Multi-scale mechanical property characterization method of shale

Abstract

The invention belongs to the field of rock mechanics, and particularly discloses a shale multi-scale mechanical property characterization method which comprises the following steps: acquiring mechanical parameters of shale under a macroscale through a triaxial compression experiment; obtaining mechanical parameters of the shale in a mesoscale through a micron indentation experiment; generating a shale surface modulus distribution diagram through a modulus imaging experiment, and combining with an SEM-EDS experiment to determine the composition and elastic modulus of microscopic minerals of the shale; a Mori-Tanaka method is used for carrying out scale upgrading on the micromechanical properties of the shale, the equivalent mechanical parameters of the shale under the macroscopic scale are obtained by integrating the volume fractions, the elastic parameters and the porosity of all phases of microminerals in the shale, and the micromechanical properties and macroscopic response of the microminerals are effectively bridged. According to the method, the anisotropy of the shale in the dimension is disclosed from multiple scales, and the mechanical behaviors and the internal relation of the shale in the macroscopic scale and the microscopic scale are comprehensively disclosed.

Description

Technical Field

[0001] The present application belongs to the field of rock mechanics, and more specifically, relates to a method for characterizing the multi-scale mechanical properties of shale. Background Art

[0002] As an important unconventional oil and gas reservoir, shale's mechanical properties play a crucial role in evaluating reservoir sweet spots, optimizing hydraulic fracturing design, predicting fracture propagation behavior, and assessing reservoir stability. However, the mechanical properties of shale exhibit significant size effects, with the research object and the factors controlling mechanical properties varying at different scales. At the microscale, the research object is a single mineral, whose mechanical properties are governed by the mineral's lattice structure; at the mesoscale, the research object is a porous, multicomponent medium, whose mechanical properties are governed by the mineral composition and microstructure; and at the macroscale, the research object is a centimeter-scale block of rock with natural fractures and bedding planes. Traditional single-scale mechanical testing methods fail to fully capture its complex mechanical response. Shale, as a complex, multiscale rock material, has mechanical properties significantly influenced by factors such as mineral composition, microstructure, and pore distribution. Traditional rock mechanics experimental methods typically focus on mechanical properties at a single scale and fail to characterize the mechanical behavior of rock under multi-scale conditions.

[0003] Currently, research on the mechanical properties of shale is primarily focused on the macroscopic centimeter specimen scale, while relatively little research has been conducted on its micromechanical properties and multiscale mechanical behavior. For example, triaxial compression experiments can characterize the mechanical properties of shale at the macroscale, but cannot reveal its microscopic mechanisms; nanoindentation experiments can characterize the mechanical properties of shale at the microscale, but are difficult to link with macroscopic mechanical properties. At the same time, while existing methods have begun to focus on the micromechanical properties and microstructure of shale, they typically study the two separately, lacking combined analysis and confined to single-scale mechanical property research. There is a lack of systematic research on the relationship between micromechanical and macromechanical properties, and there is limited application of cross-scale research and scale-up methods, making it difficult to achieve quantitative correlation of multiscale mechanical properties.

[0004] Therefore, how to comprehensively characterize the mechanical properties of shale through multi-scale experimental methods and reveal its multi-scale mechanical behavior and its microscopic mechanism remains an important issue in the field of rock mechanics. Summary of the Invention

[0005] In response to the shortcomings of the existing technology, the purpose of this application is to provide a multi-scale mechanical property characterization method for shale, revealing the dimensional anisotropy of shale from multiple scales, and comprehensively revealing the mechanical behavior of shale at the macro and micro scales and their internal connections.

[0006] To achieve the above objectives, in a first aspect, the present application provides a multi-scale mechanical property characterization method for shale, comprising the following steps: S10, obtain the mechanical parameters of shale at the macroscale through triaxial compression test; S20, obtain the mechanical parameters of shale at the mesoscale through micro-indentation experiments; S30, generate the shale surface modulus distribution map through modulus imaging experiment, and combine it with SEM-EDS experiment to determine the microscopic mineral composition of shale and the elastic modulus of minerals; S40 uses the Mori-Tanaka method to upscale the micromechanical properties of shale. By integrating the volume fraction, elastic parameters and porosity of each phase of microscopic minerals in shale, the equivalent mechanical parameters of shale at the macroscale are obtained, effectively bridging the microscopic mineral properties and macroscopic responses.

