Methane gas production equipment and methane gas production method

Abstract

We provide methane gas production equipment that can continuously produce methane gas from methane hydrate layers using gas lift. [Solution] In a methane gas production facility, a production well 2 is formed so as to communicate with the inside of a methane hydrate layer MHL at the bottom of the water, a riser pipe 11 is airtightly connected to the internal space of the production well 2, and a lift gas supply pipe 12 supplies lift gas supplied from a lift gas supply unit 32 into the riser pipe 11. When the lift gas is mixed with the water filled in the riser pipe 11 and the production well 2, the pressure inside the production well 2 is reduced, and the methane hydrate layer MHL connected to the production well 2 is At Methane hydrate (Excluding those broken up by excavation) The gas-liquid mixture of methane gas and produced water rises up the riser pipe 11. The gas flows into a gas-liquid separator that is provided above the water and connected to the upper end of the riser pipe. do.

Description

[Technical Field]

[0001] The present invention relates to a technique for producing methane gas from methane hydrate layers. [Background technology]

[0002] Methane hydrate (also known as gas hydrate), found in the deep sea and permafrost, is attracting attention as an unconventional resource. Methane hydrate is a solid substance that exists under certain temperature and pressure conditions, with guest molecules, primarily methane molecules, trapped within a cage structure (clathrate) of water molecules.

[0003] Non-Patent Document 1 proposes a method for producing methane gas from a sandy layer-type methane hydrate layer formed in a geological layer on the seabed by a depressurization method. The depressurization method involves pumping up water from a production well drilled through a methane hydrate layer, thereby reducing the pressure inside and around the production well and causing the methane hydrate to dissociate.

[0004] The methane gas and water produced by the dissociation of methane hydrate flow into the production well and are separated into gas and liquid. The water after separation is pumped up using a pump installed at the bottom of the riser, which forms the water pumping channel. The methane gas separated from the water rises in a gas flow path formed in the riser, separate from the water pumping flow path, and is extracted to the sea.

[0005] It is believed that the above-mentioned depressurization method can continuously produce methane gas from sandy methane hydrate layers. However, installing equipment such as pumps and gas-liquid separators in production wells located on the seabed at depths of several hundred meters or more can result in high manufacturing and installation costs for each piece of equipment, as well as the difficulty of maintaining the equipment.

[0006] Patent Documents 1 and 2 describe a technique for sucking up methane hydrate blocks from a shallow methane hydrate layer located on the surface of the seabed, using a method called an air bubble pump or air lift, in which transportation gas is blown into a riser (transport pipe) that transports methane hydrate. However, Patent Documents 1 and 2 do not disclose any technology that enables continuous production of methane gas by gas lift even from underground sand-type methane hydrate layers. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2002-536573 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-262083 [Non-patent literature]

[0008] [Non-Patent Document 1] Methane Hydrate Resource Development Research Consortium, "Gas Production from Methane Hydrate," Internet <URL: http: / / www.mh21japan.gr.jp / mh / 05-2 / > Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made against this background, and an object of the present invention is to provide a methane gas production facility and a methane gas production method that are capable of continuously producing methane gas from a methane hydrate layer by gas lift. [Means for solving the problem]

[0010] 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 that is embedded in the stratum so as to extend downward from the bottom of the water and that is formed so as to communicate with the methane hydrate layer; a riser pipe whose lower end side is airtightly connected to the internal space of the production well, for extracting a gas-liquid mixed fluid containing methane gas and water from the production well and sending the gas-liquid mixed fluid above the water; The lift gas supplied from the lift gas supply unit is From the connection point between the riser pipe and the production well or from a position below the connection point a lift gas supply pipe for supplying into the riser pipe; a gas-liquid separation unit that is provided above the water and connected to an upper end of the riser pipe and separates the gas-liquid mixed fluid flowing out from the riser pipe into methane gas and water, The lift gas supplied from the lift gas supply pipe is mixed with the water filled in the riser pipe and the production well, and the lift gas effect is utilized to reduce the pressure inside the production well, thereby dissociating methane hydrate (excluding methane hydrate that has been fractured by drilling) in a methane hydrate layer that communicates with the production well, and the gas-liquid mixed fluid obtained by this dissociation is allowed to flow through the lower end of the riser pipe that communicates airtight with the internal space of the production well, causing it to rise inside the riser pipe, and the pressure inside the production well is maintained at a pressure that maintains the dissociation of the methane hydrate due to the lift gas effect caused by the supply of the lift gas and the lift gas effect caused by the rise of the gas-liquid mixed fluid obtained by dissociating the methane hydrate.

