A test system and method for fracturing modification of coal and rock mass and evaluation of its effect

Abstract

The present invention relates to the technical field of coal seam fracturing and permeability enhancement, and specifically relates to a test system and method for fracturing modification of coal and rock mass and its effect evaluation. It includes a fracturing modification system, an air injection evaluation system, a backpressure monitoring system, a hydraulic servo control system, a vacuum system, and an integrated data acquisition and control system. A loading chamber with a cubic cavity is arranged at the center of the true triaxial loading system. Loading push rods penetrate through the centers of the six faces of the loading chamber. The outer ends of the loading push rods are connected to the hydraulic cylinder bodies, and the inner ends of the loading push rods are connected to the loading pressure plates located inside the loading chamber, which are respectively pressed against the six side faces of the specimen. An L-shaped pipeline cavity hole is provided inside the loading push rod. Its inner hole is located at the center of the inner end of the loading push rod, and its outer hole is located on the side near the outer end of the loading push rod. And a connecting conduit is provided inside the pipeline cavity hole. The connecting conduits in three of the six loading push rods are respectively used to connect the fracturing modification system, the air injection evaluation system, the backpressure monitoring system, and the vacuum system.

Description

Technical Field

[0001] The present invention relates to the technical field of coal seam fracturing and permeability enhancement, and specifically to a test system and method for fracturing modification of coal and rock masses and evaluation of its effects. Background Art

[0002] In recent years, with the rapid development of fracturing equipment and technology, fracturing technology has been widely applied in fields such as mine exploitation, water conservancy and hydropower engineering, oil and gas reservoir exploitation, and geothermal exploitation. Fracturing technology is one of the most effective technical means for reservoir transformation, especially hydraulic fracturing, liquid nitrogen fracturing, and carbon dioxide fracturing are widely used. In mine exploitation, fracturing technology can not only be used to transform the structure and mechanical properties of coal seam surrounding rock, weaken the hard roof of the coal seam to achieve pressure relief and impact prevention, but also be used for permeability enhancement transformation of low-permeability and high-gas coal seams, improve the permeability and conductivity of the coal seam, achieve efficient exploitation of coalbed methane, and avoid coal and rock mass dynamic disasters such as coal and gas outbursts. On the one hand, coal seam fracturing can create a large number of artificial fractures in the coal seam, form a reservoir fracture network to improve the coal seam permeability, and then improve the coalbed methane extraction efficiency; on the other hand, the fracturing medium retained in the coal seam will also undergo physical and chemical interactions with the coal body, change the pore structure and surface characteristics of the coal body, and then have different degrees of influence on the mechanical properties of the coal body and its adsorption / desorption, diffusion, and seepage characteristics.

[0003] In order to study the fracturing seepage characteristics of coal seams, domestic and foreign research scholars have developed many related devices and carried out relevant experimental studies, such as: CN110426286A "A true triaxial fracturing seepage continuous test system and method", CN111366472A "True triaxial hydraulic fracturing physical simulation device and method for variable core size", CN209542309U "A large-size true triaxial hydraulic fracturing simulation test device", CN113418852A "Ultrasonic pulse fracturing gas-containing coal body seepage experiment device and method", CN104655495A "A coal and rock high-temperature and high-pressure true triaxial fracturing seepage test device and test method". However, the above-mentioned fracturing seepage test systems mainly focus on aspects such as simulating the fracture propagation characteristics and seepage characteristics under different physical fields and different medium fracturing, and cannot comprehensively and dynamically study the aspects of coal seam fracturing modification, seepage, adsorption / desorption, and mechanical properties under in-situ conditions (true triaxial stress, temperature, moisture content, gas pressure). In particular, it cannot consider the influence of the physical and chemical modification of the fracturing medium and the coal body on its anisotropic (in the X, Y, and Z directions of the specimen) flow characteristics and mechanical properties, and at the same time lacks a test method for comprehensively and dynamically evaluating the effects of different medium fracturing on coal body modification. Summary of the Invention

[0004] The object of the present invention is to overcome the deficiencies of the existing fracturing seepage test device and fracturing effect evaluation method, and provide a test system and method for coal and rock mass fracturing modification and its effect evaluation, which can be used for experimental studies on the mechanical properties, adsorption and desorption properties, and anisotropic seepage properties of coal and rock mass under different fracturing conditions, can fully consider the influence of fracturing medium on the physical and chemical modification of coal and rock mass, and comprehensively and dynamically evaluate the fracturing modification effect of coal and rock mass.

[0005] The present invention adopts the following technical solutions: A test system for coal and rock mass fracturing modification and its effect evaluation, including a fracturing modification system, an air injection evaluation system, a backpressure monitoring system, a hydraulic servo control system, a vacuum system, and an integrated data acquisition and control system. A loading chamber with a cubic cavity is arranged at the center position of the true triaxial loading system. Loading push rods penetrate through the centers of the six faces of the loading chamber. The outer ends of the loading push rods are connected to the hydraulic cylinder bodies, and the inner ends of the loading push rods are connected to loading pressing plates located inside the loading chamber. There are six loading pressing plates in total, which are respectively pressed on the six side faces of the specimen.

[0006] Thermocouple heating rods are installed on the loading pressing plates and are controlled by the integrated data acquisition and control system.

[0007] A fracturing hole is drilled at the center position of one side face of the specimen.

[0008] A push rod sealing assembly is provided at the sliding connection between the loading push rod and the loading chamber.

[0009] An L-shaped pipeline cavity hole is provided inside the loading push rod. Its inner hole is located at the center of the inner end of the loading push rod, and its outer hole is located on the side face near the outer end of the loading push rod. And a connecting conduit is provided inside the pipeline cavity hole. The connecting conduits in three of the six loading push rods are respectively used to connect the fracturing modification system, the air injection evaluation system, the backpressure monitoring system, and the vacuum system.

[0010] Among them, the fracturing modification system provides different fracturing media for the specimen.

[0011] The air injection evaluation system injects gases under different gas pressures into the specimen.

[0012] The backpressure monitoring system is used to suck out the gas inside the specimen.

[0013] The vacuum system evacuates the loading chamber.

[0014] The integrated data acquisition and control system monitors and controls the true triaxial loading system, the fracturing modification system, the air injection evaluation system, the backpressure monitoring system, the hydraulic servo control system, and the vacuum system respectively through sensors.

[0015] The hydraulic servo control system injects or outputs hydraulic oil to the true triaxial loading system according to the test requirements.

[0016] In some embodiments, a pipeline socket is provided at the center of the loading platen, and a seepage hole is also separately provided on one side of the pipeline socket of one of the loading platens; two heating rod sockets are provided on the side of the loading platen, and four acoustic emission probe slots are respectively provided at the four corners of the loading platen.

[0017] In some embodiments, a square heat shrinkable sealing sleeve and a unidirectional high permeability slider are arranged between the sides of the sample in contact with the loading plate. The unidirectional high permeability slider is divided into three parts: upper, middle and lower. The upper part is a closed entity, the middle cavity is a diversion cavity, and the lower part is a high permeability plate covered with diversion holes. The lower high permeability plate is directly pressed on the side of the sample. A fracturing pipe joint is provided on the upper part of the unidirectional high permeability slider, and a seepage pipe joint is also provided on the upper part of the unidirectional high permeability slider on the side of the sample containing the fracturing holes. The sides of the unidirectional high permeability slider are filled and sealed by sealing strips.