[0007] The beneficial effects of this application are as follows: This application improves the rock physics characterization method based on triaxial compression experiments, micron indentation, modulus imaging technology and SEM-EDS technology, revealing the dimensional anisotropy of shale from multiple scales; and using the Mori-Tanaka method to integrate mineral volume fraction, elastic parameters and porosity, effectively bridges the microscopic mineral properties and macroscopic responses, and can verify the applicability of this method in multiphase heterogeneous shale systems. Traditional rock physics research may focus more on the basic properties of rocks, while this application reflects the physical and mechanical properties of rocks at different scales through the multi-dimensional characterization of rock physical parameters. This not only enriches the research content of rock physics, but also provides new ideas for the rock physics characterization of complex reservoirs. In addition, the characterization of multi-scale mechanical properties also promotes the cross-integration of rock physics and engineering. The combination of rock physics and engineering practice is an important direction for the future development of the field of rock physics.

[0008] As a further preferred embodiment, step S10 is specifically as follows: Prepare shale cylindrical specimens in accordance with ASTM D4543; Using the MTS rock mechanics electro-hydraulic servo system, axial loads were applied to shale samples under different confining pressures until the samples broke. The stress and strain curves, peak strength, axial and radial strains of the shale specimens were recorded, and macroscopic mechanical parameters, including elastic modulus and Poisson’s ratio, were calculated.

[0009] As a further preference, the confining pressure loading speed of the triaxial compression test is set to 150 N / S, the confining displacement loading speed is controlled at 50 mm / min, the maximum allowable confining displacement is 2 mm, the axial displacement loading speed is set to 5 mm / min, the maximum allowable displacement is 10 mm, and the axial deformation speed is controlled at 0.015 mm / min.

[0010] As a further preferred embodiment, step S20 is specifically as follows: Prepare shale thin slice samples, place the slices in a mixture of epoxy resin and curing agent for curing, and then polish them; Use a micro-indenter and probe to conduct indentation tests on shale samples; The load versus displacement curve was recorded, and the hardness and elastic modulus were calculated from the slope of the unloading phase of the load versus displacement curve.

[0011] As a further preference, the micron indentation experiment uses a Berkovich probe, the test points are arranged in a 4×4 matrix, the point spacing is greater than 300 μm, the maximum load mode is selected to be 400 mN, and the loading rate is 20 mN / s.

[0012] As a further preferred embodiment, in step S30, the step of generating a shale surface modulus distribution map through a modulus imaging experiment is specifically as follows: Prepare shale samples and perform mechanical grinding and argon ion polishing; Using the nanoDMA mode of the Bruker TI950 instrument, a certain area on the sample surface was selected for modulus imaging; The relative positions of the modulus imaging areas are recorded, and a modulus distribution image is generated.

[0013] As a further preferred embodiment, in step S30, the step of determining the microscopic mineral composition and mineral elastic modulus of the shale in combination with the SEM-EDS experiment is specifically as follows: Marking the relative position of the modulus imaging area on the sample surface; The sample surface was imaged using a scanning electron microscope (SEM) to capture the 2D morphology of the shale surface; EDS was used to analyze the elemental composition of the samples and determine the microscopic mineral types and contents of the shale.

[0014] As a further preferred embodiment, the acceleration voltage in the SEM-EDS experiment is 10 kV, the system vacuum pressure is 10 - 5 Torr, the gun vacuum pressure is 10 -8 Torr.

[0015] As a further preferred embodiment, in step S40, the Mori-Tanaka method is specifically as follows: Treat each inclusion as an isolated inclusion embedded in an “equivalent matrix” that contains the average influence of the other inclusions; The Eshelby tensor is used to describe the local strain disturbance caused by inclusions, and the average strain of inclusions and matrix is ​​related through the strain concentration tensor. The overall effective elastic modulus of the composite material is derived based on the volume average method, and the equivalent mechanical parameters of shale at the macroscale are obtained.

