Porous medium water body distribution calculation model construction method based on CT structure reconstruction

Abstract

The invention provides a porous medium water body distribution calculation model construction method based on CT structure reconstruction, and the method comprises the steps: selecting a large-scale coal sample according to a scale standard used in a laboratory coal water injection experiment; performing a water injection experiment on the large-scale coal sample according to the selected parameters and a preset water injection simulation scheme so as to simulate the distribution and flow of a water body in the porous medium; performing scanning calculation on the large-scale coal sample in the experimental environment by using a CT scanner to obtain a large-scale structure model; importing the large-scale structure model into finite element model software, and establishing a numerical simulation model; and calculating the numerical simulation model based on fluid exchange between different media to construct a porous medium water body distribution calculation model, thereby realizing research on the distribution change rule of the water body of the injected coal according to the porous medium water body distribution calculation model. Scientific basis can be provided for researching the influence of pore and crack structures on water body distribution of the water injection coal body.

Description

Technical Field

[0001] The present application relates to the technical field of coal and rock analysis, and in particular to a method for constructing a calculation model of water distribution in porous media based on CT structure reconstruction. Background Art

[0002] Low-pressure water injection into coal is an important technical means to improve the recovery rate of coal-gas combined mining. Since the water distribution process of water-injected coal samples is a complex water movement process, the pores, cracks, minerals, and matrix inside the coal samples will affect the water distribution. Therefore, it is necessary to construct a calculation model for the water distribution in porous media to visualize the water distribution inside the porous media with anisotropic structures, and then explore the water distribution law of water-injected coal samples.

[0003] In the existing technology, the structure of coal specimens is usually scanned and reconstructed through industrial CT to provide basic parameters for the water distribution calculation model. Although this method has brought convenience to theoretical calculations and numerical simulations to a certain extent, the mutual constraints between the size of CT scanned coal samples and the scanning accuracy have led to the differentiation of the research objects of the calculation model. Summary of the invention

[0004] In view of this, an embodiment of the present application provides a method for constructing a porous medium water distribution calculation model based on CT structure reconstruction to solve the above technical problems.

[0005] The present application provides a method for constructing a porous medium water distribution calculation model based on CT structure reconstruction, including: According to the scale standard used in laboratory coal water injection experiments, large-scale coal samples were selected; According to the selected parameters and the preset water injection simulation scheme, water injection experiments are carried out on large-scale coal samples to simulate the distribution and flow of water in porous media; The large-scale coal sample in the experimental environment is scanned and calculated using a CT scanner to obtain a large-scale structural model; Import the large-scale structural model into the finite element model software to establish a numerical simulation model; Based on the fluid exchange between different media, the numerical simulation model is solved to construct a calculation model for the water distribution in porous media, so as to study the changing law of water distribution in water-injected coal according to the calculation model for the water distribution in porous media.

[0006] In some embodiments, a large-scale coal sample is selected according to the scale standard used in laboratory coal water injection experiments, wherein a 10×10 cm cubic coal specimen is used.

[0007] In some embodiments, a drill hole is arranged at the center of the surface of the cubic coal specimen, and the drill hole size is 1 cm in diameter.

[0008] In some embodiments, a water injection experiment is conducted on a large-scale coal sample according to selected parameters and a preset water injection simulation scheme to simulate the distribution and flow of water in a porous medium, wherein the selected parameters include the porosity ratio of the coal matrix.

[0009] In some embodiments, the preset water injection simulation scheme is specifically: Constant pressure injection, where the initial injection pressure is 500Pa, the water saturation at the injection position is 1, and the injection study time is 2d.

[0010] In some embodiments, the use of a CT scanner to scan and calculate a large-scale coal sample in an experimental environment to obtain a large-scale structural model is specifically: The two-phase Darcy's law is applied to the anisotropic structural model reconstructed from CT scans to calculate the global anisotropic structure and then obtain a large-scale structural model.

[0011] As can be seen from the above, the embodiment of the present application scans a large-scale coal sample through a CT scanner to obtain a large-scale structural model, and imports the large-scale structural model into the finite element model software to establish a numerical simulation model. Therefore, the numerical simulation model can be solved based on the fluid exchange between different media to construct a calculation model of the water distribution in porous media, which can not only demonstrate the changing laws of fluid transfer at different positions inside the coal sample, but also visualize the water distribution inside the structurally anisotropic porous media.

