Fractured rock mass hydraulic fracturing simulation specimen and its preparation and application

Abstract

The present invention discloses a fractured rock hydraulic fracturing simulation specimen and its preparation and application, aiming to solve the technical problem that the existing specimens are difficult to use for studying the hydraulic fracturing mechanism of specific arranged primary fractures and water-injectable fractures. The specimen includes a rock-like mass cast by a highly brittle transparent resin and non-water-injected fractures and hollow water-injected fractures buried in the rock-like mass. The highly brittle transparent resin is composed of CY-39 type resin and YS-T31 curing agent in a mass ratio of 100:34, and has good rock-like properties at ‑15~‑10°C; the non-water-injected fracture is composed of two mica sheets of corresponding shapes, and the hollow water-injected fracture includes a fracture cavity of corresponding shape and a water injection channel connecting the cavity to the outside of the rock-like mass. It has the characteristics of transparency, the ability to embed various types of fractures and the high brittleness of rocks, and can clearly observe the crack initiation, expansion and evolution process of hydraulic fracturing of the specimen under various working conditions, as well as the influence and connection on non-water-injected fractures, and can be used for the study of the hydraulic fracturing mechanism of primary fractures.

Description

Technical Field

[0001] The invention relates to the technical field of rock mass simulation detection, and in particular to a fractured rock mass hydraulic fracturing simulation specimen and preparation and application thereof. Background Art

[0002] Hydraulic fracturing is an important direction of research on hydraulic coupling of engineering rock mass, and is the key theoretical basis for projects such as oil, shale and coalbed methane mining, ground stress measurement, reservoir-induced earthquake mechanism analysis, and mine water inrush prevention. At present, there are few research results on hydraulic fracturing of fractured rock mass, and the main research method is indoor testing, that is, using simulated specimens made of real rocks, mortar blocks, organic glass or photosensitive resins, and conducting research by drilling holes from the outside to the center of the specimen.

[0003] However, the above test methods all have certain shortcomings. First, the disadvantage of rock and mortar blocks is that it is difficult to observe the evolution of internal cracks, and built-in cracks cannot be made in the specimens. However, actual engineering rock masses are endowed with a large number of three-dimensional cracks and most of them are hollow, which strongly affect the deformation and strength characteristics of the rock mass. Secondly, the disadvantage of plexiglass is that it is too different from the characteristics of rocks, and its representativeness is extremely low. In addition, cracks cannot be prefabricated in it, and it can only be used to study working conditions without prefabricated cracks in the specimens. Finally, photosensitive resin is a raw material for 3D printing technology. The disadvantage is that the specimens are made by printing and curing layer by layer, which leads to poor integrity of the specimens, and it is impossible to make complex structures in them to simulate different cracks. At the same time, photosensitive resin has high plasticity and poor similarity with rocks.

[0004] In summary, it is very difficult to study the hydraulic fracturing mechanism of primary fractures based on the above-mentioned simulation specimens, which cannot simultaneously possess the characteristics of transparency, the ability to embed different types of fractures, and rock-like properties. How to accurately create three-dimensional non-water injection fractures (groups) and hollow water injection fractures (groups) with different numbers, angles, and positions inside the rock specimens, and successfully use them for water injection tests, is a technical problem that needs to be solved urgently.

[0005] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0006] The purpose of the present invention is to provide a fractured rock mass hydraulic fracturing simulation specimen and its preparation and application, so as to solve the technical problem that the existing hydraulic fracturing test specimens are difficult to use for studying the hydraulic fracturing mechanism of primary fractures.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A fractured rock hydraulic fracturing simulation specimen is designed, comprising a rock-like mass cast by high-brittle transparent resin and non-water injection fractures and hollow water injection fractures buried in the rock-like mass; the high-brittle transparent resin is composed of CY-39 resin and YS-T31 modified amine curing agent in a mass ratio of 100:34; the hollow water injection fracture comprises a fracture cavity of corresponding shape and a water injection channel for connecting the fracture cavity to the outside of the rock-like mass; the non-water injection fracture is composed of two circular or elliptical transparent mica sheets of corresponding shape.

