Sand shale interbed crack formation mechanism and evolution mode analysis method

Abstract

The invention discloses a sand shale interbed crack formation mechanism and evolution mode analysis method, which comprises the following steps of: setting a reasonable rock sample loading rate and shortening rate by using a particle flow numerical simulation technology according to a macroscopic structure style, and further implementing a rock mechanics experiment with a variable loading rate; the fracture strength and stress-strain evolution maps of sand / mudstone under different loading rate conditions are constructed, so that a sand / mudstone interbed fracture development mechanism under a strong stress rapid extrusion environment is disclosed, an interbed type artificial sample is prepared by using an underground sand / mudstone core sample from the similar perspective of three aspects of material composition, mechanical properties and an interbed structure, and the interbed type artificial sample is used for detecting the interbed fracture of the sand / mudstone. A loading type synchronous CT scanning experiment and a three-dimensional acoustic emission source positioning experiment are carried out, the rock fracture process is visualized, and the composite rock mass fracture evolution process is revealed. The method has important theoretical and practical values for improving the exploration success rate of the fractured gas reservoir, widening the exploration and development field and optimizing the reservoir transformation strategy.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field exploration and development, and in particular to a method for analyzing the formation mechanism and evolution pattern of fractures in interbedded sandstone and mudstone. Background Technology

[0002] The formation mechanism of fractures in interbedded sandstone and mudstone falls within the scope of composite rock mechanics research. The strength theory of composite rock masses can be divided into two main categories: classical strength criteria and empirical fracture criteria. The former mainly includes the Drucker-Prager criterion and the unified strength theory; the latter mainly includes the Hoek-Brown empirical strength criterion and the Yoshinaka power function criterion (Dai et al., 2011; Yin et al., 2017; Zoback and Kohli, 2019; Yang et al., 2023; Xia et al., 2024). The mechanisms of fracture propagation in bedding rock masses are mainly divided into three types: ① Cook–Gordon exfoliation, a common delamination mechanism in composite materials (Larsen et al., 2010); in isotropic materials, when the rock is heterogeneous, fractures may propagate along the weakest surfaces of the rock mass. ② Stress barrier: The stress rotation at the interface hinders the propagation of specific types of cracks, creating an unfavorable stress state locally that causes crack propagation to be blocked and terminated (Gudmundsson et al., 2006). ③ Material toughness: Defined as the amount of energy absorbed per unit crack area. When the strain energy release rate of the crack reaches the critical energy value required for propagation, the crack will penetrate the lithological interface and continue to propagate; otherwise, the crack will propagate along the interface in a deflected manner (Gudmundsson et al., 2010). Whether based on classical strength criteria or empirical fracture criteria, composite rock masses are predominantly characterized by well-developed cracks in hard rock (sandstone) and poorly developed cracks in soft rock (mudstone). These cracks mostly terminate or deflect at the lithological interface, forming the classic development pattern of interbedded sandstone and mudstone cracks (Lyu et al., 2019; Zhang et al., 2020; Xu et al., 2022; Gong et al., 2023). Therefore, studying the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures under different compression rates can improve the traditional theory of fracture formation in composite rock masses from a geomechanical perspective, and has important application value for quantitative prediction of deep fractures and large-scale discovery of tight sandstone gas reservoirs. Summary of the Invention

[0003] To address the aforementioned deficiencies in the existing technology, the present invention aims to provide a method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures, which can reveal the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures based on physical and numerical simulations.

[0004] The technical solution of this invention is: a method for analyzing the formation mechanism and evolution pattern of fractures in interbedded sandstone and mudstone, the specific implementation steps of which are as follows:

[0005] The first step is to determine the loading rate and shortening rate of rock mechanics experiments based on the macroscopic structural patterns; Rock mechanical parameters and fracture thresholds are not only related to the composition of rock samples and in-situ stress, but also controlled by experimental compression rate and formation shortening rate. Macroscopic structural patterns are closely related to structural compression rate and formation shortening rate. Using macroscopic particle flow numerical simulation technology, we can clarify the range of compression rate and formation shortening rate corresponding to various structural patterns, and clarify the influence of compression rate on rock mechanical parameters, stress-strain state and rock fracture mechanism. The specific implementation scheme for clarifying the influence of the compression rate on rock mechanical parameters, stress-strain state, and rock fracture mechanism is as follows: First, a basic structural geometric model is established based on the typical structural styles of the study area; second, referring to the triaxial mechanical simulation test results of different particles, the initial model parameters for particle flow numerical simulation are set, an initial structural model is established, and transposed into a three-dimensional discrete element elastoplastic model; third, the model is set with the same sedimentary particles to simulate the erosion and decapitation process of strata during structural evolution; finally, the deformation and compression rate of different types of structural units are revealed, laying the foundation for exploring how the compression rate affects rock mechanical parameters, stress-strain state, and rock fracture mechanism.

[0006] The second step involves conducting simultaneous digital speckle monitoring during rock mechanics experiments at different loading rates. Sandstone and mudstone samples were collected from different strata, and rock mechanics experiments were conducted at different loading rates with simultaneous digital speckle monitoring. Evolution charts of the mechanical parameters of sandstone and mudstone and the rock fracture patterns and mechanical parameters under different loading rates were established.

[0007] The steps for establishing a chart of the evolution of mechanical parameters and rock fracture patterns and mechanical parameters of sandstone and mudstone under different loading rates are as follows: Based on the stratigraphic shortening rate and stress intensity corresponding to different structural patterns, one group of sandstone and one group of mudstone samples are collected, with n samples in each group, and the sample surfaces are polished; combined with the results of paleostress field simulation of the profile, the confining pressure conditions for rock mechanics experiments are determined, and speckle patterns are sprayed onto the samples; first, a white matte primer is sprayed to enhance contrast, and then black speckle patterns are sprayed; different loading rates are set for each group of n samples, and rock mechanics experiments are carried out at different loading rates with simultaneous digital speckle monitoring; using digital holographic imaging technology, the light field in a specific three-dimensional space including the target is reconstructed by calculating and simulating the propagation path of light in space, and the three-dimensional motion tracking of particles is applied, combined with the surface morphology of the rock samples to achieve rock strain imaging, and the Young's modulus, Poisson's ratio, critical fracture threshold, energy release rate and fracture pattern of different rock samples under different loading rates are obtained, and a chart of the evolution of mechanical parameters and rock fracture patterns and mechanical parameters of sandstone and mudstone under different loading rates is established.

[0008] The aforementioned collection includes one set each of sandstone and mudstone samples, with n samples in each set, where n ≥ 5, and the sample size is 25 mm. 50mm cylinder.