[0018] In some embodiments, the fracturing modification system includes a hydraulic fracturing module, a modified hydraulic fracturing module, a liquid nitrogen fracturing module and a supercritical carbon dioxide fracturing module; the hydraulic fracturing module, the modified hydraulic fracturing module, the liquid nitrogen fracturing module and the supercritical carbon dioxide fracturing module are respectively connected to the fracturing pipe inside the sample, the hydraulic fracturing module provides water for fracturing to the fracturing pipe, the modified hydraulic fracturing module provides modifying fluid for fracturing to the fracturing pipe, the liquid nitrogen fracturing module provides liquid nitrogen for fracturing to the fracturing pipe, and the supercritical carbon dioxide fracturing module provides carbon dioxide for fracturing to the fracturing pipe.

[0019] In some embodiments, the hydraulic fracturing module includes a water tank A, a high-pressure injection pump, a pressure sensor A and a flow sensor A. The input end pipeline of the high-pressure injection pump is connected to the water tank A, and the output end pipeline is sequentially connected to a valve F, a valve B, a flow sensor A, a pressure sensor A and a connecting conduit. The high-pressure water is introduced into the fracturing pipe inside the sample through the connecting conduit for hydraulic fracturing.

[0020] The modified fluid fracturing module includes a modified fluid tank A, a modified fluid tank B, a high-pressure injection pump, a pressure sensor A and a flow sensor A. The input end pipeline of the high-pressure injection pump is connected to the modified fluid tank A and the modified fluid tank B, and the output end pipeline is connected to valve F, valve B, flow sensor A, pressure sensor A and a connecting conduit in sequence. The modifying fluid is introduced into the fracturing pipe inside the sample through the connecting conduit for fracturing modification.

[0021] The liquid nitrogen fracturing module includes a liquid nitrogen storage tank, a liquid nitrogen injector, an air compressor, a gas booster pump, a pressure buffer tank, a pressure reducing valve A, a pressure sensor A, and a flow sensor A. The gas injection port of the liquid nitrogen injector is sequentially connected to a valve L, a pressure reducing valve A, a valve K, a pressure buffer tank, a valve Q, a pressure sensor B, a gas booster pump, a valve P, and an air compressor through pipelines. The liquid injection port of the liquid nitrogen injector is sequentially connected to a valve I, a valve B, a flow sensor A, a pressure sensor A, and a connecting conduit through pipelines, and its liquid injection port is also sequentially connected to a valve J and a liquid nitrogen storage tank through a branch.

[0022] The supercritical carbon dioxide fracturing module includes a carbon dioxide gas tank, a high-pressure injection pump, a high and low temperature test chamber, a water tank A, a pressure sensor A, and a flow sensor A. The air inlet of the high and low temperature test chamber is sequentially connected to a valve E, a valve M, and a carbon dioxide gas tank through pipelines. Its pressurization port is sequentially connected to a valve G, a high-pressure injection pump, and a water tank A through pipelines. Its injection pressure port is sequentially connected to a valve H, a valve B, a flow sensor A, a pressure sensor A, and a connecting conduit through pipelines. Its pressure relief port is connected to the water tank A through a pipeline.

[0023] In some embodiments, the gas injection evaluation system includes a gas source, an air compressor, a gas booster pump, a buffer tank, a pressure reducing valve B, and a metering tank. The gas booster pump is provided with a gas source inlet, an air inlet, and an injection pressure port. Its gas source inlet is sequentially connected to a valve N and a methane gas tank through pipelines, and its gas source inlet is also respectively connected to a valve O, a valve M, and a carbon dioxide gas tank through branches. The air inlet of the gas booster pump is sequentially connected to a valve P and an air compressor through pipelines. The injection pressure port of the gas booster pump is sequentially connected to a pressure sensor B, a valve R, a buffer tank, a valve S, a pressure reducing valve B, a valve T, a flow sensor B, a valve V, a valve A, and a connecting conduit through pipelines, and a valve U and a metering tank are sequentially connected between the valve T and the valve V through a branch.

[0024] In some embodiments, the backpressure monitoring system includes an automatic backpressure pump, a water tank B, a backpressure buffer tank, a pressure sensor, a backpressure valve, a gas flowmeter, a drying filter, a gas-liquid separator, and a graduated cylinder. The inlet end of the backpressure valve is sequentially connected to a valve W, a pressure sensor D, and a connecting conduit through pipelines. Its outlet end is connected to the inlet end of the gas-liquid separator through a pipeline, and the backpressure valve is also sequentially connected to a pressure sensor C, a backpressure buffer tank, an automatic backpressure pump, and a water tank B through pipelines. The exhaust port of the gas-liquid separator is sequentially connected to a drying filter, a gas flowmeter, and an airbag through pipelines. The liquid discharge port of the gas-liquid separator is connected to the graduated cylinder through a pipeline.

[0025] In some embodiments, the vacuum system includes a vacuum pump and a pressure sensor. The vacuum pump is sequentially connected to a valve X, a pressure sensor E, a valve Y, a valve A, and a connecting conduit through pipelines.

[0026] In some embodiments, the sensors include pressure sensors A - E, flow sensors A, flow sensors B3 - 18, and strain sensors. Among them, pressure sensor A is used to monitor the injection pressure of the fracturing medium, pressure sensor B is used to monitor the output pressure of the injection port of the gas booster pump, pressure sensor C is used to monitor the output pressure of the automatic back - pressure pump, pressure sensor D is used to monitor the pressure at the gas - liquid discharge port of the true triaxial loading system, pressure sensor E is used to monitor the negative pressure of the vacuum system, flow sensor A is used to monitor the medium injection flow rate of the fracturing modification system, flow sensor B is used to monitor the medium injection flow rate of the gas injection evaluation system, and the strain sensor is used to monitor the strain of the specimen under different loading conditions.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The present invention can conduct a full - process study on aspects such as coal seam fracturing modification, seepage characteristics, adsorption - desorption characteristics, and mechanical characteristics under in - situ conditions (true triaxial stress, temperature, moisture content, gas pressure), and achieve a comprehensive evaluation of the effects of fracturing - modified coal bodies with different media.

[0029] 2. The present invention realizes the dynamic testing of the anisotropic (in the X, Y, and Z directions) permeability of the specimen before and after fracturing modification by setting unidirectional high - permeability sliders outside the six sides of the cubic specimen.

[0030] 3. The present invention can fully consider the influence of the fracturing medium on the physical and chemical modification of coal and rock masses. By injecting high - pressure fracturing medium into the coal and rock masses for fracturing, macroscopic diversion fractures are formed. At the same time, the modification effects of the fracturing medium on the microscopic pore structure, surface characteristics, and mineral composition of the coal and rock masses are considered. By measuring parameters such as the mechanical parameters, adsorption - desorption amount, and anisotropic permeability of the coal and rock masses before and after fracturing modification, the effects of different fracturing media on fracturing - modified coal and rock masses are quantitatively evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic structural diagram of the coal and rock mass fracturing modification and its effect evaluation test system of the present invention;

[0032] Figure 2 is a schematic structural diagram of the true triaxial loading system of the present invention;

[0033] Figure 3 is a schematic structural diagram of the loading platen of the present invention;

[0034] Figure 4 is a schematic structural diagram of the unidirectional high - permeability slider of the present invention;

[0035] Figure 5 is a cross - sectional view of the unidirectional high - permeability slider of the present invention.

