A method and system for determining permeability in a fractured medium

Abstract

The application discloses a fissure medium permeability measuring method and system, relates to the field of fissured rock mass permeability calculation, and combines high-resolution three-dimensional imaging of fissure medium, direct modeling based on images, finite element seepage process numerical simulation and other technologies to design a fissure medium permeability measuring method, so that the permeability of fissured rock mass in different directions can be accurately measured on the basis of considering the complexity of fissure structure and the distribution heterogeneity, and the effective measurement of the permeability of strongly fissured rock mass in different directions under the condition of non-damage can be realized by processing various three-dimensional fissure structure data bodies, the method can be applied to technical researches such as quantitative characterization of seepage characteristics of reservoirs in complex structure regions, researches on gas production rules of fissure gas reservoirs and optimization of drainage and production systems, and has important reference significance for effective exploration and efficient development and utilization of unconventional natural gas resources and other engineering practices.

Description

Technical Field

[0001] This invention relates to the field of permeability calculation of fractured rock masses, and more particularly to a method and system for measuring the permeability of fractured media. Background Technology

[0002] Reservoir permeability is a key parameter affecting natural gas accumulation and production. The low porosity and low permeability of unconventional reservoirs such as coal and shale severely limit the efficient development of unconventional natural gas resources such as coalbed methane and shale gas. Unconventional reservoirs that have undergone tectonic modification possess naturally developed fracture structures that can significantly improve overall reservoir permeability, having a crucial impact on unconventional natural gas development. However, tectonically modified reservoirs have low mechanical strength, making it difficult to prepare standard column samples suitable for permeability testing. Quantitative characterization of their permeability is a major challenge in the field of unconventional natural gas exploration and development.

[0003] Existing image-processing-based methods for calculating the permeability of fractured rock masses generally rely on empirical or functional relationships between macroscopic fracture structure parameters (fracture porosity, fracture aperture, fracture length, etc.) and permeability, and make morphological assumptions and structural simplifications to the fracture system during the calculation process. Existing permeability calculation methods that consider the fracture structure of the medium generally do not take into account the complexity and anisotropy of the fracture structure.

[0004] Therefore, the permeability values ​​calculated based on existing technologies may deviate significantly from the true permeability of fractured rock masses (especially highly fractured rock masses), and cannot be directly used for theoretical analysis of seepage characteristics of fractured rock masses and engineering practice for optimization of development schemes. Summary of the Invention

[0005] This solution addresses the problems and needs raised above by proposing a method and system for measuring the permeability of fractured media. Due to the adoption of the following technical features, it is able to achieve the above-mentioned technical objectives and bring about several other technical benefits.

[0006] One object of the present invention is to provide a method for measuring the permeability of fractured media, comprising the following steps: S10: Based on the range of fracture aperture, prepare a columnar sample of a specific size and use three-dimensional imaging technology to image the sample to obtain a two-dimensional fracture image sequence; based on the two-dimensional image sequence of fracture structure, use three-dimensional digital image processing software to generate a three-dimensional image data volume of fractured rock mass; S20: Based on the three-dimensional image data volume of the rock mass, extract the ROI of the fracture object, filter the connected fracture system from the identified fracture ROI, and obtain the fracture ROI data volume for seepage simulation and permeability calculation; based on the obtained target area fracture ROI data volume, generate the mesh model of the target area fracture ROI, and save it as an STL file containing the surface geometry information of the three-dimensional fracture structure. S30: Select a three-dimensional steady-state creeping flow model as the seepage model; import the obtained STL file into COMSOL Multiphysics software, and directly generate a three-dimensional fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution based on the surface geometry information of the three-dimensional fracture structure; automatically mesh the boundary of the generated fracture domain; add boundary layer meshes to the fracture wall of the obtained geometry, and then construct a three-dimensional model of the fracture structure. S40: Set the model dimensions, seepage inlet / outlet, and boundary conditions; S50: Conduct finite element numerical simulation of the seepage process to obtain the pressure field and velocity field within the fracture system; define the permeability calculation formula based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length in the seepage direction, and boundary area of ​​inlet and outlet; obtain the volumetric flow velocity at outlet and pressure gradient between inlet and outlet based on the obtained pressure field and velocity field data, and then calculate the permeability.

