Method and system for researching seepage characteristics of fracture surface under stress action

Abstract

The invention discloses a method and a system for researching seepage characteristics of a fracture surface under the action of stress, and relates to the technical field of rock mechanics and geological energy storage. The method comprises the following steps: selecting different lithologic samples containing continuous fracture surfaces for manufacturing samples, and measuring and summarizing fracture surface types of the samples and corresponding roughness difference characteristics; reconstructing a crack surface sample with natural crack surface characteristics of the sample by adopting a 3D printing technology; carrying out 3D printing on a cylinder sample containing a crack surface, and carrying out a permeability change test experiment under effective stress on the cylinder sample; and carrying out fracture seepage numerical simulation calculation on the cylinder sample based on a lattice Boltzmann method. The invention further provides a system for researching the seepage characteristics of the fracture surface under the stress action, and the numerical simulation of the correlation between the permeability change and roughness characteristics of the fracture surface under different stress actions is carried out according to the roughness and seepage rules of the fracture surface under the stress action based on a new method of combining the 3D printing fracture surface with digital core simulation.

Description

Technical Field

[0001] The present invention relates to the field of rock mechanics and geological energy storage technology, and more specifically, to a method and system for studying seepage characteristics of a fracture surface under stress. Background Art

[0002] In the process of geological energy storage, the surface roughness of fractures significantly affects the seepage characteristics. In the field of porous media seepage, a large number of correction formulas for fracture surface roughness have been proposed to consider the deviation between the calculation formula and the actual data.

[0003] Rough surfaces reduce seepage efficiency by increasing flow resistance and inducing non-Darcy flow, exacerbating conductivity degradation under cyclic stress and impacting energy storage cycle stability. Furthermore, the enhanced capillary effect of rough surfaces can trap non-wetting phases, improving storage safety, but potentially inducing multiphase crossflow risks. Rough fractures also optimize heat transfer or accelerate mineral dissolution-precipitation processes by increasing heat exchange area and chemical reaction active sites. However, permeability anisotropy can lead to preferential fluid flow, reducing reservoir utilization. In practical applications, it is necessary to combine multiphysics coupled models to optimize fracture roughness parameters and injection strategies to balance energy storage efficiency, long-term stability, and safety.

[0004] The distribution and size of crack surface roughness are random and disordered. How to accurately obtain crack roughness remains a problem. In addition to the crack joint roughness coefficient, fractal dimension, and crack surface undulation difference used to evaluate the crack surface roughness, there are currently three methods available to more accurately describe the crack surface morphology: The first is to combine the roughness coefficient (JRC) and fractal dimension. For example, based on the characteristics that the crack contour line satisfies the fractal characteristics, a mathematical model of fractal dimension and roughness coefficient is constructed; the second is to reduce the order of the three-dimensional rough surface of the crack, and use the power spectral density method to establish the fractal dimension and root mean square roughness of the first-order undulation or second-order protrusion respectively; the third is to convert the crack rough surface into a series of rough unit bodies with fractal characteristics, and then accumulate the calculation of the unit bodies.

[0005] Existing research has examined the correlation between fracture surface roughness and permeability, and established fracture surface permeability models for different lithologies based on seepage mechanisms. However, most work still uses flat plate models, using single parameters such as aperture or fractal dimension to simulate random fracture surfaces while ignoring the effects of stress. This approach lacks a systematic understanding of fracture surface morphology, geometric parameters, and seepage characteristics.

[0006] In addition, traditional fracture seepage research relies on natural cores or artificial synthetic models, which have problems such as uncontrollable fracture roughness, low model accuracy, and poor experimental repeatability. In addition, existing mathematical models often ignore stress and multi-factor coupling effects, resulting in a disconnect between theory and practical applications.

[0007] Finally, although 3D printing technology has been initially applied to rock model construction, existing methods still have limitations in fracture morphology restoration, wettability regulation, and stress dynamic response analysis. There is an urgent need for a high-precision, systematic research method to reveal the fracture seepage mechanism under stress. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a method and system for studying the seepage characteristics of fracture surfaces under stress, so as to solve the technical problems that current rock physics experiments cannot be repeated and the microstructure cannot be controlled.

[0009] To achieve the above objectives, the first objective of the present invention is to provide a method for studying the seepage characteristics of a fracture surface under stress, the method comprising the steps of: Step S1: Select different lithology samples with continuous fracture surfaces to prepare specimens, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the specimens; Step S2: reconstructing a crack surface sample of the sample having natural crack surface characteristics by using 3D printing technology; Step S3: 3D printing a cylindrical specimen with a crack surface, and conducting a permeability change test experiment under effective stress on the cylindrical specimen; Step S4: performing numerical simulation calculation of fracture seepage on the cylindrical specimen based on the lattice Boltzmann method.

[0010] Preferably, in step S1, the specific method of measuring and summarizing the crack surface type and corresponding roughness difference characteristics of the sample includes the following steps: Step S 11 : Use a 3D scanner to scan the crack surface of the sample to obtain 3D surface structure data; Step S 12 : The roughness of the crack surface of the sample is calculated and analyzed by combining the two-order roughness coefficient and fractal dimension method.

[0011] Preferably, in step S2, the method of reconstructing and replicating the physical model of the sample having natural fracture surface characteristics using 3D printing technology comprises the steps of: Step S 21 : The digital model of the crack surface is obtained by three-dimensional image scanning; Step S 22 : Laser sintering powder technology and light-curing resin technology are used to pre-process the digital model of the crack surface; Step S 23 : 3D printing a crack surface sample of the crack surface digital model, and cleaning the support material on the crack surface sample after printing is completed; Step S24 : The crack surface of the 3D printed crack surface sample was chemically coated using a silane coupling agent; Step S 25 : Check the consistency of the fracture surface of the fracture surface sample, and perform CT scanning imaging again to obtain a digital model of the fracture surface imaging, and compare the digital model of the fracture surface imaging with the digital model of the natural fracture surface; If the comparison is consistent, the process proceeds to step S3; if the comparison is inconsistent, the 3D printing parameters and processing method are adjusted and the 3D printing is performed again.

[0012] Preferably, in step S3, the 3D printing of the cylindrical specimen containing the crack surface comprises the following specific steps: Step S 31 : The crack surface is designed independently through MATLAB software, and the crack random generation open source program is used; Step S 32 : Define crack parameters, and randomly simulate and generate two crack surfaces through the open source program, and the first-order roughness of the two crack surfaces is consistent, and the second-order roughness of the two crack surfaces is different; Wherein: the crack parameters include crack opening, roughness coefficient and fractal dimension; Step S 33 : 3D printing of cylindrical specimens with crack surfaces.

[0013] Preferably, in step S3, the specific steps of conducting the permeability change test experiment under effective stress include: Step S 34 : A 3D-printed cylindrical sample with a fracture surface is placed in a core holder, a hand pump is connected to the core holder, and a stress with gradually increasing intervals t is applied to the cylindrical sample by the hand pump; Step S 35 : After each interval t, the pressure is stable for 30 minutes, then the valve is closed and the pipeline is disassembled; Step S 36 : Performing CT scanning on the pressurized cylindrical sample to obtain a digital image of the cylindrical sample; Step S 37 : A pressure-controlled permeability experimental system was used to carry out a permeability change test experiment under effective stress on the cylindrical sample.

[0014] Preferably, in step S4, the method for carrying out numerical simulation calculation of fracture seepage includes: Step S 41 : The 3D digital model of the cracks obtained by CT scanning under stress with gradually increasing interval time t was imported into the Palabos open source program based on the lattice Boltzmann method; Step S 42 : Calculate the macroscopic density and macroscopic velocity of the fluid in the fracture; Wherein: the calculation expression of the macroscopic density of the fluid in the crack is:

[0015] The calculation expression of the macroscopic velocity of the fluid in the fracture is:

[0016] Step S 43 : The fracture permeability is obtained by the average seepage velocity, fluid viscosity and average pressure gradient.

