A Digital Characterization Method for Shale Mineral Microstructures

Abstract

The present invention relates to a method for digital characterization of shale mineral microstructures, including experimentally determining the proportion of mineral components, the morphological characteristics of mineral microstructures, and the micro-mechanical parameters of minerals in the shale core, establishing discrete element models of clay minerals and non-clay minerals respectively based on the discrete element modeling method, and successively completing the construction of representative lamellar volume units, single shale laminations, and digital shale cores. The present invention uses the discrete element modeling method to take the clay lamellar structure as the smallest microscale to depict the discontinuous characteristics of rocks across scales, thereby realizing the digital characterization of shale mineral microstructures and their combinations, and further enabling the prediction of the mechanical behavior of shale from the perspective of the microstructures of shale minerals.

Description

Technical Field

[0001] The invention relates to the technical field of rock digital characterization, and in particular to a digital characterization method for a shale mineral microstructure. Background Art

[0002] Rocks are a mixture of various mineral particles, pores and cements. After billions of years of geological changes and complex and varied tectonic movements, a large number of randomly distributed microcracks, micropores, joints, faults and other defects have been generated inside them. Therefore, rocks are neither an ideal continuous medium, such as the existence of macro, fine and micro discontinuities, nor a discrete medium in the strict sense, such as crystalline materials. The current continuity hypothesis is mostly based on macroscopic mechanical phenomena, and does not explain complex rock mechanics and engineering problems based on the microscopic structure of rocks. It is urgent to establish the connection between microstructure and macroscopic phenomena, so as to reveal the physical nature of rock mechanics problems and guide the formulation of scientific engineering response measures.

[0003] Shale is a type of sedimentary rock that is formed by the consolidation of clastic sediments with a particle size of less than 0.0625 mm and a content of more than 50%. It has the characteristics of lamination and foliation structure. Shale is a sedimentary rock with porous clay-intercalated mineral components and mineral particles of various sizes; among which clay minerals mainly include kaolinite, illite, and montmorillonite, and the non-clay minerals included are usually quartz, feldspar, and calcite.

[0004] Petroleum engineering involves many rock mechanics problems of shale, such as the seepage mechanics of adsorption and desorption, the deformation and fragmentation of shale, the geomechanical problems of coupling ground stress with drilling, and the fracture initiation mechanism under the multi-field coupling of completion and fracturing. Shale is a sedimentary rock with complex composition, strong heterogeneity, and different structures at different scales. First, according to the multi-mineral fabric characteristics of shale, a multi-scale composition classification of shale is proposed, and the size is divided into five scales: Scale 1-microscale: the basic material unit of shale clay minerals composed of single-crystal clay mineral structures and nanopores; Scale 2-microscale: single mineral aggregates composed of single-crystal clay mineral stacks and intergranular pores, which can be used to distinguish the boundaries of single mineral bodies; Scale 3-microscale: composite media formed by clay minerals, non-clay mineral inclusions and organic matter. Scale 1, scale 2 and scale 3 are all microscales, which are mainly used to analyze the composition of porous clay media and the relationship between microstructure and mechanical properties. Scale 4 - Mesoscale: Textured mesoscopic shale flakes; Scale 5 - Macroscale: Laboratory-scale shale cylinder samples. Shale samples include millimeter and micrometer-sized particles and bedding planes, which play a key role in the geomechanical characteristics of shale and the failure of rock.

