Preparation method of multi-factor coal rock complex test piece for hydraulic fracturing experiment

Abstract

The invention belongs to the technical field of hydraulic fracturing experiments, and provides a preparation method of a multi-factor coal rock complex test piece for a hydraulic fracturing experiment. The method comprises the following steps: firstly, primarily screening coal-rock complex test piece raw materials, performing standard screening, maintaining preset water content, and weighing according to requirements; then, respectively preparing briquette coal and briquette rock materials according to a preset proportioning scheme; then, preparing various molds, coating a mold release agent on a pressing mold, putting the pressing mold into a prefabricated hole mold, pouring a molded rock material, tamping by using an angled pressing rod mold, adding a molded coal material, and tamping by using a horizontal pressing rod mold; and then, placing the pressing mold filled with the materials on a press machine, presetting the forming pressure, the loading rate and the pressure maintaining time, and demolding after pressing and forming. And finally, naturally curing the demolded test piece. The problem that in the prior art, a coal rock test piece cannot accurately simulate the stratum state can be solved.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic fracturing experiments, and in particular relates to a method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments. Background Art

[0002] Hydraulic fracturing is a crucial production-increasing measure in the fields of oil and gas extraction and coalbed methane development. The principle is to inject high-pressure fluid into the underground reservoir to form cracks in the rock, thereby increasing the permeability of the reservoir, allowing oil and gas or coalbed methane to flow more smoothly into the wellbore and achieve efficient extraction. The hydraulic fracturing experiment simulates this process in a laboratory environment. By fracturing coal and rock specimens, the crack initiation and expansion laws and the influencing factors of the fracturing effect are studied, providing a key basis for the design of fracturing schemes and parameter optimization in actual engineering. Accurate and reliable experimental results play an irreplaceable role in guiding on-site fracturing operations, improving resource extraction efficiency, and reducing mining costs.

[0003] There are two main traditional methods for preparing coal-rock composite specimens suitable for hydraulic fracturing. One is to rely on cutting the original rock and then use polyurethane glue to bond it. This method is to cut the original rock collected from the coal mine site into a suitable shape and size, and then use polyurethane glue to bond different parts of the coal and rock together to form a coal-rock composite specimen. The other method is to prepare it by pouring cement concrete. That is, first make a mold, place the coal sample in a specific position in the mold, and then pour cement concrete. After the concrete solidifies, a composite specimen containing coal and rock is formed.

[0004] In the above prior art, there are the following problems:

[0005] 1. When using polyurethane glue for bonding, it is difficult to ensure uniformity during the glue coating process. Due to uneven glue coating, the bonding strength of each part of the test piece will be inconsistent, and the weak bonding points may be damaged prematurely during the fracturing experiment, affecting the accuracy of the experimental results. In addition, polyurethane glue has special physical properties and is difficult to penetrate by water pressure. This makes it impossible to truly reflect the expansion of cracks at the interface between coal rock and adhesive when simulating actual hydraulic fracturing, resulting in a large deviation between the experimental results and the fracturing conditions under actual formation conditions.

[0006] 2. During the process of pouring cement concrete to prepare coal-rock composite specimens, it is difficult to completely expel the bubbles in the concrete. These residual bubbles will form defects inside the concrete, reduce the overall strength and uniformity of the specimen, and affect the simulation authenticity of crack expansion in the fracturing experiment. At the same time, voids are easily generated at the interface between coal rock and concrete, and the bonding effect between the two is often not ideal, forming a weak interface. This will cause the cracks to expand preferentially along the weak interface in the fracturing experiment, rather than expanding according to the mechanical properties and structure of the coal rock in the actual stratum. It is impossible to accurately simulate the stratum state and it is difficult to meet the strict requirements of the fracturing experiment for the specimen. Summary of the invention

[0007] The technical problem solved by the present invention is to provide a method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments, so as to solve the problem that the coal-rock specimens prepared in the prior art cannot accurately simulate the formation state.

