Coal multi-scale anisotropic digital core construction method

Abstract

The invention discloses a coal multi-scale anisotropic digital core construction method. The method comprises the following steps: S1, obtaining a representative body unit of a fracture network model and taking the representative body unit as a fracture digital core; s2, performing a high-pressure mercury injection test on the coal sample, determining different scales of coal sample development pores, and extracting parameter information of the coal sample development pores with different scales; s3, calculating the distribution probability of development pores of the coal samples with different scales; s4, scanning the coal samples to obtain azimuth angles and length-width ratios of coal sample development pores with different scales; s5, calculating the global pore orientation tensor of the development pores of the coal samples with different scales, and taking the tensor as the growth probability of the development pores of the coal samples with different scales; s6, according to the distribution probability, the growth probability and the parameter information of the coal sample development pores, respectively generating digital cores of the coal sample development pores with different scales; and S7, fusing the digital core of the coal sample development pores on the basis of the fracture digital core, and constructing to obtain the coal multi-scale anisotropic digital core.

Description

Technical Field

[0001] The invention relates to the field of coalbed methane exploration and development, and in particular to a method for constructing a multi-scale anisotropic digital core of coal. Background Art

[0002] Digital core technology is an interdisciplinary technology that combines geology, physics and computer science. It studies the microstructure and fluid flow characteristics of rocks by constructing a three-dimensional digital model of rocks. This technology has important application significance in coalbed methane exploration, development and improving recovery:

[0003] ① Research on microscopic seepage mechanism: Digital cores can accurately characterize the rock pore structure at the micron scale, which helps to deeply understand the flow behavior of fluids in rock pores and the microscopic seepage mechanism.

[0004] ② Macro-conductivity prediction: Through the digital core model, the macro-conductivity properties of rocks can be predicted, which is crucial for the development and production strategy formulation of coalbed methane.

[0005] ③ Dynamic simulation of coalbed methane reservoir production: Digital core technology can simulate the production dynamics of coalbed methane reservoirs, helping engineers optimize production strategies and improve the ultimate recovery rate of coalbed methane reservoirs.

[0006] At present, the methods for constructing digital cores mainly include slice combination method, X-ray stereo imaging method, numerical simulation method (process method and random method) and artificial intelligence method. Among them, the slice combination method prepares a large number of core slices and combines them, which is time-consuming and rarely used. The X-ray stereo imaging method uses an X-ray micro-imager to obtain a three-dimensional image of the core, and its equipment is expensive and the cost is high. The digital core generated by the numerical simulation method cannot accurately describe the anisotropy and pore morphology of the reservoir. The artificial intelligence method requires a large amount of training data when constructing digital cores. If the data quality is not high or the quantity is insufficient, it may affect the accuracy and generalization ability of the model.

[0007] In addition, coal reservoirs often develop multi-scale pores, including micropores (<10nm), small pores (10-100nm), transitional pores (100-1000nm) and macropores (>1000nm). Although the existing multi-scale digital core construction method provides a digital core containing pores and fractures, it does not consider the multi-scale properties of the pores themselves and the morphological characteristics of the pores, which affects the accuracy of the digital core; therefore, it is necessary to provide a digital core modeling method that contains fracture and multi-scale pore information to improve the accuracy of the digital core. Summary of the invention

[0008] The purpose of the present invention is to provide a method for constructing a multi-scale anisotropic digital core of coal based on characterizing the anisotropy and morphological differences of pores and fractures in coal, so as to construct a multi-scale and complex morphological digital core of coal with fractures, macropores, transitional pores, mesopores and micropores.

[0009] In order to achieve the above object, the technical solution of the present invention is:

[0010] A method for constructing a multi-scale anisotropic digital core of coal comprises the following steps:

[0011] S1. With the help of X-ray scanner, the fracture network model of coal sample is obtained through three-dimensional reconstruction, filtering, noise reduction and threshold segmentation. Based on the porosity, the representative volume unit (REV) of the fracture network model is extracted and used as the fracture digital core;

[0012] S2. Conduct high-pressure mercury injection test on coal samples, obtain a scatter plot of stage mercury injection amount-pore diameter through mercury injection data, determine different scales of coal sample pore development based on the scatter plot of stage mercury injection amount-pore diameter, and extract parameter information of coal sample pore development of different scales;

[0013] S3, calculating the distribution probability of the pores developed in coal samples of different scales through the parameter information of the pores developed in coal samples of different scales;

[0014] S4. Scan the coal sample with an electron microscope, and obtain the azimuth angle data and aspect ratio data of the pores of coal samples of different scales based on the electron microscope scanning photos;

[0015] S5. Calculate the global pore orientation tensor of the pores developed in coal samples of different scales based on the electron microscope scanning photos, and use it as the growth probability of the pores developed in coal samples of different scales;

[0016] S6. Generate digital cores of coal samples with different scales of pore development according to the distribution probability, growth probability and parameter information of coal samples with different scales of pore development;

[0017] S7. Through image fusion operation, digital cores of coal samples with different scales of pore development are fused on the basis of digital cores of fractures to construct multi-scale anisotropic digital cores of coal.

