Liquid nitrogen freezing combined with carbon dioxide blasting permeability enhancing device and permeability enhancing method

Abstract

A liquid nitrogen freezing combined carbon dioxide blasting permeability increasing device and permeability increasing method, comprising a freezing pipe and a permeability increasing assembly; the permeability increasing assembly is installed inside the freezing pipe; the permeability increasing assembly comprises a flexible air bag, an exciting device and a sealing positioning ring; the sealing positioning ring is sleeved outside the flexible air bag, and the flexible air bag is provided with an extension section; an end head is sleeved and installed outside the sealing positioning ring, and the sealing positioning ring fixes the extension section on the end head; the end head is fixedly connected with the freezing pipe; the exciting device is arranged inside the flexible air bag; and a fuse is installed on the exciting device. The liquid nitrogen freezing optimizes and improves the mechanical properties of the coal body, the mechanical strength is strengthened, the brittleness is greatly improved, and then the CO2 phase change blasting is performed to crack the coal seam; for the low-permeability soft coal seam, the single blasting caused by the soft coal quality is avoided, more cracks can be generated in the coal seam after the CO2 blasting, the cracking and permeability increasing effect is greatly improved, and the gas extraction is promoted.

Description

Technical Field

[0001] This invention relates to a method for enhancing the permeability of coal seams, specifically a liquid nitrogen freezing combined with carbon dioxide blasting permeability enhancement device and method, belonging to the field of gas mining technology. Background Technology

[0002] Coal mines are my country's main energy source, and gas extraction from coal seams is a crucial step in ensuring mine safety during the mining process. However, for coal seams with low permeability and soft texture, their unique physical properties, such as fractured coal body, low permeability, and high gas content, result in low gas extraction efficiency and may even lead to safety hazards such as gas accumulation and gas explosions.

[0003] To overcome the difficulties of gas drainage in low-permeability and soft coal seams, researchers have been exploring new coal seam treatment methods. Currently used coal seam permeability enhancement technologies mainly include hydraulic fracturing, hydraulic slotting, pre-splitting blasting, loosening blasting, hydraulic perforation, and liquid nitrogen freeze-thaw fracturing. All of these methods can improve coal seam permeability and enhance gas drainage to a certain extent, but they also have their own limitations.

[0004] Existing blasting technologies, including pre-splitting blasting, loosening blasting, shaped charge blasting, and CO2 phase change blasting, all suffer from poor fracturing effects due to the tendency to cause soft coal seams to collapse. Loosening blasting and pre-splitting blasting technologies produce significant pollution in the permeable coal body and are prone to localized excessive pressure or open flames, as well as gas explosions when the gas level in the roadway exceeds the limit. Furthermore, loosening blasting technology is mostly used in hard coal seams with weak outburst strength.

[0005] In recent years, liquid nitrogen freezing technology has attracted increasing attention due to its advantages such as low consumption and no pollution. Compared with other unconventional oil and gas reservoirs, coal seams are highly heterogeneous and have relatively weak mechanical properties, making them more conducive to enhancing the damage effect of liquid nitrogen at low temperatures. Therefore, applying liquid nitrogen to coal seam permeability enhancement would yield better results. However, existing liquid nitrogen fracturing technologies mainly involve cyclic freezing and thawing of liquid nitrogen. This process deteriorates the mechanical properties of coal, promotes the development of coal pores and fractures, increases permeability, and improves gas extraction rates. However, the cyclic freezing and thawing technology requires dozens of cycles to achieve the desired permeability enhancement effect, resulting in high costs, long processing times, and a decrease in oxygen concentration due to excessive nitrogen. Summary of the Invention

[0006] The purpose of this invention is to provide a liquid nitrogen freezing combined with carbon dioxide blasting permeability enhancement device and method, which combines liquid nitrogen freezing and liquid CO2 gas phase blasting into one, enabling rapid CO2 gas phase blasting in the same area after liquid nitrogen freezing, while simplifying the operation process of the two permeability enhancement methods, and has a good promoting effect on gas extraction.