[0016] As a further preferred example, the equivalent mechanical parameters of shale at the macroscopic scale include the effective shear modulus K hom , equivalent bulk modulus G hom , equivalent elastic modulus E hom and Poisson's ratio ν hom ;

[0017]

[0018]

[0019]

[0020] Where a represents each phase of minerals; K a represents the shear modulus of each phase of minerals, ; E represents elastic modulus; ν represents Poisson's ratio; G a represents the bulk modulus of each phase of minerals, ; C a Indicates the volume fraction of each phase mineral. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow chart of a multi-scale mechanical property characterization method for shale provided in an embodiment of the present application; Figure 2 is a stress-strain curve diagram of a shale sample provided in a specific embodiment of the present application; Figure 3 This is a schematic diagram of micron indentation provided in a specific embodiment of the present application; Figure 4 is a load-displacement curve diagram of a sample provided in a specific embodiment of the present application; Figure 5 It is a load-displacement curve unloading section curve diagram provided in a specific embodiment of the present application; Figure 6 Schematic diagram of the location of pyrite in the optical microscopic image of TI950 (a) and a schematic diagram of finding the location again in the SEM image (b) provided in a specific embodiment of the present application; Figure 7 These are SEM images and modulus imaging diagrams provided in the specific embodiments of the present application; wherein the red circle represents dolomite, the blue circle represents pyrite, the purple circle represents quartz, and the yellow circle represents clay minerals; Figure 8 This is a SEM-EDS image provided in a specific embodiment of the present application; wherein the blue rectangular area is the modulus imaging experimental area. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application.

[0023] To address the problem that traditional rock mechanics experimental methods are unable to characterize the mechanical properties of shale at multiple scales, this application provides a multi-scale mechanical property characterization method for shale. This method uses triaxial compression tests, micro-indentation tests, modulus imaging, and SEM-EDS technology, combined with the Mori-Tanaka method, to comprehensively characterize the mechanical properties of shale at the macro, meso, and micro scales, revealing its microscopic mechanisms and providing a theoretical basis for shale gas development and reservoir evaluation.

[0024] Figure 1 This is a flow chart of the multi-scale mechanical property characterization method of shale provided in the embodiment of the present application. Figure 1 As shown, the multi-scale mechanical property characterization method of shale provided in this embodiment includes steps S10 to S40, which are described in detail as follows: Step S10: obtaining macroscopic mechanical parameters of shale, including elastic modulus and Poisson's ratio, through triaxial compression test.

[0025] In this embodiment, step S10 can be implemented by the following steps: preparing a cylindrical shale specimen that complies with the ASTM D4543 standard; applying an axial load to the shale specimen under different confining pressures using the MTS rock mechanics electro-hydraulic servo system until the specimen ruptures; recording the stress-strain curve, peak strength, axial and radial strains of the shale specimen, and calculating macroscopic mechanical parameters, including the elastic modulus and Poisson's ratio.

[0026] Step S20: obtaining the mechanical parameters of shale at the mesoscale, including hardness and elastic modulus, through micron indentation experiments.

[0027] In this embodiment, step S20 can be implemented by the following steps: preparing a shale thin slice sample, mixing and curing it with epoxy resin and a curing agent, and polishing it; using a micron indenter and a probe to perform an indentation test on the shale sample; recording a load-displacement curve, and then calculating the hardness and elastic modulus based on the slope of the unloading stage in the load-displacement curve.

[0028] Step S30 : generating a shale surface modulus distribution map through a modulus imaging experiment, and combining it with a SEM-EDS experiment to determine the microscopic mineral composition and mineral elastic modulus of the shale.

[0029] In this embodiment, the steps of generating a modulus distribution map of the shale surface through a modulus imaging experiment in step S30 may specifically include: preparing a shale sample, and performing mechanical grinding and argon ion polishing treatments; using the nanoDMA mode of the Bruker TI950 instrument to select a certain area on the sample surface for modulus imaging; recording the relative position of the modulus imaging area, and generating a modulus distribution image.

[0030] In step S30, the steps of determining the microscopic mineral composition and mineral elastic modulus of the shale in combination with the SEM-EDS experiment can specifically be: marking the relative position of the modulus imaging area on the sample surface; using a scanning SEM electron microscope to image the sample surface and capture the 2D morphology of the shale surface; using EDS to perform elemental composition analysis on the sample to determine the microscopic mineral type and content of the shale.

[0031] In step S40, the micromechanical properties of shale are scaled up using the Mori-Tanaka method. By integrating the volume fraction, elastic parameters, and porosity of each phase of microscopic minerals in the shale, the equivalent mechanical parameters of the shale at the macroscale are obtained, effectively bridging the microscopic mineral properties and the macroscopic response.

