A reservoir constitutive model considering the impact of hydrate extraction

Abstract

This invention discloses a reservoir constitutive model considering the impact of hydrate extraction. The technical solution is as follows: The constitutive model is established by conducting CO2 replacement experiments on artificially prepared methane hydrate-containing sediment samples, and performing triaxial compression experiments on the samples before and after replacement to obtain their stress-strain curves; the applicability of the Duncan-Chang model to natural gas hydrate reservoirs under the influence of CO2 replacement is verified based on the experimental results; based on the triaxial compression experiment results under different conditions, eight model parameters are corrected using the replacement rate and initial hydrate saturation parameters; the calculation curve of the established nonlinear constitutive model of natural gas hydrate reservoirs under the influence of CO2 replacement is compared with the experimental results. The beneficial effects are: this invention considers the influence of CO2 replacement, can accurately predict reservoir damage behavior, and has good applicability, providing assistance for theoretical research, numerical modeling, and engineering design related to natural gas hydrate extraction and CO2 storage.

Description

Technical Field

[0001] This invention relates to the field of natural gas hydrate extraction technology, and in particular to a reservoir constitutive model that takes into account the impact of hydrate extraction. Background Art

[0002] Natural gas hydrate (NGH) is a high-energy-density cage-like crystalline compound formed by natural gas and water molecules under low-temperature and high-pressure conditions. Currently, the main methods for extracting NGH include: thermal activation, depressurization, chemical reagent injection, and CO2 replacement. The first three methods alter the temperature and pressure environment of the NGH reservoir, causing it to no longer meet the hydrate phase equilibrium conditions, thus promoting the decomposition of NGH and the production of CH4 gas. However, this process can significantly reduce the reservoir's resistance to deformation due to hydrate decomposition, posing potential risks of sand production, reservoir subsidence, and submarine landslides. The basic idea of ​​the CO2 replacement method is to utilize the difference in formation and stability conditions between CO2 hydrate and CH4 hydrate, bringing CO2 and CH4 hydrate into contact under suitable temperature and pressure conditions for the replacement reaction. CO2 hydrate is generated simultaneously with the decomposition of CH4 hydrate. This method obtains CH4 while simultaneously sealing CO2 on the seabed, maintaining the reservoir's mechanical stability to a certain extent. This production technology, which utilizes gas exchange reactions, will become an important medium for combining safe natural gas extraction and CO2 sequestration, thereby obtaining energy while mitigating global climate deterioration caused by CO2 emissions.

[0003] Currently, many different constitutive models for natural gas hydrate reservoirs have been established by scholars based on different methods and purposes. However, none of these constitutive models have considered the influence of CO2 replacement. At present, research on CO2 replacement mainly focuses on the kinetic mechanism of the replacement reaction and how to improve the replacement efficiency. Economic considerations play a dominant role. There is little research on the mechanical safety of reservoirs during CO2 replacement mining, especially since a constitutive model that can characterize the stress-strain relationship changes in hydrate reservoirs during CO2 replacement mining has not yet been established. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned deficiencies in the existing technology by providing a reservoir constitutive model that considers the impact of hydrate mining.

[0005] The present invention mentions a reservoir constitutive model that considers the impact of hydrate exploitation. The technical solution is as follows: the establishment process of the constitutive model mainly includes the following steps:

[0006] Step 1: Perform CO2 replacement experiments on artificially prepared methane hydrate-containing sediment samples, and conduct triaxial compression experiments on the samples before and after replacement to obtain their stress-strain curves.

[0007] Step 2: Verify the applicability of the Duncan-Chang model to natural gas hydrate reservoirs under the influence of CO2 replacement based on experimental results; the Duncan-Chang expression is as follows:

[0008] (1)

[0009] Equation (1) can also be rewritten in the following two forms:

[0010] (2)

[0011] or (3)

[0012] in, It is a deviatoric stress; The strain is axial, determined experimentally. a , b These are experimental parameters that depend on the properties of the material;

[0013] Step 3: Based on the Duncan-Chang model, and according to the triaxial compression experiment results under different conditions, the eight model parameters of the Duncan-Chang model are corrected using the displacement rate and initial hydrate saturation parameters.

