Circulating high-low temperature liquid nitrogen and pulse water injection combined fracturing anti-reflection experimental device and use method thereof

Abstract

The invention aims to provide a circulating high and low temperature liquid nitrogen and pulse water injection combined fracturing anti-reflection experimental device and a use method thereof, and belongs to the technical field of dust prevention and control. The experimental device comprises an active water preparation system, a pulse hydraulic system, a true triaxial loading system and a circulating liquid nitrogen stamping system; the active water preparation system comprises a liquid storage tank and a water tank, the pulse hydraulic system comprises a valve module, an information collection module and a water pump, the valve module comprises a ball valve, a one-way valve, a pressure release valve, a needle valve and a high-pressure electric ball valve, and the true triaxial loading system comprises a fracturing platform, an integrated operation table and a sound conversion module. The circulating liquid nitrogen stamping system comprises a gas storage module, a pressurization module and a liquid nitrogen stamping module. The problem that the effect of a coal seam single-phase fracturing method is poor is solved, the coal seam fracturing wetting period is shortened by simulating the fracturing process of pulse active water and circulating high and low temperature liquid nitrogen under different ground stresses, and dust and risk factors in the construction process are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of dust prevention and control, and particularly relates to a cyclic high and low temperature liquid nitrogen and pulsed water injection combined fracturing and permeability enhancement experimental device and a using method thereof. Background Art

[0002] The permeability of coalbed methane reservoirs in China is generally low. At the same time, with the increase of coal seam mining depth, the problem of dust prevention and control becomes more prominent. How to effectively improve the permeability of coal seams and efficiently fracture coal bodies is the key and difficult point in the current coalbed methane mining.

[0003] Nowadays, the main methods for dust prevention and control in coal mines include ventilation dust removal, coal seam water injection, spray dust reduction, chemical dust suppression, etc. Among them, the coal seam water injection technology is the most commonly used dust suppression method in the coal industry. However, factors such as the hydrophobicity of coal bodies and the low permeability of coal seams hinder the flow and wetting of water in the pores of coal bodies, restricting the application effect of the coal seam water injection technology. Single water injection cannot meet the current industry requirements.

[0004] Nowadays, the liquid nitrogen fracturing technology, as an efficient and green waterless fracturing technology, can also effectively improve the permeability of coal seams and increase the extraction rate of coalbed methane. It has been found through research that injecting liquid nitrogen into coal seams and cooperating with active water fracturing has a good fracturing effect on coal seams and has broad application prospects, but the cycle is still long and the efficiency is low.

[0005] Aiming at the current level of coal seam dust reduction technology, it is necessary to invent a coal seam fracturing experimental device combining cyclic high and low temperature liquid nitrogen and pulsed water to solve the problem that the single fracturing method of coal seams has poor effect, and to simulate the fracturing process of pulsed active water and cyclic high and low temperature liquid nitrogen on coal bodies under different in-situ stresses, shorten the coal seam fracturing and wetting cycle, and reduce dust and dangerous factors during the construction process. Summary of the Invention

[0006] Aiming at the above problems, the invention provides a cyclic high and low temperature liquid nitrogen and pulsed water injection combined fracturing and permeability enhancement experimental device and a using method thereof. The device is convenient to operate and has a reasonable structure, solves the problem of poor effect of single coal seam fracturing method, and can simulate the fracturing process of pulsed active water and cyclic high and low temperature liquid nitrogen on coal bodies under different in-situ stresses, shortens the coal seam fracturing and wetting cycle, and reduces dust and dangerous factors during the construction process.

[0007] The principle of the invention is as follows: When the pulsed active water with periodic changes acts on the coal seam, it repeatedly deforms the coal seam, forming a fatigue damage effect, so that the fracture network of the coal body is connected more quickly and efficiently. Then, low-temperature liquid nitrogen and high-temperature nitrogen are injected in a cycle, first enhancing the water-ice phase change freezing expansion effect, and then generating a huge vaporization expansion force to generate thermally induced fractures, doubling the permeability enhancement effect.

[0008] The invention adopts the following technical solutions: A cyclic high and low temperature liquid nitrogen and pulsed water injection combined fracturing and permeability enhancement experimental device, comprising an active water preparation system, a pulsed hydraulic system, a true triaxial loading system and a cyclic liquid nitrogen stamping system; The active water preparation system includes a water tank, several liquid storage tanks are arranged above the water tank, a liquid outlet is arranged at the bottom of the water tank, a flow controller is connected to the bottom of the liquid storage tank, and the conveying pipelines of the liquid storage tanks are connected to the flow controller and then converge into a pipeline and extend into the water tank. A stirrer is arranged above the water tank, one end of the stirrer is connected with a stirring rod, and stirring blades are arranged on the stirring rod.