[0007] To achieve the above objectives, the present invention provides a liquid nitrogen freezing combined with carbon dioxide explosion penetration enhancement device, comprising a freezing tube and at least one penetration enhancement component; the freezing tube has a U-shaped cross-section; the penetration enhancement component is installed inside the freezing tube; the penetration enhancement component includes a flexible airbag, an activation device, and two sealing positioning rings; the two sealing positioning rings are respectively sleeved on the outer sides of both ends of the flexible airbag, with the end of the flexible airbag extending to the outer side of the sealing positioning ring, forming an extension; an end head is sleeved on the outside of the sealing positioning ring, and the inner side of the end head and the outer side of the sealing positioning ring are threadedly connected and fixed, the sealing positioning ring pressing and fixing the extension of the flexible airbag to the end head; the outside of the end head is fixedly connected to the freezing tube; the activation device is disposed inside the flexible airbag; a ignition wire is installed on the activation device, and the ignition wire passes through the end head.

[0008] The present invention provides an inlet at one end of the freezing tube and an outlet at the other end; an outlet control valve is installed at the outlet.

[0009] The present invention comprises multiple antireflection components; multiple antireflection components are connected in series inside the freezing tube; the ignition wires of two adjacent excitation devices are connected.

[0010] The end of the present invention has an H-shaped cross-section; a circular partition is provided in the middle of the end, and the outer side of the circular partition is fixedly connected to the inner side of the end; two sealing positioning rings that are close to each other in two adjacent anti-reflective components are installed on both sides of the circular partition of the same end.

[0011] The circular partition of the present invention is equipped with a liquid filling pipe; the liquid filling pipe is arranged through the circular partition.

[0012] A liquid supply unit is connected to the inlet of the freezing tube; the liquid supply unit includes a liquid nitrogen tank, a liquid nitrogen valve, a flow monitor, a pressure detector, a heat-insulated stainless steel hose, and a liquid nitrogen distributor; the liquid nitrogen distributor is connected to the inlet; the two ends of the heat-insulated stainless steel hose are connected to the liquid nitrogen distributor and the liquid nitrogen tank; the liquid nitrogen valve, flow monitor, and pressure detector are installed on the liquid nitrogen tank.

[0013] A method for enhancing penetration using liquid nitrogen freezing combined with carbon dioxide explosion includes the following steps:

[0014] S1. Drilling: Construction of freezing blasting holes, extraction holes, and temperature measurement holes in coal seams;

[0015] S2, Water injection: Injecting water into the frozen blast holes to moisten the coal body;

[0016] S3. Establish an integrated freezing and blasting system: Place the connected liquid nitrogen freezing combined with carbon dioxide blasting and permeation enhancement device into the freezing and blasting hole, place the extraction pipe into the extraction hole, and place the temperature sensor into the temperature measuring hole; connect the freezing pipe to the liquid nitrogen distributor and connect it to the liquid nitrogen tank through a heat-insulated stainless steel hose.

[0017] S4. Sealing: Grouting is performed on the outer sections of the freezing blasting holes and extraction holes to form grouting and sealing sections;

[0018] S5. Freezing: Open the liquid nitrogen valve to allow liquid nitrogen from the liquid nitrogen tank to enter the liquid nitrogen distributor through the insulated stainless steel hose. The liquid nitrogen then enters the freezing pipe through the inlet, gradually vaporizing into nitrogen gas within the freezing pipe. This nitrogen gas absorbs heat from the coal seam, freezing and embrittled the coal. When the pressure monitor detects that the liquid nitrogen pressure has risen to a predetermined value, the liquid outlet control valve of the freezing pipe opens, and nitrogen gas is discharged through the outlet of the freezing pipe. During the injection process, the liquid nitrogen volume is adjusted using a flow monitor.

[0019] S6. Liquid filling: After freezing for a certain period of time, the temperature sensor of the temperature measuring hole reaches below 5°C and remains below 5°C for 1 hour. Then, close the liquid nitrogen valve and stop injecting liquid nitrogen. Liquid CO2 is then filled into the flexible airbag through the filling tube.

[0020] S7. Blasting: Blasting is carried out by detonating the device with a fuse. Liquid CO2 rapidly expands and turns into gas. The resulting high-pressure gas acts directly on the frozen borehole wall, causing the rock mass to crack and thus achieving rock mass fracturing.