[0014] The displacement rate is defined as follows:

[0015] (4)

[0016] In the formula, or The replacement rate; n 0 represents the initial amount of CH4, in mol; n 1 represents the amount of CH4 remaining after the displacement, in mol;

[0017] Step 3 above specifically includes the following steps:

[0018] Step 3.1: Using the substitution rate or Initial hydrate saturation S h Parameters with respect to tangent modulus E t Make corrections;

[0019] The corrected tangent modulus E t The calculation formula is:

[0020] (2)

[0021] In the formula, m 0、m 1. m 2. m 3. c 0、 c 1. a 0、 a 1 is the correction coefficient, which is obtained by fitting experimental data; p a =0.1013MPa, representing standard atmospheric pressure; c For cohesion; f It is the internal friction angle; R f For destruction ratio;

[0022] Step 3.2: Using the displacement rate and initial hydrate saturation parameters to determine the tangent Poisson's ratio v t Make corrections;

[0023] The corrected tangent Poisson's ratio v t The calculation formula is:

[0024] (3)

[0025] In the formula, G 0、 F 0、 α 0 , β 0 , α 1. α 2. β 1. β 2 is the correction coefficient, which is obtained by fitting experimental data; D These are model parameters, dimensionless;

[0026] Step 4: Compare the calculation curves of the nonlinear constitutive model of the natural gas hydrate reservoir under the influence of CO2 replacement with the experimental results to verify its accuracy.

[0027] Preferably, the artificially prepared methane hydrate-containing sediment samples have different initial hydrate saturation and need to be prepared according to the mineral composition and grain size distribution of the actual natural gas hydrate reservoir.

[0028] Preferably, the above-mentioned CO2 replacement experiment needs to be carried out under temperature and pressure conditions that allow CH4 hydrate to decompose while CO2 hydrate can exist stably.

[0029] Preferably, the above-mentioned triaxial mechanical test is a conventional triaxial compression test. Or true triaxial compression test ;in, s 1 represents the maximum effective principal stress. s 2 represents the intermediate effective principal stress. s 3 represents the minimum effective principal stress; and triaxial mechanical experiments under different effective confining pressures are required.

[0030] Preferably, step 2 specifically includes the following steps:

[0031] Step 2.1: Determine whether the stress-strain curve obtained in Step 1 satisfies the hyperbolic characteristic;

[0032] Step 2.2: If the hyperbolic characteristic is satisfied in Step 2.1, the stress-strain curve obtained from the triaxial compression experiment is fitted to the specimen. ~ Relationship;

[0033] Step 2.3: Determine and Whether the relationship is linear can be used to determine whether the Duncan-Chang model is applicable to natural gas hydrate reservoirs under the influence of CO2 replacement.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] This invention combines CO2 replacement with triaxial mechanical experiments, and uses the replacement rate and initial hydrate saturation to correct the model parameters of the Duncan-Chang model. The established nonlinear constitutive model of natural gas hydrate reservoir takes into account the influence of CO2 replacement, and can accurately predict the reservoir's destructive behavior. It can be applied to theoretical research, numerical modeling and engineering design related to CO2 replacement extraction of natural gas hydrate, and provides theoretical exploration and support for ensuring energy security and achieving carbon reduction goals. Attached Figure Description

[0036] Figure 1 This is a flowchart illustrating the process of establishing the constitutive model of the hydrate reservoir in this invention.

[0037] Figure 2 This is a schematic diagram of the triaxial mechanical experimental system for low-temperature hydrates of the present invention;

[0038] Figure 3 The stress-strain curves of the CH4 hydrate-containing sediment samples in this invention under different replacement rates are shown.

[0039] Figure 4 The stress-strain curves of CH4 hydrate-containing sediment samples in this invention under different initial hydrate saturation and effective confining pressure are shown.

