Simulated specimen for hydraulic fracturing test of crushed soft coal seam and its preparation method

Abstract

The present invention relates to the technical field of coalbed methane exploration, and more specifically to a simulated specimen for hydraulic fracturing testing of a crushed soft coal seam and a preparation method thereof, comprising: an annular roof layer, the inner wall of which is provided with a simulated wellbore; an annular coal seam interface layer, located on the outer wall of the annular roof layer; and a coal seam, located on the outer wall of the annular coal seam interface layer, wherein the outer wall of the coal seam forms the four sides of the specimen. The simulated wellbore is arranged at the center of the annular roof layer. Through the annular structure of the annular roof layer and the annular coal seam interface layer, the simulated wellbore does not need to change its own axial position. Only by setting water outlets in different directions can the constructed fracture form a predetermined angle with the minimum stress direction. In addition, multiple fractures of different angles can be formed in one specimen. By comparing the morphology of fracture formation in one specimen, the relationship between the fracture formation angle and the minimum stress direction can be more accurately obtained.

Description

Technical Field

[0001] The present invention relates to the field of coalbed methane exploration technology, in particular to a hydraulic fracturing test technology for crushed soft coal seams, and in particular to a simulation sample for hydraulic fracturing testing of crushed soft coal seams and a preparation method thereof. Background Art

[0002] Coalbed methane (CBM) development typically targets coal seams, and hydraulic fracturing is typically performed directly in these seams. While the fracturing mechanism is essentially the same as fracturing in conventional oil and gas reservoirs like sandstone, the specific physical, structural, and rock mechanical properties of coal rock result in unique crack propagation patterns in coal seams, unlike those in conventional oil and gas reservoirs. Direct fracturing in coal seams tends to produce short, wide cracks, resulting in poor fracturing effectiveness.

[0003] Prior art uses hydraulic fracturing in the roof layer of a coal seam. Fractures form in the roof layer and extend into the coal seam, creating better-shaped fractures. This has proven feasible in experiments. Furthermore, given that hydraulic fracturing fractures are always perpendicular to the minimum principal stress, the orientation of the minimum principal stress determines the direction of the fracture. At a certain depth, the minimum principal stress is horizontal. Fractures formed after hydraulic fracturing will lie in a vertical plane, forming vertical fractures. Ideally, the horizontal wellbore is aligned in the direction of the minimum principal stress, resulting in alternating transverse fractures along the axis of the horizontal wellbore.

[0004] In the actual exploration and production process, the wellbore orientation of the horizontal well must also consider other factors, such as the degree of development of natural fractures in the reservoir, reservoir physical parameters, etc., because the relationship between the wellbore axis direction and the direction of the minimum principal stress of the formation determines the shape of the fracture. Summary of the Invention

[0005] The purpose of the present invention is to propose a simulation specimen for hydraulic fracturing test of crushed soft coal seams, which can be used to test the crack expansion law at different angles in different simulated wellbores, to guide the layout angle of the wellbores, and to cooperate with natural cracks in the reservoir to form excellent gas production channels.

[0006] According to a first aspect of the present invention, a simulated sample for hydraulic fracturing testing of a crushed soft coal seam is provided, comprising:

[0007] an annular roof layer, wherein the inner wall of the annular roof layer is provided with a simulated wellbore;

[0008] an annular coal seam interface layer, located on the outer wall of the annular roof layer;

[0009] A coal seam located on the outer wall of the annular coal seam interface layer, wherein the outer wall of the coal seam forms four sides of the sample;

[0010] The annular roof layer, the annular coal seam interface layer and the upper end surface of the coal seam are flush and constitute the top surface of the simulated sample; the annular roof layer, the annular coal seam interface layer and the lower end surface of the coal seam are flush and constitute the bottom surface of the simulated sample;

[0011] The annular roof layer is made of a mixture of fine sand and cement, the coal seam interface layer is made of a mixture of gypsum, fine sand and cement, and the coal seam is made of a mixture of coal powder, fine sand and cement;

