Shale triaxial compression numerical simulation method based on seepage-mechanical coupling effect

Abstract

The invention belongs to the technical field of shale seepage mechanics and numerical simulation, and discloses a shale triaxial compression numerical simulation method based on a seepage-mechanics coupling effect. Existing numerical simulation only considers a mechanical field or a seepage field, and neglects a bidirectional coupling effect between the two fields. The problems of unreasonable boundary conditions, untight crack and seepage coupling and the like are solved, and the failure rule of the shale under the seepage action is difficult to reflect truly. According to the shale triaxial compression numerical simulation method based on the seepage-mechanical coupling effect, a shale mesoscopic model is established through a particle flow numerical method, a seepage field is solved through a finite volume method, and bidirectional coupling of the seepage field and a mechanical field is achieved. The device can dynamically simulate fracture initiation, expansion and penetration processes of shale under the action of seepage-mechanical coupling, accurately predicts mechanical properties and permeability evolution of shale, and is suitable for shale gas development, borehole wall stability analysis and underground engineering safety evaluation.

Description

Technical Field

[0001] This invention relates to the field of shale seepage mechanics and numerical simulation technology, specifically a numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling. Background Technology

[0002] Shale, as a sedimentary rock, is characterized by low porosity, extremely low permeability, and significant anisotropy in bedding, making it a major reservoir for unconventional oil and gas resources (such as shale gas and shale oil). During the exploration and development of shale gas, key engineering techniques such as horizontal drilling and hydraulic fracturing place the reservoir rocks in an extremely complex multi-physics coupled environment.

[0003] Rocks are subjected not only to three-dimensional geostress from overlying strata and engineering disturbances, but also to pore pressure changes and seepage forces caused by fracturing fluids or formation fluids. This interaction between the stress field and the seepage field (i.e., fluid-structure interaction) profoundly affects the strength, deformation characteristics, failure modes, and fracture network propagation patterns of shale.

[0004] Therefore, developing numerical simulation tools that can accurately describe the flow-mechanical coupling behavior of shale is of vital theoretical and engineering value for optimizing fracturing design, predicting production capacity, preventing wellbore instability, and assessing long-term engineering risks.

[0005] Currently, the research methods for shale mechanics and seepage characteristics are mainly divided into two categories: indoor physical experiments and numerical simulations.

[0006] Laboratory experiments are the most direct way to obtain the true physical and mechanical parameters of rocks. For example, conventional triaxial compression tests can determine the peak strength, elastic modulus, and Poisson's ratio of shale; triaxial compression-seepage coupling experiments can be used to study the evolution of shale strength, deformation, and permeability under different confining pressures and pore pressures.

[0007] Although indoor physics experiments can provide valuable experimental data, their limitations are also very prominent: First, the experimental costs are high, and it is time-consuming and laborious to prepare high-quality shale samples; second, it is often difficult to observe the entire process of microcrack initiation, propagation and penetration inside the sample in real time and without damage; third, it is difficult to simulate experimental conditions and it is difficult to accurately control all variables.

[0008] Numerical simulation methods, as an important supplement and extension to physical experiments, have been widely used due to their advantages such as low cost, high repeatability, and ability to provide full-field information. Existing numerical simulation methods mainly include the finite element method, finite difference method, boundary element method, and discrete element method.

[0009] For example, the RFPA2D-Flow system was used to simulate the fracturing process of shale under fluid-structure interaction, revealing the three stages of its failure: elasticity, yielding, and failure, as well as the anisotropy of compressive strength and elastic modulus.

[0010] However, this continuous medium method usually treats the rock as an equivalent continuum, and introduces damage variables or plastic internal variables into its constitutive relation to characterize the material's deterioration. For the treatment of seepage-mechanical coupling, a simple one-way coupling or a simplified two-way coupling model is often used.

[0011] This simplification makes it difficult for the model to accurately reflect the complex interaction between seepage channels and stress redistribution during the evolution of shale from a continuum to a discontinuum, especially in predicting fracture propagation paths and final morphology.