[0040] Figure 5 This is a comparison chart of the first experimental curve and the calculated curve after sample replacement in this invention;

[0041] Figure 6 This is a comparison chart of the second type of experimental curve and calculated curve after sample replacement in this invention;

[0042] Figure 7 This is a comparison chart of the third type of experimental curve and calculated curve after sample replacement in this invention;

[0043] Figure 8 This is a comparison chart of the fourth type of experimental curve and calculated curve after sample replacement in this invention;

[0044] In the diagram above: 1. Confining pressure pump; 2. Filling pump; 3. Hydraulic oil tank; 4. Valve; 5. Four-way valve; 6. Pressure reducing valve; 7. Pressure gauge; 8. CH4 gas cylinder; 9. CO2 gas cylinder; 10. N2 gas cylinder; 11. Control and data acquisition system; 12. Axial pressure system; 13. NaOH solution; 14. Drainage and gas collection device; 15. Pressure chamber; 16. Pressure head; 17. Sediment sample; 18. Deformation sensor; 19. Base; 20. Vent line; 21. T-junction; 22. Low-temperature cold storage. Detailed Implementation

[0045] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0046] Example 1: The reservoir constitutive model considering the impact of hydrate exploitation mentioned in this invention is established through the following steps:

[0047] Step 1: Conduct conventional triaxial compression tests on CH4 hydrate-containing sediment samples with initial hydrate saturation of 13%, 25%, and 38% at replacement times of 0 h, 6 h, 12 h, and 20 h, and effective confining pressures of 1 MPa, 2 MPa, and 3 MPa, respectively, to obtain the following results: Figure 3 and Figure 4 The stress-strain curve shown;

[0048] The CO2 displacement experiment and triaxial compression experiment mainly employ, as follows: Figure 2 The low-temperature hydrate triaxial mechanical experimental system shown is used for experiments. It includes subsystems such as a pressure loading system, a gas supply system, a gas collection system, and a temperature control system. The gas injection system includes CH4 cylinder 8, CO2 cylinder 9, N2 cylinder 10, a pressure reducing valve 6, a pressure gauge 7, and rigid pipelines. The gas collection system includes NaOH solution 13 and a drainage gas collection device 14. The temperature control system mainly refers to the low-temperature cold storage 22. This low-temperature hydrate triaxial mechanical experimental system can realize in-situ generation of hydrate-containing sediment samples, CH4-CO2 displacement experiments, and triaxial compression experiments of the samples before and after displacement.

[0049] The triaxial compression experiment was set with a loading rate of 0.25 mm·min. -1 .

[0050] The artificially prepared methane hydrate-containing sediment sample skeleton was fabricated based on the skeleton composition of rock samples collected in a certain area. The sediment mainly consists of clay and silt. In this embodiment, quartz sand particles with a particle size range of 4 μm to 125 μm were selected, and kaolin was used as the cement between the particles. The sample was fabricated according to the actual hydrate skeleton particle size distribution. f A 50×100 mm hydrate sediment sample skeleton.

[0051] After the methane hydrate-containing sediment sample skeleton is prepared, the hydrate is generated in a triaxial pressure chamber using the in-situ generation method: an excess of CH4 gas is introduced to generate methane hydrate with a fixed amount of water in the sample skeleton under low temperature and high pressure conditions.

[0052] The CO2 replacement experiment was conducted under the following temperature and pressure conditions: 278 K, 3 MPa. Under these conditions, CH4 hydrates decomposed while CO2 hydrates remained stable.

[0053] The specific steps of the CO2 replacement experiment described in this embodiment are as follows:

[0054] 1.1 After generating CH4 hydrate according to the experimental preset conditions, the CH4 hydrate-containing sediment sample was kept in a CO2 gas environment with a pressure of 3 MPa by adjusting the pressure reducing valve 6.

[0055] 1.2 Adjust the temperature of the low-temperature cold storage 22 to 278 K and begin displacement experiments for different preset times; after the displacement experiments are completed, conduct triaxial compression experiments.

