CN115436118A

CN115436118A - Fractured rock mass hydraulic fracturing simulation test piece and preparation and application thereof

Info

Publication number: CN115436118A
Application number: CN202210894018.5A
Authority: CN
Inventors: 付金伟; 郭梦迪; 雒翔宇; 李彤
Original assignee: North China University of Water Resources and Electric Power
Current assignee: North China University of Water Resources and Electric Power
Priority date: 2022-07-27
Filing date: 2022-07-27
Publication date: 2022-12-06

Abstract

The invention discloses a fractured rock mass hydraulic fracturing simulation test piece and preparation and application thereof, and aims to solve the technical problem that the conventional test piece is difficult to be used for researching the hydraulic fracturing mechanism of specifically arranged primary fractures and water-injectable fractures. The test piece comprises a quasi-rock body formed by casting high-brittleness transparent resin, and a non-water-injection crack and a hollow water-injection crack which are buried in the quasi-rock body. The high-brittleness transparent resin is prepared by mixing CY-39 type resin and YS-T31 curing agent according to the weight ratio of 100:34, and has good rock-like property at the temperature of between 15 ℃ below zero and 10 ℃ below zero; the non-water-injection crack is formed by combining two mica sheets in corresponding shapes, and the hollow water-injection crack comprises a crack cavity in a corresponding shape and a water injection channel for communicating the cavity to the outside of the rock-like body. The hydraulic fracture observation device has the characteristics of transparency, capability of embedding various fractures and high brittleness of rocks and the like, can clearly observe crack initiation, expansion and evolution processes of the hydraulic fracturing of a test piece under various working conditions and influence and connection on non-water-injection fractures, and can be used for researching a hydraulic fracturing mechanism of a primary fracture.

Description

Fractured rock mass hydraulic fracturing simulation test piece and preparation and application thereof

Technical Field

The invention relates to the technical field of rock mass simulation detection, in particular to a fractured rock mass hydraulic fracturing simulation test piece and preparation and application thereof.

Background

The hydraulic fracturing is an important direction for the hydraulic coupling research of engineering rock masses, and is the key theoretical basis of the engineering such as petroleum, shale and coal bed gas exploitation, ground stress measurement, reservoir induced seismic mechanism analysis, mine water inrush prevention and control and the like. At present, the research results on the aspect of hydraulic fracturing of fractured rock masses are few, the main research means is indoor tests, namely, simulation test pieces made of real rocks, mortar blocks, organic glass or photosensitive resin and other materials are researched by adopting a method of punching from the outside to the center of the test piece.

However, the above test methods all have certain disadvantages. Firstly, the rock and mortar blocks have the disadvantages that the evolution process of internal cracks is difficult to observe, and built-in cracks cannot be made in the test piece, but the actual engineering rock mass is provided with a large number of three-dimensional cracks and is mostly hollow, and the deformation and strength characteristics of the rock mass are strongly influenced by the three-dimensional cracks. Secondly, the organic glass has the defects of large difference with the rock characteristics, extremely low representativeness, incapability of prefabricating cracks in the organic glass and only capability of researching the working condition without prefabricating cracks in a test piece. Finally, the photosensitive resin is a raw material of the 3D printing technology, and has the disadvantages that the test piece is manufactured in a layer-by-layer printing and curing manner, so that the integrity of the test piece is poor, a complex structure cannot be manufactured therein to simulate different cracks, and the photosensitive resin has high plasticity and poor similarity to rocks.

In conclusion, the research on the hydraulic fracturing mechanism of the primary fracture based on the simulation test piece is difficult, and the simulation test piece cannot have the characteristics of transparency, capability of embedding different types of fractures and rock-like properties and the like. How to accurately manufacture three-dimensional non-water injection cracks (groups) and hollow water injection cracks (groups) with different numbers, angles and positions in a rock mass test piece, and can be successfully used for a water injection test is a technical problem to be solved urgently at present.

The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.

