Liquid nitrogen high pressure multi-stage pulse fracture full process true triaxial test system

Abstract

The present invention discloses a true triaxial test system for the whole process of liquid nitrogen high-pressure multi-stage pulse fracturing, and relates to the technical field of liquid nitrogen true triaxial fracturing coal body test system. By pre-drilling a sample coal rock, and injecting high-pressure liquid nitrogen into the pre-drilled hole, the sample is fractured by high-pressure liquid nitrogen. The crack direction and expansion will be related to the depth, position, and high-pressure liquid nitrogen injection area of ​​the crack induction hole. The crack initiation and crack direction can be detected in real time by pre-buried acoustic wave probes, and the displacement and strain of coal rock are detected by the displacement-strain monitoring technology system. The temperature and pressure of the coal sample during the whole fracturing process are detected by the coal sample temperature and pressure measuring components, and the experimental data are recorded and analyzed by the data acquisition device, data analysis module, and intelligent control terminal of the control box. The experimental system can not only study the influence of different injection pressures and flow rates, but also study liquid nitrogen fracturing under different injection times / numbers. The system is powerful and practical.

Description

Technical Field

[0001] The invention relates to the technical field of liquid nitrogen true triaxial fracturing coal body test systems, in particular to a liquid nitrogen high-pressure multi-stage pulse fracturing full-process true triaxial test system. Background Art

[0002] Coal is the main energy source and the bottom-line energy source in my country. It will remain the stabilizer and ballast of my country's energy security for a long period of time. With the increase in the intensity of coal mining and the increase in mining depth, shallow resources are becoming increasingly exhausted, and many mines have entered the deep mining stage. Most of my country's deep coal seams have the characteristics of high gas, high stress, and low permeability. In particular, low-permeability coal seams generally have developed microporous structures, strong gas adsorption capacity, and large desorption and seepage resistance, which seriously restricts the effect of coal seam gas pre-extraction. In view of the current on-site underground coal seam fracturing technology, some scholars have proposed the use of liquid nitrogen pulse fracturing. Under the combined effects of the water-ice phase change frost heave force, the liquid nitrogen gasification expansion force, and the damage of low-temperature liquid nitrogen to the coal body, the macro-cracks and micro-cracks are expanded and connected to form a fracture network to increase the permeability of the coal seam.

[0003] Liquid nitrogen has the characteristics of low temperature (-195.8℃), no pollution, simple preparation and wide source of raw materials, and has good application prospects in improving the permeability of coal seams. In order to study the changes in the pore structure and mechanical properties of coal bodies under the action of liquid nitrogen, domestic and foreign scholars have conducted some laboratory fracturing tests of liquid nitrogen injection into coal bodies. However, the current test liquid nitrogen injection pressure and flow rate are relatively low. Due to the difficulty in controlling cold preservation measures and the difficulty in achieving large temperature difference isolation, the low-flow liquid nitrogen quickly heats up and vaporizes after contacting the wall of the conveying pipeline, forming a large pressure, which poses a test risk. In some high-pressure liquid nitrogen injection pumps, due to the low boiling point (-147℃), high volatility and high phase change expansion (696 times) of liquid nitrogen, after liquid nitrogen enters the injection pump injection container, it pushes the container through high pressure to push the liquid nitrogen into the coal sample. Due to the large temperature difference in the intermediate container, it is not easy to form a good cold preservation measure and effect, which will cause the container pressure to rise suddenly and damage the liquid nitrogen injection pump. During the process of liquid nitrogen injection into coal samples, it is difficult to inject liquid nitrogen into coal sample specimens in a liquid fluid state due to its low temperature characteristics and abnormally low boiling point. In addition, conventional test sensors will shrink and deform under the ultra-low temperature of liquid nitrogen, and their sensitivity will be reduced, making it impossible to directly measure normally. In previous liquid nitrogen fracturing coal sample tests, only the destruction and deformation of coal samples before and after liquid nitrogen fracturing were usually studied. The effects of liquid nitrogen action time and injection speed on the damage evolution law are still unclear, and the influence mechanism of factors such as reservoir temperature and stress state is still not deeply studied. Therefore, it is necessary to propose a full-process liquid nitrogen high-pressure multi-stage pulse test system that takes into account reservoir stress, temperature and other factors and monitors factors such as temperature, pressure displacement and other factors in real time in coal samples to meet the above technical requirements. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a liquid nitrogen high-pressure multi-stage pulse fracturing full-process true triaxial testing system which takes into account factors such as reservoir stress and temperature and can monitor factors such as temperature, pressure displacement, etc. in real time in coal samples.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] The present invention provides a liquid nitrogen high-pressure multi-stage pulse fracturing full-process true triaxial test system, comprising a triaxial reactor, wherein the triaxial reactor comprises a reactor cavity, a reactor sealing cover and a triaxial loading rod; the reactor cavity is open at the top, a reactor inner cavity for containing a test piece is arranged inside the reactor cavity, the reactor sealing cover is arranged at the top of the reactor cavity, and the triaxial loading rod is arranged around the reactor cavity respectively;