[0011] The methane gas production facility may have the following features: (a) The lift gas is methane gas separated from the gas-liquid mixture fluid in the gas-liquid separation unit, and the lift gas supply unit is a compressor that compresses an oxygen-free gas. (b) A plurality of production wells are provided on the bottom of the water, and these production wells are connected to the common riser pipe via connecting pipes. The connection position is a connection position between the riser pipe and the connecting pipe. To do so.

[0012] Further, a methane gas production method according to another invention is a methane gas production method for producing methane gas from a methane hydrate layer, comprising: A production well is embedded in the stratum so as to extend downward from the bottom of the water and is formed so as to communicate with the inside of the methane hydrate layer, and a riser pipe whose lower end side is airtightly connected to the internal space of the production well is filled with water. , from the connection position between the riser pipe and the production well or a position below the connection position a step of mixing lift gas and reducing the pressure in the production well by utilizing the lift gas effect, thereby dissociating methane hydrate (excluding methane hydrate fractured by drilling) in a methane hydrate layer communicating with the production well; a step of introducing a gas-liquid mixed fluid containing methane gas and water obtained by dissociating the methane hydrate into the internal space of the production well through a lower end of the riser pipe that is airtightly connected to the internal space of the production well, causing the gas-liquid mixed fluid to rise inside the riser pipe and be sent above the water; and separating the gas-liquid mixture flowing out from the upper end of the riser pipe onto the water into methane gas and water, In the step of dissociating the methane hydrate, the pressure inside the production well is maintained at a pressure at which the dissociation of the methane hydrate is maintained by the lift gas effect caused by the supply of the lift gas and the lift gas effect caused by the rise of the gas-liquid mixed fluid obtained by dissociating the methane hydrate.

[0013] The method for producing methane gas may include the following features. ( c ) The lift gas is an oxygen-free gas, and the oxygen-free gas is methane gas separated from the gas-liquid mixture. ( d The methane gas separated from the gas-liquid mixture is supplied to at least one methane gas utilization facility selected from the group consisting of methane gas liquefaction facilities, synthetic gas production facilities, thermal power generation facilities, and city gas supply facilities. 。 [Effects of the Invention]

[0014] The present invention reduces the pressure in the production well by supplying lift gas into a riser pipe airtightly connected to a production well that produces methane gas from a methane hydrate layer, thereby dissociating methane hydrate in an area that communicates with the production well. As a result, the resulting gas-liquid mixture of methane gas and water rises inside the riser pipe, making it possible to produce methane gas with a simple and inexpensive configuration. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is an explanatory diagram showing a configuration example of a methane gas production facility according to an embodiment. FIG. [Figure 2] FIG. 2 is a first operational diagram of the methane gas production facility. [Figure 3] FIG. 2 is a second operational diagram of the methane gas production facility. [Figure 4] FIG. 4 is a third operational diagram of the methane gas production facility. [Figure 5] FIG. 4 is a fourth operational diagram of the methane gas production facility. [Figure 6] FIG. 10 is an explanatory diagram showing another example of the configuration of a riser provided in a methane gas production facility. [Figure 7] 1 is a first configuration example of a methane gas production facility having a riser connected to multiple production wells. [Figure 8] 1 is a second configuration example of a methane gas production facility having a riser connected to multiple production wells. DETAILED DESCRIPTION OF THE INVENTION

[0016] First, the configuration of a methane gas production facility according to an embodiment of the present invention will be described with reference to Figure 1. The methane gas production facility of this example produces methane gas from a sandy layer-type methane hydrate layer MHL formed below a geological layer GL that constitutes, for example, the seabed (bottom of the water). 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 seabed surface of this stratum GL.

[0017] The production well 2 has a structure in which a cylindrical casing 21 is disposed so as to extend downward from the seabed surface of the stratum GL toward the methane hydrate layer MHL located below the stratum GL. The casing 21 is made of, for example, a metal pipe having a diameter of several tens of centimeters to several meters, and is fixed to the stratum GL with cement (not shown).

[0018] The lower region of the casing 21 is inserted into the methane hydrate layer MHL, and a finishing layer 22 is formed in the lower region, which is in communication with the methane hydrate layer MHL via perforations, a sand screen, etc. The lower end of the casing 21 may be open toward the methane hydrate layer MHL, or may be closed by providing a casing shoe.