[0056] 1.3 The temperature of the low-temperature cold storage 22 is raised to room temperature to decompose the remaining CH4 hydrate and generated CO2 hydrate in the hydrate-containing sediment sample. The decomposed gas is then passed into an excess of NaOH solution 13, and the CO2 gas is used to drive away the residual gas in the sample pores and pipelines.

[0057] 1.4 After the NaOH solution 13 reacts with the mixed gas, the CO2 gas is completely absorbed, leaving CH4 gas. The amount of remaining CH4 is calculated by converting the volume of CH4 gas collected by the water displacement gas collection device 14. The initial amount of CH4 can be calculated based on the initial hydrate saturation.

[0058] Step 2: Verify the applicability of the Duncan-Chang model to natural gas hydrate reservoirs under the influence of CO2 replacement based on experimental results;

[0059] The Duncan-Chang expression is as follows:

[0060] (1)

[0061] Equation (1) can also be rewritten in the following two forms:

[0062] (2)

[0063] (3)

[0064] in, It is a deviatoric stress; The strain is axial, determined experimentally. a , b These are experimental parameters that are determined by the properties of the material.

[0065] Step 2 specifically includes the following steps:

[0066] Step 2.1: Figure 3 and Figure 4 The stress-strain curves of the medium sample are all hyperbolic and exhibit strain hardening characteristics, which are similar to the Duncan-Chang model that describes the stress-strain relationship of soil materials. Therefore, a modified Duncan-Chang model can be considered to describe the constitutive relationship of natural gas hydrate reservoirs under the influence of CO2 replacement.

[0067] Step 2.2: Fitting the triaxial experimental data obtained when the initial hydrate saturation was 13% to... ~ Relationship;

[0068] Step 2.3: The results show that, except for extremely small axial strain, and The relationship is linear. This indicates that the stress-strain curves of the hydrate-bearing sediment samples before and after replacement conform to the Duncan-Chang hyperbolic model. The intercept and slope of the fitted straight line segment are the model parameters under the corresponding experimental conditions. a and b Model parameters when the initial hydrate saturation is 13%. a and b The summary is shown in Table 1.

[0069] Table 1 Model Parameters a and b Summary table ( S h =13%)

[0070]

[0071] Step 3: Based on the Duncan-Chang model, and according to the triaxial compression experiment results under different conditions, the eight model parameters of the Duncan-Chang model are corrected using the displacement rate and initial hydrate saturation parameters.

[0072] Furthermore, the replacement rate is defined as follows:

[0073] (4)

[0074] In the formula, or The replacement rate; n 0 represents the initial amount of CH4, in mol; n 1 represents the amount of CH4 remaining after the displacement, in mol.

[0075] Furthermore, step 3 specifically includes the following steps:

[0076] Step 3.1: Using the displacement rate and initial hydrate saturation parameters to determine the tangent modulus E t Make corrections.

[0077] In a single experiment Tangent modulus E t The definition is as follows:

[0078] (5)

[0079] From time to time

[0080] (6)

[0081] According to equation (2) From time to time

[0082] (7)

[0083] in, E i The initial tangent modulus of the stress-strain curve is an experimental parameter. a The reciprocal of; s 1 -s 3 ) ult The limiting deviatoric stress value represented by the asymptote of the stress-strain curve is an experimental parameter. b The reciprocal of; based on the experimental data of the initial hydrate saturation of 13% in this embodiment and the parameters fitted by equations (6) and (7), the parameters are obtained. E i and( s 1 -s 3 ) ult The results are summarized in Table 2.

[0084] Table 2 Parameters E i and( s 1 -s 3 ) ult Summary table ( S h =13%)

[0085]

[0086] In the Duncan-Chang model, the initial tangent modulus E i It can be expressed as a power function of the effective confining pressure:

[0087] (8)

[0088] In the formula, p a =0.1013MPa, representing standard atmospheric pressure; K , n These are model parameters, dimensionless.

[0089] The experimental results of this embodiment show the initial tangent modulus. E i The initial tangent modulus of the Duncan-Chang model increases with increasing effective confining pressure, and also increases with increasing replacement rate under different effective confining pressures. This indicates that the initial tangent modulus of the Duncan-Chang model under the influence of replacement... E i It is unreasonable to consider only the effect of effective confining pressure. Equation (8) should include a correction term representing the effect of the replacement process.