[0036] In the figure:

[0037] 1—True triaxial loading system; 1-1—Hydraulic cylinder body; 1-2—Displacement sensor; 1-3—Loading chamber; 1-4—Valve A; 1-5—Pressure sensor A; 1-6—Flow sensor A; 1-7—Valve B; 1-8—Loading push rod; 1-9—Pipeline cavity hole; 1-10—Connecting conduit; 1-11—Push rod seal assembly; 1-12—Loading pressing plate; 1-13—Seepage hole; 1-14—Pipeline channel; 1-15—Line channel; 1-16—Square heat shrinkable sealing rubber sleeve; 1-17—Sealing rubber strip; 1-18—Rubber sleeve interface; 1-19—Unidirectional high-permeability slider; 1-20—Specimen; 1-21—Fracturing pipe; 1-22—Pipeline socket; 1-23—Acoustic emission probe slot; 1-24—Heating rod socket; 1-25—Fracturing pipe joint; 1-26—Seepage pipe joint; 1-27—Diversion chamber; 1-28—Diversion hole;

[0038] 2—Fracturing modification system; 2-1—Modifying liquid tank A; 2-2—Valve C; 2-3—Modifying liquid tank B; 2-4—Valve D; 2-5—High-pressure injection pump; 2-6—Water tank A; 2-7—High and low temperature test chamber; 2-8—Valve E; 2-9—Valve F; 2-10—Valve G; 2-11—Valve H; 2-12—Liquid nitrogen storage tank; 2-13—Valve I; 2-14—Valve J; 2-15—Liquid nitrogen injector; 2-16—Pressure buffer tank; 2-17—Valve K; 2-18—Pressure reducing valve A; 2-19—Valve L;

[0039] 3—Gas injection evaluation system; 3-1—Carbon dioxide gas tank; 3-2—Methane gas tank; 3-3—Valve M; 3-4—Valve N; 3-5—Valve O; 3-6—Air compressor; 3-7—Valve P; 3-8—Gas booster pump; 3-9—Pressure sensor B; 3-10—Valve Q; 3-11—Valve R; 3-12—Buffer tank; 3-13—Valve S; 3-14—Pressure reducing valve B; 3-15—Valve T; 3-16—Metering tank; 3-17—Valve U; 3-18—Flow sensor B; 3-19—Valve V;

[0040] 4—Back pressure monitoring system; 4-1—Water tank B; 4-2—Automatic back pressure pump; 4-3—Back pressure buffer tank; 4-4—Pressure sensor C; 4-5—Back pressure valve; 4-6—Valve W; 4-7—Pressure sensor D; 4-8—Gas flowmeter; 4-9—Dry filter; 4-10—Gas-liquid separator; 4-11—Graduated cylinder; 4-12—Gas collecting bag;

[0041] 5—Hydraulic servo control system;

[0042] 6—Vacuum system; 6-1—Vacuum pump; 6-2—Valve X; 6-3—Pressure sensor E; 6-4—Valve Y;

[0043] 7—Data acquisition and control system integration. Specific implementation mode

[0044] The following combines with the attached Figures 1-5 to further elaborate in detail the specific implementation mode of the present invention.

[0045] As Figures 1-5 shown, a coal and rock mass fracturing modification and its effect evaluation test system includes a true triaxial loading system 1, a fracturing modification system 2, an air injection evaluation system 3, a back pressure monitoring system 4, a hydraulic servo control system 5, a vacuum system 6, and a data acquisition and control system integration 7; the true triaxial loading system 1 is mainly composed of a hydraulic cylinder body 1-1, a loading chamber 1-2, a loading push rod 1-8, a loading pressing plate 1-12, a unidirectional high-permeability slider 1-19 and other auxiliary components; there are three pairs of the hydraulic cylinder bodies 1-1, one pair is respectively arranged in the X, Y, and Z directions for applying the loading and unloading of the principal stresses in three directions to the specimen 1-20, and each hydraulic cylinder body 1-1 is connected to the hydraulic servo control system 5 through an oil pipe. The hydraulic servo control system 5 can inject or output hydraulic oil into the six hydraulic cylinder bodies 1-1 respectively according to the test requirements, and control the pressure and flow rate of the hydraulic oil, and push the loading push rod 1-8 through the hydraulic cylinder body 1-1 to realize the true triaxial loading and unloading of the specimen 1-20.

[0046] The loading chamber 1-3 is a cubic cavity located at the center of the true triaxial loading system 1. Pipeline channels 1-14 and cable channels 1-15 are respectively provided on its sides. The external injection or pressure relief pipelines penetrate into the interior of the loading chamber through the pipeline channels 1-14. The data lines of the acoustic emission monitoring system, the temperature control system, the strain monitoring data lines and other cables penetrate into the interior of the loading chamber 1-3 through the cable channels 1-15 and are connected to the respective sensors. The loading push rods 1-8 vertically penetrate through the centers of the six faces of the loading chamber 1-3 respectively. Their outer ends are connected to the hydraulic cylinder heads outside the loading chamber 1-3, and their inner ends are connected to the loading pressing plates 1-12 located inside the loading chamber 1-3. And a push rod sealing assembly 1-11 is provided at the sliding connection between the loading push rod 1-8 and the loading chamber 1-3; an L-shaped pipeline cavity hole 1-9 is provided inside the loading push rod 1-8. Its inner hole is located at the center of the inner end of the loading push rod 1-8, and its outer hole is located on the side near the outer end of the loading push rod 1-8. And a connecting conduit 1-10 is provided in the pipeline cavity hole 1-9 for connecting the fracturing pipeline or the seepage pipeline. There are six loading pressing plates 1-12 in total, which are respectively pressed on the six sides of the specimen 1-20. Pipeline sockets 1-22 are provided at the centers of the loading pressing plates 1-12. And a seepage hole 1-13 is separately provided on one side of the pipeline socket 1-22 of one of the loading pressing plates 1-12. This loading pressing plate 1-12 is pressed on the side of the specimen 1-20 with a fracturing hole; two heating rod sockets 1-24 are provided on the side of the loading pressing plate 1-12, and four acoustic emission probe slots 1-23 are respectively provided at the four corners of the loading pressing plate 1-12. There are six unidirectional high-permeability sliders 1-19, which are located between the specimen 1-20 and the square heat-shrinkable sealing rubber sleeve 1-16. The unidirectional high-permeability slider 1-19 is divided into upper, middle and lower parts. The upper part is a sealed entity, the middle cavity is a diversion cavity 1-27, and the lower part is a high-permeability plate covered with diversion holes 1-28. The lower high-permeability porous plate is directly pressed on the side of the specimen 1-20. A fracturing pipe joint 1-25 is provided on the upper part of the unidirectional high-permeability slider 1-19, and a seepage pipe joint 1-26 is also provided on the upper part of the unidirectional high-permeability slider 1-19 on the side of the specimen 1-20 with a fracturing hole. The sides of the unidirectional high-permeability slider 1-19 are filled and sealed with sealing rubber strips 1-17.

[0047] The vacuum system 6 mainly includes a vacuum pump 6-1, a pressure sensor 6-2, and valves. The vacuum pump 6-1 is sequentially connected to valve X6-2, pressure sensor E6-3, valve Y6-4, and valve A1-4 through pipelines. The vacuum system 6 is used to evacuate and degas the specimen 1-20 during the test.

