Natural fracture characterization and reservoir damage evolution phase-field simulation method and related apparatus

Abstract

The present application belongs to the technical field of fracture characterization and reservoir hydraulic fracturing simulation, and discloses a natural fracture characterization and reservoir damage evolution phase field simulation method and related equipment. The method differentiates primary and secondary natural fractures based on a discrete fracture network and constructs a model, accurately characterizes the different sizes and spatial heterogeneity of the primary / secondary natural fractures in the model, characterizes the natural fractures as mechanical weak surfaces with differentiated parameters, and enables the hydraulic fractures to capture and interact with the natural fractures. In combination with a phase field damage model and a rock mechanics constitutive model, the method can accurately simulate key behaviors such as activation and turning in the natural expansion of fractures without presetting a fracture path, avoids defects such as numerical instability and low computational efficiency, and improves the simulation accuracy of the phase field method by taking into account the influence of multiple factors such as ground stress and fluid pressure. The method reduces simulation distortion, provides reliable support for fracturing design optimization, helps improve the reservoir reconstruction effect and production benefit, and ensures the smooth implementation of the fracturing process.

Description

Technical Field

[0001] This invention belongs to the field of fracture characterization and reservoir hydraulic fracturing simulation technology, specifically the method and related equipment for natural fracture characterization and reservoir damage evolution phase field simulation. Background Technology

[0002] The widespread natural fractures in reservoirs serve as both potential channels for fluid flow and weak points in mechanical processes. Their distribution patterns and mechanical properties significantly influence the propagation morphology and fracture network complexity of hydraulic fractures. Effectively characterizing natural fractures and accurately predicting their propagation paths and interactions with the natural fracture system are crucial for optimizing fracturing design and enhancing the effectiveness of unconventional reservoir stimulation. Therefore, revealing their interaction patterns and reliably predicting fracture orientation has become a key issue determining the success or failure of fracturing processes and the resulting production gains.

[0003] However, current characterization of natural fractures and simulation of the interaction between hydraulic and natural fractures using phase-field methods still have certain limitations, specifically: (1) Existing technologies, such as the fracture propagation method and apparatus disclosed in patent CN117365414A (2024.01.09), can update fracturing construction parameters in real time according to reservoir conditions; and the natural fracture activation judgment and prediction method disclosed in patent CN118484980A (2024.08.13), can construct a database of the interaction between hydraulic fracturing and natural fractures and perform machine learning to improve efficiency. However, they have limitations in terms of the geometric distribution and mechanical characterization of natural fractures, and it is difficult to truly reflect the spatial distribution law of natural fractures and the mechanical interaction behavior of weak surfaces.

[0004] (2) The prediction accuracy of the interaction between hydraulic fractures and natural fractures is insufficient. Existing numerical simulation methods, such as the literature "Finite Element Analysis of Factors Affecting Radial Hydraulic Fracturing Fracture Propagation", require pre-setting the fracture path and are difficult to simulate the activation, turning, penetration, bifurcation and fracture arrest behaviors in the natural propagation of fractures; the literature "Study on Fracture Propagation and Inter-fracture Interference of Hydraulic Fracturing in Dual Wells Based on Extended Finite Element Method", although the extended finite element method can handle discontinuous problems, it is prone to numerical instability at the intersection of complex fracture networks; the literature "Fracturing Law of Hydraulic Fracturing of Rock-like Materials Based on Discrete Element Method" can simulate fracture interaction, but the computational efficiency is low.

[0005] (3) Existing phase field method simulations, such as the literature "Study on the characteristics of fracture extension in fractured formations based on phase field method", have limitations in considering the coupling effect of geostress, fluid pressure, and natural fracture damage. Especially for highly heterogeneous reservoirs, traditional methods are difficult to achieve dynamic coupling analysis of the entire fluid-solid-damage process, resulting in distortion of the simulation of key behaviors of natural fractures such as "penetration", "turning", "activation", "bidding" and "fracture arrest". Summary of the Invention

[0006] This invention provides a method and related equipment for characterizing natural fractures and simulating the phase field of reservoir damage evolution. It solves the problems of existing technologies, which are unable to accurately characterize the heterogeneous distribution and mechanical weak surface properties of natural fractures, and lack a mechanism description of the complex interaction between hydraulic fractures and natural fractures, leading to simulation distortion.