[0012] At the same time, the embodiment of the present application obtains a numerical simulation model based on a large-scale structural model (DM-level structural model), and solves the numerical simulation model based on fluid exchange between different media, thereby reducing the impact of size effect on the simulation.

[0013] Furthermore, since the embodiment of the present application sets up a numerical simulation model to solve the porous medium water body distribution calculation model, it is very convenient to compare and verify the results of the calculation model with the experimental results, and the fluid saturation distribution calculated by simulation can provide initial state data for solving the porous medium water body distribution-ultrasonic wave propagation calculation model.

[0014] Obviously, the embodiment of the present application improves the water distribution calculation model based on coal sample structure, especially the coal sample structure with large pores and cracks, by constructing a porous media water distribution calculation model combined with CT structure reconstruction, so that it can perfectly display the changing law of fluid transfer at different positions inside the coal sample, and visualize the water distribution inside the structurally anisotropic porous medium, thereby providing a reliable basis for the water distribution law of injected coal samples under the influence of pore and crack structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application, 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 application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 It is a simulation diagram of the calculation model of porous medium water distribution provided in the embodiment of the present application; Figure 2 This is a calculation principle diagram of treating a crack as a porous medium with a porosity close to 1 provided in an embodiment of the present application; Figure 3 It is the water-gas equivalent density distribution when the water body is injected for 2 days provided in the embodiment of the present application; Figure 4 It is a distribution diagram of the fluid distribution velocity field at different water injection times provided in the embodiment of the present application. DETAILED DESCRIPTION

[0017] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present application. However, it should be clear to those skilled in the art that the present application may also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present application.

[0018] It should be understood that when used in this specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0019] It should also be understood that the terms used in this application specification are only for the purpose of describing specific embodiments and are not intended to limit the application. As used in this application specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0020] It should also be understood that the term “and / or” used in the specification and appended claims refers to any and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0021] It should also be understood that the size of the serial numbers of the steps in this embodiment does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of this application.

[0022] In order to illustrate the technical solution described in this application, a specific embodiment is provided below for illustration.

[0023] The present application provides a method for constructing a porous medium water distribution calculation model based on CT structure reconstruction, the method comprising the following steps: Step 1, select a large-scale coal sample according to the scale standard used in laboratory coal water injection experiments; Step 2: According to the selected parameters and the preset water injection simulation scheme, a water injection experiment is conducted on a large-scale coal sample to simulate the distribution and flow of water in the porous medium; Step 3, using a CT scanner to scan and calculate a large-scale coal sample under an experimental environment to obtain a large-scale structural model; Step 4, importing the large-scale structural model into the finite element model software to establish a numerical simulation model; Step 5, based on the fluid exchange between different media, solve the numerical simulation model to construct a porous medium water body distribution calculation model, so as to realize the study of the change law of water body distribution of water-injected coal according to the porous medium water body distribution calculation model.

[0024] Among them, the simulation diagram of the porous medium water body distribution calculation model obtained based on the solution provided in the embodiment of the present application is as follows: Figure 1 As shown, it is obvious that the simulation diagram based on the calculation model can realize the study of the distribution change law of water body in water-injected coal.

[0025] It should be noted that the water injection simulation scheme and parameter selection are the basis for constructing the calculation model of water distribution in porous media, that is, the simulation parameters and schemes need to be determined, and these parameters and schemes will be used to construct the model. The construction of the calculation model of water distribution in porous media depends on the water injection simulation scheme and parameter selection. The model needs these parameters to simulate the distribution and flow of water in porous media. The model is constructed to verify and analyze the effect of the water injection simulation scheme. The output of the model can be used to evaluate the rationality of parameter selection and the effectiveness of the simulation scheme. In short, the water injection simulation scheme and parameter selection are the premise and input for model construction, and the porous media water distribution calculation model based on CT structure reconstruction is a tool and platform for realizing water injection simulation. The two complement each other and serve together for the simulation and analysis of the water injection process.