[0009] The diameter of the sprayed black dots is set to 1 / 50 of the size of the sandstone or mudstone sample; for example, a 2.5 cm sample uses 0.5 mm dots.

[0010] The third step involves conducting experimental numerical simulations of rock mechanics based on particle flow to elucidate the dependence of different blocks of sandstone and mudstone samples on loading rate. To avoid the uncertainty in interpreting the fracture formation mechanism in rock mechanics experiments with different loading rates and to expand the universality of the loading rate and sandstone-mudstone mechanical response model, this study systematically reveals the dynamic mechanism of fracture evolution under the interaction of rate and lithology based on particle flow rock mechanics numerical simulation technology. Furthermore, it conducts particle flow-based rock mechanics numerical simulation experiments to solidify the universality of the loading rate and sandstone-mudstone fracture response model and further elucidates the dependence of different sandstone-mudstone samples on loading rate.

[0011] The specific implementation steps of the numerical simulation of rock mechanics experiments based on particle flow are as follows: First, a parallel bond model or a contact bond model is used to characterize the cementation effect between particles, and microscopic mechanical parameters are assigned, including normal and tangential stiffness and bond strength. Second, the macroscopic mechanical responses obtained from laboratory experiments on sandstone and mudstone blocks are used as constraints, and the microscopic parameter assignment scheme is adjusted by combining the inverse iteration method to ensure the consistency between the numerical model and the mechanical behavior of rock physics experiments. The macroscopic mechanical responses include elastic modulus and peak strength. Third, a displacement control or stress control mode is adopted, based on servo control. The system dynamically adjusts the loading rate to ensure that the experimental process meets quasi-static conditions and avoids interference from dynamic effects. The loading rate is determined through deformation sensitivity analysis and characterized in the form of strain rate, supplemented by time-step adaptive adjustment to ensure numerical stability. Finally, the system monitors particle displacement, contact force chain evolution, and bond fracture events in real time, recording stress-strain curves, fracture modes, and energy dissipation characteristics. By comparing the mechanical responses of sandstone and mudstone samples from different loading rates and different blocks, and combining the energy dissipation spectrum, the system distinguishes the changes in the proportions of elastic energy storage, bond fracture energy, and frictional dissipation, further clarifying the dependence of different blocks of sandstone and mudstone samples on the loading rate.

[0012] The fourth step reveals the formation mechanism of fractures in the interbedded sandstone and mudstone; By combining the main controlling factors of fractures, we can identify the fracture activity stages, reveal the diagenetic strength corresponding to different layers, investigate the influence of early fractures and their activity on diagenetic strength and rock mechanical parameters, interpret the influence mechanism of loading rate and stratigraphic shortening rate on the mechanical properties and fracture patterns of sandstone and mudstone, and elucidate the development mechanism of interbedded fractures in sandstone and mudstone under the comprehensive control of tectonics, fluid, diagenesis and mechanical properties.

[0013] The specific steps for elucidating the development mechanism of interbedded sandstone and mudstone fractures under the comprehensive control of tectonic-fluid-diagenetic-mechanical properties are as follows: Referring to regional tectonic evolution, stress field evolution, and major fracture-controlling factors, the evolution mechanism of fracture patterns and mechanical property differences in sandstone and mudstone under different loading rates is interpreted from the perspectives of fracture development stages and diagenetic intensity. First, typical fracture-intrusive vein or hydrothermal vein samples are collected from different strata. The geometric morphology, cross-cutting and confinement relationships, and diagenetic mineral types and structures of micron- to nanon-scale micro-fractures are observed. Fluid inclusion analysis technology in fracture filling materials is used to conduct petrographic observation of fluid inclusions in typical fracture veins, studying their types and compositions, determining the homogenization temperature and freezing point temperature of fluid inclusions, calculating the fluid inclusion trapping pressure in fracture filling materials, revealing the trapping stages of fluid inclusions, and analyzing the differences in trapping temperatures at each stage. Single-well burial... Uplift-thermal evolution history simulation and reconstruction were used to determine the thermo-compressional evolution paths of different sandstone-mudstone interbedded sections and to identify the differences in the stages of fracture activity in different sandstone-mudstone sections. Combining the results of rock mechanics experiments at different loading rates with the stages of fracture activity in different sandstone-mudstone sections, mudstone samples were selected to measure illite content and characterize diagenetic parameters, and diagenetic strength was evaluated. At the same time, typical thin sections, cathodoluminescence, and scanning electron microscopy images were selected to statistically analyze diagenetic parameters such as micropore size distribution, pore shape, the ratio of the number of intergranular contact points to the number of grains, and grain contact length. Combined with the evolution of diagenetic minerals and porosity-permeability parameters, a mineral evolution sequence and diagenetic evolution model were established. The effect of fractures on the mechanical properties of deep clastic rocks was analyzed, and the intrinsic relationship between lithology, compression rate, mechanical parameters, and fractures was identified, clarifying the fracture development mechanism of sandstone-mudstone interbedded sections under the comprehensive control of tectonics, fluid, diagenesis, and mechanical properties.

[0014] The fifth step involves conducting loading-type synchronous CT scanning experiments and digital core modeling; Loaded synchronous CT scanning experiments were conducted to investigate the fracture evolution process of samples with different lithologies; based on digital core modeling, the evolution of rock porosity and tortuosity at different stress-strain stages was revealed; and from the perspective of the evolution of pore structure during loading, the differential fracture evolution of sandstone and mudstone in different blocks was revealed.

[0015] The method described above, which reveals the differential fracturing evolution of sandstone and mudstone in different blocks from the perspective of pore structure evolution during loading, involves the following steps: First, simultaneous CT scanning technology for rock mechanics experiments is used to reveal the fracturing evolution process of sandstone and mudstone. Second, sandstone and mudstone samples are collected, and two sets of representative samples (A and B) with consistent burial depth and rock characteristics are prepared. These rock characteristics include lithology, mineral composition, and heterogeneity. Third, the sample size is reasonably determined based on the range and accuracy requirements of the CT scanning equipment, with a diameter of 2.5-5 mm and a height of 5-10 mm. Fourth, rock mechanics experiments are conducted on sample A. Based on the obtained stress-strain curves, the compressive strength σc of different samples is measured. Fifth, the scanning points for sample B are determined, and using a Zeiss X-ray microscope, the 0.3σc and 0.3σc points are selected respectively. c 0.5σ c 0.7σ c 0.8σ c 0.9σ c and σ c Seven stress points were used as scanning points to conduct synchronous micron-CT scanning experiments on sample B under stress loading conditions, acquiring two-dimensional images of rock samples at different stages and establishing corresponding three-dimensional digital core models. By setting different three-dimensional pixel screening thresholds, pore evolution models of rocks under different stress-strain states were established, and the variation laws of porosity and tortuosity at different fracture evolution stages were calculated. The tortuosity is used as a parameter to characterize the degree of bending of irregular geometry, and the larger the value, the worse the pore connectivity. Finally, a dynamic evolution model of rock pores under stress loading conditions was established, revealing the differential fracture evolution law of sandstone and mudstone in different blocks from the perspective of pore structure evolution during loading.

