A hydrate secondary generation prevention system and method for combustible ice collection pipeline

Abstract

The application discloses a hydrate secondary generation prevention and treatment system and method for combustible ice collection pipeline, which comprises a gas phase collection pipeline for conveying output gas phase medium and a liquid phase collection pipeline for conveying output liquid phase medium, the liquid phase collection pipeline is connected with a liquid phase pipeline hydrate control system for separating free gas of wellhead liquid phase produced medium. Based on the theory of sea area combustible ice mining, the application conforms to the depressurization mining mode, aims at the medium characteristics of the gas phase channel and the liquid phase channel output during the underground decomposition, combines the pressure and temperature conditions under the deep water development environment, evaluates and obtains the viewpoint that the hydrate secondary generation risk of the gas phase medium collection pipeline is smaller under the basic maintenance of 3MPa operating pressure, and the liquid phase medium collection pipeline is mainly prevented from the hydrate secondary generation, and the gas-liquid secondary separation scheme is provided to prevent the hydrate secondary generation of the liquid phase medium collection pipeline from the components.

Description

Technical Field

[0001] This invention belongs to the field of combustible ice mining and transportation technology, and specifically relates to a system and method for preventing secondary formation of hydrates in combustible ice collection pipelines. Background Technology

[0002] Natural gas hydrates can be considered as highly compressed natural gas resources, 1m 3 Natural gas hydrates can decompose and release 160–180 m³ of gas. 3 Based on standard conditions, a rough estimate suggests that the total volume of natural gas contained in global natural gas hydrate deposits is approximately 1.8 × 10⁻⁶. 16 ~2.1×10 16 m 3 This is equivalent to twice the total carbon content of all proven conventional fossil fuels globally. Therefore, hydrates are expected to replace conventional fossil fuels such as oil and coal as a new type of clean energy source in the future. Currently, offshore natural gas hydrates are still in the exploration and pilot production stage, while a large amount of basic research is being conducted. In recent years, several pilot production activities of offshore natural gas hydrates have been carried out. In particular, in 2020, my country successfully carried out the second round of pilot production of combustible ice in the South China Sea, using the "horizontal well" production mode, once again setting a record for the time and total amount of combustible ice produced, laying a solid foundation for commercial production.

[0003] Based on the formation conditions of natural gas hydrates in the sea area, the depressurization method is a relatively clear and verified approach. In the current trial production activities, this method mainly reduces the pressure of the natural gas hydrate reservoir in the sea area, breaks its stable conditions, and promotes the formation of gas and liquid phases of natural gas hydrates. The produced medium is then transported to the offshore platform for treatment and discharge through independent gas and liquid phase pipelines via downhole sand control devices, pre-separation devices, and electric submersible pumps.

[0004] Preventing secondary hydrate formation is a key issue in the extraction and transportation of methane hydrate. Influenced by reservoir formation conditions, depressurization extraction techniques, and downhole pretreatment processes, the extracted liquid phase contains a certain amount of free gas, and the extracted gas phase contains a certain amount of free water. Simultaneously, after extraction from the well, the underwater production system faces stringent pressure and temperature constraints (for example, deep-water ambient temperatures are generally 3–8°C, and the water depth for methane hydrate extraction can reach approximately 1400m). These factors all contribute to the possibility of secondary hydrate formation. Therefore, for different operating conditions of gas-liquid separation pipelines, rationally controlling and mitigating secondary hydrate formation in the transportation system is crucial for efficient and safe transportation.

[0005] Currently, there are few detailed reports on technologies for preventing secondary hydrate formation in methane hydrate harvesting systems, and some publicly available information does not specifically address solutions for preventing secondary hydrate formation in pipeline systems. Therefore, it is necessary to conduct research on hydrate secondary formation prevention systems and methods for underwater production systems using the depressurization method for methane hydrate harvesting, and to propose reasonable and feasible technical solutions to support future commercial development. Summary of the Invention

[0006] The purpose of this invention is to provide a system and method for preventing the secondary formation of hydrates in marine combustible ice collection pipelines, which can safely and efficiently address the problems mentioned above.

