A combustible ice underwater collection system and method based on horizontal well pressure reduction mining

Abstract

The application discloses a combustible ice underwater collection system and method based on horizontal well pressure reduction exploitation, and the system comprises a wellhead separation system, a pipeline transportation system, a seabed central collection system and a utility system; the wellhead separation system comprises a gas-liquid separator, gas-liquid phase inlets of the gas-liquid separator are connected with a gas phase production system and a liquid phase production system respectively, a bottom of the gas-liquid separator is connected with a sand washing system, and gas-liquid phase outlets of the gas-liquid separator are connected with the seabed central collection system through the pipeline transportation system; and the utility system is connected with the sand washing system through a pipeline. The application proposes a multi-well underwater collection system based on the demand of sea combustible ice horizontal well exploitation, combines production rules and output medium characteristics, alleviates underwater flow guarantee and sand deposition problems under large-scale development, improves underwater system production efficiency downstream of a wellhead, and optimizes downhole and underwater later operation energy consumption.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of combustible ice mining, and particularly relates to a combustible ice underwater collection system and method based on horizontal well depressurization mining. BACKGROUND

[0002] Natural gas hydrate (hereinafter referred to as hydrate) is an ice-like crystalline compound with a cage-like structure formed by contact between water molecules and small gas molecules (mainly hydrocarbon gas) under low temperature and high pressure conditions. It is currently recognized that the reserves are extremely abundant, and its distribution is mainly in deep water ocean storage. Natural gas hydrate can be regarded as a highly compressed natural gas resource. 1m 3 Natural gas hydrate can release 160-180m 3 of natural gas (standard state). Roughly estimated, the total volume of natural gas contained in global natural gas hydrate deposits is about 1.8×10 16 -2.1×10 16 m 3 , which is equivalent to twice the total amount of carbon in the proven conventional fossil fuels in the world. Therefore, the energy status of hydrate is expected to replace conventional fossil fuels such as oil and coal in the future and become a new type of clean energy. At present, marine natural gas hydrate is still in the exploration and trial mining stage, and a large amount of basic research is being carried out. In recent years, several marine natural gas hydrate trial mining activities have been carried out. In particular, China successfully carried out the second round of trial mining of combustible ice in the South China Sea in 2020, using the "horizontal well" mining mode to once again create a record of the time and total amount of combustible ice mining, laying a solid foundation for commercial mining.

[0003] According to the formation conditions of marine natural gas hydrate, the depressurization method is a relatively clear principle and a way verified by trial mining activities. In the present stage of trial mining activities, this method mainly reduces the pressure of marine natural gas hydrate reservoir, breaks its stable conditions, and promotes the formation of gas and liquid phases of natural gas hydrate. Through the sand prevention device, separation device and electric submersible pump under the well, the produced medium is transported to the offshore platform for processing and discharge through independent gas phase pipeline (channel) and liquid phase pipeline (channel).

[0004] At present, the exploitation of the combustible ice horizontal well is still in the single well depressurization mining mode, and the single well continuous production time, production scale and development economy are difficult to meet the standard of commercial exploitation, so the multi-well joint exploitation in the block will be the only way for the exploitation of the combustible ice in the sea area. In the early test mining, strict measures are taken for the downhole sand control, and good sand prevention effect is obtained, but in the subsequent large-scale development, the balance between the yield and the sand control is considered, and the downhole sand production and the sand particle size will bring more severe challenges to the underwater production system of the combustible ice. Meanwhile, there may be large underwater natural gas reservoirs in the combustible ice occurrence area of the South China Sea, and the coordinated exploitation and production of the combustible ice and conventional natural gas has great potential. In addition, the produced material of the combustible ice reservoir is basically methane and gas field water, but the effective target resource is mainly natural gas, and the ideal treatment scheme for the gas field water is a problem that cannot be avoided in the later production, and the seabed reinjection is a better idea.

[0005] Therefore, with the help of the experience of the horizontal well combustible ice test mining, combined with the requirements of the future large-scale development of the combustible ice, based on the medium characteristics of the combustible ice output, the overall planning and research and development of the underwater collection system are extremely meaningful to ensure the efficient and safe transportation of the combustible ice output. SUMMARY

[0006] The purpose of the present application is to provide a combustible ice underwater collection system and method based on horizontal well depressurization mining, which can safely and economically implement deepwater natural gas hydrate development and production.

[0007] The main technical idea of the present application is based on the horizontal well depressurization mining technology, and the multi-well coordinated production mode is used, which is based on the medium sand control, flow protection and reasonable disposal of output water, and the wellhead separation system is arranged underwater, which is used for the secondary separation of the produced natural gas, gas field water and produced sand at the wellhead (the first separation has been carried out downhole), reduces the sand and water content of the gas phase pipeline, improves the transportation efficiency and safety of the gas phase pipeline, reduces the natural gas content of the liquid phase pipeline, further reduces the hydrate production risk, realizes the slug flow receiving and processing function at the wellhead when the downhole production is unstable, and uses the liquid phase pipeline to carry the produced sand, improves the sand carrying effect, uses the umbilical cable to provide sand washing liquid and downhole injection liquid (gas), avoids the sand accumulation of the gas-liquid separator and supports the downhole production, sets up the pipeline transportation system to realize the independent transportation of the output gas and the output water, improves the transportation efficiency, sets up the central collection system to concentrate the output gas of each single well and send it to the water surface treatment system, and concentrates the output water of each single well and sends it to the injection well after pressure boosting, sets up the utility system to provide support for the underwater combustible ice collection, and the like. Thus, the purposes of effectively controlling the sand, reasonably disposing the gas field water, hydrate inhibition and high efficiency utilization of the system are achieved.

[0008] The technical scheme adopted by the present application is:

[0009] A combustible ice underwater collection system based on horizontal well pressure reduction mining, characterized in that: it comprises a wellhead separation system, a pipeline transportation system, a seabed central collection system and a utility system;

[0010] The wellhead separation system is arranged at the underwater wellhead, the wellhead separation system comprises a gas-liquid separator, gas-liquid phase inlets of the gas-liquid separator are connected with a gas phase production system and a liquid phase production system respectively, the gas-liquid separator is used for secondary separation of the produced medium downstream of each combustible ice underwater wellhead, a sand washing system is connected with the bottom of the gas-liquid separator, the sand washing system is used for removing the sand particles accumulated in the gas-liquid separator, and gas-liquid phase outlets of the gas-liquid separator are connected with the seabed central collection system through the pipeline transportation system.

[0011] The seabed central collection system is used for centralized collection of the gas phase and the liquid phase produced by each production well transported by the pipeline transportation system; the utility system is connected with the sand washing system through a pipeline, and the utility system is used for controlling the underwater single well and the seabed central collection system relying on the water surface central processing platform.

