Method and system for analyzing micro-mechanical properties of hard and brittle shale based on nanoindentation

Abstract

The present application belongs to the technical field of shale mechanics analysis, and particularly relates to a hard and brittle shale micro-mechanics performance analysis method and system based on nano-indentation. The analysis method provided by the present application is performed by combining micro-multiple scale experimental characterization and atomic scale molecular dynamics simulation. Experimental data is obtained through nano-indentation array experiments and multi-source micro-characterization, a heterogeneous molecular dynamics model is constructed, room temperature nano-indentation simulation is performed, gradient temperature is set for high temperature nano-indentation simulation and temperature effect is analyzed, and a corresponding prediction model is constructed, thereby providing micro-basis and a theoretical model for mechanical performance evaluation and prediction of deep shale under a thermal coupling environment.

Description

Technical Field

[0001] This invention belongs to the field of shale mechanical analysis technology, specifically relating to a method and system for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation. Background Technology

[0002] Shale, as a typical brittle, highly heterogeneous, and structurally complex sedimentary rock, has macroscopic mechanical properties, especially its elastic modulus, that directly affect the reservoir stimulation effect and extraction safety of shale oil and gas resources. However, traditional macroscopic mechanical experiments face significant challenges in obtaining shale mechanical parameters: core samples are prone to coupled damage from multiple mechanisms, such as microcracks, mineral deformation, and pore compression, during core extraction and preparation, making it difficult to extract a pure elastic mechanical response, thus leading to a biased understanding of its true mechanical state.

[0003] To overcome the limitations of macroscopic-scale research, nanoindentation technology has become an important tool for characterizing heterogeneous materials such as shale at the micro- and nano-scale. This is because it can directly obtain high-resolution statistical data on mechanical properties in the micrometer region and effectively correlate microscopic components with macroscopic mechanical behavior. However, experimental techniques still struggle to directly observe and quantitatively analyze the deep mechanisms of anisotropy, microscopic deformation of mineral phases, and atomic-scale displacement within materials. Molecular dynamics simulations, on the other hand, exhibit unique advantages. By constructing models at the atomic scale and dynamically simulating them, combined with modeling software and Lammps calculations, they can reveal microscopic deformation mechanisms and thermo-mechanical coupling behaviors that are difficult to capture experimentally.

[0004] Although nanoindentation technology has been used to obtain parameters such as the elastic modulus of shale, further research is needed on how to more accurately obtain the true mechanical parameters of hard and brittle shale under micro-loads and clearly reveal the micro-behavior and mechanism of its mineral phases during deformation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method and system for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation.

[0006] The technical solution of the present invention is as follows: This invention provides a method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, comprising: S1: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, we obtained the microscopic mechanical parameters of shale and established the spatial correspondence between mechanical parameters and mineral components. S2: Construct a molecular dynamics model of shale heterogeneity based on the mineral composition results of S1; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Calculate the elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship; S3: Based on the S2 nanoindentation simulation results, multiple gradient temperatures were set, the load-displacement relationship at different temperatures was recorded, the hardness and elastic modulus corresponding to each temperature were extracted, the ratio of hardness to elastic modulus was calculated, and the response consistency of shale under the action of temperature field was evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

[0007] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale using nanoindentation, the molecular dynamics simulation parameters described in S2 are set as follows: A potential function is constructed using the LJ potential, Coulomb potential, and embedding energy; Periodic boundary conditions are applied in the X, Y, and Z directions, and the size in the Z direction is expanded, dividing the system into a boundary layer, an isothermal layer, and a Newtonian layer along the Z direction.

[0008] Furthermore, using the LJ potential, Coulomb potential, and embedding energy, a potential function is constructed, which is achieved through the following formula: ; ; In the formula, U Represents the potential function. Indicates embedding capability, It is the number of all other particles in the system besides atoms in the first... i The linear superposition of electron cloud densities at each atom; This indicates the potential term, including the LJ potential and the Coulomb potential; It is the distance between two atoms; It is the first j The atom in the first i The charge density function at each atom.

[0009] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale using nanoindentation, S3 describes the construction of an elastic modulus prediction model using the temperature decay coefficient and elastic modulus, which is derived from the formula: To achieve; In the formula, Indicates temperature The elastic modulus below; Indicates the reduced modulus; This indicates the elastic modulus of the indenter; This indicates the Poisson's ratio of the shale being tested; This represents Poisson's ratio of the pressure head; Indicates the exponentially decaying term; Indicates reference temperature; This represents the temperature decay coefficient.

