Experimental system and method of cavitation cracking of coal and rock under ultrasonic vibration in high pressure water environment

Abstract

The present invention proposes an ultrasonic vibration cavitation cracking experimental system and experimental method for coal rock under high-pressure water environment. The experimental system includes an ultrasonic excitation tank with a cover plate on the top for placing coal samples, an ultrasonic excitation system, a loading system, a water injection system, a gas injection and exhaust system, and a surface crack monitoring system; the loading system applies stress to the coal sample in the ultrasonic excitation tank, the water injection system injects high-pressure water into the ultrasonic excitation tank to place the coal sample in a high-pressure water environment, the ultrasonic excitation system generates ultrasonic waves to crack the coal sample in the ultrasonic excitation tank, and the surface crack monitoring system monitors the surface cracks of the coal sample. The present invention can crack the coal sample under ultrasound or water pressure ultrasound, and after the coal sample is ultrasonically cracked, an adsorption and desorption experiment is performed to study the effects of ultrasound or water pressure ultrasound on the crack extension and adsorption of the coal sample. The present invention is a cracking experimental system integrating stress loading, high-pressure water ultrasonic vibration, and ultrasonic cracking adsorption and desorption, and has strong practicality.

Description

Technical Field

[0001] The invention belongs to the technical field of coal seam fracturing, and in particular relates to an experimental system and an experimental method for cavitation fracturing of coal rocks under load of ultrasonic vibration in a high-pressure water environment. Background Art

[0002] With the continuous development of permeability enhancement technology, the crack expansion and crack extension direction of coal bodies during the permeability enhancement process have been studied. For example, CN202222009036.9 discloses an experimental device for simulating the evolution of cracks on the surface of gas extraction boreholes and their monitoring. It lays simulated coal seams and rock formations in the experimental simulation box and introduces gas, and applies pressure loading on top to simulate the environment of gas extraction boreholes under different conditions. The aforementioned patent is a conventional uniaxial loading method, which cannot simulate the evolution of cracks on the coal surface during the ultrasonic permeability enhancement and fracturing process of coal samples. Summary of the invention

[0003] The present invention aims to solve the technical problems existing in the prior art. The first purpose of the present invention is to provide a high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system. The second purpose of the present invention is to provide an experimental method based on the aforementioned high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system.

[0004] To achieve the above-mentioned first purpose, the present invention adopts the following technical scheme: a high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system, comprising an ultrasonic excitation trough with a cover plate on the top for placing coal samples, an ultrasonic excitation system, a loading system, a water injection system, a gas injection and exhaust system, and a surface crack monitoring system; the loading system applies stress to the coal sample in the ultrasonic excitation trough, the water injection system injects high-pressure water into the ultrasonic excitation trough to place the coal sample in a high-pressure water environment, the ultrasonic excitation system generates ultrasonic waves to crack the coal sample in the ultrasonic excitation trough, the surface crack monitoring system monitors the surface cracks of the coal sample, and the gas injection and exhaust system injects and exhausts gas into the ultrasonic excitation trough to allow the coal sample to undergo adsorption and desorption tests.

[0005] In the above technical scheme, the coal sample is fixed by a loading system and simulates ground stress, the coal sample is ultrasonically fractured or ultrasonically fractured in a high-pressure water environment, and the surface cracks or internal cracks of the coal sample and the direction and extent of crack expansion are monitored by a surface crack monitoring system to obtain information on surface deformation and crack expansion of the coal sample. After the coal sample is ultrasonically fractured, an adsorption and desorption experiment is performed to study the effects of ultrasound or ultrasound with water pressure on crack expansion and adsorption of the coal sample. The present invention is a fracture experiment system that integrates stress loading, high-pressure water ultrasonic vibration, and ultrasonic fracture adsorption and desorption, and has strong practicality.

[0006] In a preferred embodiment of the present invention, the ultrasonic excitation system includes an ultrasonic generator and a plurality of ultrasonic transducers installed on the side wall of the ultrasonic excitation tank and connected to the ultrasonic generator via ultrasonic cables. The ultrasonic frequencies emitted by the plurality of ultrasonic transducers are the same or different.