[0009] Further, the pulsed hydraulic system includes a valve module, a water pump and an information collection module; The valve module includes a first ball valve, a second ball valve, a third ball valve, a check valve, a pressure relief valve, a high-pressure electric ball valve, a needle valve and a stop valve; The input end of the water pump is connected to the liquid outlet of the water tank through the first ball valve, and the output end of the water pump is connected with a liquid phase pipeline through the check valve. The liquid phase pipeline includes a main pipeline, and the main pipeline is provided with a first branch, a second branch and a third branch; the first branch is connected to the water tank through a pressure transmitter and a pressure relief valve to form a first circulating water path; the second branch is connected with an accumulator through the third ball valve; the third branch is connected to the water tank through a pressure transmitter, a flowmeter and a needle valve to form a second circulating water path; a pressure transmitter and a stop valve are arranged on the main pipeline after the third branch.

[0010] A second ball valve and a flowmeter are arranged on the main pipeline between the first branch and the second branch, and a high-pressure electric ball valve and a check valve are arranged on the main pipeline between the second branch and the third branch.

[0011] The information collection module includes a computer, a data collection system and a 485 module; the high-pressure electric ball valve is connected to the computer through the 485 module, and the flowmeter and the pressure transmitter are connected to the data collection system.

[0012] Further, the true triaxial loading system includes a fracturing platform, a servo pumping station, an integrated operation console and a sound conversion module; a specimen is arranged on the fracturing platform, a steel plate is arranged outside the specimen, an oil cylinder is arranged outside the steel plate, and the servo pumping station is respectively connected to the oil cylinder through an X-axis pipeline, a Y-axis pipeline and a Z-axis pipeline; A fracturing section is arranged in the specimen, a fracturing pipe is arranged in the fracturing section, and a sealing section is arranged at the port of the fracturing section; The integrated operation console is connected to the servo pumping station through a line; The sound conversion module includes an acoustic emission probe and an acoustic signal amplifier. The acoustic emission probe is located on the surface of the specimen, the output end of the acoustic emission probe is connected to the acoustic signal amplifier, and the acoustic signal amplifier is connected to the integrated operation console.

[0013] Furthermore, the circulating liquid nitrogen stamping system includes a gas storage module, a pressurization module, and a liquid nitrogen stamping module; the gas storage module is connected to the pressurization module through a gas-phase pipeline, and the pressurization module is connected to the liquid nitrogen stamping module through a gas-phase pipeline; The gas storage module includes a gas cylinder, and a safety valve is provided at the gas outlet of the gas cylinder, and a pressure gauge is provided on the safety valve; the pressurization module includes a gas pressurization system, and a pressure gauge and a switch are provided on the gas pressurization system. The gas-phase pipeline inside the gas pressurization system is connected to a gas storage tank through a booster pump and a pressure gauge, and the outlet of the gas storage tank is connected to a pressure regulating knob. The other end of the gas-phase pipeline inside the gas pressurization system is provided with a valve; The liquid nitrogen stamping module includes a liquid nitrogen storage tank. The gas-phase pipeline of the gas pressurization system is connected to the liquid nitrogen storage tank. A piston is provided inside the liquid nitrogen storage tank. A liquid injection head is provided at the outlet of the liquid nitrogen storage tank. The liquid injection head is connected to a heat-insulating transmission pipe. The heat-insulating transmission pipe is connected to the fracturing pipe of the true triaxial loading system through a stop valve. The heat-insulating transmission pipe is connected to the data collection system through a pressure transmitter; The end of the fracturing pipe is connected to both the liquid-phase pipeline of the pulsed hydraulic system and the gas-phase pipeline of the circulating liquid nitrogen stamping system through stop valves.

[0014] Furthermore, the stirring blades are of a short-long-short structure from top to bottom.

[0015] Furthermore, the gas cylinder stores nitrogen.

[0016] Furthermore, the outer wall of the liquid nitrogen storage tank is made of super heat-insulating material.

[0017] Furthermore, a constant-pressure energy-dissipating piece is provided at the bottom of the piston of the liquid nitrogen stamping module.

[0018] Furthermore, a remote-controlled heating sheet is provided between the heat-insulating transmission pipe and the liquid injection head.