[0026] It can be seen that the embodiment of the present application scans a large-scale coal sample through a CT scanner to obtain a large-scale structural model, imports the large-scale structural model into the finite element model software, and establishes a numerical simulation model. Therefore, the numerical simulation model can be solved based on the fluid exchange between different media to construct a calculation model of water distribution in porous media, which can not only demonstrate the changing laws of fluid transfer at different positions inside the coal sample, but also visualize the water distribution inside the structurally anisotropic porous media.

[0027] At the same time, the embodiment of the present application obtains a numerical simulation model based on a large-scale structural model (DM-level structural model), and solves the numerical simulation model based on fluid exchange between different media, thereby reducing the impact of size effect on the simulation.

[0028] Furthermore, since the embodiment of the present application sets up a numerical simulation model to solve the porous medium water body distribution calculation model, it is very convenient to compare and verify the results of the calculation model with the experimental results, and the fluid saturation distribution calculated by simulation can provide initial state data for solving the porous medium water body distribution-ultrasonic wave propagation calculation model.

[0029] Obviously, the embodiment of the present application improves the water distribution calculation model based on coal sample structure, especially the coal sample structure with large pores and cracks, by constructing a porous media water distribution calculation model combined with CT structure reconstruction, so that it can perfectly display the changing law of fluid transfer at different positions inside the coal sample, and visualize the water distribution inside the structurally anisotropic porous medium, thereby providing a reliable basis for the water distribution law of injected coal samples under the influence of pore and crack structure.

[0030] In the embodiment of the present application, step 1, according to the scale standard used in the laboratory coal water injection experiment, a large-scale coal sample is selected, wherein, in order to facilitate scanning research, the large-scale coal sample is a cubic structure, specifically, a 10×10 cm cubic coal sample can be used; To further facilitate scanning research, a drill hole with a diameter of 1 cm can be arranged at the center of the face of the cubic coal specimen.

[0031] In the embodiment of the present application, step 2, according to the selected parameters and the preset water injection simulation scheme, a water injection experiment is performed on a large-scale coal sample to simulate the distribution and flow of water in the porous medium, wherein the selected parameters include the porosity ratio of the coal matrix; Porosity calculation in coal matrix This involves quantitative analysis of the size, shape, distribution and connectivity of pores in the coal matrix. Porosity characteristics are key factors affecting fluid flow and transport in coal seams. Therefore, accurate calculation of pores is essential for understanding and predicting fluid behavior in coal seams. The pore calculation of the coal matrix provides basic data for the construction of the calculation model of water distribution in porous media. The calculation results of the pores directly affect the representation of the pore structure in the model and are one of the key input parameters for model construction. The construction of the calculation model of water distribution in porous media depends on the results of the pore calculation of the coal matrix. Accurate pore data enables the model to more realistically reflect the fluid flow and transmission characteristics in the coal seam.

[0032] The embodiment of the present application mainly analyzes the influence of the structure of the coal sample on the distribution of water when water is injected into the coal sample at ultra-low pressure. Therefore, the present embodiment only studies the distribution characteristics of parameters such as water distribution position, saturation, fluid velocity, etc. at different water injection times under a single injection condition. Based on this, the water injection simulation scheme preset in the embodiment of the present application can be specifically: The injection pressure is the constant pressure brought by the fluid horizontal surface, that is, constant pressure injection is selected, where the initial injection pressure is 500Pa, the water saturation at the injection position is 1, the injection study time is 2d, and the transient solution analysis is adopted. The solution time step is 0.001d. The spatiotemporal variation of saturation during the entire water distribution process is: As the injection time increases, the water body gradually spreads outward. When the injection time is short, the saturation of the water body is high and the water body spreads quickly. After a period of injection, the water body spread speed gradually decreases and the water body saturation gradually decreases, indicating that when the water body spreads over a large distance, the pressure at the injection point is not enough to keep the fluid spread at a high saturation. Through analysis, it can be obtained that: in the initial stage of water injection, that is, in the time period of 0.001 to 0.6 days, the water distribution in the fracture area shows a significant advantage, and the water content in the fractures around the borehole reaches a saturation of more than 0.7. As the water injection time increases, this advantage begins to weaken. When the water injection time is between 0.6 and 2 days, the wetting range of the water body gradually expands, but the area that finally reaches a saturation of more than 0.7 is relatively small, and its radius is about half of the radius of the entire water distribution area. During the water injection process, due to the gradual weakening of the effect of Darcy force, its effect is most significant in the fracture area with extremely high porosity. As the Darcy force weakens, the water transport capacity of the fracture also decreases, and the areas with water distribution farther away will transport water to the surrounding uninjected areas, which causes the boundary of the water distribution to gradually change from an irregular shape to a circle.