[0007] This invention is based on the horizontal well depressurization extraction process and aims to prevent secondary hydrate formation in the produced medium by setting up a reasonable produced medium transportation system. Specifically, a liquid phase pipeline hydrate control system is installed underwater to buffer and separate the liquid phase medium produced from the wellhead. This system significantly separates the free gas still entrained in the liquid phase pipeline after pre-separation underground, ensuring that the medium entering the underwater liquid phase transportation system is essentially a pure liquid phase, i.e., water decomposed from molten ice. This fundamentally reduces the risk of secondary hydrate formation in the liquid phase pipeline and the resulting risk of hydrate particle erosion. Simultaneously, the separated gas phase is injected into the gas phase pipeline using a small pressure differential to improve the gas phase pipeline transportation efficiency. Additionally, periodic venting valves are installed at the manifold of the central system to... The system involves releasing potentially accumulated "dead gas" into the gas phase pipeline; installing an underwater shut-off and pressure control system to isolate the liquid phase pipeline under hydrostatic pressure and release pressure in areas where free gas accumulates during shut-off operations, further controlling the secondary formation of hydrates under non-flowing conditions; and strategically installing underwater hydrate inhibitor injection systems at wellheads and manifolds to provide necessary hydrate inhibitor injection interfaces for both gas and liquid phase pipelines. Hydrate inhibitors are injected according to actual production needs, serving as an additional safeguard for controlling secondary hydrate formation underwater. This results in a hydrate secondary formation prevention system for marine combustible ice harvesting pipelines, achieving a multi-dimensional prevention system based on component control, pressure control, and inhibitor backup injection, thus efficiently controlling secondary hydrate formation.

[0008] The technical solution adopted in this invention is: a combustible ice collection pipeline hydrate secondary formation prevention system, including a gas phase collection pipeline for transporting the produced gas phase medium and a liquid phase collection pipeline for transporting the produced liquid phase medium, characterized in that: the liquid phase collection pipeline is connected to the liquid phase pipeline hydrate control system and is used to separate the free gas of the liquid phase produced medium at the wellhead;

[0009] The liquid phase pipeline hydrate control system includes a gas-liquid separator, a bypass shut-off valve, and a separator gas phase regulating valve. The liquid phase acquisition pipeline is connected to the gas-liquid separator, the liquid phase outlet of the gas-liquid separator is connected to the central manifold of the liquid phase pipeline, and the gas phase outlet of the gas-liquid separator is connected to the gas phase acquisition pipeline through a pipeline equipped with the separator gas phase regulating valve, and then connected to the central manifold of the gas phase pipeline. The gas-liquid separator is equipped with a bypass, and a bypass shut-off valve is installed on the bypass. The inlet end of the bypass is connected to the upstream pipeline of the gas-liquid separator, and its outlet end is connected to the downstream pipeline of the gas-liquid separator.

[0010] The combustible ice collection pipeline hydrate secondary formation prevention system of the present invention further includes a liquid phase pipeline hydrate control system, wherein the liquid phase pipeline hydrate control system includes a liquid phase manifold exhaust regulating valve, which is connected to the liquid phase pipeline central manifold and the gas phase pipeline central manifold, and is used to discharge the gas accumulated in the liquid phase pipeline central manifold into the gas phase pipeline system.

[0011] The combustible ice collection pipeline hydrate secondary formation prevention system of the present invention includes a shut-off valve group installed on the gas phase collection pipeline and the liquid phase collection pipeline respectively. The shut-off valve group includes a gas phase collection pipeline wellhead shut-off valve and a gas phase collection pipeline end shut-off valve installed on the gas phase collection pipeline, and a liquid phase collection pipeline wellhead shut-off valve, a liquid phase collection pipeline separation shut-off valve and a liquid phase collection pipeline end shut-off valve installed on the liquid phase collection pipeline.

[0012] The combustible ice collection pipeline hydrate secondary formation prevention system of the present invention includes a pressure control system between the gas phase collection pipeline and the liquid phase collection pipeline. The pressure control system includes a pressure relief shut-off valve for the starting liquid phase collection pipeline, a pressure relief regulating valve for the starting liquid phase collection pipeline, a pressure relief shut-off valve for the ending liquid phase collection pipeline, and a pressure relief regulating valve for the ending liquid phase collection pipeline.