[0018] Furthermore, in step S1, the representative volume units of the fracture network model include fractures and a coal matrix skeleton.

[0019] Furthermore, in step S2, after determining the different scales of the pores developed in the coal sample, the pores developed in the coal sample are divided into macropores, transitional pores, small pores and micropores according to the different scales.

[0020] Furthermore, in step S2, the parameter information is the pore volume, dominant pore size and porosity of the pores developed in the single-scale coal sample.

[0021] Furthermore, in step S3, the calculation formula for the pore distribution probability f of coal samples of different scales is:

[0022]

[0023] In the formula, v i is the volume of pores in coal samples of different scales, r i is the dominant pore size of coal samples with different scales, i is macropore, transitional pore, small pore or micropore, v is d is the total volume of the representative volume unit of the fracture network model.

[0024] Furthermore, in step S4, Image-J software is used to segment the coal sample pores in the electron microscope scanning photograph, and the azimuth and aspect ratio of the segmented coal sample pores are extracted.

[0025] Furthermore, in step S5, calculating the global pore orientation tensor of pores developed in coal samples of different scales specifically includes the following steps:

[0026] S71. Determine the geometric center or center of gravity coordinates (x c ,y c , z c );

[0027] S72. Find the endpoint coordinates (x p ,y p , z p ), calculate the normal vector t; its calculation formula is:

[0028]

[0029] S73. Normalize the normal vector t of the pores of a single coal sample and convert it into a unit normal vector n; the calculation formula is:

[0030]

[0031] S74. Calculate the global pore orientation tensor P through the unit normal vector n of all coal sample pores; the calculation formula is:

[0032]

[0033] Where N is the number of pores in the coal sample counted from electron microscope scanning photos. represents the outer product of vectors, n j is the unit normal vector of the pores in the jth coal sample.

[0034] Furthermore, in step S6, a macropore digital core is generated according to the distribution probability, growth probability and porosity of the macropores; a transition pore digital core is generated according to the distribution probability, growth probability and porosity of the transition pores; a small pore digital core is generated according to the distribution probability, growth probability and porosity of the small pores; and a micropore digital core is generated according to the distribution probability, growth probability and porosity of the micropores.

[0035] Furthermore, in step S7, the macropore digital core, transitional pore digital core, small pore digital core and micropore digital core are integrated on the basis of the fracture digital core to construct a multi-scale anisotropic digital core of coal.

[0036] Compared with the prior art, the present invention has the following advantages and positive effects:

[0037] The present invention first obtains representative volume units of a three-dimensional fracture network model as a fracture digital core, and then determines the pores and porosities of coal samples of different scales according to a stage mercury injection amount-pore diameter scatter plot; then calculates the distribution probability and growth probability of the pores of coal samples of various scales, generates digital cores of coal samples of different scales through the distribution probability, growth probability and porosity of the pores of coal samples of various scales, and finally fuses the digital cores of coal samples of different scales with the fracture digital core to obtain a multi-scale anisotropic digital core of coal; the digital core obtained by the construction method has multi-scale and complex morphological fractures, macropores, transitional pores, mesopores and micropores, can truly describe the anisotropy and morphological differences of pore fractures, effectively improves the accuracy of the digital core, and has great reference significance for coalbed methane exploration, development and improving recovery rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0039] FIG1 is a diagram showing the extraction effect of a fracture digital core; FIG1(a) is a schematic diagram of a reconstructed three-dimensional coal rock, FIG1(b) is a schematic diagram of a segmented fracture, FIG1(c) is a schematic diagram of a representative volume unit (REV) based on porosity, and FIG1(d) is a schematic diagram of a constructed fracture digital core;

[0040] Figure 2 The scatter plot of mercury injection amount and pore diameter in each stage of mercury injection data;

[0041] Figure 3 This is a schematic diagram of an electron microscope scanning photograph of a coal sample;

[0042] Figure 4 Schematic diagram of anisotropic digital core containing fractures and pores of different scales. DETAILED DESCRIPTION

[0043] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work, any modifications, equivalent substitutions, improvements, etc., should be included in the protection scope of the present invention.