[0019] A method for using an experimental device for combined fracturing and permeability enhancement by circulating high and low temperature liquid nitrogen and pulsed water injection includes the following steps: S1. Processing of the specimen: Make a specimen with dimensions of 300 mm × 300 mm × 300 mm according to the designed size, and drill a cylinder with a diameter of 18 mm and a depth of 165 mm as the fracturing section. Place the pre-processed fracturing pipe into the fracturing section and seal the hole with epoxy resin AB glue; S2. Connection of the device: After connecting the experimental device for combined coal seam fracturing by circulating high and low temperature liquid nitrogen pressurization and pulsed water, inject clear water into the water tank; S3. Preparation of active water: According to the experimental requirements, open the flow controller to obtain a quantitative active agent stock solution, and open the mixer. After stirring evenly, an active water solution is obtained; S4. Loading of the specimen: Move the specimen to the established position on the fracturing platform, start the true triaxial loading system, set the established parameters, inject hydraulic oil into the X-axis pipeline, Y-axis pipeline, and Z-axis pipeline connected to the oil cylinder through the servo pump station respectively, and control the pressure gradient to slowly and synchronously pressurize until the rated value is reached; S5. Adjustment of pulsed water flow: Keep all valves closed before starting the pulsed hydraulic system, turn on the data collection system, start the water pump and the first branch to form a first circulating water path for the first branch, start the second branch to make the water flow form a stable constant-pressure water flow, start the valve module, input the established parameters to control the opening of the valve of the high-pressure electric ball valve, and at the same time start the third branch to make the constant-pressure water flow form pulsed water flow with specific pulse waveforms such as sine wave, rectangular wave, and sawtooth wave; S6. Start of the hydraulic fracturing experiment: Open the stop valve of the liquid phase pipeline to allow the pulsed water flow to enter the specimen through the fracturing pipe until the data of the data collection system remains stable, analyze that the crack propagation no longer changes significantly, and then close the stop valve and the water pump in sequence to stop water injection; S7. Start of the liquid nitrogen fracturing experiment: Open the safety valve in the gas cylinder to provide a gas source for the gas pressurization system, turn on the switch, let nitrogen enter the gas storage tank through the booster pump, adjust the pressure regulating knob to make the nitrogen reach the rated pressure value and input it into the liquid nitrogen stamping module, open the stop valve of the gas phase pipeline, and the high-pressure nitrogen pushes the piston to inject low-temperature liquid nitrogen into the specimen. After a predetermined time, start the remote control heating sheet, and the low-temperature liquid nitrogen evaporates to form high-temperature nitrogen and enters the specimen through the fracturing pipe. Keep repeating the operation until the data of the data collection system remains stable, analyze that the crack propagation no longer changes significantly, and then close the stop valve, safety valve, and switch in sequence to stop gas supply; S8. End of the experiment: Stop the signal acquisition of the acoustic conversion module, unload the specimen and remove it from the fracturing platform; S9. Data processing: Obtain the flow rate-time curve, pressure-time curve, and acoustic emission-time curve through the data collection system and the acoustic conversion module to analyze the crack morphology under the action of water and liquid nitrogen, the crack initiation and propagation mechanism, and the influence of different loading parameters on the crack morphology and propagation.

