Coal seam permeability improvement device and method for low-permeability coal seam in underground coal mine by using variable-temperature jet

Abstract

The present application belongs to the technical field of coal seam permeability improvement in underground coal mine drilling, and provides a coal seam permeability improvement device and method for low-permeability coal seam in underground coal mine by temperature-variable jet flow, which comprises a jet flow nozzle installed at the outlet of a high-pressure water pipe, the inlet of the high-pressure water pipe being connected with a high-pressure water pump, and the high-pressure water pump being connected with a rapid heater; the jet flow nozzle is further connected with a liquid nitrogen generator through a vacuum heat insulation pipe, the inlet end of the liquid nitrogen generator being connected with a nitrogen generator, the outlet end being connected with the inlet end of a liquid nitrogen storage tank, the outlet of the liquid nitrogen storage tank being connected with a liquid nitrogen tank group, the outlet end of the liquid nitrogen tank group being connected with a cryogenic pump, and the outlet of the cryogenic pump being connected with the jet flow nozzle through a vacuum heat insulation pipe; the device effectively increases the fissure size of the drilling, improves the coal seam permeability, and increases the gas production. Meanwhile, a liquid nitrogen production device is used to directly produce liquid nitrogen, which is low in cost and can be reused. The method is simple and easy to operate, can effectively solve the loss of liquid nitrogen in the transportation process, improve the effect of liquid nitrogen on the coal body, and improve the coal seam permeability improvement effect.

Description

Technical Field

[0001] This invention belongs to the technical field of coal seam permeability enhancement in underground coal mine boreholes, specifically relating to a coal seam permeability enhancement device and method using variable temperature jets in low-permeability coal seams underground. Background Technology

[0002] Different coal seams exhibit significant differences in geological conditions and structural characteristics. Furthermore, the occurrence characteristics of coal seam gas are controlled by various geological factors, including tectonic conditions, hydrological conditions, coal seam depth, reservoir pressure, and coal seam thickness, resulting in extremely complex gas distribution. This leads to the ineffectiveness of current conventional borehole gas extraction methods, long extraction cycles, and impacts underground mining progress. Measures for protecting the mining layer are also limited by the coal seam's occurrence conditions. Against this backdrop, a series of hydraulic measures have been developed, such as hydraulic perforation, hydraulic slotting, and hydraulic cutting. However, with increasing mining depth, coal seams with complex conditions such as low permeability, high gas pressure, and high ground stress are constantly emerging. Single hydraulic permeability enhancement measures are also facing limitations in their effectiveness. Therefore, a highly efficient underground coal seam permeability enhancement method is urgently needed to improve gas extraction rates in low-permeability coal seams.

[0003] Hydraulic perforation is one of the commonly used methods in gas mining. It uses high-pressure water jets to disrupt the coal seam surrounding the borehole, inducing small outbursts within the borehole. The space created by the water flow to flush out the coal seam releases the stress in the coal, improves the permeability of the coal seam, and thus increases gas production. However, as the mining depth increases, the complexity of the coal seam gradually increases, and the mining effect of hydraulic perforation measures based on a single factor is affected, making it difficult to continue to meet the requirements. Therefore, it is necessary to improve the existing hydraulic perforation measures and improve the perforation effect through the interaction of multiple factors. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a coal seam permeability enhancement device and method using variable-temperature jet permeation in low-permeability coal seams in underground coal mines. This device effectively increases the fracture size in boreholes, thereby enhancing coal seam permeability and increasing gas production. Simultaneously, it utilizes a liquid nitrogen production device to directly produce liquid nitrogen, resulting in low cost and reusability. Vacuum-insulated pipelines can be used during liquid nitrogen transportation to reduce losses and enhance its effectiveness during the perforation process. This method is simple, easy to operate, and effectively solves the problem of liquid nitrogen loss during transportation, improving the effect of liquid nitrogen on the coal body and enhancing coal seam permeability.