[0090] The fitting obtained according to equation (8) K The value increases with increasing replacement rate. n The value decreases as the substitution rate increases, indicating that in equation (8)... K and n These are two parameters related to the substitution rate. The model parameters are corrected using the substitution rate. K and n They can be represented as:

[0091] (9)

[0092] (10)

[0093] Substituting equations (9) and (10) into equation (8), we get:

[0094] (11)

[0095] In the formula, m 0、 m 1. m 2. m 3 represents the experimental constants. Table 3 summarizes the experimental constants at different saturation levels obtained through fitting the experimental data.

[0096] Table 3. Different initial hydrate saturations E i Summary of relevant experimental constants

[0097]

[0098] The axial strain in the experiment cannot be infinitely large; the limiting deviatoric stress of the hydrate-containing sediment sample. Numerically, it will be greater than the strength of the sample. ,definition and The ratio is the destruction ratio R f Used to determine value.

[0099] (12)

[0100] Based on the triaxial compression test results and data The values ​​were calculated to obtain the values ​​under different effective confining pressures and replacement rates when the initial hydrate saturation was 13%. R f The relationship between the destruction ratio and confining pressure and replacement rate was found to be insignificant. For CH4 hydrate-containing sediment samples with an initial hydrate saturation of 13%, the destruction ratio ranged from 0.92 to 0.97 under different replacement rates, with most values ​​around 0.94. An average value of 0.940 can be taken as the destruction ratio. Similarly, for CH4 hydrate-containing sediment samples with initial hydrate saturation of 25% and 38%, the destruction ratios under different replacement rates were 0.934 and 0.937, respectively. In summary, the influence of initial hydrate saturation, effective confining pressure, and replacement rate on the destruction ratio is relatively small and the pattern is not obvious. For the purpose of simplifying the model, the destruction ratio is taken as a constant value of 0.940 in this embodiment.

[0101] Substituting equation (12) into equation (7), we get:

[0102] (13)

[0103] Substituting equations (1), (6), and (13) into equation (5), we get

[0104] (14)

[0105] According to the Mohr-Coulomb strength criterion, we have

[0106] (15)

[0107] in, c For cohesion, f It is the internal friction angle. c and f The values ​​correspond to the arctangent values ​​of the intercept and slope of the common tangent of the Mohr's circle under different effective confining pressures. The experimental results of this embodiment show that as the initial hydrate saturation and replacement rate increase, the cohesive force tends to increase, while the change in the internal friction angle is small and the change pattern is not obvious. For the purpose of simplifying the model, this embodiment takes... f =24.05°. Let the cohesion be the initial hydrate saturation. S h and replacement rate or Functions:

[0108] (16)

[0109] (17)

[0110] (18)

[0111] In the formula, c 0、 c 1. a 0、 a 1 is an experimental constant, which can be obtained by fitting the cohesive force of the sample under different experimental conditions. The expression of the fitted equation (16) is as follows:

[0112] (19)

[0113] Substituting equations (8) and (15) into equation (14) yields any stress state. s 1 , s 3 The Duncan-Chang formula for calculating the tangent modulus at time t) is:

[0114] (20)

[0115] Substituting equations (11), (16), (17), and (18) into equation (20), we obtain the formula for calculating the tangent modulus after correction using the substitution rate and initial hydrate saturation:

[0116] (twenty one)

[0117] In equation (21), besides the variables s 1 , s 3 , S h , or All other parameters can be obtained directly from the triaxial test results or obtained by fitting, following the above steps. Thus, in this embodiment, the axial stress-strain relationship of the hydrate-bearing sediment sample under the influence of CO2 replacement has been corrected, that is, the five Duncan-Chang model parameters in equation (20) are... K , n , c , f , R f The influence of the replacement rate, a parameter representing the degree of replacement, was taken into account in all cases.

