JP7062598B2 - Methane gas production equipment and methane gas production method - Google Patents

Description

The present invention relates to a technique for producing methane gas from a methane hydrate layer.

Methane hydrate (also called gas hydrate), which exists in the deep sea and frozen soil layers, is attracting attention as a non-conventional resource. Methane hydrate is a solid substance that exists in a state in which guest molecules containing methane molecules as a main component are incorporated into the cage structure (clathrate) of water molecules under predetermined temperature and pressure conditions.

Non-Patent Document 1 proposes a method for producing methane gas by a decompression method from a sand layer type methane hydrate layer formed in a stratum on the seabed.
The decompression method is a method of decomposing methane hydrate by pumping water in the production well formed so as to penetrate the methane hydrate layer to reduce the pressure inside and around the production well.

The methane gas and water generated by the decomposition of methane hydrate flow into the production well and then are separated into gas and liquid. The water after the gas-liquid separation is pumped up by using a pump provided at the bottom of the riser constituting the water pumping flow path.
Further, the methane gas separated from the water rises in the riser in the gas flow path formed separately from the water pumping flow path and is extracted to the sea.

According to the above decompression method, it is considered that methane gas can be continuously produced even from a sand layer type methane hydrate layer. On the other hand, installing equipment such as pumps and separators for separating gas and liquid in production wells located on the seabed at a depth of several hundred meters or more may increase the manufacturing and installation costs of each equipment. There is also the problem that maintenance of the equipment is difficult.

Here, Patent Documents 1 and 2 describe a surface layer type located on the surface layer of the seabed by using a method called an air bubble pump or an air lift that blows a gas for transportation into a riser (transport pipe, transportation pipe) that transports methane hydrate. A technique for sucking up methane hydrate lumps from the methane hydrate layer of the above is described.
However, Patent Documents 1 and 2 do not disclose a technique that enables continuous production of methane gas by a gas lift even from a sand layer type methane hydrate layer existing in the ground.

Special Table 2002-536573 Publication No. Japanese Patent Application Laid-Open No. 2003-262803

Methane Hydrate Resource Development Research Consortium, "Gas Production from Methane Hydrate", Internet <URL: http://www.mh21japan.gr.jp/mh/05-2/>

The present invention has been made under such a background, and an object of the present invention is to provide a methane gas production facility and a methane gas production method capable of continuously producing methane gas from a methane hydrate layer by a gas lift. It is in.

The methane gas production facility of the present invention is a methane gas production facility that produces methane gas from a methane hydrate layer.
A production well provided on the bottom of the water and formed to communicate with the methane hydrate layer,
The lower end side is airtightly communicated with the internal space of the production well, and a riser pipe for extracting the gas-liquid mixed fluid containing methane gas and water from the production well and sending it onto the water in the gas-liquid mixed state.
A lift gas supply pipe for supplying the lift gas supplied from the lift gas supply unit into the riser pipe, and
A gas-liquid separation unit provided on the water and connected to the upper end of the riser pipe to separate the gas-liquid mixed fluid flowing out of the riser pipe into methane gas and water is provided.
By mixing the lift gas supplied from the lift gas supply pipe with the water filled in the riser pipe and the production well to reduce the pressure in the production well, methane hydrate ( methane hydrate) is formed in the methane hydrate layer communicating with the production well. The gas-liquid mixed fluid obtained by decomposing (excluding those crushed by excavation) is raised through the riser pipe.
The lift gas is supplied when the methane hydrate is decomposed into a self-injection state by maintaining the depressurization in the production well due to the lift gas effect accompanying the formation of the flow in which the gas-liquid mixed fluid rises in the riser pipe. It is characterized by stopping the supply of lift gas from the pipe .

The methane gas production facility may have the following features.
(A) The lift gas is methane gas separated from the gas-liquid mixed fluid at the gas-liquid separation section. Further, the lift gas supply unit shall be a compressor that compresses oxygen-free gas.
(B) A plurality of production wells are provided on the bottom of the water, and these plurality of production wells are connected to the common riser pipe via a connecting pipe .