[0005] This invention is achieved by the following technical solution: a coal seam permeability enhancement device for low-permeability coal seams using variable-temperature jetting in underground coal mines, comprising a jet nozzle installed at the outlet of a high-pressure water pipe, the inlet of the high-pressure water pipe connected to a high-pressure water pump, and the high-pressure water pump connected to a rapid heater; the jet nozzle is also connected to a liquid nitrogen generator via a vacuum insulation pipe, the inlet end of the liquid nitrogen generator is connected to a nitrogen generator, the outlet end of the liquid nitrogen generator is connected to the inlet end of a liquid nitrogen storage tank, the outlet of the liquid nitrogen storage tank is connected to a liquid nitrogen tank group, the outlet end of the liquid nitrogen tank group is connected to a cryogenic pump, and the outlet of the cryogenic pump is connected to the jet nozzle via a vacuum insulation pipe;

[0006] The rapid heater, high-pressure water pump, cryogenic pump, liquid nitrogen storage tank, liquid nitrogen generator, and nitrogen generator are respectively connected to the PLC control system;

[0007] The vacuum insulation tube (2) has a pressure sensor on its outer wall to monitor the pressure inside the borehole.

[0008] The high-pressure water pump is equipped with a water injection valve at its outlet, and the low-temperature pump is equipped with a nitrogen injection valve at its outlet.

[0009] The jet nozzle is installed in the coal seam; the ends of the high-pressure water pipe and the vacuum insulated pipe are sealed with the coal seam; the jet nozzle is connected to the high-pressure water pipe and the vacuum insulated pipe by flanges respectively.

[0010] The pressure range of the liquid nitrogen tank group (11) is 0.6-2.8 MPa; the minimum pressure of the liquid nitrogen tank is 0.6-0.7 MPa, and the pressure difference of the liquid nitrogen tank is set to 0.5 times or 1 times the initial pressure.

[0011] The jet nozzle penetrates the coal seam to a depth of 1-2m to enhance coal seam permeability; and continues to enhance permeability with variable temperature jet at a distance of 1.5-6.5m.

[0012] The method for enhancing permeability using the aforementioned coal seam permeability enhancement device with variable temperature jet flow in low-permeability coal seams in underground coal mines comprises the following steps:

[0013] (1) Based on the underground coal seam occurrence conditions, physical property parameters, working pressure of water jet equipment and on-site requirements, determine the construction process parameters of hydraulic drilling and monitoring drilling. The hydraulic drilling size is 80-170mm and the monitoring drilling size is 42-75mm. The monitoring drillings are arranged in parallel or perpendicular to the hydraulic drillings, with a layout of 1 hydraulic drilling and 6 or 7 monitoring drillings. The spacing between the monitoring drillings and the hydraulic drillings is as follows: L1=1.2-1.6m, 1-2 monitoring holes; L2=2.0-2.2m, 2 monitoring holes; L3=2.6-3.00m, 2 monitoring holes; L4=3.2-3.6m, 2 monitoring holes.

[0014] (2) Select a high-pressure water jet nozzle with a size of 1.8-2.6mm according to the mechanical parameters of the coal seam and the requirements of field application, control the water jet pressure at 80-120MPa and the water temperature at 0-90℃;

[0015] (3) Use 80-120MPa high-pressure water jet to impact the coal seam inside the borehole. Use the high-speed impact of the water jet to break the coal seam and form a hole. At the same time, a waterproof rangefinder is set at the center of the drill rod near the nozzle to transmit the parameters of the hole-punching area and ensure the effect of water jet punching. For situations such as the presence of interbedded rock or weak surfaces in the coal seam, the water jet impact parameters are appropriately adjusted according to the punching data transmitted by the rangefinder, thereby forming a large-scale uniform damage zone inside the hole.

[0016] (4) Use the auger drill rod to transport the coal slag and residual water in the hole to the outside of the hole, and then perform secondary separation and recycling of the coal slag and residual water in the well.