[0007] In addition, the method and system for measuring the permeability of fractured media according to the present invention may also have the following technical features: In one example of the present invention, step S10 specifically includes the following steps: S11: A columnar sample for permeability measurement is obtained by cutting using low-speed wire cutting technology; S12: Perform computed tomography (CT) or focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) on the columnar samples of fractured rock mass to obtain a two-dimensional image sequence of the fractured rock mass; S13: Using ORS Dragonfly software, construct a three-dimensional image data volume of fractured rock mass based on the two-dimensional image sequence of fractured rock mass.

[0008] In one example of the present invention, step S12 specifically includes the following steps: For samples measuring a few millimeters or centimeters, computed tomography (CT) was used to obtain two-dimensional images of the crack structure with an aperture greater than a few micrometers and tens of micrometers, respectively. For target domains of tens of micrometers, focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) was used to obtain two-dimensional images of the crack structure with an aperture greater than tens of nanometers.

[0009] In one example of the present invention, step S20 specifically includes the following steps: S21: In ORS Dragonfly software, a three-dimensional image data volume is obtained by cropping to be used for seepage simulation of fractured systems; S22: Using the threshold segmentation method, the crack voxel points are extracted from the three-dimensional image data volume based on the gray value of the voxel points to obtain the crack object region ROI; S23: Perform connectivity analysis on the ROI of the fracture object region, and select the large connected fracture object region ROI as the fracture structure for seepage simulation; S24: Mesh the target fracture object region ROI to obtain the fracture structure mesh model and export it as an STL file.

[0010] In one example of the present invention, step S30 specifically includes the following steps: S31: In COMSOL Multiphysics software, select the three-dimensional steady-state creeping flow model as the seepage model; S32: Under the mesh node of the geometry module, import the STL file to import the geometric information of the crack structure mesh into the COMSOL Multiphysics software. According to the size of the generated crack mesh, set the parameter boundary segmentation, repair tolerance and detection surface in sequence. S33: Generate fracture structure domains based on fracture structure mesh geometry. For intersecting surface elements that appear during the import of fracture structure geometry information, remove intersecting surface elements by breaking boundaries and deleting boundaries / edges, and delete isolated fracture domains. S34: Improve the quality of triangular meshes on the surface of fractured structures using free tetrahedral meshes. Control the accuracy of free tetrahedral meshes by setting the maximum element size, minimum element size, maximum element growth rate, curvature factor, and resolution in narrow regions. S35: Add a boundary layer mesh at the fracture wall to avoid analytical errors caused by excessive pressure gradient at the fracture surface during seepage simulation. In this process, the boundary layer properties are controlled by setting the parameters: number of boundary layers and thickness adjustment factor. S36: Generate a geometric model of the crack structure with the required mesh element quality by using a shaped mesh.

[0011] In one example of the present invention, step S40 specifically includes the following steps: S41: Set the actual dimensions of the fracture structure geometry model; S42: In the definition node, define the boundaries of the fracture structure in the X, Y, and Z directions, and define the variables of pressure drop between the inlet and outlet, volumetric flow velocity at the outlet, and permeability; in the material node, set the fluid density and fluid viscosity; in the creeping flow node, select two boundaries in the X direction as the fluid seepage inlet and outlet, respectively, and set the inlet flow velocity.

[0012] In one example of the present invention, step S50 specifically includes the following steps: S51: Numerical simulation of the seepage process was performed using a creeping flow model to obtain the velocity and pressure fields within the fracture system; S52: Calculate the permeability in the X direction at the global node; calculate the permeability of the fractured rock mass in the Y and Z directions by setting the relative boundaries in the Y and Z directions as the inlet and outlet boundaries, respectively.

[0013] In one example of the present invention, in step S52, the expression for the pressure gradient dPdL between the inlet boundary and the outlet boundary is: In the formula: and These refer to the pressure values ​​at the inlet and outlet boundaries, respectively; L refers to the model dimension in the seepage direction.

[0014] In one example of the present invention, in step S52, the expression for permeability k is: In the formula, Indicate the flow velocity value at the outlet boundary; The dynamic viscosity of the fluid used in seepage simulation.