[0009] The embedding forming method of the hollow water injection fissure is one of the following:

[0010] (1) A crack cavity of corresponding shape is surrounded by mica sheets and gaskets, which is completely sealed with glue on all sides, and a water injection pipe is set to connect the crack cavity to the outside world; see Figure 1 (a);

[0011] (2) Using the TNSLA3908 photosensitive resin to 3D print a thin-walled cavity corresponding to the shape of the fracture cavity and the water injection channel; see Figure 1 (b);

[0012] (3) Using PVA material to 3D print a model body corresponding to the shape of the fracture cavity and the water injection channel; see Figure 1 (c).

[0013] The non-water injection fractures are divided into three types: open type, weak type, and filled type. The production plan is as follows:

[0014] (1) For fully open joints and fissures, the simulation method is to stack two identical mica sheets and apply lubricating oil in between;

[0015] (2) For weak joints and cracks, which have weak bonding strength, CY-39 epoxy resin is applied between two mica sheets;

[0016] (3) For filling-type joints and fissures, due to their low bonding strength and small tensile strength, but good friction, two mica sheets are stacked and bonded with double-sided tape.

[0017] A method for preparing a fractured rock hydraulic fracturing simulation specimen is designed, comprising the following steps:

[0018] (1) Preparation of resin casting material: Take CY-39 resin and YS-T31 modified amine curing agent in a mass ratio of 100:34, mix them evenly and remove bubbles;

[0019] (2) Preparation of fracture molds, including at least one of the following:

[0020] ① Non-water-injected fracture module: punch or cut the mica sheet into the fracture shape required for the test. Figure 2 This is the structural diagram of the steel mold. Figure 3 (a) to (d) are successfully manufactured mica sheets for processing non-water injection fracture modules;

[0021] ② Water injection fracture module:

[0022] Use mica sheets and washers to enclose a crack cavity of the shape required for the test, and set up a water injection pipe connected to the crack cavity; or,

[0023] Using the resin casting material as the printing material, 3D printing is performed to form a thin-walled cavity body corresponding to the shape of the crack cavity and the water injection channel required for the test; or,

[0024] A water-soluble model body corresponding to the shape of the fracture cavity and water injection channel required for the test is formed by 3D printing with high-temperature water-soluble PVA material;

[0025] (3) Layout of crack mold: In the specimen casting mold (such as Figure 4 Arrange and fix the fracture module or fracture module group required for the test in the test chamber;

[0026] (4) Specimen casting and curing: After the crack module is arranged, the resin casting material is poured into the specimen casting mold, and after degassing, it is dried with air at 15-20°C for 40-50 hours. When the mechanical strength of the cast specimen is ≥40 MPa, it is demolded and then dried with air at 70-80°C for 30-40 hours.

[0027] The specimen casting mold is an organic polymer silicone mold.

[0028] When the crack module is a water-soluble model body, the method further comprises the following steps:

[0029] After the specimen is cured and formed, it is placed in hot water at 80°C to dissolve the water-soluble model body made of PVA material to form corresponding crack cavities and water injection channels.

[0030] The non-water injection fracture module is a circular or elliptical thin sheet punched from a 0.1 mm thick mica sheet.

[0031] In the step (3), the crack module is positioned and fixed by:

[0032] The corresponding crack modules are pulled and connected by a thin wire, and the two ends of the thin wire are fixed on the two side walls of the specimen casting mold.

[0033] The application of the fractured rock mass simulation specimen in a hydraulic fracturing test comprises the following steps:

[0034] (1) Before the test, the fractured rock simulation specimen was placed in a -20℃ environment for 20 to 30 hours. During the test, the fractured rock simulation specimen was maintained at -15 to -10℃;

[0035] (2) Antifreeze agents to prevent freezing and dyes to facilitate observation of the expansion process of internal cracks during the experiment are added to the fracturing fluid injected into the fractured rock simulation specimen.