[0017] (5) Use a segmented temperature and pressure resistant sealing device to partially seal the water jet punching area, inject room temperature nitrogen into the sealing area, and control the injection pressure P of room temperature nitrogen. 常氮 =nP 瓦斯 , where P 瓦斯 The gas pressure inside the coal seam is given by n, where n is the pressure coefficient (n=4-7). Injected room-temperature nitrogen compresses the moisture in the coal seam, allowing it to penetrate deeper and providing a channel for liquid nitrogen to enter. After the room-temperature nitrogen injection is complete, cryogenic liquid nitrogen is injected, with the injection rate Q controlled. 液氮 =1.3-1.7πD i 2 / 4, where D i The diameter of the local borehole damage caused by the water jet impact before the i-th nitrogen injection is provided by the rangefinder on the drill pipe; at the same time, an automatic pressure relief valve is reserved on the segmented sealing device to partially relieve pressure when the high pressure generated by liquid nitrogen vaporization reaches the preset range;

[0018] (6) Due to the influence of some inorganic salt components in the underground formation water medium, the freezing temperature of the water medium is often lower than that of pure water. Therefore, when the temperature in the segmented sealing area rises to the range of -7℃ to -13℃, it basically loses its ability to freeze the coal seam. At this time, the liquid nitrogen treatment is completed.

[0019] The area was impacted again by a high-temperature and high-pressure water jet of 90-130MPa, which caused a large area of ​​damage to the frozen borehole coal seam.

[0020] (7) Repeat steps (5) and (6) to inject room temperature nitrogen and low temperature liquid nitrogen into the above area again, and then use high temperature water jets of different pressures and temperatures to repeatedly impact the area;

[0021] The water jet impact is as follows: the water flow rate is 1-2.5 m³ / min, and the injection time is 5-35 min; the first injection is a room temperature water jet with a pressure of 80-120 MPa; the second injection is a high-temperature water jet with a water medium temperature of 75-85℃ and a pressure exceeding the first injection by 10-20 MPa; thereafter, the water temperature decreases by 5-20℃ with each high-temperature, high-pressure water jet, while the pressure remains constant; the amount of cryogenic liquid nitrogen injected decreases by 10-30% each time; the initial flow rate of liquid nitrogen is 1.5-3.5 m³ / min, and the injection time is 10-30 min; the temperature of the liquid nitrogen is controlled between -200 and -205℃.

[0022] Compared with existing technologies, this invention achieves the combined action of high-temperature water jet and liquid nitrogen on the coal seam. Compared with the single effect of water jet on the coal seam, this method can produce better coal seam crushing effect, better permeability enhancement effect, and increase gas production. At the same time, it makes the liquid nitrogen less susceptible to gasification due to coal seam temperature during long-distance transportation, thus affecting the final permeability enhancement effect.

[0023] Liquid nitrogen has three main effects on coal seams. First, the cooling effect of liquid nitrogen induces thermal stress in the coal seam. Second, after being perforated by a high-temperature water jet, residual water in the borehole enters the coal seam fractures; the injected liquid nitrogen rapidly freezes this water, causing the coal seam to freeze. Third, the liquid nitrogen expands rapidly as it changes from a liquid to a gaseous phase. These three effects work together to widen the coal seam fractures and increase gas production.

[0024] Alternating water jets and liquid nitrogen applied to the coal seam improves its permeability. Liquid nitrogen can be directly generated using a nitrogen generator and liquid nitrogen producer, reducing the need for bottled liquid nitrogen and lowering costs. Simultaneously, using vacuum-insulated pipelines minimizes liquid nitrogen loss during transportation and improves the freezing, thermal stress, and expansion effects of the liquid nitrogen. A PLC control system is connected to the control panel of the rapid-fire heater to monitor the internal water temperature and flow rate. A pressure sensor at the high-pressure water pump is connected to the PLC control system to monitor the water jet pressure. The control panels of the nitrogen generator and liquid nitrogen producer are connected to the PLC control system to monitor the liquid nitrogen production process. The liquid nitrogen storage tank is connected to the PLC control system to monitor the liquid nitrogen storage capacity and pressure. A pressure sensor at the cryogenic pump is connected to the PLC control system to monitor the internal pressure of the transport pipeline in real time.