[0007] The test piece comprises a square rubber cylinder, the top of which is open, and the square rubber cylinder is used to hold the test coal sample. Coal sample loading plates are arranged around the outside of the square rubber cylinder, and the loading end of the triaxial loading rod is in contact with the coal sample loading plate; a square pressing plate is arranged on the top of the square rubber cylinder, and a heat-insulating cold cavity is arranged above the square pressing plate; the lower ends of the coaxially arranged liquid nitrogen injection rod and the nitrogen back-discharge pipeline pass through the heat-insulating cold cavity, and the rear of the square pressing plate extends into the test coal sample; the upper part of the liquid nitrogen injection rod is connected to the liquid nitrogen tank; and the heat-insulating cold cavity is provided with heat-insulating material;

[0008] It also includes a control box, a data collector, a data analysis module, an intelligent control terminal and an explosion-proof control box; the data collector, the data analysis module and the intelligent control terminal are all arranged in the control box, and the data collector, the data analysis module and the intelligent control terminal are electrically connected to the explosion-proof control box through connecting wires.

[0009] Optionally, a pipeline clamping flange is provided on the top of the thermal insulation cold cavity, the pipeline clamping flange is detachably connected to the top of the thermal insulation cold cavity, and an insulating baffle is provided between the pipeline clamping flange and the thermal insulation material; the thermal insulation material is thermal insulation cotton yarn.

[0010] Optionally, a sealing component is provided between the heat-insulating cold cavity and the nitrogen exhaust pipeline.

[0011] Optionally, a packing locking thread is provided on the lower outer wall of the nitrogen exhaust pipe, and the nitrogen exhaust pipe is connected to the square pressure plate through the packing locking thread.

[0012] Optionally, a coal sample temperature and pressure measuring component and a heating and temperature control component are provided at the bottom of the reactor cavity, and both the coal sample temperature and pressure measuring component and the heating and temperature control component are electrically connected to the data collector.

[0013] Optionally, the liquid nitrogen tank is connected to the inlet of the plunger liquid nitrogen pump through the liquid nitrogen pump inlet pipeline, and the outlet of the plunger liquid nitrogen pump is connected to the liquid nitrogen injection rod; the plunger liquid nitrogen pump is transmission-connected to an explosion-proof motor, and the explosion-proof motor is electrically connected to the explosion-proof control box.

[0014] Optionally, a second check valve, a damper, a vaporizer, a pressure gauge and a first check valve are sequentially arranged on the control pipeline of the plunger liquid nitrogen pump; and the first check valve is electrically connected to the explosion-proof control box.

[0015] Optionally, a displacement sensor and an acoustic emission sensor are provided on the loading rod of the triaxial testing machine; the loading rod of the triaxial testing machine is used to provide a load to the triaxial loading rod; the displacement sensor and the acoustic emission sensor are both electrically connected to the data collector.

[0016] Optionally, a system base is further included, and the three-axis reactor is arranged on the system base.

[0017] Optionally, a low-temperature resistant sealant is provided between the bottom of the square pressure plate and the top of the test coal sample.