[0013] The pipeline equipped with an initial liquid phase collection pipeline pressure relief shut-off valve and an initial liquid phase collection pipeline pressure relief regulating valve is used to connect the upstream section of the liquid phase outlet of the gas-liquid separator with the gas phase collection pipeline; the pipeline equipped with an end liquid phase collection pipeline pressure relief shut-off valve and an end liquid phase collection pipeline pressure relief regulating valve is used to connect the downstream section of the liquid phase outlet of the gas-liquid separator with the gas phase collection pipeline. In the shutdown state, the gas phase collection pipeline is used as a liquid phase system pressure relief medium receiving pipeline.

[0014] The combustible ice collection pipeline hydrate secondary formation prevention and control system of the present invention further includes an underwater hydrate inhibitor injection system, which includes an inhibitor injection main pipeline, an underwater central manifold injection regulating valve, a wellhead liquid phase pipeline injection regulating valve, and a wellhead gas phase pipeline injection regulating valve.

[0015] The inhibitor injection main pipeline connects the hydrate inhibitor injection unit of the surface treatment platform and the subsea system. The inhibitor injection main pipeline is connected to the liquid phase pipeline central manifold through the subsea central manifold injection regulating valve. The inhibitor injection main pipeline is connected to the liquid phase outlet of the gas-liquid separator through the wellhead liquid phase pipeline injection regulating valve. The inhibitor injection main pipeline is connected to the gas phase outlet of the gas-liquid separator through the wellhead gas phase pipeline injection regulating valve.

[0016] A method for preventing secondary hydrate formation in combustible ice collection pipelines, characterized by the following specific methods:

[0017] Methane hydrate is produced using a downhole pressure extraction method. The extracted gaseous medium is transported to the underwater central area through a gas phase acquisition pipeline. After merging with other single-well gas phase acquisition pipelines, it is transported to the water surface through a gas phase riser. The extracted liquid medium is transported to the underwater central area through a liquid phase acquisition pipeline, a liquid phase booster pump, and a liquid phase pump post-pump pipeline. After merging with other single-well liquid phase acquisition pipelines, it is transported to the water surface through a liquid phase riser. The liquid phase acquisition pipeline connects the liquid phase to a gas-liquid separator, and the separated free gas is injected into the gas phase pipeline through the separator's gas phase regulating valve.

[0018] During normal delivery, keep the main inhibitor injection pipeline open and periodically open the underwater central manifold injection regulating valve to inject small doses of hydrate inhibitor into the central manifold. Simultaneously, periodically open the liquid phase manifold exhaust regulating valve to discharge the gas accumulated in the central manifold of the liquid phase pipeline into the gas phase pipeline system.

[0019] When the gas-liquid separator needs maintenance, open the bypass shut-off valve so that the liquid phase produced medium will not undergo gas-liquid separation for a short period of time. Simultaneously, open the wellhead liquid phase pipeline injection regulating valve to continuously inject hydrate inhibitors into the liquid phase pipeline. The amount of hydrate inhibitor injected should work in synergy with the amount of hydrate inhibitor injected during downhole decomposition to ensure that the free gas in the liquid phase pipeline does not freeze during transportation. At the same time, the liquid phase manifold exhaust regulating valve should still be opened periodically.

[0020] The method for preventing secondary hydrate formation in combustible ice collection pipelines described in this invention involves a downhole booster pump supplying liquid phase at a pressure 1 MPa higher than the pressure inside the gas phase collection pipeline.

[0021] The method for preventing secondary formation of hydrates in combustible ice collection pipelines described in this invention involves closing the wellhead shut-off valves of the gas phase collection pipeline, the wellhead shut-off valve of the liquid phase collection pipeline, the shut-off valve after separation of the liquid phase collection pipeline, the end shut-off valve of the gas phase collection pipeline, and the end shut-off valve of the liquid phase collection pipeline after a planned shutdown of the single-well system, thereby dividing the single-well collection system into a wellhead area and a collection pipeline area.