Disclosure of Invention

The invention aims to provide a fractured rock mass hydraulic fracturing simulation test piece and preparation and application thereof, and aims to solve the technical problem that the conventional hydraulic fracturing test piece is difficult to be used for researching the hydraulic fracturing mechanism of a primary fracture.

In order to solve the technical problems, the invention adopts the following technical scheme:

designing a fractured rock mass hydraulic fracturing simulation test piece, which comprises a similar rock mass cast by high-brittleness transparent resin, and a non-water injection fracture and a hollow water injection fracture which are embedded in the similar rock mass; the high-brittleness transparent resin is prepared by mixing CY-39 type resin and YS-T31 modified amine curing agent according to the weight ratio of 100:34 in mass ratio; the hollow water injection fracture comprises a fracture cavity with a corresponding shape and a water injection channel for communicating the fracture cavity to the outside of the rock-like body; the non-water-injection crack is formed by combining two round or oval transparent mica sheets in corresponding shapes.

The embedding and forming method of the hollow water injection crack comprises one of the following steps:

(1) A crack cavity with a corresponding shape is surrounded by a mica sheet and a gasket, the periphery of the crack cavity is completely sealed by glue, and a water injection pipe for communicating the crack cavity to the outside is arranged; see fig. 1 (a);

(2) 3D printing and forming a thin-wall cavity body corresponding to the shapes of the fracture cavity and the water injection channel by using the TNSLA3908 type photosensitive resin; see fig. 1 (b);

(3) 3D printing and forming a model body corresponding to the shape of the crack cavity and the water injection channel by using a PVA material; see fig. 1 (c).

The non-injected water fracture is divided into an open type fracture, a soft and weak type fracture and a filling type fracture, and the manufacturing scheme is as follows:

(1) For a fully-opened joint crack, the simulation method is to stack two same mica sheets and smear lubricating oil in the middle;

(2) For the weak joint cracks, the self has weak bonding strength, and CY-39 type epoxy resin is coated between two mica sheets;

(3) For the filling type joint crack, because the bonding strength is low, the tensile strength is small, but the friction is good, two mica sheets are overlapped and bonded by using double-sided adhesive tape.

A preparation method for a fractured rock mass hydraulic fracturing simulation test piece is designed, and comprises the following steps:

(1) Preparing a resin casting material: taking CY-39 type resin and YS-T31 modified amine curing agent according to the proportion of 100:34, uniformly mixing and removing bubbles;

(2) Preparing a fracture module, wherein at least one of the following conditions is included:

(1) non-water injected fracture module: stamping or cutting a mica sheet into a crack shape required by a test, wherein FIG. 2 is a structural diagram of a steel mold, and FIGS. 3 (a) to (d) are mica sheets successfully manufactured for processing a non-water-injection crack mold;

(2) water-injected fracture module:

a crack cavity in the shape required by the test is surrounded by a mica sheet and a gasket, and a water injection pipe communicated with the crack cavity is arranged; or the like, or, alternatively,

the resin casting material is used as a printing material, and a thin-wall cavity body corresponding to the shapes of a crack cavity and a water injection channel required by the test is formed through 3D printing; or the like, or, alternatively,

3D printing and forming a water-soluble model body with the shape corresponding to the shape of a crack cavity and a water injection channel required by the test by using a high-temperature water-soluble PVA material;

(3) And (3) arranging a crack module: arranging and fixing a crack module or a crack module group required by the test in a test piece pouring mold (shown in figure 4);

(4) Pouring and curing the test piece: and (3) draining and pouring the resin casting material into a test piece casting mold after the crack module is arranged, removing air bubbles, drying the test piece casting mold by air blowing for 40-50 h at 15-20 ℃, demolding the test piece after the mechanical strength of the cast test piece is more than or equal to 40 MPa, and drying the test piece casting mold by air blowing for 30-40 h at 70-80 ℃.

The test piece casting mold is an organic polymer silica gel mold.

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

after the test piece is cured and molded, the test piece is placed in hot water of 80 ℃ to dissolve the water-soluble model body made of the PVA material so as to form a corresponding crack cavity and a water injection channel.

The non-water-injection crack module is a round or oval sheet punched by a mica sheet with the thickness of 0.1 mm.