[0048] The fracturing modification system 2 includes a hydraulic fracturing module, a modified hydraulic fracturing module, a liquid nitrogen fracturing module, and a supercritical carbon dioxide fracturing module; the hydraulic fracturing module mainly consists of a water tank A2-6, a high-pressure injection pump 2-5, valves, a pressure sensor A1-5, and a flow sensor A1-6. The input pipeline of the high-pressure injection pump 2-5 is connected to the water tank A2-6, and the output pipeline is sequentially connected to a valve F2-9, a valve B1-7, a flow sensor A1-6, a pressure sensor A1-5, and a connecting conduit 1-10. High-pressure water is introduced into the fracturing pipe 1-21 inside the specimen 1-20 through the connecting conduit 1-10 for hydraulic fracturing; the modified hydraulic fracturing module mainly consists of a modified liquid tank A2-1, a modified liquid tank B2-3, a high-pressure injection pump 2-5, valves, a pressure sensor A1-5, and a flow sensor A1-6. The input pipeline of the high-pressure injection pump 2-5 is connected to the modified liquid tank A2-1 and the modified liquid tank B2-3, and the output pipeline is sequentially connected to a valve F2-9, a valve B1-7, a flow sensor A1-6, a pressure sensor A1-5, and a connecting conduit 1-10. The modified liquid is introduced into the fracturing pipe 1-21 inside the specimen 1-20 through the connecting conduit 1-10 for fracturing modification.

[0049] The liquid nitrogen fracturing module mainly consists of a liquid nitrogen storage tank 2-12, a liquid nitrogen injector 2-15, an air compressor 3-6, a gas booster pump 3-8, a pressure buffer tank 2-16, a pressure reducing valve A2-18, valves, a pressure sensor A1-5, and a flow sensor A1-6. The gas injection port of the liquid nitrogen injector 2-15 is sequentially connected to a valve L2-19, a pressure reducing valve A2-18, a valve K2-17, a pressure buffer tank 2-16, a valve Q3-10, a pressure sensor B3-9, a gas booster pump 3-8, a valve P3-7, and an air compressor 3-6 through pipelines. The liquid injection port of the liquid nitrogen injector 2-15 is sequentially connected to a valve I2-13, a valve B1-7, a flow sensor A1-6, a pressure sensor A1-5, and a connecting conduit 1-10 through pipelines, and its liquid injection port is also sequentially connected to a valve J2-14 and a liquid nitrogen storage tank 2-12 through a branch.

[0050] The supercritical carbon dioxide fracturing module mainly consists of a carbon dioxide gas tank 3-1, a high-pressure injection pump 2-5, a high and low temperature test chamber 2-7, a water tank A2-6, valves, a pressure sensor A1-5, and a flow sensor A1-6. The air inlet of the high and low temperature test chamber 2-7 is sequentially connected to a valve E2-8, a valve M3-3, and a carbon dioxide gas tank 3-1 through pipelines. Its pressurization port is sequentially connected to a valve G2-10, a high-pressure injection pump 2-5, and a water tank A2-6 through pipelines. Its pressure injection port is sequentially connected to a valve H2-11, a valve B1-7, a flow sensor A1-6, a pressure sensor A1-5, and a connecting conduit 1-10 through pipelines. Its pressure relief port is connected to the water tank A2-6 through a pipeline;

[0051] The gas injection evaluation system 3 mainly consists of a gas source, an air compressor 3-6, a gas booster pump 3-8, a buffer tank 3-12, a pressure reducing valve B 3-14, a metering tank 3-16, valves, a pressure sensor A 1-5, and a flow sensor A 1-6; the gas booster pump 3-8 is respectively provided with a gas source inlet, an air inlet, and a pressure injection port. Its gas source inlet is connected to a methane gas tank 3-2 through pipelines in sequence via a valve N 3-4, and the gas source inlet is also respectively connected to a valve O 3-5, a valve M 3-3, and a carbon dioxide gas tank 3-1 through branch pipelines; the air inlet of the gas booster pump 3-8 is connected to the air compressor 3-6 through pipelines in sequence via a valve P 3-7; the pressure injection port of the gas booster pump 3-8 is connected to a pressure sensor B 3-9, a valve R 3-11, the buffer tank 3-12, a valve S 3-13, the pressure reducing valve B 3-14, a valve T 3-15, a flow sensor B 3-18, a valve V 3-19, a valve A 1-4, and a connecting conduit 1-10 through pipelines in sequence, and a valve U 3-17 and the metering tank 3-16 are connected in sequence through a branch pipeline between the valve T 3-15 and the valve V 3-19.

[0052] The back pressure monitoring system 4 mainly consists of an automatic back pressure pump 4-2, a water tank B 4-1, a back pressure buffer tank 4-3, a pressure sensor 4-4, a back pressure valve 4-5, valves, a gas flowmeter 4-8, a drying filter 4-9, a gas-liquid separator 4-10, and a measuring cylinder 4-11; the inlet end of the back pressure valve 4-5 is connected to the connecting conduit 1-10 through pipelines in sequence via a valve W 4-6 and a pressure sensor D 4-7, its outlet end is connected to the inlet end of the gas-liquid separator 4-10 through a pipeline, and the back pressure valve 4-5 is also connected to the pressure sensor C 4-4, the back pressure buffer tank 4-3, the automatic back pressure pump 4-2, and the water tank B 4-1 through pipelines in sequence; the exhaust port of the gas-liquid separator 4-10 is connected to the drying filter 4-9, the gas flowmeter 4-8, and a gas collecting bag 4-12 through pipelines in sequence, and the liquid discharge port of the gas-liquid separator 4-10 is connected to the measuring cylinder 4-11 through a pipeline.

[0053] The data acquisition and control system integration 7 mainly includes a temperature control subsystem, an acoustic emission monitoring subsystem, a stress and strain monitoring subsystem, an automatic control subsystem, data acquisition and processing software, and sensors; the data acquisition and control system integration 7 monitors and controls the true triaxial loading system 1, the fracturing modification system 2, the gas injection evaluation system 3, the back pressure monitoring system 4, the hydraulic servo control system 5, and the vacuum system 6 through sensors and the automatic control subsystem respectively; the sensors mainly include pressure sensors A - E, flow sensors A1 - 6, flow sensors B3 - 18, and strain sensors. Among them, pressure sensors A1 - 5 are used to monitor the injection pressure of the fracturing medium, pressure sensors B3 - 9 are used to monitor the output pressure of the injection port of the gas booster pump, pressure sensors C4 - 4 are used to monitor the output pressure of the automatic back pressure pump, pressure sensors D4 - 7 are used to monitor the pressure at the gas - liquid discharge port of the true triaxial loading system, pressure sensors E6 - 3 are used to monitor the negative pressure of the vacuum system, flow sensors A1 - 6 are used to monitor the medium injection flow of the fracturing modification system, flow sensors B3 - 18 are used to monitor the medium injection flow of the gas injection evaluation system, and strain sensors are used to monitor the strain of the specimen under different loading conditions.

