A method for preparing coal rock samples containing cracks and combined coal rock samples

Abstract

The present invention belongs to the field of coal rock technology, and in particular, relates to a method for preparing a coal rock sample containing cracks and a combined coal rock sample. The method comprises step 1, fracturing a three-dimensional coal rock sample to obtain two fractured samples; step 2, filling the two fractured samples according to a preset experimental scheme to obtain a filled sample; step 3, wire cutting the filled sample according to experimental requirements to obtain a coal rock sample containing cracks. The coal rock sample prepared by the present invention can achieve the effect of being conducive to carrying out relevant theoretical research on hydraulic fracturing in the laboratory and the influence of proppants on the flow of coalbed methane in fractures.

Description

Technical Field

[0001] The invention belongs to the technical field of coal rock, and in particular relates to a method for preparing a coal rock sample containing cracks and a combined coal rock sample. Background Art

[0002] Hydraulic fracturing is a widely used technology to increase the production of coalbed methane wells. During the hydraulic fracturing process, the induced fractures will continue to extend until the amount of fluid lost to the reservoir exceeds the amount of fluid input. However, once the fluid input stops and the pressure drops below the fracture closure pressure, the fractures may shrink or even close, causing the fracture area to no longer be available for production. To avoid this, proppants are often used to fill the reservoir fractures to ensure the openness of the coalbed methane flow path. Although the application of hydraulic fracturing in the field can successfully increase the production of coalbed methane, it is impossible to prepare coal rock samples that meet the requirements due to the complex field conditions. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a method for preparing a coal rock sample containing cracks, comprising:

[0004] Step 1, fracturing the three-dimensional coal-rock sample to obtain two fracture samples;

[0005] Step 2, performing a filling process on the two fractured samples according to a preset experimental scheme to obtain filled samples;

[0006] Step 3: perform wire cutting on the filled sample according to experimental requirements to obtain a coal rock sample containing cracks.

[0007] The beneficial effects of the present invention are as follows: in order to explore the permeability characteristics of coal reservoirs after hydraulic fracturing in the laboratory, this patent provides a sample preparation method that is more consistent with engineering practice, which is conducive to conducting relevant theoretical research on hydraulic fracturing and the influence of proppants on the flow of coalbed methane in fractures in the laboratory. This patent can lay the proppant particles more evenly, which is convenient for the laboratory to carry out relevant research on the influence of proppants on hydraulic fracturing. By changing the wire cutting position of the sample, different distribution directions of the cracks in the sample can be achieved, which is convenient for the laboratory to carry out relevant research on the influence of crack distribution on the permeability of coal samples. By combining different samples containing cracks, different areas of the coal reservoir after hydraulic fracturing are simulated, which is convenient for the laboratory to carry out permeability research on coal reservoirs after hydraulic fracturing.

[0008] Based on the above technical solution, the present invention can also be improved as follows.

[0009] Furthermore, before step 1, the method further includes:

[0010] The original coal and rock samples are processed and cut to obtain three-dimensional coal and rock specimens.

[0011] Furthermore, the fracturing process of the three-dimensional coal rock sample is specifically as follows:

[0012] The three-dimensional coal rock sample is broken by Brazilian splitting or direct shear failure.

[0013] Further, the step 2 is specifically as follows:

[0014] Filling material is evenly laid between the fracture surfaces of the two fracture samples, and the two fracture samples are spliced ​​according to the shape of the three-dimensional coal and rock samples before being fractured to obtain a filled sample.

[0015] Further, the filler comprises:

[0016] Clay minerals or proppants.

[0017] Furthermore, after step 1, the following steps are also included:

[0018] The fracture surface in each fractured sample is scanned three-dimensionally to obtain an image set corresponding to each fractured sample.

[0019] Furthermore, it also includes:

[0020] The image set and the coal rock sample containing fractures are analyzed to obtain the hydraulically fractured coal reservoir permeability analysis results.

[0021] Furthermore, it also includes:

[0022] Step 4: Combining and arranging the coal and rock samples containing fractures according to experimental requirements to obtain combined coal and rock samples with different fracture structures and filling effects.