[0018] Compared with the prior art, the present invention has achieved the following technical effects:

[0019] 1. The liquid nitrogen injection rod is a vacuum insulated tube with a sandwich design. The middle part is the liquid nitrogen injection pipeline, and the interlayer is the nitrogen exhaust pipeline, forming a through pipeline to ensure that the liquid nitrogen is injected into the coal sample in a fluid state. It can also discharge the high-pressure nitrogen vaporized from the liquid nitrogen, which can effectively solve the problem that liquid nitrogen is difficult to inject into the coal sample specimen in a liquid fluid state.

[0020] 2. The cold insulation cavity is filled with cold insulation yarn and wrapped with liquid nitrogen injection rod. After the insulation yarn is filled, the retaining ring and O-ring are installed for sealing. The cold insulation yarn is compacted with the insulation baffle. The pipeline clamping flange and adjusting nut are installed on the insulation baffle in turn to lock the injection rod for the second time, and the cold insulation cavity is locked and fixed circumferentially by the top locking stud. In addition, a polyurethane insulation layer is arranged on the outside of the three-axis reactor to ensure that the temperature transfer exchange is reduced with minimal heat exchange during the liquid nitrogen injection process, thereby reducing the liquid nitrogen consumption.

[0021] 3. Triaxial loading and temperature heating can be used to conduct a full-process liquid nitrogen high-pressure multi-stage pulse fracturing experiment that takes into account reservoir stress, temperature and other factors and monitors the temperature, pressure displacement and other factors in the coal sample in real time.

[0022] 4. The acoustic emission guide rod solves the problem that the acoustic emission sensor cannot directly contact the coal sample. The acoustic emission guide rod is embedded in the triaxial loading rod and arranged symmetrically. The coal sample temperature and pressure measurement assembly can realize the real-time monitoring of the expansion of the internal cracks and the temperature and pressure of the coal sample during the liquid nitrogen injection fracturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 It is a structural schematic diagram of the true triaxial test system of the present invention for the whole process of liquid nitrogen high pressure multi-stage pulse fracturing;

[0025] Figure 2 It is a structural schematic diagram of a triaxial reactor in the true triaxial test system of the present invention for the whole process of liquid nitrogen high-pressure multi-stage pulse fracturing;

[0026] Figure 3 It is a structural schematic diagram of the cold-insulation injection assembly in the true triaxial test system of the liquid nitrogen high-pressure multi-stage pulse fracturing process of the present invention;

[0027] Figure 4 It is a top view structural schematic diagram of the true triaxial test system of the liquid nitrogen high-pressure multi-stage pulse fracturing whole process of the present invention.

