A three-dimensional reconstruction method of internal fractures in shale samples based on free-form surfaces

Abstract

The present invention discloses a free-surface-based three-dimensional reconstruction method for internal fractures of shale samples. Based on spline surfaces and Coons surface algorithms, the vector fracture lines after the unified coordinate system are used to intuitively display and quantitatively describe the spatial morphology of the fractures in the sample after fracturing, and provide a more accurate model basis for the quantitative characterization of the geometric characteristics of the fracture surface and the numerical simulation of the seepage characteristics of the fracture surface. The present invention can improve the accuracy of the reconstruction of the fracture surface inside the rock mass, and obtain a fracture surface that conforms to the internal characteristics of the real rock mass sample.

Description

Technical Field

[0001] The invention belongs to the technical field of rock mass internal fracture surface reconstruction, and relates to a three-dimensional reconstruction method of internal pressure fractures of a shale sample based on a free-form surface. Background Art

[0002] Due to the extremely low permeability of shale oil reservoirs, hydraulic fracturing is required to promote rock fracture to form a complex fracture network, thereby improving fracture conductivity and the production of unconventional oil and gas. The morphology of hydraulic fracturing fractures affects the flow of oil and gas, proppant migration and fracture closure, so three-dimensional reconstruction of fracturing fractures is of great significance for analyzing the fracturing mechanism and evaluating the fracturing transformation effect. For conventional hydraulic fracturing physical simulation experiments where the main component of the fracturing fluid is water, the currently widely used method for fracturing fracture visualization is to first add tracers to the water, then cut the post-fracture sample, observe the area where the tracer is located with the naked eye, and finally use CAD, 3DMax, Solidworks and other drawing software to abstractly summarize the three-dimensional fracture surface. Although this method is relatively simple, the accuracy of the three-dimensional surface construction is not high, and it is generally used for qualitative analysis, and this method is not suitable for samples after gas pressure fracturing without tracers. In addition, there are technologies such as electronic computer tomography, 3D scanning, electron microscope scanning and acoustic emission that can be applied to the study of fracture observation, but the use premise and application effect of these technologies are limited. For example, although electronic computed tomography can also be used to observe three-dimensional cracks inside fractured samples, this technology is still limited by factors such as rock density, sample size, and crack width. In particular, it is more difficult to extract small cracks and secondary cracks around the drilled holes that do not penetrate the sample, which will result in the three-dimensional crack morphology inside the sample not being fully displayed. For completely fractured samples, 3D scanning and electron microscope scanning technology can only obtain data information on the fracture surface and its local area, but cannot accurately describe the overall three-dimensional crack morphology inside the sample. In addition, acoustic emission technology can be used to analyze dynamic microfractures inside rocks based on elastic waves generated by microcracks inside the material, but its positioning effect is limited by basic parameter settings, rock samples, and equipment, and cannot reflect the three-dimensional crack morphology in detail. In addition to using CAD, 3DMax, Solidworks and other drawing software to abstractly summarize the three-dimensional fracture surface, the commonly used fracture surface deterministic reconstruction algorithm is Delaunay triangulation. For example, a 3D scanner is used to scan and construct surfaces one by one along the decomposed blocks of through-fractures, and then the coordinates of the reference points before decomposition are used to transform and splice the obtained surfaces, perform Delaunay triangulation, and then establish a discrete fracture mesh model in combination with reservoir properties. The 3D surface construction accuracy of this method depends entirely on the accuracy of the original data, and is only applicable to fracture surface reconstruction under uniform data. This method is difficult to apply when all discrete points on the fracture surface cannot be fully obtained.

[0003] In summary, for samples after gas pressure fracturing without tracers, the above technical methods are not sufficient to simply and effectively display the three-dimensional cracks inside the samples after fracturing, and cannot guarantee the accuracy of the crack surface. Summary of the invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide a three-dimensional reconstruction method of internal hydraulic fractures in shale samples based on free-form surfaces, which can improve the accuracy of reconstruction of the internal fracture surface of the rock mass and obtain a fracture surface that conforms to the internal characteristics of the real rock mass sample.

