Complex fracture anisotropic elastic parameter characterization method and system

Abstract

The invention belongs to the technical field of rock physics and geophysics, and discloses a complex fracture anisotropic elastic parameter characterization method and system.The method comprises the steps that an outcrop picture of a fracture-karst geologic body is processed, a complex fracture model is obtained, and the fracture density and the fracture direction of the complex fracture model are calculated; a new Biot consolidation equation is constructed; designing an oscillation relaxation experiment; the complex fracture model is simulated, corresponding boundary conditions are given, and stress and strain fields of the complex fracture model are obtained through calculation; establishing a damage function, and substituting the stress and strain field of the complex crack model to solve the damage function to obtain elastic parameters of the complex crack model corresponding to a single frequency; different frequencies are given to realize calculation of anisotropic elastic parameters of the complex fracture. According to the invention, anisotropic elastic characterization of the complex crack rock can be realized more effectively; according to the method, the restrictive problem of conventional rock physical equivalent modeling and conventional numerical simulation estimation of the elastic parameters is effectively solved.

Description

Technical Field

[0001] The invention belongs to the technical field of rock physics and geophysics, and particularly relates to a method and system for characterizing anisotropic elastic parameters of complex fractures. Background Art

[0002] Fractures are key determinants of rock deformation capacity, fluid flow properties, and tectonic history. Understanding the elastic behavior of fractures is essential to interpreting the seismic response of rocks in terms of elasticity, and improved methods for characterizing fractures are needed.

[0003] Fractures in underground formations have complex shapes, distributions, and multiple scales, and the interactions between them lead to anisotropic characteristics of effective elastic properties. The overall effective elastic parameters of fractured rocks are usually composed of the stiffness tensors of the background surrounding rock and the fractures, which have attracted great attention because the elastic properties are often dominated by the fractures. Theoretical equivalent medium models have been widely used to analyze and calculate the effective elastic properties of fractured rocks. However, due to the dilution assumption of fracture density, these theoretical models ignore the interaction between fractures, which greatly limits their scope of application.

[0004] In the prior art, Chapman proposed an equivalent medium theory for crack length sensitivity and used the jet flow mechanism to study the frequency-dependent anisotropy of fractured porous rocks. Guo et al. combined the linear slip theory and the anisotropic Gassmann equation to analyze the equivalent stiffness matrix. The self-consistent model (SCA) and differential equivalent medium (DEM) theory are used to control the concentration of cracks to study the effects of crack interactions. Jakobsen et al. applied the T-matrix method to characterize pore-crack interactions. However, the idealization and oversimplification of real cracks severely limit the practicality of theoretical or semi-analytical models.

[0005] In order to avoid certain assumptions inherent in the effective medium theory, numerical simulation is widely used in the prior art to study the elastic properties of rocks. As an effective method with general applicability, numerical simulation can be used to characterize complex fractures with different shapes, sizes, cross-connections and connections, and to verify with equivalent theoretical methods and laboratory measurements.

[0006] In the prior art, Tang Xiaoming introduced the derivation of Biot theory for fractured media. Masson et al. used numerical experiments to analyze the correlation between material structure and seismic attenuation in porous media. Favino et al. used oscillation relaxation experimental method to characterize the velocity attenuation and scattering of P waves in fractured porous media related to frequency and incident angle.

[0007] In summary, the existing technology is mainly aimed at idealized fractures or simple fractures, and does not consider the actual interaction between fractures and fracture-cavities. The simulation accuracy of the elastic equivalent parameters of complex fractures is limited and it is difficult to calculate accurately. In addition, underground formations often contain fluids, and it is difficult to directly simulate wave-induced flow numerically (the main reason is that the main scale of wave-induced flow (characterized by the corresponding diffusion length) is much smaller than the seismic wavelength). Summary of the invention

[0008] In view of the above problems, the present invention provides a method and system for characterizing anisotropic elastic parameters of complex fractures, which adopts the following technical solutions:

[0009] A complex fracture anisotropic elastic parameter characterization method comprises the following steps: a model building step: processing an outcrop picture of a fault-karst geological body to obtain a complex fracture model, and calculating the fracture density and fracture direction of the complex fracture model; a consolidation equation building step: building a new Biot consolidation equation; an experiment design step: designing an oscillation relaxation experiment; a stress and strain field calculation step: simulating the complex fracture model based on the oscillation relaxation experiment and the new Biot consolidation equation, and calculating the stress and strain field of the complex fracture model given corresponding boundary conditions; and a single-frequency elastic parameter calculation step: building a damage function, solving the damage function through the stress and strain field of the complex fracture model to obtain the elastic parameters of the complex fracture model corresponding to a single frequency.

[0010] Furthermore, the outcrop images of the fault-karst geological body are processed to obtain a complex fracture model, including the following steps:

[0011] Binarize the outcrop images of the fault-karst geological body to obtain pixel images;

[0012] Divide the pixel image into multiple gray levels, calculate the probability distribution of the gray level of each pixel, and obtain the relative probability distribution histogram of the gray level;

[0013] According to the relative probability distribution histogram of gray levels, the pixel image is divided into multiple categories by adopting the threshold segmentation method to obtain the complex crack model.

[0014] Furthermore, the crack density and crack direction of the complex crack model are calculated, including the following steps:

[0015] When calculating the crack density, a circular scanning window is used in the complex crack model, and the total number of cracks is calculated by counting the number of endpoints of complete visible cracks in the window; in the complex crack model, an equal-area rose diagram is used instead of a histogram to represent the crack length and thus display the crack direction.

[0016] Furthermore, a new Biot consolidation equation is constructed, including the following steps:

[0017] Establish the two-dimensional Biot's spatial frequency domain consolidation equation;

[0018] According to the relationship between stress tensor and strain tensor, the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor is constructed;

[0019] According to the spatial frequency domain consolidation equation of two-dimensional Biot, as well as the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, a new Biot consolidation equation is constructed.

[0020] Furthermore, an oscillatory relaxation experiment was designed, including the following steps:

[0021] According to the relationship between the stress tensor and the strain tensor, three oscillation relaxation experiments were set up;

[0022] The first oscillation relaxation experiment is as follows: uniform vertical displacement is applied to the bottom and top boundaries of the complex crack model, the two lateral boundaries are fixed, and no shear stress is applied to determine the first lateral and vertical boundary conditions of the complex crack model;

[0023] The second oscillation relaxation experiment is as follows: applying horizontal displacement to the left and right boundaries of the complex crack model, fixing the upper and lower boundaries, and not applying shear stress, to determine the second boundary conditions in the lateral and vertical directions of the complex crack model;

[0024] The third oscillation relaxation experiment is specifically as follows: a shear experiment is carried out on the complex crack model, the top boundary and the bottom boundary of the complex crack model bear displacement, and the lateral and vertical third boundary conditions of the complex crack model are determined.

[0025] Furthermore, the complex crack model is simulated based on the oscillation relaxation experiment and the new Biot consolidation equation. Given the corresponding boundary conditions, the stress and strain fields of the complex crack model are calculated, including the following steps:

[0026] The complex crack model is simulated by the new Biot consolidation equation. After solving the new Biot consolidation equation under the corresponding boundary conditions by the finite element method, the displacement field of the finite element unit node, the stress and strain components of each unit are obtained.

[0027] The complex crack model is simulated through three oscillation relaxation experiments. According to the area of ​​the complex crack model region, the displacement field of the finite element unit node, and the stress and strain components of each unit, the average stress and strain field of the complex crack model in each average oscillation relaxation experiment is calculated.

[0028] Furthermore, a damage function is established, and the elastic parameters of the complex crack model corresponding to a single frequency are obtained by solving the damage function through the stress and strain field of the complex crack model, including the following steps:

[0029] Construct the stress-strain relationship of the complex crack model under extreme anisotropy, and determine the maximum anisotropic stiffness matrix characterizing the complex crack model according to the stress-strain relationship under extreme anisotropy;

[0030] A damage function is constructed and the six unknown components of the equivalent stiffness matrix representing the complex crack model are determined by minimizing the damage function.