[0012] In summary, the main problems with existing technologies can be summarized as follows: (1) Limitations of experimental observation: Indoor experiments cannot reveal the dynamic interaction mechanism between seepage and crack propagation in real time and intuitively at the microscale, and there are blind spots in the understanding of the underlying mechanism.

[0013] (2) Insufficient single-physics simulation: Traditional numerical simulation often only considers the mechanical field or the seepage field, ignoring the two-way feedback between the two, resulting in a large difference between the prediction results and the actual situation.

[0014] (3) Imperfect coupling mechanism: Some existing fluid-structure interaction models have problems such as simplified parameter settings, unreasonable boundary conditions, and weak coupling between cracks and seepage channels, making it difficult to truly reflect the mechanical failure law of shale under seepage.

[0015] Therefore, there is an urgent need for a numerical simulation method that can accurately describe the bidirectional coupling effect of shale seepage and mechanical field at the microscopic level and is applicable to triaxial compression conditions, so as to better reveal the failure mechanism of shale and provide theoretical basis and technical support for wellbore stability evaluation and unconventional oil and gas development. Summary of the Invention

[0016] The purpose of this invention is to provide a numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling, so as to solve the problems mentioned in the background art.

[0017] The main design concept of this invention is as follows: This invention aims to overcome the shortcomings of existing shale seepage-mechanical simulation methods and provide a numerical simulation method that can achieve bidirectional coupling of the seepage field and mechanical field at the microscale and is applicable to triaxial compression conditions of shale. This method can simultaneously consider the permeability evolution and crack propagation process of shale, thereby achieving high-precision simulation of the actual failure behavior of shale under seepage-mechanical coupling.

[0018] This invention can not only effectively reveal the initiation, expansion and connection mechanism of internal fractures in shale under the action of seepage, but also be used to predict the variation law of shale fracture pressure, peak intensity and permeability, providing scientific basis and numerical reference for unconventional oil and gas extraction, geological storage and underground engineering design.

[0019] To achieve the above objectives, the present invention provides the following technical solution: A numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling includes the following steps: Establish shale particle flow samples and calibrate their microscopic parameters; Establish the seepage field model and perform initial configuration; Based on shale particle flow samples and seepage field models, a triaxial compression numerical simulation mechanism with two-way coupling of seepage and mechanics is constructed. Run the triaxial compression numerical simulation mechanism until the simulation termination condition is met, then exit and complete the triaxial compression numerical simulation process. Output the data from the triaxial compression numerical simulation process.

[0020] More preferably, the method for establishing shale particle flow samples and calibrating their microscopic parameters includes the following steps: Two-dimensional shale particle flow samples were established using the PFC2D tool; Set the contact model, boundary conditions, and loading conditions for the shale particle flow sample; Microscopic parameters of shale particle flow samples were calibrated.

[0021] More preferably, the microscopic parameters of the calibrated shale particle flow sample include the following steps: Based on the shale particle flow sample, a triaxial compression numerical experiment was conducted to obtain the macroscopic stress-strain curve. The macroscopic stress-strain curves were compared with those from an indoor triaxial compression test under the same conditions, and the microscopic parameters of the shale particle flow sample were calibrated using the parameter inversion method.

[0022] More preferably, the establishment of the seepage field model and the initialization configuration include the following steps: A seepage field model was constructed using the FiPy solver. Generate the computational grid for the seepage field model; Configure the initial permeability of the seepage field model; Configure the upper boundary of the seepage field model as The constant pressure boundary, the lower boundary is The constant pressure boundary has a side boundary that is a zero flux boundary.

[0023] More preferably, the expression for the seepage field model is: ; in, Porosity; This represents the partial derivative of porosity with respect to time. Represents the Hamiltonian operator; Indicates penetration rate; Indicates void pressure; This represents the pressure gradient.