[0012] The simulated wellbore is configured to be cylindrical, and a plurality of water outlets are provided on the outer wall of the simulated wellbore. The cross-section of each water outlet is rectangular, and its length direction is parallel to the length direction of the simulated wellbore. One side of the simulated specimen is defined as the minimum horizontal stress direction, the minimum horizontal stress direction is defined as the X direction, and the direction perpendicular to the minimum horizontal stress direction is defined as the Y direction. The plurality of water outlets are divided into N groups, and each group of water outlets is located within a range of less than 90° from the X axis.

[0013] Along the length direction of the simulated wellbore, the simulated sample is divided into M sample layers, each sample layer corresponds to one water outlet, and an isolation layer is provided between two adjacent sample layers.

[0014] Preferably, the outer wall of the simulated wellbore is provided with two groups of water outlets, the first group of water outlets is closer to the annular coal seam interface layer than the second group of water outlets, each water outlet in the first group of water outlets and each water outlet in the second group of water outlets have the same orientation, and the first group of water outlets and the second group of water outlets are located at different heights.

[0015] Preferably, the multiple water outlets in each group of water outlets are distributed at equal angles and at equal intervals.

[0016] Preferably, 0-90° in the XY plane is defined as interval A, and the area diagonally distributed with interval A in the XY plane is defined as interval B. The outer wall of the simulated wellbore is provided with two groups of water outlets, wherein the orientation of the first group of water outlets is located in interval A, and the second group of water outlets is located in interval B.

[0017] Preferably, the first group of water outlets is located below the second group of water outlets, the outer wall of the simulated wellbore is provided with a detachable water outlet nozzle, the inside of the water outlet nozzle is provided with a water outlet, the outer wall of the simulated wellbore is provided with a socket, and the water outlet nozzle is provided with an embedded part that cooperates with the socket.

[0018] Preferably, the isolation layer comprises a fiber cloth and a metal foil layer provided on both sides of the fiber cloth, and the surface of the metal foil layer is provided with openings.

[0019] Preferably, the thickness of the sample layer is 30-50 mm.

[0020] Preferably, the thickness ratio of the annular roof layer, the annular coal seam interface layer and the coal seam is 3:1:6.

[0021] The second aspect of the present invention provides a technical solution, a method for preparing a simulated sample for hydraulic fracturing testing of a crushed soft coal seam, comprising the following steps:

[0022] Step 1: Mix coal powder, fine sand, cement and water to form a coal seam slurry, mix gypsum, fine sand, cement and water to form a coal seam interface slurry, and mix fine sand, cement and water to form a roof slurry. Then, place the coal seam slurry, coal seam interface slurry and roof slurry in separate containers of the printing equipment.

[0023] Step 2: Place the simulated wellbore upright in the center of the printing base plate;

[0024] Step 3: Printing the first sample layer; Install a water outlet nozzle at the corresponding height layer of the simulated wellbore. The first printing device uses the roof slurry to print the annular roof layer according to the predetermined path. The second printing device uses the coal seam interface slurry to print the annular coal seam interface layer according to the predetermined path. The third printing device uses the coal seam slurry to print the coal seam according to the predetermined path. After each printing layer, pressurize the formed print layer to make the printed layer surface flat until the thickness of the first sample layer is formed.

[0025] Step 4: placing an isolation layer on the surface of the first sample layer and drying the first sample layer;

[0026] Step 5: Repeat steps 3-4 until you reach the second-to-last layer.

[0027] Step 6: Repeat step 3 and dry the top sample layer to obtain a simulated sample.

