CN113250737A

CN113250737A - Coal mining system and method for gas fracturing and gas displacement

Info

Publication number: CN113250737A
Application number: CN202110579187.5A
Authority: CN
Inventors: 张健; 赵文韬; 尹玉龙; 刘练波; 张国祥; 李朝
Original assignee: Huaneng Clean Energy Research Institute
Current assignee: Huaneng Clean Energy Research Institute
Priority date: 2021-05-26
Filing date: 2021-05-26
Publication date: 2021-08-13

Abstract

The invention discloses a coal mining system and a coal mining method for gas fracturing and gas displacement, which comprises a gas source tank, a high-pressure pump and a high-pressure gas storage tank which are sequentially connected; the gas of the gas source tank is single gas of carbon dioxide, nitrogen, air and other gases, or the mixture of a plurality of gases; the output end of the high-pressure gas storage tank is divided into two paths, and the first path adopts a fracturing pipe to be sequentially connected with a high-pressure control valve and one of gas injection pipes; the second path is connected with a pressure reducing valve, a low-pressure gas storage tank and another gas injection pipe in sequence; two gas injection pipes are fixedly arranged in holes of the coal seam, and exhaust pipes are arranged around the gas injection pipes. The gas with lower cost and higher safety can be flexibly selected to crack and displace gas, and the cracking and displacing equipment is integrated, so that the cost and the occupied space are saved.

Description

Coal mining system and method for gas fracturing and gas displacement

Technical Field

The invention belongs to the field of gas control, and relates to a coal mining system and a coal mining method for gas fracturing and gas displacement.

Background

Most coal seams in China have low permeability coefficient, high gas content and high gas control difficulty, and the coal production safety is seriously influenced. The gas extraction technology is a common method for controlling gas disasters in daily production of coal mines, and the enhanced gas extraction technology and matched equipment are particularly important for high-gas mines with low gas permeability and outburst dangerous coal seam mines.

With the intensive research of people on gas treatment technology, a plurality of new coal mining methods are proposed, and the main principle is to destroy the fracture structure of the stratum and improve the air permeability, so that the high-efficiency extraction of gas is realized, and the extraction rate is improved. The existing mature mining technologies include: and (3) anti-reflection process technologies such as hydraulic punching, hydraulic fracturing, a hydraulic slotting method and deep hole presplitting blasting. However, the above techniques also have some problems, such as that the hydraulic fracturing method is easy to cause serious destabilization damage to the stratum under the condition of poor conditions of the top plate and the bottom plate, which not only consumes huge water, but also causes the fracturing fluid to pollute the underground water source; the hydraulic slotting method has obvious effect, is suitable for horizontal hole and upward hole operation, but has huge water consumption and great influence on unexplored reservoirs; the explosive blasting of the deep hole blasting method has high danger and is easy to cause gas explosion.

The existing gas fracturing and gas displacement exploitation experiment system has the following defects: (1) the research objects mainly aim at carbon dioxide, and rarely aim at air and nitrogen, which are safer and lower in cost than carbon dioxide in construction. (2) The fracturing and displacement equipment is separated independently, so that the equipment cost is increased, and the roadway space is narrow and small, so that the storage of excessive large-scale equipment is not facilitated. (3) Some devices are inconvenient to operate, cannot better control the experimental process, and even have potential safety hazards, so that great inconvenience is brought to the development of experimental research.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a coal mining system and a coal mining method for gas fracturing and gas displacement, which can flexibly select gas with lower cost and higher safety to fracture and displace gas, and the fracturing and gas displacement equipment is integrated, so that the cost and the occupied space are saved.

In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:

a coal mining system for gas fracturing and gas displacement comprises a gas source tank, a high-pressure pump and a high-pressure gas storage tank which are sequentially connected;

the gas of the gas source tank is single gas of carbon dioxide, nitrogen, air and other gases, or the mixture of a plurality of gases;

the output end of the high-pressure gas storage tank is divided into two paths, and the first path adopts a fracturing pipe to be sequentially connected with a high-pressure control valve and one of gas injection pipes; the second path is connected with a pressure reducing valve, a low-pressure gas storage tank and another gas injection pipe in sequence;

two gas injection pipes are fixedly arranged in holes of the coal seam, and exhaust pipes are arranged around the gas injection pipes.