[0007] To achieve the above objectives, the present invention provides the following technical solution: Methods for characterizing natural fractures and simulating reservoir damage evolution in phase field include: Natural cracks are generated based on discrete crack networks, and natural cracks are classified into primary cracks and secondary cracks. A natural crack system model is constructed based on the geometric parameters of primary cracks and secondary cracks. Based on solid mechanics theory, the governing equations for rock skeleton deformation and stress field are constructed, the elastic parameters of the rock matrix are defined, and a phase field damage model based on the principle of energy evolution is introduced to establish a rock mechanical constitutive model. The natural fracture system model is characterized as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and the natural fracture system model is given mechanical strength parameters and fracture toughness lower than those of the matrix. Based on Darcy's flow theory, a flow model of fluid in reservoir matrix and natural fractures is established, defining porosity, permeability parameters and injection source terms, and obtaining fluid pressure. By introducing historical field variables into the rock mechanics constitutive model, a historical field recording the maximum tensile strain energy is constructed, and the phase field governing equation is obtained. Solving the phase field governing equations yields the distribution of phase field variables characterizing the damage state of the material; The fluid pressure is coupled to the solid equilibrium equation, and the changes in pore structure and permeability caused by solid deformation are fed back to the seepage field. At the same time, the material stiffness and fluid flow properties are updated according to the phase field variable distribution.

[0008] Preferably, the geometric parameters of the natural crack include size, aperture, density, dip angle, and tortuosity. The size is randomly generated within a specified area using a statistical distribution method. The aperture is assumed to be proportional to the size based on the size ratio. The density is adjusted using a Poisson distribution. The dip angle is adjusted using a discrete crack network plugin. The tortuosity is controlled by modifying program variables to control the degree of crack curvature.

[0009] Preferably, the phase-field damage model is established based on an energy functional, which is composed of elastic strain energy, fracture energy, work done by external force and work done by fluid pressure; the elastic strain energy is decomposed into tensile and compressive components; the fracture energy is expressed by regularization through phase-field variables and their gradients; the mechanical strength parameters of the natural crack include Young's modulus and Poisson's ratio.

[0010] Preferably, the seepage model is established based on Darcy's law and the mass conservation equation and applied to the entire computational domain. The seepage model distinguishes between the reservoir domain and the fracture domain through characteristic functions, and defines their fluid density, fluid viscosity, water storage coefficient, porosity, permeability and Biot coefficient respectively.

[0011] Preferably, the historical field is a monotonically increasing function that records the maximum value of the sum of tensile elastic strain energy and the work done by the initial stress, and is used to replace the energy term in the phase field control equation.

[0012] Preferably, the phase field control equation is driven by the history field function, and the phase field variables are used to characterize the damage state of the material, so that crack propagation does not require a preset path.

[0013] A phase-field simulation system for characterizing natural fractures and simulating reservoir damage evolution includes: Model building module: used to generate natural cracks based on discrete crack network, classify natural cracks into primary cracks and secondary cracks, and build a natural crack system model based on the geometric parameters of primary cracks and secondary cracks. Model building module: used to construct the governing equations of rock skeleton deformation and stress field based on solid mechanics theory, define the elastic parameters of rock matrix, and introduce a phase field damage model based on the principle of energy evolution to establish a rock mechanical constitutive model; Embedding module: used to characterize the natural fracture system model as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and to assign the natural fracture system model mechanical strength parameters and fracture toughness lower than those of the matrix; Fluid pressure acquisition module: used to establish a fluid seepage model in reservoir matrix and natural fractures based on Darcy flow theory, define porosity, permeability parameters and injection source terms, and obtain fluid pressure; Historical Field Introduction Module: Used to introduce historical field variables into the rock mechanics constitutive model, construct a historical field that records the maximum tensile strain energy, and obtain the phase field governing equations; Solver module: Used to solve the phase field governing equations to obtain the distribution of phase field variables characterizing the damage state of the material; Simulation module: used to couple fluid pressure to solid equilibrium equations, feed back changes in pore structure and permeability caused by solid deformation to the seepage field, and update material stiffness and fluid flow properties according to phase field variable distribution.