Further, the methane gas production method according to another invention is a methane gas production method for producing methane gas from a methane hydrate layer.
Lift gas is mixed into the production well provided on the bottom of the water and formed so as to communicate with the methane hydrate layer, and the water filled in the riser pipe whose lower end side communicates airtightly with the internal space of the production well. The process of decompressing the production well and decomposing methane hydrate (excluding those crushed by excavation) in the methane hydrate layer communicating with the production well.
A step of raising a gas-liquid mixed fluid containing methane gas and water obtained by decomposing the methane hydrate in a gas-liquid mixed state via the riser tube and sending it onto water.
A step of separating the gas-liquid mixed fluid flowing out from the upper end of the riser pipe into methane gas and water on the water.
When the decompression in the production well is maintained due to the lift gas effect accompanying the formation of the flow in which the gas-liquid mixed fluid rises in the riser pipe, and the decomposition of the methane hydrate progresses into a self-injection state, the lift gas is introduced. It is characterized by including a step of stopping the supply .

The methane gas production method may have the following features.
( C ) The lift gas is an oxygen-free gas. The oxygen-free gas shall be methane gas separated from the gas-liquid mixture fluid.
( D ) The methane gas separated from the gas-liquid mixed fluid is supplied to at least one methane gas utilization facility selected from the methane gas utilization facility group consisting of the methane gas liquefaction facility, the synthetic gas production facility, the thermal power generation facility, and the city gas supply facility. To be done .

The present invention reduces the pressure in the production well by supplying lift gas into a riser pipe that is airtightly connected to the production well that produces methane gas from the methane hydrate layer, and methane hydrate in the region communicating with the production well. To disassemble. As a result, the obtained gas-liquid mixed fluid of methane gas and water rises in the riser tube, so that methane gas can be produced with a simple and inexpensive configuration.

It is explanatory drawing which shows the structural example of the methane gas production facility which concerns on embodiment. It is a 1st operation diagram of the said methane gas production facility. It is the 2nd operation diagram of the said methane gas production facility. It is a 3rd operation diagram of the said methane gas production facility. It is a 4th operation diagram of the said methane gas production facility. It is explanatory drawing which shows the other structural example of the riser provided in the methane gas production facility. It is a first configuration example of a methane gas production facility equipped with a riser connected to a plurality of productivity. It is a second configuration example of a methane gas production facility equipped with a riser connected to a plurality of productivity.

First, the configuration of the methane gas production facility according to the embodiment of the present invention will be described with reference to FIG. The methane gas production facility of this example produces methane gas from, for example, a sand layer type methane hydrate layer MHL formed on the lower side of the stratum GL constituting the seabed (water bottom).
The stratum GL containing the methane hydrate layer MHL is located on the seabed at a depth of several hundred meters or more, and the methane hydrate layer MHL is located several tens to several hundred meters below the seafloor of this stratum GL. ing.

The production well 2 has a structure in which a tubular casing 21 is arranged so as to extend downward from the seabed of the stratum GL toward the methane hydrate layer MHL located on the lower side of the stratum GL. .. The casing 21 is composed of, for example, a metal pipe having a diameter of several tens of centimeters to several meters, and is fixed to the formation GL by cement (not shown).

The lower region of the casing 21 is inserted into the methane hydrate layer MHL, and the lower region is formed with a finishing layer 22 communicating with the methane hydrate layer MHL via a perforation, a sand screen, or the like. Further, the lower end portion of the casing 21 may be opened toward the methane hydrate layer MHL, or may be closed by providing a casing shoe.

A flange-shaped riser base 23 is provided on the upper surface of the production well 2, and a flow path that penetrates the inside of the riser base 23 in the vertical direction is formed in the riser base 23. A tubing 27 is connected to the lower surface of the riser base 23 so as to communicate with the flow path. The tubing 27 is composed of a metal pipe having a diameter of several tens of centimeters to several meters, which is smaller than the inner diameter of the casing 21, and is inserted from the riser base 23 toward the internal space of the casing 21 (production well 2). ing. The lower end of the tubing 27 opens toward the lower region of the casing 21 on which the finishing layer 22 is formed, and the gap between the casing 21 and the tubing 27 at the lower end is closed by the packer 24. There is.

The riser pipe 11 is composed of, for example, a metal pipe or a flexible pipe having a diameter of several tens of centimeters to several meters, and the lower end portion of the riser pipe 11 is connected to the upper surface of the riser base 23. The riser tube 11 connected to the tubing 27 via the riser base 23 is in a state of airtight communication with the internal space of the production well 2.

