay content of the unconsolidated clayey silt hydrate sediment is 0, y is 0.166x0.841，R2＝0.973；
When the content of clay in the unconsolidated clayey silt hydrate deposit is 10%, y is 0.703x0.555，R2＝0.932；
When the clay content of the unconsolidated clayey silt hydrate sediment is 30 percent, y is 1.576x0.439，R2＝0.949；
When the clay content of weakly cemented clayey silt hydrate sediment is 10%, y is 0.8495x0.553，R2＝0.879；
Wherein y is the shear strength increase value;
x is the hydrate saturation;
R2fitting relation expressions and errors of shear strength increase values and hydrate saturation of hydrate sediment cores with different clay contents under different effective surrounding pressures.
Preferably, the fitting constants a and b and the relation between the cohesive force value and the clay content when the hydrate saturation degree is 0 comprise:
obtaining the fitting constants a and b of hydrate sediments at different clay contents and corresponding c(s)h＝0)、R2；
Fitting the fitting constants a, b and the relation between the cohesion value c (sh ═ 0) when the hydrate saturation is 0 and the clay content (cv), wherein the fitting relation comprises the following steps:
c(sh＝0)＝1.88cv+0.82，R2＝0.99；
a＝4.65cv+0.19，R2＝0.99；
b＝7.6cv 2-3.62cv+0.84，R2＝1。
preferably, the preset experimental conditions include:
under the conditions of effective confining pressure of 1MPa, 3MPa and 5MPa, hydrate saturation of 0%, 15%, 45% and 60%, clay content of 0%, 10% and 30%.
Preferably, the step of obtaining mechanical parameters of hydrate deposit under corresponding conditions based on the experimental conditions of triaxial shear test of hydrate deposit includes:
based on the experimental conditions of the triaxial shear test of the hydrate deposit, the cohesive force and the internal friction angle of the hydrate deposit can be obtained according to a Mohr circle method;
acquiring the shear strength of the hydrate deposit according to a stress-strain curve obtained by the triaxial shear test of the hydrate deposit;
wherein, for a stress-strain curve with peak strength, the peak strength is taken as the core shear strength, and for a stress value with the strain of 15 percent is taken as the core shear strength for a stress-strain curve with the strain hardening and plasticity tendency.
The invention directly obtains the change curve of axial strain with deflection stress of different types of hydrate deposit rock cores under different conditions by constructing hydrate rock core samples with different cementing strengths and carrying out a triaxial shear test to test the mechanical parameters of in-situ synthesized hydrate deposits, so as to obtain the relationship of shear strength, cohesive force, internal friction angle and the like along with the clay content, hydrate saturation and confining pressure, the calculation method of the shear strength of the hydrate deposit considering the saturation degree of the hydrate and the clay content is obtained, the problem that the shear damage of the hydrate deposit cannot be judged by the conventional M-C rule at present is solved, the hydrate saturation degree and the clay content are introduced to correct the shear damage, the shear strength of the hydrate deposit can be accurately and effectively calculated, and a judgment basis is provided for the shear failure of the hydrate deposit.
Drawings
One or more embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which elements having the same reference numeral designations represent like elements and in which the figures are not to scale unless specifically stated.
FIG. 1a is a Mohr circle with a hydrate saturation of 0% at a clay content of 30%;
FIG. 1b is a Mohr circle with a hydrate saturation of 15% at a clay content of 30%;
FIG. 1c is a Mohr circle with a hydrate saturation of 45% at a clay content of 30%;
FIG. 1d is a Mohr circle with a hydrate saturation of 60% at a clay content of 30%;
FIG. 2a is a stress-strain curve for an unconsolidated clayey silt hydrate deposit core at 30% clay content and 0% hydrate saturation;
FIG. 2b is a stress-strain curve for an unconsolidated clayey silt hydrate deposit core at 30% clay content and 15% hydrate saturation;
FIG. 2c is a stress-strain curve for an unconsolidated clayey silt hydrate deposit core at 30% clay content and 45% hydrate saturation;
FIG. 2d is a stress-strain curve for an unconsolidated clayey silt hydrate deposit core at 30% clay content and 60% hydrate saturation;
FIG. 3 is a schematic view of a Moire circle;
FIG. 4 is a fitting relational expression of shear strength increase value and hydrate saturation;
FIG. 5 is a fitting relational expression of fitting parameters to clay content;
FIG. 6 shows the comparison of the calculated shear strength value with the experimental value;
FIG. 7 is a flow chart of a method of calculating the shear strength of hydrate deposits.