[0033] In order to further quantitatively analyze the relationship between the variation of water body distribution and wetting range over time, the water body distribution distance in four directions, horizontally to the left, horizontally to the right, vertically upward, and vertically downward, was analyzed. The analysis shows that since the water distribution of the water-injected coal sample is affected by multiple factors, the contour lines of the water equivalent density during the water distribution process can characterize the water distribution law in the porous medium. Therefore, the fluid equivalent density distribution diagram after 2 days of water injection is obtained, as shown in Figure 3As shown; It can be seen that the change trend of the equivalent density contour of the fluid is significantly affected by the distribution characteristics of large-scale fractures. In areas with higher fracture density, the change rate of the fluid density contour is faster and the distribution range of the water body is wider. Further observation shows that Figure 3 It can be seen that when the equivalent density of the fluid is low, for example, in the range of 64 to 188 kg / m³, the intervals between adjacent contour lines are larger, which indicates that in the front area of ​​water distribution, the change in saturation is relatively small, and in the area close to the water injection port, the change in contour lines is more significant. However, as the fluid distribution distance increases, the area with fast flow velocity will replenish water to the area with slow flow velocity. This compensation effect gradually weakens the impact of the change in contour lines near the water injection port. Finally, a pattern is formed: near the water injection port, the distribution difference of high-density fluid is large; and at the position far from the water injection port, the distribution difference of low-density fluid is small.

[0034] In order to further quantitatively analyze the changing relationship between fluids at different positions and different equivalent density contour lines, the center of the borehole is taken as the origin, and 16 reference lines are used to divide the entire area equally, so as to obtain the data of the intersection points of different reference lines and different contour lines. The analysis shows that there is a linear relationship between the equivalent density and the distance from the reference point to the center of the borehole. The larger the equivalent density, the smaller the distance from the reference point to the center of the borehole. Since the two parameters of equivalent density and the distance from the reference point to the center of the borehole have similar changing trends in the direction, in order to quantitatively characterize the overall changing trends of the two, the average value of each contour line is taken to obtain the average relationship equation, in which the distance can be expressed by equivalent density, which can be specifically expressed as follows:

[0035] In the above formula, y represents distance, x represents density, and R represents the radius of the reference point.

[0036] Furthermore, the velocity field of water flow at different water injection times was analyzed, such as Figure 4 As shown in the figure, during the water distribution process of the injected coal sample, the change of velocity field can characterize the contribution of structure to water distribution. When the injection time is short, 0.01-0.06 d, a small high-speed flow ring is formed around the borehole. As the water distribution range expands, the flow characteristics of the fluid change rapidly, and the fluid achieves rapid flow inside the fracture. By analyzing the changes in saturation, it can be determined that the fluid flowing inside the fracture includes air and water. As the annular high-speed flow area gradually expands, the flow velocity in this area gradually decreases. When the water injection time reaches two days, the fluid velocity inside the high-speed ring is almost equal to the velocity outside the ring, and the distribution distance of the water body also reaches the maximum at this time. By comparing the distribution characteristics of the original cracks with the characteristics of the flow velocity field, it can be found that the distribution of cracks around the borehole shows a characteristic that one end is close to the borehole and the other end is far away from the borehole, that is, a pressure difference is formed at different positions of the cracks, which is the key to the rapid flow of water inside the cracks. If the cracks are long and perpendicular to the distribution direction of the water body, the water flow speed inside the cracks is not fast. This shows that the cracks can only effectively guide the fluid flow when there is a significant pressure difference inside the cracks.

[0037] In the embodiment of the present application, step 3 is to use a CT scanner to scan and calculate a large-scale coal sample in an experimental environment to obtain a large-scale structural model, which can be specifically: The two-phase Darcy's law is applied to the anisotropic structural model reconstructed from CT scans to calculate the global anisotropic structure and then obtain a large-scale structural model.