[0016] The sixth step is to conduct a multi-scale fracture acoustic emission source localization experiment on composite rock mass under the control of lithological interfaces; Under the constraints of similar material composition, mechanical properties and interlayered structure, layered samples with different sand-mud ratios and thicknesses were prepared by core sample processing and artificial sample preparation. Three-dimensional acoustic emission source localization experiments were carried out to achieve transparent analysis of the fracture development process under different loading rates. From the perspective of lithological interface and sand-mudstone thickness, the vertical propagation of fractures and the development law of fractures penetrating the lithological interface were revealed.

[0017] The steps for achieving transparent analysis of crack development processes under different loading rates are as follows: Given the extreme difficulty in obtaining sandstone-mudstone composite rock masses that meet experimental requirements under natural conditions, making it difficult to analyze the influence of lithological interfaces on rock fracturing and crack propagation, artificial sample preparation is used to investigate the influence of lithological interfaces on crack evolution. The similarity between the artificial sample and the underground sample is ensured from three aspects: material composition, mechanical properties, and interlayer structure. To ensure that the artificial sample and the underground sample have similar material compositions, core samples from different depths are first drilled and broken into fragments of 0.8-1.5 cm. Then, the rock fragments and steel balls are placed in a piston container and ground at high speed using a stirring drill to form a specified rock sample mesh size. The rock sample powder is mixed with epoxy resin and pressed layer by layer using a uniaxial compression tester. Based on previous experience in preparing samples of different strengths, the uniaxial compression strength and time parameters are reasonably set. After compaction, the artificial rock core is removed from the mold and placed in a constant temperature chamber at 80-150°C. The samples were dried at a temperature of ℃ for more than 12 hours, and artificial rock cores were drilled using a wire cutting machine. Mechanical testing was conducted to ensure that the artificially prepared samples had similar mechanical properties to the underground samples. Layered sample preparation schemes with different sand-mud ratios were determined, constrained by the macroscopic profile lithological assemblage. The influence of the interlayer ratio and thickness on fracture development in sand-mudstone interbedded layers was considered, ensuring that the prepared samples had a similar interbedded lithological structure to the macroscopic sedimentary lithology. Three types of composite rock mass samples were prepared: sand-mudstone interbedded, mudstone interbedded, and sand-mudstone interbedded. The in-situ stress environment of the rock samples was determined, and a three-dimensional acoustic emission source localization experiment was used to monitor the stress environment. The acoustic emission signals of different samples were analyzed to study the crack propagation and evolution patterns under different rock layer thickness ratios, sandstone-mudstone interlayer structure, lithological interfaces, and different rock layer thickness ratios. The acoustic emission signal processing flow was as follows: by calculating the waveform parameters of the same vibration source, the spatial location of microcrack events was obtained; based on the local region correlation imaging algorithm of the rock, the rock mass was discretized into a finite element system; by constructing a quantitative relationship between elastic wave velocity variation and rock damage, multi-scale fracture acoustic emission data processing and visualization reconstruction of damage distribution were realized; finally, the crack evolution mode of the composite rock mass was interpreted from the perspective of lithological interfaces, sandstone-mudstone thickness, and interlayer structure.

[0018] The specific steps for mixing rock sample powder with epoxy resin and pressing the sample layer by layer using a uniaxial press are as follows: mix rock sample powder and epoxy resin in a ratio of 100:9, mix manually until uniform, sieve three times, and then press the sample layer by layer.

[0019] Step 7: Clarify the fracture evolution pattern of interbedded sandstone and mudstone under different compression rates; Numerical simulation was adopted to replace physical simulation. Discrete element numerical simulation of large-scale lithological interfaces was carried out, along with loading-type CT scanning experiments and acoustic emission source localization experiments. The relationship between fracture cross-layer mode and mechanical structure and energy accumulation and dissipation was clarified, the evolution stages of fractures were quantitatively divided, and a fracture evolution model under the control of compression rate, mechanical structure and stress-strain was established.

[0020] The specific steps for establishing a fracture evolution model under the control of compression rate, mechanical structure, and stress-strain are as follows: A numerical simulation method is used in place of physical experiments to systematically analyze the control mechanism of large-scale lithological interfaces and rock combination patterns on fracture propagation. Based on the results of loaded synchronous CT scans and mechanical experiments at different loading rates, a large-scale elastoplastic damage discrete element model containing lithological interfaces is constructed using discrete element software and its FISH compiler. This enables three-dimensional reconstruction of the stress field in a single well, revealing the regulatory laws of large-scale lithological interface geometric parameters and combination configurations on the nucleation mechanism, propagation path, and stress accumulation release of tectonic fractures. A quantitative relationship is established between rock fracture and lithological interfaces, strain energy stage evolution, and single-layer / interlayer strength differences. Through comparative analysis with a single-well discrete network model, the rock fracture evolution stage of the fracture is clarified by comprehensively considering the tectonic deformation shortening, compression rate, and fracture scale distribution in each well area. This establishes the evolution law of sandstone-mudstone interbedded fractures under the control of compression rate, mechanical structure, and stress-strain.

[0021] The beneficial effects of this invention are as follows: Based on macroscopic structural patterns, using particle flow numerical simulation technology, reasonable rock sample loading rates and shortening rates are set, and then rock mechanics experiments with varying loading rates are implemented. Simultaneously, by combining fluid inclusion analysis and diagenetic strength testing, fracture strength and stress-strain evolution maps of sandstone / mudstone under different loading rates are constructed, thereby revealing the development mechanism of interbedded fractures in sandstone / mudstone under high-stress rapid compression. From the perspective of similarity in material composition, mechanical properties, and interbedded structure, interbedded artificial samples are prepared using underground sandstone / mudstone core samples. Loading-based synchronous CT scanning experiments and three-dimensional acoustic emission source localization experiments are conducted to visualize the rock fracture process and reveal the fracture evolution process of composite rock masses. This invention patent proposes a set of visualization techniques for fractures in composite rock masses with varying loading rates under macroscopic structural-sedimentary constraints, elucidating the development mechanism of interbedded fractures in sandstone / mudstone and revealing the fracture evolution process under the control of compression rate, mechanical structure, and stress-strain. This has significant theoretical and practical value for improving the success rate of fractured gas reservoir exploration, broadening the exploration and development field, and optimizing reservoir stimulation strategies. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating a method for analyzing the formation mechanism and evolution pattern of fractures in interbedded sandstone and mudstone.