The implementation, functional features and advantages of the objects of the present invention will be further explained with reference to the accompanying drawings.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.
It should be noted that, if directional indications (such as up, down, left, right, front, and back … …) are involved in the embodiment of the present invention, the directional indications are only used to explain the relative positional relationship between the components, the movement situation, and the like in a specific posture (as shown in the drawing), and if the specific posture is changed, the directional indications are changed accordingly.
In addition, if there is a description of "first", "second", etc. in an embodiment of the present invention, the description of "first", "second", etc. is for descriptive purposes only and is not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In addition, the technical solutions in the embodiments may be combined with each other, but must be realized by a person skilled in the art, and when the technical solutions are contradictory or cannot be realized, such a combination of technical solutions should not be considered to exist, and is not within the protection scope of the present invention.
The invention provides a method for calculating the shear strength of a hydrate deposit, please refer to fig. 7, which comprises the following steps:
step S100: constructing a triaxial shear test of the hydrate deposit under preset test conditions, wherein the preset test conditions comprise preset effective confining pressure, hydrate saturation and clay content;
specifically, step S100 includes:
step S110, constructing two cores of an unconsolidated clayey silt hydrate deposit and a weakly consolidated clayey silt hydrate deposit, and respectively simulating a sea area hydrate deposit and a land frozen soil hydrate deposit;
and S120, carrying out in-situ generated hydrate triaxial shear test on the two hydrate sediments under a preset experimental condition.
Wherein the preset experimental conditions comprise: under the conditions of effective confining pressure of 1MPa, 3MPa and 5MPa, hydrate saturation of 0%, 15%, 45% and 60%, clay content of 0%, 10% and 30%.
In a specific implementation, the step S120 includes:
the in-situ generated hydrate triaxial shear test was developed for two hydrate deposits (unconsolidated clayey silt hydrate deposit and weakly consolidated clayey silt hydrate deposit) under effective confining pressures of 1MPa, 3MPa and 5MPa, hydrate saturations of 0%, 15%, 45% and 60%, clay content of 0%, 10% and 30%.
Step S200: acquiring mechanical parameters of the hydrate deposit under corresponding conditions based on the experimental conditions of the triaxial shear test of the hydrate deposit, wherein the mechanical parameters of the hydrate deposit comprise shear strength, cohesive force and internal friction angle;
in a specific implementation, the step S200 includes:
based on the experimental conditions of the triaxial shear test of the hydrate deposit, the cohesive force and the internal friction angle of the hydrate deposit can be obtained according to a Mohr circle method;
acquiring the shear strength of the hydrate deposit according to a stress-strain curve obtained by the triaxial shear test of the hydrate deposit;
wherein, for a stress-strain curve with peak strength, the peak strength is taken as the core shear strength, and for a stress value with the strain of 15 percent is taken as the core shear strength for a stress-strain curve with the strain hardening and plasticity tendency.
Step S300: and establishing a corrected M-C criterion according to the mechanical parameters of the hydrate deposit.
Referring to fig. 1a to 1d and fig. 3, according to the M-C criterion, the shear strength is exactly equal to the shear strength on the envelope line where the point where the shear strength is tangent to the limit stress moire circle, the intersection point of the tangent line and the ordinate axis is the cohesive force, and the included angle between the tangent line and the abscissa axis is the inner friction angle.