[0024] More preferably, the triaxial compression numerical simulation mechanism based on shale particle flow samples and seepage field models, which couples seepage and mechanics, includes the following steps: S1. Obtain the pressure gradient by solving the seepage field model based on the set initial permeability; S2. Solve for the seepage force of the particles based on the pressure gradient; S3. Apply the seepage force to the corresponding particles of the shale particle flow sample; S4. Recalculate the permeability after particle breakage; S5. Use the recalculated permeability as input for the next round of coupling, and repeat steps S1 to S5.

[0025] More preferably, the seepage force is calculated as follows: ; in, Indicates seepage force; This is a proportionality constant, with a value of 1.0 × 10⁴. Indicates particle radius; i represents particle / unit number; Indicates the pressure gradient; Porosity.

[0026] More preferably, the permeability is calculated as follows: ; Where k0 represents the initial permeability, and its value is... ; This is an empirical coefficient, with values ​​ranging from [value missing]. ; Indicates the area of ​​the crack; This represents the initial crack area.

[0027] More preferably, the termination condition for the numerical simulation is the appearance of a through crack or a 30% decrease in peak pressure in the shale particle flow sample.

[0028] More preferably, the output triaxial compression numerical simulation process data includes stress-strain curves, crack distribution maps, and permeability coefficient distribution maps.

[0029] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves real-time, dynamic, bidirectional feedback of "seepage driving crack propagation, and cracks changing seepage paths" through fine mapping between discrete element particles and finite volume method meshes. This coupling mechanism is closer to physical reality and greatly improves the fidelity of simulating complex fracture-seepage behavior in shale.

[0030] 2. This invention can naturally and intuitively simulate the entire process of microcrack initiation, propagation, interaction and formation of macroscopic fracture surfaces in shale, breaking through the limitations of continuous medium methods in simulating fracture, and providing visual and quantitative tools for understanding the failure mechanism of shale.

[0031] 3. This invention ensures the accuracy of the numerical model in macroscopic mechanical response through a rigorous parameter calibration process that compares with indoor experimental data. This enables the simulation results to predict shale behavior not only qualitatively but also quantitatively, thus possessing the credibility of a "virtual experiment".

[0032] 4. By adjusting the boundary conditions, this invention can be easily extended to simulate complex working conditions such as triaxial stress, cyclic loading and unloading, and creep. Its output results can be directly used to guide practical engineering problems such as shale gas reservoir fracturing design, wellbore stability evaluation, and CO2 geological storage optimization, demonstrating high engineering application value. Attached Figure Description

[0033] Figure 1 This is a flowchart of the numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling of the present invention. Figure 2 This is a schematic diagram illustrating the bidirectional coupling relationship between the stress field and the seepage field in this invention; Figure 3 This invention provides a microscopic particle flow model for shale. Figure 4 A comparison of stress-strain curves of shale under triaxial compression in this invention and a numerical model (without seepage field); Figure 5 This is a distribution diagram of cracks and permeability coefficients at the final failure of the numerical model of this invention; Figure 6 This is a comparison of the stress-strain curves of the shale sample and the numerical model (seepage-mechanical coupling) under triaxial compression (0MPa confining pressure). Figure 7 This is a comparison of the stress-strain curves of the shale sample and the numerical model (seepage-mechanical coupling) under triaxial compression (5MPa confining pressure). Figure 8This is a comparison of the stress-strain curves of the shale sample and the numerical model (seepage-mechanical coupling) under triaxial compression (10MPa confining pressure). Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] In the description of this invention, it should be noted that the terms "upper," "lower," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the terms "installation" and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0036] Example 1 like Figures 1 to 8 As shown, this embodiment provides a numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling, including the following steps: Establish shale particle flow samples and calibrate their microscopic parameters; Establish the seepage field model and perform initial configuration; Based on shale particle flow samples and seepage field models, a triaxial compression numerical simulation mechanism with two-way coupling of seepage and mechanics is constructed. Run the triaxial compression numerical simulation mechanism until the simulation termination condition is met, then exit and complete the triaxial compression numerical simulation process. Output the data from the triaxial compression numerical simulation process.