[0023] Figure 2The study area shows the tectonic location and north-south trending seismic profile of the Dibei block.

[0024] Figure 3 This is a rock mechanics experimental scheme for simulating constraints using macroscopic structural particle flow.

[0025] Figure 4 This is a digital speckle monitoring experiment synchronized with rock mechanics experiments at the same loading rate.

[0026] Figure 5 Rock mechanics experiments were conducted on sandstone and mudstone samples from different blocks at different loading rates: (A) Sandstone sample from Zhongqiu Block; (B) Sandstone sample from Bozi Block; (C) Mudstone sample from Bozi Block; (D) Sandstone sample from Dibei Block; (E) Mudstone sample 1 from Dibei Block; (F) Mudstone sample 2 from Dibei Block.

[0027] Figure 6 To construct the relationship between deformation rate and different rock mechanical parameters: (A) peak stress; (B) uniaxial deviatoric stress; (C) Young's modulus.

[0028] Figure 7 The mechanism of fracture development in interbedded sandstone and mudstone under the comprehensive control of tectonic-fluid-diagenetic-mechanical properties.

[0029] Figure 8 This is a loading-type synchronous CT scan and visualization technology for rock fracture processes.

[0030] Figure 9 A sample preparation scheme for sand / mudstone interlayered samples under the constraints of similar material composition, mechanical properties, and interlayered structure.

[0031] Figure 10 The evolution mode of fractures in interbedded sandstone and mudstone under different compression rates. Detailed Implementation

[0032] The specific embodiments of the present invention are described below with reference to the accompanying drawings: This invention patent uses the interbedded sandstone and mudstone formations of the Kuqa Depression on the northern margin of the Tarim Basin in western China as an example to illustrate the specific implementation process of the invention. In the exploration history of tight oil and gas reservoirs in the Kuqa Depression, finding high-quality fracture development zones has always been the main theme of oil and gas exploration. Compared with the Bozi-Dabei and Zhongqiu blocks, the Dibei block of the Kuqa Depression is adjacent to the southern Tianshan Mountains, exhibiting a monocline structure with extremely strong paleostress and a very rapid compression rate. This block has two significant characteristics: "rapid and intense compression" and "widespread fracture development." Fractures and sand bodies are distributed in multiple cycles, with particularly significant heterogeneity. Both physical and numerical simulation results reveal a close correlation between the compression rate and the structural style, and a significant impact on the fracture mode of the rocks. Specifically, when the compression rate is slow, imbricate and sudden-formation structures are more likely to form; conversely, monocline structures tend to form. In the slow compression zone, sandstone fractures are well-developed while mudstone fractures are not, which is consistent with the classic sand / mudstone interbedded fracture development pattern. In the rapid compression zone (Dibei), imaging logging results show that mudstone fractures are well-developed while sandstone fractures are not, that is, sandstone / mudstone fractures exhibit typical differential development.

[0033] The first step is to determine the loading rate and shortening rate of rock mechanics experiments based on the macroscopic structural patterns; Using macroscopic particle flow numerical simulation technology, this study clarifies the range of compression rates and stratigraphic shortening rates corresponding to various structural styles, and elucidates the impact of compression rates on rock mechanical parameters, stress-strain states, and rock fracture mechanisms. The specific implementation scheme for clarifying the impact of compression rates on rock mechanical parameters, stress-strain states, and rock fracture mechanisms is as follows: First, a basic structural geometric model is established based on typical structural styles in the study area; second, referring to the triaxial mechanical simulation test results of different particles, the initial model parameters for particle flow numerical simulation are set, an initial structural model is established, and then transposed into a three-dimensional discrete element elastoplastic model; third, the model is set with the same sedimentary particles to simulate the erosion and decapitation process of strata during structural evolution; finally, the deformation and compression rates of different types of structural units are revealed. Figure 3 This study laid the foundation for investigating how compression rate affects rock mechanical parameters, stress-strain state, and rock fracture mechanism. Finally, the experimental schemes for rock mechanics of samples from different regions were determined (Table 1).

[0034] Table 1 Experimental schemes for the effects of different stress loading rates on rock mechanical parameters of different lithologies

[0035] The second step involves conducting simultaneous digital speckle monitoring during rock mechanics experiments at different loading rates. Sandstone and mudstone samples were collected from different strata, and rock mechanics experiments were conducted at different loading rates with simultaneous digital speckle monitoring. Evolution charts of the mechanical parameters of sandstone and mudstone and the rock fracture patterns and mechanical parameters under different loading rates were established.

[0036] The steps for establishing the evolution charts of mechanical parameters and rock fracture patterns of sandstone and mudstone under different loading rates are as follows: Based on the formation shortening rate and stress intensity corresponding to different structural patterns, one set of sandstone and one set of mudstone samples are collected, with 5 samples in each set. The sample surfaces are polished, and the sample size is 25 mm. A 50 mm cylinder was used; based on the results of the paleostress field simulation of the profile, the confining pressure conditions for the rock mechanics experiment were determined, and speckle patterns were sprayed onto the samples; a white matte primer was first sprayed to enhance the contrast, and then black speckles were sprayed, using 0.5 mm spots; different loading rates were set for each group of 5 samples (Table 1), and rock mechanics experiments were carried out at different loading rates with simultaneous digital speckle monitoring; using digital holographic imaging technology, the light field in a specific three-dimensional space including the target was reconstructed by calculating and simulating the propagation path of light in space, and the three-dimensional motion tracking of particles was applied. Combined with the surface morphology of the rock samples, rock strain imaging was achieved, and the Young's modulus, Poisson's ratio, critical fracture threshold, energy release rate, and fracture pattern of different rock samples under different loading rates were obtained. An evolution chart of the mechanical parameters of sandstone and mudstone and the rock fracture pattern and mechanical parameters under different loading rates was established. Figure 4 , Figure 5 ).

[0037] Overall, sandstone exhibits higher peak stresses than mudstone. Specifically, sandstone samples from the Bozi block show the highest peak stresses, ranging from 500 to 600 MPa, while mudstone samples from the Dibei block show the lowest, ranging from 140 to 270 MPa. The Young's modulus of sandstone and mudstone shows little difference. Sandstone samples from the Zhongqiu block have the lowest Young's modulus, ranging from 18 to 32 GPa, while mudstone samples from the Bozi block have the highest, ranging from 46 to 48 GPa. Sandstone samples from the Zhongqiu block have the lowest Poisson's ratio, ranging from 0.20 to 0.30, while mudstone samples from the Dibei block have the highest, ranging from 0.22 to 0.36. Figure 5 The stress-strain curves of the rock show that it generally exhibits brittle fracture. Some samples, under slow extrusion rates, show a brittle-ductile transition, while under rapid extrusion rates, they exhibit brittle fracture. Figure 5 ).