[0014] A computer device includes a memory, a processor, and a computer program stored in the memory, wherein the processor executes the computer program to implement the steps of a phase-field simulation method for characterizing natural fractures and simulating reservoir damage evolution.

[0015] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of a phase-field simulation method for characterizing natural fractures and simulating reservoir damage evolution.

[0016] A computer program product includes a computer program that, when executed by a processor, implements the steps of a method for characterizing natural fractures and simulating reservoir damage evolution in a phase-field model. Compared with existing technologies, this invention has the following advantages: This invention provides a method for characterizing natural fractures and simulating reservoir damage evolution in a phase-field model. By distinguishing primary and secondary natural fractures based on a discrete fracture network and constructing a model, it achieves accurate characterization of the different sizes and spatial heterogeneity of primary / secondary natural fractures in the model. Natural fractures are characterized as mechanically weak surfaces with differentiated parameters. Hydraulic fractures can capture and interact with natural fractures. Combining a phase-field damage model and a rock mechanics constitutive model, without pre-setting fracture paths, it can accurately simulate key behaviors such as activation and turning during natural fracture propagation, avoiding the defects of numerical instability and low computational efficiency. Through dynamic coupling analysis of the entire fluid-solid-damage process, taking into account the influence of multiple factors such as geostress and fluid pressure, it improves the accuracy of phase-field simulation, reduces simulation distortion, provides reliable support for fracturing design optimization, helps improve reservoir stimulation effects and production benefits, and ensures the smooth implementation of fracturing processes. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart of the method for characterizing natural fractures and simulating reservoir damage evolution according to an embodiment of the present invention; Figure 2 This is a map showing the distribution of macroscopic fractures in underground coal seams. Figure 3 (Primary / Secondary) Natural Crack Geometry Generated for Embodiments of the Present Invention; Figure 4 This is a diagram showing the distribution of the length and dip angle of primary / secondary natural cracks according to an embodiment of the present invention. Figure 5 This is a geometric modeling diagram of a natural crack in the simulation software of an embodiment of the present invention; Figure 6 The diagram shows the interaction between two natural cracks (dip angle 45°, Young's modulus 1e6Pa) and hydraulic cracks in an embodiment of the present invention, where a is the interaction diagram at 5s and b is the interaction diagram at 15s. Figure 7The diagram shows the interaction between two natural cracks (dip angle 45°, Young's modulus 5e6Pa) and hydraulic cracks in an embodiment of the present invention, where a is the interaction diagram at 5s and b is the interaction diagram at 15s. Figure 8 This is an example of the interaction diagram between multiple natural cracks (dipping angle 75°, Young's modulus 1e6Pa) and hydraulic cracks in an embodiment of the present invention, where a is the interaction diagram at 5s and b is the interaction diagram at 15s. Figure 9 The diagram shows the interaction between natural fractures (30° dip angle) and hydraulic fractures. In the diagram, a is the initial contact diagram, b is the diagram of the hydraulic fracture connecting with the natural fracture and extending along it, and c is the final extension diagram. The diagram is taken from the literature "Liangping Yi et al., Interaction between natural fractures (30° dip angle) and hydraulic fractures". Figure 10 This is a block diagram of the phase field simulation system for natural fracture characterization and reservoir damage evolution according to an embodiment of the present invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0020] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0021] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0022] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0023] like Figure 1 As shown, this invention provides a method for characterizing natural fractures and simulating the phase field of reservoir damage evolution, including: S1: Generate natural cracks based on discrete crack networks, classify natural cracks into primary cracks and secondary cracks, and construct a natural crack system model based on the geometric parameters of primary cracks and secondary cracks. S2: Based on solid mechanics theory, construct the governing equations for rock skeleton deformation and stress field, define the elastic parameters of the rock matrix, and introduce a phase field damage model based on the principle of energy evolution to establish a rock mechanical constitutive model; S3: The natural fracture system model is characterized as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and the natural fracture system model is given mechanical strength parameters and fracture toughness lower than those of the matrix. S4: Based on Darcy's flow theory, establish the flow equation of fluid in reservoir matrix and natural fractures, define porosity, permeability parameters and injection source terms, and obtain fluid pressure; S5: Introduce historical field variables into the rock mechanics constitutive model, construct a historical field that records the maximum tensile strain energy, and obtain the phase field control equation; S6: Solve the phase field control equations to obtain the distribution of phase field variables characterizing the damage state of the material; S7: Couples fluid pressure to the solid equilibrium equation, feeds back changes in pore structure and permeability caused by solid deformation to the seepage field, and updates material stiffness and fluid flow properties based on the distribution of phase field variables.