[0008] The basic scheme provided by the present invention is a method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments, comprising:

[0009] S1: Perform primary screening and standard screening on the raw materials of the coal-rock composite specimen, maintain the preset moisture content, and weigh them according to the preset hydraulic fracturing experiment weighing requirements;

[0010] S2: preparing briquette materials and briquette rock materials according to the preset briquette coal proportioning scheme and the preset briquette rock proportioning scheme respectively;

[0011] S3: prepare a pressing mold, a prefabricated hole mold, an angled pressure rod mold and a horizontal pressure rod mold, apply a release agent in the pressing mold in turn, put the prefabricated hole mold into the pressing mold, pour the molded rock material into the pressing mold, use the angled pressure rod mold to compact the molded rock material in the pressing mold, then add the briquette material into the pressing mold, and use the horizontal pressure rod mold to compact it again;

[0012] S4: placing a pressing mold containing a rock material and a coal material on a press, presetting a molding pressure, a loading rate and a holding time, performing a demoulding process after molding by a press, and obtaining a coal-rock composite specimen;

[0013] S5: The coal-rock composite specimens are naturally cured after demoulding.

[0014] Further, the S1 includes:

[0015] S1-1: Preliminarily screen and filter the raw materials of the coal-rock composite specimen, and keep the raw materials of the coal-rock composite specimen dry to a moisture content of 1-1.5%; the raw materials of the coal-rock composite specimen include coal powder, sand, cement and gypsum;

[0016] S1-2: After the initial screening, the coal powder and sand in the raw materials of the coal-rock composite specimen are sieved with a standard sieve to obtain raw materials with a particle size of 40-80 mesh.

[0017] Further, the S2 includes:

[0018] S2-1: Weigh coal powder, cement and gypsum in a ratio of 7:2:1, pour the coal powder into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and obtain briquette material after fully mixing;

[0019] S2-2: Weigh cement, sand and gypsum in a ratio of 5:4:1, pour the sand into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and obtain the rock material after fully mixing.

[0020] Further, the S3 includes:

[0021] S3-1: preparing a pressing mold, the pressing mold comprising a tray and a cylinder, the tray is tightly engaged at the bottom of the cylinder, and a release agent is coated inside the cylinder;

[0022] S3-2: preparing a prefabricated hole mold, wherein the prefabricated hole mold is cut from steel, and comprises a bottom plate and a hole column, wherein the bottom plate and the hole column are integrally formed; when the prefabricated hole mold is placed into a pressing mold, the bottom plate is at the bottom and the hole column is at the top;

[0023] S3-3: Pour the type rock material into the cylinder of the pressing mold with the prefabricated hole mold, and compact the type rock material using the angled pressing rod mold representing the angle of the formation coupling surface;

[0024] S3-4: The briquette material is introduced into the pressing mold again and compacted again using the horizontal pressing rod mold.

[0025] Further, the S4 includes:

[0026] S4-1: placing the pressing mold with the shaped rock material and the shaped coal material on the pressure iron of the press, setting the forming pressure and the loading rate and starting the pressing, and after reaching the preset target forming pressure, keeping the pressure and keeping it still according to the holding time, the forming pressure is 100KN, 200KN, 300KN, the loading rate is 500N / min, and the holding time is 30min;

[0027] S4-2: Place the pressed mold on a demoulding sleeve, and apply pressure to demould the formed coal-rock composite specimen.

[0028] Furthermore, the natural curing in S5 specifically includes: placing the demoulded test piece in a constant temperature and humidity curing box with a temperature of 20±1° C. and a humidity of 95±3% for curing.

[0029] The principles and advantages of the present invention are as follows: the preparation process in the present application includes material preparation, material mixing, mold preparation, material weighing and adding, material compacting, pressing and molding, demolding marking and natural curing. In the process of preparing the mold, prefabricated hole molds, approximate angle molds and molds with different interface angles are used to meet the contact surface conditions of coal and rock in real scenarios. Then, in the pressing and molding process, the press is used to directly press so that the molded coal and rock specimens are automatically compacted under high pressure to solve the problem of bubbles in the cast parts in the prior art. In the whole process of the above steps, the specimens are compacted by molds at different angles to form a coupling surface similar to the formation. Finally, the specimens after pressing and molding more realistically simulate and restore the construction site, and the strength of the interface is relatively high, which is conducive to subsequent hydraulic fracturing experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flowchart of an embodiment of the present invention;