[0022] Implement segmented regional pressure reduction by opening the pressure relief shut-off valve and the pressure relief regulating valve of the initial liquid phase acquisition pipeline to discharge high-pressure liquid phase medium into the gas phase pipeline, thereby reducing the pressure in the pipeline after the liquid phase pump. Subsequently, open the pressure relief shut-off valve and the pressure relief regulating valve of the final liquid phase acquisition pipeline to discharge any possible accumulated free gas in the high-pressure liquid phase pipeline into the gas phase pipeline, gradually reducing the pressure in the pipeline after the liquid phase pump. Keep the separator gas phase regulating valve open to gradually reduce the pressure of the gas-liquid separator.

[0023] The method for preventing secondary formation of hydrates in combustible ice collection pipelines described in this invention includes an underwater hydrate inhibitor injection system that connects the surface production system to the underwater system. The underwater hydrate inhibitor injection system systematically injects hydrate inhibitors into the central manifold of the liquid phase pipeline, the liquid phase outlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator.

[0024] Compared with existing technologies, the positive effects of this invention are as follows: Based on the needs of horizontal well mining of combustible ice in marine areas, and combined with the production patterns and characteristics of the produced medium, this invention proposes a set of hydrate secondary formation prevention and control systems and methods based on the non-interference of operating pressures between gas phase production pipelines and liquid phase production pipelines. From the perspectives of component control gas-liquid separation, removal of "dead gas" accumulation areas, and pressure isolation and release after shutdown, supplemented by the support of surface hydrate inhibitors, this invention provides efficient and feasible hydrate secondary formation prevention and control functions for underwater combustible ice production medium transportation pipelines from multiple dimensions.

[0025] Specifically, this manifests as follows:

[0026] (1) Setting up a scientific

[0027] This invention is based on the theory of offshore combustible ice mining and follows the depressurization mining mode. Targeting the characteristics of the gas and liquid phase channels output during downhole decomposition, and considering the pressure and temperature conditions in deep-water development environments, it evaluates and concludes that the risk of secondary hydrate formation is relatively low when the gas phase medium collection pipeline maintains a basic operating pressure of 3 MPa. The focus is on preventing secondary hydrate formation in the liquid phase medium collection pipeline. A gas-liquid secondary separation scheme is proposed to prevent secondary hydrate formation in the liquid phase medium collection pipeline from a component perspective, and periodic venting is performed in the potentially "dead gas" areas of the liquid phase collection pipeline.

[0028] Furthermore, pressure isolation measures were proposed for the collection system after the shutdown to reduce the impact of hydrostatic pressure on the collection pipeline system and significantly improve the conditions for secondary hydrate formation. Overall, through effective analysis of the risks and patterns of secondary hydrate formation in combustible ice collection systems based on depressurization mining, reasonable control measures were proposed.

[0029] (2) Excellent economic efficiency

[0030] This invention proposes a wellhead separation method targeting only the liquid phase pipeline, significantly reducing the size of the subsea separator and allowing for targeted control of the liquid phase collection pipeline components. This also significantly reduces the amount of hydrate inhibitors required to inhibit hydrate formation in the liquid phase pipeline. Simultaneously, a pressure isolation and control system effectively controls the medium pressure in the pipeline system after a single well is shut down, avoiding the need for full-line hydrate inhibitor displacement and replacement. By adopting the solution provided by this system, near-zero injection of inhibitors required for the prevention of secondary hydrate formation in combustible ice collection systems can be achieved, requiring only the supply of hydrate inhibitors needed for downhole combustible ice decomposition and production enhancement.

[0031] (3) Advanced concepts

[0032] my country has conducted multiple trial mining operations of combustible ice in its offshore areas, accumulating considerable experience and laying a solid foundation for subsequent commercial exploitation. The technical solution of this invention conforms to the horizontal well depressurization mining model, conducts a special study on the flow assurance problem of the underwater acquisition system, optimizes previous research results, and proposes key points and practices for preventing secondary hydrate formation in gas phase and liquid phase acquisition pipelines, providing valuable reference and support for subsequent large-scale development. Attached Figure Description

[0033] The present invention will be described by way of specific embodiments and with reference to the accompanying drawings, wherein

[0034] Figure 1 This is a schematic diagram of the structure of the present invention.