[0019] A flange-shaped riser base 23 is provided on the upper surface of the production well 2, and a flow path is formed in the riser base 23, penetrating the interior in the vertical direction. 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 made 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). The lower end of the tubing 27 opens toward the lower region of the casing 21 where the completion layer 22 is formed, and the gap between the casing 21 and the tubing 27 at the lower end is blocked by a packer 24.

[0020] The riser pipe 11 is configured, for example, by a metal pipe or flexible pipe having a diameter of several tens of centimeters to several meters, and the lower end of the riser pipe 11 is connected to the upper surface of the riser base 23. The riser pipe 11 connected to the tubing 27 via the riser base 23 is in airtight communication with the internal space of the production well 2.

[0021] A floater 3 is provided on the sea above the production well 2. The riser pipe 11 extends upward from the seabed surface connected to the riser base 23, through the sea, to the floater 3. The floater 3 is provided with a gas-liquid separation tank (gas-liquid separation section) 31 for receiving a gas-liquid mixture fluid (described below) extracted from the methane hydrate layer MHL via the riser pipe 11 and separating the gas-liquid mixture fluid into methane gas and water. 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 thereof (FIGS. 1 to 6 show an example in which the control valve V1 is installed upstream of the gas-liquid separation tank 31).

[0022] Furthermore, connected to the gas-liquid separation tank 31 are a line for extracting methane gas after gas-liquid separation from the upper space within the gas-liquid separation tank 31, and a line for extracting water after gas-liquid separation from the lower region within the gas-liquid separation tank 31. The methane gas extraction line is provided with a gas compressor 32 for pressurizing the methane gas extracted from the gas-liquid separation tank 31 and shipping it. Meanwhile, downstream of the water extraction line is provided with a wastewater treatment unit (not shown) for performing necessary wastewater treatment before discharging the water.

[0023] In addition, a line for extracting a portion of the pressurized methane gas branches off from the downstream side of the gas compressor 32. This branch line is connected to the upstream end of the lift gas supply pipe 12 via a flow rate control valve V2. From this perspective, the gas compressor 32 also functions as a lift gas supply unit that supplies lift gas to the lift gas supply pipe 12.

[0024] 1, a nitrogen gas supply unit 33 including a nitrogen gas tank and a gas compressor for supplying nitrogen gas to the lift gas supply pipe 12 is connected to the upstream end of the lift gas supply pipe 12 via a flow rate control valve V3. The nitrogen gas supply unit 33 also corresponds to a lift gas supply unit that supplies lift gas to the lift gas supply pipe 12.

[0025] The lift gas supply pipe 12 extends downward from the floater 3 to the seabed in the direction of extension 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 is in communication with the riser pipe 11 via a flow path formed in the riser base 23.

[0026] The lift gas supply pipe 12 functions to supply oxygen-free gases such as methane gas and nitrogen gas as lift gas into the riser pipe 11. Therefore, when the riser pipe 11 is filled with water W, the gas compressor 32 and nitrogen gas supply unit 33 that supply lift gas to the lift gas supply pipe 12 have the ability to supply lift gas at a pressure that allows a desired flow rate of lift gas to be mixed into the riser pipe 11 against the water pressure applied from the riser pipe 11 at the connection position with the riser pipe 11.

[0027] The reason for using oxygen-free gas as the lift gas is to prevent the formation of a combustible mixture when producing methane hydrate layer MHL methane gas. Therefore, this does not exclude the possibility that these lift gases may contain a trace amount of oxygen as an unavoidable component, so that a combustible mixture is not formed.

[0028] The operation of the methane gas production facility having the above-described configuration will be explained below. As shown in Figure 2, initially, the production well 2 and the riser pipe 11 are 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 a cap on the lower end of the riser pipe 11, seawater will enter the riser pipe 11. Furthermore, when the production well 2 is installed, seawater will also enter the production well 2. Before the start of methane gas production, the water W filled in the production well 2 and the riser pipe 11 is, for example, seawater.

[0029] 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 to supply nitrogen gas as a lift gas from the nitrogen gas supply unit 33 to the lift gas supply pipe 12. The nitrogen gas flows downward through the lift gas supply pipe 12 and is mixed into the water W in the riser pipe 11 via the connection point with the riser base 23.

[0030] As a result, a gas-liquid mixture of nitrogen gas bubbles B and water W is formed above the nitrogen gas mixing position. Because the specific gravity of this gas-liquid mixture is smaller than the specific gravity of the water W below the nitrogen gas mixing position, the water W below, which has a larger specific gravity, is lifted above the nitrogen gas mixing position (gas lift effect). Due to this gas lift effect, the pressure inside the production well 2 below the gas mixing position decreases, and the gas-liquid mixture begins to rise inside the riser pipe 11.

