Methods, apparatus and storage medium for determining the fracture toughness of shale

Abstract

This application provides a method, apparatus, and storage medium for determining the fracture toughness of shale, belonging to the field of computer technology. The method includes: acquiring the core column dimensions and mechanical parameters of the shale to be tested; establishing a simulation model space based on the core column dimensions, and generating an assembly of rigid particles within the simulation model space; determining the target cementation parameters between the rigid particles within the simulation model space based on the mechanical parameters; constructing a semi-circular shale simulation sample based on the assembly of rigid particles and the target cementation parameters; performing a three-point bending simulation experiment on the semi-circular shale simulation sample to obtain the axial loading peak load; determining the fracture toughness of the semi-circular shale simulation sample based on the axial loading peak load, and using the fracture toughness of the semi-circular shale simulation sample as the fracture toughness of the shale to be tested. This application is used to accurately measure the fracture toughness of ultra-deep shale, providing theoretical support for the optimized design of hydraulic fracturing in ultra-deep shale formations.

Description

Technical Field

[0001] This application relates to the field of computer technology, and specifically to a method, apparatus, and storage medium for determining the fracture toughness of shale. Background Technology

[0002] With the strategic progress made in the exploration and development of ultra-deep shale oil and gas, improving the effectiveness of hydraulic fracturing across strata has become a pressing problem to be solved in the hydraulic fracturing of ultra-deep shale reservoirs. Shale fracture toughness, as a key mechanical criterion for the initiation and propagation of complex fractures, can directly affect the hydraulic fracturing effect of ultra-deep shale reservoirs.

[0003] Shale fracture toughness refers to its ability to resist microcrack propagation, i.e., the energy absorption rate and ductility during fracture. Compared to traditional layered shale reservoirs, ultra-deep shale, due to its lower porosity and permeability and more complex high-stress environment, exhibits well-developed bedding, making vertical propagation of hydraulic fractures difficult. Therefore, it is urgent to explore measurement methods for ultra-deep shale fracture toughness to provide theoretical support for the optimized design of hydraulic fracturing in ultra-deep shale formations. Summary of the Invention

[0004] The purpose of this application is to provide a method, apparatus, and storage medium for determining the fracture toughness of shale, which can be used to accurately measure the fracture toughness of ultra-deep shale and provide theoretical support for the optimized design of hydraulic fracturing of ultra-deep shale.

[0005] To achieve the above objectives, in a first aspect, embodiments of this application provide a method for determining the fracture toughness of shale, comprising:

[0006] Obtain the core column dimensions and mechanical parameters of the shale to be tested;

[0007] A simulation model space is established based on the core column dimensions, and a collection of rigid particles is generated within the simulation model space.

[0008] Based on the mechanical parameters, determine the target bonding parameters between the rigid particles in the simulation model space;

[0009] Based on the set of rigid particles and the target cementation parameters, a semi-circular shale simulation sample is constructed.

[0010] A three-point bending simulation experiment was conducted on the semi-circular shale simulation specimen to obtain the peak axial load applied to the semi-circular shale simulation specimen.

[0011] The fracture toughness of the semi-circular shale simulation specimen is determined based on the axial loading peak load, and the fracture toughness of the semi-circular shale simulation specimen is taken as the fracture toughness of the shale to be tested.

[0012] Optionally, determining the target bonding parameters between rigid particles in the simulation model space based on the mechanical parameters includes:

[0013] Set the bonding parameters between rigid particles in the simulation model space;

[0014] A rigid wall is set around the simulation model space, and a triaxial experimental servo confining pressure is applied to the simulation model space through the rigid wall;

[0015] Real-time acquisition of stress monitoring values ​​of the rigid wall under the triaxial experimental servo confining pressure;

[0016] When the stress monitoring value of the rigid wall reaches a preset percentage relative to the preset biaxial unloading confining pressure, the simulation mechanical parameters of the simulation model space are obtained.

[0017] If the error between the simulated mechanical parameters and the mechanical parameters is less than a preset error value, the bonding parameter between the rigid particles in the simulation model space is determined as the target bonding parameter.

[0018] Optionally, constructing a semi-circular shale simulation specimen based on the set of rigid particles and the target cementation parameters includes:

[0019] A rectangular computational space is established based on the dimensions of the semi-circular shale, wherein the semi-circular shale is obtained based on the shale to be tested;

[0020] A rigid particle group, consistent with the set of rigid particles, is generated within the rectangular computational space.

[0021] The target bonding parameters are applied to the rigid particle group;

[0022] The bedding planes of the shale to be tested are divided;

[0023] The first preset parallel contact bonding model is used to process the bedding plane of the shale to be tested, and the second preset parallel contact bonding model is used to process the rigid particle group.

[0024] A wall is set around the rectangular computing space to form a closed area, and the target ultra-deep reservoir in-situ stress is applied to the rectangular computing space through the wall.

[0025] Under the action of the geostress of the target ultra-deep reservoir, the velocity of the rigid particles in the rectangular computational space is fixed, and the rectangular computational space is reduced to obtain the outer contour of the semi-circular plate.

[0026] A circular rigid wall is constructed at the top of the outer contour of the semicircular plate, and a fixed rigid wall is constructed at the bottom of the outer contour of the semicircular plate to obtain a semicircular plate shale simulation sample.

[0027] Optionally, after reducing the rectangular computational space to obtain the outer contour of the semicircular plate, the following steps are included:

[0028] Preset rigid particles are deleted at the center of the outer contour of the semicircular plate along the long side direction perpendicular to the rectangular computational space to form a preset crack.

[0029] Optionally, the method further includes:

[0030] By changing the bedding properties of the shale simulation sample, the fracture toughness of the shale simulation sample under different bedding properties is obtained, wherein the bedding properties include bedding dip angle and bedding density.