From the relationship between the shear strength envelope and the limit stress Mohr circle, it can be known that
Figure BDA0002979669500000071
After the formula (11) is arranged, the following results are obtained:
Figure BDA0002979669500000072
wherein σ1Shear strength;
c is cohesion;
Figure BDA0002979669500000073
is an internal friction angle;
σ3is effective confining pressure.
The mechanical property experiment of the hydrate deposit core deposit shows that the hydrate saturation and the clay content have obvious influence on the cohesive force of the hydrate deposit core, and have weak influence on the internal friction angle, so that the influence can be ignored. Here, the hydrate deposit core cohesion is regarded as a hydrate saturation function, and the internal friction angle is independent of the hydrate saturation, so the step S300 specifically includes:
and taking the cohesive force of the hydrate sediment core as a hydrate saturation function, wherein the corrected M-C criterion is as follows:
Figure BDA0002979669500000074
wherein, because the hydrate has a certain cementing effect on sediment particles, under a certain effective surrounding pressure, the shear strength of the core containing the hydrate (the hydrate saturation is more than 0) is greater than that of the core without the hydrate (the hydrate saturation is 0). The part with increased shear strength is apparent strength, namely the strengthening effect of the hydrate gel on shear resistance is as follows:
Figure BDA0002979669500000075
the shear strength of the clayey silt hydrate deposit at different clay contents and hydrate saturation degrees is measured through experiments. The corresponding increases in hydrate deposit core shear strength at different effective ambient pressures from 0% to 15%, 45% and 60% increase in hydrate saturation are shown in table 1.
TABLE 1 hydrate saturation increase and shear strength increase corresponding thereto
Figure BDA0002979669500000076
Figure BDA0002979669500000081
Figure BDA0002979669500000091
According to the formula (2), the increase of the shear strength is caused by the increase of cohesion caused by the cementation of the sediment particles by the hydrate under the same effective confining pressure. Theoretically, when the saturation of the hydrate is increased by the same value, the increase value of the shear strength under different effective confining pressures should be the same. However, under the influence of experimental errors, the increase values of the shear strength at different confining pressures slightly fluctuate under the same increase value of the hydrate saturation shown in table 1. According to the data in the table 1, the fitting relational expression and the error (R) of the shear strength increase value and the hydrate saturation of the hydrate sediment cores with different clay contents under different effective surrounding pressures can be obtained2) The fitting relationship is shown in fig. 4.
The fitting of the hydrate sediment cores with different clay contents and the fitting relation between the shear strength increase value and the hydrate saturation and the corresponding error under different effective surrounding pressures comprises the following steps:
when the clay content of the unconsolidated clayey silt hydrate sediment is 0, y is 0.166x0.841，R2＝0.973；
When the content of clay in the unconsolidated clayey silt hydrate deposit is 10%, y is 0.703x0.555，R2＝0.932；
When the clay content of the unconsolidated clayey silt hydrate sediment is 30 percent, y is 1.576x0.439，R2＝0.949；
When the clay content of weakly cemented clayey silt hydrate sediment is 10%, y is 0.8495x0.553，R2＝0.879；
Wherein y is the shear strength increase value;
x is the hydrate saturation;
R2fitting relation expressions and errors of shear strength increase values and hydrate saturation of hydrate sediment cores with different clay contents under different effective surrounding pressures.
The shear strength increase values and the hydrate saturation of the hydrate sediments with different clay contents satisfy the power function relationship, and the process is as follows:
based on the experimental conditions of the triaxial shear test of the hydrate deposits, measuring the shear strength of the two hydrate deposits at different clay contents and hydrate saturation degrees;
according to the shear strength of the two hydrate sediments with different clay contents and hydrate saturation, obtaining the corresponding shear strength increase value of the hydrate sediment core when the hydrate saturation is increased under different effective confining pressures;
fitting the hydrate sediment cores with different clay contents, and fitting the shear strength increase value and the hydrate saturation degree under different effective surrounding pressures and corresponding errors.