[0015] Another object of the present invention is to provide a system for measuring the permeability of fractured media, comprising: The three-dimensional image data module is configured to prepare a columnar sample of a specific size based on the range of fracture aperture and to image the sample using three-dimensional imaging technology to obtain a two-dimensional fracture image sequence; based on the two-dimensional image sequence of fracture structure, a three-dimensional image data volume of fractured rock mass is generated using three-dimensional digital image processing software. The three-dimensional fracture structure module is configured to extract the fracture object region ROI based on the three-dimensional image data volume of the rock mass, filter the connected fracture system from the identified fracture ROI, and obtain the fracture ROI data volume for seepage simulation and permeability calculation; based on the obtained target area fracture ROI data volume, generate the mesh model of the target area fracture ROI, and save it as an STL file containing the surface geometry information of the three-dimensional fracture structure. The three-dimensional fracture spatial geometry module is configured to select a three-dimensional steady-state creeping flow model as the seepage model; the obtained STL file is imported into COMSOL Multiphysics software, and a three-dimensional fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution is directly generated based on the surface geometry information of the three-dimensional fracture structure. The 3D model module for fracture structures is configured to automatically mesh the boundaries of the generated fracture domain; add boundary layer meshes to the fracture walls of the obtained geometry to construct a 3D model of the fracture structure; and set the model size, seepage inlet / outlet, and boundary conditions. The permeability calculation module is configured to perform finite element numerical simulations of the seepage process to obtain the pressure and velocity fields within the fractured system. Based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length along the seepage direction, and boundary area between inlet and outlet, a permeability calculation formula is defined. Based on the obtained pressure and velocity field data, the volumetric flow velocity at the outlet and the pressure gradient between inlet and outlet are obtained, and then the permeability is calculated.

[0016] In one example of the present invention, the three-dimensional image data module includes: The sample cutting unit is configured to cut a columnar sample for permeability measurement using a low-speed wire cutting technique. The two-dimensional image unit is configured to perform computed tomography (CT) or focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) scanning on columnar samples of fractured rock mass to obtain two-dimensional image sequences of fractured rock mass. The three-dimensional image data unit is configured to use ORS Dragonfly software to construct a three-dimensional image data volume of fractured rock mass based on a two-dimensional image sequence of fractured rock mass.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention proposes a direct modeling method based on crack images. Based on obtaining three-dimensional crack images from two-dimensional crack image sequences, a three-dimensional structural model that can fully reflect the structural complexity and heterogeneous distribution of the crack system is directly generated through meshing.

[0018] 2. In the permeability measurement method for fractured rock mass designed in this invention, the size of the fracture medium and the range of fracture aperture characterized can be freely selected. Therefore, it is applicable to the permeability measurement of fracture structures of different scales and can be used to further analyze the size effect and distribution heterogeneity of fracture permeability.

[0019] 3. This invention uses a three-dimensional fracture structure model to numerically simulate the seepage process by setting the inlet velocity or the pressure difference between the inlet and outlet. The permeability is calculated based on the pressure and velocity fields. The principle of permeability measurement is the same as that of direct experimental measurement, and the obtained permeability results are closer to the experimentally measured values.

[0020] 4. By processing various three-dimensional fracture structure data volumes, this invention can effectively determine the permeability of strongly fractured rock masses in different directions under non-destructive conditions. It can be applied to the quantitative characterization of reservoir seepage characteristics in complex structural areas, the study of gas production laws in fractured gas reservoirs, and the optimization of drainage and production systems. It has important reference value for engineering practices such as the effective exploration and efficient development and utilization of unconventional natural gas resources.

[0021] The preferred embodiments of the invention will be described in more detail below with reference to the accompanying drawings, so as to facilitate an understanding of the features and advantages of the invention. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments of the present invention will be briefly described below. The drawings are merely illustrative of some embodiments of the present invention and are not intended to limit the scope of the present invention to all embodiments.

[0023] Figure 1 This is a flowchart of a method for measuring the permeability of fractured media according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a method for measuring the permeability of fractured media according to an embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The same reference numerals in the drawings represent the same components. It should be noted that the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms “first,” “second,” and similar terms used in this patent application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, “an” or “a” and similar terms do not necessarily indicate a quantity limitation. Terms such as “comprising” or “including” mean that the element or object preceding the word encompasses the element or object listed following the word and its equivalents, without excluding other elements or objects. Terms such as “connected” or “linked” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as “upper,” “lower,” “left,” and “right” are used only to indicate relative positional relationships; these relative positional relationships may change accordingly when the absolute position of the described object changes.