[0031] Optionally, obtaining the fracture toughness of the shale simulation sample under different bedding plane properties by changing the bedding plane properties includes:

[0032] Determine the properties of one bedding plane of the shale simulation specimen, and repeat the cyclic steps until the fracture toughness of the shale simulation specimen under the properties of the last bedding plane is obtained, wherein the cyclic steps include:

[0033] A three-point bending simulation experiment was conducted on the shale simulation specimen to obtain the peak axial load applied to the shale simulation specimen under the bedding plane properties.

[0034] Based on the axial loading peak load, the fracture toughness of the shale simulation specimen under the bedding plane properties was calculated.

[0035] Optionally, the semi-circular shale simulation specimen includes a semi-circular outer contour, a circular rigid wall at the top of the semi-circular outer contour, and a fixed rigid wall at the bottom of the semi-circular outer contour. The three-point bending simulation experiment performed on the semi-circular shale simulation specimen to obtain the peak axial load applied to the semi-circular shale simulation specimen includes:

[0036] Set the axial loading speed of the circular rigid wall so that the semi-circular shale simulation sample meets the quasi-static loading condition.

[0037] An axial compressive load is applied to the circular rigid wall, the axial displacement of the circular rigid wall is recorded, and an axial load-axial displacement curve is generated based on the axial compressive load and the axial displacement.

[0038] The peak axial load applied to the semi-circular shale simulation specimen was obtained based on the axial load-axial displacement curve.

[0039] Optionally, two fixed rigid walls are provided at the bottom of the outer contour of the semicircular plate. The two fixed rigid walls are located on both sides of the center of the bottom of the outer contour of the semicircular plate and are equidistant from the center. The step of determining the fracture toughness of the semicircular plate shale simulation specimen based on the axial loading peak load includes:

[0040] The fracture toughness of the semi-circular shale simulation specimen was calculated using the fracture toughness calculation formula.

[0041] The formula for calculating fracture toughness is as follows:

[0042]

[0043]

[0044] In the formula, K iC For fracture toughness, γ is the dimensionless stress intensity factor, and P is the stress intensity factor. max denoted as the peak axial load, r as the radius of the semicircular shale simulation specimen, b as the thickness of the semicircular shale simulation specimen, a as the length of the pre-set crack on the semicircular shale simulation specimen, and S as the distance between the two fixed rigid walls.

[0045] Secondly, this application provides a device for determining the fracture toughness of shale, comprising:

[0046] The memory is configured to store instructions; and

[0047] The processor is configured to retrieve the instructions from the memory and, when executing the instructions, to implement the above-described method for determining the fracture toughness of shale.

[0048] Thirdly, this application provides a machine-readable storage medium storing instructions for causing a machine to execute the method for determining the fracture toughness of shale as described above.

[0049] The above technical solution first establishes a simulation model space using the core column dimensions of the shale to be tested, and generates a set of rigid particles within this space. Based on the mechanical parameters of the shale to be tested, the target cementation parameters between the rigid particles are determined. This allows for the pre-determination of these parameters before the three-point bending simulation experiment, making the semi-circular shale simulation sample more closely resemble actual shale and improving the accuracy of fracture toughness calculations. Simultaneously, the peak axial load applied to the semi-circular shale simulation sample is obtained using the three-point bending simulation experiment. Based on this peak axial load, the fracture toughness of the semi-circular shale simulation sample is calculated, facilitating accurate measurement of the fracture toughness of ultra-deep shale and providing theoretical support for the optimized design of hydraulic fracturing in ultra-deep shale formations.

[0050] Other features and advantages of the embodiments of this application will be described in detail in the following detailed description section. Attached Figure Description

[0051] The accompanying drawings are provided to further illustrate the embodiments of this application and form part of the specification. They are used together with the following detailed description to explain the embodiments of this application, but do not constitute a limitation on the embodiments of this application. In the drawings:

[0052] Figure 1 This paper presents an overall flowchart of a method for determining the fracture toughness of shale provided in an embodiment of this application.

[0053] Figure 2 The stress-strain curves of a rigid wall under different triaxial experimental servo confining pressures provided in the embodiments of this application are shown.

[0054] Figure 3 A simulation model of the rectangular computational space provided in an embodiment of this application is shown;

[0055] Figure 4 The stress-strain curves obtained by applying a 20 MPa experimental confining pressure to a shale simulation specimen, as provided in the embodiments of this application, are shown.

[0056] Figure 5 The semi-circular shale simulation sample provided in the embodiment of this application is shown;

[0057] Figure 6 The axial load-axial displacement curves of the three-point bending simulation experiment under different triaxial experimental servo confining pressure conditions provided in the embodiments of this application are shown.

[0058] Figure 7 This paper illustrates the relationship between the bedding angle and fracture toughness when the bedding density is 0.8 bedding planes / mm, as provided in an embodiment of this application.

[0059] Figure 8This application provides an embodiment showing the relationship between bedding plane density and fracture toughness when the bedding plane dip angle is 45 degrees.

[0060] Figure 9 This application illustrates the relationship between the mean fracture toughness and the properties of the bedding planes provided in its embodiments.

[0061] Figure 10 This application provides an example of a layered plane angle-density engineering drawing for dimensionless fracture toughness under different triaxial experimental servo confining pressures, as shown in the embodiments of this application. Detailed Implementation

[0062] The specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the embodiments of this application.

[0063] This application discloses a method for determining the fracture toughness of shale.

[0064] Reference Figure 1 A method for determining the fracture toughness of shale includes the following steps:

[0065] S110. Obtain the core column dimensions and mechanical parameters of the shale to be tested.

[0066] In this embodiment, the core column is first extracted from the shale to be tested. The core column is a columnar core sample, and the shale to be tested can be underground shale or a shale outcrop. After obtaining the core column, its dimensions are obtained by measuring it. In this embodiment, the core column dimensions are φ25mm × H50mm, indicating a core column with a diameter of 25 mm and a height of 50 mm.