[0021] S8. Gas extraction: Within 48 hours after the blasting is completed, the high-pressure CO2 gas in the frozen blast hole is allowed to fully replace and displace CH4 in the coal to enhance gas extraction; at the same time, the frozen coal body is allowed to thaw and return to normal temperature to achieve the effect of freeze-thaw cracking of the coal body; after 48 hours, gas extraction is carried out through the extraction pipe.

[0022] Preferably, the extraction hole is more than 5m away from the freezing blast hole; the temperature measuring hole is 2m away from the freezing blast hole.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) Simple operation: Compared with other combined permeability enhancement methods, it avoids the complex process of using multiple permeability enhancement methods together. Freezing and blasting can be carried out in the same borehole. The entire operation process includes freezing, blasting and extraction steps, which are clear and easy to operate. Moreover, the operating parameters can be flexibly adjusted according to the actual situation to adapt to the mining conditions of different coal seams.

[0025] 2) High economic benefits: Compared with the two permeability enhancement measures of liquid nitrogen freezing and liquid CO2 blasting carried out in different boreholes, the integrated permeability enhancement device and method greatly improves the utilization rate of liquid nitrogen and liquid CO2, and avoids poor freezing and blasting effects caused by distance between different boreholes; moreover, the flexible airbag has low manufacturing cost, is easy to bend and fold, and is convenient to transport.

[0026] 3) Coal body structure reinforcement: Through the low-temperature freezing process, the water inside the coal body freezes and expands in volume, which effectively reinforces the coal body structure, enhances its mechanical strength, and makes the coal body more stable in subsequent operations.

[0027] 4) Increased coal brittleness: The cryogenic effect of liquid nitrogen alters the internal microstructure of the coal, increasing its brittleness. This allows for the generation of more fractures in the coal seam during subsequent CO2 phase change blasting, thereby improving gas extraction efficiency.

[0028] 5) Avoiding hole collapse: The enhanced mechanical strength of the coal effectively avoids hole collapse caused by the softness of the coal during CO2 phase change blasting, ensuring the safety and efficiency of the blasting operation.

[0029] 6) Improved fracturing and permeability enhancement: The significant increase in coal brittleness allows CO2 blasting to generate more fractures in the coal seam. These fractures provide more channels for gas flow, greatly improving the fracturing and permeability enhancement effect of the coal seam. In addition, the flexible tube can be cut according to the designed length, which is suitable for borehole length. At the same time, after the flexible tube expands, it fits tightly with the borehole, reducing the gap between the traditional rigid fracturing tube and the blast hole, resulting in a good fracturing effect.

[0030] 7) Enhanced gas extraction: After blasting, the blasting pipe valve is closed for 48 hours to allow the high-pressure CO2 gas in the blast hole to fully replace and displace CH4 in the coal. This effectively enhances the gas extraction process and improves the gas recovery rate.

[0031] 8) Secondary fracturing effect: The freeze-thaw degradation after the coal seam freezes will cause the mechanical properties of the coal seam to decrease when the coal seam returns to normal temperature, thus producing the effect of secondary fracturing of the coal body, which further improves the efficiency and effect of gas extraction.

[0032] 9) High practicality: The flexible airbag and the sealing positioning ring work together to facilitate the assembly of the flexible airbag. Multiple flexible airbags can be connected in series to adapt to different drilling lengths. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the present invention;

[0034] Figure 2 This is a cross-sectional structural diagram of the present invention;

[0035] Figure 3 This is a schematic diagram illustrating the specific use of the present invention.

[0036] 1. Freezing pipe, 2. Liquid inlet, 3. Liquid outlet, 4. Liquid outlet control valve, 5. End, 6. Sealing positioning ring, 7. Flexible airbag, 8. Activation device, 9. Ignition wire, 10. Filling pipe, 11. Pipe clamp, 12. Coal seam, 13. Grouting sealing section, 14. Freezing blast hole, 15. Extraction hole, 16. Extraction pipe, 17. Temperature measuring hole, 18. Temperature sensor, 19. Liquid nitrogen tank, 20. Liquid nitrogen valve, 21. Flow monitor, 22. Pressure detector, 23. Insulated stainless steel hose, 24. Liquid nitrogen distributor. Detailed Implementation

[0037] The invention will now be further described with reference to the accompanying drawings. Example