[0026] According to a first aspect of the present invention, a method for determining the permeability of fractured media, such as... Figure 1 and Figure 2 As shown, it includes the following steps: S10: Based on the range of fracture aperture, prepare a columnar sample of a specific size and use three-dimensional imaging technology to image the sample to obtain a two-dimensional fracture image sequence; based on the two-dimensional image sequence of fracture structure, use three-dimensional digital image processing software (such as Avizo, ORS Dragonfly, etc.) to generate a three-dimensional image data volume of fractured rock mass, and preprocess it (such as removing ring artifacts or correcting grayscale unevenness through filter operation). S20: Based on the 3D image data of the rock mass, image segmentation methods such as threshold segmentation, edge detection, and deep learning are used to extract the Region of Interest (ROI). Necessary processing is then performed on the identified ROIs (e.g., removing isolated islands and background noise misidentified as fracture particles through morphological operations). Through connected component analysis, connected fracture systems are selected from the identified ROIs. As needed, the fracture structure data volume can be processed by shearing, upsampling / downsampling, etc., to obtain the fracture ROI data volume for seepage simulation and permeability calculation. Based on the obtained target region fracture ROI data volume, a mesh model of the target region fracture ROI is generated using Mesh and saved as an STL file containing the geometric information of the 3D fracture structure surface. In this step, the parameters selected for Mesh processing are set as needed to obtain a fracture mesh model that has undergone downsampling, smoothing, etc. S30: In COMSOL Multiphysics software, a three-dimensional steady-state creeping flow model is selected as the seepage model through the model wizard. The obtained STL file is imported into COMSOL Multiphysics software, and a three-dimensional fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution is directly generated based on the surface geometry information of the three-dimensional fracture structure. During this process, if necessary, the vertices, edges (line segments connected by two vertices), and boundaries (faces composed of three or more edges and faces composed of different faces) of the geometry can be processed. The boundaries of the generated fracture domain are automatically meshed using free tetrahedral meshes. Boundary layer meshes are added to the fracture walls of the obtained geometry to construct a three-dimensional model of the fracture structure. S40: Set parameters such as model size, seepage inlet / outlet, and boundary conditions; S50: Conduct finite element numerical simulation of the seepage process to obtain the pressure field and velocity field within the fracture system; define the permeability calculation formula based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length in the seepage direction, and boundary area of ​​inlet and outlet; obtain the volumetric flow velocity at outlet and pressure gradient between inlet and outlet based on the obtained pressure field and velocity field data, and then calculate the permeability.

[0027] This invention relates to a method for determining the permeability of fractured rock masses based on image processing and numerical simulation of seepage processes. It primarily utilizes the image processing module of ORS Dragonfly 3D image processing software and the seepage simulation module of COMSOL Multiphysics software. First, a geometric model of the fracture structure is directly generated by processing 3D fracture images. Then, the seepage process of the fractured system under pressure difference is simulated using the finite element method. Finally, the permeability of the fractured rock mass is calculated by processing the obtained pressure field and velocity field data.

[0028] Considering the limitations of existing image processing-based permeability calculation methods, this invention combines high-resolution three-dimensional imaging of fractured media, image-based direct modeling, and finite element numerical simulation of seepage processes to design a method for measuring the permeability of complex fractured media. This method can accurately measure the permeability of fractured rock masses in different directions, taking into account the complexity and heterogeneity of fracture structure distribution.

[0029] This permeability measurement method proposes a direct modeling approach based on fracture images. By obtaining three-dimensional fracture images from a sequence of two-dimensional fracture images, and then processing them into a grid, a three-dimensional structural model that can fully reflect the structural complexity and heterogeneous distribution of the fracture system is directly generated.

[0030] In this permeability measurement method for fractured rock masses, the size of the fracture medium and the range of fracture aperture characterized can be freely selected. Therefore, it is applicable to the permeability measurement of fracture structures of different scales and can be used to further analyze the size effect and distribution heterogeneity of fracture permeability.

[0031] This permeability measurement method involves setting the inlet flow velocity or the inlet / outlet pressure difference, and then conducting a numerical simulation of the seepage process on a three-dimensional fractured structure model. The permeability is calculated based on the treated pressure and velocity fields. The principle of permeability measurement is the same as that of direct experimental measurement, and the obtained permeability results are closer to experimentally measured values.