[0010] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale using nanoindentation, S3 describes the construction of a hardness prediction model using temperature sensitivity coefficients and hardness, as shown in the formula: To achieve; In the formula, Indicates temperature The hardness below; This indicates the load applied by the pressure head; Indicates the contact area; Indicates reference temperature; , This represents the temperature sensitivity coefficient.

[0011] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale using nanoindentation, S2, which involves statistically analyzing the Z-axis atomic number distribution and calculating the damage layer thickness, specifically: Keyframes were extracted based on nanoindentation simulation results. The number of shale matrix atoms at different Z-axis coordinates in each frame was counted to generate a Z-axis atom number distribution curve. Density is characterized by the number of atoms. By comparing the curves of the initial frame and the frame with the maximum compression depth, the boundary of the damage area with significant changes in atomic density on the Z-axis is determined. Calculate the Z-axis coordinate difference of the damaged area under the maximum compression depth frame, and obtain the overall damaged layer thickness after conversion; compare the curves of the unloaded frame and the initial frame, calculate the coordinate difference of the unrecovered area, and obtain the plastic damaged layer thickness.

[0012] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, the calculation of elastic recovery rate, plastic work, elastic work, and total deformation work according to the load-displacement relationship, as described in S2, is achieved through the following formula: ; ; ; ; In the formula, For elastic recovery rate, For maximum indentation depth, This refers to the residual push-in depth after unloading. For plastic work, This represents the load when the indenter is pressed into the ground at any depth. For elastic work, For the fitting parameters, This is the total deformation work.

[0013] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale using nanoindentation, the evaluation of the consistency of shale response under temperature field conditions, as described in S3, specifically includes: In nanoindentation testing, the ratio of hardness to elastic modulus at the target temperature is calculated and used as the first ratio. In nanoindentation simulation, the ratio of hardness to elastic modulus at the target temperature is calculated as a second ratio. The relative error at the target temperature is calculated based on the first ratio and the second ratio. Adjust the molecular dynamics simulation parameters to control the relative error to be less than a preset threshold.

[0014] Based on the above-described method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, S3 also includes: The elastic recovery rate, plastic work, elastic work, and total deformation work at different temperatures were calculated to analyze the intrinsic mechanism of elastoplastic deformation of shale at high temperatures.

[0015] This invention also provides a system for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, comprising: Test data acquisition module: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, the microscopic mechanical parameters of shale are obtained and the spatial correspondence between mechanical parameters and mineral components is established; Simulation module: Constructs a molecular dynamics model of shale heterogeneity based on the mineral composition results obtained from the test data acquisition module; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Calculate the elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship; High-temperature simulation module: Based on the nanoindentation simulation results of the simulation module, multiple gradient temperatures are set, the load-displacement relationship at different temperatures is recorded, the hardness and elastic modulus corresponding to each temperature are extracted, the ratio of hardness to elastic modulus is calculated, and the response consistency of shale under the action of temperature field is evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

[0016] Beneficial effects The analytical method provided by this invention integrates microscopic multi-scale experimental characterization with atomic-scale molecular dynamics simulation. It obtains experimental data through nanoindentation lattice experiments and multi-source microscopic characterization, constructs a heterogeneous molecular dynamics model and performs room temperature nanoindentation simulation, sets gradient temperatures to perform high temperature nanoindentation simulation and analyzes temperature effects, and constructs a corresponding prediction model. This provides a microscopic basis and theoretical model for the evaluation and prediction of the mechanical properties of deep shale under thermo-mechanical coupling environment. Attached Figure Description

[0017] Figure 1 The image shows the nanoindentation load-displacement curve for Example 1.

[0018] Figure 2 The elastic modulus and hardness of the 25 indentation points in Example 1 are shown.

[0019] Figure 3 The measured mineral composition of shale is shown in Example 1.

[0020] Figure 4 The indentation morphology features of the shale sample in Example 1 are shown.

[0021] Figure 5 The percentages of each element in the shale sample of Example 1 are shown.

[0022] Figure 6 This is a schematic diagram of shale matrix modeling in Example 1, where (a) is a three-dimensional diagram and (b) is a two-dimensional diagram.