[0032] In this example, the Mori-Tanaka method considers each inclusion as an isolated inclusion embedded in an "equivalent matrix," which includes the average influence of other inclusions. The Eshelby tensor is used to describe the local strain perturbation caused by the inclusion, and the average strain of the inclusion and the matrix is ​​related by the strain concentration tensor. The overall effective elastic modulus of the composite material is derived based on the volume averaging method, resulting in the equivalent mechanical parameters of shale at the macroscale.

[0033] The beneficial effects of this embodiment are as follows: Based on triaxial compression testing, microindentation, modulus imaging technology, and SEM-EDS technology, this embodiment improves rock physics characterization methods, revealing the dimensional anisotropy of shale at multiple scales. Furthermore, by integrating mineral volume fraction, elastic parameters, and porosity using the Mori-Tanaka method, it effectively bridges microscopic mineral properties with macroscopic responses, validating the applicability of this method to multiphase heterogeneous shale systems. Traditional rock physics research may focus more on the basic properties of rocks, while this embodiment, through the multidimensional characterization of rock physical parameters, reflects the physical and mechanical properties of rocks at different scales. This not only enriches the research content of rock physics but also provides new ideas for the rock physics characterization of complex reservoirs. Furthermore, the characterization of multiscale mechanical properties promotes the cross-integration of rock physics and engineering. The integration of rock physics and engineering practice is an important direction for the future development of the field of rock physics.

[0034] The present application is described in detail below based on specific embodiments.

[0035] This embodiment provides a method for characterizing the multi-scale mechanical properties of shale, which specifically includes the following steps: 1. Experimental sample preparation: Taking the experimental samples of this example as an example: the samples were derived from the Wufeng-Longmaxi Formation shale in the Fuling area of ​​the Sichuan Basin. In its natural state, the samples were black, uniform and dense, with no obvious pores or cracks on the surface. A small piece of the sample was taken and subjected to XRD diffraction experiments to determine the rock mineral composition.

[0036] (1) Sample preparation for triaxial compression test: The sample preparation followed the procedure of ASTM D4543. The shale rock block was made into a cylindrical specimen with a height of 50 mm and a diameter of 25 mm. Both ends of the core plug sample were carefully polished and the upper and lower ends were parallel.

[0037] (2) Micron indentation test sample preparation: Take a small piece of shale sample. The orientation of each thin slice of rock sample is the same as the orientation of the corresponding core plug sample. That is, if a specific core plug is drilled along the sample axis perpendicular to the bedding plane, the corresponding flaky rock sample is placed along the plane of the cylinder and the plane of the bedding layer. Epoxy resin is mixed with a curing agent and allowed to stand for 24 hours to cure. Then, silicon carbide paper is used to polish the cured surface parallel to the cured surface. Finally, 0.25μm and 0.05μm alumina suspensions are used to polish the surface to ensure smoothness.

[0038] (3) Sample preparation for modulus imaging experiment: A small piece of the experimental sample was cut out, mechanically polished, and then the sample surface was polished with argon ions.

[0039] 2. Specific implementation steps: Step 1: Triaxial compression test The triaxial compression test was carried out using the proprietary MTS rock mechanics electro-hydraulic servo system. This equipment can simultaneously measure the mechanical parameters, acoustic parameters and permeability properties of the sample under temperature and pressure changes. The system can provide a maximum axial pressure of 2000kN, a maximum confining pressure of 140MPa, a maximum pore pressure of 140MPa, and a maximum temperature of 200°C. Axial loads were applied to the shale samples at confining pressures of 0~80MPa until the rock broke. The experimental confining pressure loading speed was set to 150N / S, the confining displacement loading speed was controlled at 50mm / min, and the maximum allowable confining displacement was controlled at 2mm; the axial displacement loading speed was set to 5mm / min, the maximum allowable displacement was 10mm, and the axial deformation speed was controlled at 0.015mm / min. The stress-strain curves of the shale samples were recorded (see Appendix). Figure 2 ), peak strength, axial and radial strains, and calculate the elastic modulus (i.e. Young's modulus) by the formula E ) and Poisson's ratio and other macroscopic mechanical parameters.

[0040] Young's modulus E It can be calculated by formula 1 or by the slope of the stress-strain curve, that is, the static Young's modulus and Poisson's ratio It can be calculated by formula 2: (1) (2) Where σ is the normal stress; is positive strain; is the radial strain; is the axial strain.