Specifically, referring to fig. 4, the shear strength increase values and the hydrate saturation of the hydrate deposits with different clay contents satisfy a power function relationship:
△σ(sh)＝a·sh b； (3)
substituting (3) into (2) yields:
Figure BDA0002979669500000101
after finishing, the relation formula of the cohesive force of the hydrate core considering the influence of the saturation degree of the hydrate is as follows:
Figure BDA0002979669500000102
substituting (6) into (1) to obtain a corrected hydrate deposit shear strength expression considering hydrate saturation, effective confining pressure, cohesive force and internal friction angle:
Figure BDA0002979669500000103
the fitting constants a, b and the cohesion value at 0 of hydrate saturation for different clay contents are shown in Table 2.
TABLE 2 core fitting constants a, b and c(s) for hydrate deposits of different clay contentsh＝0)
Figure BDA0002979669500000104
Fitting the constants a and b and the relation between the cohesive force value and the clay content when the hydrate saturation is 0;
in concrete implementation, the fitting constants a and b and the cohesion value c(s) when the saturation of the hydrate is 0 are shown in the table 2h＝0) With clay content (c)v) The relationship of (a) was fitted as shown in fig. 5.
The relation between the fitting constants a and b and the cohesive force value when the hydrate saturation is 0 and the clay content comprises the following steps:
obtaining the fitting constants a and b of hydrate sediments at different clay contents and corresponding c(s)h＝0)、R2；
Fitting constants a, b and the relation between the cohesion value c (sh ═ 0) and the clay content (cv) at a hydrate saturation of 0, see fig. 5, the fitting relation includes:
c(sh＝0)＝1.88cv+0.82，R2＝0.99；
a＝4.65cv+0.19，R2＝0.99；
b＝7.6cv 2-3.62cv+0.84，R2＝1。
according to the fitting result, a, b and c(s) in the formula (6)h＝0) With the clay content (c) in FIG. 5v) Is characterized to obtain a corrected functionThe expression of the shear strength of the hydrate deposit by considering the clay content, the hydrate saturation, the effective confining pressure, the cohesion and the internal friction angle is as follows:
Figure BDA0002979669500000111
wherein the content of the first and second substances,
σ1(sh,σ3) Is a hydrate with a saturation of shEffective confining pressure of σ3Shear strength in time;
c(sh) Is a hydrate with a saturation of shCohesion in time;
c(sh＝0) The cohesive force is the cohesive force when the saturation of the hydrate is 0;
Figure BDA0002979669500000112
is an internal friction angle;
σ3effective confining pressure;
△σ(sh) Is a hydrate with a saturation of shThe shear strength is increased compared with the shear strength when the hydrate saturation is 0;
σ1(sh＝0,σ3) The saturation of hydrate is 0 and the effective confining pressure is sigma3Shear strength in time;
c and b are fitting constants;
shis the hydrate saturation;
Sh＝0represents a hydrate saturation of 0;
cvis the clay content.
To verify the accuracy of the modified M-C criterion proposed by the present invention, the shear strength calculated based on the modified M-C criterion was compared with the experimental results of Miyazaki, Masui, Yun, Hyodo, etc., and the results are shown in fig. 6. Since none of the above test samples contained clay, the fitting constants a, b, internal friction angle and cohesive force C (sh ═ 0) of the modified M-C criterion were 0.166, 0.841, 25.34 and 0.810, respectively.
As shown in FIG. 6, the effective confining pressure stress obtained by the previous experiment is 0.5MPa, and the shear strength at 1MPa and 3MPa is well matched with the calculation result based on the modified M-C criterion provided by the invention. Wherein the shear strength obtained by experiments of Hyodo and the like when the effective confining pressure stress is 3MPa is lower than the calculation result of the invention, but the change trend of the shear strength along with the hydrate saturation is consistent. The main reason is that the composition and experimental conditions of the hydrate deposit test piece which is experimentally researched by the predecessor are different from those of the invention, the measured shear strength is different from that of the hydrate deposit test piece, but the change trend of the shear strength along with the saturation is consistent, and the correction M-C criterion provided by the invention has certain accuracy.