[0005] As described above, the mineral structure and mechanical characteristics of shale are very complex, and it is difficult to connect the characteristics of shale at different scales and form an engineering geological body across scales. Currently, the commonly used methods for studying the mechanical characteristics of rocks are to form digital images depicting the microscopic structure through microscopic experimental means such as high-precision computed tomography (CT) and scanning electron microscope (SEM), and to establish two-dimensional or three-dimensional digital cores containing microscopic mineral, pore, and fracture characteristics with the help of numerical algorithms. This type of method focuses on depicting physical properties such as microscopic pore throats and fractures, and does not couple mechanical properties, so it is impossible to predict the mechanical behavior of shale from a physical level. Another method for studying the mechanical characteristics of rocks is to use outcrop or in-situ formation cores to test the mechanical parameters of rocks through conventional triaxial experiments in the laboratory. Due to the strong heterogeneity of the cores, the repeatability of the experimental results is poor, and carrying out a large number of core experiments with multiple factor variables consumes a lot of manpower and material resources. Secondly, due to equipment limitations, the failure mechanism of rock microscopic particles can only be described qualitatively, and it is impossible to quantitatively reveal the microscopic failure law. The third type is numerical methods based on macroscopic continuum mechanics, represented by finite element, finite difference, phase field method, etc. However, these numerical methods have poor convergence for the mechanical simulation of heterogeneous rock masses, are difficult to reproduce the discontinuous deformation characteristics of rocks, and cannot achieve cross-scale numerical simulation, and cannot finely depict the microscopic mechanism of rock failure. The last type of method uses the discrete element method to characterize discontinuous rock masses. This method conforms to the discontinuous physical properties of rocks, but currently only considers the mesoscopic particles as the basic unit, and the model fails to reflect the microscopic composition of shale minerals, resulting in insufficient understanding of the physical mechanism leading to microscopic failure of shale. Summary of the Invention

[0006] The purpose of the present invention is to provide a digital characterization method for shale mineral microstructures to solve the problem in the prior art that it is impossible to explain and predict the mechanical behavior of shale from the perspective of the microscopic structure of shale minerals.

[0007] To achieve the above purpose, the present invention adopts the following technical solutions:

[0008] The present invention provides a digital characterization method for shale mineral microstructures, including:

[0009] Select the downhole shale core of a predetermined horizon, determine the component ratio of minerals in the shale core through experiments, and obtain the morphological characteristics and microscopic mechanical parameters of the microstructures of each mineral component;

[0010] According to the component ratio and morphological characteristics of each mineral, establish discrete element models of clay minerals and non-clay minerals respectively by using the discrete element modeling method;

[0011] In the discrete element model, a set area containing clay minerals and non-clay minerals is selected, and the clay minerals and non-clay minerals are assembled and a representative lamellar volume unit is constructed according to the mineral component ratio determined by experiments.

[0012] A number of representative lamellar volume units are assembled in a random distribution manner to construct a mesoscopic-scale shale single lamina.

[0013] A number of shale single laminae are assembled in a set shape to construct a macroscopic-scale digital shale core.

[0014] Furthermore, the digital characterization method also includes a method for measuring the mineral component ratio in the shale core: sampling the selected shale core and grinding it into shale powder, analyzing the components of clay minerals and non-clay minerals in the shale core through X-ray diffraction experiments, measuring the porosity of the shale core by gas measurement method, and determining the composition ratio of each mineral component in the shale core and the porosity data of the shale core through calculation.

[0015] Furthermore, the digital characterization method also includes a method for obtaining the morphological characteristics of the microstructures of each mineral component: sampling the selected shale core and cutting it into sections, and identifying the shape, size, arrangement and combination mode, contact mode of the microstructures of each mineral component and the pore morphology of the shale core through scanning electron microscope experiments.

[0016] Furthermore, the digital characterization method also includes a method for obtaining the micro-mechanical parameters of the microstructures of each mineral component: sampling the selected shale core and polishing the surface of the shale sample, measuring the micro-mechanical parameters of the microstructures of each mineral component through nanoindentation experiments, and determining the numerical ranking of the elastic modulus of each mineral component at the microscale.

[0017] Furthermore, the digital characterization method also includes a modeling method for clay minerals: according to the mineral component ratio and morphological characteristics of clay minerals determined by experiments, it is determined that the clay minerals are represented by the stacking of a number of clay lamellar structures. Plate-shaped rigid blocks are selected as the simulated clay lamellae, and a discrete element model of the stacking of a number of simulated clay lamellae is constructed by the discrete element modeling method, and the microparameters between a number of simulated clay lamellae are calibrated by the trial-and-error method.

[0018] Furthermore, the digital characterization method also includes a modeling method for the micro-forces between a number of simulated clay lamellae: introducing the micro-mechanical parameters of clay minerals obtained by experiments into the discrete element model of clay minerals, and obtaining the van der Waals force, double-layer repulsion force and mechanical contact force between a number of simulated clay lamellae through model calculation.

