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

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

A Calculation Method of Shear Strength of Hydrate Sediments

technical field

The embodiments of the present invention relate to the technical field of oil and gas field development, and in particular, to a method for calculating the shear strength of hydrate deposits.

Background technique

Geotechnical failure mainly includes tensile failure and shear failure, among which the Mohr-Cou lomb failure criterion (M-C criterion) is a commonly used strength criterion for judging shear failure. The conventional M-C criterion treats soil shear strength as a function of cohesion, internal friction angle and effective confining pressure. The strength of hydrate sediment depends on two parts, one is the cementation of the particles by the primary filler (clay, etc.) between the reservoir particles, and the other is the cementation of the sediment particles by the solid hydrate in the pores of the hydrate reservoir. Among them, the cementation effect of hydrate on sediment particles will change with the phase transition of hydrate. When the hydrate phase equilibrium is destroyed, the hydrate will decompose, and the cementation effect of hydrate on sediment particles will be weakened. Due to the cementation of hydrates to sediment particles, the mechanical properties of hydrate sediments are different from those of conventional oil and gas reservoirs. The basic mechanical properties of hydrate sediments are a dynamic process under different hydrate saturations. Therefore, the shear strength of hydrate sediment cores is not only a function of cohesion, internal friction angle and effective confining pressure, but also related to hydrate saturation and clay content; however, the current conventional M-C criterion cannot determine whether hydrate sediments are Shear failure occurs.

SUMMARY OF THE INVENTION

The purpose of the embodiments of the present invention is to provide a method for calculating the shear strength of hydrate sediments, aiming to solve the problem that the conventional M-C criterion in the prior art cannot judge whether the hydrate sediments undergo shear damage.

In order to solve the above technical problems, embodiments of the present invention provide a method for calculating the shear strength of hydrate deposits, including:

Under preset experimental conditions, construct a hydrate sediment triaxial shear test, and the preset experimental conditions include preset effective confining pressure, hydrate saturation, and clay content;

Based on the experimental conditions of the hydrate sediment triaxial shear test, the mechanical parameters of the hydrate sediment under the corresponding conditions are obtained, and the mechanical parameters of the hydrate sediment include shear strength, cohesion, and internal friction angle. ;

According to the mechanical parameters of the hydrate deposits, the revised M-C criterion is established.

Preferably, the step of establishing the revised M-C criterion according to the mechanical parameters of the hydrate sediments specifically includes:

Taking hydrate sediment core cohesion as a function of hydrate saturation, the revised M-C criterion is:

Among them, the increase of shear strength is the apparent strength, that is, the strengthening effect of hydrate cementation against shear is:

The increase in shear strength of hydrate sediments with different clay contents and hydrate saturation satisfy a power function relationship:

△σ(sh _{)=a·s h b} ^; (3)

Substituting (3) into (2), we get:

After finishing (4), the hydrate core cohesion formula considering the influence of hydrate saturation can be obtained as:

Substituting (6) into (1), the revised expression of shear strength of hydrate deposits considering hydrate saturation, effective confining pressure, cohesion and internal friction angle is obtained:

Fitting constants a, b and the relationship between the cohesion value and the clay content when the hydrate saturation is 0;

According to the fitting results, the revised shear strength expression of hydrate deposits considering clay content, hydrate saturation, effective confining pressure, cohesion and internal friction angle is obtained:

σ ₁ ( _sh ,σ ₃ ) is the shear strength when the hydrate saturation is _sh and the effective confining pressure is σ ₃ ;

c(sh _{) is the cohesive force when the hydrate saturation is s h ;}

c(sh ₌₀ ) is the cohesive force when the hydrate saturation is 0;

为内摩擦角；

is the angle of internal friction;

σ ₃ is the effective confining pressure;

△ _σ ( _sh ) is the increase in shear strength when the hydrate saturation is sh compared to the shear strength when the hydrate saturation is 0;

σ ₁ (sh ₌₀ ,σ ₃ ) is the shear strength when the hydrate saturation is 0 and the effective confining pressure is σ ₃ ;

b, b is the fitting constant;

s _h is the hydrate saturation;

_Sh=0 means the hydrate saturation is 0;

_cv is the clay content.