[0031] The six unknown components of the equivalent stiffness matrix are simplified to obtain a system of nine equations;

[0032] The average stress and strain fields from three average oscillation relaxation experiments are input into a group of 9 equations to solve the overdetermined equations and obtain the equivalent stiffness matrix. The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

[0033] Furthermore, the method further comprises the following steps:

[0034] Based on different frequencies, the experimental design step, the stress and strain field calculation step, and the single-frequency elastic parameter calculation step are repeated to obtain the elastic parameters of the complex crack model at different frequencies.

[0035] Furthermore, the new Biot consolidation equation is as follows:

[0036]

[0037] Where P f represents fluid pressure, α represents Biot coefficient, N represents fluid coupling modulus, represents the Laplace operator of the spatial derivative, U represents the displacement tensor, i represents the gray level in the pixel image, κ represents the permeability, ω represents the angular frequency, and η represents the viscosity of the fluid.

[0038] The present invention also provides a complex fracture anisotropic elastic parameter characterization system, comprising:

[0039] The model building module is used to process the outcrop pictures of the fault-karst geological body, obtain the complex fracture model, and calculate the fracture density and fracture direction of the complex fracture model;

[0040] The first simulation module is used to construct the new Biot consolidation equation;

[0041] The second simulation module is used to design oscillatory relaxation experiments;

[0042] The first calculation module is used to simulate the complex crack model based on the oscillation relaxation experiment and the new Biot consolidation equation, and calculate the stress and strain fields of the complex crack model given the corresponding boundary conditions;

[0043] The second calculation module is used to establish a damage function, and solve the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the corresponding complex crack model under a single frequency.

[0044] Furthermore, the first simulation module is specifically used for:

[0045] Establish the two-dimensional Biot's spatial frequency domain consolidation equation;

[0046] According to the relationship between stress tensor and strain tensor, the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor is constructed;

[0047] According to the spatial frequency domain consolidation equation of two-dimensional Biot, as well as the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, a new Biot consolidation equation is constructed.

[0048] Furthermore, the second calculation module is specifically used for:

[0049] Construct the stress-strain relationship of the complex crack model under extreme anisotropy, and determine the maximum anisotropic stiffness matrix characterizing the complex crack model according to the stress-strain relationship under extreme anisotropy;

[0050] A damage function is constructed and the six unknown components of the equivalent stiffness matrix representing the complex crack model are determined by minimizing the damage function.

[0051] The six unknown components of the equivalent stiffness matrix are simplified to obtain a system of nine equations;

[0052] The average stress and strain fields from three average oscillation relaxation experiments are input into a group of 9 equations to solve the overdetermined equations and obtain the equivalent stiffness matrix. The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

[0053] Beneficial effects of the invention: The invention combines the advantages of equations and numerical oscillation relaxation experiments to effectively realize the anisotropic elastic characterization of complex fractured rocks. The invention provides a new way to understand the elastic properties of rocks, effectively overcoming the restrictive problems of conventional rock physics equivalent modeling and conventional numerical simulation estimation of elastic parameters.

[0054] Other features and advantages of the present invention will be described in the following description, and partly become obvious from the description, or be understood by implementing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0056] Figure 1 A schematic flow chart of a method for characterizing anisotropic elastic parameters of complex fractures according to an embodiment of the present invention is shown;

[0057] Figure 2 Shows an outcrop picture of a fault-karst geological body according to an embodiment of the present invention;

[0058] Figure 3 A schematic diagram of a complex fracture model in which caves and fractures are combined into one category according to an embodiment of the present invention is shown;

[0059] Figure 4 A schematic diagram of geometric properties of a complex crack model according to an embodiment of the present invention is shown;

[0060] Figure 5a A schematic diagram showing a first oscillation relaxation experiment according to an embodiment of the present invention is shown;

[0061] Figure 5b shows a schematic diagram of a second oscillation relaxation experiment according to an embodiment of the present invention;