[0028] Among them, in step 3, the first printing device uses the roof slurry to print gradually inward along the outer contour of the annular roof layer to the inner contour, the second printing device uses the coal seam interface slurry to first print the inner and outer contours of the annular coal seam interface layer and then gradually print inward, and the third printing device uses the coal seam slurry to print gradually outward along the inner contour of the coal seam to the edge of the sample;

[0029] The outer contour of the annular roof layer and the inner contour of the annular coal seam interface layer are printed simultaneously, and the outer contour of the annular coal seam interface layer and the inner contour of the coal seam are printed simultaneously.

[0030] Preferably, in step 1, the proportioning ingredients with water are obtained by premixing, and the slump of the premixed slurry is 3%-5%.

[0031] According to the above technical solution, the simulated sample for hydraulic fracturing test of crushed soft coal seams proposed by the present invention has the following significant beneficial effects:

[0032] The simulated specimen for hydraulic fracturing testing of a crushed soft coal seam of the present invention is provided with an annular roof layer and an annular coal seam interface layer, and a simulated wellbore is provided at the center of the annular roof layer. Due to the annular structure, the simulated wellbore does not need to change its own axial position. Only by providing water outlets in different directions, the constructed hydraulic fracture can form a predetermined angle with the minimum stress direction. In addition, multiple hydraulic fractures of different angles can be formed in one specimen. By comparing the morphology of the hydraulic fractures formed in one specimen, the relationship between the hydraulic fracture formation angle and the minimum stress direction can be more accurately obtained in a simulation experiment.

[0033] At the same time, the simulation sample proposed in the aforementioned technical solution of the present invention can obtain more data results in one test, which is convenient for realizing control groups with different parameters, is beneficial to the conduct of fracturing tests, saves test time and test costs, and ensures the consistency and accuracy of test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are not intended to be drawn to scale. In the accompanying drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For clarity, not every component is labeled in every figure. Embodiments of various aspects of the present invention will now be described by way of example and with reference to the accompanying drawings.

[0035] Figure 1 It is a schematic structural diagram of a simulated sample for hydraulic fracturing test of a crushed soft coal seam in the prior art.

[0036] Figure 2a It is a schematic diagram of simulating the arrangement of the wellbore parallel to the direction of minimum principal stress.

[0037] Figure 2b It is a schematic diagram of the simulated wellbore inclination and the arrangement of the minimum principal stress direction.

[0038] Figure 3 It is a top view of a simulated sample for hydraulic fracturing test of a crushed soft coal seam shown in the present invention.

[0039] Figure 4 It is a cross-sectional view of a simulated sample for hydraulic fracturing test of a crushed soft coal seam shown in the present invention.

[0040] Figure 5 It is a schematic structural diagram of the sample layer and the isolation layer shown in the present invention.

[0041] Figure 6 It is a structural schematic diagram of the simulated wellbore shown in the present invention.

[0042] Figure 7 It is a schematic diagram of the first step printing trajectory of the annular roof layer, the annular coal seam interface layer and the coal seam shown in the present invention. DETAILED DESCRIPTION

[0043] In order to better understand the technical content of the present invention, specific embodiments are given below with reference to the accompanying drawings.

[0044] Combine Figure 1 As shown in the figure, due to the low structural strength of the coal seam, direct fracturing in the coal seam is likely to form short and wide cracks, and the fracturing effect is poor. Therefore, setting the wellbore in the roof layer above the coal seam for hydraulic fracturing can extend the cracks into the coal seam, and the forming effect is good. Since natural coal seams contain natural cracks, when the hydraulic fractures can be connected with the natural cracks, better gas production effects can be achieved. However, how to control the extension direction of the cracks is currently a difficult problem.

[0045] For example Figure 2a As shown, when the wellbore is parallel to the direction of the minimum principal stress, the cracks generated are perpendicular to the wellbore. Figure 2b As shown in the figure, when the wellbore is inclined to the direction of minimum principal stress, how the crack develops requires a large number of tests. In order to ensure the uniformity of the test results, the samples are printed, which takes a long time. Therefore, there is an urgent need for a simulation sample that can obtain more test results in a small number of tests to improve test efficiency and improve the consistency and accuracy of test results.