Preferably, the input end of the gas source tank is connected with a first valve, a first filter and a second valve are connected between the gas source tank and the high-pressure pump, and a third valve and a first thermometer are connected between the high-pressure pump and the high-pressure gas storage tank.

Preferably, a safety valve and a fourth valve are arranged on the side surface of the high-pressure air storage tank.

Preferably, the high-pressure storage tank is sequentially connected with a first pressure gauge and a fifth valve through a fracturing pipe, the output end of the fifth valve is divided into two paths, and the first path is sequentially connected with a high-pressure control valve, a first check valve, a second filter and one of the gas injection pipes through the fracturing pipe;

the second path is connected with a pressure reducing valve, a second thermometer, a second pressure gauge, a low-pressure gas storage tank, a seventh valve, a second check valve, a third pressure gauge, a third filter and another gas injection pipe in sequence;

and a sixth valve is arranged on the side surface of the low-pressure air storage tank.

Preferably, the inner diameter of the fracturing pipe is 0.5-20cm, and the maximum bearable pressure is more than 30 MPa.

Preferably, the outer pipe walls of the gas injection pipe and the gas extraction pipe are located at the positions of orifices of the coal seam and are fixed to the coal seam in an orifice sealing mode, and the outer pipe walls of the gas injection pipe and the gas extraction pipe located inside coal seam holes are fixed to the sealing contact surface of the coal seam in an internal hole sealing mode.

Preferably, the circumferential surface of the gas injection pipe or the gas extraction pipe is provided with 0 or more holes, and the holes are arranged in a spiral manner, in a one-way equal interval manner or in a two-way equal interval symmetrical manner.

Preferably, the gas injection pipe and the air exhaust pipe are arranged in a multipoint way, and the gas injection pipe is positioned in the middle of the air exhaust pipes.

A coal mining method for gas fracturing and gas displacement based on any one of the systems comprises the following steps;

drilling a hole in a coal seam, and placing a gas injection pipe and an exhaust pipe;

injecting gas into the gas source tank, injecting the gas into the high-pressure pump, and then injecting pressurized gas into the high-pressure gas storage tank;

step three, if fracturing is carried out, closing the pressure reducing valve, opening the high-pressure control valve, and allowing high-pressure gas to enter the gas injection pipe to carry out gas fracturing; if the displacement is carried out, the high-pressure control valve is closed, the pressure reducing valve is opened, the gas pressure is reduced to the displacement pressure, and after the gas is injected into the low-pressure gas storage tank, the gas enters the gas injection pipe again to carry out the gas displacement.

Compared with the prior art, the invention has the following beneficial effects:

the fracturing and displacement equipment is integrated together and is used as two branches to be connected with the high-pressure gas storage tank, so that the use cost of the equipment is saved, the occupied space of a system is reduced, and the fracturing and displacement equipment is integrated together, so that the operation and the control are convenient; the gas species which is conveyed to the inside of the system through the gas source tank to be cracked or displaced can be single-species gas in carbon dioxide, nitrogen, air and other gases or mixture of multiple gases, and the gas with lower cost and higher safety can be flexibly selected.

Further, the outer walls of the gas injection pipe and the exhaust pipe are in sealing contact with the coal bed, and the gas injection pipe and the exhaust pipe are fixed in a mode of combining an internal hole sealing mode and an orifice sealing mode. The internal hole sealing is to seal a strong fracture zone to the outer side of the hole sealing section, so that the leakage of injected gas is prevented, the fracturing and displacement production effects are improved, and the extraction efficiency is improved; the orifice sealing has a fixing effect on the hole sealing pipe, and the hole sealing pipe is prevented from shaking or breaking due to high pressure, so that the fracturing or extraction effect is influenced.

Drawings

FIG. 1 is a schematic diagram of the system of the present invention;

FIG. 2 is a schematic view of the sealing method between the gas injection pipe or the gas extraction pipe and the coal seam according to the present invention;

FIG. 3 is a schematic view of a first hole distribution pattern on the side of the gas injection tube or gas extraction tube according to the present invention;

FIG. 4 is a schematic view of a second hole arrangement on the side of the gas injection tube according to the present invention;

FIG. 5 is a schematic view of a third hole distribution pattern on the side of the gas injection pipe according to the present invention;

FIG. 6 is a schematic view showing a fourth hole arrangement on the side of the gas injection pipe according to the present invention;

FIG. 7 is a top view of the gas injection tube and the gas extraction tube of the present invention;

FIG. 8 is a schematic view of a three-point arrangement of the present invention;

FIG. 9 is a schematic diagram of a four-point arrangement of the present invention;

FIG. 10 is a schematic diagram of a five-point arrangement according to the present invention.