[0012] The gas phase production system of the combustible ice underwater collection system based on horizontal well pressure reduction mining comprises a gas phase production pipeline and a gas phase wellhead shut-off valve and a gas phase wellhead regulating valve arranged on the gas phase production pipeline, the liquid phase production system comprises a liquid phase production pipeline and a liquid phase wellhead shut-off valve and a liquid phase wellhead regulating valve arranged on the liquid phase production pipeline, and the gas phase production pipeline and the liquid phase production pipeline are connected with the gas phase and the liquid phase interfaces of the wellhead respectively, so that the produced gas phase and liquid phase are transported to the gas-liquid separator.

[0013] The pipeline transportation system of the combustible ice underwater collection system based on horizontal well pressure reduction mining comprises a gas phase seabed pipeline, a liquid phase seabed pipeline, a gas phase riser and a liquid phase riser, the gas-liquid separator is connected with the seabed central collection system through the gas phase seabed pipeline and the liquid phase seabed pipeline, and the gas phase and the liquid phase collected by the seabed central collection system are output to the gas phase riser and the liquid phase riser respectively.

[0014] The gas phase production pipeline and the gas phase seabed pipeline are connected through a gas phase bypass pipeline system, the liquid phase production pipeline and the liquid phase seabed pipeline are connected through a liquid phase bypass pipeline system, and the gas phase bypass pipeline system and the liquid phase bypass pipeline system are used for the produced medium of the wellhead to directly enter the downstream gas phase seabed pipeline and the liquid phase seabed pipeline without passing through the gas-liquid separator.

[0015] The combustible ice underwater collection system based on horizontal well depressurization mining has a gas phase bypass pipeline system, which comprises a gas phase bypass pipeline and a gas phase bypass first shut-off valve, a gas phase bypass regulating valve and a gas phase bypass second shut-off valve arranged on the gas phase bypass pipeline, and the gas phase bypass pipeline is used for connecting the gas phase production pipeline and the gas phase submarine pipeline.

[0016] The combustible ice underwater collection system based on horizontal well depressurization mining has a sand washing system, which comprises a sand washing liquid inlet pipeline and a sand washing liquid outlet pipeline, the bottom of the gas-liquid separator is connected with the utility system through the sand washing liquid inlet pipeline, and the bottom of the gas-liquid separator is connected with the liquid phase submarine pipeline through the sand washing liquid outlet pipeline, a sand washing inlet shut-off valve and a sand washing liquid regulating valve are arranged on the sand washing liquid inlet pipeline, and a sand washing outlet shut-off valve, a sand washing liquid outlet regulating valve and a sand washing liquid phase pump are arranged on the sand washing liquid outlet pipeline.

[0017] The combustible ice underwater collection system based on horizontal well depressurization mining has a submarine center collection system, which comprises a gas phase collection pipe and a liquid phase collection pipe, the gas phase collection pipe is provided with a plurality of interfaces for collecting gas phase media of each gas phase submarine pipeline and outputting to a gas phase riser, a gas phase outlet shut-off valve and a center system gas phase inlet shut-off valve are arranged on each gas phase submarine pipeline, the liquid phase collection pipe is provided with a plurality of interfaces for collecting liquid phase media of each liquid phase submarine pipeline and outputting to a liquid phase riser and / or a reinjection pipeline, a liquid phase outlet shut-off valve, a liquid phase outlet booster pump and a center system liquid phase inlet shut-off valve are arranged on each liquid phase submarine pipeline, a liquid phase riser inlet shut-off valve, a liquid phase riser regulating valve and a liquid phase riser booster pump are arranged on the liquid phase riser, and a reinjection shut-off valve and a reinjection pump are arranged on the reinjection pipeline.

[0018] The combustible ice underwater collection system based on horizontal well depressurization mining has a utility system, which comprises an umbilical cable, a distributor, a sand washing liquid pipeline, a hydrate inhibitor pipeline and a lifting gas pipeline, the umbilical cable connects a water platform center with a single well wellhead system, the distributor is arranged at the wellhead and is used for separating and leading out various pipelines, energy and signal lines, the sand washing liquid pipeline is connected with the sand washing liquid inlet pipeline, the hydrate inhibitor pipeline is used for providing hydrate inhibitors for downhole and wellhead, and the lifting gas pipeline is used for providing high-pressure nitrogen gas for downhole to maintain the lifting capacity of the downhole gas phase channel and support the pressure control of the downhole system.

[0019] A combustible ice underwater collection method based on horizontal well depressurization mining, characterized by comprising the following steps:

[0020] Step one: set a gas-liquid separator at the underwater wellhead, the natural gas produced by the wellbore is transported to the gas-liquid separator through the gas phase production pipeline, and the annulus produced water is transported to the gas-liquid separator through the liquid phase production pipeline;

[0021] When the well is gradually opened, the underwater electric submersible pump is started to gradually reduce the downhole pressure, the gas phase wellhead shut-off valve and the liquid phase wellhead shut-off valve are opened, the gas phase wellhead regulating valve and the liquid phase wellhead regulating valve are slowly opened, the pressure difference between the downhole and the wellhead is established, the natural gas and water are produced from the wellhead respectively, and then enter the gas-liquid separator after pressure regulation; the gas phase outlet shut-off valve, the liquid phase outlet shut-off valve and the liquid phase outlet booster pump of the gas-liquid separator are opened, and the gas phase and liquid phase flow channels of the submarine pipeline are opened; the central system liquid phase inlet shut-off valve, the central system gas phase inlet shut-off valve and the liquid phase standpipe booster pump are opened, and the gas phase standpipe and the liquid phase standpipe lifting channel are opened; thus, the gas phase channel from the wellhead to the standpipe is continuously pressurized, and finally reaches the set operating pressure and is put into operation; the liquid phase channel from the wellhead to the standpipe is continuously watered and degassed until a stable pressure control gradient is formed, and is put into operation;

[0022] Step two: when multiple wells have been operated and a new well is ready to be put into production, before opening the well, the liquid in the liquid phase pipeline from the underwater collection area is taken out respectively, the liquid phase submarine pipeline 32 downstream of the gas-liquid separator is degassed and watered, the discharged gas enters the gas phase submarine pipeline through the gas-liquid separator, and then the well is operated according to the opening operation of step one;

[0023] Step three: when multiple wells are normally produced, the sand washing inlet shut-off valve and the sand washing liquid regulating valve are opened, the processed sand washing water is provided by the umbilical cable connected to the surface central platform, the platform sand washing water is stably supplied, the stable sand layer formed inside the gas-liquid separator is continuously and uniformly broken by the multiple nozzles inside the gas-liquid separator; the sand washing outlet shut-off valve and the sand washing liquid outlet regulating valve are opened, and continuous suction is carried out in the sand accumulation area; the discharged liquid sand mixture is pressurized by the sand washing liquid phase pump and mixed with the pressurized liquid phase; the shut-off valves of the gas and liquid from each part of the submarine central collection system are opened.