[0038] It can be seen that since the two-phase Darcy's law is applied to the anisotropic structural model reconstructed by CT scanning, the global anisotropic structure can be calculated, and the water distribution calculation model based on the coal sample structure (macroporous pores and cracks) is further improved.

[0039] Specifically, Figure 2 As shown in the figure, the cracks can be regarded as porous media with a porosity close to 1, and the two-phase Darcy law and the diffusion equation are combined to calculate the porous media, small pores in the matrix, and pores in the minerals, and obtain the calculation models of ultra-high permeability media, ultra-low permeability media, and medium permeability media. In addition, in order to reduce the calculation cost, the large-scale structural model is designed as a two-dimensional model, and different material properties can be assigned to different regions of the model. The boundary conditions are determined as follows: the middle borehole wall is a constant pressure injection boundary with an injection pressure of 500 Pa; the four sides of the model are free boundaries, and the outer boundary pressure is 0Pa.

[0040] For example, the pores and fractures in the large-scale structural model are larger pores and fractures with a pore size of more than 100 μm identified by CT scanning, the coal matrix is ​​a porous medium containing pores with a pore size range of 10-100 μm, and the mineral is a larger porous medium with a pore size of more than 90 μm identified by CT scanning.

[0041] CT scan post-processing software AVIZO can directly calculate the pore-fracture structure model to obtain the medium permeability under experimental conditions; The fluid media are identified as water and air. During the simulation, the fluid density and viscosity are selected as the viscosity parameters of standard water and air, respectively. The initial saturation of the water at the injection borehole is 1.

[0042] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0043] Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0044] The present application implements all or part of the processes in the above-mentioned embodiment method, and may also be implemented through a computer program product. When the computer program product runs on a terminal, the terminal can implement the steps in the above-mentioned method embodiments when executing the computer program product.

[0045] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A method for constructing a calculation model of porous medium water distribution based on CT structure reconstruction, characterized in that: include: According to the scale standard used in laboratory coal water injection experiments, large-scale coal samples were selected; According to the selected parameters and the preset water injection simulation scheme, water injection experiments are carried out on large-scale coal samples to simulate the distribution and flow of water in porous media; The large-scale coal sample in the experimental environment is scanned and calculated using a CT scanner to obtain a large-scale structural model; Import the large-scale structural model into the finite element model software to establish a numerical simulation model; Based on the fluid exchange between different media, the numerical simulation model is solved to construct a calculation model for the water distribution in porous media, so as to study the changing law of water distribution in water-injected coal according to the calculation model for the water distribution in porous media.

2. The method for constructing a porous medium water body distribution calculation model based on CT structure reconstruction according to claim 1, characterized in that: According to the scale standard used in the laboratory coal water injection experiment, a large-scale coal sample is selected, wherein a 10×10 cm cubic coal specimen is used.

3. The method for constructing a porous medium water body distribution calculation model based on CT structure reconstruction according to claim 2, characterized in that: A drill hole is arranged at the center of the surface of the cubic coal specimen, and the drill hole has a diameter of 1 cm.

4. The method for constructing a porous medium water body distribution calculation model based on CT structure reconstruction according to claim 1, characterized in that: According to the selected parameters and the preset water injection simulation scheme, a water injection experiment is carried out on a large-scale coal sample to simulate the distribution and flow of water in a porous medium, wherein the selected parameters include the porosity ratio of the coal matrix.

5. The method for constructing a porous medium water body distribution calculation model based on CT structure reconstruction according to claim 1, characterized in that: The preset water injection simulation scheme is specifically as follows: Constant pressure injection, where the initial injection pressure is 500Pa, the water saturation at the injection position is 1, and the injection study time is 2d.

6. The method for constructing a porous medium water body distribution calculation model based on CT structure reconstruction according to claim 1, characterized in that: The large-scale coal sample in the experimental environment is scanned and calculated using a CT scanner to obtain a large-scale structural model, specifically: The two-phase Darcy's law is applied to the anisotropic structural model reconstructed from CT scans to calculate the global anisotropic structure and then obtain a large-scale structural model.

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