The invention directly obtains the change curve of axial strain with deflection stress of different types of hydrate deposit rock cores under different conditions by constructing hydrate rock core samples with different cementing strengths and carrying out a triaxial shear test to test the mechanical parameters of in-situ synthesized hydrate deposits, so as to obtain the relationship of shear strength, cohesive force, internal friction angle and the like along with the clay content, hydrate saturation and confining pressure, the calculation method of the shear strength of the hydrate deposit considering the saturation degree of the hydrate and the clay content is obtained, the problem that the shear damage of the hydrate deposit cannot be judged by the conventional M-C rule at present is solved, the hydrate saturation degree and the clay content are introduced to correct the shear damage, the shear strength of the hydrate deposit can be accurately and effectively calculated, and a judgment basis is provided for the shear failure of the hydrate deposit.
The above description is only a preferred embodiment of the present invention, and not intended to limit the scope of the present invention, and all modifications and equivalents of the present invention, which are made by the contents of the present specification and the accompanying drawings, or directly/indirectly applied to other related technical fields, are included in the scope of the present invention.

Claims (8)

1. A method for calculating the shear strength of a hydrate deposit is characterized by comprising the following steps:
constructing a triaxial shear test of the hydrate deposit under preset experimental conditions, wherein the preset experimental conditions comprise preset effective confining pressure, hydrate saturation and clay content;
acquiring mechanical parameters of the hydrate deposit under corresponding conditions based on the experimental conditions of the triaxial shear test of the hydrate deposit, wherein the mechanical parameters of the hydrate deposit comprise shear strength, cohesive force and internal friction angle;
and establishing a corrected M-C criterion according to the mechanical parameters of the hydrate deposit.
2. The method for calculating the shear strength of the hydrate deposit according to claim 1, wherein the step of establishing the modified M-C criterion according to the mechanical parameters of the hydrate deposit specifically comprises:
and taking the cohesive force of the hydrate sediment core as a hydrate saturation function, wherein the corrected M-C criterion is as follows:
Figure FDA0002979669490000011
wherein, the part of shear strength increase is apparent strength, namely the strengthening effect of hydrate glue to shear is:
Figure FDA0002979669490000012
the shear strength increase value and the hydrate saturation of the hydrate sediment with different clay contents satisfy the power function relationship:
△σ(sh)＝a·sh b； (3)
substituting (3) into (2) yields:
Figure FDA0002979669490000013
after finishing, the relation formula of the cohesive force of the hydrate core considering the influence of the saturation degree of the hydrate is as follows:
Figure FDA0002979669490000014
substituting (6) into (1) to obtain a corrected hydrate deposit shear strength expression considering hydrate saturation, effective confining pressure, cohesive force and internal friction angle:
Figure FDA0002979669490000015
fitting the constants a and b and the relation between the cohesive force value and the clay content when the hydrate saturation is 0;
according to the fitting result, obtaining a corrected hydrate deposit shear strength expression considering the clay content, the hydrate saturation, the effective confining pressure, the cohesion and the internal friction angle:
Figure FDA0002979669490000021
σ1(sh,σ3) Is a hydrate with a saturation of shEffective confining pressure of σ3Shear strength in time;
c(sh) Is a hydrate with a saturation of shCohesion in time;
c(sh＝0) The cohesive force is the cohesive force when the saturation of the hydrate is 0;
Figure FDA0002979669490000022
is an internal friction angle;
σ3effective confining pressure;
△σ(sh) Is a hydrate with a saturation of shThe shear strength is increased compared with the shear strength when the hydrate saturation is 0;
σ1(sh＝0,σ3) The saturation of hydrate is 0 and the effective confining pressure is sigma3Shear strength in time;
a and b are fitting constants;
shis the hydrate saturation;
Sh＝0represents a hydrate saturation of 0;
cvis the clay content.