[0031] Figure 2 A schematic diagram of a mold required for preparing a coal-rock composite specimen according to an embodiment of the present invention;

[0032] Figure 3 This is a pattern diagram of a coal-rock composite specimen prepared in the present invention;

[0033] Figure 4 It is a schematic diagram of the interface state of the coal-rock composite specimen in the present invention;

[0034] Figure 5 It is a schematic diagram comparing the uniaxial compressive strength performance changes of the molded rock and the cast cement parts;

[0035] Figure 6 Schematic diagram comparing the changes in uniaxial compressive strength performance of briquette coal and raw coal. DETAILED DESCRIPTION

[0036] The following is further described in detail through specific implementation methods:

[0037] The marks in the drawings of the specification include: rock material 1, angled rod pressing mold 2, horizontal rod pressing mold 3, coal material 4, coal-rock composite specimen 5, prefabricated hole mold 6, pressing mold 7.

[0038] The embodiment is basically as shown in the attached Figure 1 As shown: A method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments, comprising:

[0039] S1: Perform primary screening and standard screening on the raw materials of the coal-rock composite specimen 5, maintain the preset moisture content, and weigh according to the preset hydraulic fracturing experiment weighing requirements, wherein S1 includes:

[0040] S1-1: Preliminarily screen and filter the raw materials of the coal-rock composite specimen 5, and keep the raw materials of the coal-rock composite specimen 5 dry to a moisture content of 1-1.5%; the raw materials of the coal-rock composite specimen 5 include coal powder, sand, cement and gypsum;

[0041] S1-2: After the initial screening, the coal powder and sand in the raw materials of the coal-rock composite specimen 5 are sieved with a standard sieve to obtain raw materials with a particle size of 40-80 mesh.

[0042] In this embodiment, when step S1 is implemented, the raw materials of the coal-rock composite specimen 5 are first prepared, including coal powder, sand, cement and gypsum, and then the coal-rock composite specimen 5 is preliminarily screened and filtered to remove larger particles and lumps so that the raw materials initially meet the subsequent processing requirements; then the raw materials of the coal-rock composite specimen 5 after the preliminary screening are dried to maintain their moisture content at 1-1.5%. In this application, natural air drying combined with the assistance of drying equipment can be adopted, and the moisture content is monitored in real time during the drying process, and the drying is stopped immediately when the requirements are met.

[0043] After the above raw materials are initially processed, they are screened again using a standard sieve to screen out raw materials with a particle size of 40-80 mesh. The raw materials are weighed according to the preset hydraulic fracturing experiment weighing requirements, and a surplus of raw materials is left during the weighing process.

[0044] Therefore, in step S1 of the present application, 1. the raw materials of the coal-rock composite specimen 5 are subjected to primary screening and standard screening to ensure the consistency and uniformity of the raw material particle size, so that the prepared coal-rock composite specimen 5 is more stable and uniform in structure and properties, thereby improving the accuracy and reliability of the hydraulic fracturing experiment structure;

[0045] 2. Maintaining the moisture content of the raw materials at 1-1.5% avoids the influence of too high or too low moisture content on the physical and mechanical properties of the coal-rock composite specimen 5, ensures the stability of the experimental conditions, and is conducive to analyzing the influence of other factors on the coal-rock composite during hydraulic fracturing.

[0046] S2: preparing the briquette material 4 and the briquette material 1 according to the preset briquette ratio scheme and the preset briquette ratio scheme respectively; wherein S2 includes:

[0047] S2-1: Weigh coal powder, cement and gypsum in a ratio of 7:2:1, pour the coal powder into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and mix thoroughly to obtain briquette material 4;

[0048] S2-2: Weigh cement, sand and gypsum in a ratio of 5:4:1, pour the sand into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and obtain the rock material 1 after fully mixing.