[0024] When the underwater reinjection condition is met, the reinjection shut-off valve and the reinjection pump of the submarine central collection system are opened, the liquid phase standpipe regulating valve opening degree is adjusted, the liquid phase distribution flow rate of the submarine central collection system to the offshore platform and the reinjection well is controlled, the pipeline downstream of the reinjection pump is connected to the newly built reinjection pipeline, and the gas phase transportation pipeline of the well can also be connected to the gas phase transportation pipeline of the natural gas hydrate production well which has been built and stopped production as a reinjection well, forming a reverse transportation channel;

[0025] When the single wellhead gas-liquid separator needs to be overhauled, the gas phase wellhead shut-off valve, the liquid phase wellhead shut-off valve, the gas phase outlet shut-off valve and the liquid phase outlet shut-off valve of the single wellhead are closed, and the gas phase bypass first shut-off valve, the gas phase bypass regulating valve, the gas phase bypass second shut-off valve, the gas phase bypass pipeline, the liquid phase bypass first shut-off valve, the liquid phase bypass regulating valve, the liquid phase bypass second shut-off valve and the liquid phase bypass pipeline are opened, so that a standby passage is formed, and the single well is temporarily produced.

[0026] Compared with the prior art, the positive effect of the present application is that based on the demand of sea area combustible ice horizontal well exploitation, a set of multi-well underwater (seabed) collection system is proposed combining with production law and output medium characteristics, which can alleviate the underwater flow guarantee and sand deposition problem under large-scale development, improve the production efficiency of the downstream underwater system of the wellhead, and optimize the energy consumption of the later operation of the downhole and underwater.

[0027] Specifically, the present application has the following advantages:

[0028] (1) scientific setting

[0029] Based on the sea area combustible ice exploitation theory, the present application conforms to the horizontal well combustible ice exploitation process successfully implemented in China in 2020, and aims at the gas, liquid and solid production law in the whole exploitation cycle, uses the downhole primary separation and booster system, considers setting the wellhead secondary separation and sand management system in the underwater system, realizes the secondary rectification, separation and sand management of the downhole output medium, improves the phase state stability of the output medium, and greatly reduces the flow guarantee risk; the central collection system is set underwater, has the functions of collecting multi-well medium, liquid phase boosting, liquid phase flow direction control, gas phase reserved boosting and the like, meets the selection demand of output water being transported to the offshore platform and directly being injected back underwater, further improves the system operation flexibility, and reduces the system energy consumption.

[0030] (2) good economy

[0031] The present application sets the mode of multi-well independent exploitation-central collection and transportation according to the demand of sea area combustible ice horizontal well exploitation, avoids the system construction of single-well independent medium transportation to the sea surface, sets two sets of liquid phase external output distribution systems in the central collection system, optimizes the scheme of liquid phase injection back to the downhole by taking advantage of the characteristics of no condensate oil output of the combustible ice gas reservoir, avoids boosting and transporting all output liquid phases to the sea surface, sets the secondary separation and sand management system at the wellhead, promotes the secondary distribution of the phase state of the output medium, reduces the water content in the downstream gas phase pipeline and the gas content in the downstream liquid phase pipeline, avoids long-term injection of hydrate inhibitors, and at the same time, the set wellhead secondary separation and sand management system provides access conditions for future downhole pressure reduction-non-separation exploitation.

[0032] (3) advanced concept

[0033] China has carried out several sea area combustible ice test mining activities, accumulated more experience, and provided a good foundation for later commercial mining. The technology of the present application conforms to the horizontal well depressurization mining mode, and the single well secondary separation and sand management, underwater central collection and liquid phase distribution, and later system and scheme of reducing hydrate inhibitor have technical feasibility and advanced concept, which further provides a wellhead separation system for the future downhole depressurization-non-separation system, and can provide reference and reference for subsequent test mining and development. BRIEF DESCRIPTION OF DRAWINGS

[0034] The present application will be described by specific embodiments and with reference to the accompanying drawings, wherein

[0035] Figure 1 It is a structural schematic diagram of the present application.

[0036] In the figure, 1 is a gas phase wellhead shut-off valve, 2 is a gas phase wellhead regulating valve, 3 is a liquid phase wellhead shut-off valve, 4 is a liquid phase wellhead regulating valve, 5 is a gas phase production pipeline, 6 is a liquid phase production pipeline, 7 is a gas-liquid separator, 8 is a gas phase outlet shut-off valve, 9 is a liquid phase outlet shut-off valve, 10 is a liquid phase outlet booster pump, 11 is a sand washing inlet shut-off valve, 12 is a sand washing liquid regulating valve, 13 is a sand washing liquid inlet pipeline, 14 is a sand washing outlet shut-off valve, 15 is a sand washing liquid outlet regulating valve, 16 is a sand washing liquid phase pump, 17 is a sand washing liquid discharge pipeline, 18 is a gas phase bypass first shut-off valve, 19 is a gas phase bypass regulating valve, 20 is a gas phase bypass second shut-off valve, 21 is a gas phase bypass pipeline, 22 is a liquid phase bypass first shut-off valve, 23 is a liquid phase bypass regulating valve, 24 is a liquid phase bypass second shut-off valve, 25 is a liquid phase bypass pipeline, 31 is a gas phase submarine pipeline, 32 is a liquid phase submarine pipeline, 33 is a gas phase vertical pipe, 34 is a liquid phase vertical pipe, 41 is a central system gas phase inlet shut-off valve, 42 is a gas phase manifold, 43 is a central system liquid phase inlet shut-off valve, 44 is a liquid phase manifold, 45 is a liquid phase vertical pipe inlet shut-off valve, 46 is a liquid phase vertical pipe regulating valve, 47 is a liquid phase vertical pipe booster pump, 48 is a reinjection shut-off valve, 49 is a reinjection pump, 50 is a reinjection pipeline, 51 is a umbilical cable, 52 is a distributor, 53 is a sand washing liquid pipeline, 54 is a hydrate inhibitor pipeline, and 55 is a lifting gas pipeline. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all. The components of the embodiments of the present application described and shown in the drawings can be arranged and designed in various different configurations.

[0038] Therefore, the following detailed description of the embodiments of the application provided in the drawings is not intended to limit the scope of the application claimed, but merely represents selected embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the application.