[0037] This invention combines the Particle Flow Code (PFC) numerical method with the Finite Volume Method (FVM) solver to achieve a coupled simulation of seepage and mechanics of shale under triaxial compression conditions.

[0038] Specifically, PFC2D (Particle Flow Code in 2 Dimensions), developed by Itasca, is used as the stress field solver. Its built-in FISH language allows users to perform custom modeling and computational control. This embodiment uses PFC2D Version 5.0.

[0039] The finite volume method solver uses FiPy, an open-source finite volume method library based on Python, as the solver for the seepage field. FiPy excels at solving partial differential equations and is easily integrated with the Python ecosystem. This embodiment uses FiPy Version 3.4.

[0040] The Python-FISH interface serves as a bridge or communication channel provided by the PFC2D software. It allows the Python environment to communicate and exchange data bidirectionally with the FISH language integrated within the PFC2D software. In this embodiment, the itasca module is imported into the Python script to implement interface coupling and data communication between PFC2D and FiPy.

[0041] Construct shale particle flow samples and calibrate their mechanical parameters: 1. Geometric model for generating shale particle flow samples To reproduce the sample size of the shale used in the indoor triaxial compression physics experiment, a rectangular sample container with a size of 50mm (width) × 100mm (height) was first generated in the discrete element PFC2D software using the wall command, based on the size of the shale used in the indoor triaxial compression physics experiment, so that the model could be used as a standard experimental rock core.

[0042] Next, the `generate` command was used to generate 9503 spherical particles in a random distribution within the sample container. To better simulate particle gradation, the particle radii were set to follow a uniform distribution, ranging from [value missing]. arrive between.

[0043] Through iterative cycles, particles are allowed to deposit and reach equilibrium under gravity. The model is then compacted by removing isolated or free particles and adjusting the position of the walls, ultimately stabilizing the macroscopic porosity of the sample at approximately 0.30. At this point, the particle aggregate forms a stable basic model.

[0044] 2. Set up the contact model for the shale particle flow sample. In triaxial compression, the particle radius has a negligible impact on the numerical simulation results of particle flow. Furthermore, macroscopic mechanical parameters of the model, such as particle density, also have a small influence on the numerical simulation results and are typically set to constant values ​​that reflect actual conditions. Common parameters for shale particle flow samples are shown in Table 1 below.

[0045] Table 1: Common parameters of shale particle flow samples ; The mechanical response of shale is determined by the interparticle contact model and its parameters. After the basic model is generated, the interparticle contact model is set as the parallel bond model (PBM). The parallel bond model is used to simulate the complete shale matrix, establishing a "cement bridge" with specific tensile and shear strengths at the particle contact points.

[0046] Based on literature review and preliminary calculations, a set of micromechanical parameters was set, thus defining the particle assembly as the matrix model. Specifically, in PFC2D, the contact model was set to a linear parallel bond model using the `contact cmat default model linearpbond` command.

[0047] Referring to the bedding distribution of shale samples in indoor experiments, a discrete fracture network (DFN) was generated at the corresponding bedding locations in the matrix model, and the interparticle contact model on this network was set as a smooth joint model (SJM). SJM can penetrate multiple particles, define specific strength and deformation characteristics, and thus control the anisotropic failure of the rock.

[0048] 3. Set the boundary conditions and loading conditions for the shale particle flow sample. After establishing the shale particle flow specimen model, a triaxial compression simulation experiment was conducted, including confining pressure application and axial loading. Confining pressure was applied by generating walls on both sides of the shale particle flow specimen model and maintaining these walls in a fixed position.

[0049] Axial loading was achieved by generating two walls at the top and bottom of the model, respectively, using a displacement-controlled loading method. The PFC2D "servo control" mechanism was employed to allow the upper and lower walls to move towards each other at a constant rate, thereby applying load and simulating a triaxial compression process until the specimen failed.