[0046] [Simulated specimen for hydraulic fracturing test in crushed soft coal seams]

[0047] Combine Figure 3 As shown, the simulated sample for hydraulic fracturing test of a crushed soft coal seam according to the embodiment of the first aspect of the present invention includes an annular roof layer 101 , an annular coal seam interface layer 102 and a coal seam 103 .

[0048] As an example, the shape of the simulated specimen is still a 300*300*300mm cubic structure, which can be placed in the pressurized chamber of a true triaxial physical simulation testing machine to simulate triaxial pressure and set one of the horizontal stresses as the minimum principal stress direction.

[0049] A simulated wellbore 10 is provided on the inner wall of the annular roof layer 101 , the annular coal seam interface layer 102 is located on the outer wall of the annular roof layer 101 , and the coal seam 103 is located on the outer wall of the annular coal seam interface layer 102 .

[0050] The outer wall of the coal seam 103 forms the four side surfaces of the sample. The upper end surfaces of the annular roof layer 101, the annular coal seam interface layer 102 and the coal seam 103 are flush and constitute the top surface of the simulated sample. The lower end surfaces of the annular roof layer 101, the annular coal seam interface layer 102 and the coal seam 103 are flush and constitute the bottom surface of the simulated sample.

[0051] In an optional embodiment, the thickness ratio of the annular roof layer 101 , the annular coal seam interface layer 102 and the coal seam 103 (with the inscribed circle of the coal seam 103 as the diameter) is 3:1:6.

[0052] In the diagram, simulated wellbore 10 is located in the center. The annular roof layer 101 extending outward corresponds to the existing roof layer, the annular coal seam interface layer 102 extending further outward corresponds to the existing interface layer, and the coal seam 103 extending further outward corresponds to the existing coal seam. When the water outlet of simulated wellbore 10 forms a fracture in annular roof layer 101, the fracture can penetrate the annular coal seam interface layer 102 and extend into the coal seam 103.

[0053] As an optional embodiment, the materials for the annular roof layer 101, the annular coal seam interface layer 102, and the coal seam 103 can be made using existing typical ingredients. For example, the annular roof layer 101 can be made using a mixture of fine sand and cement, the coal seam interface layer 103 can be made using a mixture of gypsum, fine sand, and cement, and the coal seam 103 can be made using a mixture of coal powder, fine sand, and cement. It should be understood that the proportions of the aforementioned ingredients can be prepared with reference to existing techniques and will not be further described in the embodiments of the present invention.

[0054] Furthermore, the simulated wellbore 10 is configured to be cylindrical, and a plurality of water outlets 31 are provided on the outer wall of the simulated wellbore 10. The cross-sectional shape of each water outlet 31 is rectangular, and its length direction is parallel to the length direction of the simulated wellbore 10. One side of the simulated specimen is defined as the minimum horizontal stress direction, with the minimum horizontal stress direction as the X direction and the direction perpendicular to the minimum horizontal stress direction as the Y direction. The plurality of water outlets 31 are divided into N groups, and each group of water outlets 31 is located within a range of less than 90° from the X-axis.

[0055] Along the length direction of the simulated wellbore 10 , the simulated sample is divided into M sample layers, each sample layer corresponds to a water outlet 31 , and an isolation layer 20 is provided between two adjacent sample layers.

[0056] Combine Figure 4 As shown, the angles of the water outlets 31 corresponding to the first sample layer 100a, the second sample layer 100b and the third sample layer 100c are different. Each water outlet 31 is located in the annular roof layer 101 and faces the annular coal seam interface layer 102, so that the hydraulic fracture can pass through the annular coal seam interface layer 102 and the coal seam 103 in sequence.