[0036] Compared with the prior art, the main beneficial technical effects of the present invention are:

[0037] (1) The fractured rock simulation specimen of the present invention has the characteristics of transparency, the ability to embed various types of fractures, and high brittleness, which can greatly reduce the difficulty of studying the hydraulic fracturing mechanism of primary fractures and bring great convenience to the research related to hydraulic fracturing.

[0038] (2) The fractured rock simulation specimen of the present invention has extremely high transparency, and can clearly observe the crack initiation, expansion and evolution process of water-injected fractures under water pressure, as well as the influence and connection on non-water-injected fractures, and is very suitable for the study of fracture failure laws.

[0039] (3) The present invention can conveniently and accurately prefabricate three-dimensional non-water injection cracks (groups) and hollow water injection cracks (groups) with different numbers, angles and positions inside the specimen, which can be used for experimental research on the occurrence, development and evolution of hydraulic fracturing under various working conditions.

[0040] (4) The brittleness index (ratio of compressive strength to tensile strength) of the fractured rock simulation specimen of the present invention reaches 6.6, which is more than double the brittleness index of the currently used transparent rock-like materials and is closer to the mechanical parameters and brittleness of many real rocks. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a real photo of the fracture module prepared in Example 1 of the present invention; wherein a is the water injection fracture module produced by method ①, b is the thin-walled cavity body corresponding to the water injection fracture module produced by method ②, and c is the water-soluble model body corresponding to the water injection fracture module produced by method ③.

[0042] Figure 2 This is a structural diagram of a stamping die for making mica sheets for non-water-injected fracture modules in one embodiment of the present application.

[0043] Figure 3 This is a photo of a real object used to make a non-water injection fracture module in an embodiment of the present application; wherein a is an ellipse of 12 (short axis) × 17 (long axis) mm, b is a φ15 mm circle, c is an ellipse of 15×20 mm, and d is an ellipse of 13×20 mm.

[0044] Figure 4This is a physical picture of the polymer organic silica gel casting mold used in one embodiment of the present application, with a length, width and height of 140, 70 and 80 mm respectively.

[0045] Figure 5 This is a sample diagram of a water injection fracture module produced by only one method ① in an embodiment of the present application.

[0046] Figure 6 This is a sample diagram of a water injection fracture module produced by only one method ② in an embodiment of the present application.

[0047] Figure 7 This is a sample diagram of a parallel water injection fracture module made with two parts using method ② in one embodiment of the present application.

[0048] Figure 8 This is a schematic diagram of the diverse arrangement schemes of the water injection fracture group in one embodiment of the present application, that is, a longitudinal section view of the center of the specimen.

[0049] Fig. 9 This is a picture of the hydraulic fracturing test results of a specimen containing water injection fractures in one embodiment of the present application. DETAILED DESCRIPTION

[0050] The specific implementation modes of the present invention are described below in conjunction with the accompanying drawings and examples. However, the following examples are only used to illustrate the present invention in detail and do not limit the scope of the present invention in any way.

[0051] Unless otherwise specified, the instruments and equipment involved in the following examples are all conventional instruments and equipment; unless otherwise specified, the industrial raw materials involved are all commercially available conventional industrial raw materials.

[0052] Embodiment 1: A method for preparing a fractured rock mass hydraulic fracturing simulation specimen, the main steps of which include:

[0053] 1. Preparation of resin casting material:

[0054] The CY-39 resin and the YS-T31 modified amine curing agent were fully mixed in a mass ratio of 100:34. After being evenly mixed, they were placed in a vacuum box for 30 minutes to remove bubbles in order to improve the molding quality of the specimens.