[0054] A test method for fracturing modification of coal and rock mass and its effect evaluation includes the following main steps:

[0055] (a) Specimen production and model assembly

[0056] Cut the large coal and rock samples retrieved from the on - site coal mining face into cubic specimens 1 - 20 with dimensions of 300×300×300 mm. Drill a vertical hole with a diameter of Φ10×150 mm as the fracturing hole at the center position of one side of the specimen. After the specimen is dried, insert the end with sieve holes of the fracturing pipe 1 - 21 into the fracturing hole, and use sealant to seal the fracturing hole so that the outer wall of the fracturing pipe in the sealing section is firmly sealed with the inner wall of the drilling hole; fix the bottom surface with a diversion hole of the unidirectional high - permeability slider 1 - 19 on the six sides of the specimen respectively, and make the lower end of the fracturing pipe joint 1 - 25 be hermetically connected with the fracturing pipe 1 - 21. The sides between adjacent high - permeability sliders are filled and sealed with sealant strips 1 - 17 respectively. Then put the whole into a square heat - shrinkable sealant sleeve 1 - 16 for sealing, and make the sleeve interfaces 1 - 18 be centered with the fracturing pipe joints 1 - 25 and the seepage pipe joints 1 - 26 on the unidirectional high - permeability slider 1 - 19 respectively; install the thermocouple heating rod and the acoustic emission probe into the heating rod jack 1 - 24 and the acoustic emission probe slot 1 - 23 set on the loading platen respectively. Then press the loading platen 1 - 12 on the six faces of the square heat - shrinkable sealant sleeve 1 - 16 respectively, and make the pipeline jack 1 - 22 and the seepage hole 1 - 13 on the loading platen 1 - 12 be centered with the sleeve interface 1 - 18 respectively to complete the model assembly.

[0057] (b) System connection

[0058] Connect each system component through pipelines or wires according to the set connection method. Place the assembled specimen into the loading chamber 1-3. Connect the communication conduits 1-10 in the six loading push rods 1-8 through the rubber sleeve interfaces 1-18 to the respective fracturing pipe joints 1-25. Connect the seepage pipe through the rubber sleeve interface 1-18 to the seepage pipe joint 1-26, and seal each rubber sleeve interface 1-18. Connect the fracturing modification system pipeline to the communication conduit 1-10 connected to the fracturing pipe 1-21. Connect the pipelines of the gas injection evaluation system 3 and the backpressure monitoring system 4 to the communication conduits 1-10 on the opposite sides of the specimen according to the different measured seepage directions. Connect the pipeline of the vacuum system 6 and the pipeline of the gas injection evaluation system 3 in parallel through a tee and then connect them to the communication conduit 1-10. Connect the hydraulic cylinders 1-1 of the true triaxial loading system 1 to the hydraulic servo control system 5 through oil pipes. Connect the data lines of each sensor and system control component to the data acquisition and control system integration 7.

[0059] (c)Initial parameter determination

[0060] Use the gas injection evaluation system 3 and the backpressure monitoring system 4 to conduct the specimen permeability measurement test. Under the initial in-situ conditions of the specimen 1-20, measure the initial permeability of the specimen 1-20 in the X, Y, and Z directions (set the direction parallel to the fracturing holes as the X direction) respectively. During the permeability measurement test, connect the pipeline of the gas injection evaluation system 3 through the pipeline channel 1-14 to the seepage pipe joint 1-26 of the true triaxial loading system, and connect the pipeline of the backpressure monitoring system 4 to the communication conduit 1-10 in the X direction. First, open the valves M3-3, P3-7, and R3-11. Use the gas booster pump 3-8 to boost the methane in the methane gas tank 3-2 and fill the buffer tank 3-12, and monitor the output pressure of the gas booster pump 3-8 through the pressure sensor B3-9. Open the valves S3-13, T3-15, V3-19, and A1-4, and use the pressure reducing valve B3-14 to control the inlet pressure of the specimen 1-20 P X1 and monitor the injection flow rate of the gas through the flow sensor B3-18 Q X At the same time, use the automatic backpressure pump 4-2 to inject the pressure required for the test into the backpressure buffer tank 4-3, and monitor the backpressure in real time through the pressure sensor C4-4 to keep the backpressure stable. Then open the valve W4-6 and monitor the pressure at the outlet end of the specimen 1-20 through the pressure sensor D4-7 P X2 ; According to the pressure and flow rate data collected on the data acquisition and control system integration 7, the permeability of the specimen in the X direction can be obtained according to the gas permeability calculation formula ; In the formula, K Xis the permeability in the X direction (10 -15 m 2 ); Q X is the gas flow rate (cm 3 / s); P 0 is the atmospheric pressure (taking 0.101 MPa); μ is the gas viscosity coefficient (Pa·s); L is the height of the specimen (cm); A is the cross-sectional area of the cubic specimen (cm 2 ); P X1 and P X2 are the pressure values at the inlet and outlet ends of the specimen (MPa), respectively.

[0061] Similarly, when measuring the permeability in the Y and Z directions, only need to connect the pipelines of the gas injection evaluation system 3 and the back pressure monitoring system 4 to the communication conduits 1-10 in the Y and Z directions respectively, and measure the pressure values at the inlet and outlet ends of the specimen 1-20 in the Y and Z directions P Y1 , P Y2 and P Z1 , P Z2 , as well as the gas flow rates in the Y and Z directions Q Y , Q Z , and then obtain the permeabilities of the specimen 1-20 in the Y and Z directions , .

[0062] The adsorption and desorption tests of Specimens 1 - 20 are carried out using the vacuum system 6, the gas injection evaluation system 3, the back pressure monitoring system 4 and the stress - strain monitoring system, and the methane adsorption amount, desorption amount, adsorption deformation amount and desorption deformation amount of Specimens 1 - 20 under different gas pressures are measured. When conducting the methane gas adsorption test, first open valves A1 - 4, valve X6 - 2, valve Y6 - 4, and use the vacuum pump 6 - 1 to evacuate Specimens 1 - 20 for 12 hours. Then close the vacuum system 6, open valves M3 - 3, valve P3 - 7, valve R3 - 11, use the gas booster pump 3 - 8 to boost the methane in the methane gas cylinder 3 - 2 and fill the buffer tank 3 - 12, and monitor the output pressure of the gas booster pump 3 - 8 through the pressure sensor B3 - 9. Open valves S3 - 13, valve T3 - 15, valve U3 - 17, use the pressure reducing valve B3 - 14 to inject the pressure required by the test into the metering tank 3 - 16. Close valve T3 - 15, open valves V3 - 19, A1 - 4, and introduce methane gas into Specimens 1 - 20, and monitor the pressure change in the metering tank 3 - 16 in real time until the pressure is constant. Calculate the methane adsorption amount of Specimens 1 - 20 at this pressure according to the pressure change in the metering tank 3 - 16, and at the same time monitor the strain of Specimens 1 - 20 through the strain sensor. Thus, the adsorption process at this gas pressure ends. When conducting the methane gas desorption test of Specimens 1 - 20, first close valve A1 - 4, set the back pressure of the back pressure monitoring system 4 to the standard atmospheric pressure, open valve W4 - 6, and the methane gas desorbed from the coal body of Specimens 1 - 20 enters the gas collecting bag 4 - 12 through the gas - liquid separator 4 - 10, the drying filter 4 - 9, and the gas flowmeter 4 - 8, and the instantaneous gas flow rate and total flow rate are monitored in real time through the gas flowmeter 4 - 8 to obtain the instantaneous methane desorption rate and the cumulative desorption amount, and at the same time monitor the strain of Specimens 1 - 20 through the strain sensor; according to the above steps, continue to fill the metering tank 3 - 16 with methane at different pressures, and carry out the adsorption and desorption tests of Specimens 1 - 20 under different gas pressures to obtain the methane adsorption amount, desorption amount, adsorption deformation amount and desorption deformation amount of Specimens 1 - 20 under different gas pressures.

[0063] Use the true triaxial loading system 1 to conduct uniaxial compression tests on Specimens 1 - 20 in the X, Y, and Z directions respectively until the elastic limit of Specimens 1 - 20, and obtain the dynamic stress - strain values of Specimens 1 - 20 during the entire loading process through the stress - strain monitoring system, and calculate the initial elastic modulus and Poisson's ratio of Specimens 1 - 20 in the X, Y, and Z directions.