[0050] 4. Calibration of microscopic parameters of shale particle flow samples Detailed parameter calibration is a crucial step in ensuring the accuracy of the specimen model. First, without considering seepage, the axial loading rate was set to 0.01 mm / min, and a set of PFC2D triaxial compression numerical simulation experiments configured above were run.

[0051] Then, the axial stress (calculated from the reaction force of the top wall) and axial strain (calculated from the displacement) were recorded to obtain the numerical stress-strain curve. The macroscopic stress-strain curve obtained from the above numerical simulation experiment was compared with the stress-strain curve of the indoor triaxial compression physical experiment of the shale sample.

[0052] The microscopic parameters (such as the elastic modulus of parallel bonding and the coefficient of friction) of the shale particle flow sample model were systematically adjusted using parametric inversion methods (such as trial and error and optimization algorithms) until the numerical simulation results and experimental results matched within the preset error range.

[0053] Repeat the above process until the error between the numerical simulation results and the experimental results is ≤10%, then exit and complete the triaxial compression numerical simulation experiment. At this point, a calibrated micromechanical model reflecting the true macroscopic mechanical properties of shale is obtained. The final calibrated microscopic parameters of the shale particle flow sample are shown in Table 2 below.

[0054] Table 2: Microscopic parameters of calibrated shale particle flow samples ; Construct and initialize the seepage field calculation model: 1. Construct a calculation model for the seepage field. Based on the law of conservation of mass and Darcy's law, the two-dimensional steady-state seepage control equations are established in the FiPy framework as follows: (1) In the above formula, Porosity; This represents the partial derivative of porosity with respect to time. Represents the Hamiltonian operator; Indicates penetration rate; Indicates void pressure; This represents the pressure gradient.

[0055] 2. Generate computational grid Using the center coordinates of particles in the discrete element model, a finite volume method computational mesh covering the entire sample is generated. Each mesh element can be associated with one or more particles, ensuring that physical quantities can be accurately mapped between the solid and fluid domains.

[0056] Specifically, the Python script reads the center coordinates (x_i, y_i) of all particles in the PFC2D model. Based on these coordinates, it uses FiPy's Gmsh2D or Grid2D functions to generate an irregular triangular mesh or a regular rectangular mesh covering the entire sample area.

[0057] Each grid cell contains several particles. The center coordinates of the particles are aligned with the center of the seepage cell to achieve a bidirectional mapping between FiPy and PFC2D.

[0058] 3. Configure the initialization parameters and boundary conditions of the seepage field calculation model. Initial parameters and boundary conditions were assigned to the seepage field. These parameters and boundary conditions were set based on indoor triaxial compression physics experiments. The initial parameters included: the initial permeability of the shale matrix was [value missing]. The fluid density is 1000 kg / m³, and the dynamic viscosity is... .

[0059] Boundary conditions include: the upper boundary of the model is a constant pressure boundary. The simulation assumes a constant water head at the upper boundary and a constant pressure boundary at the lower boundary. Simulates drainage from the lower end face; the side boundary is a zero-flux boundary. Simulates lateral impermeable confining pressure.

[0060] Constructing a triaxial compression numerical simulation mechanism that couples seepage and mechanics: The triaxial compression numerical simulation mechanism involving two-way coupling of seepage and mechanics is the core step of this invention, encompassing coupling effects in two directions. This includes fluid-structure interaction from the seepage field to the mechanical field, which calculates and applies seepage forces, and solid-fluid interaction from the mechanical field to the seepage field, which monitors particle breakage and updates permeability.

[0061] Fluid-structure interaction (seepage field → mechanical field): Complete the calculation and application of seepage force, and perform the following steps within each coupling time step: 1. Using the FiPy solver, based on the current permeability k and the above boundary conditions, solve the seepage control equation (1) to obtain the pressure distribution p(x,y) of the entire computational domain.

[0062] 2. For each particle i, based on its center coordinates (x_i, y_i), the pressure gradient at that point is calculated using the central difference method by querying the pressure values ​​of its own grid cell and adjacent cells. .