[0062] Figure 5c shows a schematic diagram of a third oscillatory relaxation experiment according to an embodiment of the present invention;

[0063] Figure 6 A schematic diagram showing calculation results of complex crack elastic parameters at different frequencies according to an embodiment of the present invention is shown;

[0064] Figure 7 A structural schematic diagram of a complex fracture anisotropic elastic parameter characterization system according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0066] It should be noted that the terms "first", "second", etc. in the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0067] The present invention provides a method and system for characterizing anisotropic elastic parameters of complex cracks. Anisotropic elastic parameters are obtained by using a simulation method of equations and numerical experiments based on an actual complex crack model. The method for obtaining elastic parameters by combining equations characterizing wave propagation with numerical experiments is more theoretical and has stronger applicability.

[0068] like Figure 1 As shown, a method for characterizing anisotropic elastic parameters of complex cracks includes the following steps:

[0069] S1. Model building steps: Process the outcrop images of the fault-karst geological body to obtain a complex fracture model, and calculate the fracture density and fracture direction of the complex fracture model.

[0070] The main object of the present invention is a complex crack model. Figure 2 Outcrop picture of the fault-karst geological body shown.

[0071] Wherein, S1 comprises the following steps:

[0072] S11. Binarize the outcrop image of the fault-karst geological body to obtain a pixel image.

[0073] S12, dividing the pixel image into multiple gray levels, calculating the probability distribution of the gray level of each pixel, and obtaining a relative probability distribution histogram of the gray level.

[0074] For example, the pixel image is divided into S gray levels, and there are m pixels with gray level i in the pixel image. i (i=0,1,2,…,S), then the total number of pixels is The probability of a gray level at a certain pixel is P i =m i / M, by calculating the probability distribution of the gray level of each pixel, the relative probability distribution histogram of the gray level is obtained, and the peak value and inflection point in the figure are obtained.

[0075] S13. According to the relative probability distribution histogram of the gray level, the pixel image is divided into multiple categories by adopting a threshold segmentation method to obtain a complex crack model.

[0076] For example, the pixel image can be roughly divided into N categories, and N-1 segmentation thresholds are needed. Let the threshold set t = {t k |k=1,2,3,...,N-1}, and let t 0 =0,t N =S, and given the intra-class variance of multiple thresholds

[0077]

[0078] In formula (1), ω k , Represents the probability and class variance of the kth class; their definitions are:

[0079]

[0080] When the intra-class variance is the smallest, the optimal threshold is obtained, as follows:

[0081]

[0082] In the formula represents the optimal threshold, the subscript 0 on the right side of the equation is t 1 <t 2 <...<t N-1 It represents the initial threshold value,

[0083] For example, according to the fracture network, caves and background rocks, let N = 3 to divide the pixel image into three categories. For subsequent simulation, caves and fractures are combined into one category to obtain a complex fracture model. Figure 3 Results shown.

[0084] S14. When calculating crack density, a circular scanning window is used in a complex crack model, and the total number of cracks is calculated by counting the number of endpoints of complete visible cracks in the window; an equal-area rose diagram is used instead of a histogram in a complex crack model to represent the crack length and thus display the crack direction. Figure 4 As shown, Figure 4 is the geometric property of the complex fracture model, clearly showing the density and direction of different regions

[0085] S2. Steps for constructing the consolidation equation: Constructing a new Biot consolidation equation includes the following steps:

[0086] S21. Establish the two-dimensional Biot's spatial frequency domain consolidation equation, as follows:

[0087]

[0088]

[0089] In the formula, represents the Laplace operator of the spatial derivative, σ≡(σ ij ) represents the total stress tensor, P f represents the fluid pressure, κ is the permeability, ω represents the angular frequency, W represents the average relative fluid-solid displacement tensor, and η is the viscosity of the fluid.

[0090] S22. Based on the relationship between the stress tensor and the strain tensor, the relationship between the fluid pressure and the displacement tensor and the average relative fluid-solid displacement tensor is constructed.