[0005] The present invention provides a three-dimensional reconstruction method of internal fractures of a shale sample based on a free-form surface, comprising the following steps:

[0006] Step 1: Obtain an internal crack surface slice grid image and a side distribution grid image of a rock sample containing cracks;

[0007] Step 2: Based on the color information of the internal crack surface slice grid image and the side spread grid image, extract the internal vector crack line set UI and the side vector crack line set US:

[0008] Step 3: Obtaining a corresponding positioning vector file for unifying the position information of the cracks in the internal crack surface slice grid image and the side spread grid image;

[0009] Step 4: Use the corresponding positioning vector file to unify the coordinate system of the vector crack lines inside and on the surface of the rock mass sample, and obtain the internal vector crack line set after unified coordinates. and side vector crack lines collection

[0010] Step 5: Bidirectional connection and sorting algorithm based on distance and angle is used to obtain the internal vector crack line set after obtaining unified coordinates. Connect and sort to get a new internal vector crack line collection UI * ;

[0011] Step 6: Based on the double-constrained recursive optimization algorithm of normal cross product and distance, the new internal vector crack line set UI is automatically determined * The relationship between any crack line and crack surface in the , and establish a crack line set G containing the relationship for automatic surface construction;

[0012] Step 7: Based on the boundary line construction algorithm comparing the normal vectors of the crack line and the crack surface, the boundary lines are added according to the existing crack lines, and based on the spline surface and Coons surface algorithm, several points on the crack lines of the same crack surface in the crack line set G are used as surface control points to automatically reconstruct the crack surface of the specimen.

[0013] In the free-form surface-based three-dimensional reconstruction method of internal fractures of shale samples of the present invention, step 1 is specifically as follows:

[0014] Step 1.1: Customize the slice thickness, make cutting marks along the height direction of the sample, and slice the sample after fracturing using a fine cutting machine to obtain n slices;

[0015] Step 1.2: For each slice, shoot the slice center with the camera focus to obtain the internal crack surface slice grid image of the rock sample; shoot the side distribution of the rock sample with scale through the camera to obtain the side distribution grid image of the rock sample; since the upper and lower surfaces of each slice are photographed, 2n slice grid images containing cracks are obtained to form a set of internal crack surface slice grid images and 1 side distribution grid image containing cracks.

[0016] In the free-form surface-based three-dimensional reconstruction method for internal fractures of shale samples of the present invention, step 2 is specifically as follows:

[0017] Step 2.1: In QGIS software, use the Geotrace plug-in to extract the slice vector fracture line subset from each internal fracture surface slice raster image and save it as a *.dxf file; the slice vector fracture line subsets extracted from all internal fracture surface slice raster images form the internal vector fracture line set of the entire rock mass sample UI = {ui 1 ,ui 2 ,…,ui i ,…,ui 2n};

[0018] Step 2.2: In QGIS software, use the Geotrace plug-in to extract the side vector crack line set US = {us 1 ,us 2 ,…,us i ,…,us n} and save it as a *.dxf file.

[0019] In the free-form surface-based three-dimensional reconstruction method of internal fractures of shale samples of the present invention, step 3 is specifically as follows:

[0020] Step 3.1: In QGIS software, use the Geotrace plug-in to draw two line segments on the specimen contour from each internal crack surface slice raster image as internal positioning vector segments, save them as *.dxf files, and use them as the slice vector crack line subset ui i The positioning vector file is composed of the internal positioning vector line segments extracted from all the internal fracture surface slice raster images to form the internal positioning vector line segment set of this rock sample;

[0021] Step 3.2: In QGIS software, through the Geotrace plug-in, from the side spread raster image of the rock sample, draw two parallel edges on the rock sample contour as side positioning vector segments, save them as *.dxf files, and use them as the positioning vector file of the side vector crack line set US.