[0025] This invention utilizes a combination of high-temperature water jetting and low-temperature liquid nitrogen to alternately treat coal seams within boreholes. High-pressure water jetting impacts the coal seam, creating a damaged zone around the borehole and promoting the development of primary and secondary fractures within the coal seam. Injecting room-temperature nitrogen at a certain pressure into the water jetting area drives away existing moisture, providing a channel for liquid nitrogen intrusion and increasing the treatment range and effectiveness. Subsequently, low-temperature liquid nitrogen is injected, creating thermal stress in the coal seam around the borehole. This, combined with the freezing of water within the coal seam (causing ice wedge heave) and the expansion force of the liquid nitrogen phase change, further expands the pore fractures within the borehole. Finally, high-temperature water jetting is used again to impact the aforementioned area, using the impact load of the high-temperature, high-pressure water jet to create a larger-scale damaged zone in the frozen coal seam. Simultaneously, the high-temperature water causes thermal cracking of the low-temperature coal matrix, resulting in thermal damage. This further develops pore fractures within the borehole. Repeating the combined high-temperature water jet and low-temperature nitrogen treatment, under the alternating cycle of these two processes, deteriorates the coal structure within the borehole, increases the damaged area, and fosters the development of primary and secondary fractures. These fractures interconnect, providing pathways for gas migration and thus improving the permeability of the borehole coal seam. Attached Figure Description

[0026] Figure 1 A schematic diagram of cross-layer drilling and the development of coal seam fractures;

[0027] Figure 2 This diagram illustrates the drilling along the coal seam and the development of coal seam fractures.

[0028] Figure 3 This is a schematic diagram of the layout of hydraulic drilling and monitoring boreholes; in the diagram: a) monitoring boreholes are arranged in parallel; b) is a cross-sectional view of the parallel arrangement; c) monitoring boreholes are arranged vertically.

[0029] Figure 4 Flowchart of a method for improving the permeability of coal seams inside boreholes;

[0030] In the diagram: 1-Jet nozzle; 2-Vacuum insulation pipe; 3-Sealing plug; 4-Nitrogen generator; 5-Liquid nitrogen generator; 6-Liquid nitrogen storage tank; 7-Cryogenic pump; 8-PLC control system; 9-High-pressure water pump; 10-Rapid heater; 11-Liquid nitrogen tank group; 12-High-pressure water pipe; 13-Flange connection; 14-Coal seam. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials publicly cited herein and cited by them are incorporated herein by reference.

[0033] Equivalent techniques of the specific embodiments described herein, which can be understood by those skilled in the art through conventional experiments, are all included in this application. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods.

[0034] Example 1: A coal seam permeability enhancement device using temperature-variable jet in low-permeability coal seams in underground coal mines, such as... Figure 1 As shown, the system includes a jet nozzle installed at the outlet of a high-pressure water pipe. The inlet of the high-pressure water pipe is connected to a high-pressure water pump, and the high-pressure water pump is connected to a rapid heater. The jet nozzle is also connected to a liquid nitrogen generator via a vacuum insulation pipe. The inlet of the liquid nitrogen generator is connected to a nitrogen generator, and the outlet is connected to the inlet of a liquid nitrogen storage tank. The outlet of the liquid nitrogen storage tank is connected to a liquid nitrogen tank group, and the outlet of the liquid nitrogen tank group is connected to a cryogenic pump. The outlet of the cryogenic pump is connected to the jet nozzle via a vacuum insulation pipe. The rapid heater, high-pressure water pump, cryogenic pump, liquid nitrogen storage tank, liquid nitrogen generator, and nitrogen generator are all connected to a PLC control system.

[0035] The outlet of the nitrogen generator is connected to the liquid nitrogen generator to provide the jet medium for the liquid nitrogen jetting equipment. The outlet of the liquid nitrogen generator is connected to the inlet of the liquid nitrogen storage tank. The liquid nitrogen flows through the vacuum insulated pipeline via a cryogenic pump to the jet nozzle. The internal pressure of each tank in the liquid nitrogen tank group is different. The liquid nitrogen tank group provides different internal pressures to achieve multi-stage cryogenic fracturing effect.