A floater 3 is provided on the sea above the production well 2. The riser tube 11 extends upward from the seabed connected to the riser base 23 to the floater 3 through the sea. The floater 3 receives a gas-liquid mixed fluid, which will be described later, extracted from the methane hydrate layer MHL via a riser pipe 11, and separates the gas-liquid mixed fluid into methane gas and water. A gas-liquid separation unit) 31 is provided. The upper end of the riser pipe 11 is connected to the gas-liquid separation tank 31, and a control valve V1 is installed upstream or downstream of the control valve V1 (in FIGS. 1 to 6, the control valve V1 is installed upstream of the gas-liquid separation tank 31. An example provided is shown).

Further, the gas-liquid separation tank 31 has a line for extracting methane gas after gas-liquid separation from the space on the upper side in the gas-liquid separation tank 31, and air from the lower region in the gas-liquid separation tank 31. It is connected to a line for draining water after liquid separation. The methane gas extraction line is provided with a gas compressor 32 for boosting and shipping the methane gas extracted from the gas-liquid separation tank 31. On the other hand, on the downstream side of the water extraction line, a wastewater treatment section (not shown) is provided for performing necessary wastewater treatment before discharging the water.

Further, from the downstream side of the gas compressor 32, a line for extracting a part of the boosted methane gas is branched. This branch line is connected to the upstream end of the lift gas supply pipe 12 via the flow rate adjusting valve V2. From this point of view, the gas compressor 32 also has a function as a lift gas supply unit that supplies lift gas to the lift gas supply pipe 12.

Further, as shown in FIG. 1, at the upstream end of the lift gas supply pipe 12, a nitrogen gas supply unit 33 including a nitrogen gas tank for supplying nitrogen gas to the lift gas supply pipe 12 and a gas compressor is provided to adjust the flow rate. It is connected via a valve V3. The nitrogen gas supply unit 33 also corresponds to the lift gas supply unit that supplies the lift gas to the lift gas supply pipe 12.

The lift gas supply pipe 12 is extended downward from the floater 3 to the seabed along the extension direction of the riser pipe 11. In this example, the lower end of the lift gas supply pipe 12 is connected to the riser base 23 described above and communicates with the riser pipe 11 via a flow path formed in the riser base 23.

The lift gas supply pipe 12 functions to supply methane gas or nitrogen gas, which is an oxygen-free gas, as a lift gas toward the inside of the riser pipe 11. Therefore, when the riser pipe 11 is filled with water W, the gas compressor 32 and the nitrogen gas supply unit 33 that supply the lift gas to the lift gas supply pipe 12 are connected to the riser pipe 11 side from the riser pipe 11 side. It has the ability to supply lift gas at a pressure that allows a desired flow of lift gas to be mixed into the riser pipe 11 against the applied water pressure.

The reason for adopting the oxygen-free gas as the lift gas is to prevent the formation of a combustible air-fuel mixture when producing the methane hydrate layer MHL methane gas. Therefore, it is not denied that these lift gases contain a small amount of oxygen as an unavoidable component that does not form a combustible air-fuel mixture.

Explaining the operation of the methane gas production facility having the configuration described above, as shown in FIG. 2, the inside of the production well 2 and the riser pipe 11 is initially filled with water W. When laying the riser pipe 11, if the riser pipe 11 is lowered to the connection position with the riser base 23 without covering the lower end of the riser pipe 11, seawater enters the riser pipe 11. Further, even when the production well 2 is installed, seawater enters the inside of the production well 2. Before the start of production of methane gas, the water W filled in the production well 2 and the riser pipe 11 is, for example, these seawater.

Next, as shown in FIG. 3, the control valve V1 between the riser pipe 11 and the gas-liquid separation tank 31 is opened, and the flow rate control valve V3 on the nitrogen gas supply unit 33 side is opened from the nitrogen gas supply unit 33. Nitrogen gas, which is a lift gas, is supplied to the lift gas supply pipe 12. The nitrogen gas flows downward in the lift gas supply pipe 12 and is mixed with the water W in the riser pipe 11 via the connection position with the riser base 23.

As a result, a gas-liquid mixed fluid of nitrogen gas bubbles B and water W is formed above the nitrogen gas mixing position. Since the specific gravity of this gas-liquid mixed fluid is smaller than the specific gravity of the water W on the lower side of the nitrogen gas mixing position, the lower water W having a large specific gravity is lifted to the upper side of the nitrogen gas mixing position. (Gas lift effect). Due to this gas lift effect, the pressure in the production well 2 below the gas mixing position decreases, and the gas-liquid mixed fluid starts to rise in the riser pipe 11.