[0017] Preferably, in step S 42 The governing equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture include: For the fluid flow problem in three-dimensional fractures, the Bhatnagar–Gross–Krook single relaxation model is used, and the particle velocity evolution equation can be expressed as

[0018] Where: is the particle distribution function; is the time step; is the discrete speed; is the dimensionless relaxation time; is the particle equilibrium distribution function; For each grid point, a different discrete velocity direction is given, .

[0019] Preferably, in step S 42 Among them, the control equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture also include: The D3Q19 model is used to simulate fracture surface seepage, and its corresponding particle equilibrium distribution function is defined as:

[0020] Where: is the particle equilibrium distribution function; for Directional weight coefficient; is the density of the fluid grid; is the discrete speed; is the lattice sound speed; is the velocity of the fluid grid; Among them, the lattice sound speed is the voxel side length of the digital model of the crack surface, is the time step.

[0021] Preferably, in step S 37 Among them, the pressure-controlled permeability experimental system includes: The loading system includes an annular oil cylinder for clamping the specimen, a central oil cylinder located at the center of the annular oil cylinder and directly above the specimen, a central pressure rod connected to the central oil cylinder and directly above the specimen, a pressure-controlled triaxial test bench, and an oil pump respectively connected to the annular oil cylinder and the central oil cylinder, the central oil cylinder and the central pressure rod are used to provide cyclic axial pressure to the specimen, and the annular oil cylinder is used to provide cyclic annular pressure to the specimen; the annular oil cylinder, the central oil cylinder, the central pressure rod and the specimen are all arranged in the pressure-controlled triaxial test bench, and the oil pump pumps pressure to the annular oil cylinder and the central oil cylinder under the action of the pressure-controlled triaxial test bench; An air injection system, wherein the air outlet of the air injection system is connected to the air inlet of the loading system through a high-pressure air inlet pipe; A permeability testing system comprising a gas flow measurement device connected to the gas outlets of the annular cylinder and the central cylinder via a high-pressure gas pipe; The pressure and temperature controller is used to adjust the inflation pressure and temperature in the pressure-controlled triaxial test bench according to the set pressure and temperature.

[0022] A second object of the present invention is to provide a system for studying the seepage characteristics of fracture surfaces under stress, for implementing the above-mentioned method for studying the seepage characteristics of fracture surfaces under stress. The system comprises: The fracture surface roughness characterization module is used to select different lithology samples with continuous fracture surfaces to make specimens, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the specimens; A fracture surface 3D printing model reconstruction module is used to reconstruct a fracture surface sample of the sample having natural fracture surface characteristics using 3D printing technology; 3D printing cracked surface permeability test module, used for 3D printing cylindrical specimens with cracked surfaces and conducting permeability change test experiments under effective stress on the cylindrical specimens; The fracture surface seepage numerical simulation module is used to perform fracture seepage numerical simulation calculations on the cylindrical specimen based on the lattice Boltzmann method. Compared with the prior art, the present invention has the following advantages and effects: 1. The research method of the seepage characteristics of the fracture surface under stress in this application focuses on the morphological characteristics, seepage capacity and influencing factors of the rough fracture surface under effective stress. A comprehensive method of 3D printing fracture experimental simulation combined with digital core seepage simulation is adopted. First, the types and roughness difference characteristics of the fracture surfaces of different rock types are measured and summarized, and the physical model of the natural fracture surface characteristics is reconstructed and replicated using 3D printing technology; the experimental method processing technology, parameter adjustment, and model improvement of the 3D printed fracture surface are developed, and their consistency is compared with the natural fracture surface; artificially designed simulated fracture surfaces are used to 3D print out a fracture surface physical model that systematically controls the fracture surface morphological parameters, and measure its permeability change law; combined with CT scanning and fracture surface numerical simulation, the flow channel of the rough fracture surface is intuitively visualized and studied, and the relationship between its flow characteristics and the rough characteristics of the fracture is analyzed. The mathematical relationship model of seepage is verified and improved in combination with the experimental simulation results; finally, the seepage law and influencing factors of the rough fracture surface under effective stress are summarized.

[0023] 2. The embodiment of the present invention is based on a new method of combining 3D printing of fracture surfaces with digital core simulation. The roughness of the fracture surface under stress and the seepage law are used to carry out numerical simulation of the relationship between the permeability change and roughness characteristics of the fracture surface under different stresses. Based on the simulation and experimental results, the theory of rock mechanics and seepage mechanics is applied to establish a mathematical model of porous media seepage under stress considering the roughness of the fracture. Combined with the reconstruction of the digital core model and numerical simulation, 3D printing technology and procedures for experimental research on fractured porous media are developed, providing new technical means for the microstructural characterization, mechanical characteristic analysis, and seepage capacity analysis of fractured porous media. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the process of the method for studying the seepage characteristics of the fracture surface under stress in an embodiment of the present invention; Figure 2 This is a schematic diagram of a specific process of step S1 in an embodiment of the present invention; Figure 3 This is a schematic diagram of a specific process of step S2 in an embodiment of the present invention; Figure 4 This is a schematic diagram of a specific process of step S3 in an embodiment of the present invention; Figure 5 This is a schematic diagram of a specific process of step S4 in an embodiment of the present invention; Figure 6 Schematic diagram comparing the original Berea sandstone CT model, the 3D printed prototype digital model, and the 3D printed actual sample models obtained by three different methods in an embodiment of the present invention; Figure 7 Schematic diagram comparing the pore structure of 3D printed porous rock, original sandstone, and prototype digital model in an embodiment of the present invention; Figure 8 Schematic diagram of the fracture-pore connection model in an embodiment of the present invention; Figure 9 Schematic diagram of the structure of a system for studying seepage characteristics of fracture surfaces under stress in an embodiment of the present invention; Figure 10 Schematic diagram of the structure of the pressure-controlled permeability experimental system in an embodiment of the present invention; Figure 11 Schematic diagram of calculation of average roughness Ra in an embodiment of the present invention; Figure 12 Schematic diagram of relative frequency distribution of crack pore volume distribution of samples in an embodiment of the present invention; Figure 13 This is a 3D printed schematic diagram of the frequency distribution of crack pore radius of the sample in an embodiment of the present invention.

[0025] Description of reference numerals: 1-gas injection system; 11-nitrogen tank; 12-pressure gauge; 2-loading system; 21-annular oil cylinder; 22-center oil cylinder; 23-center pressure rod; 24-pressure-controlled triaxial test bench; 25-oil pump; 3- Permeability test system; 31- Gas flow measurement equipment; 4-pressure and temperature controller; 5-cylindrical specimen; 100- Fracture surface roughness characterization module; 200- Fracture surface 3D printing model reconstruction module; 300- 3D printing fracture surface permeability experiment module; 400- Fracture surface seepage numerical simulation module. DETAILED DESCRIPTION

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention. In describing the present invention, it should be noted that the terms "include" and "comprising" as used herein should be understood as inclusive and open-ended, rather than exclusive. Specifically, when the terms "include" and "comprising" and their synonyms are used in the specification and claims, they indicate the inclusion of the specified features, steps, or components. These terms should not be interpreted as excluding the presence of other features, steps, or components.

[0027] Thanks to its advantages of high preparation efficiency, wide range of optional materials, and ability to replicate complex and fine structures, 3D printing technology has gradually been introduced into the fields of geological resources and geological engineering research, especially in experiments such as the replication, reconstruction, and testing of rock porous media models. Existing research on the design and experimentation of 3D printed fracture surface physical models can be divided into three main approaches: Fused deposition method: Directly three-dimensionally prints the matrix part of the sample in layers, leaving cracks or pore spaces. However, the problem with this method is that there is no support during the solidification process of the printed material from semi-solid to solid in the specific direction of the crack, resulting in final structural deformation, which is different from the designed crack model morphology.

[0028] The second method is to spray the binder onto the flattened powder material, skipping the pore space part according to the designed path, and then remove the unbound powder after all is completed. However, the accuracy of this method is not high and depends on the powder particle size sorting. The third method involves filling the cracks with a soluble substance, such as wax, during printing, which is then dissolved and removed after the sample is formed. This method requires careful attention to ensure that no residual filler material remains in the complex pore structure, as this could affect the experimental process and results.