[0057] In this way, each sample layer can be regarded as a test object separately, and the angle of the outlet 31 of the simulated wellbore 10 corresponding to each sample layer is different. Therefore, the expansion direction of the hydraulic fracture and the fracture length and width parameters can be simulated when the outlet 31 deviates from the minimum principal stress direction at different angles.

[0058] To ensure the effectiveness of the test, the thickness of each sample layer is 30-50mm. In this way, a simulation test block can contain 6-10 sample layers, greatly improving the consistency and accuracy of the experimental results data of a single test.

[0059] First embodiment of simulated wellbore

[0060] In an optional embodiment, a group of water outlets is provided on the outer wall of the simulated wellbore 10, i.e., 6-10 water outlets. When the outer wall of the simulated wellbore 10 includes 6 water outlets, the spacing between each water outlet is 15°. When the outer wall of the simulated wellbore 10 includes 10 water outlets, the spacing between each water outlet is 9°.

[0061] It should be understood that the higher the accuracy, the easier it is to obtain data and analyze the relationship between the minimum stress directionality and the outlet spacing. In addition, multiple sample layers in a simulated sample are subjected to the same triaxial pressure, ensuring the consistency of the test environment. The results obtained are more accurate, reflecting the actual situation and reducing system errors.

[0062] Second embodiment of simulated wellbore

[0063] Furthermore, to form a control group, two groups of water outlets were provided on the outer wall of the simulated wellbore 10. The first group of water outlets 31a was located closer to the annular coal seam interface layer 102 than the second group of water outlets 31b. To enhance the control effect, each water outlet 31 in the first group of water outlets 31a and each water outlet 31 in the second group of water outlets 31b faced the same direction.

[0064] Combine Figure 6 As shown, the first water outlet in the first group of water outlets 31a is parallel to the X-axis, and is deflected to the Y-direction at intervals of 15°. The last water outlet in the first group of water outlets 31a is parallel to the Y-axis.

[0065] The water outlets in the second group of water outlets 31b are arranged in the same manner and at the same angle as the water outlets in the first group of water outlets 31a. The difference between the first group of water outlets 31a and the second group of water outlets 31b is that each water outlet in the first group of water outlets 31a is longer and therefore closer to the annular coal seam interface layer 102. The parameter of the distance from the annular coal seam interface layer 102 can be introduced as a control test.

[0066] Furthermore, in order to facilitate the preparation of samples, the first group of water outlets 31a and the second group of water outlets 31b are located at different heights, wherein the first group of water outlets 31a is located below the sample, and the second group of water outlets 31b is located above the sample, simulating the upper opening and lower blockage of the wellbore 10.

[0067] Third embodiment of simulated wellbore

[0068] Preferably, 0-90° in the XY plane is defined as interval A, and the area diagonally distributed with interval A in the XY plane is defined as interval B. The outer wall of the simulated wellbore 10 is provided with two groups of water outlets 31, wherein the orientation of the first group of water outlets is located in interval A, and the orientation of the second group of water outlets is located in interval B.

[0069] In this way, each sample layer can be simulated by constructing two cracks by the first group of water outlets and the second group of water outlets on the wellbore 10. Optionally, the first group of water outlets and the second group of water outlets have different distances from the annular coal seam interface layer 102 or different widths, which can reflect the influence of different fracturing spacing or widths on the expansion and extension of the fracture in one sample layer.

[0070] In the above embodiment, a water outlet nozzle is detachably mounted on the outer wall of the simulated wellbore 10, a water outlet 31 is provided inside the water outlet nozzle, a socket is provided on the outer wall of the simulated wellbore 10, and an embedded portion that cooperates with the socket is provided on the water outlet nozzle.

[0071] Specific, combined Figure 6 As shown, the water outlet nozzle is arranged in a rectangular block shape, and a water outlet 31 with a rectangular cross-section is provided inside. The tail portion thereof is provided with an embedded portion that can be inserted into the socket, and the water outlet 31 is connected to the channel in the simulated wellbore 10. After water is injected into the inlet of the simulated wellbore 10, the water flows to the water outlet 31 for pressure holding. When the pressure is sufficient to form cracks in the annular roof layer 101, hydraulic fracturing is formed, and the cracks extend to the annular coal seam interface layer 102 and the coal seam 103.