As the gas-liquid mixed fluid rises in the riser pipe 11 and the pressure in the production well 2 that communicates airtightly with the riser pipe 11 decreases, the surrounding methane that communicates with the production well 2 via the finishing layer 22. The pressure in the hydrate layer MHL begins to decrease. Then, when the pressure in the methane hydrate layer MHL decreases, the methane hydrate decomposes to produce methane gas and produced water. At this time, the methane hydrate layer may contain components other than methane, for example, components such as ethane and propane, and such a mixed gas of methane and other components is also referred to as "methane gas" in the following description. Call.
A mixed fluid of methane gas and water (produced water) W generated by the decomposition of methane hydrate (it may contain water that originally existed alone in the methane hydrate layer MHL without forming methane hydrate. ) Flows into the production well 2 through the finishing layer 22, and rises in the production well 2 and the riser pipe 11 due to the lift gas effect (FIG. 4).

For example, the lower end of the riser tube 11 is connected to the riser base 23 provided on the seabed at a depth of 1000 meters, and the methane hydrate layer MHL exists at a position several tens to several hundreds meters below the riser base 23. Consider the case of doing. At this time, a water pressure of about 100 Bar (10 MPaG) is applied to the inside of the riser tube 11 near the connection position with the riser base 23. In this case, even if the pressure loss in the production well 2 is taken into consideration, if the water pressure at the connection position can be reduced to about 30 to 50 Bar (3 to 5 MPaG) by the lift gas effect, the surrounding of the finishing layer 22 It is considered that the decomposition of methane hydrate can be continuously generated.

The mixed fluid that has risen in the riser pipe 11 reaches the floater 3 side, then flows into the gas-liquid separation tank 31, and is separated into methane gas and water in the gas-liquid separation tank 31. When a sufficient amount of methane gas has flowed into the gas-liquid separation tank 31 and the gas compressor 32 can be operated, the gas compressor 32 is started and the flow rate adjusting valve V2 is opened. On the other hand, the flow rate control valve V3 on the nitrogen gas supply unit 33 side is closed to switch the lift gas from nitrogen gas to methane gas (FIG. 4).

In the methane gas production facility of this example, in addition to the lift gas effect associated with the supply of lift gas from the lift gas supply pipe 12, the mixed fluid (methane gas and production water) itself that has flowed into the production well 2 from the methane hydrate layer MHL is the riser pipe 11. The lift gas effect is also exerted by the formation of a flow that rises inside. At this time, if methane gas can be produced even if the supply of methane gas is stopped, the depressurization in the production well 2 is maintained by the lift gas effect of the mixed fluid supplied from the methane hydrate layer MHL, and the decomposition of methane hydrate is decomposed. It can be said that it is in a state of self-spraying.

When the artesian aquifer is formed in this way, the supply of methane gas (lift gas) from the lift gas supply pipe 12 may be stopped as shown in FIG. Further, when the production amount of methane gas is small and stable production cannot be achieved only by the production by self-injection, the lift gas at the flow rate required to maintain the shipping flow rate may be additionally supplied as appropriate.

Further, here, it is not essential that the artesian aquifer is always formed in the methane hydrate layer MHL around the production well 2. For example, after stopping the operation of the methane gas production facility, the methane gas production can be restarted by restarting the supply of the lift gas from the lift gas supply pipe 12 or increasing the supply flow rate of the lift gas.

Due to the action of the methane gas production equipment described above, the methane gas produced in the methane hydrate layer MHL and extracted to the gas-liquid separation tank 31 via the production well 2 and the riser pipe 11 is boosted by the gas compressor 32. It is supplied to the methane gas utilization facility in the latter stage.
Methane gas utilization equipment includes methane gas liquefaction equipment that cools and liquefies methane gas, synthetic gas production equipment that produces synthetic gas containing carbon monoxide and hydrogen by chemical reaction from methane gas, and thermal power generation equipment that burns methane gas to generate power. , At least one methane gas utilization facility selected from the methane gas utilization facility group consisting of city gas supply facilities for producing city gas by adjusting the calorific value and odorizing by adding LPG (Liquefied Petroleum Gas) to methane gas is exemplified. be able to.