[0091] The relationship between the stress tensor and the strain tensor is as follows:

[0092]

[0093]

[0094]

[0095] In the formula, δ ij The Kronecker function represented by To measure the local change of fluid content, U represents the displacement tensor and e represents the strain tensor. ij The shear modulus of dry rock skeleton and fluid-saturated rock is expressed by μ m and μ are equal.

[0096] Among them, the Biot coefficient α, fluid coupling modulus N, and Lame coefficient λ in formulas (5)-(7) are u The expression is as follows:

[0097]

[0098]

[0099]

[0100] In the formula, φ represents porosity, K s , K f , K m are the bulk moduli of the solid particles, the fluid phase and the dry rock skeleton, respectively.

[0101] Substituting formula (8) into formula (5), we have:

[0102]

[0103] According to formula (11) and (6), we can get:

[0104]

[0105] In formula (12), In the two-dimensional case, i, j = x, y.

[0106] Will Substituting into equation (6), we can obtain the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, as follows:

[0107]

[0108] S23. Based on the spatial frequency domain consolidation equation of two-dimensional Biot, as well as the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, a new Biot consolidation equation is constructed, as follows:

[0109] By combining equation (4) and equation (13), we get the new Biot consolidation equation, which is as follows:

[0110]

[0111] S3. Experimental design steps: Designing the oscillation relaxation experiment includes setting up three oscillation relaxation experiments according to the relationship between the stress tensor and the strain tensor, as follows:

[0112] S31, such as Figure 5a As shown, the first oscillation relaxation experiment is specifically performed as follows: uniform vertical displacement is applied to the bottom and top boundaries of the complex crack model, the two lateral boundaries are fixed, and no shear stress is applied, so as to determine the first lateral and vertical boundary conditions of the complex crack model.

[0113] For example, let D = (0, L x )×(0,L y ) represents the area of ​​the complex crack model, and the black arrows indicate the solid displacements in different directions. The solid displacement perpendicular to the black boundary line is zero. When performing the first oscillation relaxation experiment, uniform vertical displacements are applied to the bottom and top boundaries, the two lateral boundaries are fixed, and no shear stress is applied. Therefore, the boundary conditions in the lateral x direction and the vertical y direction of the complex crack model are determined, as shown in formula (15):

[0114]

[0115] In the formula, U represents the solid displacement, ΔU represents the displacement change, σ represents the total stress tensor, Γ represents the boundary of the model, and Γ b , Γ t , Γ l and Γ rThey are the bottom boundary, top boundary, left boundary and right boundary respectively. {ν,χ} form an orthogonal system, ν is the unit normal vector of the boundary, and χ represents the unit tangent vector.

[0116] S32, such as Figure 5b As shown in Figure 1, the second oscillation relaxation experiment is specifically as follows: applying horizontal displacement on the left and right boundaries of the complex crack model, fixing the upper and lower boundaries, and not applying shear stress, and determining the second boundary conditions in the lateral and vertical directions of the complex crack model, as shown in formula (16):

[0117]

[0118] S33, such as Figure 5c As shown in Figure 1, the third oscillation relaxation experiment is specifically as follows: a shear experiment is performed on the complex crack model, the top and bottom boundaries of the complex crack model bear displacement, and the lateral and vertical third boundary conditions of the complex crack model are determined, as shown in formula (17):

[0119]

[0120] S4, stress and strain field calculation steps: Based on the oscillation relaxation experiment and the new Biot consolidation equation, the complex crack model is simulated, and the corresponding boundary conditions are given to calculate the stress and strain field of the complex crack model, including the following steps:

[0121] S41. The complex crack model is simulated by the new Biot consolidation equation. After solving the new Biot consolidation equation under the corresponding boundary conditions by the finite element method, the displacement field of the finite element unit node, the stress and strain components of each unit are obtained.

[0122] S42. The complex crack model is simulated through three oscillation relaxation experiments. According to the area of ​​the complex crack model region, the displacement field of the finite element unit node, and the stress and strain components of each unit, the average stress and strain field of the complex crack model in each average oscillation relaxation experiment is calculated.