[0032] This permeability measurement method, by processing various three-dimensional fracture structure data volumes, can effectively measure the permeability of strongly fractured rock masses in different directions under non-destructive conditions. It can be applied to the quantitative characterization of reservoir seepage characteristics in complex structural areas, the study of gas production laws in fractured gas reservoirs, and the optimization of drainage and production systems. It has important reference value for engineering practices such as the effective exploration and efficient development and utilization of unconventional natural gas resources.

[0033] In one example of the present invention, step S10 specifically includes the following steps: S11: A columnar sample for permeability measurement is obtained by cutting using low-speed wire cutting technology; S12: Perform computed tomography (CT) or focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) on the columnar samples of fractured rock mass to obtain a two-dimensional image sequence of the fractured rock mass; S13: Using ORS Dragonfly software, construct a three-dimensional image data volume of fractured rock mass based on the two-dimensional image sequence of fractured rock mass.

[0034] In one example of the present invention, step S12 specifically includes the following steps: For samples measuring a few millimeters or centimeters, computed tomography (CT) was used to obtain two-dimensional images of the crack structure with an aperture greater than a few micrometers and tens of micrometers, respectively. For target domains of tens of micrometers, focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) was used to obtain two-dimensional images of the crack structure with an aperture greater than tens of nanometers.

[0035] In one example of the present invention, step S20 specifically includes the following steps: S21: In ORS Dragonfly software, a three-dimensional image data volume is obtained by cropping to be used for seepage simulation of fractured systems; S22: Using the threshold segmentation method, the crack voxel points are extracted from the three-dimensional image data volume based on the gray value of the voxel points to obtain the crack object region ROI; S23: Perform connectivity analysis on the ROI of the fracture object region, and select the large connected fracture object region ROI as the fracture structure for seepage simulation; S24: Mesh the target fracture object region ROI to obtain the fracture structure mesh model and export it as an STL file.

[0036] In one example of the present invention, step S30 specifically includes the following steps: S31: In COMSOL Multiphysics software, select the three-dimensional steady-state creeping flow model as the seepage model; S32: Under the mesh node of the geometry module, import the STL file to import the geometric information of the crack structure mesh into the COMSOL Multiphysics software. According to the size of the generated crack mesh, set the parameter boundary segmentation, repair tolerance and detection surface in sequence. S33: Generate fracture structure domains based on fracture structure mesh geometry. For intersecting surface elements that appear during the import of fracture structure geometry information, remove intersecting surface elements by breaking boundaries, deleting boundaries / edges, etc., and delete isolated fracture domains. S34: Improve the quality of triangular meshes on the surface of fractured structures by using free tetrahedral meshes. Control the accuracy of free tetrahedral meshes by setting parameters such as maximum element size, minimum element size, maximum element growth rate, curvature factor, and resolution in narrow regions. S35: Use boundary layer to add boundary layer mesh at the fracture wall to avoid analytical errors caused by excessive pressure gradient at the fracture surface during seepage simulation. In this process, the boundary layer properties can be controlled by setting the boundary layer number and mixing thickness adjustment factor. S36: Generate a geometric model of the crack structure with the required mesh element quality by using a shaped mesh.

[0037] In one example of the present invention, step S40 specifically includes the following steps: S41: Set the actual dimensions of the fracture structure geometry model; S42: In the definition node, define the boundaries of the fracture structure in the X, Y, and Z directions, and define variables such as pressure drop between the inlet and outlet, volumetric flow velocity at the outlet, and permeability; in the material node, set the fluid density and fluid viscosity; in the creeping flow node, select two boundaries in the X direction as the fluid seepage inlet and outlet respectively, and set the inlet flow velocity.

[0038] In one example of the present invention, step S50 specifically includes the following steps: S51: Numerical simulation of the seepage process was performed using a creeping flow model to obtain the velocity and pressure fields within the fracture system; S52: Calculate the permeability in the X direction at the global node; calculate the permeability of the fractured rock mass in the Y and Z directions by setting the relative boundaries in the Y and Z directions as the inlet and outlet boundaries, respectively.

[0039] In one example of the present invention, in step S52, the expression for the pressure gradient dPdL between the inlet boundary and the outlet boundary is: In the formula: and These refer to the pressure values ​​at the inlet and outlet boundaries, respectively, in N / m. 2 L refers to the model dimension in the seepage direction, in meters (m).

[0040] In one example of the present invention, in step S52, the expression for permeability k is: In the formula, Indicate the flow velocity at the outlet boundary, in m / s; The dynamic viscosity of the fluid used in seepage simulation, expressed in Pa·s.