[0063] 3. Pressure gradient through particle i The seepage force of the particles is calculated using the following formula: (2) In the above formula, This indicates the seepage force of the particles; This is a proportionality constant, with a value of 1.0 × 10⁴. 'i' represents the particle radius; 'i' represents the particle number.

[0064] 4. The Python script of the FiPy solver applies the seepage force as an external vector to the particles corresponding to the shale particle flow sample model in the PFC2D software by calling the commands of the Python-FISH interface, thereby introducing the effect of the seepage field into the mechanical calculation of the discrete element method.

[0065] It should be noted that during the coupled simulation, the shale particle flow sample model was loaded vertically at a rate of 0.02 mm / s in the PFC2D software, while three confining pressures of 0 MPa, 5 MPa, and 10 MPa were simulated horizontally, corresponding to three loading stages. The stress and strain values ​​of the model under different confining pressures were recorded in real time during the loading process.

[0066] Solid-fluid coupling (mechanical field → seepage field) is mainly used to update permeability, and completes the following steps within each coupling time step: 1. During the PFC2D mechanical calculation process, particle motion and bond failure are calculated based on seepage force and applied stress, and all parallel bond fracture events are monitored in real time. When a parallel bond fractures, its fracture location (center point coordinates) and fracture type (tension or shear) are recorded.

[0067] 2. After a particle breaks down, the permeability of the area it incurs increases with the increase of the fracture aperture. The permeability is updated using the following formula: (3) In the above formula, k0 represents the initial permeability, and its value is... ; This is an empirical coefficient used to convert crack density into permeability increment. It is determined through trial calculations or comparison with experiments. In this embodiment, it is taken as... ; Indicates the area of ​​the crack; This represents the initial crack area.

[0068] 3. Pass the updated permeability k value for each particle to the FiPy solver as the permeability parameter for the next coupled time step, and repeat the above two-way coupling of seepage and mechanics.

[0069] When the rate of change of stress-strain is less than 10 -4 If the current confining pressure is 0 MPa, the next loading stage will begin, for example, from the current confining pressure of 0 MPa to the next confining pressure of 5 MPa. The entire two-way coupling process continues until a through-crack appears in the model or the peak stress decreases by 30%, at which point the calculation stops, completing the entire experimental process of the triaxial compression numerical simulation mechanism of seepage and mechanical two-way coupling.

[0070] After the numerical simulation was completed, the output data was analyzed, and the corresponding stress-strain curves, crack distribution diagrams, and permeability coefficient distribution diagrams were output. The results show that: 1. Feasibility verification of the scheme: By comparing triaxial compression experiments and numerical simulations of shale under the seepage-mechanical coupling mechanism, the stress-strain curves obtained by numerical simulation under different confining pressures (0MPa, 5MPa, 10MPa) have an error of less than 10% with the results of indoor experiments. This shows that the coupled calculation of the finite volume method and the discrete element method can accurately reflect the macroscopic mechanical characteristics of shale and verify the accuracy of the coupled model.

[0071] 2. Significant coupling effect: The stress-strain curves show that the coupling effect of seepage mechanics significantly weakens the mechanical strength of shale, with the peak compressive strength decreasing by an average of about 43% (under 0 MPa confining pressure). Moreover, as the confining pressure increases, the strength recovery of shale is limited, indicating that pore fluid has a continuous weakening effect on the shale failure process.

[0072] 3. Crack and seepage evolution: In the simulation, the cracks transformed from axial tensile cracks to shear cracks. Under the influence of seepage, the cracks propagated faster and penetrated more fully. The permeability coefficient increased significantly after failure, forming interconnected channels. This indicates that seepage promoted the coordinated propagation of cracks and controlled the evolution of the seepage field.

[0073] 4. Model Reliability and Application Scenarios: The stress-strain curves obtained from numerical simulations under different confining pressures show good agreement with the results of indoor triaxial compression experiments. The peak strength, elastic modulus, and failure mode exhibit high matching degrees, verifying the accuracy and stability of the flow-mechanics two-way coupled model. This method can be used for studying the flow-fracture behavior of shale gas reservoirs, analyzing wellbore stability, and optimizing fracturing parameters.

