Method and System for Recovering Gas in Natural Gas Hydrate Exploitation

Abstract

A method for recovering gas in natural gas hydrate exploitation is disclosed, in which a gas-water mixture at a bottom of a exploitation well is delivered to an ocean surface platform through a marine riser, by adopting the gas-lift effect of methane gas derived from the dissociation of natural gas hydrate, so as to achieve a controllable flowing production of marine natural gas hydrate. In the startup stage, the pressure in the bottom of the well is decreased by the gas-lift effect of the injected gas to allow dissociation of the hydrate. In the flowing production stage, the flowing production is achieved by the gas-lift effect of the gas derived from the dissociation of the natural gas hydrate, wherein a seafloor gas tank is employed to control the flowing rate and replenish the consumed gas.

Claims

1. A method for recovering gas in natural gas hydrate exploitation, characterized in that, a gas-water mixture at a bottom of a exploitation well is delivered to an ocean surface platform through a marine riser by adopting the gas-lift effect of methane gas derived from dissociation of natural gas hydrate, so as to achieve a controllable flowing production of marine natural gas hydrate, the method comprises the following steps: step 1, startup stage: injecting a certain amount of nitrogen gas or methane gas into a seafloor gas tank by a compressor and allowing a pressure therein to be higher than a seafloor static pressure; opening an automatic control gate valve between a well head assembly and a marine riser, and an automatic control gate valve between the seafloor gas tank and a bottom of the marine riser; injecting the gas from the seafloor gas tank to the marine riser, and lifting liquid from a bottom of the well to the ocean surface platform by the gas-lift effect of the gas, so as to decrease a pressure of a seafloor hydrate layer to below a phase equilibrium pressure of the hydrate and thereby the hydrate in the seafloor hydrate layer is dissociated into methane gas and water; the gas-water mixture is driven to flow into the exploitation well by a pressure of a hydrate reservoir; step 2, flowing production stage: online detecting a liquid-gas ratio of a gas-liquid fluid produced from the hydrate reservoir by a sensor; if the liquid-gas ratio is larger than a flowing liquid-gas ratio of the gas-liquid fluid, then adding gas from the seafloor gas tank to the marine riser; if the liquid-gas ratio is smaller than the flowing liquid-gas ratio of the gas-liquid fluid, then closing the valve between the seafloor gas tank and the marine riser to stop gas supply, opening a valve between a seafloor gas-liquid cyclone separator and the marine riser to divert a portion of the gas-liquid fluid to the seafloor gas-liquid cyclone separator, adding gas separated therefrom to the seafloor gas tank after pressurizing by a booster pump to replenish the consumed gas, and returning a residual of the gas-liquid fluid to the bottom of the marine riser; after the gas-liquid fluid is lifted by its own force to the ocean surface platform, separating the gas-liquid fluid by a gas-liquid separator, wherein the water produced is discharged, and the methane gas produced is stored in a gas tank and transported away.

2. The method according to claim 1, characterized in that, the flowing liquid-gas ratio of the gas-liquid fluid increases as a production pressure at the bottom of the well increases; when provided the same production pressure, the flowing liquid-gas ratio of the gas-liquid fluid increases as the water depth decreases.

3. The method according to claim 1, characterized in that, the method for recovering gas can be applied in methods for marine natural gas hydrate exploitation including depressurization method, thermal stimulation method, chemical agent injection method, and CO.sub.2 replacement method.

4. A system for recovering gas in natural gas hydrate exploitation, characterized in that, the system comprises an ocean surface platform, a gas-liquid separator, a gas tank, a compressor, a seafloor gas tank, a booster pump, a seafloor gas-liquid cyclone separator, a gas buffer tank, a marine riser, a well head assembly, and an exploitation well; the ocean surface platform is disposed above the ocean surface; the gas-liquid separator, the gas tank and the compressor are disposed on the ocean surface platform; the exploitation well is disposed vertically above a seafloor stratum, and penetrates a seafloor sediment layer and a natural gas hydrate layer; a top of the exploitation well is connected with the well head assembly; a bottom of the marine riser is connected with the well head assembly through a first valve; a top of the marine riser is connected sequentially through pipelines with the gas-liquid separator, the gas tank and the compressor which are disposed on the ocean surface platform; the seafloor gas tank, the booster pump, the seafloor gas-liquid cyclone separator and the gas buffer tank are disposed beside the well head assembly; a gas-liquid mixture inlet of the seafloor gas-liquid cyclone separator is connected with the well head assembly through pipelines and a second valve; a liquid outlet of the seafloor gas-liquid cyclone separator is connected with the bottom of the marine riser through pipelines and a third valve; a gas outlet of the seafloor gas-liquid cyclone separator is connected sequentially with the gas buffer tank, the booster pump, a fourth valve and the seafloor gas tank through pipelines; the seafloor gas tank is connected with the compressor through a pipeline; the seafloor gas tank is connected with the bottom of the marine riser through pipelines and a fifth valve.

5. The system according to claim 4, characterized in that, a ball valve is disposed between the seafloor gas tank and the compressor.

6. The system according to claim 4, characterized in that, a sand control device is disposed in the exploitation well.

7. The system according to claim 4, characterized in that, the first valve, the second valve, the third valve, the fourth valve and the fifth valve are seafloor automatic gate valves.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 shows a schematic diagram of a system of the present invention.