[0036] To facilitate the application of liquid nitrogen into the water jet perforation, thereby increasing coal seam permeability and gas production, a sealing device is required to seal the perforation. To facilitate automated control, a PLC control system is also included, which is connected to a rapid heater, a high-pressure water pump, a cryogenic pump, a liquid nitrogen generator, and a nitrogen producer.

[0037] The method for enhancing permeability in low-permeability coal seams using a coal seam permeability enhancement device with temperature-variable jet flow in underground coal mines comprises the following steps:

[0038] (1) Based on the underground coal seam occurrence conditions, physical properties, and working pressure of the water jet equipment, determine the construction process parameters for hydraulic drilling and monitoring boreholes, and rationally arrange the borehole positions and spacing to avoid gas drainage blank areas or gas drainage overlap areas in the later stages. The commonly used hydraulic drilling size is 80-170mm, and the monitoring borehole size is 42-75mm. Based on the actual site conditions and the difficulty of monitoring borehole construction, determine whether the monitoring boreholes are arranged in parallel or vertical hydraulic drilling methods. Due to the disturbance of underground construction and the impact of rock pressure, borehole collapse and blockage are the main problems faced in the later stages, thus affecting the later coal seam permeability enhancement effect and the evaluation of the effective drainage radius.

[0039] Therefore, a borehole layout of one hydraulic drilling hole plus six or seven monitoring boreholes is adopted. Considering the actual permeability enhancement radius of the underground coal seam hydraulic treatment measures, the spacing between the monitoring boreholes and the hydraulic drilling holes is as follows: L1 = 1.2-1.6m, one monitoring borehole; L2 = 2.0-2.2m, one to two monitoring boreholes; L3 = 2.6-3.00m, two monitoring boreholes; L4 = 3.2-3.6m, two monitoring boreholes. The arrangement of the monitoring boreholes is as follows: Figure 1 As shown, only one borehole needs to be set for close-range monitoring. Even if a borehole collapse occurs, subsequent monitoring boreholes can still provide effective monitoring data. Two boreholes are set for long-range monitoring to ensure that the effective permeability radius of the coal seam can be effectively determined later.

[0040] (2) Select a high-pressure water jet nozzle with a size of 1.8-2.6mm according to the mechanical parameters of the coal seam and the requirements of field application, control the water jet pressure at 80-120MPa and the water temperature at 0-90℃;

[0041] (3) To carry out water jet drilling, the rapid heater is started through the PLC control system to heat the water. The water temperature is quickly raised to the specified temperature. The valve switch of the rapid heater is opened and the high-pressure water pump is started. After the water is pressurized by the high-pressure water pump, it reaches the jet nozzle through the vacuum insulation pipe. A high-speed water jet is formed at the jet nozzle to impact the coal seam. The high-speed impact of the water jet breaks the coal seam and finally forms a borehole of a certain size.

[0042] High-pressure water jets of 80-120MPa are used to impact the coal seam inside the borehole. The high-speed impact of the water jets breaks up the coal seam and forms a perforation hole. At the same time, a waterproof rangefinder is set at the center of the drill rod near the nozzle to transmit the parameters of the perforation area and ensure the perforation effect of the water jet. The water jet impact parameters are appropriately adjusted according to the perforation data transmitted by the rangefinder, depending on the presence of interbedded rock or weak surfaces in the coal seam. This results in a large area of ​​uniform damage and destruction within the hole.

[0043] In the first water jet punching, there is no need to heat the water; the punching is formed directly by the impact of the water jet, which facilitates the subsequent injection of liquid nitrogen.

[0044] (4) The auger drill rod is used to transport the coal slag and some residual water in the hole to the outside of the hole. The coal slag and residual water are separated and recycled. The separated coal slag enters the coal bunker, and the residual water can be reused after passing through the mine water treatment device in the mine.