[0028] Explanation of reference numerals: 1. control box; 2. data collector; 3. data analysis module; 4. intelligent control terminal; 5. displacement sensor; 6. acoustic emission sensor; 7. triaxial testing machine; 8. first check valve; 9. pressure gauge; 10. triaxial reactor; 11. nitrogen backflow pipeline; 12. liquid nitrogen injection rod; 13. cabin penetration assembly; 14. test coal sample; 15. flow meter; 16. vaporizer; 17. damper; 18. second check valve; 19. liquid nitrogen pump inlet pipeline; 20. liquid nitrogen tank; 21. plunger liquid nitrogen pump; 22. explosion-proof motor; 23. explosion-proof control box; 24. connecting wires; 25. coal sample temperature and pressure measurement assembly; 26. heating temperature control assembly; 27. system base; 28. Three-axis loading rod; 29. ​​Reactor cavity; 30. Reactor sealing cover; 31. Adjusting nut; 32. Cold-insulating cotton yarn; 33. M36 hexagonal screw; 34. Square rubber cylinder; 35. Coal sample loading plate; 36. Reactor cavity; 37. Rubber cylinder bottom plate support column; 38. Rubber cylinder bottom plate; 39. Positioning screw; 40. Breathable joint; 41. Square pressure plate; 42. Packing locking thread; 43. Sealing assembly; 44. Insulated cold cavity; 45. Retaining ring; 46. O-ring; 47. Top locking stud; 48. Insulating baffle; 49. Pipeline clamping flange; 50. M12 internal angle screw; 51. Side loading cylinder; 52. Axial loading cylinder; 53. Reactor O-ring. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] like Figures 1 to 4As shown, this embodiment provides a true triaxial test system for the whole process of liquid nitrogen high-pressure multi-stage pulse fracturing, including a triaxial reactor 10, the triaxial reactor 10 including a reactor cavity 29, a reactor sealing cover 30 and a triaxial loading rod 28; the reactor cavity 29 is open at the top, and a reactor inner cavity 36 for holding a test piece is arranged inside the reactor cavity 29, the reactor sealing cover 30 is arranged at the top of the reactor cavity 29, and the triaxial loading rods 28 are arranged around the reactor cavity 29; the test piece includes a square rubber cylinder 34, the square rubber cylinder 34 is open at the top, the square rubber cylinder 34 is used to hold the test coal sample 14, and the outside of the square rubber cylinder 34 is provided with a coal sample loading plate 35, and the loading end of the triaxial loading rod 28 is connected to the coal sample loading plate 35 touch; a square pressing plate 41 is arranged on the top of the square rubber cylinder 34, and an insulating cold cavity 44 is arranged above the square pressing plate 41; the coaxially arranged liquid nitrogen injection rod 12 and the lower end of the nitrogen back discharge pipeline 11 pass through the insulating cold cavity 44 and the square pressing plate 41 and then extend into the test coal sample 14; the upper part of the liquid nitrogen injection rod 12 is connected to the liquid nitrogen tank 20; the insulating cold cavity 44 is provided with insulating material; it also includes a control box 1, a data collector 2, a data analysis module 3, an intelligent control terminal 4 and an explosion-proof control box 23; the data collector 2, the data analysis module 3 and the intelligent control terminal 4 are all arranged in the control box 1, and the data collector 2, the data analysis module 3 and the intelligent control terminal 4 are electrically connected to the explosion-proof control box 23 through the connecting wire 24.

[0031] In a more specific embodiment, a reactor O-ring 53 is provided between the reactor cavity 29 and the reactor sealing cover 30 , which are connected by M36 hexagonal screws 33 .

[0032] A plurality of air-permeable joints 40 are provided at the bottom of the square pressure plate 41 to remove excess or overpressure gas when the test coal sample is installed, sealed and tested.

[0033] A rubber cylinder bottom plate 38 is disposed at the bottom of the square rubber cylinder 34, and a rubber cylinder bottom plate 38 is disposed between the bottom of the coal sample loading plate 35 and the top of the rubber cylinder bottom plate 38, and the rubber cylinder bottom plate 38 is used to fix the position of the test coal sample 14. A rubber cylinder bottom plate support column 37 is disposed around the rubber cylinder bottom plate 38, and the bottom of the rubber cylinder bottom plate support column 37 is connected to the bottom of the reaction kettle cavity 29 to support the square rubber cylinder 34.

[0034] A pipeline clamping flange 49 is arranged at the top of the heat-insulating cold cavity 44, and the pipeline clamping flange 49 is connected to the top of the heat-insulating cold cavity 44 by M12 internal angle screws 50, and a heat-insulating baffle 48 is arranged between the pipeline clamping flange 49 and the heat-insulating material; the heat-insulating material is heat-insulating cotton yarn. More specifically, the outer top of the pipeline clamping flange 49 is a flange plate, and a cover-type structure is arranged at the lower part of the flange plate, and the flange plate is connected to the top of the heat-insulating cold cavity 44 by M12 internal angle screws 50, and a sealing ring is arranged between the outer wall of the cover-type structure and the inner wall of the heat-insulating cold cavity 44, and the heat-insulating baffle 48 is arranged in the cover-type structure. Furthermore, an adjusting nut 50 is arranged between the top of the heat-insulating baffle 48 and the inner top of the cover-type structure, and the adjusting nut 50 is threadedly connected to the nitrogen back-discharge pipeline 11.

[0035] A retaining ring 45 and an O-ring 46 are provided on the upper portion of the outer wall of the heat-insulating cold cavity 44 for sealing with the three-axis reactor 10 .

[0036] A sealing component 43 is provided between the heat-insulating cold cavity 44 and the nitrogen exhaust pipeline 11 .