[0019] Furthermore, the digital representation method also includes a modeling method for non-clay minerals: Based on the experimentally determined proportion and morphological characteristics of non-clay mineral components, it is determined that the representation of non-clay minerals is a granular structure. Two-dimensional circular plates are selected as the simulated non-clay particles, and the discrete element modeling method is used to construct a discrete element model with a random distribution of a number of simulated non-clay particles, and the microscopic parameters between a number of simulated non-clay particles are calibrated by the trial-and-error method.

[0020] Furthermore, the digital representation method also includes the modeling of microscopic interaction forces between a number of simulated non-clay particles: The experimentally obtained microscopic mechanical parameters of non-clay minerals are introduced into the discrete element model of non-clay minerals, and the mechanical contact forces between a number of simulated non-clay particles are obtained through model calculation.

[0021] Furthermore, the digital representation method also includes a modeling method for digital shale cores: A number of shale single laminations are assembled according to a set shape, and the model parameters of the digital shale core are calibrated by the trial-and-error method.

[0022] Furthermore, the digital representation method also includes a calibration method for the model parameters of the digital shale core: The microscopic mechanical parameters of each mineral of the shale core selected by experimental determination are introduced into the discrete element model of the digital shale core. The model parameters of the digital shale core are obtained through model calculation. A triaxial compression simulation test is carried out on the digital shale core, and the simulated stress-strain curve of the compression deformation of the digital shale core is calculated. Additionally, the true stress-strain curve of the shale core selected by experimental determination is obtained. The simulated stress-strain curve is compared with the true stress-strain curve, and the model parameters of the digital shale core are adjusted based on the method of making the simulated stress-strain curve close to the true stress-strain curve.

[0023] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects:

[0024] Based on the discrete element modeling method, by first experimentally determining the real shale core sample, the proportion of mineral components, the morphological characteristics of mineral microstructures, and the microscopic mechanical parameters of the shale core are obtained. Then, discrete element models of clay mineral sheets and non-clay mineral particles are respectively established according to the model data of the shale microstructures determined by the experiment. By successively completing the construction of the representative sheet volume unit, the shale single lamination, and the digital shale core, the cross-scale digital representation from the microscopic clay mineral sheet structure, the representative sheet unit volume microstructure, the mesoscopic lamination structure, to the macroscopic shale core is realized. Thus, taking the clay mineral sheet structure as the smallest microscopic scale, the discontinuous characteristics of the rock at different scales are characterized, enabling the method of the present invention to essentially reveal the evolution mechanism of the microscopic structure during the rock deformation and failure process. At the same time, the influence of temperature and pressure changes on the properties of shale can be analyzed. With the improvement of computer computing power, the microscopic mechanical mechanism of shale rock formations at the engineering scale can be analyzed, and thus scientific engineering countermeasures can be proposed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0026] Figure 1 is a schematic flowchart of a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention;

[0027] Figure 2 is a schematic diagram of a digital modeling process of a shale mineral micro-structure provided by an embodiment of the present invention;

[0028] Figure 3 is a schematic diagram of the clay lamella and non-clay particle structures of a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention;

[0029] Figure 4 is a schematic diagram of the horizontal laminar structure of a shale core selected in a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention;

[0030] Figure 5 is a schematic diagram of a digital shale core with a horizontal laminar structure in a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention;

[0031] Figure 6 is a schematic diagram of the inclined laminar structure of a shale core selected in a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention;

[0032] Figure 7 is a schematic diagram of a digital shale core with an inclined laminar structure in a method for digital characterization of a shale mineral micro-structure provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The exemplary embodiments of the present invention will be described in more detail below with reference to the drawings. Although the exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided so that the present invention can be more thoroughly understood and the scope of the present invention can be fully conveyed to those skilled in the art.