[0074] Example 2 like Figures 1 to 8 As shown in Example 1, this example provides a numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling.

[0075] In this embodiment, PFC3D is used to generate a three-dimensional spherical (or rectangular) particle assembly. The three-dimensional model can more realistically simulate the three-dimensional expansion and complex morphology of fractures. The FiPy solver is used to generate a three-dimensional tetrahedral or hexahedral mesh. The corresponding seepage force calculation formula is updated to a three-dimensional vector, and the permeability update can be performed on the three-dimensional mesh cells, which better reflects the spatial connectivity of the fractures.

[0076] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0077] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling, characterized in that, Includes the following steps: Establish shale particle flow samples and calibrate their microscopic parameters; Establish the seepage field model and perform initial configuration; Based on shale particle flow samples and seepage field models, a triaxial compression numerical simulation mechanism with two-way coupling of seepage and mechanics is constructed. Run the triaxial compression numerical simulation mechanism until the simulation termination condition is met, then exit and complete the triaxial compression numerical simulation process. Output the data from the triaxial compression numerical simulation process.

2. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling as described in claim 1, characterized in that, The method for establishing shale particle flow samples and calibrating their microscopic parameters includes the following steps: Two-dimensional shale particle flow samples were established using the PFC2D tool; Set the contact model, boundary conditions, and loading conditions for the shale particle flow sample; Microscopic parameters of shale particle flow samples were calibrated.

3. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 2, characterized in that, The microscopic parameters of the calibrated shale particle flow sample include the following steps: Based on the shale particle flow sample, a triaxial compression numerical experiment was conducted to obtain the macroscopic stress-strain curve. The macroscopic stress-strain curves were compared with those from an indoor triaxial compression test under the same conditions, and the microscopic parameters of the shale particle flow sample were calibrated using the parameter inversion method.

4. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling as described in claim 1, characterized in that, The process of establishing the seepage field model and performing initial configuration includes the following steps: A seepage field model was constructed using the FiPy solver. Generate the computational grid for the seepage field model; Configure the initial permeability of the seepage field model; Configure the upper boundary of the seepage field model as The constant pressure boundary, the lower boundary is The constant pressure boundary has a side boundary that is a zero flux boundary.

5. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 4, characterized in that, The expression for the seepage field model is: ； in, Porosity; This represents the partial derivative of porosity with respect to time. Represents the Hamiltonian operator; Indicates penetration rate; Indicates void pressure; This represents the pressure gradient.

6. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 1, characterized in that, The triaxial compression numerical simulation mechanism based on shale particle flow samples and seepage field models, which couples seepage and mechanics, includes the following steps: S1. Obtain the pressure gradient by solving the seepage field model based on the set initial permeability; S2. Solve for the seepage force of the particles based on the pressure gradient; S3. Apply the seepage force to the corresponding particles of the shale particle flow sample; S4. Recalculate the permeability after particle breakage; S5. Use the recalculated permeability as input for the next round of coupling, and repeat steps S1 to S5.

7. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling as described in claim 6, characterized in that, The solution method for the seepage force is as follows: ； in, Indicates seepage force; This is a proportionality constant, with a value of 1.0 × 10⁴. Indicates particle radius; i represents particle number; Indicates the pressure gradient; Porosity.

8. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 6, characterized in that, The permeability is calculated as follows: ； Where k0 represents the initial permeability, and its value is... ; This is an empirical coefficient, with values ​​ranging from [value missing]. ; Indicates the area of ​​the crack; This represents the initial crack area.

9. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 1, characterized in that, The numerical simulation was terminated when a through-crack appeared in the shale particle flow sample or the peak pressure decreased by 30%.

10. The numerical simulation method for triaxial compression of shale based on seepage-mechanical coupling according to claim 1, characterized in that, The output triaxial compression numerical simulation process data includes stress-strain curves, crack distribution maps, and permeability coefficient distribution maps.

Images