[0049] In this embodiment, after the screened and processed coal powder, sand, cement and gypsum are obtained based on step S1, a clean and appropriately-capacity mixing container and mixing tool are immediately prepared, and the preparation of the briquette material 4 is first started. During the preparation of the briquette material 4, the screened coal powder, cement and gypsum are weighed in strict accordance with the ratio of 7:2:1. After weighing, firstly, 7 parts by weight of the coal powder are poured into the mixing container, and the coal powder is stirred at a low speed to make it in a uniformly dispersed state. The stirring time is controlled to be 3-5 minutes to ensure that there is no coal powder agglomeration. Then, 2 parts of cement and 1 part of gypsum are slowly added to the stirring coal powder in turn, and stirring is performed while adding to make the cement and gypsum and the coal powder initially mixed evenly. At this time, the stirring time is controlled to be 5-8 minutes. Finally, water is slowly added to the mixing container while stirring continuously. The amount of water added is determined based on experience and preliminary tests to achieve the best molding effect. At this time, the stirring time is controlled to be 10-15 minutes until the materials are fully mixed and evenly mixed to form a briquette material 4 with good plasticity and uniformity.

[0050] As for the preparation of another material, the type rock material 1, sand, cement and gypsum are weighed in a ratio of 5:4:1. First, 4 parts of sand are poured into another mixing container and stirred for 3-5 minutes to make the sand loose and uniform; then 5 parts of cement and 1 part of gypsum are added in sequence and stirred evenly for 5-8 minutes. Finally, water is added and stirred continuously for 10-15 minutes until a uniform type rock material 1 is obtained.

[0051] Therefore, in step S2 of the present application, it has the following advantages: 1. By mixing materials in a specific proportion, the physical and mechanical properties of real coal rock can be simulated, so that the prepared coal-rock composite specimen 5 is closer to the actual situation, which helps to improve the authenticity and reliability of the hydraulic fracturing experiment;

[0052] 2. Detailed stirring and mixing steps ensure that the internal components of the briquette material 4 and the briquette material 1 are evenly distributed, avoiding local component deviations, thereby ensuring the stability and consistency of the test pieces during the experiment;

[0053] 3. The preset ratio scheme and clear operation process provide experimenters with a basis to rely on during the preparation process, which is convenient for operation and quality control, improves the repeatability and accuracy of the experiment, and is conducive to comparative analysis between different experiments.

[0054] S3: prepare a pressing mold 7, a prefabricated hole mold 6, an angled pressure rod mold 2 and a horizontal pressure rod mold 3, apply a release agent in the pressing mold 7 in turn, put the prefabricated hole mold 6 into the pressing mold 7, pour the shaped rock material 1 into the pressing mold 7, use the angled pressure rod mold 2 to compact the shaped rock material 1 in the pressing mold 7, then add the shaped coal material 4 into the pressing mold 7, and compact it again using the horizontal pressure rod mold 3; wherein, S3 includes:

[0055] S3-1: preparing a pressing mold 7, wherein the pressing mold 7 comprises a tray and a cylinder, wherein the tray is tightly engaged at the bottom of the cylinder, and a release agent is applied inside the cylinder;

[0056] S3-2: prepare a prefabricated hole mold 6, the prefabricated hole mold 6 is cut from steel, the prefabricated hole mold 6 comprises a bottom plate and a hole column, the bottom plate and the hole column are integrally formed; when the prefabricated hole mold 6 is placed in the pressing mold 7, the bottom plate is at the bottom and the hole column is at the top;

[0057] S3-3: Pour the rock material 1 into the cylinder of the pressing mold 7 containing the prefabricated hole mold 6, and compact the rock material 1 using the angled pressing rod mold 2 representing the angle of the formation coupling surface;

[0058] S3-4: The briquette material 4 is introduced into the pressing mold 7 again, and compacted again using the horizontal pressing rod mold 3.

[0059] In this embodiment, step S3 is a material pressing and compacting step. After the required briquette material 4 and briquette material 1 are prepared, the required molds need to be prepared, wherein the molds include a pressing mold 7, a prefabricated hole mold 6, an angled pressing rod mold 2, and a horizontal pressing rod mold 3, such as Figure 2 As shown, when preparing the pressing mold 7, ensure that the tray and the cylinder of the pressing mold 7 are intact, install the tray at the bottom of the cylinder, ensure that the two are tightly bitten without gaps, and then use a brush or sprayer to evenly apply the release agent to the inside of the cylinder to ensure that the release agent covers the entire area to facilitate subsequent demolding of the test piece; then check the prefabricated hole mold 6 to confirm that it is made from the just cut and that the chassis and the hole column are integrally formed without deformation or cracks, place the prefabricated hole mold 6 into the pressing mold 7, with the chassis at the bottom and the hole column at the top, and place them accurately to ensure that subsequent material filling and molding are not affected. In this application, the diameter of the hole column in the prefabricated hole mold 6 is set to 50 mm, and the height is set to different heights according to the actual coal and rock conditions. The purpose is to leave a pre-hole in the test piece so that the water injection pressure head can be placed.