[0055] After a long period of extensive experimental research, it was found that the CY-39 resin [molecular formula is (C 11 H 12O3)n], the curing agent is YS-T31 modified amine curing agent, and when the mixing mass ratio of epoxy resin and curing agent is 100:34, the mechanical properties of the cast test block are very close to those of the rock, such as achieving higher brittleness at a specific temperature (-15 to -10°C), showing brittle fracture characteristics during compression tests, and the compressive tensile strength ratio can reach 6.6. The main mechanical parameters of the resin material in this example at -15 to -10°C are shown in Table 1. In addition, the test temperature of Dyskin and Wong is -50°C, and the test temperature of Song is -20°C.

[0056] Table 1 Comparison of physical and mechanical parameters of resin materials, other transparent rock-like materials and some real rocks in this example

[0057] .

[0058] As can be seen from Table 1, the mechanical parameters of the resin specimens are relatively close to those of real rocks, so they can simulate these rocks to a certain extent.

[0059] The study also found that the conventional epoxy resin and curing agent castables will generate a lot of heat during the curing process of the resin mixture, resulting in defects in the appearance of the specimen after curing, and the mechanical properties of the specimen are quite different from those of real rock. If water-injected cracks and non-water-injected cracks are embedded, the strength will be further affected. The epoxy resin and curing agent used in this example are easy to shape after a reasonable ratio, and can be made into thicker specimens with the help of a specific production process.

[0060] 2. Preparation of fracture mold:

[0061] In order to simulate and study the diverse distribution of fractures inside engineering rock mass, prefabricated fractures are divided into two categories: non-water injection fractures and hollow water injection fractures.

[0062] (1) Non-water-injected fracture modules: Non-water-injected fracture modules are made of 0.1 mm thick mica sheets, which are a kind of real rock in nature. Compared with other prefabricated fracture materials (such as aluminum sheets, copper sheets, polyethylene sheets, etc.) used by previous researchers, mica sheets have extremely low strength and rigidity, are easy to position, and will not restrict the deformation of the specimen after being buried in the specimen. Therefore, they can fully simulate the original fractures existing in natural rock mass. The shape of the prefabricated fracture can be elliptical or circular, and the rectangular mica sheets purchased on the market must be cut. For this purpose, the inventor designed and processed an integral steel mold that can punch the mica sheet into the required shape with precise size, which can avoid the corner error and stress concentration caused by manual cutting.

[0063] Specifically, for fully open joints and fissures, the simulation method is to stack two identical mica sheets and apply lubricating oil in the middle; for weak joints and fissures, which have weak bonding strength, the method used is to apply CY-39 epoxy resin in the middle of the two mica sheets; for filled joints and fissures, because of their low bonding strength and small tensile strength, but good friction, the method used is to stack and bond two mica sheets with double-sided tape.

[0064] (2) Hollow water injection fracture module: The hollow water injection fracture is connected to the water injection pipe. The material selection and manufacturing method are as follows:

[0065] ① Place two ordinary non-water-injected oval mica sheets of exactly the same size on top of each other, drill a hole in the center of one mica sheet and lead out a water injection pipe to connect it to the outside world so that water can be injected into the prefabricated cracks. Then place an equal-sized rubber ring with a thickness of about 1 mm between the two mica sheets. After stacking tightly, use high-temperature resistant glue to carefully glue and seal the edges. Strictly seal. Once the bonding is complete, the hollow cavity can be made. Figure 5 The finished specimen. When there are multiple water-injected fissures at the same time, a hole is drilled in advance in the center of another mica sheet, and a water injection pipe is led out to connect to the next water-injected fissure to form a prefabricated fissure group. When placed inside the specimen, it is positioned by drilling and pulling lines in the mold for pouring resin so that its geometric position meets the test design requirements.