[0031] As the gas-liquid mixed fluid rises inside the riser pipe 11 and the pressure inside the production well 2 that is in airtight communication with the riser pipe 11 decreases, the pressure inside the surrounding methane hydrate layer MHL that is in communication with the production well 2 via the completion layer 22 begins to decrease. When the pressure inside the methane hydrate layer MHL decreases, the methane hydrate dissociates, producing methane gas and produced water. At this time, the methane hydrate layer may contain components other than methane, such as ethane and propane, and such a mixed gas of methane and other components will also be referred to as "methane gas" in the following explanation. A mixed fluid of methane gas generated by the dissociation of methane hydrate and water (produced water) W (which may include water that did not form methane hydrate and was originally present alone in the methane hydrate layer MHL) flows into the production well 2 through the completion layer 22 and rises in the production well 2 and the riser pipe 11 due to the lift gas effect (Figure 4).

[0032] For example, consider a case where the lower end of the riser pipe 11 is connected to a riser base 23 installed on the seabed at a depth of 1,000 meters, and a methane hydrate layer MHL is located several tens to several hundreds of meters below the riser base 23. At this time, a water pressure of approximately 100 Bar (10 MPaG) is applied inside the riser pipe 11 near the connection position with the riser base 23. In this case, even taking into account the pressure loss in the production well 2, it is considered that dissociation of methane hydrate around the completion layer 22 can be continuously generated if the water pressure at the connection position can be reduced to approximately 30 to 50 Bar (3 to 5 MPaG) by the lift gas effect.

[0033] The mixed fluid that rises inside the riser pipe 11 reaches the floater 3 side and then flows into the gas-liquid separation tank 31, where it is separated into methane gas and water. When a sufficient amount of methane gas has flowed into the gas-liquid separation tank 31 and the gas compressor 32 is ready to operate, the gas compressor 32 is started and the flow control valve V2 is opened. Meanwhile, the flow control valve V3 on the nitrogen gas supply unit 33 side is closed to switch the lift gas from nitrogen gas to methane gas (Figure 4).

[0034] In the methane gas production facility of this example, in addition to the lift gas effect caused by the supply of lift gas from the lift gas supply pipe 12, the lift gas effect is also exerted by the formation of a flow in which the mixed fluid (methane gas and produced water) itself that has flowed from the methane hydrate layer MHL into the production well 2 rises inside the riser pipe 11. In this case, if methane gas can be produced even when the supply of lift gas is stopped, it can be said that the lift gas effect of the mixed fluid supplied from the methane hydrate layer MHL maintains the reduced pressure inside the production well 2, and that the system is in a self-flowing state in which the dissociation of methane hydrate progresses.

[0035] When a self-flowing state 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 Figure 5. Also, if the amount of methane gas produced by self-flowing alone is small and stable production is not possible, additional lift gas may be supplied as needed at a flow rate necessary to maintain the shipping flow rate.

[0036] Furthermore, it is not essential that a flowing state is always formed in the methane hydrate layer MHL around the production well 2. For example, after the operation of the methane gas production facility is stopped, the production of methane gas can be resumed by restarting the supply of lift gas from the lift gas supply pipe 12 or by increasing the supply flow rate of the lift gas.

[0037] By the operation of the methane gas production facility described above, 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 pressurized by the gas compressor 32 and supplied to the downstream methane gas utilization facility. Examples of methane gas utilization facilities include at least one methane gas utilization facility selected from a group consisting of methane gas liquefaction facilities that cool and liquefy methane gas, synthesis gas production facilities that produce synthesis gas containing carbon monoxide and hydrogen from methane gas through a chemical reaction, thermal power generation facilities that burn methane gas to generate electricity, and city gas supply facilities that produce city gas by adding LPG (Liquefied Petroleum Gas) to methane gas to adjust its calorific value and / or add odor.

[0038] Here, these methane gas utilization facilities may be installed outside the floater 3, for example on the land side, and a pipeline may be laid between the floater 3 and the methane gas utilization facility, and methane gas pressurized by the gas compressor 32 may be transported to the methane gas utilization facility. Alternatively, a methane gas utilization facility may be installed next to the floater 3, and the liquefied gas, synthetic gas, electricity, and city gas obtained within the floater 3 may be transported from the floater 3 to the consumer. The produced water separated from the methane gas in the gas-liquid separation tank 31 is subjected to the necessary wastewater treatment and then returned to the sea. If the produced water cannot be returned to the sea, it may be re-injected into the seabed.