Wherein: 1-a first valve; 2-a gas source tank; 3-a first filter; 4-a second valve; 5-a high pressure pump; 6-a third valve; 7-a first thermometer; 8-high pressure gas storage tank; 9-a fourth valve; 10-safety valve; 11-a first pressure gauge; 12-a fifth valve; 13-a cleavage-inducing tube; 14-high pressure control valve; 15-a first check valve; 16-a second filter; 17-a pressure reducing valve; 18-a second thermometer; 19-a second pressure gauge; 20-a low pressure gas storage tank; 21-a sixth valve; 22-a seventh valve; 23-a second check valve; 24-a third pressure gauge; 25-a third filter; 26-a gas injection pipe; 27-an exhaust tube.

Detailed Description

The invention is described in further detail below with reference to the accompanying drawings:

as shown in fig. 1, the coal mining system for gas cracking and gas displacement according to the present invention includes: the gas source device comprises a first valve 1, a gas source tank 2, a first filter 3, a second valve 4, a high-pressure pump 5, a third valve 6, a first thermometer 7, a high-pressure gas storage tank 8, a fourth valve 9, a safety valve 10, a first pressure gauge 11, a fifth valve 12, a fracturing pipe 13, a high-pressure control valve 14, a first check valve 15, a second filter 16, a pressure reducing valve 17, a second thermometer 18, a second pressure gauge 19, a low-pressure gas storage tank 20, a sixth valve 21, a seventh valve 22, a second check valve 23, a third pressure gauge 24, a third filter 25, a gas injection pipe 26 and an air extraction pipe 27.

In the system, a first valve 1 is sequentially connected with an air source tank 2, a first filter 3, a second valve 4, a high-pressure pump 5, a third valve 6, a first thermometer 7 and a high-pressure storage tank 8.

The first valve 1 is connected with an air or nitrogen conveying pipeline, can be directly connected with the atmosphere or a ventilation shaft when the fracturing or displacing gas is air, and can be connected with a nitrogen conveying pipeline or a nitrogen storage tank when the fracturing or displacing gas is nitrogen.

The gas source tank 2 is used for storing sufficient gas and providing sufficient gas source for the pressurization of the high-pressure pump. The first filter 3 is used to filter impurities in the gas. The high-pressure pump 5 is used for pressurizing the gas in the gas source tank 2 to high pressure and storing the gas in the high-pressure gas storage tank 8, and the pressure bearing of the high-pressure gas storage tank 8 is recommended to be designed to be generally more than 30 MPa.

The first thermometer 7 is used for monitoring the temperature of the high-pressure air storage tank 8. The first pressure gauge 11 is used for monitoring the gas pressure of the high-pressure gas storage tank 8. A safety valve 10 and a fourth valve 9 are arranged on the side surface of the high-pressure gas storage tank 8, and the fourth valve 9 is used for emptying gas in the high-pressure gas storage tank 8. The safety valve 10 is used for preventing the high-pressure gas storage tank 8 from exceeding rated pressure and protecting the safety of an equipment system.

The high-pressure storage tank 8 is connected to a first pressure gauge 11, a fifth valve 12, a high-pressure control valve 14, a first check valve 15, a second filter 16, and an air injection pipe 26 in this order via a cracking pipe 13.

The high pressure control valve 14 can withstand high pressures, typically up to 100MPa pressure is recommended. When the fracturing is carried out, the safety of the delivery of the high-pressure gas is ensured. The first check valve 15 prevents gas from flowing back into the system. The second filter 16 prevents coal dust, gas impurities, etc. from entering the system.

The outlet of the fifth valve 12 is also provided with a branch which is connected with a pressure reducing valve 17, a second thermometer 18, a second pressure gauge 19, a low-pressure air storage tank 20, a sixth valve 21, a seventh valve 22, a second check valve 23, a third pressure gauge 24, a third filter 25 and an air injection pipe 26 in sequence.