[0022] In the free-form surface-based three-dimensional reconstruction method for internal fractures of shale samples of the present invention, step 4 is specifically as follows:

[0023] Step 4.1: Establish a local coordinate system of the rock sample, with the center of the bottom of the rock sample as the origin, the line connecting the second end point of the first line segment in the side positioning vector line segment and the origin as the X-axis, and the line connecting the center of the bottom of the rock sample to the center of the top of the rock sample as the Z-axis, to establish a rectangular coordinate system;

[0024] Step 4.2: Use the internal positioning vector line segment set of the rock sample to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the internal vector fracture line set UI into the internal vector fracture line set after the unified coordinates of the rock sample.

[0025] Step 4.3: Use two side positioning vector segments to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the side vector crack line set US into the side vector crack line set after the unified coordinates of the rock sample

[0026] In the free-form surface-based three-dimensional reconstruction method of internal fractures of shale samples of the present invention, step 5 is specifically as follows:

[0027] Step 5.1: Loop through the internal vector crack line set after the unified coordinate system calculate Any two internal crack lines L 1 and L 2 Trend V 1 and V 2 The angle θ and calculate L 1 The first endpoint and L 2 The distance D between the first and last endpoints 11 and D 12 , calculate L 1 The end point of L 2 The distance D between the first and last endpoints 21 and D 22 Minimum distance between:

[0028] D=MIN(D 11 ,D 12 D 21 ,D22 )

[0029] Step 5.2: When and When the two internal crack lines are connected into one crack line, all internal crack lines are looped through to complete the set The connection operation; where is the preset connection angle threshold, is the preset connection distance threshold;

[0030] Step 5.3: After completing the join operation, traverse the collection again Any crack line l in 1 ,p 2 ,…,p i ,…,p n},p i is a point in l, when the first endpoint p of l 1 Less than the end point p n , define temporary line t equal to l, clear l, and add the points in t to l one by one in reverse order from n to 1. Sorting of arbitrary crack lines in ;

[0031] Step 5.4: The fracture lines after connection and sorting are combined into a new internal fracture line set Represents the jth internal vector crack line in the ith raster image.

[0032] In the free-form surface-based three-dimensional reconstruction method for internal fractures of shale samples of the present invention, step 6 is specifically as follows:

[0033] Step 6.1: Loop through the new set of internal crack lines Each crack line in At the same time, loop through each crack line in the i+1th grid image Calculate crack lines and The cross product b of the normal and calculate the crack line and The minimum plane distance d between the endpoints;

[0034] Step 6.2: When and When the two crack lines are close to parallel and their vertical projections are close to coincide, the crack lines The label of is k (0<k<n), and belong to the same crack surface, and the crack surface is marked as k; among them, is the preset facet parallel threshold, which is close to 0; is the preset facet distance threshold, which is close to 0;

[0035] Step 6.3: Create an empty set G = {g 1 ,g 2 ,…,g i ,…,g n} is used to store the crack lines used for faceting. Each subset g i are used to store several crack lines marked as i (0<i<n) for surface construction; according to the subset g i Crack line height in, truncated side vector crack line collection The side crack line in the middle and the gap between g i The side crack lines within the height of the middle crack line add g i .

[0036] In the free-form surface-based three-dimensional reconstruction method for internal fractures of shale samples of the present invention, step 7 is specifically as follows:

[0037] Step 7.1: Traverse the set G = {g 1 ,g 2 ,…,g i ,…,g n} i Perform spline surface construction, when the subset g i When the number of crack lines in is less than 2, a crack line is constructed based on the affine transformation principle and added to g i in; when i When the number of fracture lines in is less than 4, the boundary line construction algorithm based on the occurrence comparison is used to calculate the subset g i There are two crack lines m 1 ,m 2 The normal vector And calculate the subset g i The normal vector of the plane formed by any three endpoints of the crack line in Determine the relative position relationship of the crack lines, and add two crack lines according to the boundary position relationship based on the affine transformation theory to form a boundary line close to the boundary of the quadrilateral;

[0038] Step 7.2: Based on the spline surface or Coons surface, subset g i The crack lines in the image are used as constraint lines to calculate the crack surface and obtain the current subset g. i The fracture surface under constraint.