[0039] The methane gas production facility according to this embodiment has the following advantages. By supplying lift gas into the riser pipe 11 airtightly connected to the production well 2 that produces methane gas from the methane hydrate layer MHL, the pressure inside the production well 2 is reduced, and methane hydrate in the region that communicates with the production well 2 is dissociated. As a result, the resulting gas-liquid mixture fluid of methane gas and water spontaneously rises inside the riser pipe 11, making it possible to produce methane gas with a simple and inexpensive configuration.

[0040] In particular, unlike the depressurization methods proposed in the past, the methane gas production facility of this example does not require the installation of a separator to separate methane gas from produced water or a pump to pump up the produced water on the seabed where the production well 2 is located. This significantly reduces the manufacturing and installation costs of the equipment that makes up the methane gas production facility. It also makes it possible to avoid the difficulties of maintenance that accompany the installation of power equipment on the seabed.

[0041] Furthermore, when the gas-liquid mixture of methane gas and produced water flows into the production well 2 and exerts a lift gas effect, the lift gas supply flow rate can be reduced or stopped when the methane hydrate is in a self-flowing state where dissociation of the methane hydrate progresses spontaneously. As a result, it is possible to significantly reduce the operating costs of the methane gas production facility compared to the conventional depressurization method, which requires the constant pumping of produced water.

[0042] Here, the lift gas supply pipe 12 is not limited to a configuration in which it is connected to the riser base 23, but may be directly connected to the riser pipe 11. Also, for example, the lift gas supply pipe 12 may be connected to a tubing 27, and the lift gas supply pipe 12 and the riser pipe 11 may be communicated with each other via the tubing 27.

[0043] Furthermore, the lift gas supply pipe 12 is not limited to being disposed outside the riser pipe 11 as in the example described using Fig. 1 etc. For example, Fig. 6 shows an example of the configuration of a methane gas production facility in which the riser pipe 11 and the lift gas supply pipe 12a are disposed underwater so as to form a double pipe state in which the lift gas supply pipe 12a, which has a smaller diameter than the riser pipe 11, is housed inside the riser pipe 11.

[0044] The lower end of the lift gas supply pipe 12a opens inside the riser pipe 11, and the lift gas is mixed into the water W inside the riser pipe 11 from this opening. The mixed fluid of methane gas and produced water rises in the gap region between the inner circumferential surface of the riser pipe 11 and the outer circumferential surface of the lift gas supply pipe 12a and is extracted into the gas-liquid separation tank 31 on the floater 3 side. At this time, spacers may be placed at a predetermined interval 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 vertical direction inside the riser pipe 11.

[0045] 1 and the like, an example has been described in which the gas compressor 32 for shipping methane gas is used to supply methane gas as lift gas to the lift gas supply pipe 12. However, it is not essential to share the same gas compressor 32 for shipping and the compressor for supplying lift gas to the lift gas supply pipe 12, and an extraction line and compressor dedicated to supplying lift gas to the lift gas supply pipe 12 may be provided for the gas-liquid separation vessel 31.

[0046] Furthermore, in cases where the gas-liquid separation tank 31 is installed outside the methane gas production facility, for example, at a location away from the floater 3, or where it is difficult to install a nitrogen gas supply unit 33 in the floater 3, methane gas or nitrogen gas may be received as lift gas from outside. In this case, the lift gas receiving pipe installed in the floater 3 corresponds to the lift gas supply unit of this methane gas production facility. The oxygen-free gas used as the lift gas is not limited to nitrogen gas or methane gas, but may be, for example, vaporized LPG or LNG (Liquefied Natural Gas).

[0047] 7 shows an embodiment of a methane gas production facility that uses a single riser pipe 11 to produce methane gas from multiple production wells 2. In this example, multiple connecting pipes 26 are connected to a manifold section 23a provided at the lower end of the riser pipe 11, and each connecting pipe 26 is connected to each production well 2 via a connecting section 25 (Christmas tree connection).

[0048] Next, Figure 8 shows another embodiment of a methane gas production facility that uses a single riser pipe 11 to produce methane gas from multiple production wells 2. In this example, a riser base 23 is provided at the lower end of the riser pipe 11, and production wells 2 are provided below multiple connecting parts 25, and these riser bases 23 and connecting parts 25 are connected in sequence via connecting pipes 26 (daisy chain connection). Of course, it is also possible to combine the connection methods shown in Figures 7 and 8 to connect a group of multiple production wells 2, each daisy-chained, to the manifold section 23a in a Christmas tree shape, or to connect multiple connecting pipes 26 in a Christmas tree shape to multiple daisy-chained connecting sections 25 or manifold section 23.