[0024] By distinguishing primary and secondary natural fractures based on discrete fracture networks and constructing models, this approach achieves accurate characterization of the different sizes and spatial heterogeneity of primary and secondary natural fractures within the model. Natural fractures are represented as mechanically weak surfaces with differentiated parameters. Hydraulic fractures can capture and interact with natural fractures. Combining phase-field damage models and rock mechanics constitutive models, without pre-setting fracture paths, it can accurately simulate key behaviors such as activation and reversal during natural fracture propagation, avoiding numerical instability and low computational efficiency. Through dynamic coupling analysis of the entire fluid-solid-damage process, taking into account the influence of multiple factors such as geostress and fluid pressure, the accuracy of phase-field method simulation is improved, simulation distortion is reduced, and reliable support is provided for fracturing design optimization. This helps improve reservoir stimulation effects and production benefits, ensuring the smooth implementation of fracturing processes.

[0025] The specific steps are as follows: (a) Constructing primary / secondary natural fractures based on discrete fracture network (DFN).

[0026] (b) Establish a rock mechanics constitutive model that takes into account damage evolution.

[0027] (c) Characterize the discrete natural cracks generated in step (a) as mechanically weak surfaces embedded in the matrix.

[0028] (d) Establish a seepage model of fluid in porous media and cracks.

[0029] (e) Introduce historical field variables to control damage irreversibility.

[0030] (f) Achieve full fluid-solid-damage coupling to solve the phase field evolution equation to describe crack propagation.

[0031] Step (a) specifically includes: 1) Determine the geometric parameters (size, aperture, density, dip angle, tortuosity) of the primary / secondary natural cracks based on the actual engineering problem and perform geometric modeling; 2) The size of the cracks is randomly generated within a specified area using a statistical distribution method, conforming to a predetermined pattern, and employing a "power-law distribution" equation. Simulating the size and pore size distribution of cracks in nature, in the formula The minimum value of variable x is a scale parameter of the distribution. This is the power-law exponent.

[0032] 3) Crack aperture is assumed to be proportional to its size (length) based on the size ratio relationship, using the equation... The formula gives the proportional relationship between the aperture and length of a single crack, where... For the opening of the crack, For crack length, This is a scaling factor.

[0033] 4) Density control is adjusted using a Poisson distribution. Given a "area density" (number of cracks per square meter) or a "desired total number of cracks", the algorithm will randomly set points within the modeling area, with each point representing the center of a crack.

[0034] 5) The dip angle is adjusted by the DNF discrete fracture plugin to control the direction of the primary / secondary natural fractures, which determines the directionality of the natural fractures.

[0035] 6) Tortiness is controlled by modifying relevant variables in the autonomous program to control the degree of crack bending, which affects the crack's contact area, flow resistance, and mechanical strength.