3. The method for calculating the shear strength of the hydrate deposit according to claim 2, wherein the step of constructing the triaxial shear test of the hydrate deposit under the preset experimental conditions comprises the following steps:
constructing two cores of an unconsolidated clayey silt hydrate deposit and a weakly consolidated clayey silt hydrate deposit, and respectively simulating a sea area hydrate deposit and a land frozen soil hydrate deposit;
and under the preset experimental condition, carrying out in-situ generated hydrate triaxial shear test on the two hydrate sediments.
4. The method for calculating the shear strength of the hydrate deposit according to claim 3, wherein the shear strength increase values and the hydrate saturation degrees of the hydrate deposits with different clay contents satisfy a power function relationship, and the method comprises the following steps:
based on the experimental conditions of the triaxial shear test of the hydrate deposits, measuring the shear strength of the two hydrate deposits at different clay contents and hydrate saturation degrees;
according to the shear strength of the two hydrate sediments with different clay contents and hydrate saturation, obtaining the corresponding shear strength increase value of the hydrate sediment core when the hydrate saturation is increased under different effective confining pressures;
fitting the hydrate sediment cores with different clay contents, and fitting the shear strength increase value and the hydrate saturation and corresponding errors under different effective surrounding pressures.
5. The method for calculating the shear strength of the hydrate deposit according to claim 4, wherein the fitting of the hydrate deposit cores with different clay contents and the fitting relation between the shear strength increase value and the hydrate saturation and the corresponding error under different effective ambient pressures comprises the following steps:
when the clay content of the unconsolidated clayey silt hydrate sediment is 0, y is 0.166x0.841，R2＝0.973；
When the content of clay in the unconsolidated clayey silt hydrate deposit is 10%, y is 0.703x0.555，R2＝0.932；
When the clay content of the unconsolidated clayey silt hydrate sediment is 30 percent, y is 1.576x0.439，R2＝0.949；
When the clay content of weakly cemented clayey silt hydrate sediment is 10%, y is 0.8495x0.553，R2＝0.879；
Wherein y is the shear strength increase value;
x is the hydrate saturation;
R2fitting relation expressions and errors of shear strength increase values and hydrate saturation of hydrate sediment cores with different clay contents under different effective surrounding pressures.
6. The method for calculating the shear strength of the hydrate deposit according to claim 5, wherein the fitting constants a and b and the relation between the cohesive force value when the hydrate saturation degree is 0 and the clay content comprise:
obtaining the fitting constants a and b of hydrate sediments at different clay contents and corresponding c(s)h＝0)、R2；
Fitting the fitting constants a, b and the relation between the cohesion value c (sh ═ 0) when the hydrate saturation is 0 and the clay content (cv), wherein the fitting relation comprises the following steps:
c(sh＝0)＝1.88cv+0.82，R2＝0.99；
a＝4.65cv+0.19，R2＝0.99；
b＝7.6cv 2-3.62cv+0.84，R2＝1。
7. the method for calculating the shear strength of a hydrate deposit according to claim 3, wherein the preset experimental conditions comprise:
under the conditions of effective confining pressure of 1MPa, 3MPa and 5MPa, hydrate saturation of 0%, 15%, 45% and 60%, clay content of 0%, 10% and 30%.
8. The method for calculating the shear strength of the hydrate deposit according to claim 1 or 2, wherein the step of obtaining the mechanical parameters of the hydrate deposit under the corresponding conditions based on the experimental conditions of the triaxial shear test of the hydrate deposit comprises:
based on the experimental conditions of the triaxial shear test of the hydrate deposit, the cohesive force and the internal friction angle of the hydrate deposit can be obtained according to a Mohr circle method;
acquiring the shear strength of the hydrate deposit according to a stress-strain curve obtained by the triaxial shear test of the hydrate deposit;
wherein, for a stress-strain curve with peak strength, the peak strength is taken as the core shear strength, and for a stress value with the strain of 15 percent is taken as the core shear strength for a stress-strain curve with the strain hardening and plasticity tendency.
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