[0038] The third step involves conducting experimental numerical simulations of rock mechanics based on particle flow to elucidate the dependence of different blocks of sandstone and mudstone samples on loading rate. To avoid the uncertainty in interpreting the fracture formation mechanism in rock mechanics experiments with different loading rates and to expand the universality of the loading rate and sandstone-mudstone mechanical response model, this study systematically reveals the dynamic mechanism of fracture evolution under rate-lithology interaction based on particle flow rock mechanics experimental numerical simulation technology; and conducts particle flow-based rock mechanics experimental numerical simulation to solidify the universality of the loading rate and sandstone-mudstone fracture response model. Figure 6 This further clarifies the dependence of different blocks of sandstone and mudstone samples on loading rate.

[0039] With increasing tectonic deformation rate, Young's modulus, uniaxial deviatoric stress, and peak stress all increased in samples other than the mudstone samples from the Dibei block. The loading rate had the greatest impact on the sandstone of the Zhongqiu block, followed by the Bozi block, and the smallest on the Dibei block, indicating that the sandstone of the Dibei block had the strongest ability to withstand rapid compression, while the sandstone of the Zhongqiu block had the weakest. There are two main types of mudstone in the Dibei area: fractured mudstone and ordinary mudstone; the fractures developed in the mudstone can be clearly divided into two phases. In the mudstone of the Dibei block, Young's modulus, uniaxial deviatoric stress, and peak stress generally showed a logarithmic negative correlation with the tectonic deformation rate. Unlike the classic sand / mudstone interbedded fracture development model, under rapid compression, fractures developed in mudstone but not in sandstone, exhibiting a phenomenon of fracture inversion. Figure 6 ).

[0040] The fourth step reveals the formation mechanism of fractures in the interbedded sandstone and mudstone; By combining the main controlling factors of fractures, we can identify the fracture activity stages, reveal the diagenetic strength corresponding to different layers, investigate the influence of early fractures and their activity on diagenetic strength and rock mechanical parameters, interpret the influence mechanism of loading rate and stratigraphic shortening rate on the mechanical properties and fracture patterns of sandstone and mudstone, and elucidate the development mechanism of interbedded fractures in sandstone and mudstone under the comprehensive control of tectonics, fluid, diagenesis and mechanical properties.

[0041] The specific steps for elucidating the development mechanism of interbedded sandstone and mudstone fractures under the comprehensive control of tectonic-fluid-diagenetic-mechanical properties are as follows: Referring to regional tectonic evolution, stress field evolution, and major fracture-controlling factors, the evolution mechanism of fracture patterns and mechanical property differences in sandstone and mudstone under different loading rates is interpreted from the perspectives of fracture development stages and diagenetic intensity. First, typical fracture-intrusive vein or hydrothermal vein samples are collected from different strata. The geometric morphology, cross-cutting and confinement relationships, and diagenetic mineral types and structures of micron- to nanon-scale micro-fractures are observed. Fluid inclusion analysis technology in fracture filling materials is used to conduct petrographic observation of fluid inclusions in typical fracture veins, studying their types and compositions, determining the homogenization temperature and freezing point temperature of fluid inclusions, calculating the fluid inclusion trapping pressure in fracture filling materials, revealing the trapping stages of fluid inclusions, and analyzing the differences in trapping temperatures at each stage. Single-well burial... Uplift-thermal evolution history simulation and reconstruction were used to determine the thermo-compressional evolution paths of different sandstone-mudstone interbedded sections and to identify the differences in the stages of fracture activity in different sandstone-mudstone sections. Combining rock mechanics experimental results at different loading rates with the stages of fracture activity in different sandstone-mudstone sections, mudstone samples were selected to measure illite content and characterize diagenetic parameters, and diagenetic strength was evaluated. Simultaneously, typical thin sections, cathodoluminescence, and scanning electron microscopy images were selected to statistically analyze diagenetic parameters such as pore size distribution, pore shape, the ratio of intergranular contact points to the number of grains, and grain contact length. Combined with the evolution of diagenetic minerals and porosity / permeability parameters, a mineral evolution sequence and diagenetic evolution model were established. The effect of fractures on the mechanical properties of deep clastic rocks was analyzed, clarifying the intrinsic relationship between lithology, compression rate, mechanical parameters, and fractures, and elucidating the fracture development mechanism of sandstone-mudstone interbedded sections under the comprehensive control of tectonic-fluid-diagenetic-mechanical properties. Figure 7 ).

[0042] The fifth step involves conducting loading-type synchronous CT scanning experiments and digital core modeling; Loaded synchronous CT scanning experiments were conducted to investigate the fracture evolution process of samples with different lithologies; based on digital core modeling, the evolution of rock porosity and tortuosity at different stress-strain stages was revealed; and from the perspective of the evolution of pore structure during loading, the differential fracture evolution of sandstone and mudstone in different blocks was revealed.

[0043] The aforementioned method, which reveals the differential fracturing evolution of sandstone and mudstone in different blocks from the perspective of pore structure evolution during loading, involves the following specific steps: First, simultaneous CT scanning technology for rock mechanics experiments is used to reveal the fracturing evolution process of sandstone and mudstone. Second, sandstone and mudstone samples are collected, and two sets of representative samples (A and B) with consistent burial depth and rock characteristics are prepared. These rock characteristics include lithology, mineral composition, and heterogeneity. Third, the sample size is reasonably determined based on the range and accuracy requirements of the CT scanning equipment, with a diameter of 2.5-5 mm and a height of 5-10 mm. Fourth, rock mechanics experiments are conducted on sample A, and the stress-strain curves are obtained to measure the compressive strength σ of different samples. c To determine the scanning points for sample B, a Zeiss X-ray microscope was used to select 0 and 0.3σ values ​​respectively. c 0.5σ c 0.7σ c 0.8σ c 0.9σ c and σ c Seven stress points were used as scanning points to conduct synchronous micron-CT scanning experiments on sample B under stress loading conditions, obtaining two-dimensional images of rock samples at different stages and establishing corresponding three-dimensional digital core models; by setting different three-dimensional pixel screening thresholds, a porosity evolution model of the rock under different stress-strain states was established, and the variation laws of porosity and tortuosity at different fracture evolution stages were calculated. Figure 8 The tortuosity mentioned above is used as a parameter to characterize the degree of bending of irregular geometry. The larger the value, the worse the connectivity of pores and fissures. Finally, a dynamic evolution model of rock pores and fissures under stress loading is established to reveal the differential fracturing evolution law of sandstone and mudstone in different blocks from the perspective of the evolution of pore and fissure structure during loading.