[0036] Furthermore, steps (b) and (c) specifically include: 1) The phase-field model of porous elastic media is based on elastic strain energy. fracture energy External force work Together with the work done by the fluid pressure p, they constitute the energy functional. Establishing energy functionals .in The sum of the initial pressures represents the change from Status The increase in elastic potential energy of the state, Biot coefficient, Critical energy release rate, For traction, The linear strain tensor is expressed as Elastic energy density in isotropic elastic media Represented as , The Lamé constant is expressed as ,in For Young's modulus, It is Poisson's ratio.

[0037] 2) The fracture energy The equation is rewritten as Elastic strain energy It is decomposed into tensile and compressive components to ensure that the crack occurs under tensile conditions. The tensile and compressive components of the elastic energy density can be expressed as follows: Assuming the compression portion does not affect crack propagation, the elastic strain energy Can be rewritten as , The degenerate function is applicable to the energy change caused by the initial stress field. .

[0038] 3) The fracture energy Formulas, elastic strain energy Formulas and expressions Substitute into energy functional The energy functional is then rewritten using the variational method.

[0039] 4) Phase-field damage models are closely related to strain-based damage models, but they differ in their starting points. Considering the viscous regularization of the crack phase field φ, the energy functional controlling crack propagation... .in, The damage function, For elastic strain energy density, For viscosity, The critical energy release rate. For internal length scales. The crack driving force is a function of elastic strain energy density. ,in The tensile portion that retains its elastic strain energy density without damage. The strain energy threshold.

[0040] 5) Damage variables The crack phase field in the phase field damage model The function is defined as follows: By choosing the power law, the damage evolution function is defined as... Typically, the value of an exponential function .

[0041] Furthermore, step (d) specifically includes: 1) Simulate fluid flow based on Darcy's law and the standard mass conservation equation, and apply it to the entire computational domain. ,in fluid density 、S For water storage coefficient, v For flow rate, For volumetric strain, The fluid source phase; where , ,in For reservoir domain, For the fracture domain, Biot coefficient of the reservoir domain, The Biot coefficient of the fracture domain was defined; two functions were established. and Within the reservoir domain Within the fracture domain Conversely, within the reservoir domain Within the fracture domain .

[0042] 2) Expression for water storage coefficient S ,in Where is porosity, and c is fluid compressibility. Let be the bulk modulus. The fluid compressibility in the reservoir and fracture domain is denoted as . and ,have .

[0043] 3) This model does not consider the effect of gravity, Darcy velocity v The calculation formula is , For fluid viscosity, in For reservoir fluid viscosity, The fluid viscosity in the crack region; For equivalent penetration rate, and ,in For reservoir permeability, This represents the permeability of the cracked area.

[0044] 4) The Darcy velocity in 3) v Substituting the calculation formula into the global domain mass conservation equation in 1), the governing equation for fluid flow can be expressed as pressure. p function .

[0045] Furthermore, steps (e) and (f) specifically include: 1) To ensure that the fracture domain generated after fluid injection does not close, a monotonically increasing function of the historical field needs to be constructed. This makes the crack region irreversible, in which For the initial stress field, For tensile elastic strain energy, and .

[0046] 2) From step (b) The original governing equations of the phase field can be derived. However, this formula cannot guarantee irreversibility, so the historical field function in 1) is required. replace Finally, the phase field evolution equation is obtained. .

[0047] Another embodiment of the present invention provides a method for characterizing natural fractures and simulating the phase field of reservoir damage evolution: (1) Construct a geometric model (30m×30m) of primary / secondary natural fractures that closely matches real geological patterns, and analyze it as follows: Figure 2 The macroscopic fracture distribution map of the coal seam shown can determine the hierarchical phenomenon of the primary / secondary fracture system and its spatial clustering distribution characteristics in this area. Using a controllable algorithm and the DFN fracture plugin, the heterogeneous characteristics of the fracture network are accurately grasped, thereby generating a more consistent geometric model of natural fractures. (See [link to relevant documentation]). Figure 3 .