[0072] Preferably, combined Figure 5 As shown, an isolation layer 20 is provided between the two sample layers to isolate the transfer of stress and prevent the water flow and cracks of the two adjacent sample layers from affecting each other.

[0073] In an optional embodiment, the isolation layer 20 comprises a fiber cloth and metal foil layers disposed on either side of the fiber cloth, with the metal foil layers having openings on their surfaces. The fiber cloth provides ventilation, allowing moisture to evaporate through the openings in the fiber cloth and metal foil layers during drying of the coal seam sample layer. Furthermore, the fiber cloth bonds the upper and lower sample layers together, preventing them from separating and forming a cubic sample.

[0074] In an optional embodiment, the metal foil layer uses copper foil with a thickness of less than 0.1 mm and a porosity of less than 30%.

[0075]

Simulation sample preparation method

[0076] The second aspect of the present invention provides a technical solution, wherein the method for preparing a simulated sample for hydraulic fracturing testing of a crushed soft coal seam comprises the following steps:

[0077] Step 1: Mix coal powder, fine sand, cement and water to form a coal seam slurry, mix gypsum, fine sand, cement and water to form a coal seam interface slurry, and mix fine sand, cement and water to form a roof slurry. Then, place the coal seam slurry, coal seam interface slurry and roof slurry in separate containers of the printing equipment.

[0078] Step 2: Place the simulated wellbore 10 upright in the center of the printing substrate;

[0079] Step 3: Printing the first sample layer; Install a water outlet nozzle at the corresponding height layer of the simulated wellbore 10, and use the roof slurry of the first printing device to print the annular roof layer 101 according to the predetermined path. Use the coal seam 103 interface slurry of the second printing device to print the annular coal seam interface layer 102 according to the predetermined path. Use the coal seam 103 slurry to print the coal seam 103 according to the predetermined path. After each printing layer, pressurize the formed printed layer to make the printed layer surface smooth until the thickness of the first sample layer is formed.

[0080] Step 4: placing an isolation layer 20 on the surface of the first sample layer and drying the first sample layer;

[0081] Step 5: Repeat steps 3-4 until you reach the second-to-last layer.

[0082] Step 6: Repeat step 3 and dry the top sample layer to obtain a simulated sample;

[0083] The pulverized coal was ground from coal blocks in the simulated test area, and the fine sand was ground from roof rock blocks in the simulated test area. The mesh size of the pulverized coal and fine sand was 80-100. By using fine-grained raw materials as the basis for the homogeneous structural layer, the structural layer strength can be maintained at a high degree of consistency with the natural structural layer.

[0084] Optionally, a mixture of 80% coal powder, 5% fine sand, 15% cement and water in a mass ratio is used to form a coal seam slurry, a mixture of 60% gypsum, 20% fine sand, 20% cement and water in a mass ratio is used to form a coal seam interface slurry, and a mixture of 60% fine sand, 40% cement and water in a mass ratio is used to form a roof slurry, and then the coal seam slurry, coal seam interface slurry and roof slurry are placed in separate containers respectively.

[0085] First, the different slurries are pre-mixed, and then the mixed slurries are filled three times into a trumpet-shaped slump bucket with an upper opening of 100mm, a lower opening of 200mm, and a height of 300mm. After each filling, use a tamping hammer to hit the bucket wall evenly from the outside to the inside 25 times, tamp it and smooth it, then pull up the bucket. The slurry will collapse due to its own weight. The height of the highest point of the slurry after collapse is subtracted from the height of the bucket and compared with the height of the bucket to obtain the percentage value of the slump. The slump of the pre-mixed slurry is 3%-5%, and the amount of water used in each ratio is recorded to obtain the mixing ratio of water.

