System and method for cracking coal and rock strata through liquid nitrogen and water jet circulation impact

Abstract

The invention discloses a system and method for cracking a coal rock stratum through liquid nitrogen and water jet circulation impact. The system comprises a water jet impact system, a liquid nitrogen impact system and a pressure detection system, the main acting object of the system is coal rock stratum drilling, and coal rock is destroyed through low-temperature liquid nitrogen fracturing and water jet strong erosion. In the water jet impact drilling process, residues in a drilled hole can be brought out, a coal body is eroded, a crushing pit is formed, cross cracks are formed around the crushing pit, and a gas migration channel is reconstructed. The cold impact and thermal impact of the low-temperature liquid nitrogen can effectively improve the pore and fracture structure of the coal body, promote the development of cracks formed by water jet impact, and influence the permeability of the coal rock layer. And the water and the ice cyclically impact and supplement each other, so that water in pores is subjected to'water-ice 'two-phase transformation, the damage condition of the coal seam is further enhanced, the pores and cracks in the coal seam are extended, and the situation that' 1 + 1 > 2 'occurs.

Description

Technical Field

[0001] The invention belongs to the technical field of coal seam gas extraction and pressure relief and permeability enhancement, and specifically relates to a system and method for cracking coal rock layers by circulating impact of liquid nitrogen and water jets. Background Art

[0002] The main target of gas extraction in coal mines is the extraction borehole, and the fracturing effect inside the borehole directly affects the gas extraction effect. High-pressure water jet technology is used to destroy the coal seams around the borehole to induce small-scale coal seam outbursts. The space created by the water flow impact can effectively release the stress in the coal seam, thereby increasing the permeability of the coal seam. This process aims to increase the amount of gas released and thus improve the efficiency of gas extraction. However, with the increase in mining depth, the mining conditions of coal seams become more and more complex, and the problems of high gas pressure, high ground stress and low permeability of coal seams become more and more prominent. It is difficult to achieve efficient permeability enhancement of regional coal bodies using traditional water jets, resulting in poor drilling extraction effects.

[0003] The application of liquid nitrogen jet technology in the field of rock breaking is becoming more and more widespread. Its basic principle is to use the low-temperature impact zone caused by liquid nitrogen on the surface of coal rock to rapidly cool and shrink the coal rock, thereby generating stress concentration. According to the gas state equation, the volume of liquid nitrogen can expand nearly 700 times when it is vaporized under standard conditions. This expansion has a significant promoting effect on the expansion of cracks and effectively improves the crushing effect of coal rock. However, liquid nitrogen jet technology has certain limitations in practical applications. First of all, its impact effect is relatively weak. In order to achieve the ideal crushing effect, a higher working pressure is required, which undoubtedly puts higher requirements on the equipment. At the same time, the use of high pressure not only increases the cost, but also increases the danger of the operation. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of poor gas extraction effect in deep coal seams in the prior art, and to provide a system and method for cracking coal rock layers by circulating liquid nitrogen and water jets. The system and method is a coal seam permeability enhancement system and method for improving gas extraction efficiency that is easy to operate, has a wide range of practical applications, and can be recycled. It is a coal seam permeability enhancement system and method that performs cyclic impact cracking of coal seams by liquid nitrogen and water jets. Through this system and method, the rock breaking ability of water jets and the influence of liquid nitrogen on coal bodies can be combined, thereby solving the problem of poor gas extraction effect in deep coal seams.

[0005] The present invention is achieved through the following technical solutions: A system for cracking coal and rock formations by circulating impact of liquid nitrogen and water jets comprises a water jet impact system, a liquid nitrogen impact system and a pressure detection system.

[0006] The water jet impact system includes a water tank, a high-pressure water pump, a pressure controller, an air pump and a water jet impact tube. The water tank is connected to the high-pressure water pump through a pipeline with a main valve, the high-pressure water pump is connected to the pressure controller, the pressure controller is connected to the water jet impact tube through a water jet hose with a shut-off valve, the air pump is connected to the water jet hose through a pipeline with a valve, and a water jet nozzle is installed at the outlet end of the water jet impact tube.

[0007] The liquid nitrogen shock system includes a self-pressurized liquid nitrogen tank, a flow controller and a liquid nitrogen injection pipe. The self-pressurized liquid nitrogen tank is equipped with a control valve. The self-pressurized liquid nitrogen tank is connected to the flow controller through a pipeline equipped with a safety valve. The flow controller is connected to the liquid nitrogen injection pipe through a low-temperature liquid nitrogen hose equipped with a safety valve and a shut-off valve. A liquid nitrogen nozzle is installed at the outlet end of the liquid nitrogen injection pipe.