In the step (3), positioning and fixing the crack module by the following means:

and drawing and connecting the corresponding crack modules through the thin lines, and fixing the two ends of the thin lines on the two side walls of the test piece pouring mold.

The application of the fractured rock mass simulation test piece in the hydraulic fracturing test comprises the following steps:

(1) Before the test, the fractured rock mass simulation test piece is placed for 20 to 30 hours at the temperature of-20 ℃, and the fractured rock mass simulation test piece is maintained at the temperature of-15 to-10 ℃ in the test process;

(2) And an antifreeze agent for preventing icing and a coloring agent for conveniently observing the expansion process of the internal fracture in the experiment are mixed into the fracturing fluid injected into the fractured rock mass simulation test piece.

Compared with the prior art, the invention has the main beneficial technical effects that:

(1) The fractured rock mass simulation test piece has the characteristics of transparency, capability of embedding various fractures, high brittleness and the like, can greatly reduce the research difficulty of the hydraulic fracturing mechanism of the primary fracture, and brings great convenience for the related research of hydraulic fracturing.

(2) The fissured rock mass simulation test piece has high transparency, can clearly observe the crack initiation, expansion and evolution processes of the test piece water injection fissures under the water pressure and the influence and connection on the non-water injection fissures, and is very suitable for the research on fracture damage rules.

(3) The 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 in the test piece, and is used for experimental research on the generation, development and evolution rules of hydraulic fracturing under various working conditions.

(4) The brittleness index (the ratio of compressive strength to tensile strength) of the fractured rock mass simulation test piece reaches 6.6, is improved by more than one time compared with the brittleness index of the existing transparent rock material, and is more close to the mechanical parameters and brittleness of a plurality of real rocks.

Drawings

FIG. 1 is a photograph of a crack module prepared in accordance with one embodiment of the present invention; wherein a is the water injection crack module manufactured by the method (1), b is the thin-wall cavity body corresponding to the water injection crack module manufactured by the method (2), and c is the water-soluble model body corresponding to the water injection crack module manufactured by the method (3).

Fig. 2 is a structural diagram of a stamping die for manufacturing a mica sheet of a non-water-injection crack module in an embodiment of the present application.

FIG. 3 is a pictorial photograph of a non-flooding fracture module used in one embodiment of the present application; wherein a is a 12 (minor axis) × 17 (major axis) mm ellipse, b is a Φ 15 mm circle, c is a 15 × 20 mm ellipse, and d is a 13 × 20 mm ellipse.

FIG. 4 is a schematic diagram of a polymer organic silica gel casting mold used in an embodiment of the present application, wherein the length, width and height are 140 mm, 70 mm and 80 mm, respectively.

FIG. 5 is a sample of a test piece of one embodiment of the present application including only one water injection slit module made by the method (1).

FIG. 6 is a sample of a test piece of one embodiment of the present application including only one water injection slit module made by the method (2).

FIG. 7 is a sample of a test piece including two parallel water injection slit modules made by the method (2) in an example of the present application.

FIG. 8 is a schematic diagram of the diversity arrangement scheme of the water injection fracture groups in an embodiment of the present application, namely a longitudinal section view of the center of the test piece.

FIG. 9 is a photograph showing the results of a hydraulic fracturing test on a test piece containing water-injected fractures in an embodiment of the present application.

Detailed Description

The following examples are provided to illustrate the present invention in detail and are not intended to limit the scope of the present invention in any way.

The instruments and devices referred to in the following examples are conventional instruments and devices unless otherwise specified; the industrial raw materials are all conventional industrial raw materials which are sold on the market, if not specifically mentioned.

The first embodiment is as follows: the preparation method of the fractured rock mass hydraulic fracturing simulation test piece mainly comprises the following steps:

1. preparing a resin casting material:

the preparation method comprises the following steps of (1) mixing CY-39 type resin and YS-T31 modified amine curing agent according to the mass ratio of 100:34, mixing the materials fully, putting the mixture into a vacuum box for 30 minutes to remove bubbles after mixing uniformly, and improving the molding quality of the test piece.