[0041] For example: According to the attached Figure 2 In the stress-strain curve, the positive value of the X-axis represents the axial deformation, the negative value of the X-axis represents the radial deformation, and the vertical axis represents the differential stress. From the axial deformation curve, the Young's modulus is 38.08 MPa, and from the ratio of radial deformation to axial deformation, the Poisson's ratio is 0.13.

[0042] Step 2: Micron indentation test Microindentation testing was performed using a Nano Indenter (G200 / XP) using a Berkovich probe. To avoid errors caused by contact zero, a standard sample was first tested, and the instrument automatically determined the contact zero point. Based on the different colors of the shale mineral components, the position of each phase of minerals was identified using the optical microscope of the microindenter. A representative area with high flatness was selected, and a matrix of 4*4 test points with a point spacing of 300μm was tested (see attached). Figure 3 ), select the maximum load mode P max=400mN, loading rate 20mN / s, hold time 50s, unload rate 20mN / s. At the same time, to avoid mutual influence between different test positions, the distance between adjacent test points should be greater than 300μm. The load-displacement curve generated during the indenter pressing into the sample surface is recorded (see Appendix). Figure 4 ), the hardness and elastic modulus can be calculated by the obtained load and indentation depth data, and the elastic contact stiffness and contact depth can be obtained from the unloading curve using the Oliver-Pharr method. The Young's modulus and hardness can be obtained according to the load-displacement curve (see Appendix Figure 4 ) is calculated as follows: Formula 3 to Formula 5.

[0043] (3) (4) (5) (6) Where dP / dh represents the slope of the unloading curve segment of the load-displacement curve; h max represents the maximum displacement, h c Indicates the depth of the indenter below the sample surface; E r is the reduced modulus; S is the slope of the upper part of the unloading curve (e.g. Figure 4 ), also known as indentation stiffness; A c is the contact area, which can be calculated using Equation 6; H is the indentation hardness; β is a constant related to the indenter geometry (for the Berkovich indenter, β = 1.034); and P is the peak load. Considering the mechanical properties of the experimental equipment probe, the reduced modulus can be converted to Young's modulus using the following formula: (7) Where V s and V tip are the Poisson’s ratios of the sample and probe, respectively. s Assuming 0.3, E and E tip are the Young's modulus of the sample and the probe, respectively. The indenter instrument is equipped with a standard diamond probe. The standard diamond Young's modulus E is used in this study. tip =1140GPa and Poisson's ratio V tip =0.07 for calculation.

[0044] For example: Figure 5The slope of the upper part of the unloading section of the medium load-displacement curve is 0.89, that is, S=0.89, hc=4118.72nm, P=468.90mN; based on the data in the curve and the slope of the unloading section curve, the elastic modulus of the indentation point can be calculated to be 39.18GPa and the hardness is 1.38GPa.

[0045] Step 3: Modulus imaging and SEM-EDS experimental plan Modulus imaging is another mode of nanomechanical testing that can generate high-resolution modulus distribution data for a certain area of ​​the shale sample surface, revealing the changes in the modulus of a certain area of ​​various materials in the plane. The electric field force drives the indenter to scan the sample surface, and as the force acts, periodic deformation and oscillation will occur. This embodiment uses the instrument model Bruker TI950, and selects the nanoDMA mode of the instrument to obtain dynamic mechanical data, thereby generating a modulus distribution image. Each scan can generate multiple images, including morphology, hardness, storage modulus (E s ), loss modulus (E L ) and loss factor plots.

[0046] In this example, a 20*20 μm area was selected on the sample surface using the nanoDMA mode of the Bruker TI950 for modulus imaging, and the relative position of the modulus imaging area was recorded. The relative position of the modulus imaging area was recorded based on the manually calibrated cross mark, the position of the blue cross in the scanned area under the optical microscope, and the imaging characteristics of pyrite under the optical microscope. Figure 6 As shown in part (a), pyrite appears bright under an optical microscope. Based on the area marked by the modulus imaging, the same artificial cross mark and the position of the pyrite are used. In order to find the same position for subsequent SEM-EDS experiments, energy spectrum scattering imaging is performed to confirm the mineral composition of the modulus imaging area. Figure 6 As shown in part (b).