[0041] A fractured medium permeability measurement system according to a second aspect of the present invention comprises: The three-dimensional image data module is configured to prepare a columnar sample of a specific size based on the range of fracture aperture and to image the sample using three-dimensional imaging technology to obtain a two-dimensional fracture image sequence. Based on the two-dimensional image sequence of fracture structure, a three-dimensional image data volume of fractured rock mass is generated using three-dimensional digital image processing software (such as Avizo, ORS Dragonfly, etc.), and the data is preprocessed (such as removing ring artifacts or correcting grayscale unevenness through filter operations). The three-dimensional fracture structure module is configured based on the three-dimensional image data of the rock mass. It employs image segmentation methods such as thresholding, edge detection, and deep learning to extract fracture object regions (ROIs). Necessary processing is then applied to the identified ROIs (e.g., morphological operations to remove isolated islands and background noise misidentified as fracture particles). Connected component analysis is used to filter connected fracture systems from the identified ROIs. Depending on the needs, the fracture structure data volume can be processed with shearing, upsampling / downsampling, etc., to obtain fracture ROI data volumes for seepage simulation and permeability calculation. Based on the obtained target region fracture ROI data volume, a mesh model of the target region fracture ROI is generated using Mesh and saved as an STL file containing the geometric information of the three-dimensional fracture structure surface. In this step, the parameters selected for Mesh processing are set as needed to obtain fracture mesh models that have undergone downsampling, smoothing, etc. The 3D fracture spatial geometry module is configured in COMSOL Multiphysics software to select a 3D steady-state creeping flow model as the seepage model. The obtained STL file is imported into COMSOL Multiphysics software, and a 3D fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution is directly generated based on the surface geometry information of the 3D fracture structure. During this process, if necessary, the vertices, edges (line segments connecting two vertices), and boundaries (faces composed of three or more edges and faces composed of different faces) of the geometry can be processed. The boundaries of the generated fracture domain are automatically meshed using a free tetrahedral mesh. A boundary layer mesh is added to the fracture walls of the obtained geometry to construct a 3D model of the fracture structure.

[0042] The three-dimensional model module for fracture structures is configured to set parameters such as model size, seepage inlet / outlet, and boundary conditions. The permeability calculation module is configured to perform finite element numerical simulations of the seepage process to obtain the pressure and velocity fields within the fractured system. Based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length along the seepage direction, and boundary area between inlet and outlet, a permeability calculation formula is defined. Based on the obtained pressure and velocity field data, the volumetric flow velocity at the outlet and the pressure gradient between inlet and outlet are obtained, and then the permeability is calculated.

[0043] This permeability measurement system proposes a direct modeling method based on fracture images. By obtaining three-dimensional fracture images from a sequence of two-dimensional fracture images, and then processing them into a grid, a three-dimensional structural model that can fully reflect the structural complexity and heterogeneous distribution of the fracture system is directly generated.

[0044] In the permeability measurement system designed for fractured rock mass, the size of the fracture medium and the range of fracture aperture characterized can be freely selected. Therefore, it is applicable to the permeability measurement of fracture structures of different scales and can be used to further analyze the size effect and distribution heterogeneity of fracture permeability.

[0045] This permeability measurement system simulates the seepage process on a three-dimensional fractured structure model by setting the inlet flow velocity or the inlet / outlet pressure difference. The permeability is calculated based on the pressure and velocity fields. The principle of permeability measurement is the same as that of direct experimental measurement, and the obtained permeability results are closer to experimentally measured values.

[0046] This permeability measurement system can effectively measure the permeability of highly fractured rock masses in different directions under non-destructive conditions by processing various three-dimensional fracture structure data volumes. It can be applied to the quantitative characterization of reservoir seepage characteristics in complex structural areas, the study of gas production laws in fractured gas reservoirs, and the optimization of drainage and production systems. It has important reference value for engineering practices such as the effective exploration and efficient development and utilization of unconventional natural gas resources.

[0047] In one example of the present invention, the three-dimensional image data module includes: The sample cutting unit is configured to cut a columnar sample for permeability measurement using a low-speed wire cutting technique. The two-dimensional image unit is configured to perform computed tomography (CT) or focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) scanning on columnar samples of fractured rock mass to obtain two-dimensional image sequences of fractured rock mass. The three-dimensional image data unit is configured to use ORS Dragonfly software to construct a three-dimensional image data volume of fractured rock mass based on a two-dimensional image sequence of fractured rock mass.