[0034] Since the common methods for studying the mechanical characteristics of rocks cannot predict the mechanical behavior of shale from the perspective of the microscopic structure of shale minerals. The present invention provides a digital characterization method for shale mineral microstructures, including experimentally determining the mineral component ratio, the morphological characteristics of the mineral microstructures, and the microscopic mechanical parameters of the shale core, respectively establishing discrete element models of clay mineral sheets and non-clay mineral particles based on the discrete element modeling method, and successively completing the construction of representative sheet volume units, shale single laminas, and digital shale cores. The present invention uses the discrete element modeling method to take the clay mineral sheet structure as the smallest microscopic scale to depict the discontinuous characteristics of rocks across scales, thereby realizing the digital characterization of shale mineral microstructures and their combinations, and further being able to predict the mechanical behavior of shale from the perspective of the microscopic structure of shale minerals.

[0035] The following details the solution of the present invention through examples.

[0036] Embodiment

[0037] As Figures 1 - 3 shown, the present invention provides a digital characterization method for shale mineral microstructures, including:

[0038] Select a downhole shale core at a predetermined horizon, experimentally determine the component ratio of minerals in the shale core, and obtain the morphological characteristics and microscopic mechanical parameters of the microstructures of each mineral component;

[0039] According to the component ratio and morphological characteristics of each mineral, respectively establish discrete element models of clay minerals and non-clay minerals using the discrete element modeling method (refer to Figure 3 , the left is the sheet unit of clay minerals, and the right is the particle unit of non-clay minerals);

[0040] Select a set area containing clay minerals and non-clay minerals in the discrete element model, and assemble and construct a representative sheet volume unit according to the experimentally determined mineral component ratio;

[0041] Assemble a number of representative sheet volume units in a random distribution manner and construct a mesoscopic-scale shale single lamina;

[0042] Assemble a number of shale single laminas in a set shape and construct a macroscopic-scale digital shale core.

[0043] Furthermore, the digital characterization method also includes a method for determining the proportion of mineral components in a shale core: Sampling the selected shale core and grinding it into shale powder, analyzing the components of clay minerals and non-clay minerals in the shale core through X-Ray Diffraction (XRD) experiments, measuring the porosity of the shale core by gas measurement method, and determining the composition ratio of each mineral component in the shale core and the porosity data of the shale core through calculation.

[0044] Furthermore, the digital characterization method also includes a method for obtaining the morphological characteristics of the microstructures of each mineral component: Sampling the selected shale core, cutting it with an ultramicrotome knife to prepare a fresh section, and identifying the shapes, sizes, arrangement and combination methods, contact methods of the microstructures of each mineral component and the pore morphology of the shale core through Scanning Electron Microscope (SEM) experiments. By obtaining the morphological characteristics of the mineral microstructures through this experimental method, the visualization and refinement of the shale mineral microstructure can be realized, so as to facilitate the construction of a three-dimensional digital core.

[0045] Furthermore, the digital characterization method also includes a method for obtaining the micro-mechanical parameters of the microstructures of each mineral component: Sampling the selected shale core and polishing the surface of the shale sample, measuring the micro-mechanical parameters of the microstructures of each mineral component through nanoindentation experiments, and determining the numerical ranking of the elastic modulus of each mineral component at the microscale.

[0046] Furthermore, the digital characterization method also includes a modeling method for clay minerals: According to the proportion of clay mineral components determined by XRD experiments and the morphological characteristics of clay mineral components identified by SEM experiments, it is determined that the clay minerals are characterized by the stacking of several clay sheet structures. Plate-shaped rigid blocks are selected as the simulated clay sheets, and the discrete element method is used to construct a discrete element model of the stacking of several simulated clay sheets, and the microparameters between several simulated clay sheets are calibrated by the trial-and-error method.

[0047] Furthermore, the digital characterization method also includes a modeling method for the micro-forces between several simulated clay sheets: Introducing the experimentally obtained micro-mechanical parameters of clay minerals into the discrete element model of clay minerals, and calculating through the model to obtain the van der Waals force, double-layer repulsive force and mechanical contact force between several simulated clay sheets.