[0020] The beneficial effects of the present invention are as follows: The present invention provides a coal seam fracturing experimental device that combines cyclic high and low temperature liquid nitrogen with pulsed water. This device prepares active water of different types and concentrations for the experiment through an active water preparation system. The pulsed hydraulic system controls the formation of specific pulsed waveform water flows such as sine waves, rectangular waves, and sawtooth waves through a valve module, repeatedly deforming the specimen to form a fatigue damage effect for efficient fracturing. This device combines a cyclic liquid nitrogen stamping system. Nitrogen gas pressurized by a gas pressurization system pushes liquid nitrogen into the specimen. At the same time, the remote-controlled heating sheet works intermittently to provide cyclic low-temperature and high-temperature liquid nitrogen for the specimen, doubling the permeability enhancement effect. This device is equipped with a true triaxial loading system that uses a servo pump station to push an oil cylinder and act on the specimen, simulating the real coal seam geological stress conditions. It also has a sound conversion module that can monitor the situation of hydraulic and liquid nitrogen impact fracturing of the specimen and study the crack propagation law of the specimen. By setting up a data collection system, it can monitor the pressure and flow rate of the input pulsed water flow and liquid nitrogen, and explore the influence of different parameters on the fracturing specimen. This device can simulate the process of pulsed active water and high and low temperature liquid nitrogen fracturing coal samples, obtain data under different experimental conditions, and provide support for subsequent research. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic diagram of the overall structure of the experimental device of the present invention; Figure 2 is a schematic diagram of the structure of the active water preparation system of the present invention; Figure 3 is a schematic diagram of the structure of the pulsed hydraulic system of the present invention; Figure 4 is a schematic diagram of the structure of the true triaxial loading system of the present invention; Figure 5 is a schematic diagram of the structure of the cyclic liquid nitrogen stamping system of the present invention; Figure 6 is a schematic diagram of the process flow of the gas pressurization system of the present invention; Wherein: A - Active water preparation system; B - Pulse hydraulic system; C - True triaxial loading system; D - Circulating liquid nitrogen stamping system; 1 - Liquid storage tank; 2 - Flow controller; 3 - Mixer; 4 - Stirring blade; 5 - Water tank; 6 - 1 - First ball valve; 6 - 2 - Second ball valve; 6 - 3 - Third ball valve; 7 - Water pump; 8 - Check valve; 9 - Pressure relief valve; 10 - Flowmeter; 11 - Pressure transmitter; 12 - Accumulator; 13 - High - voltage electric ball valve; 14 - Needle valve; 15 - Computer; 16 - Data collection system; 17 - 485 module; 18 - Globe valve; 19 - Liquid phase pipeline; 20 - X - axis pipeline; 21 - Z - axis pipeline; 22 - Steel plate; 23 - Oil cylinder; 24 - Fracturing section; 25 - Fracturing pipe; 26 - Sealing section; 27 - Servo pump station; 28 - Integrated operation console; 29 - Acoustic emission probe; 30 - Acoustic signal amplifier; 31 - Gas cylinder; 32 - Safety valve; 33 - Pressure gauge; 34 - Gas phase pipeline; 35 - Gas boosting system; 36 - Switch; 37 - Pressure regulating knob; 38 - Injection head; 39 - Liquid nitrogen storage tank; 40 - Piston; 41 - Constant - pressure energy - discharging piece; 42 - Adiabatic transmission pipe; 43 - Booster pump; 44 - Pressure gauge; 45 - Gas storage tank; 46 - Valve; 47 - Remote control heating sheet. Detailed implementation manners

[0022] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention.

[0023] It should be noted that the accompanying drawings in this article are only for illustrative purposes and not physical drawings. To better illustrate the embodiments of the present invention, some components in the drawings will be enlarged, reduced or omitted, but it does not represent the size of the actual product.

[0024] The present invention will be further described in detail below with reference to the accompanying drawings.

[0025] As shown in the figure, a coal seam fracturing experiment device combining circulating high - low temperature liquid nitrogen and pulsed water includes an active water preparation system A, a pulse hydraulic system B, a true triaxial loading system C, and a circulating liquid nitrogen stamping system D. Wherein: The active water preparation system A prepares active water of different types and concentrations required for the experiment. The active water preparation system includes a liquid storage tank 1, a flow controller 2, a mixer 3, a stirring blade 4, and a water tank 5; The pulsed hydraulic system B provides pulsed water flow for fracturing specimens. The pulsed hydraulic system B includes a valve module, a water circuit module, an information integration module, and a water pump 7. The valve module includes a first ball valve 6-1, a second ball valve 6-2, a third ball valve 6-3, a check valve 8, a pressure relief valve 9, a high-pressure electric ball valve 13, a needle valve 14, and a stop valve 18. By controlling the valve opening, a constant-pressure water flow is formed into a pulsed water flow with a specific waveform. The water circuit module includes a liquid-phase pipeline 19, a flowmeter 10, a pressure transmitter 11, and an accumulator 12. The information integration module includes a computer 15, a 485 module 17, and a data collection system 16 to monitor the pressure and flow conditions in the pipeline. The input end of the water pump 7 is connected to the active water preparation system A. The liquid-phase pipeline 19 includes the main pipeline and a first branch, a second branch, and a third branch led out from the main pipeline. The output end of the water pump 7 is connected to the main pipeline. The pressure relief valve 9 is arranged on the first branch. The accumulator 12 is arranged on the second branch. The needle valve 14 is arranged on the third branch. The high-pressure electric ball valve 13 is arranged on the main pipeline and is located between the second branch and the third branch. The first branch is connected to the active water preparation system A to form a first circulating water circuit. The second branch is used to store energy to make the main pipeline form a constant-pressure water flow. The third branch is connected to the active water preparation system A to form a second circulating water circuit. The high-pressure electric ball valve 13 is connected to the information integration module; The true triaxial loading system C is used to apply different axial pressures and confining pressures to specimens to simulate different in-situ stress conditions of coal seams. The true triaxial loading system C includes a fracturing platform, an integrated operation console 28, and an acoustic conversion module. The fracturing platform includes an X-axis pipeline 20, a Z-axis pipeline 21, a steel plate 22, an oil cylinder 23, a fracturing section 24, a fracturing pipe 25, a sealing section 26, and a servo pump station 27. The acoustic conversion module includes an acoustic emission probe 29 and an acoustic signal amplifier 30. The X-axis pipeline 20 and the Z-axis pipeline 21 are connected to the oil cylinder 23. The integrated operation console 28 is connected to the servo pump station 27 through a line to control the servo pump station 27 to drive a plurality of the oil cylinders 23 to act on the steel plate 22 to complete operations such as automatic pressurization, pressure relief, and air defense. The fracturing pipe 25 is arranged in the specimen and is connected to the liquid-phase pipeline 19 and the gas-phase pipeline 34 through the stop valve 18 at the same time. The acoustic emission probe 29 is connected to the steel plate 22 and is connected to the integrated operation console 28 through the acoustic signal amplifier 30 to monitor the fracturing specimen conditions; The cyclic liquid nitrogen stamping system D provides low-temperature and high-temperature cyclic pressure for the fracturing specimen. The cyclic liquid nitrogen stamping system D includes a gas storage module, a boosting module, and a liquid nitrogen stamping module. The gas storage module includes a gas cylinder 31, a safety valve 32, and a pressure gauge 33. The boosting module mainly includes a gas boosting system 35. The gas boosting system 35 includes a pressure gauge 33, a switch 36, a pressure regulating knob 37, a boosting pump 43, a pressure measuring gauge 44, a gas storage tank 45, and a valve 46. The liquid nitrogen stamping module includes a liquid injection head 38, a liquid nitrogen storage tank 39, a piston 40, a constant pressure energy-discharging piece 41, an adiabatic transmission pipe 42, and a remote control heating piece 47. The gas boosting system 35 is connected to the gas storage module and the liquid nitrogen stamping module through the gas phase pipeline 34. Nitrogen is stored in the gas cylinder 31; Specifically, as Figure 2 shown, the active water preparation system A includes multiple liquid storage tanks 1 and flow controllers 2, and can configure active water of different types and concentrations according to experimental requirements. The delivery pipelines of the liquid storage tanks 1 are connected to the flow controllers 2 and then converge into a single pipeline that extends under the water tank 5, facilitating thorough mixing. Moreover, the stirring blades 4 have a short-long-short structure from top to bottom, and are more likely to form a pointed vortex during stirring, enabling more thorough mixing of the active water.