[0060] Then, the rock material 1 prepared in step S2 is poured into a pressing mold 7 with a prefabricated hole mold 6, and compacted using an angled pressing rod mold 2. In the present application, the angled pressing rod mold 2 has an inclined pressing surface, and the inclination angle of the pressing surface is fitted according to the angle of the junction between the rock and the raw coal in the actual coal rock, including 30°, 40°, 45°, etc. Taking 30° as an example, when the 30° pressing rod mold compacts the rock material 1, the upper surface of the rock material 1 in the pressing mold 7 is in a 30° state.

[0061] Similarly, after the shaped rock material 1 is compacted, the shaped coal material 4 is poured into the pressing mold 7 and evenly distributed on the shaped rock material 1 , and then the shaped coal material 4 is compacted using the horizontal pressing rod mold 3 .

[0062] Therefore, in the present application, based on step S3, the advantages include: 1. By using the prefabricated hole mold 6 and the angled pressure rod mold 2, the pore structure and the angle of the formation coupling surface in the formation can be simulated, so that the prepared coal-rock composite specimen 5 is more in line with the actual formation conditions, and the simulation accuracy of the hydraulic fracturing experiment is improved;

[0063] 2. Two material fillings and different tamping operations ensure that the rock material 1 and the coal material 4 are tightly combined, which improves the overall strength and stability of the test piece, avoids defects such as delamination and cracks, and ensures the molding quality of the test piece;

[0064] 3. Apply a release agent inside the pressing mold 7 to facilitate the subsequent removal of the test piece from the mold, reduce damage to the test piece, and facilitate subsequent processing, testing, and other operations on the test piece.

[0065] S4: placing the pressing mold 7 containing the shaped rock material 1 and the shaped coal material 4 on a press, presetting the molding pressure, loading rate and holding time, and performing demoulding after molding by the press to obtain a coal-rock composite specimen 5; wherein S4 includes:

[0066] S4-1: placing the pressing mold 7 containing the shaped rock material 1 and the shaped coal material 4 on the pressure iron of the press, setting the forming pressure and loading rate and starting the pressing, and after reaching the preset target forming pressure, holding the pressure and keeping it still according to the holding time, the forming pressure is 100KN, 200KN, 300KN, the loading rate is 500N / min, and the holding time is 30min;

[0067] S4-2: The pressing mold 7 after pressing is placed on a demoulding sleeve, and pressure is applied to demould the formed coal-rock composite specimen 5.

[0068] In this embodiment, after steps S1 to S3, it is confirmed that the pressing mold 7 containing the rock material 1 and the coal material 4 is ready, and the press is checked for normal operation to ensure that the surface of the pressurized iron is flat and free of debris to avoid affecting the pressing effect. At the same time, a demolding sleeve is prepared to ensure that its size is compatible with the pressing mold 7. Then, the pressing mold 7 containing the material is carefully placed on the pressurized iron of the press to ensure that the mold is placed stably and centered. On the operation panel of the press, the molding pressure is set to 100KN, 200KN or 300KN, and the loading rate is set to 500N / min. After the setting is completed, the press is started to start pressing. During the pressing process, the pressure display of the press and the status of the mold are closely observed to ensure that the pressing process proceeds smoothly. When the pressure reaches the preset target molding pressure, the timing is started, and the pressure is maintained and allowed to stand for 30 minutes to allow the material to be fully compacted and formed under constant pressure.