[0044] The sixth step is to conduct a multi-scale fracture acoustic emission source localization experiment on composite rock mass under the control of lithological interfaces; Under the constraints of similar material composition, mechanical properties and interlayered structure, layered samples with different sand-mud ratios and thicknesses were prepared by core sample processing and artificial sample preparation. Three-dimensional acoustic emission source localization experiments were carried out to achieve transparent analysis of the fracture development process under different loading rates. From the perspective of lithological interface and sand-mudstone thickness, the vertical propagation of fractures and the development law of fractures penetrating the lithological interface were revealed.

[0045] The steps for achieving transparent analysis of crack development processes under different loading rates are as follows: Given the extreme difficulty in obtaining sandstone-mudstone composite rock masses that meet experimental requirements under natural conditions, making it difficult to analyze the influence of lithological interfaces on rock fracturing and crack propagation, artificial sample preparation is used to investigate the influence of lithological interfaces on crack evolution. The similarity between the artificial sample and the underground sample is ensured from three aspects: material composition, mechanical properties, and interlayer structure. To ensure that the artificial sample and the underground sample have similar material compositions, core samples from different depths are first drilled and broken into fragments of 0.8-1.5 cm. Then, the rock fragments and steel balls are placed in a piston container and ground at high speed using a stirring drill to form a specified rock sample mesh size. The rock sample powder is mixed with epoxy resin and pressed layer by layer using a uniaxial compression tester. Based on previous experience in preparing samples of different strengths, the uniaxial compression strength and time parameters are reasonably set. After compaction, the artificial rock core is removed from the mold and placed in a constant temperature chamber at 80-150°C. The samples were dried at a temperature of ℃ for more than 12 hours, and artificial rock cores were drilled using a wire cutting machine. Mechanical testing was conducted to ensure that the artificial samples had similar mechanical properties to the underground samples. Layered sample preparation schemes with different sand-mud ratios were determined, constrained by the macroscopic profile lithology combination. The influence of the interlayer ratio and thickness on fracture development in sand-mudstone interbedded layers was considered, ensuring that the prepared samples had a similar interbedded lithology to the macroscopic sedimentary lithology. Three types of composite rock mass samples were prepared: sand-mudstone interbedded, mudstone-sand interbedded, and sand-mudstone interbedded. Figure 9 The in-situ stress environment of the rock samples was determined, and the acoustic emission signals of different samples were monitored using a three-dimensional acoustic emission source localization experiment. The evolution of crack propagation under different rock layer thicknesses, sandstone-mudstone interbedded structures, lithological interfaces, and different rock layer thickness ratios was analyzed. The acoustic emission signal processing flow was as follows: by calculating the waveform parameters of the same vibration source, the spatial location of microcrack events was obtained. Based on the local region correlation imaging algorithm of the rock, the rock mass was discretized into a finite element system. By constructing a quantitative relationship between elastic wave velocity variation and rock damage, multi-scale fracture acoustic emission data processing and visualization reconstruction of damage distribution were realized. Finally, the fracture evolution mode of the composite rock mass was interpreted from the perspectives of lithological interfaces, sandstone-mudstone thickness, and interbedded structures.

[0046] The specific steps for mixing rock sample powder with epoxy resin and pressing the sample layer by layer using a uniaxial press are as follows: mix rock sample powder and epoxy resin in a ratio of 100:9, mix manually until uniform, sieve three times, and then press the sample layer by layer.

[0047] Step 7: Clarify the fracture evolution pattern of interbedded sandstone and mudstone under different compression rates; Numerical simulation was adopted to replace physical simulation. Discrete element numerical simulation of large-scale lithological interfaces was carried out, along with loading-type CT scanning experiments and acoustic emission source localization experiments. The relationship between fracture trans-layer mode and mechanical structure and energy accumulation and dissipation was clarified, the evolution stages of fractures were quantitatively divided, and a fracture evolution model controlled by compression rate, mechanical structure and stress-strain was established. Figure 10 ).

[0048] The present invention has been described above by way of example, but the present invention is not limited to the specific embodiments described above. Any modifications or variations made based on the present invention are within the scope of protection claimed by the present invention.

Claims

1. A method for analyzing the genetic mechanism and evolution pattern of fractures in interbedded sandstone and mudstone, characterized in that: The steps to achieve this are as follows: The first step is to determine the loading rate and shortening rate of rock mechanics experiments based on the macroscopic structural patterns; Rock mechanical parameters and fracture thresholds are not only related to the composition of rock samples and in-situ stress, but also controlled by experimental compression rate and formation shortening rate. Macroscopic structural patterns are closely related to structural compression rate and formation shortening rate. Using macroscopic particle flow numerical simulation technology, we can clarify the range of compression rate and formation shortening rate corresponding to various structural patterns, and clarify the influence of compression rate on rock mechanical parameters, stress-strain state and rock fracture mechanism. The second step involves conducting simultaneous digital speckle monitoring during rock mechanics experiments at different loading rates. Sandstone and mudstone samples were collected from different strata, and rock mechanics experiments were conducted at different loading rates with simultaneous digital speckle monitoring. Evolution charts of the mechanical parameters of sandstone and mudstone and the rock fracture patterns and mechanical parameters under different loading rates were established. The third step involves conducting experimental numerical simulations of rock mechanics based on particle flow to elucidate the dependence of different blocks of sandstone and mudstone samples on loading rate. To avoid the uncertainty in interpreting the fracture formation mechanism in rock mechanics experiments with different loading rates and to expand the universality of the loading rate and sandstone-mudstone mechanical response model, this study systematically reveals the dynamic mechanism of fracture evolution under the interaction of rate and lithology based on particle flow rock mechanics numerical simulation technology. Furthermore, it conducts particle flow-based rock mechanics numerical simulations to solidify the universality of the loading rate and sandstone-mudstone fracture response model and further elucidates the dependence of different sandstone-mudstone samples on loading rate. The fourth step reveals the formation mechanism of fractures in the interbedded sandstone and mudstone; By combining the main controlling factors of fractures, we can identify the fracture activity stages, reveal the diagenetic strength corresponding to different layers, investigate the influence of early fractures and their activity on diagenetic strength and rock mechanical parameters, interpret the influence mechanism of loading rate and stratigraphic shortening rate on the mechanical properties and fracture patterns of sandstone and mudstone, and elucidate the development mechanism of interbedded fractures in sandstone and mudstone under the comprehensive control of tectonics, fluids, diagenesis and mechanical properties. The fifth step involves conducting loading-type synchronous CT scanning experiments and digital core modeling; Loaded synchronous CT scanning experiments were conducted to investigate the fracture evolution process of samples with different lithologies; based on digital core modeling, the evolution of rock porosity and tortuosity at different stress-strain stages was revealed; and from the perspective of the evolution of pore structure during loading, the differential fracture evolution of sandstone and mudstone in different blocks was revealed. The sixth step is to conduct a multi-scale fracture acoustic emission source localization experiment on composite rock mass under the control of lithological interfaces; Under the constraints of similar material composition, mechanical properties and interlayered structure, layered samples with different sand-mud ratios and thickness ratios were prepared by core sample processing and artificial sample preparation. Three-dimensional acoustic emission source localization experiments were carried out to achieve transparent analysis of the fracture development process under different loading rates. From the perspective of lithological interface and sand-mudstone thickness, the vertical propagation of fractures and the development law of fractures penetrating the lithological interface were revealed. Step 7: Clarify the evolution pattern of fractures in interbedded sandstone and mudstone under different compression rates; Numerical simulation was adopted to replace physical simulation. Discrete element numerical simulation of large-scale lithological interfaces was carried out, along with loading-type CT scanning experiments and acoustic emission source localization experiments. The relationship between fracture cross-layer mode and mechanical structure and energy accumulation and dissipation was clarified, the evolution stages of fractures were quantitatively divided, and a fracture evolution model under the control of compression rate, mechanical structure and stress-strain was established.

2. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: The first step, which clarifies the influence of the compression rate on rock mechanical parameters, stress-strain state, and rock fracture mechanism, is implemented as follows: First, a basic structural geometric model is established based on typical structural styles in the study area; second, referring to the triaxial mechanical simulation test results of different particles, the initial model parameters for particle flow numerical simulation are set, an initial structural model is established, and transposed into a three-dimensional discrete element elastoplastic model; third, the model is set with the same sedimentary particles to simulate the erosion and decapitation process of strata during structural evolution; finally, the deformation and compression rate of different types of structural units are revealed, laying the foundation for exploring how the compression rate affects rock mechanical parameters, stress-strain state, and rock fracture mechanism.

3. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In the second step, the steps for establishing the evolution charts of mechanical parameters of sandstone and mudstone and rock fracture patterns and mechanical parameters under different loading rates are as follows: Based on the formation shortening rate and stress intensity corresponding to different structural patterns, a set of sandstone and mudstone samples are collected, with n samples in each set, and the sample surfaces are polished. Based on the results of paleostress field simulation of the cross-section, the confining pressure conditions for rock mechanics experiments were determined, and speckle patterns were sprayed onto the samples. First, a white matte primer was sprayed to enhance contrast, and then black speckle patterns were sprayed. Different loading rates were set for each group of n samples, and rock mechanics experiments were carried out at different loading rates with simultaneous digital speckle monitoring. Using digital holographic imaging technology, the light field in a specific three-dimensional space including the target was reconstructed by calculating and simulating the propagation path of light in space. Three-dimensional particle motion tracking was applied, and rock strain imaging was achieved by combining the surface morphology of the rock samples. The Young's modulus, Poisson's ratio, critical fracture threshold, energy release rate, and fracture pattern of different rock samples under different loading rates were obtained, and evolution charts of the mechanical parameters of sandstone and mudstone and the rock fracture patterns and mechanical parameters under different loading rates were established.

4. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 3, characterized in that: The aforementioned collection includes one set each of sandstone and mudstone samples, with n samples in each set, where n ≥ 5, and the sample size is 25 mm. A 50 mm cylinder; the diameter of the sprayed black dots is set to 1 / 50 of the size of the sandstone and mudstone samples.

5. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In the third step, the numerical simulation of rock mechanics experiments based on particle flow is implemented in the following ways: First, a parallel bonding model or a contact bonding model is used to characterize the cementation effect between particles, and microscopic mechanical parameters are assigned, including normal and tangential stiffness and bond strength. Second, the macroscopic mechanical responses obtained from laboratory experiments on sandstone and mudstone blocks in different blocks are used as constraints, and the microscopic parameter assignment scheme is adjusted by combining the inverse iteration method to ensure the consistency between the numerical model and the mechanical behavior of rock physics experiments. The macroscopic mechanical responses include elastic modulus and peak strength. Third, a displacement control or stress control mode is adopted, based on servo... The loading rate was adjusted in real time by the control system to ensure that the experimental process met the quasi-static conditions and to avoid interference from dynamic effects. The loading rate was determined by deformation sensitivity analysis and characterized in the form of strain rate, with time-step adaptive adjustment to ensure numerical stability. Finally, the particle displacement, contact force chain evolution, and bond fracture events were monitored in real time, and stress-strain curves, fracture modes, and energy dissipation characteristics were recorded. By comparing the mechanical responses of sandstone and mudstone samples from different loading rates and different blocks, and combining the energy dissipation spectrum, the changes in the proportions of elastic energy storage, bond fracture energy, and frictional dissipation were distinguished, further clarifying the dependence of different blocks of sandstone and mudstone samples on the loading rate.

6. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: The fourth step, elucidating the development mechanism of interbedded sandstone and mudstone fractures under the comprehensive control of tectonic-fluid-diagenetic-mechanical properties, involves the following specific steps: Referring to regional tectonic evolution, stress field evolution, and the main controlling factors of fractures, the evolution mechanism of fracture patterns and mechanical property differences in sandstone and mudstone under different loading rates is interpreted from the perspectives of fracture development stages and diagenetic intensity. First, typical fracture-intrusive vein or hydrothermal vein samples are collected from different layers. The geometric morphology, cross-cutting and confinement relationships, and diagenetic mineral types and structures of micron- to nanon-scale micro-fractures are observed. Fluid inclusion analysis technology in fracture filling materials is used to conduct petrographic observation of fluid inclusions in typical fracture veins, studying their types and compositions, determining the homogenization temperature and freezing point temperature of fluid inclusions, calculating the fluid inclusion trapping pressure in fracture filling materials, revealing the trapping stages of fluid inclusions, and analyzing the differences in trapping temperatures at each stage. Single-well analysis is then conducted. The burial-uplift-thermal evolution history was simulated and reconstructed to determine the temperature and pressure evolution paths of different sandstone-mudstone interbedded sections and to identify the differences in the stages of fracture activity in different sandstone-mudstone sections. Combining the results of rock mechanics experiments at different loading rates with the stages of fracture activity in different sandstone-mudstone sections, mudstone samples were selected to measure illite content and characterize diagenetic parameters, and diagenetic strength was evaluated. At the same time, typical thin sections, cathodoluminescence, and scanning electron microscopy images were selected to statistically analyze diagenetic parameters such as micropore size distribution, pore shape, the ratio of the number of intergranular contact points to the number of grains, and grain contact length. Combined with the evolution of diagenetic minerals and porosity-permeability parameters, a mineral evolution sequence and diagenetic evolution model were established. The effect of fractures on the mechanical properties of deep clastic rocks was analyzed, and the intrinsic relationship between lithology, compression rate, mechanical parameters, and fractures was identified, clarifying the fracture development mechanism of sandstone-mudstone interbedded sections under the comprehensive control of tectonics, fluid, diagenesis, and mechanical properties.