[0037] A packing locking thread 42 is provided on the lower outer wall of the nitrogen backflow pipeline 11 , and the nitrogen backflow pipeline 11 is connected to the square pressure plate 41 through the packing locking thread 42 .

[0038] The bottom of the reactor cavity 29 is provided with a coal sample temperature and pressure measuring assembly 25 and a heating and temperature control assembly 26, which are both electrically connected to the data collector 2. More specifically, the coal sample temperature and pressure measuring assembly 25 passes through the rubber cylinder bottom plate 38 and the bottom of the square rubber cylinder 34, the heating and temperature control assembly 26 passes through the reactor cavity 29, and the heating and temperature control assembly 26 is located around the rubber cylinder bottom plate 38.

[0039] The liquid nitrogen tank 20 is connected to the inlet of the plunger liquid nitrogen pump 21 through the liquid nitrogen pump inlet pipeline 19, and the outlet of the plunger liquid nitrogen pump 21 is connected to the liquid nitrogen injection rod 12; the plunger liquid nitrogen pump 21 is transmission-connected to the explosion-proof motor 22, and the explosion-proof motor 22 is electrically connected to the explosion-proof control box 23.

[0040] The control pipeline of the plunger liquid nitrogen pump 21 is provided with a second check valve 18 , a damper 17 , a vaporizer 16 , a pressure gauge 9 and a first check valve 8 in sequence; the first check valve 8 is electrically connected to the explosion-proof control box 23 .

[0041] The loading rod of the triaxial testing machine 7 is provided with a displacement sensor 5 and an acoustic emission sensor 6; the loading rod of the triaxial testing machine 7 is used to provide a load to the triaxial loading rod 28; the displacement sensor 5 and the acoustic emission sensor 6 are both electrically connected to the data collector 2. Furthermore, a lateral loading cylinder 51 of the triaxial testing machine 7 is provided with a displacement sensor 5 and an acoustic emission sensor 6, another lateral loading cylinder 51 of the triaxial testing machine 7 is provided with an acoustic emission sensor 6, and an axial loading cylinder 52 of the triaxial testing machine 7 is provided with a displacement sensor 5.

[0042] The liquid nitrogen high-pressure multi-stage pulse fracturing full-process true triaxial testing system in this embodiment further includes a system base 27 , and the triaxial reactor 10 is disposed on the system base 27 .

[0043] A low-temperature resistant sealant is provided between the bottom of the square pressing plate 41 and the top of the test coal sample 14 .

[0044] The outlet end of the nitrogen backflow pipeline 11 is provided with a pressure gauge 9 and a first check valve 8 in sequence.

[0045] The explosion-proof control box 23 is provided with a pressure gauge 9 and a flow meter 15 for monitoring the nitrogen pressure and flow during the test.

[0046] In a further specific embodiment, the liquid nitrogen injection rod 12 is a vacuum insulated tube with a sandwich design, a liquid nitrogen injection pipeline in the middle, and a nitrogen backflow pipeline 11, forming a through pipeline to ensure that the liquid nitrogen is injected into the coal sample in a fluid state, and can also discharge the high-pressure nitrogen vaporized from the liquid nitrogen, which can effectively solve the problem that liquid nitrogen is difficult to inject into the coal sample specimen in a liquid fluid state.

[0047] A plurality of penetration components 13 are provided through the reactor cavity 29, and the penetration components 13 are used to introduce some connecting lines from the outside into the reactor cavity 36. Interfaces and air inlet interfaces can also be reserved, and some external air inlet pipelines can be connected to cooperate with the air permeable joints 40, so that coal sample permeability test experiments and other functional experiments can be carried out subsequently.

[0048] The coal sample temperature and pressure measuring assembly 25 is placed in 9 temperature and pressure measuring holes. Each borehole has 3 pressure and temperature probes in the vertical direction, with a total of 3*9=27 measuring points, which can realize all-round three-dimensional real-time monitoring of the internal pressure and temperature of the coal sample during the whole process of liquid nitrogen fracturing.