[0069] Finally, after the holding time is over, the pressing mold 7 after pressing is removed from the press and placed on the demolding sleeve. The pressurizing device of the press is adjusted to a position suitable for the mold, the press is started again, and the pressurization is slowly applied. As the pressure increases, the formed coal-rock composite specimen 5 gradually comes out of the pressing mold 7. After demolding is completed, the coal-rock composite specimen 5 is taken out, and the entire preparation process is completed.

[0070] Therefore, under the pressing and demoulding of step S4, the advantages are: 1. By presetting the forming pressure, loading rate and holding time, the physical properties such as density and strength of the coal-rock composite specimen 5 can be accurately controlled. Different forming pressures can simulate the compaction degree of coal and rock under different geological conditions, providing reliable experimental specimens for studying hydraulic fracturing under different formation conditions;

[0071] 2. Clear pressure parameters and operating steps standardize the experimental process and improve experimental efficiency. At the same time, the same operating conditions can ensure that the specimens prepared each time have similar physical properties, improve the repeatability of the experiment, and facilitate comparative analysis between different experimental results;

[0072] 3. Reasonable demoulding operation and the use of demoulding sleeves can ensure the smooth demoulding of the specimen while protecting the integrity of the specimen to the greatest extent, reduce the damage to the specimen caused by the demoulding process, and provide high-quality specimens for subsequent hydraulic fracturing experiments.

[0073] S5: The demoulded coal-rock composite specimen 5 is subjected to natural curing. In this embodiment, the natural curing is specifically to place the demoulded specimen in a constant temperature and humidity curing box with a temperature of 20±1° C. and a humidity of 95±3% for curing.

[0074] like Figure 3 and Figure 4As shown, the actual object state and interface state of the coal-rock composite specimen 5 prepared using the technical solution of the present application.

[0075] At the same time, in order to demonstrate the advantages of the technical solution of the present application, as shown in Table 1 below, various performance indicators of the type rock prepared by the technical solution of the present application are compared with various performance indicators of the cast cement parts, and various performance indicators of the type coal prepared by the technical solution of the present application are compared with various performance indicators of the raw coal. When the error between the uniaxial compressive strength is ±2MPa, it means that the preset error indicator is met; when the error between the elastic modulus is 6%, it means that the preset error indicator is met; when the error between the Poisson's ratio is 5%, it means that the preset error indicator is met;

[0076] Table 1

[0077] Dimensions (mm) Uniaxial compressive strength (MPa) Elastic modulus(GPa) Poisson's ratio Casting cement parts 100.66*51.83 32.51 17.21 0.19 Type rock 99.87*51.82 30.98 16.32 0.21 raw coal 99.43*51.32 4.72 3.63 0.32 Coal Shaped 100.66*51.83 3.83 3.56 0.31

[0078] in:

[0079] 1. In the type rock prepared by pressing in this scheme, when the size is close, such as Figure 5 As shown, the uniaxial compressive strength is 30.98MPa, the elastic modulus is 16.23GPa, and the Poisson's ratio is 0.21. Compared with the uniaxial compressive strength of the cast cement part of 32.51MPa, the error is 1.53MPa, which meets the preset error index; compared with the elastic modulus of the cast cement part of 17.21GPa, the error is 0.98GPa, which meets the preset error index; compared with the Poisson's ratio of the cast cement part of 0.19, the error is 0.02, which meets the preset error index;

[0080] 2. In the briquette prepared by this scheme, when the size is close, Figure 6 As shown, the uniaxial compressive strength is 3.83MPa, the elastic modulus is 3.56GPa, and the Poisson's ratio is 0.31. Compared with the uniaxial compressive strength of the raw coal of 4.72MPa, the error is 0.89MPa, which meets the preset error index; compared with the elastic modulus of the raw coal of 3.63GPa, the error is 0.07GPa, which meets the preset error index; compared with the Poisson's ratio of the raw coal of 0.32, the error is 0.01, which meets the preset error index;

[0081] According to the above experimental results, cast cement parts are a better choice in the prior art that can replace the rock part in the coal-rock complex in a real coal mine underground, and raw coal is the coal mine part that can characterize the coal-rock complex in a real coal mine underground. The rock mold and coal mold prepared by the scheme of the present application are close to the various properties of cast cement and raw coal in terms of their various properties. Therefore, rock mold can be used to replace cast cement parts, and coal mold can be used to replace raw coal. In addition, in the present scheme, during the pressing process of rock mold and coal mold, the coupling surface state of coal rock in the real scene is restored through the angled pressing rod mold 2, so that more accurate experimental results can be represented during the hydraulic fracturing experiment.