[0023] Another technical solution of the present invention to solve the above technical problem is as follows: a coal rock sample, characterized in that it is made by any of the above-mentioned methods for preparing a coal rock sample containing fractures.

[0024] The beneficial effects of the present invention are: in order to explore the permeability characteristics of coal seams in hydraulic fracturing areas in the laboratory, this patent provides a sample preparation method that is more consistent with engineering practice, which is conducive to conducting relevant theoretical research on hydraulic fracturing and the influence of proppants on the flow of coalbed methane in cracks in the laboratory. This patent can lay proppant particles more evenly, which is convenient for the laboratory to carry out relevant research on the influence of proppants on hydraulic fracturing. By changing the line cutting position of the sample, different distribution directions of the cracks in the sample can be achieved, which is convenient for the laboratory to carry out relevant research on the influence of crack distribution on the permeability of coal samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of a process for preparing a coal rock sample containing fractures according to an embodiment of the present invention;

[0026] Figure 2 A schematic diagram of the preparation steps provided in an embodiment of a method for preparing a coal rock sample containing fractures according to the present invention. DETAILED DESCRIPTION

[0027] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0028] like Figure 1 As shown, a method for preparing a coal rock sample containing fractures comprises:

[0029] Step 1, fracturing the three-dimensional coal-rock sample to obtain two fracture samples;

[0030] Step 2, performing a filling process on the two fractured samples according to a preset experimental scheme to obtain filled samples;

[0031] Step 3: perform wire cutting on the filled sample according to experimental requirements to obtain a coal rock sample containing cracks.

[0032] In some possible implementations, in order to explore the permeability characteristics of coal seams in hydraulic fracturing areas in the laboratory, this patent provides a sample preparation method that is more consistent with engineering practice, which is conducive to conducting relevant theoretical research on hydraulic fracturing and the effect of proppants on the flow of coalbed methane in fractures in the laboratory. This patent can lay proppant particles more evenly, which is convenient for the laboratory to carry out relevant research on the effect of proppants on hydraulic fracturing. By changing the line cutting position of the sample, different distribution directions of the cracks in the sample can be achieved, which is convenient for the laboratory to carry out relevant research on the effect of crack distribution on the permeability of coal samples.

[0033] It should be noted that hydraulic fracturing technology, as one of the important technical measures to improve coal seam permeability and water and gas conductivity, has become an important way to increase coalbed methane production. However, due to the complex field conditions, a method for preparing a laboratory hydraulic fracturing physical model is now provided, which is helpful for studying the permeability law of coal reservoirs under the influence of hydraulic fracturing.

[0034] The objective of the present invention is achieved through the following technical solutions:

[0035] A method for preparing a coal rock sample containing fractures and its application, comprising the following steps:

[0036] a. Process and cut the original coal and rock samples collected on site into cubic coal and rock specimens;

[0037] b. Perform Brazilian splitting or direct shear failure on the cubic coal rock sample, so that the coal sample will produce tensile fracture through Brazilian splitting test, or shear fracture through direct shear failure, and obtain two new samples after fracture, each of which has a fracture surface;

[0038] c. Perform three-dimensional scanning on the fracture surface formed by the breaking process to obtain multiple three-dimensional scanning images.

[0039] d. In order to study the influence of the coverage of proppant in the fracture surface on the permeability of the sample, a corresponding experimental scheme can be designed (the experimental scheme involved here is designed based on the needs of subsequent experiments. If it is necessary to study the "influence of proppant in the fracture surface on the permeability of the sample", the proppant is laid between the two fracture surfaces. Based on the experimental scheme, the proppant laying area can be divided, and proppant with different densities can be laid in different laying areas.), fillers (such as clay minerals, proppant, etc.) are evenly laid between the two fracture surfaces, and the fracture surface of the sample is aligned and fixed according to the initial position at the time of destruction to prevent the sample from scattering;

[0040] e. Cut the fixed sample into cubes or cylinders according to the shape required by the experiment through a wire cutting machine to obtain coal rock samples containing cracks. By changing the cutting position, different distribution directions of cracks in the coal sample can be achieved.