Preferably, the step of constructing a hydrate sediment triaxial shear test under preset experimental conditions:

Two types of cores, unconsolidated clayey silt hydrate deposits and weakly cemented clayey silt hydrate deposits, were constructed to simulate marine hydrate deposits and terrestrial permafrost hydrate deposits, respectively;

Under preset experimental conditions, the in-situ hydrate triaxial shear test was carried out on the two hydrate deposits.

Preferably, the increase in shear strength of the hydrate deposits with different clay contents and the hydrate saturation satisfy a power function relationship, and the obtaining process is as follows:

Based on the experimental conditions of the hydrate sediment triaxial shear test, the shear strengths of the two hydrate sediments at different clay contents and hydrate saturation were measured;

According to the shear strength of the two hydrate sediments at different clay content and hydrate saturation, under different effective confining pressures, when the hydrate saturation increases, the corresponding increase in shear strength of the hydrate sediment core is obtained;

Fitting the hydrate sediment cores with different clay contents, under different effective confining pressures, the fitting relationship between the increase in shear strength and the hydrate saturation and the corresponding errors.

Preferably, for the fitting of hydrate sediment cores with different clay contents, under different effective confining pressures, the fitting relationship between the increase in shear strength and the hydrate saturation and the corresponding errors include:

When the clay content of the unconsolidated clayey silt hydrate deposits is 0, y=0.166x ^0.841 and R ² =0.973;

When the clay content of the unconsolidated clayey silt hydrate deposit is 10%, y=0.703× ^0.555 , R ² =0.932;

When the clay content of unconsolidated clay silt hydrate deposits is 30%, y=1.576× ^0.439 , R ² =0.949;

When the clay content of weakly cemented clay silt hydrate deposits is 10%, y=0.8495× ^0.553 , R ² =0.879;

Among them, y is the increase value of shear strength;

x is the hydrate saturation;

R ² is the expression and error of the fitting relationship between the increase in shear strength and the hydrate saturation for hydrate sediment cores with different clay contents under different effective confining pressures.

Preferably, the fitting constants a, b and the relationship between the cohesion value and the clay content when the hydrate saturation is 0, include:

Obtain the fitting constants a, b and the corresponding c (sh ₌ 0) and R2 of hydrate sediments with different clay contents;

Fit the relationship between the fitting constants a, b and the cohesion value c (sh=0) and the clay content (cv) when the hydrate saturation is 0. The fitting relationship includes:

c(sh ₌₀ )= _1.88cv ⁺ 0.82, R2=0.99;

a=4.65c _v +0.19, R ² =0.99;

b=7.6c _v ² -3.62c _v +0.84, R ² =1.

Preferably, the preset experimental conditions include:

The effective confining pressure is 1MPa, 3MPa and 5MPa, the hydrate saturation is 0%, 15%, 45% and 60%, and the clay content is 0%, 10% and 30%.

Preferably, the step of obtaining the mechanical parameters of the hydrate sediment under the corresponding conditions based on the experimental conditions of the hydrate sediment triaxial shear test, includes:

Based on the experimental conditions of the hydrate sediment triaxial shear test, the cohesion and internal friction angle of the hydrate sediment can be obtained according to the Mohr circle method;

According to the stress-strain curve obtained by the triaxial shear test of the hydrate deposit, the shear strength of the hydrate deposit is obtained;

Among them, for the stress-strain curve with peak strength, the peak strength is taken as the core shear strength, and for those showing strain hardening and plastic trends, the stress value when the strain is 15% is taken as the core shear strength.