[0086] Specifically, in step 2, the simulated wellbore 10 is placed upright in the center of the printing substrate and fixed with glue or other auxiliary devices. A base is provided on the periphery of the substrate, and a cavity for accommodating the substrate is provided on the base. A driving cylinder is provided at the bottom of the substrate, and the driving cylinder can drive the substrate to move in the vertical direction. In the initial state, the upper end surface of the substrate is flush with the upper end surface of the base. Every time the printing device prints a layer, the substrate moves downward a certain distance, and the pressure plate is pressed on the surface of the printed layer to make its surface flat.

[0087] During the printing process, in order to ensure that the interfaces between the annular roof layer 101, the annular coal seam interface layer 102 and the coal seam 103 are clear, the boundaries are printed first so that the boundaries abut against each other and do not expand outwards to cause unclear boundaries.

[0088] Specifically, in step 3, combined with Figure 7 As shown, the first printing device uses the roof slurry to print gradually inward along the outer contour of the annular roof layer 101 to the inner contour, as shown in track A, the first step of printing by the first printing device; the second printing device uses the coal seam interface slurry to first print the inner and outer contours of the annular coal seam interface layer 102 and then gradually print inward, as shown in track B, the first step of printing by the second printing device, jumping from the inner contour of the annular coal seam interface layer 102 to the outer contour of the annular coal seam interface layer 102; the third printing device uses the coal seam slurry to print gradually outward along the inner contour of the coal seam 103 to the edge of the sample, as shown in track C, the first step of printing by the third printing device;

[0089] The outer contour of the annular roof layer 101 and the inner contour of the annular coal seam interface layer 102 are printed simultaneously, and the outer contour of the annular coal seam interface layer 102 and the inner contour of the coal seam 103 are printed simultaneously.

[0090] In the above embodiment, an ultrasonic vibration component is provided on the pressing plate, which is used to make the slurry uniform and maintain the consistency of the sample when the pressing plate is pressed against the surface of the printed layer.

[0091] Furthermore, the drying device uses an electric heating plate, that is, a heating wire is provided inside the pressing plate, and the printed layer is dried by heat conduction of the pressing plate.

[0092] In combination with the above embodiments, the present invention sets an annular roof layer and an annular coal seam interface layer, and the simulated wellbore is set in the central position of the annular roof layer. Through the annular structure, the simulated wellbore does not need to change its own axial position. Only by setting water outlets in different directions can the constructed fracture form a predetermined angle with the minimum stress direction. In addition, in one sample, multiple fractures with different angles can be formed. By comparing the morphology of the fractures formed in one sample, the relationship between the fracture formation angle and the minimum stress direction can be obtained more accurately.

[0093] The simulated sample proposed in the above technical solution of the present invention can obtain more data results in one test, which is convenient for realizing control groups with different parameters, is beneficial to the conduct of fracturing tests, saves test time and test costs, and ensures the consistency and accuracy of test results.