[0029] To solve the above technical problems, please refer to Figure 1-13 As shown, in one embodiment, the present invention provides a method for studying the seepage characteristics of a fracture surface under stress, the method comprising the steps of: Step S1: Select different lithology samples with continuous fracture surfaces to prepare specimens, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the specimens; Step S2: reconstructing a crack surface sample of the sample having natural crack surface characteristics by using 3D printing technology; Step S3: 3D printing a cylindrical specimen 5 containing a crack surface, and conducting a permeability change test experiment under effective stress on the cylindrical specimen 5; Step S4: performing numerical simulation calculation of fracture seepage on the cylindrical specimen 5 based on the lattice Boltzmann method.

[0030] In the technical solution of the embodiment of the present invention, focusing on the morphological characteristics, seepage capacity and influencing factors of rough fracture surfaces under effective stress, a comprehensive method of 3D printing fracture experimental simulation combined with digital core seepage simulation is adopted. First, the types and roughness difference characteristics of fracture surfaces of different lithologies are measured and summarized, and the physical model that replicates the characteristics of natural fracture surfaces is reconstructed using 3D printing technology; experimental method processing technology, parameter adjustment, and model improvement of 3D printed fracture surfaces are developed, and their consistency is compared with natural fracture surfaces; artificially designed simulated fracture surfaces are used to 3D print out a fracture surface physical model that systematically controls the fracture surface morphological parameters, and its permeability change law is measured; combined with CT scanning and fracture surface numerical simulation, the flow channel of the rough fracture surface is intuitively visualized and studied, and the relationship between its flow characteristics and the rough fracture characteristics is analyzed. The mathematical relationship model of seepage is comprehensively verified and improved with the experimental simulation results; finally, the seepage law and influencing factors of the rough fracture surface under effective stress are summarized.

[0031] Therefore, the embodiment of the present invention is based on a new method of combining 3D printing of fracture surfaces with digital core simulation. The roughness of the fracture surface under stress and the seepage law are used to carry out numerical simulation of the relationship between the permeability change and roughness characteristics of the fracture surface under different stresses. Based on the simulation and experimental results, the theory of rock mechanics and seepage mechanics is applied to establish a mathematical model of porous medium seepage under stress considering the fracture roughness. Combined with the reconstruction of the digital core model and numerical simulation, 3D printing technology and procedures for experimental research on fractured porous media are developed, providing new technical means for the microstructural characterization, mechanical characteristic analysis, and seepage capacity analysis of fractured porous media.

[0032] It should be explained that the crack surface roughness refers to the unevenness of the crack surface contour and tiny protrusions.

[0033] More specifically, see Figure 2 As shown, in a preferred embodiment of the present invention, in step S1, the specific method of measuring and summarizing the crack surface type and the corresponding roughness difference characteristics of the sample includes the following steps: Step S 11 : Use a 3D scanner to scan the crack surface of the sample to obtain 3D surface structure data.

[0034] In this step, the crack surface of the specimen is first scanned using a 3D scanner. The 3D scanner acquires spatial information of the crack surface by emitting light (such as laser) and receiving reflected light.

[0035] Using a 3D scanner, the specimen is scanned from various angles to fully capture the three-dimensional structural features of the crack surface. The coordinate data of each surface point is recorded to generate 3D surface structure data. This data, which includes details such as the crack surface shape and concavity, provides the raw data foundation for subsequent roughness analysis.

[0036] The use of a 3D scanner can accurately capture the three-dimensional structural data of the crack surface. Compared with traditional two-dimensional measurement methods, it can more comprehensively reflect the true geometry of the crack surface, providing a data foundation for subsequent roughness analysis. This precise data acquisition method helps to improve the accuracy of the understanding of crack surface characteristics and avoid analytical errors caused by missing or inaccurate data.

[0037] Step S 12 : The roughness of the crack surface of the sample is calculated and analyzed by combining the two-order roughness coefficient and fractal dimension method.

[0038] This step uses a two-order roughness coefficient method. The first-order roughness coefficient reflects the larger-scale roughness characteristics of the fracture surface and is determined by calculating the height variation of the fracture surface within a macroscopic range. This involves statistically analyzing the height values ​​of large areas in the 3D scan data, such as calculating indicators such as average height deviation.

[0039] The second-order roughness coefficient focuses on the surface roughness at a finer scale, usually analyzing the surface texture and tiny undulations in the microscopic area, such as by calculating the height difference between adjacent points or using a specific filtering algorithm to extract detail information and quantify the roughness.

[0040] At the same time, the fractal dimension method is introduced. It is important to note that fractal dimension is a measure of the complexity and irregularity of an object. For a crack surface, its fractal dimension can reflect the complexity of the surface. By converting the three-dimensional data of the crack surface into a fractal parameter model and calculating its fractal dimension, the roughness of the crack surface can be quantitatively analyzed in another dimension.

[0041] Finally, the calculation results of the second-order roughness coefficient and fractal dimension are combined to comprehensively analyze the roughness difference characteristics of the specimen crack surface and fully characterize the roughness state of the crack surface.

[0042] In this embodiment, three typical samples containing continuous fracture surfaces were selected from conventional sandstone, carbonate rock, and shale cores, respectively. For each type of sample, the two halves of the spliced ​​sample were cut into specimens with a length of 5.08 cm and a width of 2.54 cm. The fracture surfaces of the samples were then scanned using a high-resolution three-dimensional scanner to obtain three-dimensional surface structure data, which provided a basis for digital reconstruction and consistency verification of 3D printed fracture surfaces. Finally, the roughness of the fracture surfaces of the samples was quantitatively analyzed. Specifically, by adopting a two-order roughness coefficient combined with a fractal dimension method, the correlation and variation function between different parameters were analyzed, and the fracture surfaces and heterogeneity of different rock types were summarized and compared.

[0043] The combination of two-order roughness coefficients and fractal dimension allows for analysis of fracture surface roughness at different scales and angles. The two-order roughness coefficients reflect the degree of roughness at both the macro and micro scales, respectively, while the fractal dimension further quantifies surface complexity and irregularities, resulting in a more comprehensive and accurate characterization of fracture surface roughness. This multi-method approach can provide a deeper understanding of the roughness characteristics of fracture surfaces, providing more reliable roughness data for related research and applications.

[0044] More specifically, see Figure 3 As shown, in a preferred embodiment of the present invention, in step S2, the method of reconstructing the fracture surface sample of the sample having natural fracture surface characteristics by using 3D printing technology includes the steps of: Step S 21 : The digital model of the crack surface is obtained by three-dimensional image scanning; In this step, a 3D scanning device is used to perform a comprehensive, high-precision scan of the specimen's fracture surface, capturing detailed information such as its geometry and texture. This generates a digital model of the fracture surface. This provides a precise digital basis for subsequent 3D printing, ensuring that the printed fracture surface sample can reproduce the characteristics of the natural fracture surface to the greatest extent possible.

[0045] Step S 22 : Laser sintering powder technology and light-curing resin technology are used to pre-process the digital model of the crack surface; In this step, laser sintering powder technology uses a laser to sinter powdered materials into a solid form, helping to establish the basic shape and some details of the crack surface. Meanwhile, photocurable resin technology uses ultraviolet light to rapidly solidify the resin material, further refining the structure of the crack surface. This preprocessing process optimizes the digital model's printing parameters, improving print quality and efficiency while also resolving issues with support and precision that may arise during the printing process.

[0046] Step S 23: 3D printing a crack surface sample of the crack surface digital model, and cleaning the support material on the crack surface sample after printing is completed; In this step, a 3D printer is used to deposit powdered or photosensitive resin materials layer by layer based on the preprocessed digital model of the fracture surface, creating a fracture surface sample with the characteristics of a natural fracture surface. After printing, the support material on the fracture surface sample is removed to remove the auxiliary support added during the printing process to ensure structural integrity and stability, giving the fracture surface sample an appearance and morphology that is more similar to its natural state.