[0007] The above technical solution installs several ultrasonic transducers on the side wall of the ultrasonic excitation tank, which has good ultrasonic effect. Moreover, the ultrasonic frequencies of several ultrasonic transducers are the same or different. High-pressure water multi-frequency ultrasonic vibration can be realized according to the situation to observe the crack conditions of coal samples.

[0008] In a preferred embodiment of the present invention, a plurality of ultrasonic transducers are arranged on the side wall of the ultrasonic excitation groove at intervals in the height direction and in the circumferential direction; and / or the side wall of the ultrasonic excitation groove is a double-layer structure with a cavity, the cavity is filled with water, and the ultrasonic transmitting end of the ultrasonic transducer is located in the cavity.

[0009] The above technical solution makes ultrasound more uniform and has a better fracturing effect; the ultrasonic transducer directly contacts water to generate ultrasound, which is beneficial to the propagation of ultrasound, causing the water molecules in the ultrasonic excitation tank to vibrate and produce a focusing effect, which is beneficial to the fracturing of coal samples.

[0010] In a preferred embodiment of the present invention, the loading system includes a hydraulic cylinder, a hydraulic rod of the hydraulic cylinder passes through a cover plate and extends downward into the ultrasonic excitation groove, and the lower end of the hydraulic rod is fixedly connected to a pressure plate located above the coal sample and capable of covering the upper surface of the coal sample.

[0011] According to the above technical solution, the loading load of the hydraulic cylinder is large, the reliability is high, and the economy is better.

[0012] In another preferred embodiment of the present invention, the cover plate is detachably fixed to the top wall of the ultrasonic excitation groove, the cover plate is sleeved outside the hydraulic rod and sealedly connected to the outer wall of the hydraulic rod, and the cover plate is a detachable structure composed of at least two sub-plates; or the cover plate is fixed to the hydraulic rod, and the outer wall of the cover plate is vertically slidably connected and sealed to the inner wall of the ultrasonic excitation groove.

[0013] The above technical solution provides two implementation methods for connecting the cover plate to the ultrasonic excitation tank and the hydraulic rod, which facilitates the removal of the cover plate from the ultrasonic excitation tank.

[0014] In another preferred embodiment of the present invention, the water injection system includes a first water injection pipe connected to the ultrasonic excitation tank and communicating with the interior of the ultrasonic excitation tank, the inlet of the first water injection pipe is connected to a pressurized water source, and the first water injection pipe is provided with a first pressure regulating valve for regulating pressure.

[0015] In the above technical solution, the first water injection pipe is used to inject high-pressure water into the ultrasonic excitation tank, and by controlling the first pressure regulating valve to adjust the pressure, a certain water pressure can be applied to the environment where the coal sample is located.

[0016] In another preferred embodiment of the present invention, a water outlet pipe communicating with the interior of the ultrasonic excitation tank is also installed at the bottom of the side wall of the ultrasonic excitation tank, and a water outlet valve is provided on the water outlet pipe.

[0017] The above technical solution, by providing a water outlet pipe and a water outlet valve, facilitates the discharge of water in the ultrasonic excitation tank after the ultrasonic cracking coal sample experiment.

[0018] In another preferred embodiment of the present invention, the gas injection and exhaust system includes an injection pipe and an outlet pipe connected to the ultrasonic excitation tank and communicating with the interior thereof, the air inlet of the air injection pipe is connected to the air storage tank, and a reference tank connected to the air inlet of the air injection pipe is also connected downstream of the air storage tank, an air regulating valve is provided on the air injection pipe, the outlet pipe can be connected to a vacuum pump, and an outlet valve is provided on the outlet pipe; the ultrasonic excitation tank is also connected to a pressure sensor for detecting the air pressure therein, and the signal output end of the pressure sensor is connected to a pressure gauge.