[0123] For example, by solving formulas (3) and (14) under the corresponding boundary conditions through the finite element method, after obtaining the displacement field of the finite element unit node, the stress and strain components of each unit are calculated using formulas (7) and (12). Thus, the average stress and strain field of the complex crack model in each average oscillation relaxation experiment is calculated using formulas (18) and (19) through these parameters.

[0124]

[0125]

[0126] In the formula, m = 1 to 3 represents the Figure 4 The mth oscillatory relaxation experiment shown, and denote the average stress and strain fields of the mth relaxation experiment respectively, and S is the area of ​​region D of the complex crack model.

[0127] S5. Single-frequency elastic parameter calculation steps: Establish a damage function, and solve the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the complex crack model corresponding to a single frequency.

[0128] S51. Construct a stress-strain relationship of a complex crack model under extreme anisotropy, and determine a maximum anisotropic stiffness matrix characterizing the complex crack model based on the stress-strain relationship under extreme anisotropy.

[0129] For example, considering the case of extreme anisotropy, under plane strain conditions, the stress-strain relationship is as follows:

[0130]

[0131] According to formula (20), three relaxation experiments can obtain 9 equations, and the maximum anisotropic stiffness matrix M of the two-dimensional complex crack model has 6 unknowns (M ij , i≤j).

[0132] S52. Construct a damage function and determine the six unknown components of the equivalent stiffness matrix that characterizes the complex crack model by minimizing the damage function.

[0133] In order to determine the components in the equivalent stiffness matrix, the following damage function is first constructed:

[0134]

[0135] By minimizing the damage function F 1 (M) Obtain the equivalent stiffness matrix M ij There are 6 unknown components.

[0136] S53. Simplify the six unknown components of the equivalent stiffness matrix to obtain a system of equations consisting of nine equations.

[0137] For example, according to formula (22), the 6 unknown components are solved and simplified to obtain the equation system consisting of 9 equations (23)-(28), which are as follows:

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145] S54. The average stress and strain fields in the three average oscillation relaxation experiments are respectively input into a group of 9 equations to solve the overdetermined equations to obtain an equivalent stiffness matrix. The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

[0146] For example, the average stress and strain fields in three average oscillation relaxation experiments are substituted as coefficients to solve the above formulas (23)-(28) to obtain the equivalent stiffness matrix M ij The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

[0147] S6, full-frequency elastic parameter calculation step: Repeat the above experimental design steps, stress and strain field calculation steps, and single-frequency elastic parameter calculation steps (steps S3-S5) based on different frequencies to obtain the elastic parameters of the complex crack model at different frequencies, such as Figure 6 shown.

[0148] The existing equivalent theoretical modeling mainly uses simplified fractures such as coin-shaped and needle-shaped fractures as the main objects, which is difficult to apply to complex fractures and does not consider the interaction between fractures. In addition, conventional numerical methods fail to simulate the influence of saturated rock fluid on elastic parameters, resulting in unclear changes in elastic parameters under high fracture density, which is inconsistent with the actual situation.

[0149] The present invention compares and verifies the results obtained in the above steps S1-S6 with the equivalent theoretical method, which shows that the present invention can better and more effectively achieve the anisotropic elastic characterization of complex fractured rocks. The characterization method of the present invention is highly practical, applicable to complex fractured rocks, and has outstanding advantages.

[0150] Based on the above complex fracture anisotropic elastic parameter characterization method, such as Figure 7 As shown, the present invention also provides a complex fracture anisotropic elastic parameter characterization system, including a model building module, a first simulation module, a second simulation module, a first calculation module and a second calculation module.

[0151] Among them, the model building module is used to process the outcrop pictures of the fault-karst geological body, obtain the complex crack model, and calculate the crack density and crack direction of the complex crack model; the first simulation module is used to construct a new Biot consolidation equation; the second simulation module is used to design an oscillation relaxation experiment; the first calculation module is used to simulate the complex crack model based on the oscillation relaxation experiment and the new Biot consolidation equation, and calculate the stress and strain field of the complex crack model given the corresponding boundary conditions; the second calculation module is used to establish a damage function, and solve the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the corresponding complex crack model under a single frequency.