[0008] The pressure detection system comprises a pressure sensor and a pressure recorder. The pressure sensor is respectively arranged at the outlet of the water jet nozzle and the outlet of the liquid nitrogen nozzle. The pressure sensor is electrically connected to the pressure recorder.

[0009] As a preferred technical solution, the system of the present invention also includes a nozzle fixing device, and a first through hole and a second through hole are provided on the nozzle fixing device; one end of the first through hole is a first water inlet, and the other end is a first water outlet, wherein the first water inlet is connected to the outlet end of the water jet hose, and the first water outlet is connected to the inlet end of the water jet impact tube; one end of the second through hole is a second water inlet, and the other end is a second water outlet, wherein the second water inlet is connected to the outlet end of the low-temperature liquid nitrogen hose, and the second water outlet is connected to the inlet end of the liquid nitrogen injection tube.

[0010] As a preferred technical solution, the nozzle fixing device includes a base and a plug, both of which are cylindrical. The diameter of the plug is smaller than the diameter of the base. The inner end face of the plug is fixedly connected to the inner end face of the base and the two are coaxially arranged. The first water inlet and the second water inlet are opened on the outer end face of the base, and the first water outlet and the second water outlet are opened on the outer end face of the plug.

[0011] As a preferred technical solution, the water jet impact pipe adopts a seamless stainless steel pipe, and is threadedly connected to the water jet nozzle and the nozzle fixing device. Polytetrafluoroethylene (raw tape) is used as a filler for sealing during the threaded connection; the liquid nitrogen injection pipe adopts a seamless stainless steel pipe, and is threadedly connected to the liquid nitrogen nozzle and the nozzle fixing device. Polytetrafluoroethylene is used as a filler for sealing during the threaded connection.

[0012] A method for impact fracturing coal strata by the above system specifically comprises the following steps: (1) Collect coal samples in the area where permeability enhancement is required underground, and measure their mechanical properties in the laboratory to determine the water jet pressure and liquid nitrogen pressure.

[0013] (2) Drill a hole in the area where transparency enhancement is required and remove some of the coal slag in the hole.

[0014] (3) After checking the air tightness of the system, place the water jet impact tube and liquid nitrogen injection tube into the drilled hole and secure them firmly.

[0015] (4) Open the main valve on the pipeline between the water storage tank and the high-pressure water pump, start the high-pressure water pump, and open the stop valve on the water jet hose. Control the jet water pressure through the pressure controller and adjust it to the required pressure according to the data displayed by the pressure recorder.

[0016] (5) After the water jet impacts for 5 minutes, turn off the high-pressure water pump, and then turn off the main valve and the stop valve in turn. Use high-pressure water jet technology to efficiently crush coal and rock. Through the kinetic energy of the high-pressure water jet, the impact force is applied to the surface of the rock formation to achieve the crushing of the rock formation and the generation of cracks. For areas with relatively complete structures in the rock formation, the high-pressure water jet can effectively break and create cracks. At the same time, for the existing natural cracks in the rock formation, the injection of high-pressure water jets will further promote the expansion of these cracks, thereby improving the crushing effect of the rock formation.

[0017] (6) Open the valve and start the air pump. After 1 minute, close the air pump and the valve in turn. The air pump presses out the residual water in the tube to prevent the residual water in the tube from condensing and blocking the water jet tube when spraying low-temperature liquid nitrogen. (7) After standing still for 3-5 minutes, open the control valve on the self-pressurized liquid nitrogen tank and the stop valve on the cryogenic liquid nitrogen hose. Control the liquid nitrogen flow rate through the flow controller and adjust it to the required pressure according to the data displayed on the pressure recorder.