[0019] In the above technical scheme, the gas storage tank is the gas source for providing gas, the reference tank plays the role of stabilizing the pressure, the gas injection pipe is used to inject gas into the ultrasonic excitation tank, and the gas regulating valve adjusts the pressure, so as to apply a certain air pressure to the environment in which the coal sample is located, which is beneficial for the adsorption test of the coal sample. The pressure sensor is used to detect the air pressure in the ultrasonic excitation tank; the air outlet pipe is used to evacuate the ultrasonic excitation tank and discharge the gas in the ultrasonic excitation tank for analytical testing.

[0020] In another preferred embodiment of the present invention, artificial speckles are sprayed on the surface of the coal sample, a visualization window is provided on the side wall of the ultrasonic excitation groove in an area corresponding to the artificial speckles, and the surface crack monitoring system includes a lighting device for illuminating the artificial speckles and a camera device for photographing the artificial speckles.

[0021] In the above technical solution, artificial speckle cooperates with the visualization window to form DIC visualization speckle, and the lighting device illuminates the artificial speckle, which makes it easier for the camera device to monitor the surface cracks of the coal sample.

[0022] To achieve the second purpose, the present invention adopts the following technical solution: an experimental method of a high-pressure water environment coal rock loaded ultrasonic vibration cavitation cracking experimental system comprises the following steps:

[0023] S1, controlling the loading system to apply stress to the coal sample according to the characteristics of the coal sample to simulate the ground stress;

[0024] S2, close the cover;

[0025] S3, starting the ultrasonic excitation system, cracking the coal sample by ultrasonic vibration, and the surface crack monitoring system monitors the information of the surface deformation and crack extension of the coal sample; or before the ultrasonic vibration, injecting high-pressure water into the ultrasonic excitation tank through the water injection system to put the coal sample in a high-pressure water environment, and cracking the coal sample by ultrasonic vibration cavitation;

[0026] S4, after the coal sample is cracked, the water in the ultrasonic excitation tank is drained;

[0027] S5, evacuating the ultrasonic excitation tank;

[0028] S6, injecting gas into the ultrasonic excitation tank through the gas injection and exhaust system to reach the set pressure, and the coal sample is subjected to adsorption experiment;

[0029] S7, increasing the gas pressure in the ultrasonic excitation tank, and performing a step adsorption test until the gas pressure in the ultrasonic excitation tank reaches the highest adsorption pressure, and then exhausting the gas in the ultrasonic excitation tank through the gas injection and exhaust system, and performing a desorption experiment on the coal sample;

[0030] S8, obtain the amount of gas adsorbed by the coal sample using the following formula:

[0031]

[0032] Among them, Q n Q is the cumulative amount of gas adsorbed by the coal sample after the nth gas injection; n-1 V is the cumulative amount of gas adsorbed by the coal sample after the n-1th gas filling; m is the gas molar volume; T is the experimental system temperature; R is the gas constant; m is the coal sample mass; V c V is the volume of the reference tank connected in series to the air inlet of the ultrasonic excitation tank; f is the volume of the ultrasonic excitation tank; p n0 The initial pressure of the reference tank of the gas injection and exhaust system; n0 is the adsorption pressure p n0 and gas compressibility factor under temperature T; p n-1 is the adsorption equilibrium pressure after the n-1th gas filling; z n-1 is the adsorption pressure p n-1 and temperature T under the conditions of CH4 compression coefficient; p n is the adsorption equilibrium pressure; n is the adsorption pressure p n and gas compressibility factor under temperature T conditions.

[0033] The present invention can be used to study the effects of ultrasound or ultrasound with water pressure on crack expansion and adsorption of coal samples, and can also obtain the amount of gas adsorbed by the coal samples by calculation, and more intuitively obtain the effects of ultrasound or ultrasound with water pressure on the adsorption of coal samples.

[0034] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0036] Figure 1 Schematic diagram of the structure of the experimental system of the embodiment.

[0037] Figure 2 yes Figure 1 Schematic diagram of the structure after omitting the surface crack monitoring system.