[0039] It should be noted that the embodiments in the application and the features in the embodiments can be combined with each other without conflict.

[0040] It should be noted that: similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0041] In the description of the embodiments of the application, it should be noted that the indicated position or position relationship is based on the position or position relationship shown in the drawings, or the position or position relationship commonly placed when the product of the application is used, or the position or position relationship commonly understood by those skilled in the art, or the position or position relationship commonly placed when the product of the application is used, which is only for the convenience of describing the application and simplifying the description, and is not intended to indicate or imply that the indicated device or element must have a particular position, be constructed and operated in a particular position, and therefore cannot be understood as a limitation on the application. In addition, the terms "first", "second" are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0042] In the description of the embodiments of the application, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set", "connected" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be directly connected, or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances; the drawings in the embodiments are used to clearly and completely describe the technical solutions in the embodiments of the application, and obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. The components of the embodiments of the application described and shown in the drawings here can be arranged and designed in various different configurations.

[0043] As Figure 1 As shown in the figure, a combustible ice underwater collection system based on horizontal well depressurization mining includes a wellhead separation system, a pipeline transportation system, a seabed central collection system and a utility system, through the setting of the system, the sea area natural gas hydrate of horizontal well depressurization mining is realized high efficient collection, centralized collection.

[0044] The wellhead separation system is arranged at the underwater wellhead, that is, each wellhead of the single well is provided with a corresponding wellhead separation system, the wellhead separation system comprises a gas-liquid separator 7, gas-liquid phase inlets of the gas-liquid separator 7 are connected with a gas phase production system and a liquid phase production system respectively, the gas-liquid separator 7 is used for carrying out secondary separation of the produced medium downstream of each combustible ice underwater wellhead (primary separation has been carried out underground), further improving the efficiency of gas-liquid separation, reducing the amount of gas entrained in the liquid phase, reducing the amount of sand in the gas phase, and using the liquid phase with greater density to transport the sand produced underground, the bottom of the gas-liquid separator 7 is connected with a sand washing system, the sand washing system is used for removing the sand particles accumulated in the gas-liquid separator 7, avoiding the accumulation of sand particles in the separator, and the gas-liquid phase outlets of the gas-liquid separator 7 are connected with a seabed central collection system through a pipeline transportation system; the seabed central collection system is used for collecting the gas phase and the liquid phase produced by each production well transported by the pipeline transportation system; a utility system is connected with the sand washing system through a pipeline, and the utility system is used for controlling the underwater single well and the seabed central collection system relying on the above-water central processing platform.

[0045] Specifically, the gas phase production system comprises a gas phase production pipeline 5, a gas wellhead shut-off valve 1 and a gas wellhead regulating valve 2 arranged on the gas phase production pipeline 5, the gas wellhead shut-off valve is a remote electric shut-off valve ball valve, which is used for controlling the opening and closing of the underground gas phase production pipeline; the gas wellhead regulating valve is a remote electric regulating valve, which controls the pressure of the underground gas phase medium by adjusting the opening degree in cooperation with the underground lifting system; the liquid phase production system comprises a liquid phase production pipeline 6, a liquid wellhead shut-off valve 3 and a liquid wellhead regulating valve 4 arranged on the liquid phase production pipeline 6, the liquid wellhead shut-off valve is a remote electric shut-off valve ball valve, which is used for controlling the opening and closing of the underground liquid phase production pipeline; the liquid wellhead regulating valve is a remote electric regulating valve, which is used for controlling the liquid phase pressure entering the gas-liquid separator by adjusting the opening degree, so as to keep consistent with the gas phase pressure; the gas phase production pipeline 5 and the liquid phase production pipeline 6 are connected with the gas phase and liquid phase interfaces of the wellhead respectively, so as to transport the produced gas phase and liquid phase to the gas-liquid separator 7 respectively; the gas-liquid separator is a horizontal separator, which is used for carrying out secondary separation of the produced medium (natural gas, water, sand), reducing the water and sand content in the natural gas, reducing the natural gas content in the water, carrying the settled sand by using the separated water, improving the carrying efficiency of the sand, and the sand washing nozzle is arranged in the gas-liquid separator, which is used for dispersing and disturbing the settled sand at the bottom of the separator by using high-pressure sand washing liquid, and supporting the suspended sand liquid discharge.

[0046] The pipeline transportation system comprises a gas-phase submarine pipeline 31, a liquid-phase submarine pipeline 32, a gas-phase riser 33 and a liquid-phase riser 34, which are mainly used for transporting the natural gas and the liquid phase separated from the wellhead on the seabed and underwater. The gas-liquid separator 7 is connected to the submarine central collection system through the gas-phase submarine pipeline 31 and the liquid-phase submarine pipeline 32. The gas phase and the liquid phase collected by the submarine central collection system are output to the gas-phase riser 33 and the liquid-phase riser 34, respectively. The gas-phase riser is connected to the underwater central collection system and the water surface treatment facility. The liquid-phase riser is connected to the underwater central collection system and the water surface treatment facility. The liquid phase is transported when there is no injection well underwater at present, and the liquid-phase transportation pipeline for a single well is converted in the later stage. The pipeline is used for providing pure sand washing water for underwater sand washing after treatment on the platform. At the same time, the later-stage liquid-phase injection connection submarine pipeline is connected to the underwater central collection system and the later-stage target injection well, which is used for transporting the produced liquid collected underwater to the injection well.

[0047] Specifically, the gas-phase submarine pipeline and the liquid-phase submarine pipeline are designed according to the maximum production of a single well. The liquid-phase submarine pipeline has an inner wall erosion allowance of at least 2 mm. The gas-phase riser and the liquid-phase riser are designed according to the maximum estimated production of the underwater system.

[0048] The submarine central collection system comprises a gas-phase manifold 42 and a liquid-phase manifold 44. The gas-phase manifold 42 is provided with multiple interfaces for collecting the gas-phase medium of each gas-phase submarine pipeline 31 and outputting to the gas-phase riser 33. A gas-phase outlet shut-off valve 8 and a central system gas-phase inlet shut-off valve 41 are arranged on each gas-phase submarine pipeline 31. The liquid-phase manifold 44 is provided with multiple interfaces for collecting the liquid-phase medium of each liquid-phase submarine pipeline 32 and outputting to the liquid-phase riser 34 and / or the injection pipeline 50. A liquid-phase outlet shut-off valve 9, a liquid-phase outlet booster pump 10 and a central system liquid-phase inlet shut-off valve 43 are arranged on each liquid-phase submarine pipeline 32. A liquid-phase riser inlet shut-off valve 45, a liquid-phase riser regulating valve 46 and a liquid-phase riser booster pump 47 are arranged on the liquid-phase riser 34. An injection shut-off valve 48 and an injection pump 49 are arranged on the injection pipeline 50. The submarine central collection system is mainly used for collecting the natural gas produced by each production well and sending to the gas-phase riser. The collected liquid phase is distributed in flow rate. A small part is transported to the water surface through the liquid-phase riser booster pump and used for underwater sand washing after treatment. The remaining part is boosted by the injection pump and transported to the liquid-phase injection pipeline.