[0026] Specifically, a first ball valve 6-1 is provided on the pipeline between the water tank 5 and the water pump 7, and a check valve 8 is provided between the output end of the water pump 7 and the first branch, such that the active water can only flow out along the water pump 7, preventing the active water from flowing back into the water pump 7. A pressure transmitter 11 and a pressure relief valve 9 are provided on the first branch, and the output end of the first branch is connected to the water tank 5, thereby forming a first circulating water path, enabling the water pump 7 to operate for a long time. A second ball valve 6-2 and a flowmeter 10 are provided on the main pipeline between the first branch and the second branch to control and measure the flow rate of the main pipeline. A third ball valve 6-3 and an accumulator 12 are successively provided on the second branch, thereby regulating the pressure in the pipeline to form a constant-pressure water flow in the main pipeline. A high-pressure electric ball valve 13 and a check valve 8 are provided on the main pipeline between the second branch and the third branch. The high-pressure electric ball valve 13 is connected to the information collection module through a line to control the change in its valve opening, such that the constant-pressure water flow forms a pulsed water flow with a specific waveform. After the check valve 8, a third branch is led out from the main pipeline and connected to the water tank 5 to form a second circulating water path. A pressure transmitter 11, a second flowmeter 10, and a needle valve 14 are successively provided on the third branch. The needle valve 14 is used to finely adjust the formation of the waveform of the auxiliary water path. A pressure transmitter 11 and a stop valve 18 are provided on the main pipeline after the third branch to continuously monitor the input water pressure and control the on-off of the water path with the specimen through the stop valve 18. The formed pulsed water flow enters the interior of the specimen through the fracturing pipe 25.

[0027] Specifically, the true triaxial loading system C controls the servo pump station 27 through the integrated operation console 28 to drive multiple oil cylinders 23 to act on the steel plate 22 to complete operations such as automatic pressurization, pressure relief, and air defense, simulating the coal body under in-situ stress. The acoustic emission probe 29 is arranged on the surface of the specimen, and its output end is connected to the acoustic signal amplifier 30 to obtain and amplify the original acoustic signal of the specimen. The integrated operation console 28 is connected to the acoustic signal amplifier 30 to convert and process the acoustic signal, which is more convenient for monitoring the crack propagation law of the specimen.