[0045] (5) To ensure the local low-temperature treatment effect of the coal seam inside the borehole, a segmented high-temperature and high-pressure resistant sealing device is used to partially seal the water jet perforation area. First, room-temperature nitrogen is injected into the sealing area through a nitrogen injection pipe. The pressure of the room-temperature nitrogen is used to allow water to enter the deep coal seam, and at the same time, it can provide a channel for low-temperature liquid nitrogen to enter the coal seam, thereby improving the range and effect of low-temperature treatment. The injection pressure P of the room-temperature nitrogen is controlled. 常氮 =nP 瓦斯 , where P 瓦斯 Let n be the internal gas pressure of this coal seam, and n be the pressure coefficient, n=4-7.

[0046] Afterwards, cryogenic liquid nitrogen is injected into the area. The pre-prepared liquid nitrogen is stored in a liquid nitrogen storage tank through a nitrogen generator and liquid nitrogen generator. When liquid nitrogen needs to be injected, the valve of the liquid nitrogen storage tank is opened, and the cryogenic pump is started through the PLC control system to transport the liquid nitrogen to the jet nozzle through the vacuum insulation pipe. The liquid nitrogen is then injected into the hole drilled by the water jet through the drill bit.

[0047] Considering that some liquid nitrogen may infiltrate the coal seam, the amount of liquid nitrogen injected, Q, should be controlled. 液氮 =1.3-1.7πD i 2 / 4, where D i The diameter of the local borehole damage caused by the water jet impact before the i-th nitrogen injection is mainly provided by the rangefinder. To prevent the danger caused by the volume expansion of liquid nitrogen vaporization, an automatic pressure relief valve is reserved on the segmented sealing device to partially depressurize when the high pressure generated by liquid nitrogen vaporization reaches a preset range.

[0048] Based on this, localized cooling and freezing treatment using cryogenic liquid nitrogen is used to transform the water medium in the pores and fissures inside the coal seam into ice medium in a short time, forming an ice wedge freezing and swelling effect on the coal seam. At the same time, due to the heterogeneity of the coal body itself, there are large differences in the heat transfer coefficients of different parts, generating thermal stress inside under a large temperature gradient, as well as the huge pressure generated by the vaporization of liquid nitrogen. Under the combined effect of these three factors, the pores and fissures of the coal seam are expanded and connected.

[0049] Liquid nitrogen has three main effects on coal seams. First, its cooling effect induces thermal stress in the coal seam. Second, after being jet-perforated by high-temperature water jets, the number and size of fractures within the boreholes increase. Residual water from the boreholes enters these fractures, and the injected liquid nitrogen rapidly freezes this water, causing the coal seam to freeze. Subsequent jet perforation further accelerates the melting of the frozen portion, resulting in a dramatic temperature change that further increases the number and size of fractures. Third, the rapid expansion of liquid nitrogen as it transitions from a liquid to a gaseous phase, with the gas phase's volume far exceeding that of the liquid phase, further enlarges and increases the number of fractures. These three effects combined significantly improve coal seam fracturing, substantially increasing the number and quantity of fractures, enhancing permeability, and ultimately increasing gas production.

[0050] (6) Due to the influence of some inorganic salt components in the underground formation water medium, the freezing temperature of the water medium is often lower than that of pure water. Therefore, when the temperature in the segmented sealing area rises to the range of -7℃ to -13℃, it basically loses its ability to freeze the coal seam. At this time, the liquid nitrogen treatment is completed. Then, the above area is impacted again with a high-temperature water jet of 90-130MPa. The impact of the high-temperature and high-pressure water jet creates a large-scale damage zone on the frozen borehole coal seam. At the same time, under the action of high-temperature water, the low-temperature coal matrix undergoes thermal cracking, thus forming thermal damage. On this basis, the pores and fractures in the coal seam inside the borehole are further developed.

[0051] (7) Repeat steps 5-6 to inject cryogenic liquid nitrogen into the area again, and then repeatedly impact the area with high-temperature water jets at different pressures. Repeat the above process multiple times.