Here, these methane gas utilization facilities are installed outside the floater 3, for example, on the land side, a pipeline is laid between the floater 3 and the methane gas utilization facility, and the methane gas boosted by the gas compressor 32 is used for methane gas. A configuration for transporting to equipment may be adopted. Further, a methane gas utilization facility may be provided in the floater 3 to transport the liquefied gas, the synthetic gas, the electric power and the city gas obtained in the floater 3 from the floater 3 to the consumption destination.
The produced water separated from the methane gas in the gas-liquid separation tank 31 is returned to the sea after undergoing necessary wastewater treatment. If the produced water cannot be returned to the sea, it may be re-injected into the seabed.

According to the methane gas production facility according to the present embodiment, the following effects are obtained. By supplying lift gas into the riser pipe 11 that is airtightly connected to the production well 2 that produces methane gas from the methane hydrate layer MHL, the inside of the production well 2 is depressurized and the region that communicates with the production well 2 is used. Decomposes methane hydrate. As a result, the obtained gas-liquid mixed fluid of methane gas and water spontaneously rises in the riser pipe 11, so that methane gas can be produced with a simple and inexpensive configuration.

In particular, unlike the conventional decompression method proposed, the methane gas production facility of this example has a separator that separates methane gas and production water on the seabed where the production well 2 is provided, and a pump for pumping the production water. There is no need to provide it. Therefore, the manufacturing cost and the installation cost of each device constituting the methane gas production facility can be significantly reduced. In addition, it is possible to avoid the difficulty of maintenance due to the installation of power equipment on the seabed.

Furthermore, when the gas-liquid mixed fluid of methane gas and production water flows into the production well 2 and exerts the lift gas effect, and the decomposition of methane hydrate progresses spontaneously, the lift gas is used. It is also possible to reduce or stop the supply flow. As a result, it is possible to significantly reduce the operating cost of the methane gas production facility as compared with the conventional decompression method that requires pumping of the production water at all times.

Here, the lift gas supply pipe 12 is not limited to the configuration connected to the riser base 23, and may be directly connected to the riser pipe 11. Further, for example, the lift gas supply pipe 12 may be connected to the tubing 27, and the lift gas supply pipe 12 and the riser pipe 11 may communicate with each other via the tubing 27.

Further, the lift gas supply pipe 12 is not limited to the case where the lift gas supply pipe 12 is arranged outside the riser pipe 11 as in the example described with reference to FIG. For example, in FIG. 6, the riser pipe 11 and the lift gas supply pipe 12a are placed in the sea so as to be in the state of a double pipe accommodating the lift gas supply pipe 12a having a diameter smaller than that of the riser pipe 11 inside the riser pipe 11. An example of the configuration of the arranged methane gas production facility is shown.

The lower end of the lift gas supply pipe 12a opens in the riser pipe 11, and the lift gas is mixed into the water W in the riser pipe 11 from the opening. The mixed fluid of methane gas and the produced water rises in the gap region between the inner peripheral surface of the riser pipe 11 and the outer peripheral surface of the lift gas supply pipe 12a, and is extracted to the gas-liquid separation tank 31 on the floater 3 side. At this time, spacers are arranged at predetermined intervals between the riser pipe 11 and the lift gas supply pipe 12a so that the gap region through which the mixed fluid flows is stably formed in the riser pipe 11 in the vertical direction. You may.

Further, in the example described with reference to FIG. 1 and the like, an example of supplying methane gas as lift gas to the lift gas supply pipe 12 by using a gas compressor 32 for shipping methane gas has been described. However, it is not an indispensable requirement to share the shipping gas compressor 32 and the lift gas supply compressor to the lift gas supply pipe 12, and the gas-liquid separation tank 31 is dedicated to the lift gas supply to the lift gas supply pipe 12. A sampling line and a compressor may be provided.

Further, when the gas-liquid separation tank 31 is provided outside the methane gas production facility, for example, at a position away from the floater 3, or when it is difficult to provide the nitrogen gas supply unit 33 in the floater 3, from the outside. Methane gas or nitrogen gas may be accepted as the lift gas. In this case, the lift gas receiving pipe provided in the floater 3 corresponds to the lift gas supply section of the methane gas production facility.
The oxygen-free gas used for the lift gas is not limited to nitrogen gas or methane gas, and may be, for example, vaporized LPG or LNG (Liquefied Natural Gas).

Further, FIG. 7 shows an embodiment of a methane gas production facility that produces methane gas from a plurality of production wells 2 using one riser pipe 11. In this example, a plurality of connecting pipes 26 are connected to the manifold portion 23a provided at the lower end of the riser pipe 11, and each connecting pipe 26 is connected to each production well 2 via the connecting portion 25. Yes (Christmas tree connection).