[0066] ② After determining the water injection crack arrangement plan, use 3D printing technology to print out a thin-walled structure with a water injection crack cavity and a water injection pipe. The thin wall thickness is 0.2 mm, which plays a supporting role during pouring and is made of resin material, which is consistent with the strength of the resin specimen. Figure 6 and Figure 7 The finished test piece. Due to 3D printing technology, there will be residual liquid resin in the thin-walled cavity. A small hole with a diameter of 2 mm is opened at the center of the bottom of the crack to remove the residual liquid resin inside the thin-walled cavity. After cleaning, the hole is filled. The advantage of this method is that the size and shape of the crack can be accurately controlled, and the crack arrangement is more regular. When fixing it to the mold, you only need to locate the overall crack, and there is no need to consider the arrangement and layout between the cracks.

[0067] ③ PVA material can be dissolved in water above 80℃, and hot water at 80℃ will not affect the performance of the molded resin specimen. Therefore, PVA material is selected to produce cracks and water injection pipes through 3D printing. After the specimen is cast and cured, it is placed in hot water to dissolve the cracks and water injection pipes made of PVA material in water, and then water is injected into the interior with a syringe and rinsed clean to obtain a specimen with water injection cracks and water injection pipes inside.

[0068] 3. Layout of crack modules:

[0069] Diversified arrangement of fractures (groups): Prefabricated fractures can be divided into hollow water injection fractures and ordinary non-water injection fractures, as well as different numbers, sizes, relative positions, and angles. Therefore, various arrangement schemes can be formed, such as Figure 8 As shown, the hydraulic fracturing mechanism and engineering site simulation are studied from more experimental conditions. Taking the condition where the fixed fracture shape is an ellipse, the size is 20×13 mm, and the angle with the vertical direction is 45° as an example, the available layout schemes are listed in detail as follows:

[0070] (1) Condition with a single water injection fracture

[0071] The number of hollow water injection fractures is 1, see Figure 8 (a) The number of common non-water injection cracks can be freely set according to research needs. By changing the number of common cracks, we can compare and study their influence on the crack initiation and development stress of single water injection crack specimens, as well as the initiation, expansion and penetration of secondary cracks. Mainly including:

[0072] ① Single water injection fracture; ② Water injection fractures and ordinary fractures are parallel with a certain offset; ③ Water injection fractures and ordinary fractures intersect at a large angle and are arranged in a goose-tail shape.

[0073] (2) Condition with two water injection fractures

[0074] The number of hollow water injection fractures is 2, see Figure 8 (b) The number of ordinary non-water injection cracks can still be freely set according to research needs. As the number of cracks increases, the size of the specimen can be appropriately increased to avoid boundary effects. The main working conditions include: ① The two cracks are at the same elevation, and are guided to the upper surface of the specimen by their own water injection pipes and arranged side by side, so there are two external water injection channels; ② Based on working condition ①, an ordinary crack is added below; ③ The two water injection cracks are directly opposite each other, and the water injection pipe connects the two in series. The water injection pipe connects the centers of the two, and the offset can be changed; ④ The two water injection cracks are directly opposite each other, but are connected to the upper and lower surfaces of the specimen by their own water injection pipes, so the laboratory needs to apply water pressure on both the upper and lower surfaces at the same time; ⑤ Based on working condition ④, an ordinary crack is added between the two water injection cracks, and they are arranged in parallel, and the offset can be changed.

[0075] (3) Condition with three water injection fractures

[0076] There are 3 hollow water injection fractures. Figure 8(c) The number of ordinary non-water injection fractures can still be freely set according to research needs. As the number of fractures increases, the specimen size can be further increased to avoid boundary effects. The main working conditions include: ① Three fractures are at different elevations and are connected in series by a water injection pipe. The water injection pipe connects the centers of the three fractures and the staggered distance is variable; ② Two water injection fractures are at the same elevation, side by side, and each leads to a water injection pipe. Another water injection fracture is arranged in series with one of the water injection fractures; ③ Two water injection fractures are at the same elevation, side by side, and each leads to a water injection pipe. Another water injection fracture is added to face the two water injection fractures up and down, and an additional water injection pipe is led out on the opposite side; ④ On the basis of working condition ③, an additional non-water injection fracture is added, and the four fractures are arranged in two layers.