Through a large amount of long-term experimental researches, the CY-39 type resin [ the molecular formula is (C) ₁₁ H ₁₂ O ₃ ）n]The curing agent is YS-T31 modified amine curing agent, and when the mixing mass ratio of the epoxy resin to the curing agent is 100:34, the mechanical properties of the cast test block and the rock are extremely close, for example, higher brittleness is realized at a specific temperature (-15 to-10 ℃), a brittle fracture characteristic is presented when a compression test is carried out, and the specific pressure-tensile strength can reach 6.6. The main mechanical parameters of the resin material of this example at-15 to-10 ℃ are shown in Table 1, and the test temperatures of Dyskin and Wong are-50 ℃ and the test temperature of Song is-20 ℃.

TABLE 1 comparison of the physico-mechanical parameters of the resin material and other rock-like materials of the transparent type and of a part of the real rock in this example

。

As can be seen from table 1, the resin samples are closer to the mechanical parameters of real rocks, so that the rocks can be simulated to some extent.

Researches also find that the castable formed by the conventional epoxy resin, the curing agent and the proportion can generate a lot of heat in the curing process of the resin mixture, so that the appearance of a test piece after the curing is finished is defective, and the mechanical property of the test piece is greatly different from that of real rock. If the water injection cracks and the non-water injection cracks are embedded, the strength is further influenced. The epoxy resin and the curing agent adopted by the embodiment are reasonably proportioned and are easy to mold, and a test piece with larger thickness can be manufactured by a specific manufacturing process.

2. Preparation of a crack module:

for simulating and researching the distribution condition of various cracks in an engineering rock mass, the prefabricated cracks are divided into two types: non-water-injected fractures and hollow water-injected fractures.

(1) Non-water injected fracture module: the non-water-injection crack module is made of a mica sheet with the thickness of 0.1 mm, the mica sheet is a real rock in the nature, and compared with other prefabricated crack materials (such as aluminum sheets, copper sheets, polyethylene sheets and the like) adopted by researchers in the past, the mica sheet is extremely low in strength and rigidity, convenient to position and incapable of limiting the deformation of a test piece after being buried in the test piece, so that the non-water-injection crack module can completely simulate the primary cracks existing in a natural rock mass. The shape of the prefabricated crack can be oval or round, and rectangular mica sheets purchased in the market need to be cut, so that the inventor designs and processes an integral steel die, the mica sheets can be punched into the required shape, the size is accurate, and corner errors and stress concentration caused by manual shearing can be avoided.

Specifically, for a fully-opened joint crack, the simulation method is to stack two same mica sheets and coat lubricating oil in the middle; for the weak joint crack, the self has weak bonding strength, and CY-39 type epoxy resin is coated between two mica sheets; for the filling type joint crack, because the bonding strength is low, the tensile strength is small, but the friction is good, two mica sheets are overlapped and bonded by using double-sided adhesive tape.

(2) Hollow water-injected fracture module: the hollow water injection crack is connected with a water injection pipe, and the material selection and the manufacturing method are as follows:

(1) two common non-water-injection oval mica sheets with the same size are stacked up and down, a hole is drilled in the center of each mica sheet, a water injection pipe is led out, the mica sheets are communicated with the outside, water can be injected into the prefabricated crack, an equal-size rubber thin ring is placed between the two mica sheets, the thickness of the rubber thin ring is about 1 mm, after the two mica sheets are tightly laminated, the edges of the two mica sheets are carefully bonded and sealed by high-temperature-resistant glue, the edges of the two mica sheets are strictly sealed, the hollow cavity can be completely bonded, and a test piece manufactured according to a figure 5 is shown. When a plurality of water injection cracks exist simultaneously, a hole is drilled in the center of another mica sheet in advance, and a water injection pipe is led out to be connected to the next water injection crack so as to form a prefabricated crack group. When the device is placed in a test piece, the device is positioned in a mold for pouring resin in a drilling and wire drawing mode, so that the geometric position of the device meets the requirement of test design.