[0049] Specifically, the gas phase outlet shut-off valve is a remote electric shut-off valve for connecting the gas-liquid separator and the gas phase submarine pipeline; the central system gas phase inlet shut-off valve is a remote electric shut-off valve for controlling a single gas phase submarine pipeline to enter the central collection system; the liquid phase outlet shut-off valve is a remote electric shut-off valve for connecting the gas-liquid separator and the liquid phase submarine pipeline; the liquid phase outlet booster pump is a remote electric pump for pressurizing the separated liquid phase and delivering it to the central collection system; further, in order to avoid the influence of the central collection system main pump failure on the underwater collection system production, the liquid phase outlet booster pump has the ability to directly lift the separated liquid phase to the water surface; the central system liquid phase inlet shut-off valve is a remote electric shut-off valve for controlling a single liquid phase submarine pipeline to enter the central collection system; the liquid phase standpipe inlet shut-off valve is a remote electric shut-off valve for controlling the opening and closing of the liquid phase standpipe; the liquid phase standpipe regulating valve is a remote electric regulating valve for regulating the liquid amount distributed into the liquid phase standpipe, and the remaining part enters the reinjection pipeline; the liquid phase standpipe booster pump is a centralized booster pump after the liquid phase central collection, for uniformly lifting the liquid phase collected by the collection system to the water surface treatment facility; the liquid phase standpipe booster pump is preferably a screw pump to adapt to the fluid characteristics containing liquid and sand; the reinjection shut-off valve is arranged at the reinjection pipeline inlet for controlling the opening and closing of the reinjection pipeline; the reinjection pump is used to pressurize the liquid phase that needs to be reinjected underground to meet the reinjection pressure requirement; the reinjection pipeline is connected to the reinjection wellhead to provide a channel for the flow of reinjection medium.

[0050] The utility system includes an umbilical cable 51, a distributor 52, a sand washing liquid pipeline 53, a hydrate inhibitor pipeline 54, a lifting gas pipeline 55, and other power, instrument, and control cables, which are used to rely on the water surface central treatment platform to provide power, control signals, communication signals, hydrate inhibitors, sand washing water, and the like for each single well and the central collection system.

[0051] Specifically, the umbilical cable 51 connects the water surface platform center and the single wellhead system to provide multiple shared engineering medium, energy, and signal transmission functions; the distributor 52 is arranged at the wellhead to separate and connect various pipelines, energy, and signal lines; the sand washing liquid pipeline 53 is connected with the sand washing liquid inlet pipeline 13 to provide high-pressure purified water for the gas-liquid separator to perform bottom sand washing; the hydrate inhibitor pipeline 54 is used to provide hydrate inhibitors for the downhole and wellhead, on the one hand to improve the hydrate decomposition effect, and on the other hand to provide hydrate inhibitors for the wellhead and downstream collection system; the lifting gas pipeline 55 is used to provide high-pressure nitrogen gas for the downhole to maintain the lifting capacity of the downhole gas phase channel and support the pressure control of the downhole system. In addition, power, communication, and control signal lines are connected to corresponding valves and equipment.

[0052] In the embodiment, the gas phase production pipeline 5 is connected with the gas phase subsea pipeline 31 through a gas phase bypass pipeline system, and the liquid phase production pipeline 6 is connected with the liquid phase subsea pipeline 32 through a liquid phase bypass pipeline system, which are used for the wellhead output medium to directly enter the downstream gas phase subsea pipeline 31 and liquid phase subsea pipeline 32 without passing through the gas-liquid separator 7.

[0053] Specifically, the gas phase bypass pipeline system includes a gas phase bypass pipeline 21, a gas phase bypass first shut-off valve 18, a gas phase bypass regulating valve 19 and a gas phase bypass second shut-off valve 20 arranged on the gas phase bypass pipeline 21, which are mainly used for providing a temporary gas phase flow channel when the gas-liquid separator is maintained, and the gas phase bypass pipeline 21 is used for connecting the gas phase production pipeline 5 with the gas phase subsea pipeline 31; the liquid phase bypass pipeline system includes a liquid phase bypass pipeline 25, a liquid phase bypass first shut-off valve 22, a liquid phase bypass regulating valve 23 and a liquid phase bypass second shut-off valve 24 arranged on the liquid phase bypass pipeline 25, which are mainly used for providing a temporary liquid phase flow channel when the gas-liquid separator is maintained, and the liquid phase bypass pipeline 25 is used for connecting the liquid phase production pipeline 6 with the liquid phase subsea pipeline 32.

[0054] The sand washing system includes a sand washing liquid inlet pipeline 13 and a sand washing liquid outlet pipeline 17, the bottom of the gas-liquid separator 7 is connected with the utility system through the sand washing liquid inlet pipeline 13, and the bottom of the gas-liquid separator 7 is connected with the liquid phase subsea pipeline 32 through the sand washing liquid outlet pipeline 17, a sand washing inlet shut-off valve 11 and a sand washing liquid regulating valve 12 are arranged on the sand washing liquid inlet pipeline 13, and a sand washing outlet shut-off valve 14, a sand washing liquid outlet regulating valve 15 and a sand washing liquid phase pump 16 are arranged on the sand washing liquid outlet pipeline 17. By arranging two-phase pumps at the bottom outlet of the separator and a plurality of suction pipes in the separator, the liquid phase with high sand content at the bottom of the separator is continuously sucked, the sucked sand-containing liquid phase is mixed with the liquid phase pressurized by the liquid phase pump, and then is output to the central collection system.

[0055] Specifically, the sand washing inlet shut-off valve is a remote electric shut-off valve, which is used for connecting or isolating the sand washing water supplied by the umbilical cable with the gas-liquid separator; the sand washing liquid regulating valve is a remote electric regulating valve, which is used for regulating the pressure of the sand washing water to meet the sand washing demand in the separator; the sand washing liquid inlet pipeline provides a channel for the sand washing liquid to enter the separator; the sand washing outlet shut-off valve is a remote electric shut-off valve, which is used for controlling the opening and closing of the sand-containing liquid outlet channel; the sand washing liquid outlet regulating valve is a remote electric regulating valve, which is used for regulating the pressure and discharge flow of the sand-containing liquid; the sand washing liquid phase pump is a remote electric pump, which is used for pressurizing and outputting the sand-containing liquid; and the sand washing liquid outlet pipeline connects the sand washing liquid phase pump, and inputs the pressurized medium into the liquid phase subsea pipeline.