[0048] (2) Establish a rock mechanics constitutive model considering damage evolution. Define the elastic parameters of the rock matrix: Young's modulus (E0 = 60 GPa), Poisson's ratio (E0 = 60 GPa), etc. =0.3), a phase-field damage model based on the principle of energy evolution is introduced, whose evolution is driven by historical field variables to describe the progressive damage and fracture process of the matrix under tensile stress. The porous elastic medium phase-field model is based on elastic strain energy. fracture energy External force work Together with the work done by the fluid pressure p, they constitute the energy functional. Establishing energy functionals .

[0049] in The sum of the initial pressures represents the change from Status to The increase in elastic potential energy of the state, Biot coefficient, =200N is the critical energy release rate. For traction, The linear strain tensor is expressed as Elastic energy density in isotropic elastic media Represented as ,in and All are Lamé constants. Damage variables The crack phase field in the phase field damage model The function is defined as follows: By choosing the power law, the damage evolution function is defined as... Typically, the value of an exponential function .

[0050] (3) The primary / secondary natural cracks are characterized as mechanically weak surfaces embedded in the matrix in (2). Figure 3 The differential geometry of the generated primary and secondary cracks is shown. Figure 4 The distribution of primary and secondary natural fracture lengths and angles differs. Primary fractures typically have an angle of 75° and a length of 8m, while secondary fractures typically have an angle of 35° and a length of 4m. Within the framework of solid mechanics, they are assigned mechanical strength parameters significantly lower than those of the matrix (Young's modulus of primary / secondary natural fractures E=1e6Pa / 4e6Pa, Poisson's ratio). =0.25) to represent different interface stiffnesses, reflecting their physical characteristics of preferential tensile failure under stress.

[0051] (4) Construct the primary / secondary crack morphology generated by programming in (3) into a model geometry, such as Figure 5 (a) shows two natural fractures and an injection source perforation hole. Figure 5 (b) shows the three injection source perforations at the center, each with a perforation length of 0.8 m; the other curves represent primary / secondary natural fractures. Based on Darcy's flow theory, flow equations for fluids in the reservoir matrix and fracture network were established and applied to the entire computational domain. ,in For fluid density (1000 kg / m³) 、S For water storage coefficient, v For flow rate, For volumetric strain, The fluid source phase; where , ,in For reservoir domain, For the fracture domain, Biot coefficient of the reservoir domain, The Biot coefficient of the fracture domain was defined; two functions were established. and Within the reservoir domain Within the fracture domain Conversely, within the reservoir domain Within the fracture domain The matrix porosity (0.05) and permeability (1e-15m³) were set respectively, and an injection source term (discharge rate = 20kg / (m³ / s)) was set at the perforation cluster location to simulate fracturing fluid injection (viscosity = 1e-3Pa·s).

[0052] (5) To verify the accuracy of the model, a parametric comparison experiment was conducted, with only two natural crack dip angles (45°) preset. See [link to relevant documentation]. Figure 5(a); The Young's modulus of natural cracks was simulated with two different strengths: high (5e6 Pa) and low (1e6 Pa), while all other parameters remained unchanged. See [reference needed]. Figure 6 , Figure 7 The results show that high-strength natural fractures induce direct penetration of hydraulic fractures, while low-strength natural fractures guide the hydraulic fractures to propagate along their interfaces. This comparison is consistent with classical mechanics laws, effectively validating the accuracy and parameter sensitivity of this model in predicting fracture interaction modes.

[0053] (6) Based on Figure 5 (b) Setting key parameters for natural cracks (Young's modulus 1×10⁻⁶) 6 A numerical model was constructed using a 75° inclination angle (Pa) for calculations. Following a fully coupled fluid-solid-damage phase-field simulation, the results of the interactive propagation of hydraulic fractures and natural fractures are shown in [reference needed]. Figure 8 .

[0054] (7) To highlight the significant advantages of this patented method in simulating complex crack interaction behavior, such as Figure 9 As shown, this figure is cited from the literature "Liangping Yi et al.", in which... Figure 9 The diagrams show the interaction between natural and hydraulic fractures at a dip angle of 30° in the same model. A is the initial contact diagram, b is the diagram showing the hydraulic fracture connecting with and extending along the natural fracture, and c is the final extension diagram, demonstrating the interaction mode between hydraulic and natural fractures under single or simply defined natural fracture conditions.