[0018] (8) After 5 minutes of liquid nitrogen shock, the control valve and stop valve on the self-pressurized liquid nitrogen tank are closed in turn, and the safety valve on the pipeline between the self-pressurized liquid nitrogen tank and the flow controller and the safety valve on the cryogenic liquid nitrogen hose are opened to release the high pressure generated in the pipeline due to the gasification of liquid nitrogen during the transmission process. Liquid nitrogen cold shock technology plays an important role in this invention method. By scattering liquid nitrogen to the surface of coal rock, the temperature of the rock formation can be quickly reduced, resulting in significant thermal stress inside the rock formation, thereby inducing the formation of cracks. At the same time, the intervention of high-pressure water jet technology, using its water wedge effect, can further promote the expansion of cracks. This high-pressure water wedge effect, together with the low-temperature cracks induced by liquid nitrogen cold shock, acts on the rock formation together, making the crack expansion more rapid and effective. In this process, the synergistic effect of liquid nitrogen and high-pressure water jet not only improves the efficiency of crack formation, but also reduces the pressure requirements for equipment, thereby reducing operating costs and potential dangers. Through this technology, a through fracture network with a high fracture rate can be formed around the main wellbore, facilitating subsequent mining activities.

[0019] (9) After 30 minutes of rest, repeat the above steps (4) to (7) to perform cyclic impact fracture on the coal rock layer. After repeating 3 to 5 times, the coal body is subjected to alternating cycles of liquid nitrogen and water jet treatment. The damaged area of ​​the coal body increases, primary and secondary cracks are developed, and the cracks are interconnected, thereby improving the permeability enhancement effect of the coal seam.

[0020] As a preferred technical solution, in step (2), the drilling includes through-layer drilling and along-layer drilling.

[0021] As a preferred technical solution, in step (3), after the water jet impact tube and the liquid nitrogen injection tube are placed in the borehole, the plug of the nozzle fixing device is inserted and fixed in the borehole opening, and the base of the nozzle fixing device abuts against the coal wall outside the borehole opening.

[0022] The technical solution of the present invention is to first quickly destroy the coal rock structure by water jet, forming a large impact crater and cracks inside the coal body; then degrade the coal body by liquid nitrogen, making the coal body fragile, reducing the mechanical parameters of the coal body, and promoting the development of cracks; then the impact crater can be enlarged again by water jet impact and promoting the development of cracks; then the coal body can be degraded by liquid nitrogen again to promote the development of cracks. Through this cycle, a better gas migration channel can be formed to ensure efficient and practical drilling and efficient gas extraction.

[0023] Compared with the prior art or similar devices, the present invention has the following beneficial effects: 1) The water wedge effect generated by the high-pressure water jet is used to promote the further expansion of existing cracks, thereby enhancing the cracks formed by the liquid nitrogen cold shock. In addition, the cold shock effect of liquid nitrogen will also spread to the cracks broken by the high-pressure water jet, so that these cracks are further expanded, and the crack-making effect of the high-pressure water jet is strengthened. This interaction and enhancement mechanism enables the two jet technologies to work together effectively, improve the efficiency of rock formation fracture, and finally form a fracture network with high fracture rate and good penetration around the main wellbore. The present invention combines liquid nitrogen and water jet, fully utilizes the advantages of both in breaking coal and rock, and reasonably arranges the implementation sequence of the two methods of hydraulic impact and liquid nitrogen frost heave to achieve the front and back coordination between the methods, so as to achieve the effect of the previous step providing convenience or enhancing effect for the subsequent steps, and realizes the coal seam permeability enhancement in a more reasonable and scientific way, and its coal rock breaking efficiency exceeds that of a single method. Through cyclic impact, coal and rock can be better broken, the pores and cracks of coal and rock can be increased, the gas migration channel can be improved, and the gas extraction efficiency can be improved.

[0024] 2) The present invention has a simple structure and is easy to implement. The device is easy to operate. The equipment can be designed and adjusted according to actual conditions and has a wide range of engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments of the present invention are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0026] Figure 1 It is a schematic diagram of the structure of the system of the present invention.

[0027] Figure 2 It is a schematic diagram of the connection between the water jet impact tube and the liquid nitrogen injection tube and the nozzle fixing device in the system of the present invention.