The invention constructs hydrate core samples with different cementation strengths, carries out triaxial shear test to test the mechanical parameters of in-situ synthetic hydrate sediments, and directly obtains the axial strain of different types of hydrate sediment cores under different conditions. The relationship between shear strength, cohesion, internal friction angle, etc. with clay content, hydrate saturation, and confining pressure can be obtained, and the hydrate deposition considering hydrate saturation and clay content can be obtained. The method for calculating the shear strength of hydrate deposits solves the problem that the current conventional M-C criterion cannot judge the shear failure of hydrate sediments. By introducing hydrate saturation and clay content to correct them, hydrate sediments can be accurately and effectively calculated. The shear strength provides a basis for judging the shear failure of hydrate sediments.

Description of drawings

One or more embodiments are exemplified by the pictures in the corresponding drawings, and these exemplifications do not constitute limitations of the embodiments, and elements with the same reference numerals in the drawings are denoted as similar elements, Unless otherwise stated, the figures in the accompanying drawings do not constitute a scale limitation.

Figure 1a shows the Mohr circle when the hydrate saturation is 0% when the clay content is 30%;

Figure 1b shows the Mohr circle when the clay content is 30% and the hydrate saturation is 15%;

Fig. 1c shows the Mohr circle when the hydrate saturation is 45% when the clay content is 30%;

Figure 1d shows the Mohr circle when the clay content is 30% and the hydrate saturation is 60%;

Fig. 2a shows the stress-strain curve of the unconsolidated clayey silt hydrate sediment core when the clay content is 30% and the hydrate saturation is 0%;

Fig. 2b shows the stress-strain curve of unconsolidated clayey silt hydrate sediment core when the clay content is 30% and the hydrate saturation is 15%;

Fig. 2c shows the stress-strain curve of the unconsolidated clayey silt hydrate sediment core when the clay content is 30% and the hydrate saturation is 45%;

Fig. 2d shows the stress-strain curve of the unconsolidated clayey silt hydrate sediment core when the clay content is 30% and the hydrate saturation is 60%;

Fig. 3 is the schematic diagram of Mohr's circle;

Figure 4 is the fitting relationship expression between the increase in shear strength and the hydrate saturation;

Fig. 5 is the fitting relation expression of fitting parameter and clay content;

Figure 6 is the comparison result between the calculated value of shear strength and the experimental value;

Fig. 7 is a flow chart of a method for calculating the shear strength of hydrate deposits.

The realization, functional characteristics and advantages of the present invention will be further described with reference to the accompanying drawings in conjunction with the embodiments.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

It should be noted that if there are directional indications (such as up, down, left, right, front, back, etc.) involved in the embodiments of the present invention, the directional indications are only used to explain a certain posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication also changes accordingly.

In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for the purpose of description, and should not be construed as indicating or implying Its relative importance or implicitly indicates the number of technical features indicated. Thus, a feature delimited with "first", "second" may expressly or implicitly include at least one of that feature. In addition, the technical solutions between the various embodiments can be combined with each other, but must be based on the realization by those of ordinary skill in the art. When the combination of technical solutions is contradictory or cannot be realized, it should be considered that the combination of such technical solutions does not exist. , is not within the scope of protection required by the present invention.

The present invention provides a method for calculating the shear strength of hydrate deposits. Please refer to Figure 7. The method for calculating the shear strength of hydrate deposits includes:

Step S100: constructing a hydrate sediment triaxial shear test under preset experimental conditions, where the preset experimental conditions include preset effective confining pressure, hydrate saturation, and clay content;

Specifically, step S100 includes:

Step S110, constructing two cores of uncemented clayey silt hydrate deposits and weakly cemented clayey silt hydrate deposits, respectively simulating sea hydrate deposits and terrestrial frozen soil hydrate deposits;

Step S120, under preset experimental conditions, perform an in-situ hydrate triaxial shear test on the two types of hydrate deposits.

The preset experimental conditions include: the effective confining pressure is 1MPa, 3MPa and 5MPa, the hydrate saturation is 0%, 15%, 45% and 60%, and the clay content is 0%, 10% and 30% condition.