7. The method for analyzing the genetic mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In the fifth step, the specific steps for revealing the differential fracturing evolution of sandstone and mudstone in different blocks from the perspective of the evolution of pore structure during loading are as follows: Using simultaneous CT scanning technology for rock mechanics experiments, the fracturing evolution process of sandstone and mudstone is revealed; firstly, sandstone and mudstone samples are collected, and two sets of representative samples, A and B, with consistent burial depth and rock characteristics are prepared. The rock characteristics include lithology, mineral composition, and heterogeneity; based on the range and accuracy requirements of the CT scanning equipment, the sample size is reasonably determined, with a diameter of 2.5-5 mm and a height of 5-10 mm; rock mechanics experiments are conducted on sample A, and the rock stress-strain curve is obtained based on the experiment, measuring the compressive strength σ of different samples. c To determine the scanning points for sample B, a Zeiss X-ray microscope was used to select 0 and 0.3σ values ​​respectively. c 0.5σ c 0.7σ c 0.8σ c 0.9σ c and σ c Seven stress points were used as scanning points to conduct synchronous micron-CT scanning experiments on sample B under stress loading conditions, acquiring two-dimensional images of rock samples at different stages and establishing corresponding three-dimensional digital core models. By setting different three-dimensional pixel screening thresholds, pore evolution models of rocks under different stress-strain states were established, and the variation laws of porosity and tortuosity at different fracture evolution stages were calculated. The tortuosity is used as a parameter to characterize the degree of bending of irregular geometry, and the larger the value, the worse the pore connectivity. Finally, a dynamic evolution model of rock pores under stress loading conditions was established, revealing the differential fracture evolution law of sandstone and mudstone in different blocks from the perspective of pore structure evolution during loading.

8. The method for analyzing the genetic mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In the sixth step, the steps to achieve transparent analysis of the crack development process under different loading rates are as follows: Given that it is extremely difficult to obtain sandstone-mudstone composite rock masses that meet the experimental requirements in natural conditions, it is difficult to analyze the influence of lithological interfaces on rock fracture and crack propagation; artificial sample preparation is adopted to explore the influence of lithological interfaces on crack evolution; the similarity between artificial samples and underground samples is ensured from three aspects: material composition, mechanical properties, and interlayer structure. To ensure that the artificially prepared samples have a similar material composition to the underground samples, core samples from different depths were first drilled and broken into fragments of 0.8-1.5 cm. These fragments, along with steel balls, were then placed in a piston container and ground at high speed using a stirring drill to achieve the specified mesh size. The rock sample powder was mixed with epoxy resin and then pressed layer by layer using a uniaxial compression tester. Based on previous experience preparing samples of different strengths, the uniaxial compression strength and time parameters were rationally set. After compaction, the artificial core samples were removed from the mold and placed in a constant temperature chamber at 80-150°C. The samples were dried at a temperature of ℃ for more than 12 hours, and artificial rock cores were drilled using a wire cutting machine. Mechanical testing was conducted to ensure that the artificially prepared samples had similar mechanical properties to the underground samples. Layered sample preparation schemes with different sand-mud ratios were determined, constrained by the macroscopic profile lithological assemblage. The influence of the interlayer ratio and thickness on fracture development in sand-mudstone interbedded layers was considered, ensuring that the prepared samples had a similar interbedded lithological structure to the macroscopic sedimentary lithology. Three types of composite rock mass samples were prepared: sand-mudstone interbedded, mudstone interbedded, and sand-mudstone interbedded. The in-situ stress environment of the rock samples was determined, and a three-dimensional acoustic emission source localization experiment was used to monitor the stress environment. The acoustic emission signals of different samples were analyzed to study the crack propagation and evolution patterns under the interlayered structure of sandstone and mudstone, lithological interfaces, and different rock layer thickness ratios. The acoustic emission signal processing workflow was as follows: by calculating the waveform parameters of the same vibration source, the spatial location of microcrack events was obtained; based on the local region correlation imaging algorithm of the rock, the rock mass was discretized into a finite element system; by constructing a quantitative relationship between elastic wave velocity variation and rock damage, multi-scale fracture acoustic emission data processing and visualization reconstruction of damage distribution were realized; finally, the crack evolution mode of the composite rock mass was interpreted from the perspectives of lithological interfaces, sandstone and mudstone thickness, and interlayered structure.

9. The method for analyzing the formation mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In the sixth step, the rock sample powder is mixed with epoxy resin adhesive, and the sample is pressed layer by layer using a uniaxial press. The specific steps are as follows: the rock sample powder and epoxy resin adhesive are mixed at a ratio of 100:9, manually kneaded until uniform, and then pressed layer by layer after sieving three times.

10. The method for analyzing the genetic mechanism and evolution pattern of interbedded sandstone and mudstone fractures according to claim 1, characterized in that: In step seven, the specific implementation steps for establishing a fracture evolution model under the control of compression rate, mechanical structure, and stress-strain are as follows: A numerical simulation method is used in place of physical experiments to systematically analyze the control mechanism of large-scale lithological interfaces and rock combination patterns on fracture propagation; based on the results of loaded synchronous CT scans and mechanical experiments at different loading rates, a large-scale elastoplastic damage discrete element model containing lithological interfaces is constructed using discrete element software and its FISH compiler to achieve three-dimensional reconstruction of the stress field in a single well. This reveals the regulatory laws of large-scale lithological interface geometric parameters and combination configurations on the nucleation mechanism, propagation path, and stress accumulation release of tectonic fractures, establishing a quantitative relationship between rock fracture and lithological interfaces, strain energy stage evolution, and single-layer / interlayer strength differences; through comparative analysis with a single-well discrete network model, the rock fracture evolution stage of the fracture is clarified by comprehensively considering the tectonic deformation shortening, compression rate, and fracture scale distribution in each well area, establishing the evolution law of sandstone-mudstone interbedded fractures under the control of compression rate, mechanical structure, and stress-strain.

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