[0064] (d)In - situ condition simulation

[0065] First, evacuate the specimen through the vacuum system 6 for 12 hours. According to the actual in-situ conditions of the coal seam, set the corresponding triaxial loading parameters, temperature, moisture content, gas pressure and other conditions. Apply the principal stress to the three pairs of specimens 1-20 respectively through the hydraulic servo control system 5 to simulate the in-situ stress state of the coal seam. Use the temperature control subsystem to control the electrothermal couple heating rod in the loading platen 1-12 to heat the specimens 1-20 to the required test temperature to simulate the in-situ temperature state of the coal seam. Use the hydraulic fracturing module of the fracturing modification system to inject a certain amount of water into the coal and rock specimens 1-20 to simulate the in-situ water content of the coal seam. Use the gas injection evaluation system 3 to fill methane with a certain pressure into the specimens 1-20 to simulate the in-situ gas pressure conditions of the coal seam.

[0066] (e)Fracturing modification

[0067] Close the gas injection evaluation system 3 and the back pressure monitoring system 4, and open the fracturing modification system 2. Use the hydraulic fracturing module, modified hydraulic fracturing module, liquid nitrogen fracturing module and supercritical carbon dioxide fracturing module in the fracturing modification system 2 to conduct fracturing tests with different fracturing media on the specimens 1-20. During the test, first inject the fracturing medium into the specimens 1-20 at high pressure to fracture the coal and rock mass. At the same time, monitor the acoustic emission changes of the specimens 1-20 during the fracturing process through the acoustic emission monitoring system, and use the three-dimensional acoustic emission positioning technology to determine the crack initiation, propagation and evolution parameters inside the specimens 1-20. Then maintain a certain fracturing medium injection pressure to conduct pressure maintenance modification on the coal and rock mass, so that different fracturing media can fully undergo physical and chemical interactions with the coal and rock mass.

[0068] The fracturing modification test mainly includes hydraulic fracturing test, modified hydraulic fracturing test, liquid nitrogen fracturing test and supercritical carbon dioxide fracturing test. First, keep all the valves in the fracturing modification system 2 in the closed state. When conducting the hydraulic fracturing test, first open the valve B1-7 and the valve F2-9, and use the high-pressure injection pump to inject the water in the water tank A2-6 into the coal and rock specimens 1-20 for fracturing and permeability enhancement, and monitor the injection pressure and flow rate in real time through the pressure sensor A1-5 and the flow sensor A1-6 installed on the fracturing pipeline.

[0069] When conducting the modified hydraulic fracturing test, open valve C2-2, valve B1-7, and valve F2-9. First, use the high-pressure injection pump 2-5 to inject the modified fluid A in the modified fluid tank A2-1 into the coal-rock sample 1-20 for fracture enhancement and permeability improvement, and use the pressure sensor A1-5 and flow sensor A1-6 installed on the fracturing pipeline to monitor the injection pressure and flow rate of the modified fluid in real time. Then, maintain a certain injection pressure for a period of time to perform pressure maintenance and modification on the coal-rock mass, so that the modified fluid fully undergoes physical and chemical reactions with the coal-rock mass. In addition, a synergistic permeability improvement test with two modified fluids can also be carried out. The specific method is that after the modification of the modified fluid A is completed, first use the backpressure measurement system 4 to discharge the modified fluid A, close valve C2-2 after the discharge is completed, open valve D2-4, and then use the high-pressure injection pump 2-5 to inject the modified fluid B in the modified fluid tank B2-3 into the coal-rock sample 1-20 for secondary pressure maintenance and modification to achieve the synergistic modification of the two modified fluids.

[0070] When conducting the liquid nitrogen fracturing test, first open valve J2-14, inject the liquid nitrogen in the liquid nitrogen storage tank 2-12 into the liquid nitrogen injector 2-15. After the liquid nitrogen injector 2-15 is filled with liquid nitrogen, close valve J2-14, open valve P3-7 and valve Q3-10, inject high-pressure air into the pressure buffer tank 2-16, open valve L2-19, valve I2-13, and valve B17, use the pressure reducing valve A2-18 to control the output pressure of the pressure buffer tank 2-16, and inject the liquid nitrogen in the liquid nitrogen injector 2-15 into the sample 1-20 at the pressure required for the test for liquid nitrogen fracture enhancement and permeability improvement, and use the pressure sensor A1-5 and flow sensor A1-6 installed on the fracturing pipeline to monitor the injection pressure and flow rate of the liquid nitrogen in real time. Then, maintain a certain injection pressure for a period of time to perform pressure maintenance and modification on the coal-rock mass, so that the liquid nitrogen fully undergoes physical and chemical reactions with the coal-rock mass.

[0071] When conducting the supercritical carbon dioxide fracturing test, first open valve E2-8 and valve M3-3, inject the carbon dioxide gas in the carbon dioxide gas tank 3-1 into the injector in the high and low temperature test chamber 2-7. After it is filled, close valve E2-8 and valve M3-3, open valve G2-10, use the high-pressure injection pump 2-5 to increase the pressure of the injector in the high and low temperature test chamber 2-7 to the critical pressure of 7.4 MPa, and adjust the temperature of the high and low temperature test chamber 2-7 to the critical temperature of 31.3 °C to make the carbon dioxide in the injector in a supercritical state. Then, open valve H2-11 and valve B1-7, use the high-pressure injection pump 2-5 to inject the supercritical carbon dioxide into the sample 1-20 for fracture enhancement and permeability improvement, and use the pressure sensor A1-5 and flow sensor A1-6 installed on the fracturing pipeline to monitor the injection pressure and flow rate of the supercritical carbon dioxide in real time. Then, maintain a certain injection pressure for a period of time to perform pressure maintenance and modification on the coal-rock mass, so that the supercritical carbon dioxide fully undergoes physical and chemical reactions with the coal-rock mass.

[0072] (f)Effect evaluation

[0073] After the fracturing modification test is completed, close the fracturing modification system 2, open the gas injection evaluation system 3 and the back pressure monitoring system 4, and use the back pressure monitoring system 4 to flow back the fracturing medium in the specimens 1-20. After the flow back is completed, first measure the adsorption and desorption amounts of the specimens 1-20 after fracturing under different gas pressure conditions and their permeabilities in the X, Y, and Z directions again according to the method described in step (c), and then measure the elastic moduli and Poisson's ratios of the specimens 1-20 after fracturing in the X, Y, and Z directions respectively according to the method described in step (c); according to the changes in the adsorption and desorption amounts, permeabilities in different directions, elastic moduli, and Poisson's ratios of the specimens 1-20 before and after fracturing modification, quantitatively evaluate the effects of different fracturing media on fracturing and modifying coal and rock masses.

[0074] It is obvious that the above description and record are only examples and not intended to limit the disclosure, application, or use of the present invention. Although the embodiments have been described in the examples and illustrated in the drawings, the present invention does not limit the specific examples described in the drawings and the examples as the currently considered best mode for implementing the teachings of the present invention. The scope of the present invention will include any embodiment falling within the foregoing specification and the appended claims.