[0047] Step S 24 : The crack surface of the 3D printed crack surface sample was chemically coated using a silane coupling agent; In this step, a silane coupling agent is used to chemically coat the 3D-printed crack surface. This agent forms a chemical coating on the crack surface, improving its physical and chemical properties, such as hardness, wear resistance, and corrosion resistance. This enhances the sample's stability and service life. It may also affect properties such as wettability, providing more suitable surface properties for subsequent research or applications.

[0048] Step S 25 : Check the consistency of the fracture surface of the fracture surface sample, and perform CT scanning imaging again to obtain a digital model of the fracture surface imaging, and compare the digital model of the fracture surface imaging with the digital model of the natural fracture surface; If the comparison is consistent, the process proceeds to step S3; if the comparison is inconsistent, the 3D printing parameters and processing method are adjusted and the 3D printing is performed again.

[0049] In this step, the consistency of the crack surface of the crack surface sample is checked, including whether the size, shape, roughness and other aspects are consistent with the natural crack surface. Then CT scanning imaging is performed again to obtain a digital model of the crack surface imaging, and it is compared with the digital model of the natural crack surface. CT scanning can obtain detailed structural information of the interior and surface of the sample. By comparing the two digital models, it can be accurately judged whether the 3D printed crack surface sample accurately restores the characteristics of the natural crack surface. If the comparison is consistent, it means that the 3D printing is successful, and the printed crack surface sample is highly similar to the natural crack surface in structure and characteristics, meeting the requirements of subsequent research or application, and entering step S3; if the comparison is inconsistent, the cause is analyzed, which may be unreasonable printing parameter settings, inadequate pretreatment, etc., and then the 3D printing parameters and processing methods are adjusted, and 3D printing is repeated until a crack surface sample that meets the requirements is obtained.

[0050] Using the method in step S2, an accurate digital model of the natural fracture surface is obtained through 3D scanning. This is then combined with laser sintering powder technology and light-curing resin technology for pre-processing, followed by 3D printing and subsequent cleaning, chemical coating, and other fine processing steps to produce a highly accurate and realistic fracture surface sample. This sample can more realistically reflect the physical structure and surface properties of the natural fracture surface, providing a reliable experimental model for geological research, material performance testing, and other related fields.

[0051] When the structure and properties of 3D-printed fracture surface samples are highly consistent with natural fracture surfaces, the research and test results using them will be more representative and can more accurately simulate the fracture surface behavior and related physical and chemical processes in actual geological environments, such as fluid seepage and crack expansion, thereby providing more reliable data support and technical basis for geological disaster prediction, oil and gas exploration and exploitation, underground engineering design, etc.

[0052] During the inspection and comparison process, if differences are found between the printed fracture surface samples and the natural fracture surface, the performance and quality of 3D printing technology in preparing fracture surface samples can be continuously improved by adjusting and optimizing the 3D printing parameters and processing methods. This will help promote the application and development of 3D printing technology in geological simulation and related fields, and also provide experience for 3D printing of other complex structures.

[0053] More specifically, see Figure 4 As shown, in a preferred embodiment of the present invention, in step S3, the 3D printing of the cylindrical specimen 5 containing the crack surface specifically comprises the following steps: Step S 31 : The crack surface is independently designed through MATLAB software, and the crack is randomly generated using an open source program.

[0054] In this step, the fracture surface was independently designed using MATLAB software. Leveraging its powerful numerical calculation and graphics processing capabilities, combined with an open-source program for random fracture generation, this provided the data foundation and design ideas for subsequent fracture surface simulations. This open-source program allowed for the introduction of randomness into the design process, simulating the complexity and diversity of fracture surfaces found in nature.

[0055] Step S 32 : Define crack parameters, and randomly simulate and generate two crack surfaces through the open source program, and the first-order roughness of the two crack surfaces is consistent, and the second-order roughness of the two crack surfaces is different; Wherein: the crack parameters include crack opening, roughness coefficient and fractal dimension.

[0056] In this step, fracture parameters are defined, including fracture aperture, roughness coefficient, and fractal dimension. These parameters determine the geometric characteristics and physical properties of the fracture surface. Two fracture surfaces are randomly generated using an open-source program. Their first-order roughness is set to be the same, but their second-order roughness is different. The first-order roughness reflects the macroscopic surface undulations of the fracture surface, while the second-order roughness reflects the microscopic fine texture characteristics. This setting can simulate fracture surfaces with different geological conditions or different origins, making the generated fracture surface samples targeted and representative.

[0057] Step S 33 : 3D printed cylindrical specimen with crack surface 5.

[0058] In this step, the simulated crack surface data is 3D printed to obtain a cylindrical specimen 5 containing a crack surface, where the length of the cylindrical specimen 5 is 5.08 cm and the diameter is 2.54 cm. In this way, 3D printing technology can convert the virtual crack surface model into a physical specimen, realizing the transformation from digital design to physical entity, and providing actual test samples for subsequent experimental research.

[0059] Thus, through independent design and random simulation generation methods, a variety of fracture surfaces with different roughness characteristics can be generated, providing a rich sample for studying the mechanical, seepage and other characteristics of fracture surfaces under different conditions. This diverse simulation method helps to deeply understand the behavior and differences of fracture surfaces in different geological environments, and provides a broad experimental data basis for related research. Defining fracture parameters and performing precise simulation generation ensures that key features such as the first-order and second-order roughness of the fracture surface are generated according to predetermined requirements. This precise control ensures that the generated fracture surface samples have clear parameter basis in terms of physical characteristics, which can meet the precise requirements of fracture surface characteristics for specific experiments or studies, and improve the repeatability and reliability of the experiment. Using 3D printing technology, the virtual fracture surface model can be quickly converted into an actual physical specimen, greatly shortening the production cycle of experimental samples. This enables researchers to carry out experimental research more efficiently and observe and analyze physical phenomena under different fracture surface characteristics in a timely manner. The printed cylindrical specimen 5 containing the fracture surface can be used as a basic experimental sample to obtain basic data on the fracture surface during mechanical loading, fluid seepage and other processes. At the same time, by comparing the experimental results with the numerical simulation results, the fracture surface related models and theories can be verified and optimized, further promoting the in-depth development of fracture surface research.

[0060] More specifically, see Figure 4 As shown, in a preferred embodiment of the present invention, in step S3, the specific steps of conducting a permeability change test experiment under effective stress on the cylindrical sample 5 include: Step S 34: A 3D printed cylindrical sample 5 containing a fracture surface is placed in a core holder, a hand pump is connected to the core holder, and a stress with a gradually increasing interval time t is applied to the cylindrical sample 5 through the hand pump.

[0061] In this step, the 3D printed cylindrical sample 5 with a crack surface is placed in a small core holder. The core holder can fix the sample and provide a uniform confining pressure environment to simulate the actual stress state of underground rocks.

[0062] A hand pump is connected to the core holder and used to apply gradually increasing stress at intervals of time t to the cylindrical specimen 5. The hand pump can precisely control the magnitude and speed of stress application. By gradually increasing stress, the changes in the rock fracture surface under different effective stress conditions are simulated.

[0063] Step S 35 : After each interval t, wait for the pressure to stabilize for 30 minutes and then close the valve and dismantle the pipeline.

[0064] During this step, stress is applied after a time interval of t, and the pressure is allowed to stabilize for 30 minutes to ensure that the specimen reaches mechanical equilibrium under the current stress conditions and that the crack surface fully adapts to the applied stress, allowing for accurate measurement of the specimen's properties. The valve is then closed and the pipeline removed in preparation for the next CT scan, preventing unstable pressure from affecting the equipment and specimen during the scan.

[0065] Step S 36 : Perform CT scanning on the pressurized cylindrical sample 5 to obtain a digital image of the cylindrical sample 5.