[0048] It should be noted that the fractured medium permeability measurement system of the present invention can also perform any of the processes described in the fractured medium permeability measurement method previously described, and the specific details are not repeated here.

[0049] The foregoing description, with reference to preferred embodiments, details an exemplary implementation of the method and system for measuring the permeability of fractured media proposed in this invention. However, those skilled in the art will understand that various modifications and alterations can be made to the above specific embodiments without departing from the concept of this invention, and various combinations can be made to the various technical features and structures proposed in this invention without exceeding the protection scope of this invention, which is determined by the appended claims.

Claims

1. A method of fracture medium permeability determination, characterized by, Includes the following steps: S10: Based on the range of fracture aperture, prepare a columnar sample of a specific size and use three-dimensional imaging technology to image the sample to obtain a two-dimensional fracture image sequence; based on the two-dimensional image sequence of fracture structure, use three-dimensional digital image processing software to generate a three-dimensional image data volume of fractured rock mass; S20: Based on the three-dimensional image data volume of the rock mass, extract the ROI of the fracture object, filter the connected fracture system from the identified fracture ROI, and obtain the fracture ROI data volume for seepage simulation and permeability calculation; based on the obtained target area fracture ROI data volume, generate the mesh model of the target area fracture ROI, and save it as an STL file containing the surface geometry information of the three-dimensional fracture structure. S30: Select a three-dimensional steady-state creeping flow model as the seepage model; import the obtained STL file into COMSOL Multiphysics software, and directly generate a three-dimensional fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution based on the surface geometry information of the three-dimensional fracture structure; automatically mesh the boundary of the generated fracture domain; add boundary layer meshes to the fracture wall of the obtained geometry, and then construct a three-dimensional model of the fracture structure. S40: Set the model dimensions, seepage inlet / outlet, and boundary conditions; S50: Conduct finite element numerical simulation of the seepage process to obtain the pressure field and velocity field within the fracture system; define the permeability calculation formula based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length in the seepage direction, and boundary area of ​​inlet and outlet; obtain the volumetric flow velocity at outlet and pressure gradient between inlet and outlet based on the obtained pressure field and velocity field data, and then calculate the permeability.

2. The method for determining the permeability of fractured media according to claim 1, characterized in that, Step S10 specifically includes the following steps: S11: A columnar sample for permeability measurement is obtained by cutting using low-speed wire cutting technology; S12: Perform computed tomography (CT) or focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) on the columnar samples of fractured rock mass to obtain a two-dimensional image sequence of the fractured rock mass; S13: Using ORS Dragonfly software, construct a three-dimensional image data volume of fractured rock mass based on the two-dimensional image sequence of fractured rock mass.

3. The method for determining the permeability of fractured media according to claim 2, characterized in that, Step S12 specifically includes the following steps: For samples measuring a few millimeters or centimeters, computed tomography (CT) was used to obtain two-dimensional images of the crack structure with an aperture greater than a few micrometers and tens of micrometers, respectively. For target domains of tens of micrometers, focused ion beam-electron beam dual-beam scanning electron microscopy (FIB-SEM) was used to obtain two-dimensional images of the crack structure with an aperture greater than tens of nanometers.

4. The method for determining the permeability of fractured media according to claim 1, characterized in that, Step S20 specifically includes the following steps: S21: In ORS Dragonfly software, a three-dimensional image data volume is obtained by cropping to be used for seepage simulation of fractured systems; S22: Using the threshold segmentation method, the crack voxel points are extracted from the three-dimensional image data volume based on the voxel point gray values ​​to obtain the crack object region ROI; S23: Perform connectivity analysis on the ROI of the fracture object region, and select the large connected fracture object region ROI as the fracture structure for seepage simulation; S24: Mesh the target fracture object region ROI to obtain the fracture structure mesh model and export it as an STL file.