[0035] The diagram shows the following markings: 1 for gas phase acquisition pipeline, 2 for liquid phase acquisition pipeline, 3 for liquid phase booster pump, 4 for liquid phase pump post-pump pipeline, 5 for gas phase pipeline central manifold, 6 for gas phase riser, 7 for liquid phase pipeline central manifold, 8 for liquid phase riser, 11 for gas-liquid separator, 12 for bypass shut-off valve, 13 for separator gas phase regulating valve, 14 for liquid phase manifold exhaust regulating valve, 21 for gas phase acquisition pipeline wellhead shut-off valve, 22 for liquid phase acquisition pipeline wellhead shut-off valve, 23 for liquid phase acquisition pipeline after separation shut-off valve, and 24 for gas phase acquisition pipeline. 25 is the end shut-off valve of the liquid phase acquisition pipeline, 26 is the beginning shut-off valve of the gas phase riser, 27 is the beginning shut-off valve of the liquid phase riser, 28 is the pressure relief shut-off valve of the beginning liquid phase acquisition pipeline, 29 is the pressure relief regulating valve of the beginning liquid phase acquisition pipeline, 31 is the pressure relief shut-off valve of the end liquid phase acquisition pipeline, 32 is the pressure relief regulating valve of the end liquid phase acquisition pipeline, 41 is the main pipeline for inhibitor injection, 42 is the injection regulating valve of the subsea central manifold, 43 is the injection regulating valve of the wellhead liquid phase pipeline, and 44 is the injection regulating valve of the wellhead gas phase pipeline. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0037] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0038] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0039] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0040] In the description of the embodiments of the present invention, it should be noted that the indicated orientation or positional relationship is based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of the invention is conventionally placed during use, or the orientation or positional relationship in which those skilled in the art conventionally understand it during use. This is only for the convenience of describing the present invention and simplifying the description, and is not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of the present invention. Furthermore, the terms "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0041] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set" and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the present invention based on the specific circumstances.

[0042] like Figure 1As shown, a system for preventing secondary hydrate formation in a combustible ice collection pipeline is described. It should be noted that this prevention system is mainly based on a typical combustible ice collection process. This typical combustible ice collection system includes a gas phase collection pipeline 1, a liquid phase collection pipeline 2, a liquid phase booster pump 3, a liquid phase pump post-pump pipeline 4, a gas phase pipeline central manifold 5, a gas phase riser 6, a liquid phase pipeline central manifold 7, and a liquid phase riser 8. Its basic principle is to rely on the depressurization method for extraction, and to distribute the produced gas phase medium (natural gas, free water) and liquid phase medium (water, free natural gas) through single wells. At the underwater central location (multi-well extraction), they are collected through a central manifold and transported to the surface platform for further processing using unified gas phase risers and liquid phase risers.

[0043] This invention is based on a typical process for collecting combustible ice. The liquid phase collection pipeline 2 is connected to the liquid phase pipeline hydrate control system. Under normal operating conditions, it is used to separate the free gas in the liquid phase produced from the wellhead, control the content of free gas in the liquid phase pipeline from the composition, significantly improve the flow stability of the downstream liquid phase pipeline system, and reduce the probability of secondary formation of natural gas hydrate slurry by liquid phase entrainment in the downstream liquid phase pipeline system.

[0044] The liquid phase pipeline hydrate control system includes a gas-liquid separator 11, a bypass shut-off valve 12, a separator gas phase regulating valve 13, and a liquid phase manifold exhaust regulating valve 14. The liquid phase collection pipeline 2 is connected to the gas-liquid separator 11. The liquid phase outlet of the gas-liquid separator 11 is connected to the central manifold 7 of the liquid phase pipeline. The gas phase outlet of the gas-liquid separator 11 is connected to the gas phase collection pipeline 1 through a pipeline equipped with the separator gas phase regulating valve 13, and then connected to the central manifold 5 of the gas phase pipeline. The separator gas phase regulating valve is used to overcome the operating pressure difference between the liquid phase pipeline and the gas phase pipeline, thereby controlling the vertical gas-liquid separator. The separated free natural gas is transported to the gas phase pipeline. The gas-liquid separator 11 is equipped with a bypass, and a bypass shut-off valve 12 is installed on the bypass. This bypass shut-off valve provides a backup bypass for the vertical gas-liquid separator, ensuring normal production even during maintenance or malfunctions. The inlet of the bypass is connected to the upstream pipeline of the gas-liquid separator 11, and its outlet is connected to the downstream pipeline of the gas-liquid separator 11. The liquid phase manifold exhaust regulating valve 14 connects the liquid phase pipeline central manifold 7 and the gas phase pipeline central manifold 5, and is used to discharge the gas accumulated in the liquid phase pipeline central manifold 7 into the gas phase pipeline system. Therefore, the liquid phase pipeline hydrate control system significantly reduces the risk of secondary formation and blockage of natural gas hydrates in the liquid phase pipeline during normal production from the perspective of component control.