[0049] Furthermore, the methane gas production facility of this example is not limited to the case where methane gas is produced from a methane hydrate layer MHL located on the seabed, but may of course be installed in a methane hydrate layer MHL located on the bottom of a lake to produce methane gas. [Explanation of symbols]

[0050] B. Air bubbles MHL methane hydrate layer W water 11 Riser pipe 12, 12a Lift gas supply pipe 2 Production wells 31 Gas-liquid separation tank 32 Gas Compressor

Claims

1. In methane gas production facilities that produce methane gas from methane hydrate layers, a production well that is embedded in the stratum so as to extend downward from the bottom of the water and that is formed so as to communicate with the methane hydrate layer; a riser pipe whose lower end side is airtightly connected to the internal space of the production well, for extracting a gas-liquid mixed fluid containing methane gas and water from the production well and sending the gas-liquid mixed fluid above the water; a lift gas supply pipe for supplying the lift gas supplied from the lift gas supply unit into the riser pipe from a connection position between the riser pipe and the production well or a position below the connection position; a gas-liquid separation unit that is provided above the water and connected to an upper end of the riser pipe, and that separates the gas-liquid mixed fluid that has flowed out of the riser pipe into methane gas and water, A methane gas production facility characterized in that lift gas supplied from the lift gas supply pipe is mixed with water filled in the riser pipe and the production well, and the pressure inside the production well is reduced by utilizing the lift gas effect, thereby dissociating methane hydrate (excluding methane hydrate that has been fractured by drilling) in a methane hydrate layer communicating with the production well, and the gas-liquid mixed fluid obtained by this dissociation is caused to flow through the lower end of the riser pipe that communicates airtight with the internal space of the production well and rise inside the riser pipe, and the pressure inside the production well is maintained at a pressure that maintains the dissociation of the methane hydrate by the lift gas effect caused by the supply of the lift gas and the lift gas effect caused by the rise of the gas-liquid mixed fluid obtained by dissociating the methane hydrate.

2. 2. The methane gas production facility according to claim 1, wherein the lift gas is methane gas separated from the gas-liquid mixed fluid in the gas-liquid separation section.

3. 2. The methane gas production facility according to claim 1, wherein the lift gas supply unit is a compressor that compresses an oxygen-free gas.

4. 2. The methane gas production facility according to claim 1, wherein 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 connecting pipes, and the connection position is a connection position between the riser pipe and the connecting pipe.

5. A methane gas production method for producing methane gas from a methane hydrate layer, a step of injecting lift gas into a production well embedded in a stratum so as to extend downward from the bottom of the water and formed so as to communicate with the inside of a methane hydrate layer, and into a riser pipe whose lower end is airtightly connected to the internal space of the production well, from a connection position between the riser pipe and the production well or a position below the connection position, reducing the pressure inside the production well by utilizing the lift gas effect, and dissociating methane hydrate (excluding methane hydrate that has been fractured by drilling) in the methane hydrate layer that communicates with the production well; a step of introducing a gas-liquid mixed fluid containing methane gas and water obtained by dissociating the methane hydrate into the internal space of the production well through a lower end of the riser pipe that is airtightly connected to the internal space of the production well, causing the gas-liquid mixed fluid to rise inside the riser pipe and be sent above the water; and separating the gas-liquid mixture flowing out from the upper end of the riser pipe onto the water into methane gas and water, a methane gas production method, characterized in that in the step of dissociating the methane hydrate, the pressure inside the production well is maintained at a pressure at which dissociation of the methane hydrate is maintained by a lift gas effect caused by the supply of the lift gas and a lift gas effect caused by the rise of a gas-liquid mixed fluid obtained by dissociating the methane hydrate.

6. 6. The method for producing methane gas according to claim 5, wherein the lift gas is an oxygen-free gas.

7. 7. The method for producing methane gas according to claim 6, wherein the oxygen-free gas is methane gas separated from the gas-liquid mixture fluid.

8. 6. The methane gas production method according to claim 5, wherein the methane gas separated from the gas-liquid mixed fluid is supplied to at least one methane gas utilization facility selected from a group of methane gas utilization facilities consisting of a methane gas liquefaction facility, a synthesis gas production facility, a thermal power generation facility, and a city gas supply facility.

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