[0044] This embodiment discloses a method for constructing a multi-scale anisotropic digital core of coal, which comprises the following steps:

[0045] 1. With the help of X-ray scanner coal sample, the three-dimensional fracture network model of coal rock is obtained through three-dimensional reconstruction, filtering, noise reduction and threshold segmentation, as shown in Figure 1(a) and Figure 1(b);

[0046] 2. Based on the porosity, the representative volume unit (REV) of the three-dimensional fracture network model is extracted. The representative volume unit includes fractures and coal matrix skeleton, with a total volume of v d , taking the representative volume unit as the fracture digital core, as shown in Figure 1(c) and Figure 1(d);

[0047] 3. To generate pores of different scales in the coal matrix skeleton representing the volume unit of the three-dimensional fracture network model, it is necessary to determine the distribution probability f, growth probability x and porosity φ of pores of different scales;

[0048] 4. Grind the coal sample to about 80 meshes and conduct high-pressure mercury injection test to obtain the stage mercury injection amount-pore diameter scatter plot of mercury injection data, such as Figure 2 As shown (in Figure 2 In the figure, pore scale 1 represents micropores, pore scale 2 represents transitional pores, and pore scale 3 represents macropores); based on this analysis, it is concluded that the coal sample in this test only develops micropores, transitional pores and macropores;

[0049] 5. Extract the pore structure information of micropores, transitional pores and macropores respectively, including pore volume v, dominant pore diameter r and porosity φ;

[0050] 6. The distribution probability of pores of different scales is:

[0051]

[0052] In the formula, v i represents the volume of pores of different sizes, r iRepresents the dominant pore size of pores of different scales, i is h, g, w, where h represents macropores, g represents transitional pores, and w represents micropores.

[0053] 7. The coal samples were polished with 800 mesh, 1200 mesh, 2000 mesh and 5000 mesh sandpaper in turn, and then argon ion polished, followed by scanning electron microscope observation test. The observation results are as follows: Figure 3 As shown in the figure, the morphological characteristics of coal sample pores were observed by scanning electron microscopy, and the electron microscope scanning photos were segmented using Image-J software to extract the growth azimuth θ and aspect ratio I of coal rock pores, analyze their development anisotropy, and calculate their pore growth probability;

[0054] 8. The pore growth probability can be characterized by the pore orientation, and a tensor is introduced to determine the main growth orientation of the pores. The growth probability of pores of various scales can be characterized by the global pore orientation tensor P of macropores, transitional pores and micropores. The calculation steps of the global pore orientation tensor P are as follows:

[0055] 8.1. Calculate the unit normal vector of the pore, which refers to the directional vector pointing from the pore center to the pore wall or pore mouth; the calculation steps are as follows:

[0056] (1) Use Image-J software to segment the pores of coal samples in electron microscope scanning photos;

[0057] (2) Determine the geometric center or centroid coordinates (x c ,y c , z c );

[0058] (3) Determine the normal vector of a single pore; first find the endpoint coordinates of the pore's long axis (x p ,y p , z p ), and then find the normal vector t:

[0059]

[0060] (4) Normalized normal vector t: convert the normal vector of a single pore into a unit normal vector n;

[0061]

[0062] 8.2. Calculate the global pore orientation tensor P;

[0063]

[0064] Where N is the number of macropores, transitional pores, and micropores counted from electron microscope scanning photos. represents the outer product of vectors, n jis the unit normal vector of the jth pore of different scales.

[0065] Based on the above calculations, the global pore orientation tensor of macropores, the global pore orientation tensor of transitional pores and the global pore orientation tensor of micropores are obtained respectively.

[0066] 9. Generate macropore digital core, transition pore digital core and micropore digital core respectively according to the porosity, distribution probability and growth probability (global pore orientation tensor) of macropores, transition pores and micropores.

[0067] 10. Through the image fusion method, the fracture digital core, macro digital core, transition hole digital core and micropore digital core are fused to construct a multi-scale anisotropic digital core of coal, such as Figure 4 shown.

[0068] The present invention first obtains representative volume units of a three-dimensional fracture network model as a fracture digital core, and then determines the pores and porosities of coal samples of different scales according to a stage mercury injection amount-pore diameter scatter plot; then calculates the distribution probability and growth probability of the pores of coal samples of various scales, generates digital cores of coal samples of different scales through the distribution probability, growth probability and porosity of the pores of coal samples of various scales, and finally fuses the digital cores of coal samples of different scales with the fracture digital core to obtain a multi-scale anisotropic digital core of coal; the digital core obtained by the construction method has multi-scale and complex morphological fractures, macropores, transitional pores, mesopores and micropores, can truly describe the anisotropy and morphological differences of pore fractures, effectively improves the accuracy of the digital core, and has great reference significance for coalbed methane exploration, development and improving recovery rate.