[0077] (4) n water injection fractures can be set according to experimental requirements

[0078] The number of water injection fractures is set according to the experimental requirements, and some non-water injection fractures are added appropriately. A more complex distribution of rock fractures has been simulated. The connection point between the water injection pipe and the fracture can also be arranged according to the experimental and research requirements.

[0079] Fixing of cracks on the mold: Drill two small holes at the corresponding positions of the projection points of the left and right long side panels of the specimen mold, and then use soft cotton thread to pass through each small hole in sequence. Two holes form a group with a spacing of 5 mm. One or two side-by-side cracks can be located. Use a thin wire to pull, connect and fix the cracks. The two ends of the thin wire are fixed in the holes on the two side panels to achieve accurate positioning and realize the arrangement of water-injected cracks and non-water-injected cracks under different working conditions. After the water-injected cracks and non-water-injected cracks are fixed on the mold, the pouring step is performed.

[0080] 4. Specimen casting and curing:

[0081] The specimen casting mold with the crack fixed is placed on a horizontal workbench, and the mixture prepared and debubbled in step (2) is poured into the specimen mold using a glass rod; the specimen mold is placed in a vacuum box again for debubbling for 30 minutes; then it is placed in a constant temperature forced air drying oven at 18°C ​​for curing for 48 hours. When the specimen has a mechanical strength of 40 MPa, the mold can be removed; the demoulded specimen is placed in a constant temperature forced air drying oven at a constant temperature of 75°C for curing for 36 hours to complete the production.

[0082] The specimen casting mold uses a polymer silicone casting mold, which has the following advantages: high dimensional accuracy, and the mold size and mold thickness can be designed according to the experimental requirements. The mold can be made in one piece, there is no splicing gap, and the leak-proof effect is good. In addition, it is not easy for bubbles to adhere to the silicone mold wall, and the bubble content of the produced specimen is low. It is easy to demould without damaging the appearance of the specimen, maintaining the high permeability and integrity of the specimen, and greatly improving the quality of the specimen. Compared with existing glass molds, acrylic molds, plastic molds, steel molds, etc., it has high practicality, is easy to drill holes, and is convenient for the arrangement of cracks.

[0083] Example 2: Hydraulic fracturing test application

[0084] 1. Preparation of fractured rock simulation specimens

[0085] (1) Creation and arrangement of cracks

[0086] The water injection fracture was made by using method ② in the hollow water injection fracture module in Example 1, and a single fracture was selected as the water injection fracture working condition. The fracture size was selected to be 13 (minor axis) × 20 (major axis) mm, the thickness was 2 mm, and the thin wall thickness was 0.2 mm. An elliptical mica sheet with a size of 13 × 20 mm was made using a steel mold; it was used as a non-water injection fracture module.

[0087] The water injection fissure was arranged at the center of the specimen, that is, at a height of 70 mm on the long side of the specimen, and the non-water injection fissure was 10 mm directly below the water injection fissure.

[0088] (2) Production of polymer silicone mold

[0089] A rectangular silicone mold with dimensions of 140 × 70 × 80 mm was made, with an opening at 140 × 70 mm.

[0090] (3) Arrangement and fixation of water injection fractures and non-water injection fractures

[0091] First, draw the projection of the crack on the side of the specimen in the drawing software and adjust the position of the crack. According to the projection diagram, drill holes at the corresponding positions of the projection points on the left and right long side panels of the specimen mold. To fix a crack, two holes need to be drilled on both sides of the long plate of the mold. Then, use soft cotton thread to pass through each small hole in sequence, and then apply high-temperature resistant super glue on the cotton thread to fix the crack firmly.