[0023] Figure 7 Example 1 shows a nanoindentation model, where (a) is a three-dimensional view and (b) is a side projection.

[0024] Figure 8 The load-displacement curves are from the nanoindentation simulation at room temperature in Example 1.

[0025] Figure 9 The shear stress distribution during the nanoindentation process at room temperature in Example 1 is shown in (a) as 0 frames, (b) as 150 frames, (c) as 300 frames, (d) as 400 frames, (e) as 500 frames, and (f) as 600 frames.

[0026] Figure 10 This is a schematic diagram of twinning in the shale model at room temperature in Example 1, where (a) is an unenlarged view and (b) is a partially enlarged view.

[0027] Figure 11The Z-axis atomic number distribution curve is shown for the entire process of nanoindentation in Example 1.

[0028] Figure 12 The load-displacement curves are simulated by nanoindentation at different temperatures in Example 1.

[0029] Figure 13 The elastic modulus and hardness of shale at different temperatures are shown in Example 1.

[0030] Figure 14 The elastic recovery rate is shown in Example 1 at different temperatures.

[0031] Figure 15 This is the relationship between indentation deformation work and temperature for different types of examples in Example 1. Detailed Implementation

[0032] The following examples are intended to illustrate the present invention, and not to further limit the invention.

[0033] Example 1 This embodiment provides a method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, including: S1: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, we obtained the microscopic mechanical parameters of shale and established the spatial correspondence between the mechanical parameters and mineral components.

[0034] In the specific implementation process, the operation is as follows.

[0035] S11: A Nano Indenter G200 nanoindentation testing system was used, equipped with a diamond Bose triangular pyramid indenter. The experimental sample was shale from the Daqing area, and the testing process employed a quasi-static loading mode. Load-displacement curves were automatically recorded throughout the process, such as... Figure 1 As shown.

[0036] S12: Combination Figure 1 Then, based on the Oliver-Pharr method, the hardness and elastic modulus were calculated from the load-displacement curves of 25 indentation points. To reduce errors, the highest and lowest points obtained due to experimental errors were discarded. The graphs generally show a normal distribution. The average elastic modulus of the shale was 90.42 GPa, and the average hardness was 4.75 GPa. The data distribution is relatively concentrated, reflecting the accuracy of the measured mechanical parameters of the hard and brittle shale. Figure 2 As shown.

[0037] S13: For shale samples with lattice indentations, each indentation point was analyzed in depth using microscopic combined measurement methods such as X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM).

[0038] In this embodiment, a Bruker D8AA25 X-ray vcc diffractometer was used to obtain the test spectrum of the rock core. Analysis of the spectral peaks yielded the following shale and clay mineral composition: Figure 3 As shown, the core sample contained 43.9% quartz and 23.3% clay minerals, the main component of which was illite, with a 2:1 layered silicate structure.

[0039] like Figure 4 The image shows the morphological features of SEM indentations. Taking the marked points as an example, the morphology of the indentations is illustrated. Figure 4 After performing EDS testing on the marked points, the percentage results of each element are as follows: Figure 5 As shown.

[0040] The above multi-source microscopic characterization confirms that the high modulus region is associated with the enrichment of brittle minerals such as quartz.

[0041] S2: Construct a molecular dynamics model of shale heterogeneity based on the mineral composition results of S1; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Based on the load-displacement relationship, calculate the elastic recovery rate, plastic work, elastic work, and total deformation work.

[0042] In the specific implementation process, the operation is as follows.

[0043] S21: Based on the XRD and EDS analysis results of the shale samples, the volume fractions of the model components were precisely set: quartz accounted for 50%, providing rigid support as a load-bearing framework; illite accounted for 30%, characterizing the plastic deformation and anisotropy of clay minerals; and kerogen accounted for 20%, serving as a continuous matrix to encapsulate and cement inorganic minerals. The crystal structures of quartz and illite were derived from mineral crystallography databases. Quartz and illite nanocrystals obtained by cutting from these databases were randomly embedded in proportion to construct an initial heterogeneous composite model. For example... Figure 6 The image shows the final basic unit cell model obtained.

[0044] To reveal the mechanical response and failure mechanism of shale under complex stress conditions at the atomic scale through nanoindentation simulation, this invention employs molecular dynamics simulation methods and combines materials computation and geophysics for modeling.