[0028] In the figure: 1-water storage tank, 2-main valve, 3-high-pressure water pump, 4-pressure controller, 5-self-pressurized liquid nitrogen tank, 6-flow control meter, 7-stop valve, 8-drilling hole, 9-liquid nitrogen nozzle, 10-water jet nozzle, 11-coal rock layer, 12-safety valve, 13-pressure recorder, 14-pressure sensor, 15-nozzle fixing device, 15-1-base, 15-2-plug, 16-liquid nitrogen injection pipe, 17-water jet impact pipe, 18-low-temperature liquid nitrogen hose, 19-water jet hose, 20-first water inlet, 21-first water outlet, 22-second water inlet, 23-second water outlet, 24-air pump, 25-valve. DETAILED DESCRIPTION

[0029] The embodiments of the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore only used as examples, and the protection scope of the present invention cannot be limited thereto. It should be noted that, unless otherwise specified, the technical terms or scientific terms used in this application should be the common meanings understood by those skilled in the art to which the present invention belongs. Example 1

[0030] This embodiment provides a system for cracking coal and rock formations by circulating liquid nitrogen and water jets. Figure 1 and Figure 2 As shown, it includes a water jet impact system, a liquid nitrogen impact system, a pressure detection system and a nozzle fixing device 15.

[0031] The water jet impact system includes a water tank 1, a high-pressure water pump 3, a pressure controller 4, an air pump 24 and a water jet impact pipe 17. The water tank 1 is connected to the high-pressure water pump 3 through a pipeline provided with a main valve 2, the high-pressure water pump 3 is connected to the pressure controller 4, the pressure controller 4 is connected to the water jet impact pipe 17 through a water jet hose 19 provided with a shut-off valve 7, the air pump 24 is connected to the water jet hose 19 through a pipeline provided with a valve 25, and a water jet nozzle 10 is installed at the outlet end of the water jet impact pipe 17. The water jet impact pipe 17 adopts a seamless stainless steel pipe and is threadedly connected to the water jet nozzle 10. Polytetrafluoroethylene is used as a filler for sealing during the threaded connection.

[0032] The liquid nitrogen impact system includes a self-pressurized liquid nitrogen tank 5, a flow control meter 6 and a liquid nitrogen injection pipe 16. The self-pressurized liquid nitrogen tank 5 is provided with a control valve. The self-pressurized liquid nitrogen tank 5 is connected to the flow control meter 6 through a pipeline provided with a safety valve 12. The flow control meter 6 is connected to the liquid nitrogen injection pipe 16 through a low-temperature liquid nitrogen hose 18 provided with a safety valve 12 and a stop valve 7. Safety valves 12 are provided between the self-pressurized liquid nitrogen tank 5 and the flow control meter 6, and between the flow control meter 6 and the liquid nitrogen injection pipe 16. The safety valve 12 is used to release the high pressure generated in the pipeline due to the gasification of liquid nitrogen during the transmission process; a liquid nitrogen nozzle 9 is installed at the outlet end of the liquid nitrogen injection pipe 16. The liquid nitrogen injection pipe 16 adopts a seamless stainless steel pipe and is threadedly connected to the liquid nitrogen nozzle 9. Polytetrafluoroethylene is used as a filler for sealing during the threaded connection.

[0033] The pressure detection system includes a pressure sensor 14 and a pressure recorder 13, and the pressure sensor 14 is electrically connected to the pressure recorder 13; the pressure sensor 14 is respectively arranged at the outlet of the water jet nozzle 10 and the outlet of the liquid nitrogen nozzle 9, and the pressure sensor 14 can detect the pressure of the water jet and the liquid nitrogen at the outlet, and record the pressure of the water jet and the liquid nitrogen outflow through the pressure recorder 13.

[0034] The nozzle fixing device 15 is provided with a first through hole and a second through hole; one end of the first through hole is a first water inlet 20, and the other end is a first water outlet 21, wherein the first water inlet 20 is connected to the outlet end of the water jet hose 19, and the first water outlet 21 is connected to the inlet end of the water jet impact tube 17, and the water jet impact tube 17 and the first water outlet 21 are connected by a threaded connection, and polytetrafluoroethylene is used as a filler for sealing during the threaded connection; one end of the second through hole is a second water inlet 22, and the other end is a second water outlet 23, wherein the second water inlet 22 is connected to the outlet end of the low-temperature liquid nitrogen hose 18, and the second water outlet 23 is connected to the inlet end of the liquid nitrogen injection tube 16, and the liquid nitrogen injection tube 16 and the second water outlet 23 are connected by a threaded connection, and polytetrafluoroethylene is used as a filler for sealing during the threaded connection. The nozzle fixing device 15 includes a base 15-1 and a plug 15-2, both of which are cylindrical. The diameter of the plug 15-2 is smaller than the diameter of the base 15-1. The inner end surface of the plug 15-2 is fixedly connected to the inner end surface of the base 15-1 and the two are coaxially arranged. The first water inlet 20 and the second water inlet 22 are opened on the outer end surface of the base 15-1, and the first water outlet 21 and the second water outlet 23 are opened on the outer end surface of the plug 15-2. Example 2

[0035] This embodiment provides a method for impact fracturing coal strata by using the system in the above embodiment 1, which specifically includes the following steps: (1) Collect coal samples in the area where permeability enhancement is required underground, and measure their mechanical properties in the laboratory to determine the water jet pressure and liquid nitrogen pressure.