[0066] In this step, a CT scan is performed on the pressurized cylindrical specimen 5. CT scanning can penetrate the specimen and obtain detailed structural information on its interior and surface, including the morphology, distribution, and size of cracks. The scan results are then converted into digital images, providing a visual and quantitative data foundation for subsequent permeability analysis, facilitating the observation and analysis of crack changes under different stress conditions.

[0067] Step S 37 : A pressure-controlled permeability experimental system was used to carry out a permeability change test experiment under effective stress on cylindrical sample 5.

[0068] In this step, a pressure-controlled permeability test system was used to test the permeability changes under effective stress on cylindrical specimen 5. The flow characteristics and permeability of fluids in rock fractures vary under different combinations of fracture surface morphology, roughness, confining pressure, and effective stress. This system allows for precise measurement of the permeability of specimens under varying stresses, allowing for the study of the relationship between effective stress and permeability.

[0069] By gradually increasing stress and performing measurements, we can delve deeper into the impact of effective stress changes on fracture surface morphology and permeability, providing key data support for establishing seepage theory in fractured oil and gas reservoirs and improving oil and gas recovery. CT scans capture digital images that clearly demonstrate microstructural changes in fracture surfaces. Combined with permeability test data, they effectively combine visualization of fracture surface changes with quantitative permeability analysis, providing researchers with more comprehensive and in-depth methods and tools for analyzing fracture surface characteristics.

[0070] This technical solution integrates multiple specialized equipment, including a core holder, hand pump, CT scanner, and a pressure-controlled permeability testing system, leveraging the strengths of each device and enabling efficient testing. By applying stress and stabilizing it before performing the CT scan and permeability test, the experimental process is optimized, improving both efficiency and data accuracy.

[0071] The obtained permeability data can be used as basic data for numerical simulation to verify and improve the seepage model of fractured reservoirs, improve the accuracy of simulation, and provide a reliable basis for the optimization of oil and gas field development plans and groundwater resource evaluation.

[0072] More specifically, see Figure 5 As shown, in a preferred embodiment of the present invention, in step S4, the method for performing numerical simulation calculation of fracture seepage on the cylindrical specimen based on the lattice Boltzmann method includes the following steps: Step S 41 : The three-dimensional digital model of the 3D-printed crack obtained by CT scanning at gradually increasing intervals t and under different stresses was imported into the Palabos open source program based on the lattice Boltzmann method.

[0073] In this step, the 3D-printed digital models of the cracks, obtained through CT scanning at varying stresses and increasing intervals (t), were imported into the Palabos open-source program, based on the lattice Boltzmann method. This step is the foundation of the entire numerical simulation. The Palabos open-source program provides algorithmic support and a numerical calculation framework for subsequent simulation calculations. The accurate 3D digital model ensures the geometric realism of the simulation, making the results more accurate than actual physical conditions.

[0074] It's important to note that the Lattice Boltzmann Method (LBM) is a numerical simulation method based on microscopic particle dynamics. It simulates the motion and collisions of particles in a fluid to determine the distribution of macroscopic physical quantities. The basic idea behind this method is to treat the fluid as a collection of particles moving at a constant velocity at discrete lattice points and interacting with other particles during collisions.

[0075] The core of the lattice Boltzmann method is the Boltzmann equation:

[0076] Where: f( , , ) is the particle distribution function, which indicates that ,speed and time The particle density under Represents the collision term, which describes the interaction between particles.

[0077] To simplify the calculation, LBM introduces discrete velocity and lattice models to discretize the continuous velocity space and position space, thereby converting the Boltzmann equation into a series of discrete equations.

[0078] The main steps of the lattice Boltzmann method are: Initialization: At the beginning of the simulation, the particle distribution function at each grid point is initialized according to the initial conditions (such as the initial velocity and density of the fluid).

[0079] Collision process: At each time step, the particle distribution function is updated according to the collision operator. The collision operator determines the interaction rules between particles. The Bhatnagar-Gross-Krook (BGK) model is usually used to simplify the collision process.

[0080] Propagation process: The updated particle distribution function propagates to adjacent grid points on the grid at discrete speeds.

[0081] Macroscopic calculation: Calculate macroscopic physical quantities (such as velocity, density, etc.) based on the particle distribution function. These quantities can be used to analyze the macroscopic behavior of the fluid.

[0082] Therefore, the lattice Boltzmann method has the characteristics of a mesoscopic model between the microscopic molecular dynamics model and the macroscopic continuous model, and has the advantages of simple description of fluid interactions, easy setting of complex boundaries, easy parallel calculation, and easy implementation of the program.

[0083] Step S 42 : Calculate the macroscopic density and macroscopic velocity of the fluid in the fracture; Wherein: the calculation expression of the macroscopic density of the fluid in the crack is:

[0084] The calculation expression of the macroscopic velocity of the fluid in the fracture is:

[0085] In this step, the macroscopic density and macroscopic velocity of the fluid in the fracture are calculated using the relevant formulas in the lattice Boltzmann method. Macroscopic density reflects the mass of the fluid per unit volume, while macroscopic velocity reflects the overall flow rate of the fluid in the fracture. These two physical quantities are fundamental parameters describing the motion of the fluid in the fracture. By calculating them, we can gain a preliminary understanding of the basic flow conditions of the fluid within the fracture.

[0086] Step S 43 : The fracture permeability is obtained by the average seepage velocity, fluid viscosity and average pressure gradient.

[0087] In this step, the fracture permeability is calculated using the average seepage velocity, fluid viscosity, and average pressure gradient according to Darcy's law and other related theories. In this embodiment, the Palabos open-source program, which is compatible with Linux platforms, is preferably used to send the simulation task to a supercomputer platform, thereby improving the computational speed and efficiency of the numerical simulation. The simulation results of multiple variables are then compared with the seepage test results of the 3D-printed fracture surface.

[0088] It is understandable that permeability is an important indicator to measure the ability of fluid to penetrate into porous media or fractures. This calculation can combine the flow characteristics of the fluid with the physical properties of the fracture to quantitatively describe the permeability of the fluid in the fracture.

[0089] Therefore, using high-precision CT scan 3D digital models and the Palabos open-source program based on the lattice Boltzmann method, it is possible to accurately simulate the seepage process of fluids in fractures. This method can capture the influence of complex fracture geometry on fluid flow, including the constraints and guidance effects of fracture shape, size, and connectivity on seepage. This provides a powerful tool for studying fracture seepage mechanisms and contributes to a deeper understanding of fluid flow patterns in fractured rocks.

[0090] The calculated macroscopic density and macroscopic velocity provide detailed data for in-depth analysis of the flow characteristics of fluids in fractures. Based on this data, researchers can further explore the flow patterns of fluids under different stress conditions. For example, when stress changes cause changes in fracture morphology, how does the fluid adapt to these changes and adjust its flow state? This can reveal the interaction mechanism between fluids and fractures.

[0091] By calculating fracture permeability using average seepage velocity, fluid viscosity, and average pressure gradient, we can more accurately assess fracture permeability. This is crucial for practical engineering applications such as predicting oil and gas reservoir production, assessing groundwater flow, and analyzing fluid seepage risks in underground engineering projects. It also provides a scientific basis for the design, development, and management of related projects.

[0092] Comparing the calculated permeability with experimental test results verifies the accuracy of the numerical simulation model. If the simulation results agree well with the experimental data, the model can reliably simulate the fracture seepage process. If there are discrepancies, the model can be optimized and adjusted based on the experimental results to further improve the simulation accuracy and make it more realistic.

[0093] More specifically, see Figure 5 As shown, in a preferred embodiment of the present invention, step S 42 The governing equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture include: For the fluid flow problem in three-dimensional fractures, the Bhatnagar–Gross–Krook single relaxation model is used, and the particle velocity evolution equation can be expressed as

[0094] Where: is the particle distribution function; is the time step; is the discrete speed; is the dimensionless relaxation time; is the particle equilibrium distribution function; For each grid point, a different discrete velocity direction is given, .