[0094] While the present invention has been disclosed above with reference to preferred embodiments, this is not intended to limit the present invention. Persons skilled in the art will readily appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A simulated sample for hydraulic fracturing test of crushed soft coal seam, characterized in that: include: an annular roof layer (101), wherein the inner wall of the annular roof layer (101) is provided with a simulated wellbore (10); an annular coal seam interface layer (102), located on the outer wall of the annular roof layer (101); A coal seam (103) is located on the outer wall of the annular coal seam interface layer (102), wherein the outer wall of the coal seam (103) forms four sides of the sample; The upper end surfaces of the annular roof layer (101), the annular coal seam interface layer (102) and the coal seam (103) are flush and constitute the top surface of the simulated sample; the lower end surfaces of the annular roof layer (101), the annular coal seam interface layer (102) and the coal seam (103) are flush and constitute the bottom surface of the simulated sample; The annular roof layer (101) is made of a mixture of fine sand and cement, the coal seam (103) interface layer is made of a mixture of gypsum, fine sand and cement, and the coal seam (103) is made of a mixture of coal powder, fine sand and cement. The thickness ratio of the annular roof layer (101), the annular coal seam interface layer (102) and the coal seam (103) is 3:1:6; The simulated wellbore (10) is configured to be cylindrical, and the outer wall of the simulated wellbore (10) is provided with a plurality of water outlets (31), each water outlet (31) has a rectangular cross-section, and its length direction is parallel to the length direction of the simulated wellbore (10), and one side of the simulated specimen is defined as the minimum horizontal stress direction, the minimum horizontal stress direction is defined as the X direction, and the direction perpendicular to the minimum horizontal stress direction is defined as the Y direction. The plurality of water outlets (31) are divided into N groups, and each group of water outlets (31) is located within a range of less than 90° from the X axis; Along the length direction of the simulated wellbore (10), the simulated sample is divided into M sample layers, each sample layer corresponds to one of the water outlets (31), and an isolation layer (20) is provided between two adjacent sample layers, the isolation layer (20) comprising a fiber cloth and a metal foil layer provided on both sides of the fiber cloth, the surface of the metal foil layer being provided with openings.

2. The simulated sample for hydraulic fracturing test of crushed soft coal seam according to claim 1, characterized in that: A water outlet nozzle is detachably mounted on the outer wall of the simulated wellbore (10), a water outlet (31) is provided inside the water outlet nozzle, a plug hole is provided on the outer wall of the simulated wellbore (10), and the water outlet nozzle is provided with an embedded portion that cooperates with the plug hole.

3. The simulated sample for hydraulic fracturing test of crushed soft coal seam according to claim 1 or 2, characterized in that: The thickness of the sample layer is 30-50 mm.

4. The method for preparing a simulated sample for hydraulic fracturing test of a crushed soft coal seam according to claim 2, characterized in that: The following steps are involved: Step 1: Mix coal powder, fine sand, cement and water to form a coal seam slurry, mix gypsum, fine sand, cement and water to form a coal seam interface slurry, and mix fine sand, cement and water to form a roof slurry. Then, place the coal seam slurry, coal seam interface slurry and roof slurry in separate containers of the printing equipment. Step 2: Place the simulated wellbore (10) upright in the center of the printing substrate; Step 3: Printing the first sample layer; installing a water outlet nozzle at a corresponding height layer of the simulated wellbore (10), a first printing device using a roof slurry to print an annular roof layer (101) according to a predetermined path, a second printing device using a coal seam (103) interface slurry to print an annular coal seam interface layer (102) according to a predetermined path, and a third printing device using a coal seam (103) slurry to print a coal seam (103) according to a predetermined path, and after each printing layer is printed, pressurizing the formed printing layer to make the surface of the printing layer flat, until the thickness of the first sample layer is formed; Step 4: placing an isolation layer (20) on the surface of the first sample layer and drying the first sample layer; Step 5: Repeat steps 3-4 until you reach the second-to-last layer. Step 6: Repeat step 3 and dry the top sample layer to obtain a simulated sample; Wherein, in step 3, the first printing device uses the roof slurry to print gradually inward along the outer contour of the annular roof layer (101) to the inner contour, the second printing device uses the coal seam interface slurry to first print the inner and outer contours of the annular coal seam interface layer (102) and then gradually print inward, and the third printing device uses the coal seam slurry to print gradually outward along the inner contour of the coal seam (103) to the edge of the sample; The outer contour of the annular roof layer (101) and the inner contour of the annular coal seam interface layer (102) are printed simultaneously, and the outer contour of the annular coal seam interface layer (102) and the inner contour of the coal seam (103) are printed simultaneously.

5. The method for preparing a simulated sample according to claim 4, wherein: In step 1, the water ratio is obtained by premixing, and the slump of the premixed slurry is 3%-5%.

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