[0038] The figure marks in the drawings of the specification include: ultrasonic excitation tank 10, cover plate 11, visualization window 12, cavity 13, ultrasonic excitation system 20, ultrasonic generator 21, ultrasonic cable 22, ultrasonic transducer 23, loading system 30, hydraulic cylinder 31, hydraulic rod 32, pressure plate 33, water injection system 40, first water injection pipe 41, first pressure regulating valve 42, second water injection pipe 43, second pressure regulating valve 44, water outlet pipe 45, water outlet valve 46, gas injection and exhaust system 50, gas injection pipe 51, gas regulating valve 52, gas outlet pipe 53, gas outlet valve 54, pressure sensor 55, pressure gauge 56, gas storage tank 57, reference tank 58, lighting device 61, camera device 62, coal sample A, artificial speckle B. DETAILED DESCRIPTION

[0039] Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0040] In the description of the present invention, it is to be understood that the terms “longitudinal”, “lateral”, “vertical”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside” and “outside” etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0041] In the description of the present invention, unless otherwise specified and limited, it should be noted that the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal connection between two components. It can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances.

[0042] Embodiment 1

[0043] This embodiment provides a high-pressure water environment coal rock loaded ultrasonic vibration cavitation cracking experimental system, such as Figure 1 and Figure 2 As shown, in a preferred embodiment, the experimental system includes an ultrasonic excitation tank 10 with a cover plate 11 on the top for placing the coal sample A, an ultrasonic excitation system 20, a loading system 30, a water injection system 40, a gas injection and exhaust system 50, and a surface crack monitoring system. The coal sample A is placed in the ultrasonic excitation tank 10. The inner diameter of the ultrasonic excitation tank 10 is slightly larger than the outer diameter of the coal sample A, and the coal sample A can be fixed, and can also be used to fill high-pressure water. The loading system 30 applies stress to the coal sample A in the ultrasonic excitation tank 10 to simulate the ground stress of the coal sample A. The water injection system 40 injects high-pressure water into the ultrasonic excitation tank 10 to place the coal sample A in a high-pressure water environment. The ultrasonic excitation system 20 generates ultrasonic waves to crack the coal sample A in the ultrasonic excitation tank 10. The surface crack monitoring system monitors the surface cracks of the coal sample A. The gas injection and exhaust system 50 injects and exhausts gas into the ultrasonic excitation tank 10 to allow the coal sample A to undergo an adsorption and desorption test.

[0044] In the present invention, the loading system 30 includes a hydraulic cylinder 31, a hydraulic rod 32 of the hydraulic cylinder 31 passes through the cover plate 11 and extends downward into the ultrasonic excitation tank 10, and the lower end of the hydraulic rod 32 is fixedly connected to a pressure plate 33 located above the coal sample A and capable of covering the upper surface of the coal sample A, that is, the diameter of the pressure plate 33 is not less than the diameter of the coal sample A, thereby ensuring that the pressure plate 33 can apply pressure to all surfaces on the top of the coal sample A.

[0045] In the present invention, the water injection system 40 includes a first water injection pipe 41 connected to the cover plate 11 of the ultrasonic excitation tank 40 and communicating with the interior of the ultrasonic excitation tank 10. The first water injection pipe 41 is a high-pressure water pipe. The inlet of the first water injection pipe 41 is connected to a pressurized water source. The pressurized water source is high-pressure water delivered by a water pump or high-pressure water obtained in other forms. The first water injection pipe 41 is provided with a first pressure regulating valve 42 for regulating the pressure to ensure the pressure of the water in the ultrasonic excitation tank 10 (after pressurization by the water pump, the pressure is maintained by the first pressure regulating valve 42). Preferably, a water outlet pipe 45 communicating with the interior of the ultrasonic excitation tank 10 is also installed at the bottom of the side wall of the ultrasonic excitation tank 10. A water outlet valve 46 is provided on the water outlet pipe 45. After the experiment is over, the water outlet valve 46 is opened to discharge the water in the ultrasonic excitation tank 10 through the water outlet pipe 45.

[0046] In the present invention, the ultrasonic excitation system 20 includes an ultrasonic generator 21 arranged outside the ultrasonic excitation groove 10 and a plurality of ultrasonic transducers 23 installed on the side wall of the ultrasonic excitation groove 10 and connected to the ultrasonic generator 21 through an ultrasonic cable 22. The ultrasonic frequencies emitted by the plurality of ultrasonic transducers 23 are the same or different. The plurality of ultrasonic transducers 23 are arranged on the side wall of the ultrasonic excitation groove 10 at intervals in the height direction and in the circumferential direction.