[0056] At the same time, the system also sets pressure transmitters at key points to monitor the pressure values of various points, and provides key parameters for the operation of the collection system.

[0057] The working principle of the present application is as follows:

[0058] (1) After the underwater natural gas hydrate is gasified after pressure reduction at the well bottom, it is decomposed into natural gas, water and part of the sand, and after passing through the sand prevention system, the separation system and the lifting system at the well bottom, the liquid phase is transported from the wellbore to the seabed surface and connected to the liquid phase channel of the underwater gas production tree, and the gas phase is transported from the annulus to the seabed surface and connected to the gas phase channel of the underwater gas production tree. This part of the structure is outside the scope of the system structure, but the system considers providing hydrate inhibitor and lifting gas supply for downhole production, and matching downhole production during well opening, operation and maintenance, and providing collaborative functions.

[0059] (2) The natural gas and water produced by each single well only undergoes one downhole separation, and the gas phase contains a small amount of liquid phase and sand; the liquid phase contains a small amount of gas phase and sand. At the same time, during downhole production, it is inevitable to produce wellbore slug flow, therefore, in order to improve the production efficiency of single well, reduce the erosion risk of sand in gas phase pipeline, improve the transportation efficiency of downstream seabed pipeline and riser, and support the liquid phase reinjection function of the central collection system, a gas-liquid separator is arranged at the underwater wellhead; in order to alleviate the sand deposition risk of the gas-liquid separator, a sand flushing and discharging system is arranged in the gas-liquid separator, and the discharged sand is connected to the liquid phase seabed pipeline through pressurization and transported through the liquid phase seabed pipeline. At the same time, gas phase bypass and liquid phase bypass are arranged in the underwater wellhead area, and temporary gas phase and liquid phase transportation is carried out when the gas-liquid separator or the sand discharging system needs to be overhauled.

[0060] (3) In the seabed central collection system, a multi-well collection function is provided, mainly considering improving the overall collection capacity of the combustible ice block; a centralized pressurization function is provided for the collected liquid phase; the collected gas phase has not been considered for centralized pressurization, and when the production scale is sufficient, an underwater compressor can be considered to be arranged in the central collection system. Since the conditions of combustible ice accumulation are relatively clean, the produced materials are formation water, natural gas (mainly methane) and formation sand, which can be directly reinjected, thus avoiding the waste of energy consumption caused by lifting all the produced media to the water surface for treatment.

[0061] The present application also provides a combustible ice underwater collection method based on horizontal well pressure reduction mining, comprising the following steps:

[0062] Step one: a gas-liquid separator is arranged at the underwater wellhead, and the natural gas produced in the downhole wellbore is transported to the gas-liquid separator through the gas phase production pipeline, and the produced water in the annulus is transported to the gas-liquid separator through the liquid phase production pipeline.

[0063] The basic method of the undersea mining of the sea area combustible ice to the seabed is that the sea area combustible ice production is carried out under the well to reduce the pressure, and the pressure reduction makes the combustible ice deposit decompose to generate natural gas, produced water, and carry out part of the sand particles. The decomposed products are separated by the undersea gas-liquid primary separation and the undersea electric submersible pump liquid phase pressure boosting, the natural gas is transported to the seabed through the wellbore, and the produced water is transported to the seabed through the annulus. Affected by the separation efficiency, the natural gas (containing a small amount of water and sand) and the produced water (containing a small amount of natural gas and sand) transported to the seabed after the undersea combustible ice decompression are theoretically multiphase medium, not single-phase fluid. The pressure of the decomposed natural gas and the produced water is 3-5 MPa, the natural gas is discharged by the residual pressure without pressure boosting under the well, and the produced water is boosted by the electric submersible pump under the well; at the same time, the umbilical cable provided by the water surface provides the lifting gas and the increased liquid (hydrate inhibitor) for the undersea production control.

[0064] When the first single well is initially put into production, the undersea wellbore (the ideal medium is mainly natural gas) and the undersea annulus (the ideal medium is mainly water) have not yet established a stable production balance. The internal pressure of the wellbore is the termination pressure after completion, which needs to be reduced to induce the undersea natural gas hydrate decomposition. When the well is gradually opened, the undersea electric submersible pump is opened to gradually reduce the undersea pressure, the gas phase wellhead shut-off valve and the liquid phase wellhead shut-off valve are opened, the gas phase wellhead regulating valve and the liquid phase wellhead regulating valve are slowly opened, the pressure difference between the undersea and the wellhead is established, the natural gas and the water are respectively produced from the wellhead, and are adjusted to enter the gas-liquid separator; the gas phase outlet shut-off valve, the liquid phase outlet shut-off valve and the liquid phase outlet booster pump of the gas-liquid separator are opened, the gas phase and the liquid phase flow channels of the seabed pipeline are opened; the liquid phase inlet shut-off valve, the gas phase inlet shut-off valve of the central system and the liquid phase standpipe booster pump are opened, the gas phase standpipe and the liquid phase standpipe lifting channels are opened; thus, the gas phase channel from the wellhead to the standpipe is continuously pressurized, and finally reaches the set operating pressure and is put into operation; the liquid phase channel from the wellhead to the standpipe is continuously watered and degassed until a stable pressure control gradient is formed and is put into operation; at the same time, the umbilical cable connected to the single well is used to inject the hydrate inhibitor into the undersea, and to provide the supply of the lifting gas (nitrogen) required by the undersea.

[0065] Step two: when several wells have been operated and a new well is ready to be put into production, before the well is opened, the liquid in the subsea gathering area is taken separately from the liquid phase pipeline, the liquid phase subsea pipeline 32 downstream of the gas-liquid separator liquid phase outlet is vented and filled with water, the gas discharged enters the gas phase subsea pipeline through the gas-liquid separator; then the subsea electric submersible pump is started to gradually reduce the downhole pressure, the gas phase wellhead shut-off valve and the liquid phase wellhead shut-off valve are opened, the gas phase wellhead regulating valve and the liquid phase wellhead regulating valve are slowly opened, the pressure difference between the downhole and the wellhead is established, the natural gas and water are produced at the wellhead and then enter the gas-liquid separator after pressure regulation; the gas phase outlet shut-off valve, the liquid phase outlet shut-off valve and the liquid phase outlet booster pump are opened, the gas phase and liquid phase flow passages of the subsea pipeline are opened; when the pressure of the gas phase subsea pipeline reaches the gas phase pressure of the subsea gathering area, the central system gas phase inlet shut-off valve is opened; when the pressure of the liquid phase subsea pipeline reaches the liquid phase pressure of the subsea gathering area, the central system liquid phase inlet shut-off valve is opened. Thus, the new wellhead is put into operation. At the same time, the hydrate inhibitor is injected into the well through the umbilical cable connected to the single well, and the supply of lifting gas (nitrogen) required by the well is provided.