[0039] The present invention discloses a three-dimensional reconstruction method of internal hydraulic fractures of shale samples based on free surfaces. Based on spline surfaces and Coons surface algorithms, vector fracture lines in a unified coordinate system are used to intuitively display and quantitatively describe the spatial morphology of sample fractures after hydraulic fracturing, providing a more accurate model basis for the quantitative characterization of the geometric characteristics of the hydraulic fracture surface and the numerical simulation research on the seepage characteristics of the fracture surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a flow chart of a method for three-dimensional reconstruction of internal fractures of a shale sample based on a free-form surface of the present invention. DETAILED DESCRIPTION

[0041] like Figure 1 As shown, a three-dimensional reconstruction method of internal hydraulic fractures of a shale sample based on a free-form surface of the present invention comprises the following steps:

[0042] Step 1: Obtain the internal crack surface slice grid image and the side distribution grid image of the rock sample containing cracks. Step 1 is specifically as follows:

[0043] Step 1.1: Customize the slice thickness, make cutting marks along the height direction of the sample, and slice the sample after fracturing using a fine cutting machine to obtain n slices;

[0044] Step 1.2: For each slice, shoot the slice center with the camera focus to obtain the internal crack surface slice grid image of the rock sample; shoot the side distribution of the rock sample with scale through the camera to obtain the side distribution grid image of the rock sample; since the upper and lower surfaces of each slice are photographed, 2n slice grid images containing cracks are obtained to form a set of internal crack surface slice grid images and 1 side distribution grid image containing cracks.

[0045] In this embodiment, the slice thickness is first customized, and then cutting marks are made along the height direction of the cylindrical sample, and finally the sample after fracturing is sliced ​​using a fine cutting machine to obtain circular slices. The method for obtaining the crack surface slice grid image is: first, the circular slice is placed in a marked circle with scales and angles, and then the camera focus is directly on the center of the slice to shoot, so as to obtain the original data of the crack on the slice surface. The method for obtaining the crack side distribution grid image is: first, a transparent thermoplastic film is wrapped on the side of the cylindrical sample, and then the side crack is outlined to the transparent thermoplastic film, and finally the transparent thermoplastic film is spread according to the cutting mark points in the height direction of the sample, and the spread area with scales is photographed with a camera to obtain the original data of the crack on the side of the slice.

[0046] The internal crack surface slice grid image and the side distribution grid image of the rock sample containing cracks include color information, and the color information includes the value (r, g, b) of each point in the three RGB channels.

[0047] Step 2: Based on the color information of the internal crack surface slice grid image and the side spread grid image, extract the internal vector crack line set UI and the side vector crack line set US. Step 2 is specifically as follows:

[0048] Step 2.1: In QGIS software, use the Geotrace plug-in to extract the slice vector fracture line subset from each internal fracture surface slice raster image and save it as a *.dxf file; the slice vector fracture line subsets extracted from all internal fracture surface slice raster images form the internal vector fracture line set of the entire rock mass sample UI = {ui 1 ,ui 2 ,…,ui i ,…,ui 2n};

[0049] Step 2.2: In QGIS software, use the Geotrace plug-in to extract the side vector crack line set US = {us 1 ,us 2 ,…,us i ,…,us n} and save it as a *.dxf file.

[0050] Step 3: Obtain the corresponding positioning vector file used to unify the position information of the cracks in the internal crack surface slice grid image and the side spread grid image. Step 3 is specifically as follows:

[0051] Step 3.1: In QGIS software, use the Geotrace plug-in to draw two line segments on the specimen contour from each internal crack surface slice raster image as internal positioning vector segments, save them as *.dxf files, and use them as the slice vector crack line subset ui i The positioning vector file is composed of the internal positioning vector line segments extracted from all the internal fracture surface slice raster images to form the internal positioning vector line segment set of this rock sample;

[0052] Step 3.2: In QGIS software, through the Geotrace plug-in, from the side spread raster image of the rock sample, draw two parallel edges on the rock sample contour as side positioning vector segments, save them as *.dxf files, and use them as the positioning vector file of the side vector crack line set US.