[0028] Specifically, nitrogen is stored in the gas cylinder 31. The gas pressurization system 35 can pressurize the input nitrogen through the booster pump 43 and perform secondary pressure adjustment by rotating the pressure regulating knob 37, making it more convenient and intuitive to obtain the required gas pressure for the experiment. The outer wall of the liquid nitrogen storage tank 39 is made of super heat-insulating material, and the adiabatic transmission pipe is connected to the true triaxial loading system C and the circulating liquid nitrogen stamping system D to prevent the loss of temperature and gas pressure of high and low temperature liquid nitrogen during the stamping of the specimen. A constant pressure energy release sheet 41 is arranged below the liquid nitrogen stamping module and is opened when the gas pressure in the liquid nitrogen storage tank 39 is too high to reduce the safety risk. A remote control heating sheet 47 is arranged between the adiabatic transmission pipe 42 and the liquid injection head 38 to ensure the cyclic injection of low and high temperature liquid nitrogen. The dual action of the water-ice phase change freeze-thaw effect and the vaporization expansion force increases the fatigue damage of the coal body and accelerates the permeability enhancement effect.

[0029] A method for using a coal seam fracturing experimental device combining circulating high and low temperature liquid nitrogen and pulsed water includes the following steps: S1. Processing of the specimen: A specimen of 300mm×300mm×300mm is made according to the designed dimensions, and a cylinder with a diameter of 18mm and a depth of 165mm is drilled as the fracturing section 24. The pre-processed fracturing pipe 25 is placed into the fracturing section 24 and sealed with epoxy resin AB glue. S2. Connection of the device: After connecting the coal seam fracturing experimental device combining circulating high and low temperature liquid nitrogen pressurization and pulsed water, clean water is injected into the water tank 5. S3. Preparation of active water: According to the experimental requirements, the flow controller 2 is opened to obtain a quantitative active agent stock solution, and the stirrer 3 is opened to stir evenly to obtain an active water solution. S4. Loading of the specimen: The specimen is moved to the established position in the fracturing platform, the true triaxial loading system C is started, the established parameters are set, and hydraulic oil is injected into the X-axis pipeline 20, Y-axis pipeline, and Z-axis pipeline 21 connected to the multiple oil cylinders 23 respectively through the servo pump station 27, and the pressure gradient is controlled to slowly and synchronously pressurize until the rated value is reached. S5. Regulation of pulsed water flow: Keep all valves closed before starting the pulsed hydraulic system B. Turn on the data collection system 16, start the water pump 7 and the first branch to form a first circulating water path in the first branch. Start the second branch to make the water flow form a stable constant-pressure water flow. Start the valve module, input the established parameters to control the opening degree of the valve of the high-pressure electric ball valve 13. At the same time, start the third branch to make the constant-pressure water flow form specific pulsed waveform water flows such as sine waves, rectangular waves, and sawtooth waves; S6. Start the hydraulic fracturing experiment: Open the stop valve 18 of the liquid phase pipeline 19 to make the pulsed water flow enter the specimen through the fracturing pipe 25 until the data of the data collection system 16 remains stable. Analyze that the crack propagation no longer changes significantly. Then close the stop valve 18 and the water pump 7 in sequence to stop water injection; S7. Start the liquid nitrogen fracturing experiment: Open the safety valve 32 in the gas cylinder 31 to provide a gas source for the gas boosting system 35. Turn on the switch 36, and nitrogen enters the gas storage tank 45 through the boosting pump 43. Adjust the pressure regulating knob 37 to make the nitrogen reach the rated pressure value and input it into the liquid nitrogen stamping module. Open the stop valve 18 of the gas phase pipeline 34, and the high-pressure nitrogen pushes the piston 40 to inject low-temperature liquid nitrogen into the specimen. After a predetermined time, start the remote control heating sheet 47, and the low-temperature liquid nitrogen evaporates to form high-temperature nitrogen, which enters the specimen through the fracturing pipe 25. Continuously repeat the operation until the data of the data collection system remains stable. Analyze that the crack propagation no longer changes significantly. Then close the stop valve 18, the safety valve 32, and the switch 36 in sequence to stop gas supply; S8. End of the experiment: Stop the signal acquisition of the acoustic conversion module, unload the specimen and remove it from the fracturing platform; S9. Data processing: Obtain the flow time curve, pressure time curve, and acoustic emission time curve through the data collection system 16 and the acoustic conversion module to analyze the crack morphology, crack initiation and propagation mechanism under the action of water and liquid nitrogen, and the influence of different loading parameters on the crack morphology and propagation.

[0030] It should be noted that, in this document, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the said element.