[0095] At the beginning of the simulation, the particle distribution function at each grid point is initialized based on the geometric characteristics of the 3D fracture and the initial state of the fluid (such as initial velocity and density). Simultaneously, key parameters such as the dimensionless relaxation time in the Bhatnaga–Gross–Krook (BGK) single relaxation model are determined. This dimensionless relaxation time is a crucial model parameter that influences physical properties such as fluid viscosity and must be appropriately selected based on the actual flow problem.

[0096] For fluid flow in three-dimensional fractures, the particle velocity evolution equation of the BGK single relaxation model is applied. Within each time step, the particle distribution function is updated according to this equation. Specifically, each term in the equation reflects the particle motion, collision, and equilibrium process in different discrete velocity directions.

[0097] Among them, the particle distribution function Describes the quantity state of particles at different positions, speeds and times; discrete speed Specifies the direction of movement of particles on the grid; dimensionless relaxation time Controls the speed of particle collision process; particle equilibrium distribution function It represents the theoretical value of particle distribution in a state of complete collision equilibrium.

[0098] Based on the updated particle distribution function, the macroscopic density and macroscopic velocity of the fluid in the fracture are calculated using corresponding formulas (usually by summing the particle distribution functions in different discrete velocity directions). Macroscopic density reflects the mass of the fluid per unit volume, while macroscopic velocity reflects the overall flow speed of the fluid in the fracture. These two macroscopic physical quantities can intuitively describe the macroscopic flow state of the fluid in the fracture.

[0099] It's important to note that the BGK single-relaxation model, a commonly used lattice Boltzmann model, effectively simulates the complex physical processes of fluid flow in three-dimensional fractures. By comprehensively accounting for multiple factors, such as particle motion and collisions, through the particle velocity evolution equation, it can accurately capture the details of fluid flow in fractures, such as velocity distribution and density variations. This is of great significance for in-depth research on the seepage mechanisms and understanding of fluid flow behavior in fractures.

[0100] This model boasts a relatively simple form and excellent numerical stability, ensuring simulation accuracy while improving computational efficiency. This makes it suitable for large-scale, long-term simulations, providing powerful support for studying fluid flow in complex fracture networks and multi-physics coupling. For example, in the numerical simulation of oil and gas reservoirs and groundwater flow, it can effectively handle complex fracture systems and simulate long production processes.

[0101] When dealing with complex geometric structures such as three-dimensional fractures, the particle velocity evolution equation can naturally adapt to the fracture shape and orientation on a discrete velocity and spatial grid, eliminating the need for tedious processing of complex geometric boundaries. This makes this method uniquely advantageous for simulating fluid flow in fractures with irregular shapes and complex connectivity, accurately reflecting the influence of fracture geometry on fluid flow.

[0102] Because the lattice Boltzmann method inherently possesses excellent parallelism, the BGK single-relaxation model also leverages its high efficiency within this parallel computing framework. The computations at each lattice point are relatively independent and can be easily distributed to different processors or computing units for parallel processing. This significantly shortens simulation time, improves the efficiency of research and engineering applications, and better meets the demands of fast simulation and real-time analysis in real-world problems.

[0103] More specifically, see Figure 5 As shown, in a preferred embodiment of the present invention, step S 42 Among them, the control equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture also include: The D3Q19 model is used to simulate fracture surface seepage, and its corresponding particle equilibrium distribution function is defined as:

[0104] Where: is the particle equilibrium distribution function; for Directional weight coefficient; is the density of the fluid grid; is the discrete speed; is the lattice sound speed; is the velocity of the fluid grid; Among them, the lattice sound speed is the voxel side length of the digital model of the crack surface, is the time step.

[0105] The D3Q19 model is used to simulate fracture surface seepage. This model features 19 discrete velocity directions in three-dimensional space, enabling a more detailed description of the motion of fluid particles within the fracture. The particle equilibrium distribution function is defined, which comprehensively considers factors such as the density, velocity, and lattice acoustic velocity of the fluid grid points and is the key equation for simulating equilibrium fluid flow.

[0106] The relationship between the lattice acoustic velocity and the voxel side length of the digital fracture surface model was clarified. As a key parameter in the model, the lattice acoustic velocity significantly impacts the accuracy and stability of the simulation. Furthermore, weight coefficients for 19 discrete directions were determined. These weight coefficients are pre-set constants based on the mathematical derivation and physical meaning of the model and are used to weight the particle distribution in different directions during the calculation process.

[0107] It is understood that the D3Q19 model provides a richer range of velocity directions, enabling more accurate capture of flow details in three-dimensional fractures, such as anisotropy in velocity distribution and vortex structures. This helps improve the accuracy of simulations of fracture seepage phenomena and provides more reliable numerical results for in-depth studies of complex flows in fractures.

[0108] When simulating seepage problems in complex geometric structures like fractures, the D3Q19 model is more adaptable to changes in fracture shape and direction. Through appropriate meshing and model parameter settings, it can accurately simulate fluid flow in different directions, thereby more realistically reflecting the actual seepage conditions in fractures.

[0109] A reasonable definition of the particle equilibrium distribution function helps ensure the stability of numerical calculations. During the simulation process, this function can effectively constrain the evolution of the particle distribution function, prevent problems such as numerical divergence, and ensure the smooth progress of the simulation process.

[0110] The D3Q19 model, within the framework of the lattice Boltzmann method, exhibits excellent parallel computing capabilities. The calculations at each lattice point are relatively independent, enabling the model to run efficiently in a large-scale parallel computing environment, significantly reducing simulation time and improving research efficiency.

[0111] More specifically, see Figure 10 As shown, in a preferred embodiment of the present invention, step S 37 The pressure-controlled permeability experimental system includes a gas injection system 1, a loading system 2, a permeability testing system 3, and a pressure and temperature controller 4, wherein: The air outlet of the gas injection system 1 is connected to the air inlet of the loading system 2 through a high-pressure air inlet pipe; the loading system 2 includes an annular oil cylinder 21, a central oil cylinder 22, a central pressure rod 23, a pressure-controlled triaxial test bench 24 and an oil pump 25, wherein: the annular oil cylinder 21 is used to clamp the sample, the central oil cylinder 22 is located at the center of the annular oil cylinder 21 and is located directly above the sample, the central pressure rod 23 is connected to the central oil cylinder 22 and is located directly above the sample, the oil pump 25 is connected to the annular oil cylinder 21 and the central oil cylinder 22 respectively, the central oil cylinder 22 and the central pressure rod 23 are used to provide cyclic axial pressure to the sample, and the annular oil cylinder 21 is used to provide cyclic annular pressure to the sample; the annular oil cylinder 21, the central oil cylinder 22, the central pressure rod 23 and the sample are all arranged in the pressure-controlled triaxial test bench 24, and the oil pump 25 pumps pressure to the annular oil cylinder 21 and the central cylinder 22 under the action of the pressure-controlled triaxial test bench 24; The permeability testing system 3 includes a gas flow measurement device 31 connected to the gas outlets of the annular cylinder 21 and the central cylinder 22 through a high-pressure gas pipe; The pressure and temperature controller 4 is used to adjust the inflation pressure and temperature in the pressure-controlled triaxial test platform 24 according to the set pressure and temperature.

[0112] Specifically, the gas injection system 1 consists of a nitrogen tank 11 and a pressure gauge 12. The gas outlet of the gas injection system 1 is connected to the gas inlet of the loading system 2 via a high-pressure gas inlet pipe. The loading system 2 includes an annular cylinder 21, a central cylinder 22, a central pressure rod 23, a pressure-controlled triaxial test bench 24, and an oil pump 25. The annular cylinder 21 is used to circumferentially clamp the specimen. The central cylinder 22 is located at the center of the annular cylinder 21 and directly above the specimen. The central pressure rod 23 is connected to the central cylinder 22 and directly above the specimen. The oil pump 25 is connected to the annular cylinder 21 and the central cylinder 22, respectively. The central cylinder 22 and the central pressure rod 23 are used to provide cyclic axial pressure to the specimen, while the annular cylinder 21 is used to provide cyclic annular pressure. Through the coordinated action of the annular cylinder 21 and the central cylinder 22, cyclic axial and annular pressures can be applied to the specimen, simulating the complex stress state of the specimen in the actual underground geological environment. This is of great significance for studying the mechanical behavior and seepage characteristics of rock specimens under multi-axial stress, helping to more realistically reflect actual engineering problems. The annular cylinder 21, central cylinder 22, central pressure rod 23, and specimen are all placed within a pressure-controlled triaxial test bench 24. Oil pump 25, driven by the pressure-controlled triaxial test bench 24, pumps pressure into the annular cylinder 21 and central cylinder 22.