The pressure reducing valve 17 is used for reducing the high-pressure gas in the high-pressure gas storage tank 8 to the pressure required by displacement, and the pressure is regulated and then output and stored in the low-pressure gas storage tank 20. The low pressure gas tank 20 is used to store low pressure gas. The second thermometer 18 is used to monitor the temperature of the low pressure reservoir 20. The second pressure gauge 19 is used to monitor the pressure of the low pressure reservoir 20. The sixth valve 21 is used to evacuate the low pressure reservoir 20. The second check valve 23 prevents gas from flowing back into the system. The low triple pressure gauge 24 is used to monitor the gas pressure during displacement. The third filter 25 prevents coal dust from entering the system. The gas injection pipe 26 is used for injecting gas into the rock mass during fracturing or displacement.

A safety valve 10 and a fourth valve 9 are designed on the side surface of the high-pressure gas storage tank 8; the low pressure air storage tank 20 is provided with a sixth valve 21 on the side.

The fracturing pipe 13 is pressure-resistant and thick in pipe diameter, can bear the pressure of high-pressure gas, can bear more than 30MPa at most, and the inner diameter of the fracturing pipe 13 is 0.5-20cm, so that the conveying safety of the high-pressure gas is ensured, and the production effect is ensured.

The gas species to be sent to the inside of the system for cracking or displacement through the gas source tank 2 can be carbon dioxide, nitrogen, a single gas of air and other gases, or a mixture of a plurality of gases.

As shown in FIG. 2, the contact surfaces of the outer walls of the gas injection pipe 26 and the extraction pipe 27 and the coal seam seal are fixed in a manner of combining internal hole sealing and orifice sealing. The outer pipe wall of the gas injection pipe and the gas extraction pipe is located at the position of a coal seam orifice and is fixed with the coal seam in an orifice sealing mode, and the outer pipe wall of the gas injection pipe and the gas extraction pipe located inside a coal seam hole is fixed with the coal seam sealing contact surface in an internal hole sealing mode. The internal hole sealing is to seal a strong fracture zone to the outer side of the hole sealing section, so that the leakage of injected gas is prevented, the fracturing and displacement production effects are improved, and the extraction efficiency is improved; the orifice sealing has a fixing effect on the hole sealing pipe, and the hole sealing pipe is prevented from shaking or breaking due to high pressure, so that the fracturing or extraction effect is influenced.

The side wall of the gas injection pipe 26 can flexibly select different distribution modes of the holes according to production requirements. As shown in fig. 3, the side of the gas injection pipe 26 is not provided with a hole, and the injected gas is directly output from the end of the gas injection pipe 26 for blasting; as shown in fig. 4, the holes are arranged on the side surface of the gas injection pipe 26 in a spiral manner, the hole distribution phase difference and the hole distribution density can be set according to the production requirement; as shown in fig. 5, the holes distributed on the side surface of the gas injection pipe 26 are arranged in a unidirectional equal interval manner, and the interval can be flexibly designed according to production requirements; as shown in fig. 6, the holes distributed on the side of the gas injection pipe 26 are symmetrically arranged at equal intervals in both directions, and the intervals are set according to production requirements. In addition, the technical parameters of each type of gas injection pipe, such as length, pipe diameter thickness, aperture, hole number, density, phase angle and the like, can be designed according to specific requirements.

As shown in fig. 7, a plan view of the gas injection pipe 26 and the gas extraction pipe 27 is shown. 26 is a gas injection pipe, which is used for injecting gas into the coal seam when the gas is cracked or displaced; and 27 is an exhaust pipe used for extracting gas.

The gas injection pipe 26 and the air extraction pipe 27 can be arranged in a multi-point method, that is, the gas injection pipe 26 is arranged among the air extraction pipes 27, the number proportion and the distance between the gas injection pipe 26 and the air extraction pipes 27 can be flexibly designed according to requirements, and the arrangement mode of the gas injection pipe and the air extraction pipes can be one of surrounding type, staggered type, three-point method arrangement, four-point method arrangement and five-point method arrangement.