Claims

1. A test system for fracturing modification of coal and rock mass and its effect evaluation, characterized in that: It includes a fracturing modification system (2), a gas injection evaluation system (3), a backpressure monitoring system (4), a hydraulic servo control system (5), a vacuum system (6), and a data acquisition and control system integration (7). A loading chamber (1-3) with a cubic cavity is provided at the central position of the true triaxial loading system (1). Loading push rods (1-8) penetrate through the centers of the six faces of the loading chamber (1-3). The outer ends of the loading push rods (1-8) are connected to hydraulic cylinder bodies (1-1), and the inner ends of the loading push rods (1-8) are connected to loading pressure plates (1-12) located inside the loading chamber (1-3). There are six loading pressure plates (1-12) in total, which are respectively pressed against the six side faces of the specimen (1-20). Thermocouple heating rods are installed on the loading pressure plates (1-12) and are controlled by the data acquisition and control system integration (7). A fracturing hole is drilled at the central position of one side face of the specimen (1-20). A push rod sealing assembly (1-11) is provided at the sliding connection between the loading push rod (1-8) and the loading chamber (1-3). An L-shaped pipeline cavity hole (1-9) is provided inside the loading push rod (1-8). Its inner hole is located at the center of the inner end of the loading push rod (1-8), and its outer hole is located on the side face near the outer end of the loading push rod (1-8). And a connecting conduit (1-10) is provided inside the pipeline cavity hole (1-9). The connecting conduits (1-10) inside three of the six loading push rods (1-8) are respectively used to connect the fracturing modification system (2), the gas injection evaluation system (3), the backpressure monitoring system (4), and the vacuum system (6). The fracturing modification system (2) provides different fracturing media for the specimen (1-20). The gas injection evaluation system (3) injects gases under different gas pressures into the specimen (1-20). The backpressure monitoring system (4) is used to suck out the gas inside the specimen (1-20). The vacuum system (6) evacuates the loading chamber (1-3). The data acquisition and control system integration (7) monitors and controls the true triaxial loading system (1), the fracturing modification system (2), the gas injection evaluation system (3), the backpressure monitoring system (4), the hydraulic servo control system (5), and the vacuum system (6) respectively through sensors. The hydraulic servo control system (5) injects or outputs hydraulic oil to the true triaxial loading system (1) according to the test requirements. Pipeline jacking holes (1-22) are provided at the centers of the loading pressure plates (1-12), and a seepage hole (1-13) is separately provided on one side of the pipeline jacking hole (1-22) of one of the loading pressure plates (1-12). Two heating rod jacking holes (1-24) are provided on the side faces of the loading pressure plates (1-12), and four acoustic emission probe slots (1-23) are provided at the four corners of the loading pressure plates (1-12). A square heat-shrinkable sealant sleeve (1-16) and a unidirectional high-permeability slider (1-19) are provided between the side surfaces of the specimen (1-20) in contact with the loading pressing plate (1-12). The unidirectional high-permeability slider (1-19) is divided into upper, middle, and lower parts. The upper part is a sealed entity, the middle cavity is a diversion cavity (1-27), and the lower part is a high-permeability plate covered with diversion holes (1-28). The high-permeability plate in the lower part is directly pressed on the side surface of the specimen (1-20). A fracturing pipe joint (1-25) is provided on the upper part of the unidirectional high-permeability slider (1-19), and a seepage pipe joint (1-26) is also provided on the upper part of the unidirectional high-permeability slider (1-19) on the side of the specimen (1-20) with a fracturing hole. The side surfaces of the unidirectional high-permeability slider (1-19) are filled and sealed by sealing rubber strips (1-17). On the side surfaces of the loading chamber (1-3), a pipeline channel (1-14) and a circuit channel (1-15) are respectively provided. An external injection pressure pipeline or a pressure relief pipeline penetrates into the interior of the loading chamber through the pipeline channel (1-14). The data lines of the acoustic emission monitoring system, the temperature control system, and the strain monitoring data line penetrate into the interior of the loading chamber (1-3) through the circuit channel (1-15) and are connected to the respective sensors.

2. The coal and rock mass fracturing modification and its effect evaluation test system according to claim 1, characterized in that: The fracturing modification system (2) includes a hydraulic fracturing module, a modified hydraulic fracturing module, a liquid nitrogen fracturing module, and a supercritical carbon dioxide fracturing module. The hydraulic fracturing module, the modified hydraulic fracturing module, the liquid nitrogen fracturing module, and the supercritical carbon dioxide fracturing module are respectively connected to the fracturing pipe (1-21) inside the specimen (1-20). The hydraulic fracturing module provides water for fracturing to the fracturing pipe (1-21), the modified hydraulic fracturing module provides modified liquid for fracturing to the fracturing pipe (1-21), the liquid nitrogen fracturing module provides liquid nitrogen for fracturing to the fracturing pipe (1-21), and the supercritical carbon dioxide fracturing module provides carbon dioxide for fracturing to the fracturing pipe (1-21).

3. The fracturing modification and effect evaluation test system for coal and rock masses according to claim 2, characterized in that: The hydraulic fracturing module includes a water tank A (2-6), a high-pressure injection pump (2-5), a pressure sensor A (1-5), and a flow sensor A (1-6). The pipeline at the input end of the high-pressure injection pump (2-5) is connected to the water tank A (2-6), and the pipeline at the output end is sequentially connected to a valve F (2-9), a valve B (1-7), a flow sensor A (1-6), a pressure sensor A (1-5), and a connecting conduit (1-10). High-pressure water is introduced into the fracturing pipe (1-21) inside the specimen (1-20) through the connecting conduit (1-10) for hydraulic fracturing. The modified hydraulic fracturing module includes a modified liquid tank A (2-1), a modified liquid tank B (2-3), a high-pressure injection pump (2-5), a pressure sensor A (1-5), and a flow sensor A (1-6). The pipeline at the input end of the high-pressure injection pump (2-5) is connected to the modified liquid tank A (2-1) and the modified liquid tank B (2-3). The pipeline at the output end is successively connected to a valve F (2-9), a valve B (1-7), a flow sensor A (1-6), a pressure sensor A (1-5), and a connecting conduit (1-10). The modified liquid is introduced into the fracturing pipe (1-21) inside the specimen (1-20) through the connecting conduit (1-10) for fracturing modification. The liquid nitrogen fracturing module includes a liquid nitrogen storage tank (2-12), a liquid nitrogen injector (2-15), an air compressor (3-6), a gas booster pump (3-8), a pressure buffer tank (2-16), a pressure reducing valve A (2-18), a pressure sensor A (1-5), and a flow sensor A (1-6). The gas injection port of the liquid nitrogen injector (2-15) is successively connected to a valve L (2-19), a pressure reducing valve A (2-18), a valve K (2-17), a pressure buffer tank (2-16), a valve Q (3-10), a pressure sensor B (3-9), a gas booster pump (3-8), a valve P (3-7), and an air compressor (3-6) through pipelines. The liquid injection port of the liquid nitrogen injector (2-15) is successively connected to a valve I (2-13), a valve B (1-7), a flow sensor A (1-6), a pressure sensor A (1-5), and a connecting conduit (1-10) through pipelines. And its liquid injection port is also successively connected to a valve J (2-14) and a liquid nitrogen storage tank (2-12) through a branch. The supercritical carbon dioxide fracturing module includes a carbon dioxide gas tank (3-1), a high-pressure injection pump (2-5), a high and low temperature test chamber (2-7), a water tank A (2-6), a pressure sensor A (1-5), and a flow sensor A (1-6). The air inlet of the high and low temperature test chamber (2-7) is successively connected to a valve E (2-8), a valve M (3-3), and a carbon dioxide gas tank (3-1) through pipelines. Its pressurizing port is successively connected to a valve G (2-10), a high-pressure injection pump (2-5), and a water tank A (2-6) through pipelines. Its pressure injection port is successively connected to a valve H (2-11), a valve B (1-7), a flow sensor A (1-6), a pressure sensor A (1-5), and a connecting conduit (1-10) through pipelines. Its pressure relief port is connected to the water tank A (2-6) through a pipeline.