[0113] The permeability testing system 3 includes a gas flow measurement device 31, which is connected to the air outlet of the annular cylinder 21 and the central cylinder 22 through a high-pressure gas pipe. It can accurately measure the flow of gas in the sample, thereby calculating the permeability of the sample. The system can monitor the changes in gas flow in real time, providing researchers with accurate permeability data of the sample under different stress and temperature conditions, which is helpful for in-depth analysis of the seepage mechanism of samples such as rocks. The pressure and temperature controller 4 adjusts the inflation pressure and temperature in the pressure-controlled triaxial test bench 24 according to the set pressure and set temperature to ensure that the environmental conditions are stable and controllable during the experiment. This is crucial to ensure the reliability and repeatability of the experimental results, because under different pressure and temperature conditions, the mechanical and seepage properties of samples such as rocks may change significantly. Stable experimental conditions help to eliminate external interference factors and make the experimental results more scientific and credible.

[0114] This controlled-pressure permeability experimental system integrates gas injection, loading, permeability testing, and pressure and temperature control functions, providing a comprehensive, integrated experimental platform for studying the mechanical and seepage properties of rock and other specimens. This integrated design reduces the complex connections and conversions between experimental devices, improves experimental efficiency and data accuracy, and facilitates researchers to obtain a variety of relevant data within the same experimental cycle.

[0115] See also Figure 9As shown, another embodiment of the present invention further provides a system for studying seepage characteristics of fracture surfaces under stress, which is used to implement the above-mentioned method for studying seepage characteristics of fracture surfaces under stress. The system includes: The fracture surface roughness characterization module 100 is used to select different lithology samples with continuous fracture surfaces to prepare samples, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the samples; The fracture surface 3D printing model reconstruction module 200 is used to reconstruct the fracture surface sample of the sample with natural fracture surface characteristics by using 3D printing technology; The 3D printing permeability test module 300 for a cracked surface is used to 3D print a cylindrical specimen 5 with a cracked surface and perform a permeability change test experiment under effective stress on the cylindrical specimen 5; The fracture surface seepage numerical simulation module 400 is used to perform fracture seepage numerical simulation calculations on the cylindrical sample 5 based on the lattice Boltzmann method.

[0116] In this system, samples of different lithologies containing continuous fracture surfaces are first prepared. Next, the fracture surface types and corresponding roughness differences of these samples are measured and summarized. This step utilizes a variety of measurement techniques, such as 3D scanners and stylus profilometers, to obtain detailed geometric information and roughness parameters of the fracture surfaces, providing accurate baseline data for subsequent model reconstruction.

[0117] Using 3D printing technology, based on the data and information obtained from the fracture surface roughness characterization module, a fracture surface sample with natural fracture surface characteristics is reconstructed. This involves processing and optimizing the digital model of the fracture surface to ensure it is suitable for 3D printing. Then, using appropriate printing materials and techniques, a physical model is generated that is highly similar to the natural fracture surface in terms of geometry, size, and roughness.

[0118] To test permeability changes under different stresses, a cylindrical specimen 5 containing a fracture surface was 3D printed. Then, permeability changes under effective stress were tested on this cylindrical specimen 5. During the experiment, stress conditions were controlled to simulate the stress state in a real geological environment. The flow of fluid in the fractures, including parameters such as flow rate and pressure, was measured. The permeability changes were then calculated to investigate the influence of effective stress on permeability.

[0119] Finally, a numerical simulation of fracture seepage was performed on the cylindrical specimen 5 based on the lattice Boltzmann method. Specifically, the geometric model and relevant parameters of the 3D-printed specimen were input into numerical simulation software. The flow of fluid in the fracture was simulated on a computer. Key parameters such as the macroscopic density, macroscopic velocity, and permeability of the fluid were obtained through simulation and compared with experimental results.

[0120] To verify the effectiveness of the method for studying the seepage characteristics of fracture surfaces under stress, in a preferred embodiment of the present invention, porous samples with natural rock pore and fracture structures were prepared using 3D printing technology to simulate the seepage behavior of complex structural surfaces and provide an experimental model for oil and gas development and geological engineering. The specific implementation process is as follows: Step 1: Pore and crack structure design and 3D modeling In this step, we first need to complete the three-dimensional data collection of the sample for the pore fracture structure design. Then, we need to construct a digital model of the porous rock using three-dimensional software and perform model optimization. The details are as follows: Data acquisition: Nano-CT scanning of natural sandstone samples was performed to obtain three-dimensional data of fracture pore structure including fracture pore shape, connectivity, pore size distribution, etc. Figure 7 shown.

[0121] Model optimization: Based on CT data, a digital model of porous rock was constructed using computer-aided design (CAD) software. The fracture pore parameters were adjusted to a porosity of 30% to 40% and a pore size of 0.1 to 10 μm to ensure fracture pore connectivity, such as Figure 8 Color distribution shown.

[0122] Step 2: 3D printing to prepare porous samples In this step, the printing material is selected as light-curing resin (SLA) for high-precision crack pore structure simulation; the key pore crack surface is sandblasted, and the sandblasting process uses The blasting process simulates the roughness of natural fractures using a 0.5 MPa pressure and a 3-minute sandblasting time. Printing parameters include a layer thickness of 5-10 μm, a scanning speed of 300 mm / s, and a light source intensity of 800-1200 mW / cm². Post-printing processing includes degreasing and cleaning to remove support materials and ensure that the fracture pores are unobstructed. Finally, multi-material partitioned printing is performed using a multi-nozzle printer. The upper layer is a resin material with a fracture porosity of 40% to simulate a high-permeability layer, and the lower layer is a gypsum-quartz sand composite material with a fracture porosity of 20% to simulate a low-permeability layer.

[0123] Step 3: Pore fracture surface seepage simulation experiment In this step, the sample is fixed on the seepage test bench, with pressure sensors and flow meters connected at both ends, and water or oil injected as a simulated fluid; Next, set the parameters as follows: Effective stress: normal pressure, 15MPa, 30MPa, 45MPa, 60MPa; Fluid viscosity: 1-10 cP. In the embodiment of the present invention, water is used as the simulated fluid. Flow rate range: 0.1-10 mL / min (adjusted according to material type); Then, if Figure 11 As shown in Figure 2, the average roughness of the crack surface of the 3D printed sample is calculated by the formula:

[0124] Where L represents the sample length and f(x) represents the crack surface roughness curve.

[0125] Finally, data recording and analysis are performed, the flow rate and flow rate under different pressures are recorded, and the permeability (K value) is calculated.

[0126] For example, when measuring the permeability of a sample, water is used as the simulated fluid and the sample cross-sectional area is 1 cm 2 , according to the data in the following table and Darcy's formula:

[0127] Where: k represents the permeability, A is the cross-sectional area, ΔP Indicates the pressure difference, μ represents the fluid viscosity, L is the sample length, the permeability can be calculated, and the calculation results are shown in Table 1 below.

[0128] Table 1 Permeability results

[0129] Step 4: Comparative Analysis like Figure 12 、 Figure 13 As shown, through the above steps, combined with the fracture pore radius of the sandstone prototype and the 3D-printed analog, the relative frequency of the pore volume distribution and the pore radius frequency distribution of the 3D-printed sample indicate that the resin sample is closer to the fracture pore characteristics of natural rock. Compared with the natural rock sample, the permeability of the printed sample is less than 15% different from that of the medium- and low-permeability natural sandstone used in this invention, and the fracture pore connectivity is good, which can effectively simulate the seepage behavior of complex fracture surfaces. Compared with the gypsum / quartz sand sample, the seepage path of the resin sample is more similar to the actual scene of natural rock.