[0066] Step three: when several wells are in normal production, the downhole pressure reduction production is relatively stable, and the wellhead area basically maintains normal production. The sand washing inlet shut-off valve and the sand washing liquid regulating valve are opened, the treated sand washing water is provided by the umbilical cable connected to the surface central platform, the stable sand layer formed in the gas-liquid separator is continuously and uniformly broken by using multiple nozzles inside the gas-liquid separator; the sand washing outlet shut-off valve and the sand washing liquid outlet regulating valve are opened, continuous suction is carried out in the sand accumulation area, and the discharged liquid sand mixture is mixed with the pressurized liquid phase; the shut-off valves for incoming gas and liquid of the subsea central gathering system are opened. When the subsea injection conditions are met, the injection shut-off valve and the injection pump of the subsea central gathering system are opened, and the liquid phase standpipe regulating valve opening degree is adjusted to control the liquid phase distribution flow rate of the subsea central gathering system to the offshore platform and the injection well. The pipeline downstream of the injection pump is connected to the newly built injection pipeline, or the gas phase pipeline of the natural gas hydrate production well that has been built and is out of production is connected to form a reverse transport channel. When the wells are in normal production, the gas phase transport pressure downstream of the separator is controlled at 3-4 MPa, and the system can not consider adding the hydrate inhibitor at the seabed according to the temperature of about 3°C of the deep sea 1000 m current.

[0067] When the single wellhead gas-liquid separator needs to be repaired, the gas phase wellhead shut-off valve, the liquid phase wellhead shut-off valve, the gas phase outlet shut-off valve and the liquid phase outlet shut-off valve of the single wellhead are closed, the gas phase bypass first shut-off valve, the gas phase bypass regulating valve, the gas phase bypass second shut-off valve, the gas phase bypass pipeline, the liquid phase bypass first shut-off valve, the liquid phase bypass regulating valve, the liquid phase bypass second shut-off valve and the liquid phase bypass pipeline are opened to form a backup path, so that the single well is temporarily produced.

[0068] The application is not restricted to the described specific embodiments, but extends to any novel one, or any novel combination of the features disclosed in this specification, and to any novel method or process steps disclosed in this specification or any novel combination of the steps.

Claims

1. A combustible ice underwater collection system based on pressure reduction mining of a horizontal well, characterized in that: The wellhead separation system, the pipeline transportation system, the seabed central collection system and the utility system are included. The wellhead separation system is arranged at the underwater wellhead, that is, the wellhead of each single well is provided with a corresponding wellhead separation system, the wellhead separation system includes a gas-liquid separator (7), the gas-liquid phase inlets of the gas-liquid separator (7) are connected with the gas phase production system and the liquid phase production system respectively, the gas-liquid separator (7) is used for secondary separation of the produced medium downstream of each underwater wellhead of combustible ice, the bottom of the gas-liquid separator (7) is connected with a sand washing system, the sand washing system is used for removing the sand accumulated in the gas-liquid separator (7), and the gas-liquid phase outlet of the gas-liquid separator (7) is connected with the seabed central collection system through the pipeline transportation system. The gas phase production pipeline (5) of the gas phase production system is connected with the gas phase seabed pipeline (31) of the pipeline transportation system through a gas phase bypass pipeline system, the liquid phase production pipeline (6) of the liquid phase production system is connected with the liquid phase seabed pipeline (32) of the pipeline transportation system through a liquid phase bypass pipeline system, and the gas phase bypass pipeline system and the liquid phase bypass pipeline system are used for the wellhead produced medium to directly enter the downstream gas phase seabed pipeline (31) and the liquid phase seabed pipeline (32) without passing through the gas-liquid separator (7). The seabed central collection system is used for collecting and concentrating the gas phase and the liquid phase produced by each production well transported by the pipeline transportation system, the seabed central collection system includes a gas phase collecting pipe (42) and a liquid phase collecting pipe (44), the liquid phase collecting pipe (44) is communicated with the liquid phase standpipe (34) and the reinjection pipeline (50) of the pipeline transportation system respectively, the flow of the collected liquid phase is distributed, a small part is transported to the water surface through the liquid phase standpipe booster pump (47) and is used for underwater sand washing after treatment, and the remaining part is pressurized through the reinjection pump (49) and is transported to the liquid phase reinjection pipeline (50). The utility system is connected with the sand washing system through a pipeline, the utility system is used for controlling and managing each single well and the seabed central collection system relying on the onshore central processing platform, the sand washing system includes a sand washing liquid inlet pipeline (13) and a sand washing liquid discharge pipeline (17), the bottom of the gas-liquid separator (7) is connected with the utility system through the sand washing liquid inlet pipeline (13), and the bottom of the gas-liquid separator (7) is connected with the liquid phase seabed pipeline (32) through the sand washing liquid discharge pipeline (17). The gas-liquid separator (7) is a horizontal separator, is used for secondary separation of the produced medium, carries the settled sand by using the separated water, the sand washing nozzle is arranged in the gas-liquid separator (7), the settled sand at the bottom of the separator is scattered and disturbed by using the high-pressure sand washing liquid, and the suspended sand liquid is discharged.

2. The combustible ice underwater mining system based on the pressure reduction mining of the horizontal well according to claim 1, characterized in that: The gas phase production system comprises a gas phase production pipeline (5), a gas phase wellhead shut-off valve (1) and a gas phase wellhead regulating valve (2) arranged on the gas phase production pipeline (5), the liquid phase production system comprises a liquid phase production pipeline (6), a liquid phase wellhead shut-off valve (3) and a liquid phase wellhead regulating valve (4) arranged on the liquid phase production pipeline (6), and the gas phase production pipeline (5) and the liquid phase production pipeline (6) are connected with a wellhead gas phase and liquid phase interface respectively to convey the produced gas phase and liquid phase to a gas-liquid separator (7) respectively.

3. The subsea combustible ice production system based on pressure reduction by horizontal well according to claim 2, characterized in that: The pipeline conveying system comprises a gas phase submarine pipeline (31), a liquid phase submarine pipeline (32), a gas phase riser (33) and a liquid phase riser (34), the gas-liquid separator (7) is connected with a submarine central gathering system through the gas phase submarine pipeline (31) and the liquid phase submarine pipeline (32), and the submarine central gathering system outputs the gathered gas phase and liquid phase to the gas phase riser (33) and the liquid phase riser (34) respectively.