[0067] Mechanical parameters are used to describe the mechanical properties of the shale under test. In this embodiment, the mechanical parameters include biaxial peak strength, elastic modulus, and Poisson's ratio. In specific implementation, a triaxial compression test is first performed on the core column of the shale under test to obtain stress-strain curves. The stress-strain curves represent the rock strain under different stresses and are used to reflect the elastic and plastic deformation characteristics of the rock, including information such as elastic modulus, yield strength, and plastic deformation stage.

[0068] Biaxial peak strength represents the strength of the shale under test when subjected to biaxial shear stress. It is obtained from the stress-strain curve. Specifically, the biaxial peak strength is calculated from the maximum stress point in the stress-strain curve and the stress applied to the shale under test in the experiment. The method for calculating the biaxial peak strength based on the maximum stress point in the stress-strain curve and the stress applied to the shale under test in the experiment adopts existing technical methods and will not be elaborated here.

[0069] The elastic modulus and Poisson's ratio are obtained from half of the biaxial peak strength during the loading stage of the stress-strain curve. Specifically, the loading stage refers to the period before the peak strength is reached. The slope of the stress-strain curve at half the biaxial peak strength during the loading stage represents the elastic modulus. At half the biaxial peak strength during the loading stage of the stress-strain curve, the transverse strain and axial strain are obtained. Poisson's ratio is calculated from the transverse strain and axial strain, where Poisson's ratio = transverse strain / axial strain.

[0070] S120. Establish a simulation model space based on the core column size, and generate a collection of rigid particles within the simulation model space.

[0071] The simulation model space refers to the virtual space used for calculations and simulations. It is defined by a set of boundary conditions to constrain the physical phenomena and processes in the simulation. Specifically, if the core column size is φ25mm×H50mm, a two-dimensional rectangular simulation model boundary with a width of 25mm and a height of 50mm is generated. The space enclosed by the rectangular simulation model boundary is the simulation model space.

[0072] After the simulation model space is constructed, a set of rigid particles is generated in the simulation model space using preset simulation tools. Rigid particles refer to tiny objects with rigid properties used in physical simulation.

[0073] S130. Based on the mechanical parameters, determine the target bonding parameters between rigid particles in the simulation model space.

[0074] Since the mechanical parameters of the shale under test reflect the interaction forces and the nature of the forces between rigid particles, the target cementation parameters between rigid particles in the simulation model space can be determined by the mechanical parameters of the shale under test. Specifically, the target cementation parameters include the cementation strength and the cementation strength ratio between rigid particles.

[0075] S140. Based on the collection of rigid particles and the target cementation parameters, construct a semi-circular shale simulation specimen, wherein the semi-circular shale simulation specimen includes the outer contour of the semi-circular plate, the circular rigid wall at the top of the outer contour of the semi-circular plate, and the fixed rigid wall at the bottom of the outer contour of the semi-circular plate.

[0076] A semi-circular shale simulation specimen refers to an experimental model used to study the mechanical properties of shale under test. After determining the target cementation parameters between rigid particles, the target cementation parameters of the rigid particle assembly can be set, thereby constructing a semi-circular shale simulation specimen. Specifically, the semi-circular shale simulation specimen includes a semi-circular outer contour, a circular rigid wall at the top of the semi-circular outer contour, and a fixed rigid wall at the bottom of the semi-circular outer contour.

[0077] S150. A three-point bending simulation experiment was conducted on the semi-circular shale simulation specimen to obtain the peak axial load applied to the semi-circular shale simulation specimen.

[0078] The three-point bending simulation experiment is used to study the bending performance and mechanical behavior of semi-circular shale. In the three-point bending simulation experiment, a crack is pre-placed on the semi-circular shale simulation specimen and it is placed between two support points. A loading point is applied in the middle of the semi-circular shale simulation specimen to apply force to generate strain and stress, thereby obtaining the peak axial load applied to the semi-circular shale simulation specimen. The peak axial load refers to the maximum load applied to the semi-circular shale simulation specimen, that is, the highest load value reached in the three-point bending simulation experiment.

[0079] S160. Based on the peak axial load, determine the fracture toughness of the semi-circular shale simulation specimen and use the fracture toughness of the semi-circular shale simulation specimen as the fracture toughness of the shale to be tested.

[0080] In this embodiment, there are two fixed rigid body walls at the bottom of the outer contour of the semicircular plate. The two fixed rigid body walls are located on both sides of the center of the bottom of the outer contour of the semicircular plate and are equidistant from the center.

[0081] Specifically, the fracture toughness of the semi-circular shale simulation specimen is determined based on the peak axial load, including:

[0082] S161. The fracture toughness of the semi-circular shale simulation specimen was calculated using the fracture toughness calculation formula.

[0083] The formula for calculating fracture toughness is as follows:

[0084]

[0085]

[0086] In the formula, K iC For fracture toughness, γ is the dimensionless stress intensity factor, and P is the stress intensity factor. max The axial loading peak load is given by r, which is the radius of the semicircular shale simulation specimen. In this embodiment, the radius of the semicircular shale simulation specimen is the radius of the semicircle in the outer contour of the semicircular plate. b is the thickness of the semicircular shale simulation specimen, a is the length of the pre-set crack on the semicircular shale simulation specimen, and S is the distance between the two support points used to place the semicircular shale simulation specimen, i.e., the distance between the two fixed rigid walls.