[0130] Table 2 Descriptive statistics of fracture pore radius of sandstone prototype and 3D printed analogues

[0131] Although the present invention is disclosed as above, the scope of protection disclosed by the present invention is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention, and these changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A method for studying the seepage characteristics of a fracture surface under stress, characterized in that: The research method comprises the steps of: Step S1: Select different lithology samples with continuous fracture surfaces to prepare specimens, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the specimens; Step S2: reconstructing a crack surface sample of the sample having natural crack surface characteristics by using 3D printing technology; Step S3: 3D printing a cylindrical specimen with a crack surface, and conducting a permeability change test experiment under effective stress on the cylindrical specimen; Step S4: performing numerical simulation calculation of fracture seepage on the cylindrical specimen based on the lattice Boltzmann method.

2. The method for studying seepage characteristics of fracture surfaces under stress according to claim 1, characterized in that: In step S1, the specific method for measuring and summarizing the crack surface type and corresponding roughness difference characteristics of the sample includes the following steps: Step S 11 : Use a 3D scanner to scan the crack surface of the sample to obtain 3D surface structure data; Step S 12 : The roughness of the crack surface of the sample is calculated and analyzed by combining the two-order roughness coefficient and fractal dimension method.

3. The method for studying the seepage characteristics of fracture surfaces under stress according to claim 1, characterized in that: In step S2, the method of reconstructing the fracture surface sample of the sample having natural fracture surface characteristics using 3D printing technology includes the following steps: Step S 21 : The digital model of the crack surface is obtained by three-dimensional image scanning; Step S 22 : Laser sintering powder technology and light-curing resin technology are used to pre-process the digital model of the crack surface; Step S 23 : 3D printing a crack surface sample of the crack surface digital model, and cleaning the support material on the crack surface sample after printing is completed; Step S 24 : The crack surface of the 3D printed crack surface sample was chemically coated using a silane coupling agent; Step S 25 : Check the consistency of the fracture surface of the fracture surface sample, and perform CT scanning imaging again to obtain a digital model of the fracture surface imaging, and compare the digital model of the fracture surface imaging with the digital model of the natural fracture surface; If the comparison is consistent, the process proceeds to step S3; if the comparison is inconsistent, the 3D printing parameters and processing method are adjusted and the 3D printing is performed again.

4. The method for studying seepage characteristics of fracture surfaces under stress according to claim 1, characterized in that: In step S3, the 3D printing of the cylindrical specimen containing the crack surface comprises the following specific steps: Step S 31 : The crack surface is designed independently through MATLAB software, and the crack random generation open source program is used; Step S 32 : Define crack parameters, and randomly simulate and generate two crack surfaces through the open source program, and the first-order roughness of the two crack surfaces is consistent, and the second-order roughness of the two crack surfaces is different; Wherein: the crack parameters include crack opening, roughness coefficient and fractal dimension; Step S 33 : 3D printing of cylindrical specimens with crack surfaces.

5. The method for studying seepage characteristics of fracture surfaces under stress according to claim 1, characterized in that: In step S3, the specific steps of conducting a permeability change test experiment under effective stress on the cylindrical sample include: Step S 34 : A 3D-printed cylindrical sample with a fracture surface is placed in a core holder, a hand pump is connected to the core holder, and a stress with gradually increasing intervals t is applied to the cylindrical sample by the hand pump; Step S 35 : After each interval t, the pressure is stable for 30 minutes, then the valve is closed and the pipeline is disassembled; Step S 36 : Performing CT scanning on the pressurized cylindrical sample to obtain a digital image of the cylindrical sample; Step S 37 : A pressure-controlled permeability experimental system was used to carry out a permeability change test experiment under effective stress on the cylindrical sample.

6. The method for studying seepage characteristics of fracture surfaces under stress according to claim 1, characterized in that: In step S4, the method for performing numerical simulation calculation of fracture seepage on the cylindrical specimen based on the lattice Boltzmann method includes the following steps: Step S 41 : The 3D printed crack 3D digital model obtained by CT scanning at different stresses with gradually increasing interval time t was imported into the Palabos open source program based on the lattice Boltzmann method; Step S 42 : Calculate the macroscopic density and macroscopic velocity of the fluid in the fracture; Wherein: the calculation expression of the macroscopic density of the fluid in the crack is: The calculation expression of the macroscopic velocity of the fluid in the fracture is: Step S 43 : The fracture permeability is obtained by the average seepage velocity, fluid viscosity and average pressure gradient; Where: is the particle distribution function; For each grid point, a different discrete velocity direction is given, ; is the density of the fluid grid.

7. The method for studying seepage characteristics of fracture surfaces under stress according to claim 6, characterized in that: In step S 42 The governing equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture include: For the fluid flow problem in three-dimensional fractures, the Bhatnagar–Gross–Krook single relaxation model is used, and the particle velocity evolution equation can be expressed as Where: is the time step; is the discrete speed; is the dimensionless relaxation time; is the particle equilibrium distribution function.

8. The method for studying seepage characteristics of fracture surfaces under stress according to claim 7, characterized in that: In step S 42 Among them, the control equations for calculating the macroscopic density and macroscopic velocity of the fluid in the fracture also include: The D3Q19 model is used to simulate fracture surface seepage, and its corresponding particle equilibrium distribution function is defined as: Where: for Directional weight coefficient; is the lattice sound speed; is the velocity of the fluid grid; Among them, the lattice sound speed is the voxel side length of the digital model of the crack surface, is the time step.

9. The method for studying seepage characteristics of fracture surfaces under stress according to claim 5, characterized in that: In step S 37 Among them, the pressure-controlled permeability experimental system includes: The loading system (2) comprises an annular oil cylinder (21) for clamping the sample, a central oil cylinder (22) located at the center of the annular oil cylinder (21) and directly above the sample, a central pressure rod (23) connected to the central oil cylinder (22) and directly above the sample, a pressure-controlled triaxial test bench (24), and an oil pump (25) respectively connected to the annular oil cylinder (21) and the central oil cylinder (22), wherein the central oil cylinder (22) and the central pressure rod (23) are used to provide cyclic axial pressure to the sample, and the annular oil cylinder (21) is used to provide cyclic annular pressure to the sample; the annular oil cylinder (21), the central oil cylinder (22), the central pressure rod (23) and the sample are all arranged in the pressure-controlled triaxial test bench (24), and the oil pump (25) pumps pressure to the annular oil cylinder (21) and the central oil cylinder (22) under the action of the pressure-controlled triaxial test bench (24); An air injection system (1), wherein the air outlet of the air injection system (1) is connected to the air inlet of the loading system (2) via a high-pressure air inlet pipe; A permeability testing system (3) comprising a gas flow measurement device (31) connected to the gas outlets of the annular oil cylinder (21) and the central oil cylinder (22) via a high-pressure gas pipe; The pressure and temperature controller (4) is used to adjust the inflation pressure and temperature in the pressure-controlled triaxial test bench (24) according to the set pressure and temperature.

10. A system for studying seepage characteristics of fracture surfaces under stress, characterized in that: The system is used to implement the method for studying seepage characteristics of fracture surfaces under stress as described in any one of claims 1 to 9, comprising: The fracture surface roughness characterization module is used to select different lithology samples with continuous fracture surfaces to make specimens, and measure and summarize the fracture surface types and corresponding roughness difference characteristics of the specimens; A fracture surface 3D printing model reconstruction module is used to reconstruct a fracture surface sample of the sample having natural fracture surface characteristics using 3D printing technology; 3D printing cracked surface permeability test module, used for 3D printing cylindrical specimens with cracked surfaces and conducting permeability change test experiments under effective stress on the cylindrical specimens; The fracture surface seepage numerical simulation module is used to carry out fracture seepage numerical simulation calculations on the cylindrical specimen based on the lattice Boltzmann method.

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