[0047] Preferably, the side wall of the ultrasonic excitation tank 10 is a double-layer structure having a cavity 13, the cavity 13 is filled with water, and the ultrasonic transmitting end of the ultrasonic transducer 23 is located in the cavity 13, so that the ultrasonic transducer 23 is in direct contact with the water. Specifically, water can be packaged in the cavity 13 in advance, or water can be added during the experiment. Preferably, the water injection system 40 also includes a second water injection pipe 43 connected to the top wall of the ultrasonic excitation tank 10 and communicating with the interior of the cavity 13, the inlet of the second water injection pipe 43 is connected to the water source, and the second water injection pipe 43 is provided with a second pressure regulating valve 44 for adjusting the pressure. The second pressure regulating valve 44 is opened, and water is injected into the cavity 13 through the second water injection pipe 43. The water in the cavity 13 may be pressurized or unpressurized, depending on the experimental content and experimental conditions. Ultrasound acts directly in the water, and water is used as the ultrasonic transmission medium, which can better bring out the cavitation effect and transform the coal sample cracks. The pressurized water in the cavity 13 (after being pressurized by a water pump, the pressure is maintained by the second pressure regulating valve 44) will further promote the cavitation effect.

[0048] In the present invention, the gas injection and exhaust system 50 includes a gas injection pipe 51 and a gas outlet pipe 53 connected to the cover plate 11 of the ultrasonic excitation tank 10 and communicating with the inside of the ultrasonic excitation tank 10. The gas inlet of the gas injection pipe 51 is connected to the gas storage tank 57. The downstream of the gas storage tank 57 is also connected to a reference tank 58 connected to the gas inlet of the gas injection pipe 51, that is, the reference tank 58 is connected in series to the gas inlet of the ultrasonic excitation tank 10. The gas injection pipe 51 is provided with a gas regulating valve 52 located downstream of the reference tank 58. The gas outlet pipe 53 can be connected to a vacuum pump, and the gas outlet pipe 53 is provided with a gas outlet valve 54. The cover plate 11 of the ultrasonic excitation tank 10 is also connected to a pressure sensor 55 for detecting the air pressure therein, and the signal output end of the pressure sensor 55 is connected to a pressure gauge 56.

[0049] In the present invention, artificial speckles B are sprayed on the surface of the coal sample A, and a visualization window 12 is provided on the side wall of the ultrasonic excitation tank 10 in the area corresponding to the artificial speckles B, so as to facilitate observation of the coal sample A in the ultrasonic excitation tank 10. Specifically, the visualization window 12 can be made of polycarbon amorphous polymer. The surface crack monitoring system is a DIC visualization surface crack monitoring system, including an illumination device 61 for illuminating the artificial speckles B through the visualization window 12 and a camera device 62 for photographing the artificial speckles B through the visualization window 12.

[0050] In one embodiment of the present invention, the cover plate 11 is detachably fixed to the top wall of the ultrasonic excitation groove 10, such as by bolts, and a sealing ring is pressed between the cover plate 11 and the ultrasonic excitation groove 10; the cover plate 11 is sleeved outside the hydraulic rod 32 and is sealed to the outer wall of the hydraulic rod 32, and the cover plate 11 is a detachable structure composed of at least two sub-plates to facilitate the removal of the cover plate 11 from the ultrasonic excitation groove 10 and the hydraulic rod 32.