[0048] Furthermore, the digital representation method also includes a modeling method for non-clay minerals: Based on the proportion of non-clay mineral components determined by X-ray diffraction experiments and the morphological characteristics of non-clay mineral components identified by scanning electron microscope experiments, it is determined that the representation of non-clay minerals is a granular structure. Two-dimensional circular plates are selected as the simulated non-clay particles, and the discrete element method is used to construct a discrete element model with a random distribution of a number of simulated non-clay particles, and the microscopic parameters between a number of simulated non-clay particles are calibrated by the trial-and-error method. Among them, only the main components of non-clay minerals are considered, such as quartz and feldspar.

[0049] Furthermore, the digital representation method also includes the modeling of microscopic interaction forces between a number of simulated non-clay particles: The microscopic mechanical parameters of non-clay minerals obtained from experiments are introduced into the discrete element model of non-clay minerals, and the mechanical contact forces between a number of simulated non-clay particles are obtained through model calculations.

[0050] Furthermore, the digital representation method also includes a modeling method for digital shale cores: A number of shale single laminations are assembled according to a set shape, and the model parameters of the digital shale core are calibrated by the trial-and-error method.

[0051] Furthermore, the digital representation method also includes a calibration method for the model parameters of the digital shale core: The microscopic mechanical parameters of each mineral of the shale core selected by experimental determination are introduced into the discrete element model of the digital shale core, and the model parameters of the digital shale core are obtained through model calculations. A triaxial compression simulation test is carried out on the digital shale core and the simulated stress-strain curve of the compression deformation of the digital shale core is calculated. Additionally, the true stress-strain curve of the shale core selected by experimental determination is obtained, and the simulated stress-strain curve is compared with the true stress-strain curve, and the model parameters of the digital shale core are adjusted based on the method of making the simulated stress-strain curve close to the true stress-strain curve.

[0052] Among them, the model parameters of the digital shale core include effective modulus, normal-tangential stiffness ratio, tensile strength, cohesion, internal friction angle, friction coefficient, and cementation parameters, etc.

[0053] To further understand the content of the present invention, the following will give examples:

[0054] Such as Figure 4 and Figure 5 As shown, an implementation mode is a digital model of a shale mineral micro-structure with a horizontal lamination structure. First, obtain the shale micro-structure model data, and the specific steps are as follows:

[0055] S1. Select a shale core with a horizontal bedding structure and determine its mineral components, including clay and non-clay mineral components, through X-ray diffraction experiments. Among them, the proportions of quartz, feldspar, and clay minerals are 30.3%, 10.4%, and 45.7% respectively. Among the clay minerals, the proportions of illite and kaolinite are 43.2% and 30.5% respectively, and only the main mineral components are characterized in digital representation. The porosity of the shale is measured to be 5% by gas measurement method. In the discrete element digital modeling, the porosity between the generated particles is set to 5%.

[0056] S2. Obtain the shapes, sizes, arrangement and combination methods, contact methods of clay minerals and non-clay minerals in the shale, as well as the pore morphology of the shale core through scanning electron microscope experiments. The clay minerals are mainly in sheet structures, and the non-clay minerals are mainly in granular forms; the size range of the clay minerals is 0.2 - 1 μm, and the size range of quartz and feldspar minerals is 2 - 10 μm; the clay minerals show sheet stacking and flocculent cementation, mainly in edge and face contact, while other minerals are mainly in point contact.

[0057] S3. Determine the mechanical properties of the shale minerals. Through nanoindentation experiments, the Young's modulus of the clay minerals is about 6 GPa, the Young's modulus of quartz is about 104 GPa, and the Young's modulus of feldspar is about 62 GPa. It is found that at the microscale, the order of the elastic modulus values is clay minerals < feldspar < quartz.