(2) After the water injection crack arrangement scheme is determined, a 3D printing technology is used for printing a thin-wall structure with a water injection crack cavity and a water injection pipe, the thickness of the thin wall is 0.2 mm, the thin wall plays a supporting role during pouring, the thin wall is made of a resin material, the strength of the thin wall is consistent with that of a resin test piece, and the test piece is shown in the figure 6 and the figure 7. Because of the 3D printing technology, residual liquid resin exists in the thin-wall cavity, a small hole with the diameter of 2 mm is formed in the center of the bottom of the crack and used for removing the residual liquid resin in the thin-wall cavity, and the hole is completed after the cleaning is finished. The method has the advantages that the sizes and the shapes of the cracks can be accurately controlled, and the crack arrangement mode is more regular. When the die is fixed, only the whole cracks need to be positioned, and arrangement among the cracks do not need to be considered.

(3) The PVA material can be dissolved in water with the temperature of more than 80 ℃, and the hot water with the temperature of 80 ℃ can not influence the performance of the molded resin test piece. Therefore, a PVA material is selected, and the crack and the water injection pipe are manufactured through 3D printing. After the pouring, curing and forming of the test piece are finished, the test piece is placed in hot water to enable the cracks and the water injection pipe made of the PVA material to be dissolved in water, then water is injected into the test piece again by using an injector, and the test piece is washed clean, so that the test piece with the water injection cracks and the water injection pipe inside is obtained.

3. And (3) arranging a crack module:

diversity arrangement of fissures (groups): the prefabricated fractures have the difference between hollow water injection fractures and common non-water injection fractures, and the difference between different numbers, different sizes, different relative positions and different angles, so that various arrangement schemes can be formed, as shown in fig. 8, and the hydraulic fracturing mechanism and the simulation engineering site are researched from more experimental working conditions. Now, taking the working condition that the shape of the fixed crack is an ellipse, the size of the fixed crack is 20 × 13 mm, and the included angle between the fixed crack and the vertical direction is 45 degrees as an example, the available arrangement schemes are listed in detail as follows:

(1) Including single water injection fracture condition

The number of the hollow water injection cracks is 1, see figure 8 (a), and the number of the common non-water injection cracks can be freely set according to research needs. By changing the change of the number of the common cracks, the influence rule of the crack initiation and the development stress of the single-injection crack test piece and the initiation, the expansion and the communication of the secondary cracks can be contrastively researched. The method mainly comprises the following steps:

(1) a single water injection crack; (2) the water injection crack is parallel to the common crack and has a certain offset; (3) the water injection crack and the common crack are crossed in a large angle, and the goose tail type is arranged.

(2) Working condition containing two water injection cracks

The number of hollow water injection cracks is 2, as shown in figure 8 (b), and the number of common non-water injection cracks is still freely set according to the research needs. As the number of cracks increases, the size of the test piece can be increased appropriately to avoid the boundary effect. The main working conditions include: (1) the two cracks are positioned at the same elevation, the upper surface of the test piece is guided by respective water injection pipes and are arranged side by side, so that 2 external water injection channels are arranged; (2) on the basis of the working condition (1), a common crack is additionally arranged below the working condition; (3) the two water injection cracks are opposite up and down, the water injection pipe connects the two cracks in series, the water injection pipe is connected with the centers of the two cracks, and the offset distance is variable; (4) the two water injection cracks are opposite up and down, but are respectively connected to the upper surface and the lower surface of the test piece through respective water injection pipes, so that the upper surface and the lower surface of a laboratory need to simultaneously apply water pressure; (5) on the basis of the working condition (4), a common crack is additionally arranged between the two water injection cracks and arranged in parallel, so that the offset can be changed.