[0092] (4) Casting of test pieces

[0093] CY-39 resin and YS-T31 modified amine curing agent were mixed in a mass ratio of 100:34 and placed in a vacuum box for 30 minutes to remove bubbles in order to improve the molding quality of the specimens.

[0094] Place the polymer silicone mold with fixed cracks on the workbench, and use a glass rod to guide the prepared and treated mixture into the mold for casting; after casting, put the mold into the vacuum box again for debubbling treatment for 30 minutes, and then put it into a 18°C ​​constant temperature forced air drying box for curing for 36 hours to complete the curing process.

[0095] (5) Demolding and curing

[0096] When the mechanical strength reaches 40 MPa, the mold is removed, and the appearance of the specimen is not damaged during demolding. The demolded specimen is placed in a 75℃ constant temperature forced air drying oven for curing for 36 hours to complete the specimen production.

[0097] 2. Hydraulic fracturing test

[0098] The experimental equipment mainly includes water injection equipment and uniaxial compression device. The water injection equipment can maintain constant water pressure. Before the test, the specimen must be kept in a refrigerator at -20℃ for 24 hours to ensure that the specimen has the brittleness required for the experiment. During the experiment, dry ice must be placed around the specimen to maintain low temperature conditions (within the range of -15 to -10℃), and an infrared thermometer is used to monitor temperature changes. An appropriate amount of antifreeze is added to the fracturing fluid to prevent freezing. At the same time, in order to facilitate the observation of the expansion process of internal cracks in the experiment, blue dye is also added.

[0099] Under uniaxial compression, a fracturing test with an internal water pressure of 2 MPa was carried out. The test results are shown in Table 2 and Fig. 9 As shown. Water pressure should be applied slowly to the predetermined value. In the process of the water pressure steadily rising to the predetermined value, no cracks occurred in the specimens; after the water pressure reached the predetermined value, the water pressure was kept stable by the pressure-maintaining system, and the specimens were continuously loaded axially. When the axial stress increases to a certain value, cracks initiate at the upper and lower ends of the long axis of the preset crack at the same time, and the growth rate is very fast, and finally stabilizes. As the axial pressure continues to increase, the cracks begin to expand horizontally, and the vertical expansion rate is slow. When the cracks expand horizontally to the surface of the specimen, they do not penetrate in the vertical direction.

[0100] Table 2 Crack initiation and penetration stress under two internal water pressures

[0101] Water pressure / MPa Crack initiation stress / MPa Lateral crack penetration stress / MPa 0 41.5 86.2 4 5.8 17.2

[0102] This specimen is highly transparent, and the entire process of crack initiation and development can be clearly observed. By observing and analyzing the specimen, it can be seen that under the 2 MPa water pressure condition, no small cracks appeared during the rupture process. This shows that water pressure has an inhibitory effect on the generation of small cracks. In addition, the crack initiation stress of the specimen under 2 MPa water pressure is reduced by 86% compared with the condition without water pressure, and the peak strength is reduced by 80%. The crack initiation stress and peak strength are much smaller than those under the condition without water, indicating that water pressure has a significant reducing effect on the crack initiation stress and peak strength, and the greater the water pressure, the more obvious it is.

[0103] The present invention has been described in detail above with reference to the accompanying drawings and embodiments. However, those skilled in the art will appreciate that, without departing from the concept of the present invention, the specific parameters in the above embodiments may be changed, or related components, structures and materials may be replaced by equivalents, thereby forming multiple specific embodiments, which are all within the common variation range of the present invention and will not be described in detail one by one here.