[0118] Step 3.2: Using the displacement rate and initial hydrate saturation parameters to determine the tangent Poisson's ratio v t Make corrections.

[0119] To establish a complete constitutive model, axial strain also needs to be discussed. and radial strain The relationship between the tangent Poisson's ratio and the tangent. v t Defined as:

[0120] (twenty two)

[0121] In a conventional triaxial compression test, axial strain and radial strain There is also a hyperbolic relationship between them, which is expressed as:

[0122] (twenty three)

[0123] in and D These are model parameters, dimensionless. Substituting equation (23) into equation (22) yields:

[0124] (twenty four)

[0125] hour, , v i The initial tangent Poisson's ratio. The CH4 hydrate-containing sediment sample with an initial hydrate saturation of 13% ( or Axial strain = 0) and radial strain The experimental data were fitted according to equation (23) to obtain the model parameters under different effective confining pressures. and D The value of . The experimental results of this embodiment show D The value has no significant relationship with the effective confining pressure. Other initial hydrate saturation and replacement rates... and D The values ​​also follow the same pattern described above, and all the fitted values... D The values ​​are all between 1 and 4, and are concentrated in the range of 1 to 4. To simplify the model, the parameters in this embodiment are... D The value is set to a constant of 2.

[0126] v i and s The relationship of 3 is shown in equation (25):

[0127] (25)

[0128] In the formula, G The initial tangent Poisson's ratio at standard atmospheric pressure; F To characterize v i Follow s 3. Experimental parameters for the rate of change. Based on the parameters... The experimental data and equation (25) can be fitted to obtain S h =13%, or When =0, the parameter G and F Similarly, the values ​​can be fitted to obtain the parameters under all experimentally preset initial hydrate saturation and displacement rate conditions. G and F The value of . Assume G and F It is the initial hydrate saturation. S h and replacement rate or The function is expressed in the following form:

[0129] (26)

[0130] (27)

[0131] (28)

[0132] (29)

[0133] (30)

[0134] (31)

[0135] Parameters can be fitted based on experimental data. G 0、 F 0、 α 0 , β 0 , α 1. α 2. β 1. β The value of 2. Substituting into equations (26) to (31) respectively, the expressions for equations (26) and (29) are as follows:

[0136] (32)

[0137] (33)

[0138] Substituting equations (32) and (33) into equation (25) yields the initial tangent Poisson ratio.

[0139] Substituting equations (3), (6), (13), (15), and (25) into equation (24), we obtain the expression for the tangent Poisson's ratio:

[0140] (34)

[0141] Substituting equations (11), (16), (17), (26)~(31) into equation (34), we obtain the formula for calculating the tangent Poisson's ratio after correction for the substitution rate and the initial hydrate saturation:

[0142] (35)

[0143] Besides variables s 1 , s 3 , S h , or All other parameters can be obtained directly from the triaxial test results or obtained by fitting, following the steps described above. Combining equations (35) and (21), all eight model parameters in the Duncan-Chang model have been determined in this embodiment. K , n , c , f , R f , D , G , FThe corrections are as follows. Constitutive models for natural gas hydrate reservoirs considering the effects of CO2 substitution have been established.

[0144] Step 4: Compare the calculation curves of the nonlinear constitutive model of the natural gas hydrate reservoir under the influence of CO2 replacement with the experimental results to verify its accuracy.

[0145] Figure 5-8 This is a comparison of stress-strain experimental results and calculated curves for a CH4 hydrate-containing sediment sample with an initial hydrate saturation of 13% under different effective confining pressures and replacement rates. The results show a good fit.

[0146] This invention designs and implements an integrated experiment combining CH4-CO2 replacement and triaxial compression, significantly reducing errors in the establishment of constitutive models for hydrate reservoirs under the influence of CO2 replacement mining due to experimental conditions and operations. The calculated curves under different experimental conditions fit the experimental results well, especially at the yield failure stage near peak stress, where the fitting accuracy is even higher, indicating that the model can accurately predict the stress-strain state at reservoir failure. The constitutive model establishment method proposed in this invention can provide support for subsequent theoretical exploration and engineering applications.