[0055] As can be seen from the comparison, the method of this patent can not only reproduce the simple interactive phenomena in the literature, but also break through the limitation of traditional methods that require pre-setting the crack path. Through the DFN plugin, it can characterize natural cracks that are more in line with the geological distribution law, and realize the complex competitive behavior and prediction of activation, turning, penetration, bifurcation and crack arrest among multiple cracks, multiple groups and arbitrary dip angles.

[0056] like Figure 10 As shown, this invention provides a phase-field simulation system for natural fracture characterization and reservoir damage evolution, comprising: Model building module: used to generate natural cracks based on discrete crack network, classify natural cracks into primary cracks and secondary cracks, and build a natural crack system model based on the geometric parameters of primary cracks and secondary cracks. Model building module: used to construct the governing equations of rock skeleton deformation and stress field based on solid mechanics theory, define the elastic parameters of rock matrix, and introduce a phase field damage model based on the principle of energy evolution to establish a rock mechanical constitutive model; Embedding module: used to characterize the natural fracture system model as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and to assign the natural fracture system model mechanical strength parameters and fracture toughness lower than those of the matrix; Fluid pressure acquisition module: used to establish the flow equation of fluid in reservoir matrix and natural fractures based on Darcy flow theory, define porosity, permeability parameters and injection source terms, and obtain fluid pressure; Historical Field Introduction Module: Used to introduce historical field variables into the rock mechanics constitutive model, construct a historical field that records the maximum tensile strain energy, and obtain the phase field governing equations; Solver module: Used to solve the phase field governing equations to obtain the distribution of phase field variables characterizing the damage state of the material; Simulation module: used to couple fluid pressure to solid equilibrium equations, feed back changes in pore structure and permeability caused by solid deformation to the seepage field, and update material stiffness and fluid flow properties according to phase field variable distribution.

[0057] A computer device is provided according to an embodiment of the present invention. This computer device includes a processor, a memory, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps in the various method embodiments described above. Alternatively, when the processor executes the computer program, it implements the functions of each module / unit in the various device embodiments described above.

[0058] The computer program can be divided into one or more modules / units, which are stored in the memory and executed by the processor to complete the present invention.

[0059] The computer device may be a desktop computer, laptop, handheld computer, or cloud server, etc. The computer device may include, but is not limited to, a processor and memory.

[0060] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc.

[0061] The memory can be used to store the computer program and / or module, and the processor implements various functions of the computer device by running or executing the computer program and / or module stored in the memory, and by calling the data stored in the memory.

[0062] If the modules / units integrated into the computer device are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory, random access memory, electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content included in the computer-readable medium can be appropriately added or removed according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, computer-readable media do not include electrical carrier signals and telecommunication signals.

[0063] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. 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 basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects.

[0064] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for characterizing natural fractures and simulating reservoir damage evolution in a phase-field manner, characterized in that, include: Natural cracks are generated based on discrete crack networks, and natural cracks are classified into primary cracks and secondary cracks. A natural crack system model is constructed based on the geometric parameters of primary cracks and secondary cracks. Based on solid mechanics theory, the governing equations for rock skeleton deformation and stress field are constructed, the elastic parameters of the rock matrix are defined, and a phase field damage model based on the principle of energy evolution is introduced to establish a rock mechanical constitutive model. The natural fracture system model is characterized as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and the natural fracture system model is given mechanical strength parameters and fracture toughness lower than those of the matrix. Based on Darcy's flow theory, a flow model of fluid in reservoir matrix and natural fractures is established, defining porosity, permeability parameters and injection source terms, and obtaining fluid pressure. By introducing historical field variables into the rock mechanics constitutive model, a historical field recording the maximum tensile strain energy is constructed, and the phase field governing equation is obtained. Solving the phase field governing equations yields the distribution of phase field variables characterizing the damage state of the material; The fluid pressure is coupled to the solid equilibrium equation, and the changes in pore structure and permeability caused by solid deformation are fed back to the seepage field. At the same time, the material stiffness and fluid flow properties are updated according to the phase field variable distribution.