Claims

1. A method for preparing a fractured rock mass hydraulic fracturing simulation specimen, characterized in that: The specific steps include: (1) Preparation of resin casting material: Mix CY-39 resin and YS-T31 modified amine curing agent in a mass ratio of 100:34 and place in a vacuum machine for 30 minutes to remove bubbles; (2) Preparation of fracture molds, including at least one of the following: ① Non-water injection fracture module: Take the rectangular mica sheets purchased on the market and punch or cut them into the crack shape required for the test. According to the different types of cracks to be simulated, namely open type, weak type, and filled type, they are made according to the following methods: The non-water injection fissures are composed of circular or elliptical transparent mica sheets of corresponding shapes. The thickness of a single mica sheet is 0.1 mm. It is formed by stamping with a steel mold and has neat edges to avoid stress concentration caused by burrs and size defects. The following different schemes are used to simulate three types of joint fissures: For fully open joints and fissures, the simulation method is to stack two identical mica sheets and apply lubricating oil in between; For weak joints and fissures, which have weak bonding strength, CY-39 epoxy resin is applied between two mica sheets. For filling type joints and fissures, due to their low bonding strength and small tensile strength, but good friction, two mica sheets are stacked and bonded with double-sided tape; ② Water injection fracture module: Use mica sheets and washers to enclose a crack cavity of the shape required for the test, and set up a water injection pipe connected to the crack cavity; or, Using TNSLA3908 photosensitive resin as the printing material, 3D printing is used to form a thin-walled cavity corresponding to the shape of the fracture cavity and water injection channel required for the test; or, Using high-temperature water-soluble PVA as the material, a water-soluble model body corresponding to the shape of the fracture cavity and water injection channel required for the test was 3D printed; When the water injection fracture module is a water-soluble PVA model body, the following treatment steps are also included: After the specimen is cured and formed, it is placed in 80°C hot water for half an hour to completely dissolve the water-soluble model body printed by the PVA material, thereby forming corresponding crack cavities and water injection channels; (3) Fracture module layout: Arrange and fix the fracture modules or fracture module groups required for the test in the specimen casting mold; (4) Specimen casting and curing: After the crack module is arranged, the resin casting material is poured into the specimen casting mold, and after degassing, it is dried with air at 15-20°C for 40-50 hours. When the mechanical strength of the specimen is ≥40MPa, it is demolded and then dried with air at 70-80°C for 30-40 hours.

2. The method for preparing a fractured rock mass hydraulic fracturing simulation specimen according to claim 1, characterized in that: In step (3), the specimen casting mold is an organic polymer silicone mold, which does not react with the resin, is easy to demould and can be reused; at the same time, the crack module is positioned and fixed by the following method: the corresponding crack modules are pulled and connected by a thin wire, and the two ends of the thin wire are fixed to the two side walls of the specimen casting mold.

3. The method for preparing a fractured rock mass hydraulic fracturing simulation specimen according to claim 1, characterized in that: The fractured rock hydraulic fracturing simulation specimen is highly transparent, and the entire evolution law of internal cracks and damage can be clearly observed with the naked eye. Specifically, it includes a rock-like mass cast by a highly brittle transparent resin material and non-water-injected fractures or non-water-injected fracture groups and / or water-injected fractures or water-injected fracture groups buried inside the rock-like mass. The ratio of the compressive and tensile strengths of the simulation specimen is as high as 6.6, and the progressive expansion process of cracks in shale formations during oil and gas extraction under water pressure is presented through real experiments.

4. The method for preparing a fractured rock hydraulic fracturing simulation specimen according to claim 1, characterized in that: The water injection fissure is a hollow body, including a fissure cavity of a corresponding shape and a water injection channel for connecting the cavity to the outside of the rock body; the preparation and molding method is any one of the following three methods: (1) A closed crack cavity of corresponding shape is formed by mica sheets and gaskets, and a water injection pipe is provided to connect the crack cavity to the outside; (2) Using TNSLA3908 photosensitive resin as raw material and 3D printing technology, a thin-walled cavity corresponding to the shape of the fracture cavity and the water injection channel is printed, with a wall thickness of 0.2 mm; (3) Using high-temperature water-soluble polyvinyl alcohol as a raw material and adopting 3D printing technology, a model body corresponding to the shape of the fracture cavity and the water injection channel is printed.

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