Claims

1. A method for constructing multi-scale anisotropic digital core of coal, characterized by: The following steps are involved: S1. With the help of X-ray scanner, the fracture network model of coal sample is obtained through three-dimensional reconstruction, filtering, noise reduction and threshold segmentation. Based on the porosity, the representative volume unit of the fracture network model is extracted and used as the fracture digital core; S2. Conduct high-pressure mercury injection test on coal samples, obtain a scatter plot of stage mercury injection amount-pore diameter through mercury injection data, determine different scales of coal sample pore development based on the scatter plot of stage mercury injection amount-pore diameter, and extract parameter information of coal sample pore development of different scales; S3, calculating the distribution probability of the pores developed in coal samples of different scales through the parameter information of the pores developed in coal samples of different scales; S4. Scan the coal sample with an electron microscope, and obtain the azimuth angle data and aspect ratio data of the pores of coal samples of different scales based on the electron microscope scanning photos; S5. Calculate the global pore orientation tensor of the pores developed in coal samples of different scales based on the electron microscope scanning photos, and use it as the growth probability of the pores developed in coal samples of different scales; S6. Generate digital cores of coal samples with different scales of pore development according to the distribution probability, growth probability and parameter information of coal samples with different scales of pore development; S7. Through image fusion operation, digital cores of coal samples with different scales of pore development are fused on the basis of digital cores of fractures to construct multi-scale anisotropic digital cores of coal.

2. The method for constructing a multi-scale anisotropic digital core of coal according to claim 1, characterized in that: In the step S1, the representative volume units of the fracture network model include fractures and a coal matrix skeleton.

3. The method for constructing a multi-scale anisotropic digital core of coal according to claim 1, characterized in that: In step S2, after determining the different scales of the pores developed in the coal sample, the pores developed in the coal sample are divided into macropores, transitional pores, small pores and micropores according to the different scales.

4. The method for constructing a multi-scale anisotropic digital core of coal according to claim 3, characterized in that: In step S2, the parameter information is the pore volume, dominant pore size and porosity of the pores developed in the single-scale coal sample.

5. The method for constructing a multi-scale anisotropic digital core of coal according to claim 4, characterized in that: In step S3, the calculation formula for the probability f of pore distribution of coal samples of different scales is: In the formula, v i is the volume of pores in coal samples of different scales, r i is the dominant pore size of coal samples with different scales, i is macropore, transitional pore, small pore or micropore, v is d is the total volume of the representative volume unit of the fracture network model.

6. The method for constructing a multi-scale anisotropic digital core of coal according to claim 5, characterized in that: In step S4, Image-J software is used to segment the coal sample pores in the electron microscope scanning photo, and the azimuth angle and aspect ratio of the segmented coal sample pores are extracted.

7. The method for constructing a multi-scale anisotropic digital core of coal according to claim 6, characterized in that: In step S5, calculating the global pore orientation tensor of pores developed in coal samples of different scales specifically includes the following steps: S71. Determine the geometric center or center of gravity coordinates (x c ,y c , z c ); S72. Find the endpoint coordinates (x p ,y p , z p ), calculate the normal vector t; its calculation formula is: S73. Normalize the normal vector t of the pores of a single coal sample and convert it into a unit normal vector n; the calculation formula is: S74. Calculate the global pore orientation tensor P through the unit normal vector n of all coal sample pores; the calculation formula is: Where N is the number of pores in the coal sample counted from electron microscope scanning photos. represents the outer product of vectors, n j is the unit normal vector of the pores in the jth coal sample.

8. The method for constructing a multi-scale anisotropic digital core of coal according to claim 7, characterized in that: In step S6, a macropore digital core is generated according to the distribution probability, growth probability and porosity of macropores; a transition pore digital core is generated according to the distribution probability, growth probability and porosity of transition pores; a small pore digital core is generated according to the distribution probability, growth probability and porosity of small pores; and a micropore digital core is generated according to the distribution probability, growth probability and porosity of micropores.

9. The method for constructing a multi-scale anisotropic digital core of coal according to claim 8, characterized in that: In the step S7, the macropore digital core, transitional pore digital core, small pore digital core and micropore digital core are integrated on the basis of the fracture digital core to construct a multi-scale anisotropic digital core of coal.

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