[0053] Step 4: Use the corresponding positioning vector file to unify the coordinate system of the vector crack lines inside and on the surface of the rock mass sample, and obtain the internal vector crack line set after unified coordinates. and side vector crack lines collection Step 4 is as follows:

[0054] Step 4.1: Establish a local coordinate system of the rock sample, with the center of the bottom of the rock sample as the origin, the line connecting the second end point of the first line segment in the side positioning vector line segment and the origin as the X-axis, and the line connecting the center of the bottom of the rock sample to the center of the top of the rock sample as the Z-axis, to establish a rectangular coordinate system;

[0055] Step 4.2: Use the internal positioning vector line segment set of the rock sample to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the internal vector fracture line set UI into the internal vector fracture line set after the unified coordinates of the rock sample.

[0056] Step 4.3: Use two side positioning vector segments to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the side vector crack line set US into the side vector crack line set after the unified coordinates of the rock sample

[0057] Step 5: Bidirectional connection and sorting algorithm based on distance and angle is used to obtain the internal vector crack line set after obtaining unified coordinates. Connect and sort to obtain a new internal vector crack line set UI*. Step 5 is as follows:

[0058] Step 5.1: Loop through the internal vector crack line set after the unified coordinate system calculate Any two internal crack lines L 1 and L 2 Trend V 1 and V 2 The angle θ and calculate L 1 The first endpoint and L 2 The distance D between the first and last endpoints 11 and D 12 , calculate L 1 The end point of L 2 The distance D between the first and last endpoints 21 and D 22 Minimum distance between:

[0059] D=MIN(D 11 ,D 12 D 21 ,D 22 )

[0060] Step 5.2: When and When the two internal crack lines are connected into one crack line, all internal crack lines are looped through to complete the set The connection operation; where is the preset connection angle threshold, is the preset connection distance threshold;

[0061] Step 5.3: After completing the join operation, traverse the collection again Any crack line l in 1 ,p 2 ,…,p i ,…,p n},p i is a point in l, when the first endpoint p of l 1 Less than the end point p n , define temporary line t equal to l, clear l, and add the points in t to l one by one in reverse order from n to 1. Sorting of arbitrary crack lines in ;

[0062] Step 5.4: The fracture lines after connection and sorting are combined into a new internal fracture line set i∈[1,2n],j∈[1,n], Represents the jth internal vector crack line in the ith raster image.

[0063] Step 6: Based on the double-constrained recursive optimization algorithm of normal cross product and distance, the new internal vector crack line set UI is automatically determined * The relationship between any crack line and crack surface in the , and establish a crack line set G containing the relationship for automatic surface construction, step 6 is specifically as follows:

[0064] Step 6.1: Loop through the new set of internal crack lines Each crack line in At the same time, loop through each crack line in the i+1th grid image Calculate crack lines and The cross product b of the normal and calculate the crack line and The method for calculating the plane distance d is the same as the calculation process in step 5.1 and will not be repeated here.

[0065] Step 6.2: When and When the two crack lines are close to parallel and their vertical projections are close to coincide, the crack lines The label of is k (0<k<n), and belong to the same crack surface, and the crack surface is marked as k; among them, is the preset facet parallel threshold, which is close to 0; is the preset facet distance threshold, which is close to 0;

[0066] Step 6.3: Create an empty set G = {g 1 ,g 2 ,…,g i ,…,g n} is used to store the crack lines used for faceting. Each subset g i are used to store several crack lines marked as i (0<i<n) for surface construction; according to the subset g i Crack line height in, truncated side vector crack line collection The side crack line in the middle and the gap between g i The side crack lines within the height of the middle crack line add g i .