[0147] The above description is merely a partial preferred embodiment of the present invention. Any person skilled in the art can modify the above-described technical solutions or modify them into equivalent technical solutions. Therefore, any simple modifications or equivalent transformations made based on the technical solutions of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for establishing a reservoir constitutive model considering the impact of hydrate exploitation, characterized by: The following steps are involved: Step 1: Perform CO2 replacement experiments on artificially prepared methane hydrate-containing sediment samples, and conduct triaxial compression experiments on the samples before and after replacement to obtain their stress-strain curves. Step 2: Verify the applicability of the Duncan-Chang model to natural gas hydrate reservoirs under the influence of CO2 replacement based on experimental results; the Duncan-Chang expression is as follows: （1） Equation (1) can also be rewritten in the following two forms: （2） or (3) in, It is a deviatoric stress; The strain is axial, determined experimentally. a , b These are experimental parameters that depend on the properties of the material; Step 3: Based on the Duncan-Chang model, and according to the triaxial compression experiment results under different conditions, the eight model parameters of the Duncan-Chang model are corrected using the displacement rate and initial hydrate saturation parameters. The displacement rate is defined as follows: （4） In the formula, η The replacement rate; n 0 represents the initial amount of CH4, in mol; n 1 represents the amount of CH4 remaining after the displacement, in mol; Step 3 specifically includes the following steps: Step 3.1: Using the substitution rate η Initial hydrate saturation S h Parameters with respect to tangent modulus E t Make corrections; The corrected tangent modulus E t The calculation formula is: （2） In the formula, m 0、 m 1. m 2. m 3. c 0、 c 1. a 0、 a 1 is the correction coefficient, which is obtained by fitting experimental data; p a =0.1013MPa, representing standard atmospheric pressure; c For cohesion; φ It is the internal friction angle; R f For destruction ratio; Step 3.2: Using the displacement rate and initial hydrate saturation parameters to determine the tangent Poisson's ratio v t Make corrections; The corrected tangent Poisson's ratio v t The calculation formula is: （3） In the formula, G 0、 F 0、 α 0 , β 0 , α 1. α 2. β 1. β 2 is the correction coefficient, which is obtained by fitting experimental data; D These are model parameters, dimensionless; Step 4: Compare the calculation curves of the nonlinear constitutive model of the natural gas hydrate reservoir under the influence of CO2 replacement with the experimental results to verify its accuracy.

2. The method for establishing a reservoir constitutive model considering the impact of hydrate exploitation according to claim 1, characterized in that: The artificially prepared methane hydrate-containing sediment samples have different initial hydrate saturation and need to be prepared according to the mineral composition and grain size distribution of the actual natural gas hydrate reservoir.

3. The method for establishing a reservoir constitutive model considering the impact of hydrate exploitation according to claim 1, characterized in that: The CO2 replacement experiment must be carried out under temperature and pressure conditions that allow CH4 hydrates to decompose while CO2 hydrates can exist stably.

4. The method for establishing a reservoir constitutive model considering the impact of hydrate exploitation according to claim 1, characterized in that: The triaxial mechanics experiment is a conventional triaxial compression experiment. Or true triaxial compression test ;in, σ 1 represents the maximum effective principal stress. σ 2 represents the intermediate effective principal stress. σ 3 represents the minimum effective principal stress; and triaxial mechanical experiments under different effective confining pressures are required.

5. The method for establishing a reservoir constitutive model considering the impact of hydrate exploitation according to claim 1, characterized in that: Step 2 specifically includes the following steps: Step 2.1: Determine whether the stress-strain curve obtained in Step 1 satisfies the hyperbolic characteristic; Step 2.2: If the hyperbolic characteristic is satisfied in Step 2.1, the stress-strain curve obtained from the triaxial compression experiment is fitted to the specimen. ~ Relationship; Step 2.3: Determine and Whether the relationship is linear can be used to determine whether the Duncan-Chang model is applicable to natural gas hydrate reservoirs under the influence of CO2 replacement.

Images