(3) Working condition containing three water injection cracks

The number of the hollow water injection cracks is 3, as shown in figure 8 (c), and the number of the common non-water injection cracks is still freely set according to the research needs. As the number of cracks increases, the specimen size can be further increased to avoid boundary effects. The main working conditions include: (1) the three cracks are at different elevations and are connected in series by a water injection pipe, the water injection pipe is connected with the centers of the three cracks, and the offset distance is variable; (2) the two water injection cracks are positioned at the same elevation and are arranged side by side, and a water injection pipe is led out from each water injection crack. Another water injection crack is arranged in series with one of the water injection cracks; (3) the two water injection cracks are positioned at the same elevation and are arranged side by side, and a water injection pipe is led out from each water injection crack. A water injection crack is added to be vertically opposite to the two water injection cracks, and a water injection pipe is additionally led out from the opposite side; (4) under the working condition (3), a non-water injection crack is additionally arranged, and the four cracks are arranged in two layers.

(4) Can be provided with n water injection fractures according to the experimental requirements

And setting the number of water injection fractures according to experimental requirements, and properly adding some non-water injection fractures. More complex rock mass fracture distributions have been simulated. Wherein the position of the connecting point of the water injection pipe and the fracture can also be arranged according to the experiment and research requirements.

Fixing the crack on the mold: the projection point of two long side face corresponds punishment and do not drills out two apertures about the test piece mould, then passes every aperture in order with soft cotton thread, and two holes are a set of, and the interval is 5 mm, can fix a position one or two cracks side by side, with fine rule tractive, connect and fixed crack, during the hole of both sides face is fixed in at the fine rule both ends, play the effect of accurate positioning, realize that the water injection crack under the different operating mode arranges and the non-water injection crack arranges. And after the water injection crack and the non-water injection crack are fixed on the mold, the step of pouring is carried out.

4. Pouring and maintaining the test piece for molding:

placing the test piece pouring mold with the fixed cracks on a horizontal working table, and pouring the mixture prepared in the step (2) and subjected to bubble removal treatment into the test piece mold by using a glass rod for drainage; placing the test piece mold into a vacuum box again for 30 minutes to remove bubbles; then placing the test piece into a constant-temperature air-blast drying oven at 18 ℃ for curing for 48 hours, and removing the die when the test piece has the mechanical strength of 40 MPa; and (5) placing the demoulded test piece into a constant-temperature air-blast drying oven with the constant temperature of 75 ℃ for curing for 36 hours, and then finishing the manufacturing.

The polymer silica gel casting mold is selected for use to test piece casting mold, and its benefit lies in: the size precision is high, can design mould size and mould thickness by oneself according to the experiment needs. The mould can be made in an integrated manner, splicing gaps do not exist, and the leakage-proof effect is good. And the silica gel mould wall is difficult for attaching the bubble, and the test piece bubble content of making is low. The drawing of patterns is convenient, can not harm the test piece outward appearance, keeps test piece high permeability and integrality, makes the test piece quality obtain very big promotion. Compared with the existing glass mold, acrylic mold, plastic mold, steel mold and the like, the crack drilling tool has very high practicability, is easy to drill, and is convenient for crack arrangement.

Example two: application of hydraulic fracturing test

1. Preparation of fractured rock mass simulation test piece

(1) Making and laying out fractures

The water injection fracture is manufactured by the method (2) in the hollow water injection fracture module in the first embodiment, and the single fracture is selected as the working condition of the water injection fracture. The dimensions of the slit were selected to be 13 (minor axis). Times.20 (major axis) mm, the thickness 2 mm, and the thickness 0.2 mm. Manufacturing an oval mica sheet with the size of 13 multiplied by 20 mm by using a steel mould; as a non-water injected fracture module.

The water injection crack is arranged in the center of the test piece, namely the height of the long edge of the test piece is 70 mm, and the non-water injection crack is 10 mm under the water injection crack.

(2) Manufacture of high-molecular organic silica gel mold

A rectangular silicone rubber mold having a size of 140X 70X 80 mm was prepared, and an opening was formed at a position of 140X 70 mm.

(3) Arrangement and fixation of water-injected fractures and non-water-injected fractures

Firstly, drawing a projection drawing of the crack on the side surface of the test piece in drawing software, and adjusting the position of the crack. Drilling holes at corresponding positions of projection points of left and right long side plate surfaces of a test piece die according to a projection drawing, drilling two holes at two sides of the long plate of the die for fixing a crack, sequentially penetrating each small hole with a soft cotton thread, and coating high-temperature-resistant super glue on the cotton thread to firmly fix the crack.