As shown in fig. 8, the three-point arrangement is such that the gas injection pipe 26 is located at the midpoint of 2 extraction pipes; FIG. 9 shows a four-point arrangement with the gas injection tube 26 at the midpoint of an equilateral triangle of 3 extraction tubes 27; FIG. 10 shows a five-point arrangement, with the gas injection tube 26 located at the midpoint of a square of 4 extraction tubes 27.

The specific embodiment of the system of the invention is introduced as follows:

(1) firstly, drilling is carried out, and the arrangement mode of the gas injection pipe 26 and the extraction pipe 27 can be selected according to the scheme provided by the figures 8-10, and the selection can be flexibly carried out according to the production requirement. FIG. 8 is a three-hole arrangement with the gas injection tube 26 at the midpoint of the 2 extraction tubes; FIG. 9 shows a four-hole arrangement with the gas injection tube 26 at the midpoint of an equilateral triangle of 3 extraction tubes 27; FIG. 10 shows a five-hole arrangement with the gas injection tube 26 at the midpoint of the square of 4 extraction tubes 27. In addition, the specific hole distribution quantity, the hole distribution mode, the spacing interval between the gas injection pipe and the gas extraction pipe, the arrangement mode and the like of the gas injection pipe 26 and the gas extraction pipe 27 can be set according to the production requirements.

(2) The selection of the hole distribution in the gas injection tube 26 and the gas extraction tube 27 is made in accordance with the schemes provided in FIGS. 3-6. FIG. 3 is a view of a cartridge with blasting or gas injection at the end; as shown in fig. 5, the holes are arranged in a spiral manner, the hole distribution phase difference and the hole distribution density are set according to the production requirement; FIG. 5 is arranged in a unidirectional equal interval mode, and the interval can be flexibly designed according to production requirements; FIG. 6 is a bidirectional symmetrical arrangement with equal intervals, and the intervals are set according to production requirements. The length, the aperture, the number of holes, the density, the phase angle, the size of the holes and other technical parameters of each type of gas injection pipe can be automatically adjusted according to specific requirements.

(3) Then hole sealing is carried out, as shown in fig. 2, a mode of combining inner hole sealing and orifice sealing is adopted, and the inner hole sealing is adopted to seal a strong fracture zone to the outer side of a hole sealing section, so that gas leakage is prevented, and the extraction efficiency is improved; the orifice sealing is adopted to fix the hole sealing pipe, so that the hole sealing pipe is prevented from shaking or breaking due to high pressure, and the extraction effect is improved.

(4) And (5) subsequently entering an equipment operation stage, as shown in figure 1, connecting the equipment, checking the air tightness of the system, and preparing for an experiment. The first valve 1 is opened to inject gas, the gas source tank 2 is filled with gas, and the gas is filtered by the first filter 3. The second valve 4 is opened to inject gas into the high pressure pump 5. The fourth valve 9 is closed, the third valve 6 is opened to inject pressurized gas into the high pressure gas tank 8, the pressure is adjusted, and the gas temperature is observed.

(5) When the fracturing is performed, the pressure reducing valve 17 is closed, the fifth valve 12 is opened, the high pressure control valve 14 is opened, and the high pressure gas enters the gas injection pipe 26 to perform gas fracturing.

(6) If displacement is performed, the sixth valve 21 is closed, the fifth valve 12 is opened, the pressure reducing valve 17 is opened, the seventh valve 22 is opened, the gas pressure is reduced to the displacement pressure, the gas is injected into the low-pressure gas storage tank 20, and the gas enters the gas injection pipe 26 for gas displacement.

(7) And similarly, changing the production scheme and continuing the subsequent production scheme.

Research shows that the gas fracturing and displacement technology has obvious anti-reflection effect and also has the following characteristics: the extraction time is shortened, and the extraction cost is reduced; the medium used for high-pressure gas blasting is air or nitrogen, so that the medium can be recycled, and the material cost is basically avoided; the air or nitrogen used for blasting cannot increase air pollution sources, increase the discharge amount of toxic gases and carbon dioxide, and generate no sparks, so that the possibility of igniting the gas due to sparks is avoided, and meanwhile, a large amount of high-pressure gas can be expanded to dilute the gas escaping due to blasting permeability increase, so that the local gas is prevented from exceeding the limit; and fourthly, after drilling, feeding the drill holes into a gas injection pipe and an exhaust pipe, reducing the labor intensity of workers, arranging blasting drill holes according to needs and being not limited by the inclination angle of the drill holes.