When specifically implemented, the step S120 includes:

Under the conditions of effective confining pressure of 1MPa, 3MPa and 5MPa, hydrate saturation of 0%, 15%, 45% and 60%, and clay content of 0%, 10% and 30%, the two hydrate sediments ( Uncemented clayey silt hydrate deposits and weakly cemented clayey silt hydrate deposits) carry out in-situ hydrate triaxial shear tests.

Step S200: Based on the experimental conditions of the hydrate sediment triaxial shear test, the mechanical parameters of the hydrate sediment under the corresponding conditions are obtained, and the mechanical parameters of the hydrate sediment include shear strength, cohesion, internal friction angle;

When specifically implemented, the step S200 includes:

Based on the experimental conditions of the hydrate sediment triaxial shear test, the cohesion and internal friction angle of the hydrate sediment can be obtained according to the Mohr circle method;

According to the stress-strain curve obtained by the triaxial shear test of the hydrate deposit, the shear strength of the hydrate deposit is obtained;

Among them, for the stress-strain curve with peak strength, the peak strength is taken as the core shear strength, and for those showing strain hardening and plastic trends, the stress value when the strain is 15% is taken as the core shear strength.

Step S300: Establish a revised M-C criterion according to the mechanical parameters of the hydrate deposit.

Please refer to Figure 1a to Figure 1d and Figure 3. According to the M-C criterion, the point on the envelope where the shear strength is tangent to the Mohr circle of ultimate stress is on the envelope, and the shear stress value is exactly equal to the shear strength. The intersection of the coordinate axes is the cohesion force, and the angle between it and the abscissa axis is the internal friction angle.

According to the relationship between the shear strength envelope and the Mohr circle of ultimate stress, it can be known that

After arranging formula (11), we get:

Among them, σ ₁ is the shear strength;

c is the cohesion;

为内摩擦角；

is the angle of internal friction;

σ ₃ is the effective confining pressure.

According to the experiment on the mechanical properties of hydrate sediment core sediments, it can be seen that the hydrate saturation and clay content have a significant effect on the cohesion of the hydrate sediment core, but have a weak effect on the internal friction angle, which can be ignored. Here, the cohesion of the hydrate sediment core is regarded as a function of the hydrate saturation, and the internal friction angle has nothing to do with the hydrate saturation. Therefore, the step S300 specifically includes:

Taking hydrate sediment core cohesion as a function of hydrate saturation, the revised M-C criterion is:

Among them, because hydrate has a certain cementation effect on sediment particles, under a certain effective confining pressure, the shear strength of the core containing hydrate (hydrate saturation > 0) is higher than that without hydrate (hydrate saturation). =0) The shear strength of the core should be large. The increase in shear strength is the apparent strength, that is, the strengthening effect of hydrate cementation against shear is:

The shear strength of clay silt hydrate sediments with different clay content and hydrate saturation has been measured experimentally. Under different effective confining pressures, when the hydrate saturation increases from 0% to 15%, 45% and 60%, the corresponding increase in shear strength of hydrate sediment cores is shown in Table 1.

Table 1 The increase value of hydrate saturation corresponding to the increase value of shear strength

According to formula (2), under the same effective confining pressure, the increase of shear strength is caused by the increase of cohesion caused by the cementation of hydrate to sediment particles. Theoretically, when the hydrate saturation increases by the same value, the increase in shear strength should also be the same under different effective confining pressures. However, affected by the experimental error, under the same hydrate saturation increase value shown in Table 1, the increase value of shear strength under different confining pressures fluctuates slightly. According to the data in Table 1, the hydrate sediment cores with different clay contents can be obtained. Under different effective confining pressures, the fitting relationship expression and error (R ² ) of the increase in shear strength and hydrate saturation, the fitting See Figure 4 for the relationship.