[0087] The shale fracture toughness determination method provided in this embodiment first establishes a simulation model space based on the core column size of the shale to be tested, and generates a set of rigid particles within the simulation model space. The target cementation parameters between the rigid particles are determined based on the mechanical parameters of the shale to be tested. This pre-determines the target cementation parameters between the rigid particles before conducting the three-point bending simulation experiment, making the semi-circular shale simulation sample closer to the actual shale and improving the accuracy of fracture toughness calculation. Simultaneously, the peak axial load applied to the semi-circular shale simulation sample is obtained using the three-point bending simulation experiment, and the fracture toughness of the semi-circular shale simulation sample is calculated based on the peak axial load. This facilitates accurate measurement of the fracture toughness of ultra-deep shale, providing theoretical support for the optimized design of hydraulic fracturing in ultra-deep shale formations.

[0088] In one embodiment of this invention, determining the target bonding parameters between rigid particles in the simulation model space based on mechanical parameters includes the following steps:

[0089] S131. Set the bonding parameters between rigid particles in the simulation model space.

[0090] First, the set of rigid particles in the simulation model is divided into sub-strips of the same width according to the actual bedding dip angle of the shale to be tested, and cementation with preset cementation parameters is added between the rigid particles.

[0091] In this embodiment, in order to reproduce the weak surface features of real shale bedding, different bonding strengths are set between rigid particles in the sub-strips and between rigid particles in different sub-strips, and a preset bonding strength ratio is set. Specifically, the bonding parameters include bonding strength and bonding strength ratio.

[0092] S132. Set up a rigid wall around the simulation model space, and apply a three-axis experimental servo confining pressure to the simulation model space through the rigid wall.

[0093] Rigid walls refer to wall structures used to restrict the deformation of the simulation model space and bear confining pressure. Rigid walls are set up around the simulation model space to form a closed region. Bidirectional triaxial experimental servo confining pressure is applied to the rigid walls. Bidirectional refers to both the horizontal and vertical directions. Triaxial experimental servo confining pressure means that in a triaxial experiment, a servo system applies pressure to the simulation model space through the rigid walls, maintaining it under a certain confining pressure state. The confining pressure refers to the uniform pressure applied around the rigid walls, and its magnitude and stability can be controlled by the servo system.

[0094] S133. Real-time acquisition of stress monitoring values ​​of rigid walls under triaxial experimental servo confining pressure.

[0095] Stress monitoring values ​​refer to the pressure value and direction experienced by a rigid wall when subjected to triaxial experimental servo confining pressure.

[0096] S134. When the stress monitoring value of the rigid wall reaches the preset percentage of the biaxial unloading confining pressure, obtain the simulation mechanical parameters of the simulation model space.

[0097] Biaxial unloading confining pressure refers to the confining pressure applied perpendicular to the loading direction to a rigid wall in a triaxial experiment. When the stress monitoring value of the rigid wall reaches a preset percentage of the preset biaxial unloading confining pressure, it indicates that the rigid wall has reached the preset degree of confining pressure unloading after being subjected to the confining pressure. At this point, the wall servo is stopped, that is, the application of confining pressure to the rigid wall is stopped, and the simulation mechanical parameters in the simulation model space are obtained according to the preset monitoring system. Specifically, the simulation mechanical parameters include biaxial peak strength, elastic modulus, and Poisson's ratio. The acquisition of simulation mechanical parameters is carried out using the same method as the acquisition of mechanical parameters in step S110 above, and will not be described in detail here.

[0098] S135. When the error between the simulated mechanical parameters and the mechanical parameters is less than the preset error value, the bonding parameters between the rigid particles in the simulation model space are determined as the target bonding parameters.

[0099] The error value between the simulated mechanical parameters and the actual mechanical parameters refers to the difference between the two. If the error value between the simulated mechanical parameters and the actual mechanical parameters is less than the preset error value, it indicates that the simulated mechanical parameters are close to the actual mechanical parameters. At this time, the bonding parameters between the rigid particles in the simulation model space are determined as the target bonding parameters.

[0100] In this embodiment, in addition to setting the bonding parameters between rigid particles in the simulation model space, it is also necessary to set the properties, radius range, and biaxial confining pressure accuracy coefficient of the rigid particles. Among them, the properties of the rigid particles include particle density and global / local damping coefficient; setting the radius range of the rigid particles is used to ensure that the confining pressure monitored on the rigid wall has high resolution.

[0101] Specifically, when setting the bonding parameters between rigid particles in the simulation model space, the following steps also need to be performed:

[0102] A parallel contact bonding model is applied to the matrix portion of the simulation model space, where the matrix portion refers to the region in the simulation model space excluding cracks or bedding planes; a second preset parallel contact bonding model is used to apply bonding to the contact points between rigid particles in the matrix; a first preset parallel contact bonding model is used to apply bonding to the contact points at the bedding planes, i.e., the sub-strip boundaries, in the simulation model space; the parameters of the first preset parallel contact bonding model are set, including effective modulus, normal-tangential stiffness ratio, tensile strength, cohesion, internal friction angle, and friction coefficient; the number of microcracks, microcrack propagation morphology, and types of microcracks generated in the simulation model space are statistically analyzed.

[0103] In this embodiment, the first preset parallel contact bonding model is a weakened second preset parallel contact bonding model, that is, the bonding of the first preset parallel contact bonding model is weaker than the bonding of the second preset parallel contact bonding model.

[0104] In this embodiment, when the percentage of the stress monitoring value to the preset biaxial unloading confining pressure reaches a preset percentage, the simulated mechanical parameters are obtained, and the simulated mechanical parameters are compared with the mechanical parameters to determine the cementation parameters between rigid particles as the target cementation parameters. This makes the semi-circular shale simulation sample closer to the actual shale and improves the accuracy of fracture toughness calculation.

[0105] In one embodiment of this invention, a semi-circular shale simulation sample is constructed based on the aggregate of rigid particles and the target cementation parameters, including the following steps:

[0106] S141. Establish a rectangular computational space based on the dimensions of the semi-circular shale, where the semi-circular shale is obtained based on the shale to be measured.