[0047] SEM imaging was used to qualitatively capture the 2D morphology of the shale surface, and EDS analysis was used to quantify the distribution of elemental composition. Before SEM imaging, the sample surface was connected to the tray with conductive tape. In order to obtain appropriate EDS resolution and related dimensions, the acceleration voltage needs to be adjusted. Using a lower acceleration voltage can limit the interaction volume and improve the resolution of the surface signal. Using a higher acceleration voltage can detect deeper elemental information. The acceleration voltage used in this example is 10kV. The system vacuum pressure is 10 -5 Torr, the gun vacuum pressure is 10 -8 The mineral type and content of the sample at the microscopic scale can be clearly determined by the analysis of SEM electron microscope-EDS energy map.

[0048] The storage stiffness S of the shale sample is calculated based on the modulus imaging experiment: (8) Where F0 is the electric field force of the Bruker TI950 device; h is the displacement of the Berkovich indenter tip; ω is the phase difference between F0 and h; K s is the system stiffness; m tip is the mass of the indenter tip; φ is the oscillation frequency of the dynamic force. Similar to nanoindentation, the storage modulus E s It can be derived from the following formula, where A represents the contact area between the indenter and the sample: (9) For modulus mapping, the displacement of the indenter tip is very small, so the spherical indenter tip assumption is applied and the contact area is considered as a circle with radius a: (10) Where R = the radius of the indenter tip; F = the actual contact force between the indenter tip and the specimen. Finally, the storage modulus can be calculated by combining the formula: (11) Calculate the mineral Young's modulus E, the formula is: (12) E s is the storage modulus, E is the Young's modulus, γ represents the Poisson's ratio of each phase of minerals, E tip =1140GPa and Poisson's ratio γ tip =0.07.

[0049] According to the above formula and principle, the modulus map can be directly obtained, and the elastic modulus of a single mineral can be obtained according to the modulus map image and SEM-EDS image (see Appendix). Figure 7 , attached Figure 8 ).

[0050] Step 4: Scale Up The Mori-Tanaka method is combined with the Mori-Tanaka method to upscale the micromechanical properties of shale. This method treats each inclusion as an isolated inclusion embedded in an "equivalent matrix," which includes the average influence of other inclusions. The Eshelby tensor is used to describe the local strain perturbation caused by the inclusion, and the strain concentration tensor is used to relate the average strain of the inclusion and the matrix. Finally, the overall effective elastic modulus of the composite material is derived based on the volume average method. This method reveals its multi-scale mechanical behavior, as shown in the following formula: (13) (14) In the formula, a=0, 1, 2, 3 represent the minerals of each phase, etc.K a represents the shear modulus of each phase of minerals, G a It represents the bulk modulus of each phase of minerals. The value of Young's modulus (E) comes from the modulus imaging data. The value of Poisson's ratio (ν) can be obtained from the literature. The equivalent shear modulus K hom , equivalent bulk modulus G hom , equivalent elastic modulus E hom , Poisson's ratio ν hom Calculated by formulas 15 to 18, C a is the volume fraction of each phase mineral.

[0051] (15) (16) (17) (18) For example: in a shale sample, the volume fractions of dolomite, quartz, pyrite, feldspar, calcite, clay minerals and TOC are 13%, 31%, 1.7%, 3.3%, 2.1%, 38 and 6% respectively; the elastic moduli are 93.64 GPa, 84.2 GPa, 102.87 GPa, 68.57 GPa, 83.76 GPa, 50.18 GPa and 15 GPa respectively; and the Poisson's ratios are 0.3, 0.08, 0.13, 0.32, 0.31, 0.37 and 0.22 respectively.

[0052] According to formulas (13)-(18), the equivalent elastic modulus of the shale sample is 36.30 GPa, and the equivalent Poisson's ratio is 0.25.

[0053] The beneficial effects of this embodiment are: (1) This embodiment combines the mechanical property characterization methods of macroscale (triaxial compression test), mesoscale (micrometer indentation test) and microscale (modulus imaging technology) for the first time, and combines SEM-EDS technology to reveal the microscopic mineral composition and pore structure of shale, and systematically studies the multi-scale mechanical properties of shale; (2) Traditional rock mechanics research often only focuses on the mechanical properties of a single scale, while this embodiment can comprehensively reveal the mechanical behavior of shale at the macroscale and microscale and their internal connections through multi-scale experimental methods.