The fitting relationship of the hydrate sediment cores with different clay contents, under different effective confining pressures, the fitting relationship between the increase in shear strength and the hydrate saturation and the corresponding errors include:

When the clay content of the unconsolidated clayey silt hydrate deposits is 0, y=0.166x ^0.841 and R ² =0.973;

When the clay content of the unconsolidated clayey silt hydrate deposit is 10%, y=0.703× ^0.555 , R ² =0.932;

When the clay content of unconsolidated clay silt hydrate deposits is 30%, y=1.576× ^0.439 , R ² =0.949;

When the clay content of weakly cemented clay silt hydrate deposits is 10%, y=0.8495× ^0.553 , R ² =0.879;

Among them, y is the increase value of shear strength;

x is the hydrate saturation;

R ² is the expression and error of the fitting relationship between the increase in shear strength and the hydrate saturation for hydrate sediment cores with different clay contents under different effective confining pressures.

The increase in shear strength of hydrate sediments with different clay contents and hydrate saturation satisfy a power function relationship, and the process is as follows:

Based on the experimental conditions of the hydrate sediment triaxial shear test, the shear strengths of the two hydrate sediments at different clay contents and hydrate saturation were measured;

According to the shear strength of the two hydrate sediments at different clay content and hydrate saturation, under different effective confining pressures, when the hydrate saturation increases, the corresponding increase in shear strength of the hydrate sediment core is obtained;

Fitting the hydrate sediment cores with different clay contents, under different effective confining pressures, the fitting relationship between the increase in shear strength and the hydrate saturation and the corresponding errors.

Specifically, referring to Fig. 4, the increase in shear strength of hydrate sediments with different clay contents and hydrate saturation satisfy a power function relationship:

△σ(sh _{)=a·s h b} ^; (3)

Substituting (3) into (2), we get:

After finishing (4), the hydrate core cohesion formula considering the influence of hydrate saturation can be obtained as:

Substituting (6) into (1), the revised expression of shear strength of hydrate deposits considering hydrate saturation, effective confining pressure, cohesion and internal friction angle is obtained:

Table 2 shows the fitting constants a, b and cohesion values when the hydrate saturation is 0 under different clay contents.

Table 2. Fitting constants a, b and c of hydrate sediment cores with different clay contents (sh _{= 0} )

Fitting constants a, b and the relationship between the cohesion value and the clay content when the hydrate saturation is 0;

In the specific implementation, fitting is performed according to the fitting constants a, b in Table 2 and the relationship between the cohesion value c (sh ₌₀ ) and the clay content ( _cv ) when the hydrate saturation is 0, as shown in Figure 5 .

The fitting constants a, b and the relationship between the cohesion value and the clay content when the hydrate saturation is 0 include:

Obtain the fitting constants a, b and the corresponding c(sh ₌₀ ) and R2 of hydrate sediments with different clay contents;

Fit the relationship between the fitting constants a, b and the cohesion value c (sh=0) and the clay content (cv) when the hydrate saturation is 0, see Figure 5, and the fitting relationship includes:

c(sh ₌₀ )= _1.88cv ⁺ 0.82, R2=0.99;

a=4.65c _v +0.19, R ² =0.99;

b=7.6c _v ² -3.62c _v +0.84, R ² =1.

According to the fitting results, a, b and c (sh _{= 0} ) in formula (6) are characterized by the function of clay content (c _v ) in Fig. 5, and the corrected clay content and hydrate saturation are obtained. , the effective confining pressure, cohesion and internal friction angle of the hydrate sediment shear strength expression:

in,

σ ₁ ( _sh ,σ ₃ ) is the shear strength when the hydrate saturation is _sh and the effective confining pressure is σ ₃ ;

c(sh _{) is the cohesive force when the hydrate saturation is s h ;}

c(sh ₌₀ ) is the cohesive force when the hydrate saturation is 0;

为内摩擦角；

is the angle of internal friction;

σ ₃ is the effective confining pressure;

△ _σ ( _sh ) is the increase in shear strength when the hydrate saturation is sh compared to the shear strength when the hydrate saturation is 0;

σ ₁ (sh ₌₀ ,σ ₃ ) is the shear strength when the hydrate saturation is 0 and the effective confining pressure is σ ₃ ;

c, b are fitting constants;

s _h is the hydrate saturation;

_Sh=0 means the hydrate saturation is 0;

_cv is the clay content.