[0107] In this embodiment, the semi-circular shale refers to a standard open semi-circular shale, with a semi-circular opening and two parallel edges, used for three-point bending simulation experiments. It should be noted that in this embodiment, both the semi-circular shale and the core column are rock samples obtained by sampling the shale to be tested.

[0108] Similar to the simulation model space, the rectangular computational space is a rectangular region defined in computer simulation, serving as a virtual space. Various physical parameters, boundary conditions, and initial conditions can be set within the rectangular computational space to simulate the mechanical behavior of a semi-circular shale slab.

[0109] S142. Generate a rigid particle group that is consistent with the set of rigid particles within a rectangular computational space.

[0110] To make the simulated semi-circular shale sample closely resemble the actual semi-circular shale, a rigid particle group needs to be generated in the rectangular computational space that is consistent with the set of rigid particles in the simulation model space.

[0111] S143. Apply target bonding parameters to the rigid particle group.

[0112] Set the bonding parameters between rigid particles in the rigid body object group as the target bonding parameters.

[0113] S144. Divide the bedding planes of the shale to be tested.

[0114] In this embodiment, the bedding planes of the shale to be tested are divided according to the actual rock sample of the semi-circular shale.

[0115] S145. The first preset parallel contact bonding model is used to process the bedding plane of the shale to be tested, and the second preset parallel contact bonding model is used to process the rigid particle group.

[0116] In this embodiment, the first preset parallel contact bonding model is a weakened second preset parallel contact bonding model, that is, the bonding of the first preset parallel contact bonding model is weaker than the bonding of the second preset parallel contact bonding model.

[0117] Rigid particle groups are processed using the second preset parallel contact bonding model. Since bedding planes differ from rigid particle groups in mechanical properties and contact behavior, bedding planes refer to parallel or nearly parallel fractures or fault planes in the shale to be tested. Therefore, bedding planes usually have lower shear strength and higher shear deformation capacity. Thus, the first preset parallel contact bonding model is used to process the bedding planes of the shale to be tested.

[0118] S146. Set up walls around the rectangular computational space to make the rectangular computational space a closed area, and apply the target ultra-deep reservoir in-situ stress to the rectangular computational space through the walls.

[0119] The geostress of the target ultra-deep reservoir refers to the geostress state in the rock layer located at a relatively deep underground depth. Geostress is the stress on the rocks inside the Earth.

[0120] A wall is set up around the computational space to enclose the rectangular computational space into a closed area, and the target ultra-deep reservoir in-situ stress is applied to the rectangular computational space through the wall to simulate the actual geological environment and original stress field of the shale to be tested.

[0121] Before applying the target ultra-deep reservoir in-situ stress to the rectangular computational space through the wall, it is necessary to set the accuracy coefficient for applying the target ultra-deep reservoir in-situ stress.

[0122] S147. Under the action of geostress in the target ultra-deep reservoir, the velocity of rigid particles in the rectangular computational space is fixed, and the rectangular computational space is reduced to obtain the outer contour of the semi-circular plate.

[0123] Under the influence of geostress in the target ultra-deep reservoir, the velocity of rigid particles within a fixed rectangular computational space is used to simulate the static or quasi-static loading conditions of the shale under test in the simulation, that is, to simulate the stress state and deformation behavior of the shale under test in the actual environment.

[0124] In this embodiment, the velocity of the rigid particles can be fixed by setting the velocity parameter of the rigid particles to 0 or a fixed value.

[0125] S148. Construct a circular rigid wall at the top of the outer contour of the semicircular plate and a fixed rigid wall at the bottom of the outer contour of the semicircular plate to obtain a semicircular plate shale simulation sample.

[0126] In this embodiment, two fixed rigid walls are provided as the fulcrum for the three-point bending of the semicircular shale simulation sample. The two fixed rigid walls are located on both sides of the bottom center of the outer contour of the semicircular plate. The distance between the two fixed rigid walls and the bottom center of the outer contour of the semicircular plate is the same. In addition, the radius of the two fixed rigid walls is the same as the radius of the circular rigid wall at the top of the outer contour of the semicircular plate.

[0127] This embodiment simulates the contact behavior and cementation characteristics between rigid particles by generating a rigid particle group consistent with the set of rigid particles within a rectangular computational space and applying target cementation parameters. The bedding planes of the shale under test are divided, and a first preset parallel contact bonding model is used to process the bedding planes, while a second preset parallel contact bonding model is used to process the rigid particle group. This facilitates a more accurate simulation of the bedding plane characteristics and contact behavior between rigid particles in the shale. By setting walls to enclose the rectangular computational space and applying the target ultra-deep reservoir in-situ stress, the in-situ stress state of the rock in actual ultra-deep reservoirs can be simulated.

[0128] In one embodiment of this invention, after reducing the rectangular computational space to obtain the outer contour of the semicircular plate, the following steps are included:

[0129] S210. At the center of the outer contour of the semicircular plate, delete the preset rigid body particles along the long side direction perpendicular to the rectangular computation space to form a preset crack.

[0130] In this embodiment, preset rigid particles are deleted at the center of the outer contour of the semi-circular plate along the long side perpendicular to the rectangular computational space to form a pre-set crack for the subsequent initiation and propagation of microcracks. Specifically, the initiation and propagation of microcracks refer to the formation and gradual expansion of tiny cracks in the shale under test. The initiation and propagation of microcracks can be used to understand the failure mechanism of the shale under test, assess its strength and stability, and predict its deformation and failure.

[0131] In one embodiment of this invention, the method further includes the following steps:

[0132] S310. By changing the bedding properties of the shale simulation sample, the fracture toughness of the shale simulation sample under different bedding properties is obtained, wherein the bedding properties include the bedding dip angle and the bedding density.