(4) Pouring test piece

The preparation method comprises the following steps of (1) mixing CY-39 type resin and YS-T31 modified amine curing agent according to the mass ratio of 100:34, mixing, putting into a vacuum box for 30 minutes after mixing to remove bubbles so as to improve the molding quality of the test piece.

Placing a macromolecular silica gel mold with the crack fixed on a workbench, and guiding the prepared and treated mixture into the mold by a glass rod for casting and molding; after pouring, the mould is placed in a vacuum box again for degassing treatment for 30 minutes, and then is placed in a constant-temperature air-blowing drying box at the temperature of 18 ℃ for curing for 36 hours to complete the curing process.

(5) Demolding and curing

And when the mechanical strength of the test piece is 40 MPa, the mold is removed, and the appearance of the test piece is ensured not to be damaged during mold removal. And (5) placing the demoulded test piece into a 75-DEG C constant-temperature air-blast drying oven for curing for 36 hours, and then completing the manufacture of the test piece.

2. Hydraulic fracturing test

The experimental facilities are mainly water injection equipment and unipolar compressor arrangement, and water injection equipment can keep water pressure invariable. Before the test, the test piece must be stored for 24 hours in a refrigerator at the temperature of-20 ℃ to ensure that the test piece has the brittleness required by the test, dry ice is placed around the test piece to keep the low temperature condition (within the range of-15 to-10 ℃) during the test, and an infrared thermometer is used for monitoring the temperature change. The fracturing fluid is doped with a proper amount of antifreeze to prevent icing, and simultaneously, a blue dye is also doped to facilitate the observation of the expansion process of internal cracks in the experiment.

Fracture test with internal water pressure of 2 MPa was carried out under uniaxial compression, and the test results are shown in Table 2 and FIG. 9. Water pressure is slowly applied to a predetermined level. In the process that the water pressure stably rises to a preset value, the test piece is not cracked; and after the water pressure reaches a preset value, keeping the water pressure stable through the pressure maintaining system, and then continuously loading the axial direction of the test piece. When the axial stress is increased to a certain value, the long axis of the preset crack simultaneously grows cracks at the upper end and the lower end, the growth speed is very high, and finally the crack tends to be stable. And as the axial pressure continues to increase, the crack begins to expand along the transverse direction, the vertical expansion rate is slow, and when the crack transversely expands to the surface of the test piece, the crack is not communicated in the vertical direction.

TABLE 2 crack initiation and penetration stresses at two internal water pressures

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

The test piece is highly transparent, the whole process of crack germination and development can be clearly observed, and the test piece can be observed and analyzed, so that no impurity and small cracks appear in the cracking process under the working condition of 2 MPa water pressure. The water pressure has an inhibiting effect on the generation of small cracks. In addition, the crack initiation stress of the test piece under the water pressure of 2 MPa is reduced by 86% compared with the water pressure, the peak intensity is reduced by 80%, the crack initiation stress and the peak intensity are both far smaller than the water-free working condition, the water pressure has a remarkable reduction effect on the crack initiation stress and the peak intensity, and the water pressure is more obvious when being larger.

While the invention has been described in detail with reference to the drawings and examples, it will be understood by those skilled in the art that various changes in the details of construction and materials may be made therein without departing from the spirit of the invention, and it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A fractured rock mass hydraulic fracturing simulation test piece is characterized in that the test piece is highly transparent, and the whole-course evolution law of internal cracks and damages can be clearly observed by naked eyes, and the test piece specifically comprises a similar rock mass formed by casting a high-brittleness transparent resin material and a non-water injection crack or a non-water injection crack group and/or a water injection crack group buried in the similar rock mass; the ratio of the compressive strength to the tensile strength of the simulation test piece is up to 6.6, and a crack progressive expansion process of the shale formation in the oil and gas exploitation process under the action of water pressure is presented through a real test.