[0049] The heat-insulating cold cavity 44 is filled with heat-insulating cotton yarn 32. After the heat-insulating cotton yarn is fully filled, the retaining ring 45 and the O-ring 46 are installed for sealing. The heat-insulating cotton yarn 32 is compacted with the heat-insulating baffle 48. The pipeline clamping flange 49 and the adjusting nut 50 are installed on the heat-insulating baffle 48 in sequence to perform secondary locking and fixing of the injection rod. The heat-insulating cold cavity 44 is circumferentially locked and fixed by the top locking stud, and the M12 internal angle screw 50 is axially locked and fixed. After that, the reactor sealing cover 30 and the reactor cavity 29 are sealed by the reactor O-ring 53, and the two are locked by the M36 hexagonal screw 33. In addition, an insulating layer is arranged on the outside of the three-axis reactor 10 to ensure that during the liquid nitrogen injection process, the temperature transfer exchange is reduced with minimal heat exchange to reduce the liquid nitrogen consumption.

[0050] The size of the coal sample is 150mm*150mm*150mm; the O-ring is 46mm dense, the cylinder material is low-temperature resistant polytetrafluoroethylene, the axial test force is 1500kN, the maximum triaxial loading pressure is 25MPa, and the specimen simulation temperature is -196℃~200℃. The whole process of liquid nitrogen high-pressure multi-stage pulse fracturing experiment can be carried out taking into account factors such as reservoir stress and temperature and real-time monitoring of factors such as temperature, pressure displacement in the coal sample.

[0051] The liquid nitrogen injection pump uses an explosion-proof motor 22 control system to control the motor speed. The explosion-proof motor 22 uses a servo motor, which can accurately adjust the injection speed of liquid nitrogen and measure the injection amount of liquid nitrogen. The plunger liquid nitrogen pump 21 is a horizontal piston vacuum pump with a flow rate of 0-70L / h (gasification volume), an inlet pressure of 0.02-1.2MPa, an outlet pressure of 0-45MPa, and a design temperature of -196°C.

[0052] The system uses a low-temperature self-pressurized liquid nitrogen tank 20 to store industrial liquid nitrogen. The capacity of the liquid nitrogen tank 20 is 1000L. The self-pressurized liquid nitrogen tank 20 is made of high-quality stainless steel. The container is equipped with a booster system, which can generate pressure and discharge liquid continuously. It consists of a control system and a tank body. The control system mainly consists of an inlet / discharge valve, a booster valve, a vent valve, a double safety valve, a liquid level gauge, a pressure gauge 9, and 4 casters with independent brakes.

[0053] A displacement sensor 5 and an acoustic emission sensor 6 are provided. The acoustic emission sensor 6 and the displacement sensor 5 are embedded in the triaxial loading rod 28. The acoustic emission sensor 6 is arranged symmetrically, and the displacement sensor 5 is arranged adjacently.