5. The method for determining the permeability of fractured media according to claim 1, characterized in that, Step S30 specifically includes the following steps: S31: In COMSOL Multiphysics software, select the three-dimensional steady-state creeping flow model as the seepage model; S32: Under the mesh node of the geometry module, import the STL file to import the geometric information of the crack structure mesh into the COMSOL Multiphysics software. According to the size of the generated crack mesh, set the parameter boundary segmentation, repair tolerance and detection surface in sequence. S33: Generate fracture structure domains based on fracture structure mesh geometry. For intersecting surface elements that appear during the import of fracture structure geometry information, remove intersecting surface elements by breaking boundaries and deleting boundaries / edges, and delete isolated fracture domains. S34: Improve the quality of triangular meshes on the surface of fractured structures using free tetrahedral meshes. Control the accuracy of free tetrahedral meshes by setting the maximum element size, minimum element size, maximum element growth rate, curvature factor, and resolution in narrow regions. S35: Add a boundary layer mesh at the fracture wall to avoid analytical errors caused by excessive pressure gradient at the fracture surface during seepage simulation. In this process, the boundary layer properties are controlled by setting the parameters: number of boundary layers and thickness adjustment factor. S36: Generate a geometric model of the crack structure with the required mesh element quality by using a shaped mesh.

6. The method for determining the permeability of fractured media according to claim 1, characterized in that, Step S40 specifically includes the following steps: S41: Set the actual dimensions of the fracture structure geometry model; S42: In the definition node, define the boundaries of the fracture structure in the X, Y, and Z directions, and define the variables of pressure drop between the inlet and outlet, volumetric flow velocity at the outlet, and permeability; in the material node, set the fluid density and fluid viscosity; in the creeping flow node, select two boundaries in the X direction as the fluid seepage inlet and outlet, respectively, and set the inlet flow velocity.

7. The method for determining the permeability of fractured media according to claim 1, characterized in that, Step S50 specifically includes the following steps: S51: Numerical simulation of the seepage process was performed using a creeping flow model to obtain the velocity and pressure fields within the fracture system; S52: Calculate the permeability in the X direction at the global node; calculate the permeability of the fractured rock mass in the Y and Z directions by setting the relative boundaries in the Y and Z directions as the inlet and outlet boundaries, respectively.

8. The method for determining the permeability of fractured media according to claim 7, characterized in that, In step S52, the expression for the pressure gradient dPdL between the inlet and outlet boundaries is: where: and Pin and Pout refer to the pressure values at the inlet and outlet boundaries, respectively; L refers to the model size in the direction of flow.

9. The method for determining the permeability of fractured media according to claim 7, characterized in that, In step S52, the expression for permeability k is: wherein denotes the value of the flow rate at the outlet boundary; denotes the dynamic viscosity of the fluid used for the seepage simulation.

10. A fracture medium permeability measurement system characterized by, include: The three-dimensional image data module is configured to prepare a columnar sample of a specific size based on the range of fracture aperture and to image the sample using three-dimensional imaging technology to obtain a two-dimensional fracture image sequence; based on the two-dimensional image sequence of fracture structure, a three-dimensional image data volume of fractured rock mass is generated using three-dimensional digital image processing software. The three-dimensional fracture structure module is configured to extract the fracture object region ROI based on the three-dimensional image data volume of the rock mass, filter the connected fracture system from the identified fracture ROI, and obtain the fracture ROI data volume for seepage simulation and permeability calculation; based on the obtained target area fracture ROI data volume, generate the mesh model of the target area fracture ROI, and save it as an STL file containing the surface geometry information of the three-dimensional fracture structure. The three-dimensional fracture spatial geometry module is configured to select a three-dimensional steady-state creeping flow model as the seepage model; the obtained STL file is imported into COMSOL Multiphysics software, and a three-dimensional fracture spatial geometry that fully reflects the complexity and heterogeneity of fracture distribution is directly generated based on the surface geometry information of the three-dimensional fracture structure. The 3D model module for fracture structures is configured to automatically mesh the boundaries of the generated fracture domain; add boundary layer meshes to the fracture walls of the obtained geometry to construct a 3D model of the fracture structure; and set the model size, seepage inlet / outlet, and boundary conditions. The permeability calculation module is configured to perform finite element numerical simulations of the seepage process to obtain the pressure and velocity fields within the fractured system. Based on the functional relationship between volumetric flow velocity and permeability, pressure gradient between inlet and outlet, fracture length along the seepage direction, and boundary area between inlet and outlet, a permeability calculation formula is defined. Based on the obtained pressure and velocity field data, the volumetric flow velocity at the outlet and the pressure gradient between inlet and outlet are obtained, and then the permeability is calculated.

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