[0025] FIG. 2 shows the relationship between the production pressure at the bottom of the well and the flowing liquid-gas ratio.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

[0026] As shown in FIG. 1, an ocean surface platform 9 is set up using prior art technology where a marine gas hydrate reservoir is located. A vertical exploitation well 13 is drilled above a seafloor stratum and penetrates a seafloor sediment layer and a natural gas hydrate layer. A sand control device 14 is disposed in the exploitation well. The top of the exploitation well is connected with a well head assembly 12. Also provided is a marine riser 10. The bottom of the marine riser 10 is connected with a well head assembly 12 through a seafloor automatic gate valve 205. The top of the marine riser 10 is connected sequentially through pipelines with a gas-liquid separator 8, a gas tank 7 and a compressor 6 which are disposed on the ocean surface platform 9. a seafloor gas tank 1, a seafloor gas-liquid cyclone separator 11, a gas buffer tank 4 and a booster pump 3 are disposed beside the well head assembly 12. A gas-liquid mixture inlet of the seafloor gas-liquid cyclone separator 11 is connected with the well head assembly 12 through pipelines and a seafloor automatic gate valve 204. A liquid outlet of the seafloor gas-liquid cyclone separator 11 is connected with the bottom of the marine riser 10 through pipelines and a seafloor automatic gate valve 203. A gas outlet at the top of the seafloor gas-liquid cyclone separator 11 is connected sequentially through pipelines with the gas buffer tank 4, the booster pump 3, a seafloor automatic gate valve 202 and the top of the seafloor gas tank 1. The seafloor gas tank 1 is connected through a pipeline with the compressor 6 disposed on the ocean surface platform 9. The seafloor gas tank 1 is connected with the bottom of the marine riser 10 through pipelines and a seafloor automatic gate valve 201.

[0027] When the hydrate exploitation is performed via the depressurization method, a certain amount of nitrogen gas or methane gas is first injected into the seafloor gas tank 1 by a compressor 6 so as to allow the pressure therein to be higher than the seafloor static pressure. Then the seafloor automatic gate valves 205 and 201 are opened, and the gas is injected from the seafloor gas tank 1 to the bottom of the marine riser 10. The gas will go upwards by its own buoyancy after injected into the marine riser 10 and lift the liquid from the bottom of the exploitation well 13 to the ocean surface platform by the gas-lift effect, so as to decrease the pressure in the bottom of the well and the pressure of the seafloor hydrate layer to below a phase equilibrium pressure of the hydrate, and thereby the hydrate at the seafloor hydrate layer is dissociated into methane gas and water which will be driven to flow into the bottom of the exploitation well 13 by the pressure-gradient force of the hydrate reservoir.

[0028] When the amount of the water and methane gas produced from the seafloor hydrate layer reaches a certain value, the gas-liquid fluid produced from the hydrate reservoir can flow to the ocean surface platform 9 through the marine riser 10 under the gas-lift effect of the methane gas therein. Then the seafloor automatic gate valve 201 between the seafloor gas tank 1 and the marine riser 10 is closed so as to stop injecting the gas, and thereby the hydrate exploitation enters the flowing production stage.

[0029] In the flowing production stage, a liquid-gas ratio of the gas-liquid fluid produced from the hydrate reservoir is detected online by a sensor.

[0030] If the liquid-gas ratio is larger than a flowing liquid-gas ratio of the gas-liquid fluid, then the seafloor automatic gate valve 201 is opened to add gas from the seafloor gas tank 1 to the bottom of the marine riser 10. If the liquid-gas ratio is smaller than the flowing liquid-gas ratio of the gas-liquid fluid, then the seafloor automatic gate valve 201 is closed to stop gas supply, and the seafloor automatic gate valves 202, 203 and 204 are opened to divert a portion of the gas-liquid fluid to the seafloor gas-liquid cyclone separator 11. Gas separated therefrom is added to the seafloor gas tank 1 after pressurized by the gas buffer tank 4 and the booster pump 3 to replenish the consumed gas, and a residual of the gas-liquid fluid is returned to the bottom of the marine riser 10. The gas-liquid fluid is then lifted by its own force to the ocean surface platform.

[0031] After the gas-liquid fluid flows to the ocean surface platform is separated by the gas liquid separator 8, the water produced is discharged, and the methane gas produced is stored in a gas tank 7 and transported away.

[0032] As shown in FIG. 2, for a natural gas hydrate reservoir at the depth of 2000 meters (which is a sum of the lengths of the marine riser and the production well) provided with a marine riser having an internal diameter of 200 millimeters, when the recovery rate of the gas-liquid fluid is 37.5 kg/s, if a production pressure of 8.0 MPa is employed at the bottom of the well, then the flowing liquid-gas ratio is 13.5 kg H.sub.2O/m.sup.3 CH.sub.4, and therefore we shall control the liquid-gas ratio of the fluid at the bottom of the marine riser 10 to below 13.5 kg H.sub.2O/m.sup.3 CH.sub.4 to allow the flowing production. If a production pressure of 6.0 MPa is employed at the bottom of the well, then the flowing liquid-gas ratio is 9 kg H.sub.2O/m.sup.3 CH.sub.4, and therefore we shall control the liquid-gas ratio of the fluid at the bottom of the marine riser 10 to below 9 kg H.sub.2O/m.sup.3 CH.sub.4 to allow the flowing production.

[0033] The above is a detailed description of a feasible embodiment of the present invention, which is not used to limit the present invention. Any equivalent embodiment or modification that not departs from the spirit of the present invention shall fall within the scope of the present invention.