[0067] Step 7: Based on the boundary line construction algorithm of the comparison of the normal vectors of the crack line and the crack surface, the boundary line is added according to the existing crack line, and based on the spline surface and Coons surface algorithm, the points on several crack lines of the same crack surface in the crack line set G are used as surface control points to automatically reconstruct the crack surface of the sample. Step 7 is as follows:

[0068] Step 7.1: Traverse the set G = {g 1 ,g 2 ,…,g i ,…,g n} i Perform spline surface construction, when the subset g i When the number of crack lines in is less than 2, a crack line is constructed based on the affine transformation principle and added to g i in; when i When the number of fracture lines in is less than 4, the boundary line construction algorithm based on the occurrence comparison is used to calculate the subset g i There are two crack lines m 1 ,m 2 The normal vector And calculate the subset g i The normal vector of the plane formed by any three endpoints of the crack line Determine the relative position relationship of the crack lines (close to parallel, close to vertical), and add two crack lines according to the boundary position relationship based on the affine transformation theory to form a boundary line close to the boundary of the quadrilateral;

[0069] Step 7.2: Based on the spline surface or Coons surface, subset g i The crack lines in the image are used as constraint lines to calculate the crack surface and obtain the current subset g. i The fracture surface under constraint.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the concept of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A three-dimensional reconstruction method of internal fractures of shale samples based on free-form surfaces, characterized in that: The steps include: Step 1: Obtain an internal crack surface slice grid image and a side distribution grid image of a rock sample containing cracks; Step 2: Based on the color information of the internal crack surface slice grid image and the side spread grid image, extract the internal vector crack line set UI and the side vector crack line set US: Step 3: Obtaining a corresponding positioning vector file for unifying the position information of the cracks in the internal crack surface slice grid image and the side spread grid image; Step 4: Use the corresponding positioning vector file to unify the coordinate system of the vector crack lines inside and on the surface of the rock mass sample, and obtain the internal vector crack line set after unified coordinates. and side vector crack lines collection Step 5: Bidirectional connection and sorting algorithm based on distance and angle is used to obtain the internal vector crack line set after obtaining unified coordinates. Connect and sort to get a new internal vector crack line collection UI * ; Step 6: Based on the double-constrained recursive optimization algorithm of normal cross product and distance, the new internal vector crack line set UI is automatically determined * The relationship between any crack line and crack surface in the , and establish a crack line set G containing the relationship for automatic surface construction; Step 7: Based on the boundary line construction algorithm comparing the normal vectors of the crack line and the crack surface, the boundary lines are added according to the existing crack lines, and based on the spline surface and Coons surface algorithm, several points on the crack lines of the same crack surface in the crack line set G are used as surface control points to automatically reconstruct the crack surface of the specimen.

2. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 1, characterized in that: Step 1 is as follows: Step 1.1: Customize the slice thickness, make cutting marks along the height direction of the sample, and slice the sample after fracturing using a fine cutting machine to obtain n slices; Step 1.2: For each slice, shoot the slice center with the camera focus to obtain the internal crack surface slice grid image of the rock sample; shoot the side distribution of the rock sample with scale through the camera to obtain the side distribution grid image of the rock sample; since the upper and lower surfaces of each slice are photographed, 2n slice grid images containing cracks are obtained to form a set of internal crack surface slice grid images and 1 side distribution grid image containing cracks.

3. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 2, characterized in that: Step 2 is as follows: Step 2.1: In QGIS software, use the Geotrace plug-in to extract the slice vector fracture line subset from each internal fracture surface slice raster image and save it as a *.dxf file; the slice vector fracture line subsets extracted from all internal fracture surface slice raster images form the internal vector fracture line set of the entire rock mass sample UI = {ui 1 ,ui 2 ,…,ui i ,…,ui 2n }; Step 2.2: In QGIS software, use the Geotrace plug-in to extract the side vector crack line set US = {us1, us2, …, us i ,…,us n } and save it as a *.dxf file.

4. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 3, characterized in that: Step 3 is as follows: Step 3.1: In QGIS software, use the Geotrace plug-in to draw two line segments on the specimen contour from each internal crack surface slice raster image as internal positioning vector segments, save them as *.dxf files, and use them as the slice vector crack line subset ui i The positioning vector file is composed of the internal positioning vector line segments extracted from all the internal fracture surface slice raster images to form the internal positioning vector line segment set of this rock sample; Step 3.2: In QGIS software, through the Geotrace plug-in, from the side spread raster image of the rock sample, draw two parallel edges on the rock sample contour as side positioning vector segments, save them as *.dxf files, and use them as the positioning vector file of the side vector crack line set US.

5. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 4, characterized in that: Step 4 is as follows: Step 4.1: Establish a local coordinate system of the rock sample, with the center of the bottom of the rock sample as the origin, the line connecting the second end point of the first line segment in the side positioning vector line segment and the origin as the X-axis, and the line connecting the center of the bottom of the rock sample to the center of the top of the rock sample as the Z-axis, to establish a rectangular coordinate system; Step 4.2: Use the internal positioning vector line segment set of the rock sample to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the internal vector fracture line set UI into the internal vector fracture line set after the unified coordinates of the rock sample. Step 4.3: Use two side positioning vector segments to transform the coordinate system through translation, rotation, scaling, and symmetry operations to convert the side vector crack line set US into the side vector crack line set after the unified coordinates of the rock sample 6. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 5, characterized in that: Step 5 is as follows: Step 5.1: Loop through the internal vector crack line set after the unified coordinate system calculate The angle θ between the strike directions V1 and V2 of any two internal crack lines L1 and L2 is calculated, and the distance D between the head end point of L1 and the head end point and tail end point of L2 is calculated. 11 and D 12 , calculate the distance D between the end point of L1 and the beginning and end points of L2 21 and D 22 Minimum distance between: D=MIN(D 11 ,D 12 D 21 ,D 22 ) Step 5.2: When and When the two internal crack lines are connected into one crack line, all internal crack lines are looped through to complete the set The connection operation; where is the preset connection angle threshold, is the preset connection distance threshold; Step 5.3: After completing the join operation, traverse the collection again Any crack line l={p1,p2,…,p i ,…,p n },p i is a point in l, when the first endpoint p1 of l is smaller than the last endpoint p n , define temporary line t equal to l, clear l, and add the points in t to l one by one in reverse order from n to 1. Sorting of arbitrary crack lines in ; Step 5.4: The fracture lines after connection and sorting are combined into a new internal fracture line set Represents the jth internal vector crack line in the ith raster image.

7. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 1, characterized in that: Step 6 is as follows: Step 6.1: Loop through the new set of internal crack lines Each crack line in At the same time, loop through each crack line in the i+1th grid image Calculate crack lines and The cross product b of the normal and calculate the crack line and The minimum plane distance d between the endpoints; Step 6.2: When and When the two crack lines are close to parallel and their vertical projections are close to coincide, the crack lines The label of is k (0<k<n), and belong to the same crack surface, and the crack surface is marked as k; among them, is the preset facet parallel threshold, which is close to 0; is the preset facet distance threshold, which is close to 0; Step 6.3: Create an empty set G = {g1, g2, ..., g i ,…,g n } is used to store the crack lines used for faceting. Each subset g i are used to store several crack lines marked as i (0<i<n) for surface construction; according to the subset g i Crack line height in, truncated side vector crack line collection The side crack line in the middle and the gap between g i The side crack lines within the height of the middle crack line add g i .

8. The method for three-dimensional reconstruction of internal fractures of shale samples based on free-form surfaces according to claim 1, characterized in that: Step 7 is as follows: Step 7.1: Traverse the set G = {g1, g2, ..., g i ,…,g n } i Perform spline surface construction, when the subset g i When the number of crack lines in is less than 2, a crack line is constructed based on the affine transformation principle and added to g i middle; When i When the number of fracture lines in is less than 4, the boundary line construction algorithm based on the occurrence comparison is used to calculate the subset g i There are two crack lines m1 and m2 with normal vectors And calculate the subset g i The normal vector of the plane formed by any three endpoints of the crack line in Determine the relative position relationship of the crack lines, and add two crack lines according to the boundary position relationship based on the affine transformation theory to form a boundary line close to the boundary of the quadrilateral; Step 7.2: Based on the spline surface or Coons surface, subset g i The crack lines in the image are used as constraint lines to calculate the crack surface and obtain the current subset g. i The fracture surface under constraint.

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