[0054] Before the experiment, the experimental coal sample is firstly cored and made. The experimental coal sample is a cubic coal sample of 150*150*150mm. Then a hole is drilled on the specimen with a diameter of 15mm and a depth of 70mm. Nine temperature and pressure measuring holes are opened below the drill hole, and the pressure measuring holes are arranged 3*3. After the coal sample is made, the coal sample is placed inside the rubber cylinder, and the injection pipeline is embedded in the experimental coal sample through the square pressing plate 41. After the injection pipeline and the top plate are combined into an assembly, they are tightly fitted to the coal sample specimen, and the packing locking thread 42 is used to fix the injection pipeline. Then, the gap between the square pressing plate 41 and the rubber cylinder is sealed with low-temperature resistant sealant, and then the assembly is placed in a cool and ventilated place to dry for about 36 hours. After the sealant is dried, the square rubber cylinder 34 is placed on the rubber cylinder bottom plate 38 in the three-axis reactor 10, and the rubber cylinder bottom plate 38 is fixed by the rubber cylinder bottom plate support column 37 and the rubber cylinder bottom plate 38 below. During the installation process, the air joint 40 is opened, and the air joint 40 is closed after the position of the rubber cylinder is fixed. Then, the coal sample temperature and pressure measuring assembly 25 is placed in 9 temperature and pressure measuring holes. There are 3 pressure and temperature probes in each borehole in the vertical direction, totaling 3*9=27 measuring points, which can realize the full-dimensional and real-time monitoring of the internal pressure and temperature of the coal sample during the whole process of liquid nitrogen fracturing. After the position of the rubber cylinder is fixed, the sealing assembly 43 and the heat-insulating cold cavity 44 are successively installed through the injection pipeline, and the heat-insulating cold cavity 44 is filled with cold-insulating cotton yarn 32. After the heat-insulating cotton yarn is filled, the retaining ring 45 and the O-ring 46 are installed for sealing, and the heat-insulating cotton yarn 32 is compacted with the heat-insulating baffle 48. The pipeline clamping flange 49 and the adjusting nut 50 are successively installed on the heat-insulating baffle 48 to lock and fix the injection rod for the second time, and the heat-insulating cold cavity 44 is circumferentially locked and fixed by the top locking stud, and the M12 internal angle screw 50 is axially locked and fixed. After that, the reactor sealing cover 30 and the reactor cavity 29 are sealed by the reactor O-ring 53, and the M36 hexagon screw 33 is used to lock the two. In addition, an insulating layer is arranged on the outside of the triaxial reactor 10 to ensure that the temperature transfer exchange is reduced with minimal heat exchange during the liquid nitrogen injection process. After the coal sample is installed, the reactor cavity 36 is injected with water for pressure measurement through the cabin penetration assembly. After the pressure measurement is completed, the triaxial testing machine 7 is turned on. The lateral loading cylinder 51 and the axial loading cylinder 52 jointly load the loading plate on the outside of the specimen rubber cylinder through the triaxial loading rod 28 to provide the maximum principal stress and the intermediate principal stress. The reactor cavity 36 provides net water pressure loading to provide a confining pressure sealing environment for the rubber cylinder outside the specimen, and the heating and temperature control assembly 26 is turned on to simulate the reservoir temperature.

[0055] After the ground stress loading system and coal sample are installed, the liquid nitrogen injection system is debugged and operated. First, open the liquid nitrogen tank 20 to fill the liquid. The liquid nitrogen pump fills the container with liquid. Please open the vent valve first, connect the metal hose for infusion to the liquid inlet valve, open the liquid inlet valve, and then add liquid nitrogen from the liquid inlet valve. Observe the pressure gauge 9. After the filling is completed, close the liquid inlet valve. Open the explosion-proof control box 23 and the explosion-proof motor 22, adjust the motor speed, and increase the speed to more than 600rpm. Then open the damper 17, the vaporizer 16 and the check valve in turn to pre-cool the liquid nitrogen pump. At the same time, open the data acquisition device 2, the data analysis module 3, the intelligent control terminal 4, the pressure gauge 9, the flow meter 15, the displacement sensor 5 and the acoustic emission sensor 6 of the control box 1 for data collection and analysis. When the external pipeline of the liquid nitrogen pump is frosted for 3 minutes, it means that the liquid nitrogen pump has been cooled. Subsequently, the liquid inlet valve was opened to carry out the whole process experiment of liquid nitrogen high-pressure multi-stage pulse fracturing under true triaxial loading. During the experiment, the pressure gauge 9 and the check valve on the backflow pipeline were opened to form a through pipeline to ensure that the liquid nitrogen was injected into the coal sample in a fluid state and the high-pressure nitrogen gas vaporized from the liquid nitrogen could also be discharged.

[0056] The liquid nitrogen high-pressure multi-stage pulse fracturing full-process true triaxial test system in the present invention pre-drills a sample coal rock and injects high-pressure liquid nitrogen into the pre-drilled hole. The sample is fractured by high-pressure liquid nitrogen. The crack direction and expansion are related to the depth, position, and high-pressure liquid nitrogen injection of the crack induction hole. The crack initiation and crack direction can be detected in real time by pre-buried acoustic wave probes. The displacement and strain of coal rock are detected by the displacement-strain monitoring technology system. The temperature and pressure of the coal sample during the whole fracturing process are detected by the coal sample temperature and pressure measuring component 25, and the experimental data are recorded and analyzed by the data acquisition device 2, data analysis module 3, and intelligent control terminal 4 of the control box 1. The experimental system can not only study the influence of different injection pressures and flow rates, but also study liquid nitrogen fracturing under different injection times / numbers. The system is powerful and practical.