[0058] After obtaining the above shale microstructural model data, then carry out the digital modeling of the shale microstructural body, and the specific steps are as follows:

[0059] S1. Modeling of clay minerals. The height and width of the construction area of the clay minerals are 10 μm and 5 μm respectively. Generate clay sheets inside the above closed area to represent illite and kaolinite. The clay sheets are selected as plate-shaped rigid body blocks, and the unit density ρ b = 2500 kg / m 3 , and the damping coefficient f b = 0.7. The van der Waals force between the sheets is calculated according to the micro weak force model, and the double-layer repulsive force is calculated according to the mid-plane potential method. Among the mechanical contact forces, the mutual contact of the units is defined as the parallel cementation contact model to realize the cementation between the clays, and the simulation results are compared with the strength parameters measured in the laboratory according to the trial-and-error method to adjust the model strength parameters of the rock sample. Finally, set the following model parameters: effective modulus = 6 GPa, normal-tangential stiffness ratio tensile strength cohesion c fj = 10 MPa, internal friction angle and friction coefficient μ fj = 0.5.

[0060] S2. Non-clay mineral modeling. The sizes of quartz and feldspar are larger than those of clay minerals. Two-dimensional circular plate ordinary particles are used for modeling, and only the mechanical contact forces between particles are considered. The contact model is also set as the parallel bond model, and the effective moduli of quartz and feldspar are 104 GPa and 62 GPa respectively.

[0061] S3. Assembly of representative lamellar volume elements. Microscopic representative lamellar volume elements in a certain range (height 200 μm, width 50 μm) are divided. The proportions of clay minerals, quartz, and feldspar are set as 45.7%, 30.3%, and 10.4% respectively according to the experimental test results. The indentation experiment of the microstructural body of the representative lamellar volume element is simulated, and the parameters in the contact model are adjusted by the trial-and-error method to be close to the results of the indentation experiment.

[0062] S4. Assembly of shale single laminations. The representative lamellar volume elements are assembled into a single lamination with a mesoscopic scale of height 2 mm and width 0.5 mm according to the Weibull random distribution.

[0063] S5. Construction of digital cores of shale with horizontal laminar structures. For the shale core with horizontal laminar structures, the single laminations are stacked horizontally to form a shale experimental sample with a height of 50 mm and a width of 25 mm. A servo confining pressure of 20 MPa is applied to the sample through the walls around the sample model, and the discrete element simulation of the conventional triaxial experiment is carried out. The stress-strain curve of the shale compression deformation is calculated by programming, and this curve is compared with the stress-strain curve measured by the experiment of the real core to study the microscopic mechanism of the shale compression deformation.

[0064] Through the above steps, the digital characterization of the shale mineral microstructural body with a horizontal laminar structure can be realized.

[0065] As Figure 6 and Figure 7 shown, another implementation is a digital model of a shale mineral microstructural body with a inclined laminar structure. First, the model data of the shale microstructural body is obtained, and the specific steps are as follows:

[0066] S1. The downhole shale core of an oilfield has an inclined laminar structure. Its mineral components are determined by X-ray diffraction experiment. The proportions of quartz, feldspar, and clay minerals are 26.3%, 8.4%, and 55.6% respectively. Among the clay minerals, the proportions of illite and kaolinite are 20.2% and 40.5% respectively. In terms of digital characterization, other trace minerals are ignored. The porosity of the shale is measured by the gas measurement method to be 5%. In the discrete element digital modeling, the porosity between particles is also 5%.

[0067] S2. Obtain the shapes, sizes, arrangement and combination modes, contact modes of clay minerals and non-clay minerals in shale, as well as the pore morphology of the shale core through scanning electron microscope experiments. The clay minerals are mainly in sheet structures, and the non-clay minerals are mainly in granular forms; the size range of the clay minerals is 0.2 - 1 μm, and the size range of quartz and feldspar minerals is 2 - 10 μm; the clay minerals show sheet stacking and flocculent cementation, mainly in edge and face contacts, while other minerals are mainly in point contacts.

[0068] S3. Determine the mechanical properties of shale minerals. Through nanoindentation experiments, the Young's modulus of the clay minerals is about 6 GPa, the Young's modulus of quartz is about 104 GPa, and the Young's modulus of feldspar is about 62 GPa. It is found that at the microscale, the order of the elastic modulus values is clay minerals < feldspar < quartz.