[0152] The present invention obtains anisotropic elastic parameters by using a simulation method of equations and numerical experiments based on an actual complex fracture model. The method of obtaining elastic parameters by combining equations characterizing wave propagation and numerical experiments is more theoretical and has stronger applicability. It can effectively realize the anisotropic elastic characterization of complex fractured rocks and provides a new way to recognize and understand the elastic properties of rocks.

[0153] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for characterizing anisotropic elastic parameters of complex cracks, characterized in that: The following steps are involved: Model building steps: Process the outcrop pictures of the fault-karst geological body to obtain a complex fracture model, and calculate the fracture density and fracture direction of the complex fracture model; Steps for constructing the consolidation equation: Construct a new Biot consolidation equation; Experimental design steps: Design an oscillatory relaxation experiment; Stress and strain field calculation steps: simulate the complex crack model based on the oscillation relaxation experiment and the new Biot consolidation equation, give the corresponding boundary conditions, and calculate the stress and strain field of the complex crack model; Calculation steps of single-frequency elastic parameters: establish a damage function, solve the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the complex crack model corresponding to a single frequency.

2. The complex crack anisotropic elastic parameter characterization method according to claim 1, characterized in that: Processing the outcrop images of the fault-karst geological body to obtain a complex fracture model includes the following steps: Binarize the outcrop images of the fault-karst geological body to obtain pixel images; Divide the pixel image into multiple gray levels, calculate the probability distribution of the gray level of each pixel, and obtain the relative probability distribution histogram of the gray level; According to the relative probability distribution histogram of gray levels, the pixel image is divided into multiple categories by adopting the threshold segmentation method to obtain the complex crack model.

3. The complex crack anisotropic elastic parameter characterization method according to claim 2, characterized in that: Calculating the fracture density and fracture direction of a complex fracture model includes the following steps: When calculating the crack density, a circular scanning window is used in the complex crack model, and the total number of cracks is calculated by counting the number of endpoints of complete visible cracks in the window; in the complex crack model, an equal-area rose diagram is used instead of a histogram to represent the crack length and thus display the crack direction.

4. The complex crack anisotropic elastic parameter characterization method according to claim 1, characterized in that: Constructing a new Biot consolidation equation includes the following steps: Establish the two-dimensional Biot's spatial frequency domain consolidation equation; According to the relationship between stress tensor and strain tensor, the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor is constructed; According to the spatial frequency domain consolidation equation of two-dimensional Biot, as well as the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, a new Biot consolidation equation is constructed.

5. The complex crack anisotropic elastic parameter characterization method according to claim 1, characterized in that: Design an oscillatory relaxation experiment, including the following steps: According to the relationship between the stress tensor and the strain tensor, three oscillation relaxation experiments were set up; The first oscillation relaxation experiment is as follows: uniform vertical displacement is applied to the bottom and top boundaries of the complex crack model, the two lateral boundaries are fixed, and no shear stress is applied to determine the first lateral and vertical boundary conditions of the complex crack model; The second oscillation relaxation experiment is as follows: applying horizontal displacement to the left and right boundaries of the complex crack model, fixing the upper and lower boundaries, and not applying shear stress, to determine the second boundary conditions in the lateral and vertical directions of the complex crack model; The third oscillation relaxation experiment is specifically as follows: a shear experiment is carried out on the complex crack model, the top boundary and the bottom boundary of the complex crack model bear displacement, and the lateral and vertical third boundary conditions of the complex crack model are determined.