[0057] It should be noted that it is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention, and any figure mark in the claims should not be regarded as limiting the claims involved.

[0058] The present specification uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. Liquid nitrogen high pressure multi-stage pulse fracture full process true triaxial test system, characterized by: The invention comprises a triaxial reactor, wherein the triaxial reactor comprises a reactor cavity, a reactor sealing cover and a triaxial loading rod; the top of the reactor cavity is open, a reactor inner cavity for holding a test piece is arranged inside the reactor cavity, the reactor sealing cover is arranged on the top of the reactor cavity, and a triaxial loading rod is arranged around the reactor cavity; The test piece comprises a square rubber cylinder, the top of which is open, and the square rubber cylinder is used to hold the test coal sample. Coal sample loading plates are arranged around the outside of the square rubber cylinder, and the loading end of the triaxial loading rod is in contact with the coal sample loading plate; a square pressing plate is arranged on the top of the square rubber cylinder, and a heat-insulating cold cavity is arranged above the square pressing plate; the lower ends of the coaxially arranged liquid nitrogen injection rod and the nitrogen back-discharge pipeline pass through the heat-insulating cold cavity, and the rear of the square pressing plate extends into the test coal sample; the upper part of the liquid nitrogen injection rod is connected to the liquid nitrogen tank; and the heat-insulating cold cavity is provided with heat-insulating material; It also includes a control box, a data collector, a data analysis module, an intelligent control terminal and an explosion-proof control box; the data collector, the data analysis module and the intelligent control terminal are all arranged in the control box, and the data collector, the data analysis module and the intelligent control terminal are electrically connected to the explosion-proof control box through connecting wires; The liquid nitrogen tank is connected to the inlet of the plunger liquid nitrogen pump through the liquid nitrogen pump inlet pipeline, and the outlet of the plunger liquid nitrogen pump is connected to the liquid nitrogen injection rod; the plunger liquid nitrogen pump is transmission-connected to the explosion-proof motor, and the explosion-proof motor is electrically connected to the explosion-proof control box; A second check valve, a damper, a vaporizer, a pressure gauge and a first check valve are sequentially arranged on the control pipeline of the plunger liquid nitrogen pump; the first check valve is electrically connected to the explosion-proof control box; Nine temperature and pressure measuring holes are opened on the test coal sample, and each of the temperature and pressure measuring holes has three pressure and temperature probes in the vertical direction.

2. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: A pipeline clamping flange is arranged on the top of the thermal insulation cold cavity, and the pipeline clamping flange is detachably connected to the top of the thermal insulation cold cavity. A heat insulation baffle is arranged between the pipeline clamping flange and the thermal insulation material; the thermal insulation material is thermal insulation cotton yarn.

3. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 2 is characterized in that: A sealing component is provided between the heat-insulating cold cavity and the nitrogen exhaust pipeline.

4. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: A packing locking thread is arranged on the lower outer wall of the nitrogen exhaust pipeline, and the nitrogen exhaust pipeline is connected to the square pressure plate through the packing locking thread.

5. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: A coal sample temperature and pressure measuring component and a heating and temperature control component are arranged at the bottom of the reaction kettle cavity, and both the coal sample temperature and pressure measuring component and the heating and temperature control component are electrically connected to the data collector.

6. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: A displacement sensor and an acoustic emission sensor are arranged on the loading rod of the triaxial testing machine; the loading rod of the triaxial testing machine is used to provide a load to the triaxial loading rod; the displacement sensor and the acoustic emission sensor are both electrically connected to the data collector.

7. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: It also includes a system base, and the three-axis reaction kettle is arranged on the system base.

8. The liquid nitrogen high pressure multi-stage pulse fracturing full process true triaxial testing system according to claim 1 is characterized in that: A low-temperature resistant sealant is arranged between the bottom of the square pressing plate and the top of the test coal sample.

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