[0045] S22: First, the potential function describes the interactions between atoms and molecules; therefore, in nanoindentation simulations, the selection of the potential function plays a decisive role in the simulation results. To ensure the accuracy and reliability of the results, preferably, the molecular dynamics simulation parameters are set as follows: A potential function is constructed using the LJ potential, Coulomb potential, and embedding energy; Periodic boundary conditions are applied in the X, Y, and Z directions, and the size in the Z direction is expanded, dividing the system into a boundary layer, an isothermal layer, and a Newtonian layer along the Z direction.

[0046] Furthermore, using the LJ potential, Coulomb potential, and embedding energy, a potential function is constructed, which is achieved through the following formula: ; ; In the formula, U Represents the potential function. Indicates embedding capability, It is the number of all other particles in the system besides atoms in the first... i The linear superposition of electron cloud densities at each atom; This indicates the potential term, including the LJ potential and the Coulomb potential; It is the distance between two atoms; It is the first j The atom in the first i The charge density function at each atom.

[0047] Periodic boundary conditions are applied in the X, Y, and Z directions to ensure the infinite scalability of the simulated system and eliminate boundary effects. To ensure the physical rationality and thermodynamic stability of the model structure, an energy optimization and equilibrium process is executed, which is as follows: (1) Geometric structure optimization: The conjugate gradient method is used to minimize the energy of the system and eliminate unreasonable atomic overlap and local stress. The convergence criterion is that the energy change is less than 1×10. -10 eV / atom.

[0048] (2) Cyclic annealing: Five cyclic annealing processes are carried out in the range of 300K to 600K, with a relaxation of 50ps in each stage, to help the system overcome local potential barriers, promote atomic rearrangement, and obtain a more stable global low-energy configuration.

[0049] (3) Isothermal and isobaric relaxation: Long-term MD relaxation (298 K, 0.1 MPa) was performed under the NPT ensemble. The organic-inorganic interaction was described using the COMPASS force field with a time step of 1 fs, a cutoff radius of 12.5 Å, and a relaxation time of 300 ps. The evolution of the total energy and density of the system was monitored in real time during this process. After relaxation, the system energy converged to a stable plateau, and the calculated density reached 2.50 g / cm³.3 The actual density of hard and brittle shale is 2.4-2.7 g / cm³. 3 The reliability of the experiment was verified.

[0050] Building upon this, the present invention further expands the Z-axis dimension of the model to accommodate the indenter, and divides the system along the Z-axis into a boundary layer (with fixed atoms at the bottom 1 nm), a isothermal layer (with adjacent 1 nm atoms temperature-controlled using a velocity calibration method), and a Newtonian layer (with the remaining atoms following Newton's laws of motion). The indenter is made of diamond and is treated as a rigid body in the simulation.

[0051] The simulation process is carried out in a canonical system NVE. The initial state of the model established by the nanoindentation simulation is as follows: Figure 7 As shown.

[0052] Combination such as Figure 8 The load-displacement curves shown reveal the entire deformation process of the matrix as the indentation depth increases. First, elastic deformation occurs, with point A representing the elastoplastic critical point. At this point, the indentation force decreases significantly due to the release of deformation energy from the initial dislocation nucleation. Subsequently, dislocation motion causes the matrix to enter the plastic stage. The unloading curve does not coincide with the loading curve, reflecting permanent plastic deformation. The load initially decreases rapidly and then gradually slows down; the adhesion of amorphous atoms to the indenter also results in a negative load value. The simplification of the potential function amplifies the deviation in the mechanical response. The final simulation yielded a shale hardness of 4.63 GPa and an elastic modulus of 86.42 GPa, which highly agrees with the S1 test values, verifying the reliability of the simulation.

[0053] S23: Shear strain distribution of the material at different indentation depths throughout the nanoindentation process, as shown in... Figure 9 As shown, the upper and lower parts of each frame correspond to the truncated side view and top view of the shale matrix, respectively.

[0054] To further investigate the nucleation and movement of dislocations during the plastic stage, this invention colors atoms according to the magnitude of shear strain: red atoms represent high-strain regions, and yellow atoms represent low-strain regions. Despite the symmetrical Boehringer indenter, the shear strain of the matrix still exhibits asymmetry, indicating anisotropy within the shale.

[0055] Defective atoms within the matrix can form dislocation loops, which undergo four stages as the probe displacement increases: dislocation loop initiation stage, dislocation loop growth stage, dislocation loop proliferation stage, and dislocation loop maintenance stage.