[0038] like Figure 1 , Figure 2 As shown, a liquid nitrogen freezing combined with carbon dioxide explosion penetration enhancement device includes a freezing tube 1 and at least one penetration enhancement component; the freezing tube 1 has a U-shaped cross-section; the penetration enhancement component is installed inside the freezing tube 1; the penetration enhancement component includes a flexible airbag 7, an activation device 8, and two sealing positioning rings 6; the two sealing positioning rings 6 are respectively sleeved on the outer sides of both ends of the flexible airbag 7, and the end of the flexible airbag 7 extends to the outer side of the sealing positioning ring 6, forming an extension; an end head 5 is sleeved on the outside of the sealing positioning ring 6, and the inner side of the end head 5 and the outer side of the sealing positioning ring 6 are threadedly connected and fixed, the sealing positioning ring 6 compresses and fixes the extension of the flexible airbag 7 onto the end head 5, sealing and fixing the flexible airbag 7; the outside of the end head 5 is fixedly connected to the freezing tube 1; the activation device 8 is disposed inside the flexible airbag 7; a fuse 9 is installed on the activation device 8, and the fuse 9 passes through the end head 5.

[0039] The flexible airbag 7 is made of rubber or plastic materials and has a certain degree of deformation capability.

[0040] In this embodiment, one end of the freezing pipe 1 is provided with a liquid inlet 2 and the other end is provided with a liquid outlet 3. Liquid nitrogen is injected from the liquid inlet 2 and flows out from the liquid outlet 3. A liquid outlet control valve 4 is installed at the liquid outlet 3 to control the flow rate of liquid nitrogen so that it can be fully vaporized and cooled inside the coal body.

[0041] In this embodiment, multiple anti-reflection components are configured; these components are connected in series inside the freezing pipe 1; and the ignition wires 9 of two adjacent ignition devices 8 are connected. This example uses two sets connected in series, but the number of sets can be increased or decreased depending on the drilling conditions to adapt to mine conditions.

[0042] In this embodiment, the cross-sectional shape of the end 5 is H-shaped; a circular partition is provided in the middle of the end 5, and the outer side of the circular partition is fixedly connected to the inner side of the end 5; two sealing positioning rings 6 close to each other in two adjacent anti-reflection components are installed on both sides of the circular partition of the same end 5.

[0043] In this embodiment, a filling pipe 10 is installed on the circular partition. The filling pipe 10 is a one-way valve, which allows liquid CO2 to flow in but not out, preventing leakage during the filling process. The filling pipe 10 passes through the circular partition. The length of the filling pipe 10 can be extended according to the sealing requirements so that liquid CO2 can be injected after sealing.

[0044] In this embodiment, the inlet 2 of the freezing tube 1 is connected to a liquid supply unit; the liquid supply unit includes a liquid nitrogen tank 19, a liquid nitrogen valve 20, a flow monitor 21, a pressure detector 22, a heat-insulated stainless steel hose 23, and a liquid nitrogen distributor 24; the liquid nitrogen distributor 24 is connected to the inlet 2; the two ends of the heat-insulated stainless steel hose 23 are connected to the liquid nitrogen distributor 24 and the liquid nitrogen tank 19; the liquid nitrogen valve 20, the flow monitor 21, and the pressure detector 22 are installed on the liquid nitrogen tank 19. Example

[0045] like Figure 3 As shown, a method for enhancing penetration using liquid nitrogen freezing combined with carbon dioxide explosion includes the following steps:

[0046] S1. Drilling: Drilling freezing blasting hole 14, extraction hole 15, and temperature measuring hole 17 in coal seam 12. In this embodiment, the extraction hole 15 is more than 5m away from the freezing blasting hole 1 to prevent the possible collapse caused by blasting from affecting the extraction pipe 16; the temperature measuring hole 17 is 2m away from the freezing blasting hole 1.

[0047] S2, Water injection: Water is injected into the frozen blast hole 14 to moisten the coal body;

[0048] S3. Establish an integrated freezing and blasting system: Place the connected liquid nitrogen freezing combined with carbon dioxide blasting and permeation enhancement device into the freezing and blasting hole 14, place the extraction pipe 16 into the extraction hole 15, and place the temperature sensor into the temperature measuring hole 17; the freezing pipe 1 is connected to the liquid nitrogen distributor 24 and connected to the liquid nitrogen tank 19 through the heat-insulated stainless steel hose 23.