[0069] After obtaining the above data of the shale microstructural model, the digital modeling of the shale microstructural body is then carried out. The specific steps are as follows:

[0070] S1. Modeling of clay minerals. The height and width of the construction area of the clay minerals are 10 μm and 5 μm respectively. Clay sheets are generated inside the above closed area to represent illite and kaolinite. The clay sheets are selected as plate-shaped rigid body elements, and the unit density ρ b = 2500 kg / m 3 , and the damping coefficient f b = 0.7. The van der Waals force between the sheets is calculated according to the micro weak force model, and the double-layer repulsive force is calculated by the mid-plane potential method. Among the mechanical contact forces, the mutual contact of the elements is defined as a parallel cementation contact model to realize the cementation between the clays, and the simulation results are compared with the strength parameters measured in the laboratory according to the trial-and-error method to adjust the model strength parameters of the rock sample. Finally, the following model parameters are set: effective modulus = 6 GPa, normal-tangential stiffness ratio tensile strength cohesion c fj = 10 MPa, internal friction angle and friction coefficient μ fj = 0.5.

[0071] S2. Modeling of non-clay minerals. The sizes of quartz and feldspar are larger than those of clay minerals. Two-dimensional circular plate ordinary particles are used for modeling, and only the mechanical contact forces between the particles are considered. The contact model is also set as a parallel cementation model, and the effective moduli of quartz and feldspar are 104 and 62 GPa respectively.

[0072] S3. Assembly of representative lamellar volume units. Microscopic representative lamellar volume units in a certain range (height 200 μm, width 50 μm) are divided, and the proportions of clay minerals, quartz, and feldspar are set to 55.6%, 26.3%, and 8.4% respectively according to the experimental test results. The indentation experiment of the microstructural body of the representative lamellar volume unit is simulated, and the parameters in the contact model are adjusted by the trial-and-error method to be close to the indentation experiment results.

[0073] S4. Assembly of shale single laminae. The representative lamellar volume units are assembled into single laminae with a mesoscopic scale of height 2 mm and width 0.5 mm according to the Weibull random distribution.

[0074] S5. Construction of digital cores of shale with horizontal laminar structures. For the shale core with horizontal laminations, the single laminae are stacked horizontally to form a shale experimental sample with a height of 50 mm and a width of 25 mm. A servo confining pressure of 15 MPa is applied to the sample through the walls around the sample model, and the discrete element simulation of the conventional triaxial experiment is carried out. It is compared with the experiment of a real core with a 30° inclined lamination, and the microscopic mechanism of the compression deformation of shale with an inclined lamination structure is studied.

[0075] Through the above steps, the digital characterization of the shale mineral microstructural body with an inclined lamination structure can be realized.

[0076] A method for digital characterization of a shale mineral microstructural body according to the present invention first conducts experimental measurements on real shale core samples to obtain the mineral component ratio, the morphological characteristics of the mineral microstructural body, and the microscopic mechanical parameters of the shale core. Then, discrete element models of clay mineral laminae and non-clay mineral particles are respectively established based on the experimental measurement data of the shale microstructural body model. By successively completing the construction of representative lamellar volume units, shale single laminae, and digital shale cores, the cross-scale digital characterization from the microscopic clay mineral lamina structure, the representative lamellar unit volume microstructural body, the mesoscopic lamination structure, to the macroscopic shale core is realized. Thus, taking the clay mineral lamina structure as the smallest microscopic scale, the discontinuous characteristics of the rock across scales are characterized, enabling the method of the present invention to essentially reveal the evolution mechanism of the microscopic structure during the rock deformation and failure process. At the same time, the influence of temperature and pressure changes on the properties of shale can be analyzed, and with the improvement of computer computing power, the microscopic mechanical mechanism of shale rock formations at the engineering scale can be analyzed, thereby proposing scientific engineering countermeasures.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. However, such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A digital characterization method for shale mineral microstructures, characterized in that, The digital representation method includes: Selecting downhole shale cores at a predetermined horizon, experimentally determining the component ratios of minerals in the shale cores, and obtaining the morphological characteristics and micro-mechanical parameters of the microstructures of each mineral component; According to the component ratios and morphological characteristics of each mineral, using the discrete element modeling method to establish discrete element models of clay minerals and non-clay minerals respectively; Selecting an aggregate area containing clay minerals and non-clay minerals in the discrete element model, and assembling and constructing a representative lamellar volume unit of clay minerals and non-clay minerals according to the experimentally determined mineral component ratios; Assembling a number of representative lamellar volume units in a random distribution manner and constructing a mesoscopic-scale shale single lamina; Assembling a number of shale single laminae according to a set shape and constructing a macroscopic-scale digital shale core; It also includes a modeling method for clay minerals: According to the experimentally determined component ratios and morphological characteristics of clay minerals, it is determined that the representation of clay minerals is formed by stacking a number of clay lamellar structures. Plate-shaped rigid blocks are selected as simulated clay laminae, and a discrete element model of the stacking of a number of simulated clay laminae is constructed using the discrete element modeling method, and the micro-parameters between a number of simulated clay laminae are calibrated by the trial-and-error method; It also includes a modeling method for non-clay minerals: According to the experimentally determined component ratios and morphological characteristics of non-clay minerals, it is determined that the representation of non-clay minerals is a granular structure. Two-dimensional circular plates are selected as simulated non-clay particles, and a discrete element model of the random distribution of a number of simulated non-clay particles is constructed using the discrete element modeling method, and the micro-parameters between a number of simulated non-clay particles are calibrated by the trial-and-error method; It also includes a modeling method for the digital shale core: Assembling a number of shale single laminae according to a set shape, and calibrating the model parameters of the digital shale core by the trial-and-error method.