2. The fractured rock mass hydraulic fracturing simulation test piece according to claim 1, wherein the non-water injection fracture is formed by combining circular or oval transparent mica sheets with corresponding shapes, the thickness of each mica sheet is 0.1 mm, the mica sheets are formed by stamping through a steel die, the edges are neat, so that stress concentration caused by burrs and size defects is avoided, and the following different schemes are adopted to simulate three types of joint fractures:

for a fully-opened joint crack, the simulation method is to stack two same mica sheets and coat lubricating oil in the middle;

for the weak joint cracks, the self has weak bonding strength, and CY-39 type epoxy resin is coated between two mica sheets;

for the filling type joint cracks, because the filling type joint cracks have low bonding strength and low tensile strength but better frictional property, two mica sheets are overlapped and bonded by using double-sided adhesive tape.

3. The fractured rock mass hydraulic fracturing simulation test piece according to claim 1, wherein the water injection fracture is a hollow body and comprises a fracture cavity with a corresponding shape and a water injection channel for communicating the cavity to the outside of the rock-like body; the preparation and forming method is any one of the following three methods:

(1) A mica sheet and a gasket are enclosed into a closed crack cavity with a corresponding shape, and a water injection pipe for communicating the crack cavity to the outside is arranged;

(2) TNSLA3908 type photosensitive resin is used as a raw material, a 3D printing technology is adopted, a thin-wall cavity body corresponding to the shape of the crack cavity and the shape of the water injection channel is formed by printing, and the wall thickness is 0.2 mm;

(3) And (3) printing and forming a model body corresponding to the shapes of the fracture cavity and the water injection channel by using high-temperature water-soluble polyvinyl alcohol as a raw material and adopting a 3D printing technology.

4. A preparation method of a fractured rock mass hydraulic fracturing simulation test piece is characterized by comprising the following specific steps:

(1) Preparing a resin casting material: taking CY-39 type resin and YS-T31 modified amine curing agent according to the proportion of 100:34, and placing the mixture in a vacuum machine for 30 minutes to remove bubbles;

(2) Preparing a fracture module, wherein at least one of the following components is included:

(1) non-water injected fracture module:

punching or cutting a rectangular mica sheet purchased from the market into a crack shape required by the test, and manufacturing the crack by adopting the method disclosed in claim 2 according to different types of simulated cracks, namely open type, weak type and filling type;

(2) water-injected fracture module:

TNSLA3908 type photosensitive resin is used as a printing material, and a thin-wall cavity body corresponding to a crack cavity and a water injection channel required by a test is formed by 3D printing; or the like, or, alternatively,

taking high-temperature water-soluble PVA as a material, and 3D printing and forming a water-soluble model body corresponding to the shapes of a crack cavity and a water injection channel required by the test;

(3) And (3) arranging a crack module: arranging and fixing a crack module or a crack module group required by the test in a test piece pouring mold;

(4) Pouring and maintaining the test piece for molding: and (3) draining and pouring the resin casting material into a test piece pouring mold after the crack module is arranged, removing bubbles, performing air blast drying at 15-20 ℃ for 40-50 h, demolding when the mechanical strength of the test piece is more than or equal to 40 MPa, and performing air blast drying at 70-80 ℃ for 30-40 h.

5. The preparation method of the fractured rock mass simulation test piece according to claim 4, wherein when the water injection fracture module is a water-soluble PVA model body, the method further comprises the following treatment steps:

after the test piece is cured and molded, the test piece is placed in hot water at 80 ℃ for half an hour, so that a water-soluble model body printed by the PVA material is completely dissolved, and a corresponding crack cavity and a water injection channel are formed.

6. The method for preparing the fractured rock mass simulation test piece according to claim 4, wherein in the step (3), the test piece pouring mold is an organic polymer silica gel mold which does not react with resin, is easy to demould and can be repeatedly used; simultaneously positioning and fixing the crack module by the following modes: and drawing and connecting the corresponding crack modules through the thin lines, and fixing the two ends of the thin lines on the two side walls of the test piece pouring mold.