4. The subsea combustible ice production system based on pressure reduction by horizontal well according to claim 1, characterized in that: The gas phase bypass pipeline system comprises a gas phase bypass pipeline (21), a gas phase bypass first shut-off valve (18), a gas phase bypass regulating valve (19) and a gas phase bypass second shut-off valve (20) arranged on the gas phase bypass pipeline (21), and the gas phase bypass pipeline (21) is used for connecting the gas phase production pipeline (5) with the gas phase submarine pipeline (31); the liquid phase bypass pipeline system comprises a liquid phase bypass pipeline (25), a liquid phase bypass first shut-off valve (22), a liquid phase bypass regulating valve (23) and a liquid phase bypass second shut-off valve (24) arranged on the liquid phase bypass pipeline (25), and the liquid phase bypass pipeline (25) is used for connecting the liquid phase production pipeline (6) with the liquid phase submarine pipeline (32).

5. The subsea combustible ice production system based on pressure reduction by horizontal well according to claim 3, characterized in that: A sand washing liquid inlet shut-off valve (11) and a sand washing liquid regulating valve (12) are arranged on the sand washing liquid inlet pipeline (13), and a sand washing outlet shut-off valve (14), a sand washing liquid outlet regulating valve (15) and a sand washing liquid pump (16) are arranged on the sand washing liquid outlet pipeline (17).

6. The subsea combustible ice production system based on pressure reduction by horizontal well according to claim 3, characterized in that: The gas phase manifold (42) is provided with multiple interfaces for gathering the gas phase medium of each gas phase submarine pipeline (31) and outputting to the gas phase riser (33), and a gas phase outlet shut-off valve (8) and a central system gas phase inlet shut-off valve (41) are arranged on each gas phase submarine pipeline (31) respectively, the liquid phase manifold (44) is provided with multiple interfaces for gathering the liquid phase medium of each liquid phase submarine pipeline (32) and outputting to the liquid phase riser (34) and / or a reinjection pipeline (50), a liquid phase outlet shut-off valve (9), a liquid phase outlet booster pump (10) and a central system liquid phase inlet shut-off valve (43) are arranged on each liquid phase submarine pipeline (32) respectively, a liquid phase riser inlet shut-off valve (45), a liquid phase riser regulating valve (46) and a liquid phase riser booster pump (47) are arranged on the liquid phase riser (34), and a reinjection shut-off valve (48) and a reinjection pump (49) are arranged on the reinjection pipeline (50).

7. The subsea combustible ice production system based on pressure reduction by horizontal well according to claim 5, characterized in that: The utility system comprises a umbilical cable (51) connecting a water platform center and a single wellhead system, a distributor (52) arranged at the wellhead for separating and connecting various pipelines, energy and signal pipelines, a sand washing liquid pipeline (53) connected with a sand washing liquid inlet pipeline (13), a hydrate inhibitor pipeline (54) for providing hydrate inhibitors for the downhole and wellhead, and a lifting gas pipeline (55) for providing high pressure nitrogen for the downhole to maintain the lifting capacity of the gas phase passage in the downhole and support the pressure control of the downhole system.

8. A method for collecting combustible ice in a horizontal well depressurization production system according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: Step one: arranging a gas-liquid separator at the underwater wellhead, and conveying the natural gas produced by the downhole wellbore to the gas-liquid separator through a gas phase production pipeline, and conveying the produced water in the annulus to the gas-liquid separator through a liquid phase production pipeline; When gradually opening the well, the underwater electric submersible pump is started to gradually reduce the downhole pressure, the gas phase wellhead shut-off valve and the liquid phase wellhead shut-off valve are opened, the gas phase wellhead regulating valve and the liquid phase wellhead regulating valve are slowly opened, the pressure difference between the downhole and the wellhead is established, the natural gas and the water are produced from the wellhead and enter the gas-liquid separator after pressure regulation, the gas phase outlet shut-off valve, the liquid phase outlet shut-off valve and the liquid phase outlet booster pump of the gas-liquid separator are opened, the gas phase and liquid phase flow passages of the subsea pipeline are opened, the liquid phase inlet shut-off valve, the gas phase inlet shut-off valve of the central system and the liquid phase standpipe booster pump are opened, the gas phase standpipe and the liquid phase standpipe lifting passages are opened, the gas phase passage from the wellhead to the standpipe is continuously pressurized, the final operating pressure is reached, and the system is put into operation; the liquid phase passage from the wellhead to the standpipe is continuously watered and degassed until a stable pressure control gradient is formed, and the system is put into operation; Step two: when multiple wells have been operated and a new well is ready to be put into production, before opening the well, the liquid in the liquid phase pipeline of the underwater collection area is taken out respectively, the liquid phase subsea pipeline 32 downstream of the gas-liquid separator is degassed and watered, the discharged gas enters the gas phase subsea pipeline through the gas-liquid separator, and then the well is operated according to the opening operation of step one; Step three: when multiple wells are in normal production, the sand washing inlet shut-off valve and the sand washing liquid regulating valve are opened, the treated sand washing water is provided by the umbilical cable connected with the surface central platform, the platform sand washing water is stably supplied, the stable sand layer formed in the gas-liquid separator is continuously and uniformly broken by the multiple nozzles in the gas-liquid separator, the sand washing outlet shut-off valve and the sand washing liquid outlet regulating valve are opened, the sand in the sand accumulation area is continuously pumped out, and the discharged liquid sand mixture is mixed with the pressurized liquid phase; the shut-off valves of the gas and liquid from the subsea central collection system are opened.

9. The method according to claim 8, wherein the method further comprises: When the water has the reinjection condition, the reinjection pump and the reinjection cut-off valve of the seabed central gathering system are opened, and the liquid phase standpipe regulating valve opening is adjusted to control the liquid phase distribution flow of the seabed central gathering system to the offshore platform and the reinjection well, and the pipeline downstream of the reinjection pump is connected to the newly built reinjection pipeline; when the built and stopped natural gas hydrate production well is used as the reinjection well, the gas phase conveying pipeline connected to the well is accessed to form a reverse conveying channel; When the single wellhead gas-liquid separator needs to be overhauled, the gas phase wellhead cut-off valve, the liquid phase wellhead cut-off valve, the gas phase outlet cut-off valve and the liquid phase outlet cut-off valve of the single wellhead are closed, and the gas phase bypass first cut-off valve, the gas phase bypass regulating valve, the gas phase bypass second cut-off valve, the gas phase bypass pipeline, the liquid phase bypass first cut-off valve, the liquid phase bypass regulating valve, the liquid phase bypass second cut-off valve and the liquid phase bypass pipeline are opened to form a standby path, so that the single well is temporarily produced.

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