[0054] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A multi-scale mechanical property characterization method for shale, characterized in that: The steps include: S10, obtain the mechanical parameters of shale at the macroscale through triaxial compression test; S20, obtain the mechanical parameters of shale at the mesoscale through micro-indentation experiments; S30, generate the shale surface modulus distribution map through modulus imaging experiment, and combine it with SEM-EDS experiment to determine the microscopic mineral composition of shale and the elastic modulus of minerals; S40 uses the Mori-Tanaka method to upscale the micromechanical properties of shale. By integrating the volume fraction, elastic parameters and porosity of each phase of microscopic minerals in shale, the equivalent mechanical parameters of shale at the macroscale are obtained, effectively bridging the microscopic mineral properties and macroscopic responses.

2. The multi-scale mechanical property characterization method of shale according to claim 1, characterized in that: Step S10 is specifically as follows: Prepare shale cylindrical specimens in accordance with ASTM D4543; Using the MTS rock mechanics electro-hydraulic servo system, axial loads were applied to shale samples under different confining pressures until the samples broke. The stress and strain curves, peak strength, axial and radial strains of the shale specimens were recorded, and macroscopic mechanical parameters, including elastic modulus and Poisson’s ratio, were calculated.

3. The multi-scale mechanical property characterization method of shale according to claim 1 or 2, characterized in that: The confining pressure loading speed of the triaxial compression test was set to 150 N / S, the confining displacement loading speed was controlled at 50 mm / min, the maximum allowable confining displacement was 2 mm, the axial displacement loading speed was set to 5 mm / min, the maximum allowable displacement was 10 mm, and the axial deformation speed was controlled at 0.015 mm / min.

4. The multi-scale mechanical property characterization method of shale according to claim 1, characterized in that: Step S20 is specifically as follows: Prepare shale thin slice samples, place the slices in a mixture of epoxy resin and curing agent for curing, and then polish them; Use a micro-indenter and probe to conduct indentation tests on shale samples; The load versus displacement curves were recorded, and the hardness and elastic modulus were calculated from the slope of the unloading phase of the load versus displacement curves.

5. The multi-scale mechanical property characterization method of shale according to claim 1 or 4, characterized in that: The micron indentation experiment uses a Berkovich probe, the test point layout is a 4×4 matrix, the point spacing is greater than 300 μm, the maximum load mode is 400 mN, and the loading rate is 20 mN / s.

6. The multi-scale mechanical property characterization method of shale according to claim 1, characterized in that: In step S30, the step of generating a shale surface modulus distribution map through a modulus imaging experiment is specifically as follows: Prepare shale samples and perform mechanical grinding and argon ion polishing; Using the nanoDMA mode of the Bruker TI950 instrument, a certain area on the sample surface was selected for modulus imaging; The relative positions of the modulus imaging areas are recorded, and a modulus distribution image is generated.

7. The multi-scale mechanical property characterization method of shale according to claim 1 or 6, characterized in that: In step S30, the steps of determining the microscopic mineral composition and elastic modulus of the shale in combination with the SEM-EDS experiment are specifically as follows: Marking the relative position of the modulus imaging area on the sample surface; The sample surface was imaged using a scanning electron microscope (SEM) to capture the 2D morphology of the shale surface; EDS was used to analyze the elemental composition of the samples and determine the microscopic mineral types and contents of the shale.

8. The multi-scale mechanical property characterization method of shale according to claim 1, characterized in that: The acceleration voltage in the SEM-EDS experiment was 10 kV, and the system vacuum pressure was 10 -5 Torr, the gun vacuum pressure is 10 -8 Torr.

9. The multi-scale mechanical property characterization method of shale according to claim 1, characterized in that: In step S40, the Mori-Tanaka method is specifically as follows: Treat each inclusion as an isolated inclusion embedded in an "equivalent matrix" that contains the average influence of other inclusions; The Eshelby tensor is used to describe the local strain disturbance caused by inclusions, and the average strain of inclusions and matrix is ​​related through the strain concentration tensor. The overall effective elastic modulus of the composite material is derived based on the volume average method, and the equivalent mechanical parameters of shale at the macroscale are obtained.

10. The multi-scale mechanical property characterization method of shale according to claim 1 or 9, characterized in that: The equivalent mechanical parameters of shale at the macroscale include the effective shear modulus K hom , equivalent bulk modulus G hom , equivalent elastic modulus E hom and Poisson's ratio ν hom ; Where a represents each phase of minerals; K a represents the shear modulus of each phase of minerals, ; E represents elastic modulus; ν represents Poisson's ratio; G a represents the bulk modulus of each phase of minerals, ; C a Indicates the volume fraction of each phase mineral.

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