[0036] (2) Drill a hole 8 in the coal-rock layer 11 in the area where permeability enhancement is required, and remove some coal slag in the hole; in specific implementation, the drilling hole 8 includes through-layer drilling and along-layer drilling. Use a drilling rig to drill holes at the specified position and designed angle. During the drilling process, it is necessary to closely monitor the verticality and depth of the borehole to ensure the quality of the drilling.

[0037] (3) After checking the air tightness of the system, place the water jet impact tube 17 and the liquid nitrogen injection tube 16 in the borehole 8 and fix them firmly. In the specific implementation, after the water jet impact tube 17 and the liquid nitrogen injection tube 16 are placed in the borehole 8, the plug 15-2 of the nozzle fixing device 15 is inserted and fixed in the hole of the borehole 8, and the base 15-1 of the nozzle fixing device 15 abuts against the coal wall outside the hole of the borehole 8.

[0038] (4) Before starting the injection, it is necessary to check whether the high-pressure water pump 3 and related equipment are in good working condition and confirm that there are no leaks or damaged parts. At the same time, ensure that the water source is sufficient and the pressure is stable, connect the outlet pipe of the high-pressure water pump 3 to the inlet pipe of the borehole, and ensure that the connection is tight to prevent leakage. Select a suitable connection method according to the characteristics of the borehole or injection object. Adjust the pressure and flow of the high-pressure water pump 3 according to the area and purpose of injection. By adjusting the knob or switch on the pressure controller 4, the output pressure can be increased or decreased, and the main valve 2 can be adjusted to control the flow rate. When the high-pressure water pump 3 is started, the high-pressure water is injected into the borehole or other area to be injected. During the injection process, the water pressure and flow are closely monitored to ensure that they remain within the set parameter range. During the injection process, the water pressure changes and the water output of the rock formation are monitored in real time, and the fracturing parameters are adjusted according to the actual situation to ensure that the high-pressure water can effectively fracture the coal rock formation.

[0039] (5) Open the main valve 2 on the pipeline between the water tank 1 and the high-pressure water pump 3, start the high-pressure water pump 3, and open the stop valve 7 on the water jet hose 19. Control the jet water pressure through the pressure controller 4 and adjust it to the required pressure according to the data displayed by the pressure recorder.

[0040] (6) After the water jet impacts for 5 minutes, turn off the high-pressure water pump 3, and then turn off the main valve 2 and the stop valve 7. The high-pressure water pump 3 increases the water pressure to tens to hundreds of MPa, forming a high-speed water jet. When the high-speed water jet impacts the coal rock layer for the first time, a high-pressure area will be generated at the contact point, causing microcracks on the surface of the coal rock layer. With the continuous impact of the water jet, these microcracks will expand and merge under the action of stress concentration. The impact energy of the water jet is converted into the fracture energy of the coal rock layer, prompting the cracks to expand deeper and around.

[0041] (7) Open valve 25 and start air pump 24. After 1 minute, close air pump 24 and valve 25 in turn. Air pump 24 presses out the residual water in the tube to prevent the residual water in the tube from condensing and blocking the water jet tube when spraying low-temperature liquid nitrogen.

[0042] (8) After standing still for 3-5 minutes, open the control valve on the self-pressurized liquid nitrogen tank 5, and open the stop valve 7 on the cryogenic liquid nitrogen hose 18. Control the liquid nitrogen flow through the flow controller 6, and adjust to the required pressure according to the data displayed by the pressure recorder 13. During the injection process, slowly open the built-in valve and start liquid nitrogen injection. Closely monitor the flow and pressure changes of liquid nitrogen to ensure a smooth injection process. Monitor the pressure and injection volume of the liquid nitrogen tank in real time and adjust if necessary. Avoid risks caused by overinjection or excessive pressure. After completing the predetermined injection volume, slowly close the built-in valve and stop liquid nitrogen injection. Ensure that the liquid nitrogen in all equipment and pipelines has completely stopped flowing.