6. The complex crack anisotropic elastic parameter characterization method according to claim 1, characterized in that: The complex crack model is simulated based on the oscillation relaxation experiment and the new Biot consolidation equation. Given the corresponding boundary conditions, the stress and strain fields of the complex crack model are calculated, including the following steps: The complex crack model is simulated by the new Biot consolidation equation. After solving the new Biot consolidation equation under the corresponding boundary conditions by the finite element method, the displacement field of the finite element unit node, the stress and strain components of each unit are obtained. The complex crack model is simulated through three oscillation relaxation experiments. According to the area of ​​the complex crack model region, the displacement field of the finite element unit node, and the stress and strain components of each unit, the average stress and strain field of the complex crack model in each average oscillation relaxation experiment is calculated.

7. The method for characterizing anisotropic elastic parameters of complex cracks according to claim 1, characterized in that: Establishing a damage function, solving the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the complex crack model corresponding to a single frequency, including the following steps: Construct the stress-strain relationship of the complex crack model under extreme anisotropy, and determine the maximum anisotropic stiffness matrix characterizing the complex crack model according to the stress-strain relationship under extreme anisotropy; A damage function is constructed and the six unknown components of the equivalent stiffness matrix representing the complex crack model are determined by minimizing the damage function. The six unknown components of the equivalent stiffness matrix are simplified to obtain a system of nine equations; The average stress and strain fields from three average oscillation relaxation experiments are input into a group of 9 equations to solve the overdetermined equations and obtain the equivalent stiffness matrix. The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

8. The method for characterizing anisotropic elastic parameters of complex cracks according to any one of claims 1 to 7, characterized in that: The following steps are also included: Based on different frequencies, the experimental design step, the stress and strain field calculation step, and the single-frequency elastic parameter calculation step are repeated to obtain the elastic parameters of the complex crack model at different frequencies.

9. The method for characterizing anisotropic elastic parameters of complex cracks according to any one of claims 1 to 7, characterized in that: The new Biot consolidation equation is as follows: Where P f represents fluid pressure, α represents Biot coefficient, N represents fluid coupling modulus, represents the Laplace operator of the spatial derivative, U represents the displacement tensor, i represents the gray level in the pixel image, κ represents the permeability, ω represents the angular frequency, and η represents the viscosity of the fluid.

10. A complex fracture anisotropic elastic parameter characterization system, characterized in that: include: The model building module is used to process the outcrop pictures of the fault-karst geological body, obtain the complex fracture model, and calculate the fracture density and fracture direction of the complex fracture model; The first simulation module is used to construct the new Biot consolidation equation; The second simulation module is used to design oscillatory relaxation experiments; The first calculation module is used to simulate the complex crack model based on the oscillation relaxation experiment and the new Biot consolidation equation, and calculate the stress and strain fields of the complex crack model given the corresponding boundary conditions; The second calculation module is used to establish a damage function, and solve the damage function through the stress and strain field of the complex crack model to obtain the elastic parameters of the corresponding complex crack model under a single frequency.

11. The complex fracture anisotropic elastic parameter characterization system according to claim 10, characterized in that: The first simulation module is specifically used for: Establish the two-dimensional Biot's spatial frequency domain consolidation equation; According to the relationship between stress tensor and strain tensor, the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor is constructed; According to the spatial frequency domain consolidation equation of two-dimensional Biot, as well as the relationship between fluid pressure and displacement tensor and average relative fluid-solid displacement tensor, a new Biot consolidation equation is constructed.

12. The complex fracture anisotropic elastic parameter characterization system according to claim 10, characterized in that: The second calculation module is specifically used for: Construct the stress-strain relationship of the complex crack model under extreme anisotropy, and determine the maximum anisotropic stiffness matrix characterizing the complex crack model according to the stress-strain relationship under extreme anisotropy; A damage function is constructed and the six unknown components of the equivalent stiffness matrix representing the complex crack model are determined by minimizing the damage function. The six unknown components of the equivalent stiffness matrix are simplified to obtain a system of nine equations; The average stress and strain fields from three average oscillation relaxation experiments are input into a group of 9 equations to solve the overdetermined equations and obtain the equivalent stiffness matrix. The elastic parameters of the complex crack model are characterized by the maximum anisotropic stiffness matrix corresponding to a single frequency in the equivalent stiffness matrix.

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