[0056] When the indentation depth reaches 3.2 Å, dislocation nuclei first germinate within the matrix; shear strain nucleates in the contact region between the indenter and the matrix and propagates inward into the matrix along a 45° direction. Due to the strain gradient effect in the nanoindentation, the shear strain decreases unevenly from the center to the edge of the contact region between the matrix and the indenter. As the indentation depth increases to 13 Å, the dislocation emission frequency significantly increases, and multiple extended stacking faults form closed dislocation loop structures through dislocation interactions.

[0057] During the process of increasing the indentation depth from 23.7 Å to the maximum indentation depth of 50 Å, from Figure 9 (c) to Figure 9 In (e), it can be observed that increasing the compressive depth leads to the evolution of more dislocation loops, and adjacent dislocation loops are prone to cross-linking and interlocking, radiating towards the substrate bottom via detachment. Subsurface damage intensifies and propagates from both sides of the contact as stress waves. The final unloading stage can be achieved from... Figure 9 In (f), it was observed that the dislocation loop range decreased and elastic recovery occurred.

[0058] S24: Common-nearest-neighbor analysis was performed on a shale model with an indentation depth of 40 Å, revealing twinning induced by nanoindentation. For example... Figure 10 As shown, the twinned structure exhibits an alternating arrangement of multiple layers of FCC atoms and an intermediate layer of HCP atoms. Through... Figure 10 The magnified image of the intermediate twin boundary clearly shows that the FCC atoms are symmetrically distributed relative to the intermediate HCP atomic layer, which is a typical twinning structure. Shale has a low stacking fault energy, and the stacking fault energy is positively correlated with temperature, meaning that a lower deformation temperature leads to a lower stacking fault energy. Therefore, twins are more likely to form inside shale under low-temperature conditions.

[0059] Throughout the simulation, no dislocations were generated in the matrix during elastic deformation. When dislocations began to form, the matrix entered the plastic deformation stage. During this stage, the total dislocation length increased with increasing indentation depth. At certain indentation moments, the decrease in the total dislocation length was primarily attributed to dislocations escaping from the matrix surface or sides.

[0060] S25: After indentation testing, shale matrix not only develops indentations on its surface, but also suffers varying degrees of damage internally. This damage primarily results from interatomic compression, causing a change in density within the damaged layer relative to its initial state. Figure 11 This process of change is clearly revealed.

[0061] Preferably, the step of statistically analyzing the Z-axis atom number distribution and calculating the damage layer thickness specifically involves: Keyframes were extracted based on nanoindentation simulation results. The number of shale matrix atoms at different Z-axis coordinates in each frame was counted to generate a Z-axis atom number distribution curve. Density is characterized by the number of atoms. By comparing the curves of the initial frame and the frame with the maximum compression depth, the boundary of the damage area with significant changes in atomic density on the Z-axis is determined. Calculate the Z-axis coordinate difference of the damaged area under the maximum compression depth frame, and obtain the overall damaged layer thickness after conversion; compare the curves of the unloaded frame and the initial frame, calculate the coordinate difference of the unrecovered area, and obtain the plastic damaged layer thickness.

[0062] Figure 11 The results show that when the indentation depth is 5 nm, the thickness of the plastic damage layer in the shale matrix is ​​about 3 nm, which clarifies the range of subsurface damage caused by indentation.

[0063] S26: To further investigate the influence of different indentation depths on the behavior of single nanoindentation, this invention is based on... Figure 8 The load-displacement curves were used to calculate the elastic recovery rate and the work done to resist deformation.

[0064] Preferably, the calculation of elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship is achieved through the following formula: ; ; ; ; In the formula, For elastic recovery rate, For maximum indentation depth, This refers to the residual push-in depth after unloading. For plastic work, This represents the load when the indenter is pressed into the ground at any depth. For elastic work, For the fitting parameters, This is the total deformation work.

[0065] By combining the above formulas with the markings in the Lammps software, the indentation displacement and residual displacement of the target atom can be obtained. It can then be calculated that the elastic recovery rate is 28.7% and the total deformation work is 119.938 eV when the maximum indentation depth is 53 Å at room temperature.