[0082] The above are only embodiments of the present invention. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the relevant field know all the common technical knowledge in the technical field to which the invention belongs before the application date or priority date, can obtain all the existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the enlightenment given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments, characterized in that: include: S1: Perform primary screening and standard screening on the raw materials of the coal-rock composite specimen, maintain the preset moisture content, and weigh them according to the preset hydraulic fracturing experiment weighing requirements; S2: preparing briquette materials and briquette rock materials according to the preset briquette coal proportioning scheme and the preset briquette rock proportioning scheme respectively; S3: prepare a pressing mold, a prefabricated hole mold, an angled pressure rod mold and a horizontal pressure rod mold, apply a release agent in the pressing mold in turn, put the prefabricated hole mold into the pressing mold, pour the molded rock material into the pressing mold, use the angled pressure rod mold to compact the molded rock material in the pressing mold, then add the briquette material into the pressing mold, and use the horizontal pressure rod mold to compact it again; S4: placing a pressing mold containing a rock material and a coal material on a press, presetting a molding pressure, a loading rate and a holding time, performing a demoulding process after molding by a press, and obtaining a coal-rock composite specimen; S5: The coal-rock composite specimens are naturally cured after demoulding.

2. The method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiment according to claim 1, characterized in that: The S1 includes: S1-1: Preliminarily screen and filter the raw materials of the coal-rock composite specimen, and keep the raw materials of the coal-rock composite specimen dry to a moisture content of 1-1.5%; the raw materials of the coal-rock composite specimen include coal powder, sand, cement and gypsum; S1-2: After the initial screening, the coal powder and sand in the raw materials of the coal-rock composite specimen are sieved with a standard sieve to obtain raw materials with a particle size of 40-80 mesh.

3. The method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiment according to claim 2, characterized in that: The S2 includes: S2-1: Weigh coal powder, cement and gypsum in a ratio of 7:2:1, pour the coal powder into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and obtain briquette material after fully mixing; S2-2: Weigh cement, sand and gypsum in a ratio of 5:4:1, pour the sand into a mixing container and mix evenly, add cement and gypsum and mix evenly, then add water while stirring, and obtain the rock material after fully mixing.

4. The method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiment according to claim 3, characterized in that: The S3 includes: S3-1: preparing a pressing mold, the pressing mold comprising a tray and a cylinder, the tray is tightly engaged at the bottom of the cylinder, and a release agent is coated inside the cylinder; S3-2: preparing a prefabricated hole mold, wherein the prefabricated hole mold is cut from steel, and comprises a bottom plate and a hole column, wherein the bottom plate and the hole column are integrally formed; when the prefabricated hole mold is placed into a pressing mold, the bottom plate is at the bottom and the hole column is at the top; S3-3: Pour the type rock material into the cylinder of the pressing mold with the prefabricated hole mold, and compact the type rock material using the angled pressing rod mold representing the angle of the formation coupling surface; S3-4: The briquette material is introduced into the pressing mold again and compacted again using the horizontal pressing rod mold.

5. The method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiment according to claim 4, characterized in that: The S4 includes: S4-1: placing the pressing mold with the shaped rock material and the shaped coal material on the pressure iron of the press, setting the forming pressure and the loading rate and starting the pressing, and after reaching the preset target forming pressure, keeping the pressure and keeping it still according to the holding time, the forming pressure is 100KN, 200KN, 300KN, the loading rate is 500N / min, and the holding time is 30min; S4-2: Place the pressed mold on a demoulding sleeve, and apply pressure to demould the formed coal-rock composite specimen.

6. A method for preparing a multi-factor coal-rock composite specimen for hydraulic fracturing experiments according to claim 5, characterized in that: The natural curing in S5 specifically includes: placing the demoulded test piece in a constant temperature and humidity curing box with a temperature of 20±1° C. and a humidity of 95±3% for curing.

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