[0133] The dip angle of the bedding plane refers to the angle between the bedding plane in the shale being tested and the horizontal plane; the bedding plane density refers to the number or density of bedding planes in the shale being tested.

[0134] The fracture toughness of shale simulation samples under different bedding properties was obtained to demonstrate the influence of bedding properties on fracture toughness, that is, the change of fracture toughness under different bedding properties.

[0135] In one embodiment of this invention, the fracture toughness of the shale simulation sample under different bedding plane properties is obtained by changing the bedding plane properties, including the following steps:

[0136] S311. Determine the properties of one of the bedding planes of the shale simulation specimen, and repeat the cyclic steps until the fracture toughness of the shale simulation specimen under the properties of the last bedding plane is obtained.

[0137] The loop steps include:

[0138] S312. A three-point bending simulation experiment was conducted on the shale simulation sample to obtain the peak axial load applied to the shale simulation sample under the bedding plane properties.

[0139] S313. Based on the peak axial load, the fracture toughness of the shale simulation specimen under the bedding plane properties is calculated.

[0140] In this embodiment, the influence of bedding plane properties on the fracture toughness of the shale under test can be investigated by changing the bedding plane properties of the shale simulation sample. Therefore, one bedding plane property of the shale simulation sample is first set, and the fracture toughness of the shale simulation sample under this bedding plane property can be calculated. By changing the bedding plane properties and repeating the cyclic steps, the fracture toughness of the shale simulation sample under different bedding plane properties can be obtained. Specifically, the bedding plane properties include the bedding plane dip angle and the bedding plane density. In practice, the bedding plane dip angle and the bedding plane density need to be determined separately. That is, when investigating the influence of the bedding plane dip angle on the fracture toughness of the shale under test, the bedding plane density needs to be kept consistent to demonstrate the influence of different bedding plane dip angles on the fracture toughness of the shale simulation sample. Similarly, when investigating the influence of the bedding plane density on the fracture toughness of the shale under test, the bedding plane dip angle needs to be kept consistent to demonstrate the influence of different bedding plane densities on the fracture toughness of the shale simulation sample.

[0141] In addition to exploring the fracture toughness of shale simulation samples under different bedding plane properties, this embodiment can also investigate the influence of different bedding plane properties on microcracks on shale simulation samples in three-point bending simulation experiments by changing the bedding plane properties, including the number of microcracks, propagation rate, and morphology of propagation along the bedding plane; furthermore, the fracture toughness obtained from the three-point bending simulation experiment can be displayed and expressed in graphical form, i.e., in the form of an engineering drawing, to facilitate the visualization of experimental results and improve the applicability of the three-point bending simulation experiment, thus providing support for shale research.

[0142] In one embodiment of this invention, the semi-circular shale simulation specimen includes a semi-circular outer contour, a circular rigid wall at the top of the semi-circular outer contour, and a fixed rigid wall at the bottom of the semi-circular outer contour. A three-point bending simulation experiment is performed on the semi-circular shale simulation specimen to obtain the peak axial load applied to it. This includes the following steps:

[0143] S151. Set the axial loading speed of the circular rigid wall so that the semi-circular shale simulation sample meets the quasi-static loading condition.

[0144] Quasi-static loading conditions refer to conditions where the dynamic effects during loading are negligible. In this embodiment, the axial loading speed of the circular rigid wall is set to make the semi-circular shale slab meet the quasi-static loading conditions.

[0145] S152. Apply an axial compressive load to the circular rigid wall, record the axial displacement of the circular rigid wall, and generate an axial load-axial displacement curve based on the axial compressive load and axial displacement.

[0146] Axial compressive load refers to the pressure applied along the axial direction to a circular rigid wall. After applying an axial compressive load to the circular rigid wall, the axial displacement of the circular rigid wall is recorded in real time using a preset monitoring tool. An axial load-axial displacement curve is generated based on the axial compressive load and axial displacement. The axial load-axial displacement curve is the relationship curve between the axial compressive load and the axial displacement, used to represent the axial displacement of the semi-circular shale simulation sample under different axial compressive loads.

[0147] S153. Obtain the peak axial load applied to the semi-circular shale simulation specimen based on the axial load-axial displacement curve.

[0148] The maximum value point in the axial load-axial displacement curve represents the peak axial load applied to the semi-circular shale simulation specimen.

[0149] This embodiment sets the axial loading velocity of the circular rigid wall to make the semi-circular shale simulation specimen meet the quasi-static loading conditions; by applying an axial compressive load and recording the axial displacement of the circular rigid wall, an axial load-axial displacement curve is generated; by analyzing this curve, the peak axial load applied to the semi-circular shale simulation specimen is obtained to simulate the mechanical behavior of actual rock during loading and to obtain the relevant mechanical parameters, i.e., the peak axial load.

[0150] The method for determining the fracture toughness of shale is further illustrated below through specific examples:

[0151] This embodiment uses shale with a bedding plane dip angle of 0° to measure fracture toughness. First, the triaxial experimental servo confining pressure is determined. (Refer to...) Figure 2 The figures show the stress-strain curves of a rigid wall under different triaxial experimental servo confining pressures in specific experiments. When the confining pressure is between 0 and 20 MPa, the slope of the post-peak segment is relatively large, and the tested shale still exhibits high brittleness. When the confining pressure increases to 40 MPa, the stress-strain curve of the tested shale begins to show a yield plateau after the peak, exhibiting more ideal ductility characteristics. Therefore, 40 MPa can be used as the triaxial experimental servo confining pressure. When the confining pressure continues to increase to 60 MPa, the stress-strain curve of the tested shale shows a phenomenon similar to that of the 40 MPa confining pressure, indicating that the triaxial experimental servo confining pressure should be greater than or equal to 40 MPa.