In order to verify the accuracy of the revised M-C criterion proposed in the present invention, the shear strength calculated based on the revised M-C criterion was compared with the experimental results of Miyazaki, Masui, Yun, Hyodo, etc. The results are shown in Figure 6. Since the above test samples do not contain clay, the fitting constants a, b, internal friction angle and cohesion c (sh=0) in the modified M-C criterion are 0.166, 0.841, 25.34 and 0.810, respectively.

As shown in Figure 6, the effective confining compressive stress obtained by previous experiments is 0.5 MPa, and the shear strength at 1 MPa and 3 MPa is in good agreement with the calculation results based on the modified M-C criterion proposed by the present invention. Among them, the shear strength obtained by Hyodo et al. was lower than the calculation result of the present invention when the effective confining compressive stress was 3MPa, but the variation trend of shear strength with hydrate saturation was consistent. This is mainly because the composition and experimental conditions of the hydrate sediment specimens studied by previous experiments are different from those of the present invention, and the measured shear strength is different from this paper, but the variation trend of shear strength with saturation is consistent, indicating that The revised M-C criterion proposed by the present invention has certain accuracy.

The invention constructs hydrate core samples with different cementation strengths, carries out triaxial shear test to test the mechanical parameters of in-situ synthetic hydrate sediments, and directly obtains the axial strain of different types of hydrate sediment cores under different conditions. The relationship between shear strength, cohesion, internal friction angle, etc. with clay content, hydrate saturation, and confining pressure can be obtained, and the hydrate deposition considering hydrate saturation and clay content can be obtained. The method for calculating the shear strength of hydrate deposits solves the problem that the current conventional M-C criterion cannot judge the shear failure of hydrate sediments. By introducing hydrate saturation and clay content to correct them, hydrate sediments can be accurately and effectively calculated. The shear strength provides a basis for judging the shear failure of hydrate sediments.

The above are only the preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Under the inventive concept of the present invention, the equivalent structure transformation made by the contents of the description and drawings of the present invention, or directly/indirectly applied to other All relevant technical fields are included in the scope of patent protection 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:

wherein, the part of shear strength increase is apparent strength, namely the strengthening effect of hydrate glue to shear is:

the shear strength increase value and the hydrate saturation of the hydrate sediment with different clay contents satisfy the power function relationship:

△σ(s_h)＝a·s_h ^b； (3)

substituting (3) into (2) yields:

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:

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:

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:

σ₁(s_h,σ₃) Is a hydrate with a saturation of s_hEffective confining pressure of σ₃Shear strength in time;

c(s_h) Is a hydrate with a saturation of s_hCohesion in time;

c(s_h＝0) The cohesive force is the cohesive force when the saturation of the hydrate is 0;

is an internal friction angle;

σ₃effective confining pressure;

△σ(s_h) Is a hydrate with a saturation of s_hThe shear strength is increased compared with the shear strength when the hydrate saturation is 0;

σ₁(s_h＝0,σ₃) The saturation of hydrate is 0 and the effective confining pressure is sigma₃Shear strength in time;

a and b are fitting constants;

s_his the hydrate saturation;

S_h＝0represents a hydrate saturation of 0;

c_vis 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 0.166x^0.841，R²＝0.973；

When the content of clay in the unconsolidated clayey silt hydrate deposit is 10%, y is 0.703x^0.555，R²＝0.932；

When the clay content of the unconsolidated clayey silt hydrate sediment is 30 percent, y is 1.576x^0.439，R²＝0.949；

When the clay content of weakly cemented clayey silt hydrate sediment is 10%, y is 0.8495x^0.553，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(s_h＝0)＝1.88c_v+0.82，R²＝0.99；

a＝4.65c_v+0.19，R²＝0.99；

b＝7.6c_v ²-3.62c_v+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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