[0051] In another embodiment of the present invention, the cover plate 11 is fixedly connected to the hydraulic rod 32, the outer wall of the cover plate 11 is vertically slidably connected and sealed to the inner wall of the ultrasonic excitation tank 10, and a sealing ring is provided between the outer wall of the cover plate 11 and the inner wall of the ultrasonic excitation tank 10. Before the experiment, the cover plate 11 and the pressure plate 33 are located above the outside of the ultrasonic excitation tank 10. After people place the coal sample A in the ultrasonic excitation tank 10, the hydraulic cylinder 31 moves the pressure plate 33 downward through the hydraulic rod 32. The pressure plate 33 contacts the top of the coal sample A and applies pressure to it. At the same time, the hydraulic rod 32 makes the cover plate 11 move downward with the pressure plate 33. The cover plate 11 closes the top opening of the ultrasonic excitation tank 10, so that the inside of the ultrasonic excitation tank 10 is a closed environment, providing conditions for the subsequent injection of high-pressure water; after the experiment, the hydraulic rod 32 makes the pressure plate 33 and the cover plate 11 move upward together, open the opening at the top of the ultrasonic excitation tank 10, and take out the coal sample A.

[0052] Before the experiment, the surface of coal sample A is sprayed with paint to produce artificial speckle B, and coal sample A is placed in the center of the ultrasonic excitation tank 10. The hydraulic rod 32 is used to apply a certain stress to fix the coal sample A. Water is injected into the ultrasonic excitation tank 10 through the first water injection pipe 41, and water is injected into the cavity 13 through the second water injection pipe 43. The first pressure regulating valve 42 is controlled to apply a certain water pressure to the environment where the coal sample A is located. When the experimental environment is reached, the ultrasonic generator 21 is turned on, and ultrasound is performed through multiple ultrasonic transducers 23, such as using multi-frequency ultrasonic vibration to crack the coal sample A. At the same time, the lighting device 61 illuminates the coal sample A, and the camera device 62 is used to monitor the surface cracks of the coal sample A until the end of the experiment.

[0053] The present invention is a fracturing experimental system integrating stress loading, high-pressure water ultrasonic vibration, and ultrasonic fracturing adsorption and desorption. The function of the present invention is to perform ultrasound on coal sample A in a high-pressure water environment to form an ultrasonic cavitation effect, so as to cause the coal sample A to fracture, observe the surface cracks or internal cracks of the coal sample A and the direction and degree of crack expansion, so as to study the influence of ultrasound or ultrasound with water pressure on the expansion of coal sample cracks; and through adsorption and desorption experiments, study the influence of ultrasound or ultrasound with water pressure on the adsorption of coal samples.

[0054] Embodiment 2

[0055] This embodiment provides an experimental method based on the experimental system of embodiment 1, including the following steps:

[0056] S1, according to the coal sample characteristics, the loading system 30 is controlled to apply stress to the coal sample A to simulate the ground stress. Specifically, the ground stress can be obtained according to the geological data or coal seam data provided by the coal mining enterprise. If the enterprise has not extracted it, the ground stress can be determined according to the depth of the coal seam where the coal sample is located.

[0057] S2, closing the cover 11.

[0058] S3, start the ultrasonic excitation system 20, crack the coal sample A through ultrasonic vibration, and the camera device 62 of the surface crack monitoring system records the surface deformation and crack propagation information of the coal sample A; or before the ultrasonic vibration, inject high-pressure water into the ultrasonic excitation tank 10 through the first water injection pipe 41 of the water injection system 40 to put the coal sample in a high-pressure water environment, and crack the coal sample A through ultrasonic vibration cavitation in the high-pressure water environment. During ultrasonic fracturing in the high-pressure water environment, the water pressure can be adjusted according to experimental requirements.

[0059] S4, after the coal sample is cracked, the water in the ultrasonic excitation tank 10 is drained through the water outlet pipe 45.

[0060] S5, the vacuum pump is connected to the air outlet pipe 53, and the ultrasonic excitation tank 10 is evacuated through the air outlet pipe 53.

[0061] S6, open the valve of the gas storage tank 57 and the gas regulating valve 52, the gas provided by the gas storage tank 57 enters the reference tank 58, and the gas (such as CO2, CH4, N2) is injected into the ultrasonic excitation tank 10 through the gas injection pipe 51 to reach the set pressure, and the coal sample A adsorbs the gas to conduct an adsorption experiment. According to the experimental requirements, for example, about 1.5MPa of gas can be injected first, and the pressure in the ultrasonic excitation tank 10 is detected by the pressure sensor 55. After the pressure stabilizes, the gas pressure in the reference tank 58 is recorded, and the equilibrium state is reached, and the pressure at equilibrium is recorded.