[0052] The first treatment uses room-temperature water, followed by a high-temperature water jet at 75-85℃. Subsequent jets exhibit a decreasing water jet temperature, while the jet pressure increases progressively with the extent of damage. Simultaneously, the injection pressure of room-temperature nitrogen is kept constant, and the amount of liquid nitrogen injected increases progressively with the range of hydraulic perforation. Through alternating cycles of high-pressure water jet impact and low-temperature liquid nitrogen, the coal seam structure deteriorates, the damaged area increases, and primary and secondary fractures develop and interconnect, thereby improving the permeability of the coal seam. The amount of low-temperature liquid nitrogen injected decreases by 10-30% each time.

[0053] In the device and control method for improving coal seam permeability using variable temperature jets, the development status of coal seam fractures is determined by a pressure sensor on the outer wall of a vacuum insulated tube, and the injection volume of water and liquid nitrogen is determined by the pressure inside the borehole.

[0054] To achieve automated control, the PLC control system is connected to the rapid heater 10 and the liquid nitrogen storage tank 6 to monitor the internal temperature, pressure, and storage capacity. Simultaneously, the PLC control system can control the heating program of the heater. The PLC control system is also connected to pressure sensors at the high-pressure water pump 9 and the cryogenic pump 7 to monitor the pressure of the jet medium. Furthermore, the PLC control system is connected to a pressure sensor on the outer wall of the vacuum insulation tube 2 inside the borehole to monitor the pressure inside the borehole. The control panels of the nitrogen generator 4 and the liquid nitrogen generator 5 are connected to the PLC control system to monitor parameters such as flow rate and pressure during the liquid nitrogen manufacturing process, ensuring normal production.

[0055] This method achieves the simultaneous impact of two jet media, high-temperature water jet and liquid nitrogen, on the coal seam, effectively improving coal seam permeability and increasing gas production. Compared to using only water jet, this method is more effective. Furthermore, the use of vacuum-insulated pipelines effectively prevents liquid nitrogen loss during transportation, allowing the liquid nitrogen to better penetrate the coal seam.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for enhancing permeability by using a coal seam permeability enhancement device with variable temperature jet in a low-permeability coal seam in a coal mine, wherein the device includes a jet nozzle (1) installed at the outlet of a high-pressure water pipe (12), a high-pressure water pump (9) connected to the inlet of the high-pressure water pipe (12), and a rapid heater (10) connected to the high-pressure water pump (9). The jet nozzle (1) is also connected to a liquid nitrogen generator (5) via a vacuum insulation pipe (2). The inlet end of the liquid nitrogen generator (5) is connected to a nitrogen generator (4), and the outlet end is connected to the inlet end of a liquid nitrogen storage tank (6). The outlet of the liquid nitrogen storage tank (6) is connected to a liquid nitrogen tank group (11), and the outlet end of the liquid nitrogen tank group (11) is connected to a cryogenic pump (7). The outlet of the cryogenic pump (7) is connected to the jet nozzle (1) via the vacuum insulation pipe (2). The rapid heater (10), high-pressure water pump (9), cryogenic pump (7), liquid nitrogen storage tank (6), liquid nitrogen generator (5) and nitrogen generator (4) are respectively connected to the PLC control system (8); The outer wall of the vacuum insulation tube (2) is equipped with a pressure sensor, which is connected to the PLC control system. Its features are: The steps are as follows: (1) Based on the underground coal seam occurrence conditions, physical property parameters, working pressure of water jet equipment and on-site requirements, determine the construction process parameters of hydraulic drilling and monitoring drilling. The hydraulic drilling size is 80-170mm and the monitoring drilling size is 42-75mm. The monitoring drillings are arranged in parallel or perpendicular to the hydraulic drillings, with a layout of 1 hydraulic drilling and 6 or 7 monitoring drillings. The spacing between the monitoring drillings and the hydraulic drillings is as follows: L1=1.2-1.6m, 1-2 monitoring holes; L2=2.0-2.2m, 2 monitoring holes; L3=2.6-3.00m, 2 monitoring holes; L4=3.2-3.6m, 2 monitoring holes. (2) Select a high-pressure water jet nozzle with a size of 1.8-2.6mm according to the mechanical parameters of the coal seam and the requirements of field application, control the water jet pressure at 80-120MPa and the water temperature at 0-90℃; (3) Use 80-120MPa high-pressure water jet to impact the coal seam in the hydraulic borehole. When using water jet for the first time, it is not necessary to heat the water. Control the water temperature to 15-30℃. Use the high-speed impact of the water jet to break the coal seam and form a hole. At the same time, a waterproof rangefinder is set at the center of the drill rod near the jet nozzle to transmit the parameters of the hole area and ensure the water jet hole effect. For the case of interbedded rock or weak surface in the coal seam, adjust the water jet impact parameters appropriately according to the hole data transmitted by the rangefinder, so as to form a large area of ​​uniform damage zone in the hydraulic borehole. (4) Use the auger drill rod to transport the coal slag and residual water in the hydraulic borehole to the outside of the hydraulic borehole, and then perform secondary separation and recycling of the coal slag and residual water in the well. (5) Use a segmented temperature and pressure resistant sealing device to partially seal the water jet punching area, inject room temperature nitrogen into the sealing area, and control the injection pressure P of room temperature nitrogen. 常氮 =nP 瓦斯 , where P 瓦斯 The gas pressure inside the coal seam is given by n, where n is the pressure coefficient (n=4-7). Injected room-temperature nitrogen compresses the moisture in the coal seam, allowing it to penetrate deeper and providing a channel for liquid nitrogen to enter. After the room-temperature nitrogen injection is complete, cryogenic liquid nitrogen is injected, with the injection rate Q controlled. 液氮 =1.3-1.7πD i 2 / 4, where D i The diameter of the local borehole damage caused by the water jet impact before the i-th nitrogen injection is provided by the rangefinder on the drill pipe; at the same time, an automatic pressure relief valve is reserved on the segmented sealing device to partially relieve pressure when the high pressure generated by liquid nitrogen vaporization reaches the preset range; (6) Due to the influence of some inorganic salt components in the underground formation water medium, the freezing temperature of the water medium is often lower than that of pure water. Therefore, when the temperature in the segmented sealing area rises to the range of -7℃ to -13℃, it basically loses its ability to freeze the coal seam. At this time, the liquid nitrogen treatment is completed. The second impact of a 90-130MPa high-temperature and high-pressure water jet into the aforementioned area caused a large-scale damage zone to the frozen borehole coal seam. (7) Repeat steps (5) and (6) to inject room temperature nitrogen and low temperature liquid nitrogen into the above area again, and then use high temperature water jets with different pressures and temperatures to repeatedly impact the area.