[0031] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments described herein, but rather will conform to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A combined fracturing and permeability enhancement experimental device for circulating high and low temperature liquid nitrogen and pulse water injection, characterized in that: It includes active water preparation system (A), pulse hydraulic system (B), true triaxial loading system (C) and circulating liquid nitrogen punching system (D); The active water preparation system (A) comprises a water tank (5), a plurality of liquid storage tanks (1) are arranged above the water tank (5), a liquid outlet is arranged at the bottom of the water tank (5), a flow controller (2) is connected to the bottom of the liquid storage tank (1), a delivery pipeline of the liquid storage tank (1) is connected to the flow controller (2) and then merged into a pipeline extending into the water tank (5), a stirrer (3) is arranged above the water tank (5), one end of the stirrer (3) is connected to a stirring rod, and a stirring blade (4) is arranged on the stirring rod.

2. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 1 is characterized by: The pulse hydraulic system (B) comprises a valve module, a water pump (7) and an information collection module; The valve module comprises a first ball valve (6-1), a second ball valve (6-2), a third ball valve (6-3), a one-way valve (8), a pressure relief valve (9), a high-pressure electric ball valve (13), a needle valve (14) and a stop valve (18); The input end of the water pump (7) is connected to the liquid outlet of the water tank (5) via a first ball valve (6-1); the output end of the water pump (7) is connected to a liquid phase pipeline (19) via a one-way valve (8); the liquid phase pipeline (19) comprises a main pipeline, and the main pipeline is provided with a first branch, a second branch and a third branch; the first branch is connected to the water tank (5) via a pressure transmitter (11) and a pressure relief valve (9) to form a first circulating water circuit; the second branch is connected to an accumulator (12) via a third ball valve (6-3); the third branch is connected to the water tank (5) via a pressure transmitter (11), a flow meter (10) and a needle valve (14) to form a second circulating water circuit; the main pipeline after the third branch is provided with a pressure transmitter (11) and a stop valve (18); A second ball valve (6-2) and a flow meter (10) are provided on the main pipe between the first branch and the second branch, and a high-pressure electric ball valve (13) and a one-way valve (8) are provided on the main pipe between the second branch and the third branch.

3. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 2 is characterized by: The information collection module comprises a computer (15), a data collection system (16) and a 485 module (17); the high-pressure electric ball valve (13) is connected to the computer (15) via the 485 module (17), and the flow meter (10) and the pressure transmitter (11) are connected to the data collection system (16).

4. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 1 is characterized by: The true triaxial loading system (C) comprises a fracturing platform, a servo pump station (27), an integrated operating table (28) and a sound conversion module; a sample is provided on the fracturing platform, a steel plate (22) is provided on the outside of the sample, an oil cylinder (23) is provided on the outside of the steel plate (22), and the servo pump station (27) is connected to the oil cylinder (23) via an X-axis pipeline (20), a Y-axis pipeline and a Z-axis pipeline (21) respectively; The sample is provided with a fracturing section (24), a fracturing tube (25) is provided in the fracturing section (24), and a sealing section (26) is provided at the end of the fracturing section (24); The integrated operating console (28) is connected to the servo pump station (27) via a line; The acoustic conversion module comprises an acoustic emission probe (29) and an acoustic signal amplifier (30); the acoustic emission probe (29) is located on the surface of the sample; the output end of the acoustic emission probe (29) is connected to the acoustic signal amplifier (30); and the acoustic signal amplifier (30) is connected to the integrated operating table (28).

5. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 4 is characterized by: The circulating liquid nitrogen stamping system (D) comprises a gas storage module, a pressure boosting module and a liquid nitrogen stamping module; the gas storage module is connected to the pressure boosting module via a gas phase pipeline (34), and the pressure boosting module is connected to the liquid nitrogen stamping module via a gas phase pipeline (34); The gas storage module comprises a gas cylinder (31), a gas outlet of the gas cylinder (31) is provided with a safety valve (32), and the safety valve (32) is provided with a pressure gauge (33); the booster module comprises a gas booster system (35), the gas booster system (35) is provided with a pressure gauge (33) and a switch (36), a gas phase pipeline inside the gas booster system is connected to the gas storage tank (45) via a booster pump (43) and a pressure gauge (44), the outlet of the gas storage tank (45) is connected to a pressure regulating knob (37), and the other end of the gas phase pipeline (34) inside the gas booster system is provided with a valve (46); The liquid nitrogen stamping module comprises a liquid nitrogen storage tank (39), a gas phase pipeline (34) of a gas pressurizing system is connected to the liquid nitrogen storage tank (39), a piston (40) is provided in the liquid nitrogen storage tank (39), an injection head (38) is provided at the outlet of the liquid nitrogen storage tank (39), the injection head (38) is connected to an insulated transmission pipe (42), the insulated transmission pipe (42) is connected to a fracturing pipe (25) of a true triaxial loading system (D) via a stop valve (18), and the insulated transmission pipe (42) is connected to a data collection system (16) via a pressure transmitter (11); The end of the fracturing pipe (25) is connected to the liquid phase pipeline (19) of the pulse hydraulic system (B) and the gas phase pipeline (34) of the circulating liquid nitrogen punching system (D) via a stop valve (18).

6. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 1 is characterized by: The stirring blade (4) has a short-long-short structure from top to bottom.

7. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 5 is characterized by: Nitrogen is stored in the gas cylinder (31); and the outer wall of the liquid nitrogen storage tank (39) is made of a super-strong heat-insulating material.

8. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 5 is characterized by: A constant pressure energy release plate (41) is provided at the bottom of the piston (40) of the liquid nitrogen stamping module.

9. The combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection according to claim 5 is characterized by: A remote control heating plate (47) is provided between the heat-insulating transmission pipe (42) and the liquid injection head (38).

10. A method for using the combined fracturing and permeability enhancement experimental device of circulating high and low temperature liquid nitrogen and pulse water injection as claimed in any one of claims 1 to 9, characterized in that: The steps include: S1. Processing of the sample: a sample of 300 mm×300 mm×300 mm is prepared according to the designed size, and a cylinder with a diameter of 18 mm and a depth of 165 mm is drilled as a fracturing section (24). A pre-processed fracturing tube (25) is placed in the fracturing section (24) and the hole is sealed with epoxy resin AB glue; S2. Connecting the device: After connecting the coal seam fracturing experimental device combining circulating high and low temperature liquid nitrogen pressurization and pulse water, inject clean water into the water tank (5); S3, preparation of active water, according to the experimental requirements, open the flow controller (2), obtain a quantitative active agent stock solution, turn on the stirrer (3), and stir evenly to obtain an active aqueous solution; S4, loading the sample, moving the sample to a predetermined position in the fracturing platform, starting the true triaxial loading system (C), setting predetermined parameters, and injecting hydraulic oil into the X-axis pipeline (20), the Y-axis pipeline and the Z-axis pipeline (21) connected to the oil cylinder (23) through the servo pump station (27), respectively, and controlling the pressure gradient to slowly and synchronously increase the pressure until the rated value is reached; S5, regulating the pulse water flow, keeping all valves closed before starting the pulse hydraulic system (B), starting the data collection system (16), starting the water pump (7) and the first branch, so that the first branch forms a first circulating water path, starting the second branch, so that the water flow forms a stable constant pressure water flow, starting the valve module, inputting predetermined parameters to control the valve opening of the high-pressure electric ball valve (13), and simultaneously starting the third branch, so that the constant pressure water flow forms a specific pulse waveform water flow of a sine wave, a rectangular wave and a sawtooth wave; S6, the hydraulic fracturing experiment begins, the stop valve (18) of the liquid phase pipeline (19) is opened, and the pulsed water flow enters the sample through the fracturing pipe (25), until the data of the data collection system (16) remains stable and the crack extension analysis shows no obvious change, the stop valve (18) and the water pump (7) are closed in sequence, and the water injection is stopped; S7, the liquid nitrogen fracturing experiment begins, the safety valve (32) in the gas cylinder (31) is opened to provide a gas source for the gas boosting system (35), the switch (36) is turned on, the nitrogen enters the gas storage tank (45) through the boosting pump (43), the pressure regulating knob (37) is adjusted to make the nitrogen reach the rated pressure value and input into the liquid nitrogen stamping module, the stop valve (18) of the gas phase pipeline (34) is opened, the high-pressure nitrogen pushes the piston (10), and the low-temperature liquid nitrogen is injected into the sample. After a predetermined time, the remote control heating plate (47) is started, the low-temperature liquid nitrogen evaporates to form high-temperature nitrogen and enters the sample through the fracturing pipe (25), and the operation is repeated until the data of the data collection system remains stable and the crack extension analysis no longer changes significantly, and the stop valve (18), the safety valve (32) and the switch (36) are closed in turn to stop the gas supply; S8, the experiment is over, the acoustic conversion module is stopped from collecting signals, the sample is unloaded and removed from the fracturing platform; S9. Data processing: derive flow time curve, pressure time curve and acoustic emission time curve through the data collection system (16) and the acoustic conversion module to analyze the crack morphology under the action of hydraulic pressure and liquid nitrogen, the crack initiation and expansion mechanism and the influence of different loading parameters on the crack morphology and expansion.

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