[0062] S7, increase the gas pressure in the ultrasonic excitation tank 10, conduct a step adsorption test, until the gas pressure in the ultrasonic excitation tank 10 reaches the highest adsorption pressure, then discharge the gas in the ultrasonic excitation tank 10 through the exhaust pipe 53 of the gas injection and exhaust system 50, and conduct a desorption experiment on the coal sample A. For example, the set step gas pressure is 1.5MPa, 2MPa, 2.5MPa, 3MPa, 3.5MPa, 4MPa, conduct a step adsorption test, let the adsorption reach equilibrium at each pressure, and then continue to inject gas into the ultrasonic excitation tank 10 until the equilibrium is 4MPa, and 4MPa is the highest adsorption pressure.

[0063] S8, obtain the amount of gas adsorbed by the coal sample using the following formula:

[0064]

[0065] Among them, Q n Q is the cumulative amount of gas adsorbed by the coal sample after the nth gas injection, in mL / g; n-1 V is the cumulative amount of gas adsorbed by the coal sample after the n-1th gas injection, in mL / g; m is the molar volume of the gas, V m =22.4×103mL / mol; T is the experimental system temperature, unit K; R is the gas constant, R=8.3144J / (mol·K); m is the mass of the coal sample, unit g; Vc is the volume of the reference tank connected in series to the air inlet of the ultrasonic excitation tank, in L; V f is the volume of the ultrasonic excitation tank (the volume of the cavity where the coal sample A is placed), in L; p n0 is the initial pressure of the reference tank, in MPa; z n0 is the adsorption pressure p n0 The gas compressibility factor at the temperature T, dimensionless unit; p n-1 is the adsorption equilibrium pressure after the n-1th gas filling, unit: MPa; z n-1 is the adsorption pressure p n-1 and temperature T under the conditions of CH4 compression coefficient; p n is the adsorption equilibrium pressure after the nth gas filling, in MPa; z n is the adsorption pressure p n and temperature T under the conditions of gas compressibility factor, unit is dimensionless.

[0066] In the description of this specification, the description with reference to the terms "preferred embodiment", "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0067] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. High-pressure water environment coal rock loaded ultrasonic vibration cavitation cracking experimental system, characterized by: It includes an ultrasonic excitation tank with a cover plate on the top for placing coal samples, an ultrasonic excitation system, a loading system, a water injection system, a gas injection and exhaust system, and a surface crack monitoring system; The loading system applies stress to the coal sample in the ultrasonic excitation tank, the water injection system injects high-pressure water into the ultrasonic excitation tank to place the coal sample in a high-pressure water environment, the ultrasonic excitation system generates ultrasonic waves to crack the coal sample in the ultrasonic excitation tank, the surface crack monitoring system monitors surface cracks of the coal sample, and the gas injection and exhaust system injects and exhausts gas into the ultrasonic excitation tank to allow the coal sample to undergo an adsorption and desorption test; The ultrasonic excitation system includes an ultrasonic generator and a plurality of ultrasonic transducers installed on the side wall of the ultrasonic excitation tank and connected to the ultrasonic generator through ultrasonic cables, wherein the ultrasonic frequencies emitted by the plurality of ultrasonic transducers are the same or different; The side wall of the ultrasonic excitation groove is a double-layer structure with a cavity, the cavity is filled with water, and the ultrasonic transmitting end of the ultrasonic transducer is located in the cavity.

2. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 1 is characterized in that: A plurality of ultrasonic transducers are arranged on the side wall of the ultrasonic excitation groove at intervals in the height direction and in the circumferential direction.

3. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 1 is characterized in that: The loading system comprises a hydraulic cylinder, a hydraulic rod of the hydraulic cylinder passes through the cover plate and extends downward into the ultrasonic excitation groove, and the lower end of the hydraulic rod is fixedly connected to a pressure plate located above the coal sample and capable of covering the upper surface of the coal sample.

4. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 3 is characterized in that: The cover plate is detachably fixed to the top wall of the ultrasonic excitation groove, the cover plate is sleeved outside the hydraulic rod and is sealed to the outer wall of the hydraulic rod, and the cover plate is a detachable structure composed of at least two sub-plates; Alternatively, the cover plate is fixedly connected to the hydraulic rod, and the outer wall of the cover plate is vertically slidably connected and sealed to the inner wall of the ultrasonic excitation groove.

5. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 1 is characterized in that: The water injection system includes a first water injection pipe connected to the ultrasonic excitation tank and communicating with the interior thereof, the inlet of the first water injection pipe is connected to a pressurized water source, and a first pressure regulating valve for regulating pressure is provided on the first water injection pipe.

6. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 5 is characterized in that: A water outlet pipe communicating with the interior of the ultrasonic excitation tank is also installed at the bottom of the side wall of the ultrasonic excitation tank, and a water outlet valve is arranged on the water outlet pipe.

7. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to claim 1 is characterized in that: The gas injection and exhaust system includes a gas injection pipe and a gas outlet pipe connected to the ultrasonic excitation tank and communicating with the inside thereof, the gas inlet of the gas injection pipe is connected to the gas storage tank, and a reference tank connected to the gas inlet of the gas injection pipe is also connected to the downstream of the gas storage tank, the gas injection pipe is provided with a gas regulating valve, the gas outlet pipe can be connected to a vacuum pump, and the gas outlet pipe is provided with a gas outlet valve; The ultrasonic excitation tank is also connected to a pressure sensor for detecting the air pressure therein, and the signal output end of the pressure sensor is connected to the pressure meter.

8. The high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracturing experimental system according to any one of claims 1-7, characterized in that: Artificial speckles are sprayed on the surface of the coal sample, and a visualization window is provided on the side wall of the ultrasonic excitation groove in an area corresponding to the artificial speckles. The surface crack monitoring system includes a lighting device for illuminating the artificial speckles and a camera device for photographing the artificial speckles.

9. An experimental method based on the high-pressure water environment coal rock loaded ultrasonic vibration cavitation fracture experimental system according to any one of claims 1 to 8, characterized in that: The steps include: S1, controlling the loading system to apply stress to the coal sample according to the characteristics of the coal sample to simulate the ground stress; S2, close the cover; S3, starting the ultrasonic excitation system, cracking the coal sample by ultrasonic vibration, and the surface crack monitoring system monitors the information of the surface deformation and crack extension of the coal sample; or before the ultrasonic vibration, injecting high-pressure water into the ultrasonic excitation tank through the water injection system to put the coal sample in a high-pressure water environment, and cracking the coal sample by ultrasonic vibration cavitation; S4, after the coal sample is cracked, the water in the ultrasonic excitation tank is drained; S5, evacuating the ultrasonic excitation tank; S6, injecting gas into the ultrasonic excitation tank through the gas injection and exhaust system to reach the set pressure, and the coal sample is subjected to adsorption experiment; S7, increasing the gas pressure in the ultrasonic excitation tank, and performing a step adsorption test until the gas pressure in the ultrasonic excitation tank reaches the set maximum adsorption pressure, and then exhausting the gas in the ultrasonic excitation tank through the gas injection and exhaust system, and performing a desorption experiment on the coal sample; S8, obtain the amount of gas adsorbed by the coal sample using the following formula: Among them, Q n Q is the cumulative amount of gas adsorbed by the coal sample after the nth gas injection; n-1 V is the cumulative amount of gas adsorbed by the coal sample after the n-1th gas filling; m is the gas molar volume; T is the experimental system temperature; R is the gas constant; m is the coal sample mass; V c V is the volume of the reference tank connected in series to the air inlet of the ultrasonic excitation tank; f is the volume of the ultrasonic excitation tank; p n0 is the initial pressure of the reference tank; n0 is the adsorption pressure p n0 and gas compressibility factor under temperature T; p n-1 is the adsorption equilibrium pressure after the n-1th gas filling; z n-1 is the adsorption pressure p n-1 and temperature T under the conditions of CH4 compression coefficient; p n is the adsorption equilibrium pressure after the nth gas filling; z n is the adsorption pressure p n and gas compressibility factor under temperature T conditions.

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