[0041] f. In order to simulate the distribution characteristics of coal rock after actual hydraulic fracturing on site, the coal rock samples containing fractures are combined and arranged according to the geological characteristics of the site to obtain combined coal rock samples with different fracture structures and filling effects.

[0042] Since deep coal reservoirs usually have low or ultra-low permeability, hydraulic fracturing is often required to achieve economical production of coalbed methane. In this process, proppants such as sand are usually injected to support the opening of hydraulic fractures to increase coalbed methane production. The sample preparation method provided by this patent is used to prepare coal samples that simulate the hydraulic fracturing effect on site, produce cracks in different distribution directions and contain proppants, which is convenient for the laboratory to carry out research on related issues such as the permeability of hydraulically fractured coal reservoirs.

[0043] Existing patents usually use the method of drilling and injecting high-pressure water to prepare hydraulic fracturing samples. This method cannot control the distribution of cracks in coal samples. This patent can achieve different distribution directions of cracks in the sample by changing the cutting position of the sample in step e. Compared with existing patents, step d of this patent can lay the proppant particles more evenly, which is convenient for the laboratory to carry out related research on the impact of proppant on hydraulic fracturing. In addition, adding three-dimensional laser scanning of the fracture surface during sample preparation and importing the scanning results into numerical simulation software is conducive to carrying out related numerical simulation work.

[0044] Embodiment 1, as Figure 2 As shown, step 1: select a piece of original coal rock of appropriate size on site. To ensure the stability of the coal sample during the cutting process, cover the bottom of the sample with plaster. Then install the original coal rock on the platform of the wire saw. During the cutting process, carefully identify and mark all visible cracks to avoid visible cracks in the cut cube sample, so as to obtain a coal rock cube sample A without visible cracks.

[0045] Step 2: Perform Brazilian splitting, direct shear failure and other fracture treatments on the cubic sample A. Taking the direct shear failure experiment as an example, use a strain-controlled direct shear apparatus, align the upper and lower shear boxes, insert the pins, and place the sample in the shear box. After the sample is installed, turn the handwheel to make the front end of the upper box contact the dynamometer. Add the pressure cover, steel ball, and pressure frame in sequence. After applying the vertical load, immediately pull out the pin, adjust the dial indicator to zero, and turn the handwheel until the coal rock sample is destroyed and a through crack appears. After the shearing is completed, the vertical pressure of the shear box is removed to obtain samples A1 and A2.

[0046] Step 3: Use a 3D laser scanner to scan the fracture surfaces of sample A1 and sample A2 respectively. The scanning results can be used for numerical simulation work matching the physical model experiment.

[0047] Step 4: According to the requirements of the subsequent experimental design, choose whether to lay clay minerals or proppants between the fractures of sample A1 and sample A2, and further design the proppant laying area. Proppants of different densities can be laid in different areas. Then align and fix the fracture surfaces of sample A1 and sample A2 to obtain sample A3.

[0048] Step 5: Perform wire cutting on sample A3 and change the position of wire cutting to achieve different distribution directions of cracks in the sample.

[0049] Step 6: If it is necessary to study the permeability of the coal reservoir after hydraulic fracturing, the cut coal rock sample B1 will be subjected to mechanical and permeability experiments alone. If it is necessary to study the permeability of the coal reservoir and the rock roof and floor after hydraulic fracturing, the cut coal rock sample C2 will be combined with the rock roof C1 and the rock floor C3 to obtain a coal rock sample with fractures combined with the rock, and further study its mechanical response characteristics and permeability evolution law under load.

[0050] Preferably, in any of the above embodiments, before step 1, the step further includes:

[0051] The original coal and rock samples are processed and cut to obtain three-dimensional coal and rock specimens.

[0052] Preferably, in any of the above embodiments, the fracturing process of the three-dimensional coal-rock sample is specifically:

[0053] The three-dimensional coal rock sample is broken by Brazilian splitting or direct shear failure.