[0066] S3: Based on the S2 nanoindentation simulation results, multiple gradient temperatures were set, the load-displacement relationship at different temperatures was recorded, the hardness and elastic modulus corresponding to each temperature were extracted, the ratio of hardness to elastic modulus was calculated, and the response consistency of shale under the action of temperature field was evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

[0067] Considering that the formation temperature gradient changes continuously with depth, the thermal expansion and contraction effect continuously disturbs the interatomic spacing and intermolecular forces inside the shale, reshaping the elastic parameters; the shale skeleton structure generates compression or stretching, causing the elastic parameters to exhibit nonlinear fluctuations. Therefore, this invention further studies the influence of high temperature conditions on the nanoindentation process of shale.

[0068] In the specific implementation process, the operation is as follows.

[0069] S31: Based on the S2 nanoindentation simulation results, data was obtained during the simulation process by changing the temperature parameter, and plotted as follows. Figure 12 The load-displacement curves at different temperatures are shown.

[0070] Shale at around 400°C has the lowest hardness and elastic modulus, consistent with current technological findings. Increased temperature intensifies atomic motion and reduces interatomic bonding forces, causing the material to soften and become more prone to deformation. According to... Figure 12 The elastic modulus and hardness of shale at different temperatures were calculated, such as... Figure 13 As shown.

[0071] The results show that with increasing temperature, the load, elastic modulus, and hardness all gradually decrease.

[0072] S32: To verify the applicability of the constructed interatomic potential function under high temperature conditions, this invention compares the ratio of hardness to elastic modulus under test and simulation conditions based on the Oliver-Pharr method, and systematically evaluates the consistency of the response of the intrinsic mechanical behavior of shale under the action of temperature field.

[0073] Preferably, the evaluation of the consistency of shale response under the influence of a temperature field specifically includes: In nanoindentation testing, the ratio of hardness to elastic modulus at the target temperature is calculated and used as the first ratio. In nanoindentation simulation, the ratio of hardness to elastic modulus at the target temperature is calculated as a second ratio. The relative error at the target temperature is calculated based on the first ratio and the second ratio. Adjust the molecular dynamics simulation parameters to control the relative error to be less than a preset threshold.

[0074] The calculation results are shown in Table 1. By comparing the first and second ratios of shale at 80℃, 150℃, and 200℃, it was found that the errors between the test results and the simulation results were all within 5%. This indicates that the constructed interatomic potential function can be effectively applied to the simulation of nanoindentation under high-temperature conditions, thus verifying its reliability and applicability in the study of high-temperature mechanical behavior.

[0075] Table 1 Comparison results of the first ratio and the second ratio

[0076] S33: The aforementioned method utilizes the temperature decay coefficient and elastic modulus to construct an elastic modulus prediction model, which is based on the formula: To achieve this.

[0077] In the formula, Indicates temperature The elastic modulus below; Indicates the reduced modulus; This indicates the elastic modulus of the indenter; This indicates the Poisson's ratio of the shale being tested; The Poisson's ratio represents the pressure head; expressed through the exponential decay term. This reflects the material softening effect caused by increased temperature. Indicates reference temperature; The temperature decay coefficient represents the sensitivity of shale to temperature. A smaller value indicates a slower decrease in elastic modulus with temperature. Based on simulation data, the temperature decay coefficient... =0.0014℃ -1 When the indenter is made of diamond, =1140GPa, =0.07.

[0078] This invention uses an exponential decay term The goodness of fit of this formula reflects the material softening effect caused by increased temperature. The value is approximately 0.96, indicating that the model has high reliability.

[0079] The aforementioned model utilizes temperature sensitivity coefficients and hardness to construct a hardness prediction model, which is derived through the formula: To achieve this.

[0080] In the formula, Indicates temperature The hardness below; This indicates the load applied by the pressure head; Indicates the contact area; Indicates reference temperature; , Indicates the temperature sensitivity coefficient. The hardness decreases primarily in the low-to-medium temperature range. Enhance nonlinear attenuation in the high-temperature region.

[0081] Combining simulation data, =0.004℃ -1 , =1.5×10 -7 ℃ -1 The goodness of fit of this formula The value is approximately 0.976, indicating that the model has high reliability. When the simulation data is substituted, the prediction error is less than 5%.

[0082] S34: Calculate the elastic recovery rate at different temperatures, as well as the plastic work, elastic work, and total deformation work at different temperatures, and analyze the intrinsic mechanism of elastoplastic deformation of shale at high temperatures.