[0043] (9) After 5 minutes of liquid nitrogen shock, the control valve and stop valve 7 on the self-pressurized liquid nitrogen tank 5 are closed in turn, and the safety valve 12 on the pipeline between the self-pressurized liquid nitrogen tank 5 and the flow control meter 6 and the safety valve 12 on the cryogenic liquid nitrogen hose 18 are opened to release the high pressure generated in the pipeline due to the gasification of liquid nitrogen during the transmission process. Liquid nitrogen causes the water in the cracks caused by high-energy shock to condense and increase in volume during the freezing process. This change further exacerbates the expansion of the cracks. Under the synergistic effect of high-pressure water jets and low-temperature shock of liquid nitrogen, a large and interconnected crack network can be formed on the surface of the coal rock layer.

[0044] (10) After 30 minutes of rest, repeat the above steps (5) to (8) to perform cyclic impact fracture on the coal rock layer 11. After 3-5 times of repetition, after the alternating cycle of liquid nitrogen and water jet treatment, the damaged area of ​​the coal body increases, primary and secondary cracks are developed, and the cracks are interconnected, thereby improving the permeability enhancement effect of the coal seam. In this step, it is necessary to rest for 30 minutes after the liquid nitrogen impact. This is because the water in the borehole 8 and the crack will expand in volume during the freezing process, which will squeeze the surrounding rock mass, increase the degree of rock mass fragmentation, and thus increase the permeability of the coal body. After 30 minutes of rest, the effect of liquid nitrogen will reach the best state, more fine cracks can be generated, and the cracks in the cutting area will be more developed.

[0045] In the combined application of water jet and liquid nitrogen in the above method, the impact effect of high-energy high-pressure water jet combined with the impact effect of liquid nitrogen jet can significantly enhance the destructive power of both. The low temperature effect of liquid nitrogen can trigger the formation of cracks, while the high-pressure water jet can penetrate these cracks caused by low temperature. With the help of the wedging effect of the water jet, the cracks can be further extended. In addition, the cracks caused by the impact of high-pressure water jet will continue to expand under the influence of the low temperature of liquid nitrogen. Therefore, the interaction between high-pressure water jet and liquid nitrogen jet can effectively enhance their respective cracking effects.

[0046] In summary, the main target of the system of the present invention is the borehole 8 of the coal rock layer 11, and the coal rock is destroyed by the low-temperature liquid nitrogen fracturing and the strong erosion of the water jet. In the process of water jet impacting the borehole 8, the residue in the borehole 8 can be brought out and the coal body can be eroded to form a crushing pit and cross cracks around the crushing pit, and the gas migration channel can be reconstructed. The cold shock and thermal shock of low-temperature liquid nitrogen can effectively improve the pore and crack structure of the coal body, promote the development of cracks formed by water jet impact, and affect the permeability of the coal rock layer 11. The cyclic impact of the two complements each other, which can cause the water in the pores to undergo a "water-ice" two-phase transition, further enhance the damage to the coal seam, extend the pores and cracks in the coal seam, and create a "1+1>2" situation.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some or all of the technical features may be replaced by equivalents. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. A system for cracking coal and rock formations by circulating liquid nitrogen and water jets, characterized in that: Including water jet impact system, liquid nitrogen impact system and pressure detection system; The water jet impact system includes a water tank, a high-pressure water pump, a pressure controller, an air pump and a water jet impact pipe. The water tank is connected to the high-pressure water pump through a pipeline provided with a main valve, the high-pressure water pump is connected to the pressure controller, the pressure controller is connected to the water jet impact pipe through a water jet hose provided with a shut-off valve, the air pump is connected to the water jet hose through a pipeline provided with a valve, and a water jet nozzle is installed at the outlet end of the water jet impact pipe; The liquid nitrogen impact system includes a self-pressurized liquid nitrogen tank, a flow controller and a liquid nitrogen injection pipe. The self-pressurized liquid nitrogen tank is equipped with a control valve. The self-pressurized liquid nitrogen tank is connected to the flow controller through a pipeline provided with a safety valve. The flow controller is connected to the liquid nitrogen injection pipe through a low-temperature liquid nitrogen hose provided with a safety valve and a stop valve. A liquid nitrogen nozzle is installed at the outlet end of the liquid nitrogen injection pipe. The pressure detection system comprises a pressure sensor and a pressure recorder. The pressure sensor is respectively arranged at the outlet of the water jet nozzle and the outlet of the liquid nitrogen nozzle. The pressure sensor is electrically connected to the pressure recorder.