[0152] Table 1 below shows the experimental parameters in the triaxial test, including the confining pressure, peak strength, elastic modulus, and Poisson's ratio. As shown in Table 1, with the confining pressure increasing to 40 MPa, the elastic modulus, Poisson's ratio, and peak strength of the tested shale all increase. When the confining pressure is in the range of 40–60 MPa, the shale softens due to its greater plasticity. In the specific implementation, the mechanical parameters obtained from a 20 MPa confining pressure experiment were selected as the basis for preparing the shale simulation specimen.

[0153] Table 1

[0154] #1 0MPa 45.7MPa 10.2 GPa 0.24 #2 20MPa 100.4MPa 12.4 GPa 0.26 #3 40MPa 122.3MPa 15.7 GPa 0.29 #4 60MPa 118.7MPa 12.1 GPa 0.28

[0155] Reference Figure 3 , Figure 3 A simulation model of a rectangular computational space is shown. The simulation model includes a matrix 1 and a stratification surface 2. The matrix 1 includes multiple matrix particles, and the stratification surface 2 includes multiple stratification surface particles. The cementation in the matrix particles is strong cementation, and the cementation in the stratification surface particles is weak cementation.

[0156] Reference Figure 4 After applying a 20MPa confining pressure to the shale simulation sample, the stress-strain curve was obtained, and the mechanical parameters could be obtained from the stress-strain curve.

[0157] Table 2 below shows the target bonding parameters obtained from the mechanical parameters in a specific implementation.

[0158] Table 2

[0159]

[0160] After determining the target cementation parameters, a semi-circular shale simulation specimen was constructed. In specific implementation, the constructed semi-circular shale simulation specimen is as follows: Figure 5 As shown, the axial load-axial displacement curves of the three-point bending simulation experiment under different triaxial experimental servo confining pressure conditions are as follows: Figure 6 As shown, when the bedding plane density is 0.8 planes / mm, the relationship between the bedding plane dip angle and fracture toughness is as follows: Figure 7 As shown, when the dip angle of the bedding plane is 45 degrees, the relationship between the bedding plane density and the fracture toughness is as follows: Figure 8 As shown.

[0161] In practice, when the bedding plane density is the same but the bedding plane dip angle is different, the relationship between the bedding plane dip angle and the fracture toughness is fitted, and the relationship between the average fracture toughness of the semi-circular shale simulation sample and the triaxial experimental servo confining pressure is expressed by the following formula (1):

[0162] K′=0.0036σ c +0.0946 (R 2 =0.9353) (1)

[0163] In the formula, σ c The confining pressure applied to the semi-circular plate simulation specimen is K′, which is the normalized dimensionless mean fracture toughness. R 2 The correlation coefficient is used to represent the accuracy of the fit.

[0164] When the bedding planes have different densities but the same dip angle, the relationship between the bedding plane dip angle and the fracture toughness is fitted, and the relationship between the mean fracture toughness of the semi-circular shale simulation sample and the triaxial experimental servo confining pressure is expressed by the following equation (2):

[0165] K′=0.0047σ c +0.0828 (R 2 =0.9754) (2)

[0166] The relationship between the servo confining pressure and fracture toughness in different triaxial experiments was fitted to obtain the relationship between the mean fracture toughness and the servo confining pressure in the triaxial experiments, which is expressed by the following equation (3):

[0167] K′=a σ P A +b σ P D (3)

[0168] In the formula, P A For the dimensionless bedding plane angle, P D Let a be the dimensionless bedding surface density. σ and b σ These represent the changes in weighting coefficients with confining pressure.

[0169] The influence coefficient of the bedding plane dip angle on fracture toughness as a function of the triaxial test servo confining pressure is expressed by the following equation (4):

[0170] a σ =0.0038σ c +0.0994 (R 2 =0.987) (4)

[0171] The effect coefficient of bedding density on fracture toughness as a function of triaxial experimental servo confining pressure is expressed by the following equation (5):

[0172] b σ =0.0019σ c +0.0175 (R 2 =0.906) (5)

[0173] Table 3 below shows the fracture toughness of the semi-circular shale simulation specimens calculated in a specific implementation.

[0174] Table 3

[0175]

[0176]

[0177] By fitting the relationship between fracture toughness and bedding plane properties, a reference relationship between the mean fracture toughness and the variation of bedding plane properties is obtained. Figure 9 Furthermore, bedding plane angle-density engineering plots for dimensionless fracture toughness under different triaxial experimental servo confining pressures were drawn, with reference to... Figure 10 .

[0178] This application also discloses a device for determining the fracture toughness of shale, which may include:

[0179] The memory is configured to store instructions; and

[0180] The processor is configured to retrieve the instructions from the memory and, when executing the instructions, to implement the above-described method for determining the fracture toughness of shale.

[0181] Specifically, the processor can be a central processing unit (CPU). Of course, depending on the actual use, other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), off-the-shelf programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. can also be used. The general-purpose processor can be a microprocessor or any conventional processor, etc. This application does not limit this.

[0182] This application also discloses a machine-readable storage medium storing a program that, when executed by a processor, implements the above-described method for determining shale fracture toughness.

[0183] The computer program can be stored in a machine-readable medium. The computer program includes computer program code, which can be in the form of source code, object code, executable file, or certain middleware. The machine-readable medium includes any entity or device capable of carrying computer program code, recording media, USB flash drive, portable hard drive, magnetic disk, optical disk, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the machine-readable medium includes, but is not limited to, the above-mentioned components.

[0184] The shale fracture toughness determination method described in the above embodiments is stored in the machine-readable storage medium and loaded and executed on the processor to facilitate the storage and application of the above method.