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims

1. a coal mining system for gas cracking and gas displacement, characterized in that it comprises a gas source tank (2), a high-pressure pump (5) and a high-pressure gas storage tank (8) connected in sequence;

The gas in the gas source tank (2) is a single type of gas among carbon dioxide, nitrogen, air and other gases, or a mixture of multiple gases;

The output end of the high-pressure gas storage tank (8) is divided into two paths, the first path adopts a fracturing pipe (13) and is sequentially connected with a high-pressure control valve (14) and one of the gas injection pipes (26); the second path is sequentially connected with a decompression pipe (13). valve (17), low-pressure air tank (20) and another gas injection pipe (26);

The two gas injection pipes (26) are fixedly arranged in the holes of the coal seam, and a gas extraction pipe (27) is arranged around the gas injection pipes (26).

2. The coal mining system for gas cracking and gas displacement according to claim 1, wherein the input end of the gas source tank (2) is connected with the first valve (1), the gas source tank (2) and the high pressure A first filter (3) and a second valve (4) are connected between the pump (5), and a third valve (6) and a first thermometer are connected between the high-pressure pump (5) and the high-pressure gas storage tank (8). (7).

3 . The coal mining system for gas cracking and gas displacement according to claim 1 , wherein a safety valve ( 10 ) and a fourth valve ( 9 ) are provided on the side of the high-pressure gas storage tank ( 8 ). 4 .

4. The coal mining system for gas cracking and gas displacement according to claim 1, characterized in that, the high-pressure storage tank (8) is sequentially connected with the first pressure gauge (11), the first pressure gauge (11), the first pressure The output ends of the five valves (12) and the fifth valve (12) are divided into two paths, the first path is connected to the high pressure control valve (14), the first check valve (15), the second one through the fracturing pipe (13) in sequence a filter (16) and one of the gas injection pipes (26);

The second path is sequentially connected with a pressure reducing valve (17), a second thermometer (18), a second pressure gauge (19), a low-pressure gas storage tank (20), a seventh valve (22), and a second check valve (23) ), a third pressure gauge (24), a third filter (25) and another gas injection pipe (26);

A sixth valve (21) is provided on the side of the low-pressure air storage tank (20).

5 . The coal mining system for gas cracking and gas displacement according to claim 1 , wherein the inner diameter of the cracking pipe ( 13 ) is 0.5-20 cm, and the maximum can withstand more than 30 MPa. 6 .

6. The coal mining system for gas cracking and gas displacement according to claim 1, characterized in that, the outer walls of the gas injection pipe (26) and the gas extraction pipe (27) are located at the position of the coal seam orifice, and the coal seam adopts the hole The gas injection pipe (26) and the gas extraction pipe (27) are located inside the coal seam hole and the outer wall of the pipe and the coal seam sealing contact surface are fixed by means of internal hole sealing.

7. The coal mining system for gas cracking and gas displacement according to claim 1, wherein the gas injection pipe (26) is provided with 0 or more holes on the peripheral surface, and the plurality of holes are arranged in a spiral pattern, One-way equal-spaced arrangement or two-way equal-spaced symmetrical arrangement.

8. The coal mining system for gas cracking and gas displacement according to claim 1, characterized in that, the gas injection pipe (26) and the gas extraction pipe (27) are arranged by a multi-point method, and the gas injection pipe (26) is located in a multi-point method. The middle position of each air suction pipe (27).

9. A coal mining method for gas cracking and gas displacement based on the system of any one of claims 1-8, characterized in that, comprising the following steps;

Step 1, drill holes in the coal seam, and place the gas injection pipe (26) and the gas extraction pipe (27);

Step 2, inject gas into the gas source tank (2), inject the gas into the high-pressure pump (5), and then inject the pressurized gas into the high-pressure gas storage tank (8);

Step 3, if fracturing is performed, then the pressure reducing valve (17) is closed, the high-pressure control valve (14) is opened, the high-pressure gas enters the gas injection pipe (26), and gas fracturing is performed; if the displacement is performed, the high-pressure control valve (14) is closed. 14), open the pressure reducing valve (17), the gas pressure is reduced to the displacement pressure, and after the gas is injected into the low-pressure gas storage tank (20), the gas enters the gas injection pipe (26) for gas displacement.