2. The system for cracking coal and rock formations by circulating impact of liquid nitrogen and water jets according to claim 1 is characterized in that: It also includes a nozzle fixing device, on which a first through hole and a second through hole are provided; one end of the first through hole is a first water inlet, and the other end is a first water outlet, wherein the first water inlet is connected to the outlet end of the water jet hose, and the first water outlet is connected to the inlet end of the water jet impact tube; one end of the second through hole is a second water inlet, and the other end is a second water outlet, wherein the second water inlet is connected to the outlet end of the low-temperature liquid nitrogen hose, and the second water outlet is connected to the inlet end of the liquid nitrogen injection tube.

3. The system for cracking coal and rock formations by circulating impact of liquid nitrogen and water jet according to claim 2, characterized in that: The nozzle fixing device includes a base and a plug, both of which are cylindrical. The diameter of the plug is smaller than the diameter of the base. The inner end surface of the plug is fixedly connected to the inner end surface of the base and the two are coaxially arranged. The first water inlet and the second water inlet are opened on the outer end surface of the base, and the first water outlet and the second water outlet are opened on the outer end surface of the plug.

4. The system for cracking coal and rock formations by circulating impact of liquid nitrogen and water jets according to claim 3 is characterized in that: The water jet impact pipe is made of seamless stainless steel pipe, and is threadedly connected to the water jet nozzle and nozzle fixing device. Polytetrafluoroethylene is used as filler for sealing during the threaded connection; the liquid nitrogen injection pipe is made of seamless stainless steel pipe, and is threadedly connected to the liquid nitrogen nozzle and nozzle fixing device. Polytetrafluoroethylene is used as filler for sealing during the threaded connection.

5. A method for impact fracturing a coal formation by the system of claim 4, characterized in that: The following steps are involved: (1) Collect coal samples in the area where permeability enhancement is required underground, and measure their mechanical properties in the laboratory to determine the water jet pressure and liquid nitrogen pressure; (2) Drill a hole in the area where the transparency needs to be increased and remove some of the coal slag in the hole; (3) After checking the air tightness of the system, place the water jet impact tube and liquid nitrogen injection tube in the drilled hole and secure them firmly; (4) Open the main valve on the pipeline between the water storage tank and the high-pressure water pump, start the high-pressure water pump, and open the stop valve on the water jet hose. Control the jet water pressure through the pressure controller and adjust it to the required pressure according to the data displayed by the pressure recorder; (5) After the water jet impacts for 5 minutes, turn off the high-pressure water pump, and then close the main valve and stop valve in turn; (6) Open the valve and start the air pump. After 1 minute, turn off the air pump and valve in turn. The air pump presses out the residual water in the tube to prevent the residual water in the tube from condensing and blocking the water jet tube when spraying low-temperature liquid nitrogen. (7) After standing still for 3-5 minutes, open the control valve on the self-pressurized liquid nitrogen tank and the stop valve on the cryogenic liquid nitrogen hose. Control the liquid nitrogen flow rate through the flow controller and adjust it to the required pressure according to the data displayed on the pressure recorder. (8) After 5 minutes of liquid nitrogen shock, close the control valve and stop valve on the self-pressurized liquid nitrogen tank in turn, and open the safety valve on the pipeline between the self-pressurized liquid nitrogen tank and the flow controller and the safety valve on the cryogenic liquid nitrogen hose to release the high pressure generated in the pipeline due to the gasification of liquid nitrogen during the transmission process; (9) After 30 minutes of rest, repeat the above steps (4) to (7) to perform cyclic impact fracture on the coal rock layer. After repeating 3 to 5 times, the coal body is subjected to alternating cycles of liquid nitrogen and water jet treatment. The damaged area of ​​the coal body increases, primary and secondary cracks are developed, and the cracks are interconnected, thereby improving the permeability enhancement effect of the coal seam.

6. The method for impact fracturing coal strata according to claim 5, characterized in that: In step (2), the drilling includes through-layer drilling and along-layer drilling.

7. The method for impact fracturing coal strata according to claim 5, characterized in that: In step (3), after the water jet impact tube and the liquid nitrogen injection tube are placed in the borehole, the plug of the nozzle fixing device is inserted and fixed in the borehole opening, and the base of the nozzle fixing device abuts against the coal wall outside the borehole opening.

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