2. The digital characterization method of a shale mineral microstructural body according to claim 1, wherein It also includes a method for determining the component ratios of minerals in the shale core: Sampling the selected shale core and grinding it into shale powder, analyzing the components of clay minerals and non-clay minerals in the shale core through X-ray diffraction experiments, measuring the porosity of the shale core by gas measurement method, and calculating to determine the composition ratios of each mineral component in the shale core and the porosity data of the shale core.

3. A method for digital characterization of a shale mineral micro-structure according to claim 1, characterized in that, It also includes a method for obtaining the morphological characteristics of the microstructures of each mineral component: Sampling the selected shale core and cutting it into sections, and identifying the shape, size, arrangement combination mode, contact mode of the microstructures of each mineral component and the pore morphology of the shale core through scanning electron microscope experiments.

4. A digital characterization method for a shale mineral micro-structure according to claim 1, characterized in that, It also includes a method for obtaining the micro-mechanical parameters of the microstructures of each mineral component: Sampling the selected shale core and polishing the surface of the shale sample, measuring the micro-mechanical parameters of the microstructures of each mineral component through nano-indentation experiments, and determining the numerical sorting of the elastic modulus of each mineral component at the micro-scale.

5. A digital characterization method for a shale mineral microstructural body according to claim 1, characterized in that It also includes a modeling method for the micro-interaction forces between a number of simulated clay laminae: Introducing the experimentally obtained micro-mechanical parameters of clay minerals into the discrete element model of clay minerals, and obtaining the van der Waals force, double-layer repulsive force and mechanical contact force between a number of simulated clay laminae through model calculation.

6. The digital characterization method of a shale mineral microstructural body according to claim 1, characterized in that It also includes the modeling of simulating the microscopic forces between non-clay particles: introducing the microscopic mechanical parameters of non-clay minerals obtained from experiments into the discrete element model of non-clay minerals, and obtaining several mechanical contact forces between simulated non-clay particles through model calculation.

7. A method for digital characterization of a shale mineral microstructure according to claim 1, characterized in that, It also includes the calibration method for the parameters of the digital shale core model: introducing the microscopic mechanical parameters of each mineral of the shale core selected by experimental determination into the discrete element model of the digital shale core, obtaining the model parameters of the digital shale core through model calculation, conducting a triaxial compression simulation test on the digital shale core and calculating the simulated stress-strain curve of the compression deformation of the digital shale core, and separately obtaining the true stress-strain curve of the shale core selected by experimental determination, comparing the simulated stress-strain curve with the true stress-strain curve, and adjusting the model parameters of the digital shale core based on the method of making the simulated stress-strain curve close to the true stress-strain curve.

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