2. The method for characterizing natural fractures and simulating reservoir damage evolution according to claim 1, characterized in that, The geometric parameters of natural cracks include size, aperture, density, dip angle, and tortuosity. The size is randomly generated within a specified area using a statistical distribution method. The aperture is assumed to be proportional to the size based on the size ratio. The density is adjusted using a Poisson distribution. The dip angle is adjusted using a discrete crack network plugin. The tortuosity is controlled by modifying program variables to control the degree of crack curvature.

3. The method for characterizing natural fractures and simulating reservoir damage evolution according to claim 1, characterized in that, The phase-field damage model is based on energy functionals, which are composed of elastic strain energy, fracture energy, work done by external forces and work done by fluid pressure; the elastic strain energy is decomposed into tensile and compressive components. The fracture energy is expressed in a regularized manner through phase field variables and their gradients; the mechanical strength parameters of the natural cracks include Young's modulus and Poisson's ratio.

4. The method for characterizing natural fractures and simulating reservoir damage evolution according to claim 1, characterized in that, The seepage model is established based on Darcy's law and the mass conservation equation and applied to the entire computational domain. The seepage model distinguishes between the reservoir domain and the fracture domain through characteristic functions, and defines their fluid density, fluid viscosity, water storage coefficient, porosity, permeability and Biot coefficient respectively.

5. The method for characterizing natural fractures and simulating reservoir damage evolution according to claim 1, characterized in that, The historical field is a monotonically increasing function that records the maximum value of the sum of tensile elastic strain energy and the work done by the initial stress, and is used to replace the energy term in the phase field governing equation.

6. The method for characterizing natural fractures and simulating reservoir damage evolution according to claim 1, characterized in that, The phase field control equation is driven by the history field function, and the phase field variables are used to characterize the damage state of the material. Crack propagation does not require a preset path.

7. A phase-field simulation system for characterizing natural fractures and simulating reservoir damage evolution, characterized in that, include: Model building module: used to generate natural cracks based on discrete crack network, classify natural cracks into primary cracks and secondary cracks, and build a natural crack system model based on the geometric parameters of primary cracks and secondary cracks. Model building module: used to construct the governing equations of rock skeleton deformation and stress field based on solid mechanics theory, define the elastic parameters of rock matrix, and introduce a phase field damage model based on the principle of energy evolution to establish a rock mechanical constitutive model; Embedding module: used to characterize the natural fracture system model as a mechanically weak surface of the rock matrix embedded in the rock mechanics constitutive model, and to assign the natural fracture system model mechanical strength parameters and fracture toughness lower than those of the matrix; Fluid pressure acquisition module: used to establish a fluid seepage model in reservoir matrix and natural fractures based on Darcy flow theory, define porosity, permeability parameters and injection source terms, and obtain fluid pressure; Historical Field Introduction Module: Used to introduce historical field variables into the rock mechanics constitutive model, construct a historical field that records the maximum tensile strain energy, and obtain the phase field governing equations; Solver module: Used to solve the phase field governing equations to obtain the distribution of phase field variables characterizing the damage state of the material; Simulation module: used to couple fluid pressure to solid equilibrium equations, feed back changes in pore structure and permeability caused by solid deformation to the seepage field, and update material stiffness and fluid flow properties according to phase field variable distribution.

8. A computer device, comprising a memory, a processor, and a computer program stored in the memory, characterized in that, The processor executes the computer program to implement the steps of the natural fracture characterization and reservoir damage evolution phase field simulation method according to any one of claims 1-6.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the steps of the phase-field simulation method for natural fracture characterization and reservoir damage evolution as described in any one of claims 1-6.

10. A computer program product, comprising a computer program, characterized in that, When executed by a processor, the computer program implements the steps of the phase-field simulation method for natural fracture characterization and reservoir damage evolution as described in any one of claims 1-6.

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