[0049] S4. Sealing: Grouting is performed on the outer sections of the freezing blast hole 14 and the extraction hole 15 to form the grouting and sealing section 13.

[0050] S5. Freezing: Open the liquid nitrogen valve 20 to allow liquid nitrogen in the liquid nitrogen tank 19 to enter the liquid nitrogen distributor 24 through the heat-insulated stainless steel hose 23, and then enter the freezing pipe 1 from the inlet 2. The liquid nitrogen gradually vaporizes into nitrogen gas in the freezing pipe 1, absorbing heat from the coal seam 12 and freezing and embrittled the coal seam 12. When the pressure monitor 22 on the liquid nitrogen tank 19 detects that the liquid nitrogen pressure has risen to a predetermined value, the liquid outlet control valve 4 of the return gas section of the freezing pipe 1 opens, and the nitrogen gas is discharged from the outlet 3 through the freezing pipe 1. During the liquid injection process, the liquid nitrogen volume is adjusted by the flow monitor 21 on the liquid nitrogen tank 19.

[0051] S6. Liquid filling: After freezing for a certain period of time, the temperature of the temperature sensor in the temperature measuring hole 17 reaches below 5°C and remains below 5°C for 1 hour. Then, the liquid nitrogen valve 20 is closed to stop the injection of liquid nitrogen. Liquid CO2 is then filled through the filling tube 10 and enters the flexible airbag 7. When multiple anti-permeability components are installed in series, the bottom filling tube 10 is installed in a unidirectional air intake manner from the outside to the inside. Carbon dioxide enters the other flexible airbags 7 through the middle filling tube 10 without leakage. Ultimately, the flexible airbags 7 of the entire liquid nitrogen freezing combined with carbon dioxide explosion anti-permeability device contain liquid CO2 at the same pressure.

[0052] S7. Blasting: Blasting is carried out by detonating the trigger device 8 through the fuse 9. The liquid CO2 rapidly expands and turns into gas. The high-pressure gas formed directly acts on the frozen hole wall, causing the rock mass to crack and achieving rock mass fracturing.

[0053] S8. Gas extraction: Within 48 hours after the blasting is completed, the high-pressure CO2 gas in the frozen blast hole 14 is allowed to fully replace and displace CH4 in the coal body to achieve the purpose of enhanced gas extraction; at the same time, the frozen coal body is allowed to fully thaw and return to normal temperature to achieve the effect of freeze-thaw cracking of the coal body; after 48 hours, gas extraction is carried out through the extraction pipe 16.