[0054] Preferably, in any of the above embodiments, step 2 is specifically:

[0055] Filling material is evenly laid between the fracture surfaces of the two fracture samples, and the two fracture samples are spliced ​​according to the shape of the three-dimensional coal and rock samples before being fractured to obtain a filled sample.

[0056] Preferably, in any of the above embodiments, the filler comprises:

[0057] Clay minerals or proppants.

[0058] Preferably, in any of the above embodiments, after step 1, the following steps are further included:

[0059] The fracture surface in each fractured sample is scanned three-dimensionally to obtain an image set corresponding to each fractured sample.

[0060] Preferably, in any of the above embodiments, it further comprises:

[0061] The image set and the coal rock sample containing fractures are analyzed to obtain the hydraulically fractured coal reservoir permeability analysis results.

[0062] Preferably, in any of the above embodiments, it further comprises:

[0063] Step 4: Combining and arranging the coal and rock samples containing fractures according to experimental requirements to obtain combined coal and rock samples with different fracture structures and filling effects.

[0064] Another technical solution of the present invention to solve the above technical problem is as follows: a combined coal rock sample, characterized in that it is made by any of the above-mentioned methods for preparing a coal rock sample containing fractures.

[0065] In some possible implementations, in order to explore the permeability characteristics of coal seams in hydraulic fracturing areas in the laboratory, this patent provides a sample preparation method that is more consistent with engineering practice, which is conducive to conducting relevant theoretical research on hydraulic fracturing and the effect of proppants on the flow of coalbed methane in fractures in the laboratory. This patent can realize different schemes for laying proppant particles, which is convenient for the laboratory to carry out relevant research on the effect of proppants on hydraulic fracturing. By changing the wire cutting position of the sample, different distribution directions of the cracks in the sample can be achieved, which is convenient for the laboratory to carry out relevant research on the effect of crack distribution on the permeability of coal samples.

[0066] The reader should understand that in the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0067] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the method embodiments described above are only illustrative, for example, the division of steps is only a logical function division, and there may be other division methods in actual implementation, such as multiple steps can be combined or integrated into another step, or some features can be ignored or not executed.

[0068] If the above method is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention is essentially or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions to enable a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk and other media that can store program codes.

[0069] The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for preparing a coal rock sample containing cracks, characterized in that: include: Step 1, fracturing the three-dimensional coal-rock sample to obtain two fracture samples; Step 2, performing a filling process on the two fractured samples according to a preset experimental scheme to obtain filled samples; Step 3, performing wire cutting on the filled sample to obtain a coal rock sample containing cracks; The specific method of fracturing the three-dimensional coal and rock samples is as follows: The three-dimensional coal rock sample is fractured by Brazilian splitting or direct shearing; The step 2 is specifically as follows: Filling material is evenly laid between the fracture surfaces of the two fracture samples, and the two fracture samples are spliced ​​according to the shape of the three-dimensional coal and rock samples before being fractured to obtain a filled sample.

2. The method for preparing a coal rock sample containing cracks according to claim 1, characterized in that: The step 1 also includes: The original coal and rock samples are processed and cut to obtain three-dimensional coal and rock specimens.

3. The method for preparing a coal rock sample containing cracks according to claim 1, characterized in that: The filler comprises: Clay minerals or proppants.

4. The method for preparing a coal rock sample containing cracks according to claim 1, characterized in that: After step 1, also include: The fracture surface in each fractured sample is scanned three-dimensionally to obtain an image set corresponding to each fractured sample.

5. The method for preparing a coal rock sample containing cracks according to claim 4, characterized in that: Also includes: The image set and the coal rock sample containing fractures are analyzed to obtain the hydraulically fractured coal reservoir permeability analysis results.

6. The method for preparing a coal rock sample containing cracks according to claim 1, characterized in that: Also includes: Step 4: Combining and arranging the coal and rock samples containing fractures according to experimental requirements to obtain combined coal and rock samples with different fracture structures and filling effects.

7. A combined coal-rock sample, characterized in that: The method is manufactured and assembled by a method for preparing a coal rock sample containing fractures as claimed in any one of claims 1 to 6.

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