[0083] from Figure 14 It can be seen that the elastic recovery rate of the rock increases within the temperature range of 26.85℃ to 150℃. This phenomenon is mainly due to the fact that within this temperature range, the mineral particles of the rock undergo slight deformation due to thermal expansion, which leads to the compression and closure of primary pores and microcracks. However, the local stress concentration caused by thermal expansion promotes dislocation movement, thereby leading to the accumulation of plastic deformation.

[0084] During nanoindentation, the material undergoes irreversible plastic deformation during the loading phase, while during the unloading phase, its elastic recovery ability is limited due to the suppression effect of thermal expansion stress.

[0085] Within the temperature range of 150℃ to 300℃, the elastic recovery rate shows a decreasing trend. When the temperature reaches 150℃ to 200℃, the rock enters the thermal fracturing threshold stage. The difference in thermal expansion between mineral particles leads to the failure of the cemented structure, and microcracks rapidly propagate and interconnect, forming a macroscopic crack network.

[0086] This process reduces the overall stiffness of the material (manifested as a decrease in the elastic modulus), but crack propagation releases some of the internal stress. During the unloading phase, the elastic recovery capability is enhanced due to the temporary reduction in crack opening.

[0087] When the temperature rises to 400℃, the elastic recovery rate increases significantly. This change is due to the combined effects of quartz phase transformation, ductile failure, and mineral phase transformation, which intensifies the irreversibility of plastic deformation.

[0088] from Figure 15 As can be seen, the plastic work done by rocks to resist deformation decreases continuously with increasing temperature, with a decrease of about 24%. At high temperatures, dislocation climb accelerates, and the dynamic recovery mechanism dominates, reducing the plastic strain energy stored in dislocations. Increasing temperature also weakens interatomic forces, reducing the yield strength of rocks and reducing the energy required for plastic deformation. The elastic work initially increases slightly and then gradually decreases, with an overall decrease of about 17%. The total deformation work generally shows a decreasing trend, with a decrease of about 32%. Increasing temperature leads to material softening, reducing the external force required for the same indentation depth and thus reducing the total deformation work.

[0089] The analytical method provided by this invention integrates microscopic multi-scale experimental characterization with atomic-scale molecular dynamics simulation. It obtains experimental data through nanoindentation lattice experiments and multi-source microscopic characterization, constructs a heterogeneous molecular dynamics model and performs room temperature nanoindentation simulation, sets gradient temperatures to perform high temperature nanoindentation simulation and analyzes temperature effects, and constructs a corresponding prediction model. This provides a microscopic basis and theoretical model for the evaluation and prediction of the mechanical properties of deep shale under thermo-mechanical coupling environment.

[0090] This invention also provides a system for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, comprising: Test data acquisition module: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, the microscopic mechanical parameters of shale are obtained and the spatial correspondence between mechanical parameters and mineral components is established; Simulation module: Constructs a molecular dynamics model of shale heterogeneity based on the mineral composition results obtained from the test data acquisition module; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Calculate the elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship; High-temperature simulation module: Based on the nanoindentation simulation results of the simulation module, multiple gradient temperatures are set, the load-displacement relationship at different temperatures is recorded, the hardness and elastic modulus corresponding to each temperature are extracted, the ratio of hardness to elastic modulus is calculated, and the response consistency of shale under the action of temperature field is evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

[0091] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, characterized in that, include: S1: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, we obtained the microscopic mechanical parameters of shale and established the spatial correspondence between mechanical parameters and mineral components. S2: Construct a molecular dynamics model of shale heterogeneity based on the mineral composition results of S1; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Calculate the elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship; S3: Based on the S2 nanoindentation simulation results, multiple gradient temperatures were set, the load-displacement relationship at different temperatures was recorded, the hardness and elastic modulus corresponding to each temperature were extracted, the ratio of hardness to elastic modulus was calculated, and the response consistency of shale under the action of temperature field was evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

2. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, The molecular dynamics simulation parameters described in S2 are set as follows: A potential function is constructed using the LJ potential, Coulomb potential, and embedding energy; Periodic boundary conditions are applied in the X, Y, and Z directions, and the size in the Z direction is expanded, dividing the system into a boundary layer, an isothermal layer, and a Newtonian layer along the Z direction.

3. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 2, characterized in that, Using the LJ potential, Coulomb potential, and embedding energy, a potential function is constructed, which is realized by the following formula: ； ； In the formula, U Represents the potential function. Indicates embedding capability, It is the number of all other particles in the system besides atoms in the first... i The linear superposition of electron cloud densities at each atom; This indicates the potential term, including the LJ potential and the Coulomb potential; It is the distance between two atoms; It is the first j The atom in the first i The charge density function at each atom.

4. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, S3 describes the construction of an elastic modulus prediction model using the temperature decay coefficient and elastic modulus, based on the formula: To achieve; In the formula, Indicates temperature The elastic modulus below; Indicates the reduced modulus; This indicates the elastic modulus of the indenter; This indicates the Poisson's ratio of the shale being tested; This represents Poisson's ratio of the pressure head; Indicates the exponentially decaying term; Indicates reference temperature; This represents the temperature decay coefficient.

5. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, S3 describes the use of temperature sensitivity coefficient and hardness to construct a hardness prediction model, which is based on the formula: To achieve; In the formula, Indicates temperature The hardness below; This indicates the load applied by the pressure head; Indicates the contact area; Indicates reference temperature; , This represents the temperature sensitivity coefficient.

6. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, The statistical Z-axis atom number distribution described in S2 is used to calculate the damage layer thickness, specifically as follows: Keyframes were extracted based on nanoindentation simulation results. The number of shale matrix atoms at different Z-axis coordinates in each frame was counted to generate a Z-axis atom number distribution curve. Density is characterized by the number of atoms. By comparing the curves of the initial frame and the frame with the maximum compression depth, the boundary of the damage region with significant changes in atomic density on the Z-axis is determined. Calculate the Z-axis coordinate difference of the damaged area under the maximum compression depth frame, and obtain the overall damaged layer thickness after conversion; compare the curves of the unloading completion frame and the initial frame, calculate the coordinate difference of the unrecovered area, and obtain the plastic damaged layer thickness.

7. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, S2 describes the calculation of elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship, which is achieved through the following formula: ； ； ； ； In the formula, For elastic recovery rate, For maximum indentation depth, This refers to the residual push-in depth after unloading. For plastic work, This represents the load when the indenter is pressed into the ground at any depth. For elastic work, For fitting parameters, This is the total deformation work.

8. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, The evaluation of the consistency of shale response under temperature field conditions, as described in S3, specifically includes: In nanoindentation testing, the ratio of hardness to elastic modulus at the target temperature is calculated and used as the first ratio. In nanoindentation simulation, the ratio of hardness to elastic modulus at the target temperature is calculated as a second ratio. The relative error at the target temperature is calculated based on the first ratio and the second ratio. Adjust the molecular dynamics simulation parameters to control the relative error to be less than a preset threshold.

9. The method for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation according to claim 1, characterized in that, S3 also includes: The elastic recovery rate, plastic work, elastic work, and total deformation work at different temperatures were calculated to analyze the intrinsic mechanism of elastoplastic deformation of shale at high temperatures.

10. A system for analyzing the micromechanical properties of hard and brittle shale based on nanoindentation, characterized in that, include: Test data acquisition module: Using nanoindentation lattice experiments and multi-source microscopic characterization tests, the microscopic mechanical parameters of shale are obtained and the spatial correspondence between mechanical parameters and mineral components is established; Simulation module: Constructs a molecular dynamics model of shale heterogeneity based on the mineral composition results obtained from the test data acquisition module; Nanoindentation simulation was performed based on the molecular dynamics model and molecular dynamics simulation parameters of heterogeneous shale. Based on the results of nanoindentation simulations, the microscopic deformation and damage mechanisms were analyzed. Specific procedures included: Based on the shear stress distribution characteristics at different indentation depths, the evolutionary process of dislocation loop germination, growth, proliferation, and maintenance was traced. Statistical analysis of the atomic number distribution along the Z-axis was performed to calculate the thickness of the damaged layer. Calculate the elastic recovery rate, plastic work, elastic work, and total deformation work based on the load-displacement relationship; High-temperature simulation module: Based on the nanoindentation simulation results of the simulation module, multiple gradient temperatures are set, the load-displacement relationship at different temperatures is recorded, the hardness and elastic modulus corresponding to each temperature are extracted, the ratio of hardness to elastic modulus is calculated, and the response consistency of shale under the action of temperature field is evaluated. A model for predicting elastic modulus is constructed using the temperature decay coefficient and the elastic modulus. A hardness prediction model is constructed using temperature sensitivity coefficient and hardness.

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