2. The method according to claim 1, characterized in that: The high-pressure water pump is equipped with a water injection valve at its outlet, and the low-temperature pump is equipped with a nitrogen injection valve at its outlet.

3. The method according to claim 1, characterized in that: The jet nozzle (1) is installed in the coal seam; the ends of the high-pressure water pipe (12) and the vacuum insulation pipe (2) are provided with sealing plugs (3) between them and the coal seam; the jet nozzle (1) is connected to the high-pressure water pipe (12) and the vacuum insulation pipe (2) by flanges respectively.

4. The method according to claim 1, characterized in that: The jet nozzle penetrates the coal seam to a depth of 1-2m to enhance coal seam permeability; and continues to enhance permeability with variable temperature jet at a distance of 1.5-6.5m.

5. The method according to claim 1, characterized in that: The water jet impact is as follows: the water flow rate is 1-2.5 m³ / min, and the injection time is 5-35 min; the first water jet is at room temperature with a pressure of 80-120 MPa, the second water jet is at high temperature with a water medium temperature of 75-85℃ and a pressure exceeding the first by 10-20 MPa, and thereafter the water temperature in the high-temperature and high-pressure water jet decreases by 5-20℃ each time, while the pressure remains constant; the amount of cryogenic liquid nitrogen injected each time decreases by 10-30% each time; the initial flow rate of liquid nitrogen is 1.5-3.5 m³ / min, and the injection time is 10-30 min; the temperature of liquid nitrogen is controlled between -200 and -205℃.

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