Next, FIG. 8 shows another embodiment of a methane gas production facility that produces methane gas from a plurality of production wells 2 using one riser pipe 11. In this example, the riser base 23 provided at the lower end of the riser pipe 11 and the production wells 2 are provided below the plurality of connecting portions 25, respectively, and the riser base 23 and the connecting portion 25 are connected via the connecting pipe 26. It is configured to be connected in order (daisy chain connection).
Further, by combining the connection methods shown in FIGS. 7 and 8, a group of a plurality of production wells 2 connected in a daisy chain can be connected to the manifold portion 23a in a Christmas tree shape, or a plurality of production wells 2 connected in a daisy chain can be connected to each other. Of course, a plurality of connecting pipes 26 may be connected to the connecting portion 25 and the manifold portion 23 in a Christmas tree shape.

The methane gas production facility of this example is not limited to the case of producing methane gas from the methane hydrate layer MHL existing on the seabed, and may be provided in the methane hydrate layer MHL existing on the lake bottom to produce methane gas. Of course.

B Bubble MHL Methane hydrate layer W Water 11 Riser tube 12, 12a
Lift gas supply pipe 2 Production well 31 Gas-liquid separation tank 32 Gas compressor

Claims (8)

In a methane gas production facility that produces methane gas from a methane hydrate layer
A production well provided on the bottom of the water and formed to communicate with the methane hydrate layer,
The lower end side is airtightly communicated with the internal space of the production well, and a riser pipe for extracting the gas-liquid mixed fluid containing methane gas and water from the production well and sending it onto the water in a gas-liquid mixed state.
A lift gas supply pipe for supplying the lift gas supplied from the lift gas supply unit into the riser pipe, and
A gas-liquid separation unit provided on the water and connected to the upper end of the riser pipe to separate the gas-liquid mixed fluid flowing out of the riser pipe into methane gas and water is provided.
By mixing the lift gas supplied from the lift gas supply pipe with the water filled in the riser pipe and the production well to reduce the pressure in the production well, methane hydrate ( methane hydrate) is formed in the methane hydrate layer communicating with the production well. The gas-liquid mixed fluid obtained by decomposing (excluding those crushed by excavation) is raised through the riser pipe.
The lift gas is supplied when the methane hydrate is decomposed into a self-injection state by maintaining the depressurization in the production well due to the lift gas effect accompanying the formation of the flow in which the gas-liquid mixed fluid rises in the riser pipe. A methane gas production facility characterized by stopping the supply of lift gas from pipes . The methane gas production facility according to claim 1, wherein the lift gas is methane gas separated from the gas-liquid mixed fluid at the gas-liquid separation unit. The methane gas production facility according to claim 1, wherein the lift gas supply unit is a compressor that compresses oxygen-free gas. The methane gas production facility according to claim 1, wherein a plurality of production wells are provided on the bottom of the water, and the plurality of production wells are connected to the common riser pipe via a connecting pipe. In the methane gas production method that produces methane gas from the methane hydrate layer,
Lift gas is mixed into the production well provided on the bottom of the water and formed so as to communicate with the methane hydrate layer, and the water filled in the riser pipe whose lower end side communicates airtightly with the internal space of the production well. The process of decompressing the production well and decomposing methane hydrate (excluding those crushed by excavation) in the methane hydrate layer communicating with the production well.
A step of raising a gas-liquid mixed fluid containing methane gas and water obtained by decomposing the methane hydrate in a gas-liquid mixed state via the riser tube and sending it onto water.
A step of separating the gas-liquid mixed fluid flowing out from the upper end of the riser pipe into methane gas and water on the water.
When the decompression in the production well is maintained due to the lift gas effect accompanying the formation of the flow in which the gas-liquid mixed fluid rises in the riser pipe, and the decomposition of the methane hydrate proceeds into a self-injection state, the lift gas is introduced. A method for producing methane gas , which comprises a step of stopping the supply . The methane gas production method according to claim 5 , wherein the lift gas is an oxygen-free gas. The methane gas production method according to claim 6 , wherein the oxygen-free gas is methane gas separated from the gas-liquid mixture fluid. The methane gas separated from the gas-liquid mixed fluid is supplied to at least one methane gas utilization facility selected from the methane gas utilization facility group consisting of methane gas liquefaction facility, syngas production facility, thermal power generation facility, and city gas supply facility. The methane gas production method according to claim 5 , wherein the methane gas is produced.

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