[0185] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0186] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0187] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0188] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0189] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0190] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, like read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0191] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0192] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0193] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A method for determining the fracture toughness of shale, characterized in that, include: Obtain the core column dimensions and mechanical parameters of the shale to be tested; A simulation model space is established based on the core column dimensions, and a collection of rigid particles is generated within the simulation model space. Based on the mechanical parameters, determine the target bonding parameters between the rigid particles in the simulation model space; Based on the set of rigid particles and the target cementation parameters, a semi-circular shale simulation sample is constructed. A three-point bending simulation experiment was conducted on the semi-circular shale simulation specimen to obtain the peak axial load applied to the semi-circular shale simulation specimen. Based on the axial loading peak load, the fracture toughness of the semi-circular shale simulation specimen is determined, and the fracture toughness of the semi-circular shale simulation specimen is taken as the fracture toughness of the shale to be tested. The step of constructing a semi-circular shale simulation sample based on the set of rigid particles and the target cementation parameters includes: A rectangular computational space is established based on the dimensions of the semi-circular shale, wherein the semi-circular shale is obtained based on the shale to be tested; A rigid particle group, consistent with the set of rigid particles, is generated within the rectangular computational space. The target bonding parameters are applied to the rigid particle group; The bedding planes of the shale to be tested are divided; The first preset parallel contact bonding model is used to process the bedding plane of the shale to be tested, and the second preset parallel contact bonding model is used to process the rigid particle group. A wall is set around the rectangular computing space to form a closed area, and the target ultra-deep reservoir in-situ stress is applied to the rectangular computing space through the wall. Under the action of the geostress of the target ultra-deep reservoir, the velocity of the rigid particles in the rectangular computational space is fixed, and the rectangular computational space is reduced to obtain the outer contour of the semi-circular plate. A circular rigid wall is constructed at the top of the outer contour of the semicircular plate, and a fixed rigid wall is constructed at the bottom of the outer contour of the semicircular plate to obtain a semicircular plate shale simulation sample.

2. The method for determining the fracture toughness of shale according to claim 1, characterized in that, Determining the target bonding parameters between rigid particles in the simulation model space based on the mechanical parameters includes: Set the bonding parameters between rigid particles in the simulation model space; A rigid wall is set around the simulation model space, and a triaxial experimental servo confining pressure is applied to the simulation model space through the rigid wall; Real-time acquisition of stress monitoring values ​​of the rigid wall under the triaxial experimental servo confining pressure; When the stress monitoring value of the rigid wall reaches the preset percentage of the biaxial unloading confining pressure, the simulation mechanical parameters of the simulation model space are obtained; If the error between the simulated mechanical parameters and the mechanical parameters is less than a preset error value, the bonding parameter between the rigid particles in the simulation model space is determined as the target bonding parameter.

3. The method for determining the fracture toughness of shale according to claim 1, characterized in that, After reducing the rectangular computational space to obtain the outer contour of the semicircular plate, the process includes: Preset rigid particles are deleted at the center of the outer contour of the semicircular plate along the long side direction perpendicular to the rectangular computational space to form a preset crack.

4. The method for determining the fracture toughness of shale according to claim 1, characterized in that, The method further includes: By changing the bedding properties of the shale simulation sample, the fracture toughness of the shale simulation sample under different bedding properties is obtained, wherein the bedding properties include bedding dip angle and bedding density.

5. The method for determining the fracture toughness of shale according to claim 4, characterized in that, The method of obtaining the fracture toughness of the shale simulation sample under different bedding plane properties by changing the bedding plane properties includes: Determine the properties of one bedding plane of the shale simulation specimen, and repeat the cyclic steps until the fracture toughness of the shale simulation specimen under the properties of the last bedding plane is obtained, wherein the cyclic steps include: A three-point bending simulation experiment was conducted on the shale simulation specimen to obtain the peak axial load applied to the shale simulation specimen under the bedding plane properties. Based on the axial loading peak load, the fracture toughness of the shale simulation specimen under the bedding plane properties was calculated.

6. The method for determining the fracture toughness of shale according to claim 1, characterized in that, The semi-circular shale simulation specimen includes a semi-circular outer contour, a circular rigid wall at the top of the semi-circular outer contour, and a fixed rigid wall at the bottom of the semi-circular outer contour. A three-point bending simulation experiment is performed on the semi-circular shale simulation specimen to obtain the peak axial load applied to the specimen, including: Set the axial loading speed of the circular rigid wall so that the semi-circular shale simulation sample meets the quasi-static loading condition. An axial compressive load is applied to the circular rigid wall, the axial displacement of the circular rigid wall is recorded, and an axial load-axial displacement curve is generated based on the axial compressive load and the axial displacement. The peak axial load applied to the semi-circular shale simulation specimen was obtained based on the axial load-axial displacement curve.

7. The method for determining the fracture toughness of shale according to claim 6, characterized in that, Two fixed rigid walls are provided at the bottom of the outer contour of the semicircular plate. The two fixed rigid walls are located on both sides of the center of the bottom of the outer contour of the semicircular plate and are equidistant from the center. The step of determining the fracture toughness of the semicircular plate shale simulation specimen based on the axial loading peak load includes: The fracture toughness of the semi-circular shale simulation specimen was calculated using the fracture toughness calculation formula. The formula for calculating fracture toughness is as follows: ； ； In the formula, K iC For fracture toughness, It is a dimensionless stress intensity factor. For axial loading peak load, r The radius of the semi-circular shale simulation sample is given. b denoted as , where 'a' represents the thickness of the semi-circular shale simulation sample, and 'a' represents the length of the pre-placed cracks on the semi-circular shale simulation sample. S This is the distance between two fixed rigid walls.

8. A device for determining the fracture toughness of shale, characterized in that, include: The memory is configured to store instructions; as well as The processor is configured to retrieve the instructions from the memory and, when executing the instructions, to implement the method for determining shale fracture toughness according to any one of claims 1 to 7.

9. A machine-readable storage medium storing instructions thereon, characterized in that, This instruction is used to cause the machine to perform the method for determining the fracture toughness of shale according to any one of claims 1 to 7.

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