[0054] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A liquid nitrogen freezing combined with carbon dioxide blasting penetration enhancement method, implemented using a liquid nitrogen freezing combined with carbon dioxide blasting penetration enhancement device, the device comprising a freezing tube (1) and at least one penetration enhancement component; the freezing tube (1) has a U-shaped cross-section; the penetration enhancement component is installed inside the freezing tube (1); the penetration enhancement component comprises a flexible airbag (7), an excitation device (8), and two sealing positioning rings (6); the two sealing positioning rings (6) are respectively sleeved on the outer sides of both ends of the flexible airbag (7), and the ends of the flexible airbag (7) extend to the sealing positioning rings. The outer side of the positioning ring (6) is the extension section; the outer side of the sealing positioning ring (6) is fitted with an end (5), the inner side of the end (5) and the outer side of the sealing positioning ring (6) are threaded and fixed, and the sealing positioning ring (6) squeezes and fixes the extension section of the flexible airbag (7) onto the end (5); the outer side of the end (5) is fixedly connected to the freezing tube (1); the activation device (8) is set inside the flexible airbag (7); the activation device (8) is equipped with a fuse (9), and the fuse (9) passes through the end (5); The freezing tube (1) is provided with an inlet (2) at one end and an outlet (3) at the other end; an outlet control valve (4) is installed at the outlet (3); The inlet (2) of the freezing tube (1) is connected to a liquid supply unit; the liquid supply unit includes a liquid nitrogen tank (19), a liquid nitrogen valve (20), a flow monitor (21), a pressure detector (22), a heat-insulated stainless steel hose (23), and a liquid nitrogen distributor (24); the liquid nitrogen distributor (24) is connected to the inlet (2); the two ends of the heat-insulated stainless steel hose (23) are connected to the liquid nitrogen distributor (24) and the liquid nitrogen tank (19); the liquid nitrogen valve (20), the flow monitor (21), and the pressure detector (22) are installed on the liquid nitrogen tank (19); Its features are, Includes the following steps: S1. Drilling: Construct freezing blasting holes (14), extraction holes (15), and temperature measuring holes (17) in the coal seam (12). S2, Water injection: Inject water into the frozen blast hole (14) to moisten the coal body; S3. Establish an integrated freezing and blasting system: Place the connected liquid nitrogen freezing and carbon dioxide blasting penetration enhancement device into the freezing blasting hole (14), place the extraction pipe (16) into the extraction hole (15), and place the temperature sensor into the temperature measuring hole (17); connect the freezing pipe (1) to the liquid nitrogen distributor (24), and connect it to the liquid nitrogen tank (19) through the heat-insulated stainless steel hose (23). S4. Sealing: Grouting is performed on the outer sections of the freezing blast hole (14) and the extraction hole (15) to form a grouting and sealing section (13). S5. Freezing: Open the liquid nitrogen valve (20) to allow the liquid nitrogen in the liquid nitrogen tank (19) to enter the liquid nitrogen distributor (24) through the heat-insulated stainless steel hose (23), and enter the freezing tube (1) from the inlet (2). The liquid nitrogen gradually vaporizes into nitrogen gas in the freezing tube (1), absorbing the heat of the coal seam (12) and freezing and embrittles the coal seam (12). When the pressure detector (22) detects that the liquid nitrogen pressure has risen to the predetermined value, the liquid outlet control valve (4) of the freezing tube (1) is opened, and the nitrogen gas is discharged from the outlet (3) through the freezing tube (1). During the liquid injection process, the amount of liquid nitrogen is adjusted by the flow monitor (21). S6, Liquid filling: After freezing for a certain period of time, the temperature of the temperature sensor of the temperature measuring hole (17) reaches below 5°C and remains below 5°C for 1 hour. Then, close the liquid nitrogen valve (20) and stop injecting liquid nitrogen. Liquid CO2 is then filled into the flexible airbag (7) through the liquid filling tube (10) set on the end (5). S7, Blasting: Blasting is carried out by detonating the ignition device (8) through the fuse (9). The liquid CO2 rapidly expands and transforms into gas. The high-pressure gas formed directly acts on the frozen hole wall, causing the rock mass to crack and achieving the cracking of the rock mass. S8. Extraction: Within 48 hours after the blasting is completed, the high-pressure CO2 gas in the frozen blast hole (14) is allowed to fully replace and drive the CH4 in the coal, while the frozen coal body is allowed to fully thaw and return to normal temperature. After 48 hours, gas is extracted through the extraction pipe (16).

2. The method for enhancing penetration by combining liquid nitrogen freezing with carbon dioxide explosion as described in claim 1, characterized in that, Multiple anti-reflection components are configured; multiple anti-reflection components are connected in series inside the freezing tube (1); the ignition wires (9) of two adjacent excitation devices (8) are connected.

3. The method for enhancing permeability by combining liquid nitrogen freezing with carbon dioxide explosion according to claim 1, characterized in that, The cross-sectional shape of the end (5) is H-shaped; a circular partition is provided in the middle of the end (5), and the outer side of the circular partition is fixedly connected to the inner side of the end (5); the two sealing positioning rings (6) that are close to each other in the two adjacent anti-reflection components are installed on both sides of the circular partition of the same end (5).

4. The method for enhancing penetration by combining liquid nitrogen freezing with carbon dioxide explosion according to claim 3, characterized in that, A filling pipe (10) is installed on the circular partition; the filling pipe (10) is arranged through the circular partition.

5. The method for enhancing penetration by combining liquid nitrogen freezing with carbon dioxide explosion according to claim 1, characterized in that, The extraction hole (15) is more than 5m away from the freezing blast hole (14); the temperature measuring hole (17) is 2m away from the freezing blast hole (14).

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