4. The fracturing modification of coal and rock mass and its effect evaluation test system according to claim 1, characterized in that: The gas injection evaluation system (3) includes a gas source, an air compressor (3-6), a gas booster pump (3-8), a buffer tank (3-12), a pressure reducing valve B (3-14), and a metering tank (3-16); the gas booster pump (3-8) is provided with a gas source inlet, an air inlet, and an injection pressure port. Its gas source inlet is connected to a valve N (3-4) and a methane gas tank (3-2) in sequence through a pipeline, and the gas source inlet is also connected to a valve O (3-5), a valve M (3-3), and a carbon dioxide gas tank (3-1) respectively through a branch; the air inlet of the gas booster pump (3-8) is connected to the air compressor (3-6) in sequence through a pipeline via a valve P (3-7); the injection pressure port of the gas booster pump (3-8) is connected to a pressure sensor B (3-9), a valve R (3-11), the buffer tank (3-12), a valve S (3-13), the pressure reducing valve B (3-14), a valve T (3-15), a flow sensor B (3-18), a valve V (3-19), a valve A (1-4), and a connecting conduit (1-10) in sequence through a pipeline, and a valve U (3-17) and the metering tank (3-16) are connected in sequence through a branch between the valve T (3-15) and the valve V (3-19).

5. The coal and rock mass fracturing modification and its effect evaluation test system according to claim 1, characterized in that: The backpressure monitoring system (4) includes an automatic backpressure pump (4-2), a water tank B (4-1), a backpressure buffer tank (4-3), a pressure sensor C (4-4), a backpressure valve (4-5), a gas flowmeter (4-8), a dry filter (4-9), a gas-liquid separator (4-10), and a graduated cylinder (4-11); the inlet end of the backpressure valve (4-5) is connected to a valve W (4-6), a pressure sensor D (4-7), and the connecting conduit (1-10) in sequence through a pipeline, its outlet end is connected to the inlet end of the gas-liquid separator (4-10) through a pipeline, and the backpressure valve (4-5) is also connected to the pressure sensor C (4-4), the backpressure buffer tank (4-3), the automatic backpressure pump (4-2), and the water tank B (4-1) in sequence through a pipeline; the exhaust port of the gas-liquid separator (4-10) is connected to the dry filter (4-9), the gas flowmeter (4-8), and an air collecting bag (4-12) in sequence through a pipeline, and the liquid discharge port of the gas-liquid separator (4-10) is connected to the graduated cylinder (4-11) through a pipeline.

6. The coal and rock mass fracturing modification and its effect evaluation test system according to claim 1, characterized in that: The vacuum system (6) includes a vacuum pump (6-1) and a pressure sensor E (6-3). The vacuum pump (6-1) is connected to a valve X (6-2), the pressure sensor E (6-3), a valve Y (6-4), a valve A (1-4), and the connecting conduit (1-10) in sequence through a pipeline.

7. The coal and rock mass fracturing modification and its effect evaluation test system according to claim 1, characterized in that: The sensors include pressure sensors A - E, flow sensors A(1 - 6), flow sensors B(3 - 18), and strain sensors. Among them, pressure sensor A(1 - 5) is used to monitor the injection pressure of the fracturing medium, pressure sensor B(3 - 9) is used to monitor the output pressure of the gas booster injection port, pressure sensor C(4 - 4) is used to monitor the output pressure of the automatic backpressure pump, pressure sensor D(4 - 7) is used to monitor the pressure at the gas - liquid discharge port of the true triaxial loading system, pressure sensor E(6 - 3) is used to monitor the negative pressure of the vacuum system, flow sensor A(1 - 6) is used to monitor the medium injection flow rate of the fracturing modification system, flow sensor B(3 - 18) is used to monitor the medium injection flow rate of the gas injection evaluation system, and the strain sensor is used to monitor the strain of the specimen under different loading conditions.

8. A test method for the fracturing modification and effect evaluation test system of coal and rock mass according to claim 1, characterized in that: It includes the following steps, It includes the following steps, S1: Fabricate the specimen and assemble the model; S2: Connect the true triaxial loading system (1), fracturing modification system (2), gas injection evaluation system (3), backpressure monitoring system (4), hydraulic servo control system (5), vacuum system (6), and data acquisition and control system integration (7) together; S3: Use the gas injection evaluation system (3) and the backpressure monitoring system (4) to conduct the permeability measurement test of the specimen; Use the vacuum system (6), gas injection evaluation system (3), and backpressure monitoring system (4) to conduct the adsorption - desorption test of the specimen (1 - 20), and measure the methane adsorption amount, desorption amount, adsorption deformation amount, and desorption deformation amount of the specimen (1 - 20) under different gas pressures; Use the true triaxial loading system (1) to conduct uniaxial compression tests on the specimen (1 - 20) in the X, Y, and Z directions respectively until the elastic limit of the specimen (1 - 20), obtain the dynamic stress - strain values of the specimen (1 - 20) during the entire loading process through the stress - strain monitoring system, and calculate the initial elastic modulus and Poisson's ratio of the specimen (1 - 20) in the X, Y, and Z directions; S4: In - situ condition simulation. Evacuate the specimen through the vacuum system (6), and set the corresponding triaxial loading parameters, temperature, moisture content, and gas pressure according to the actual in - situ conditions of the coal seam. Apply principal stresses to the three pairs of opposite sides of the specimen (1 - 20) through the hydraulic servo control system (5) to simulate the in - situ stress state of the coal seam. Use the data acquisition and control system integration (7) to control the electric thermocouple heating rod in the loading platen (1 - 12) to heat the specimen (1 - 20) to the required test temperature to simulate the in - situ temperature state of the coal seam. Use the hydraulic fracturing module of the fracturing modification system to inject a certain amount of water into the coal - rock specimen (1 - 20) to simulate the in - situ water content of the coal seam. Use the gas injection evaluation system (3) to fill a certain pressure of methane into the specimen (1 - 20) to simulate the in - situ gas pressure condition of the coal seam; S5: Close the gas injection evaluation system (3) and the back pressure monitoring system (4), turn on the fracturing modification system (2), and use the hydraulic fracturing module, modified hydraulic fracturing module, liquid nitrogen fracturing module, and supercritical carbon dioxide fracturing module in the fracturing modification system (2) to conduct fracturing tests on the specimens (1 - 20) with different fracturing media; S6: After the fracturing modification test is completed, close the fracturing modification system (2), turn on the gas injection evaluation system (3) and the back pressure monitoring system (4), use the back pressure monitoring system (4) to conduct backflow of the fracturing media in the specimens (1 - 20). After the backflow is completed, first measure the adsorption and desorption amounts of the fractured specimens (1 - 20) under different gas pressure conditions and their permeabilities in the X, Y, and Z directions again according to the method described in step (c), and then measure the elastic moduli and Poisson's ratios of the fractured specimens (1 - 20) in the X, Y, and Z directions respectively according to the method described in step (c); According to the changes in the adsorption and desorption amounts, permeabilities in different directions, elastic moduli, and Poisson's ratios of the specimens (1 - 20) before and after fracturing modification, quantitatively evaluate the effects of different fracturing media on fracturing and modifying coal and rock masses.

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