[0045] Specifically, in the system shown, the vertical gas-liquid separator 11 is connected to the liquid phase collection pipeline 2, used to buffer and separate the liquid phase medium (containing free gas) transported by the liquid phase collection pipeline 2, separating free gas and water. Preferably, it is a vertical separator, and the liquid particle size in the gas phase is less than 200μm. The bypass shut-off valve 12 and its pipeline connect the liquid phase collection pipeline 2 and the liquid phase pump downstream pipeline 4, providing a bypass for the vertical gas-liquid separator 11, ensuring that production is not affected during maintenance of the vertical gas-liquid separator 11. The separator gas phase regulating valve 13 connects the gas phase outlet of the vertical gas-liquid separator 11 to the gas phase collection pipeline 1, preferably maintaining a pressure difference of approximately 1MPa, used for... The free gas separated by the vertical gas-liquid separator 11 is injected into the gas phase acquisition pipeline 1. Furthermore, when the pressure difference is large, a small dose of hydrate inhibitor can be injected in conjunction with the wellhead gas phase pipeline injection regulating valve 44. The liquid phase manifold exhaust regulating valve 14 is set at the liquid phase central manifold to periodically discharge the "dead gas" that may accumulate at the central manifold due to the manifold structure into the gas phase pipeline system, so as to avoid secondary freezing and blockage of natural gas hydrates in the "dead gas" of the liquid phase central manifold. Furthermore, a small dose of hydrate inhibitor can be injected in conjunction with the underwater central manifold injection regulating valve 42. The opening frequency of the liquid phase manifold exhaust regulating valve 14 is preferably 2 hours / time.

[0046] The gas phase acquisition pipeline 1 and the liquid phase acquisition pipeline 2 are respectively equipped with a shut-off valve group. The shut-off valve group includes a gas phase acquisition pipeline wellhead shut-off valve 21 and a gas phase acquisition pipeline end shut-off valve 24 installed on the gas phase acquisition pipeline 1, and a liquid phase acquisition pipeline wellhead shut-off valve 22, a liquid phase acquisition pipeline separation shut-off valve 23 and a liquid phase acquisition pipeline end shut-off valve 25 installed on the liquid phase acquisition pipeline 2. A gas phase riser start shut-off valve 26 is installed on the gas phase riser 6, and a liquid phase riser start shut-off valve 27 is installed on the liquid phase riser 8.

[0047] A pressure control system is installed between the gas phase acquisition pipeline 1 and the liquid phase acquisition pipeline 2. The pressure control system includes a pressure relief shut-off valve 28 at the beginning of the liquid phase acquisition pipeline, a pressure relief regulating valve 29 at the beginning of the liquid phase acquisition pipeline, a pressure relief shut-off valve 31 at the end of the liquid phase acquisition pipeline, and a pressure relief regulating valve 32 at the end of the liquid phase acquisition pipeline. The pipeline with the pressure relief shut-off valve 28 and the pressure relief regulating valve 29 at the beginning of the liquid phase acquisition pipeline is used to connect the upstream section of the liquid phase outlet of the gas-liquid separator 11 with the gas phase acquisition pipeline 1. The pipeline with the pressure relief shut-off valve 31 and the pressure relief regulating valve 32 at the end of the liquid phase acquisition pipeline is used to connect the downstream section of the liquid phase outlet of the gas-liquid separator 11 with the gas phase acquisition pipeline 1. In the off-line state, the gas phase acquisition pipeline 1 is used as a receiving pipeline for the pressure relief medium of the liquid phase system.

[0048] By coordinating the shut-off valve assembly with the pressure control system, it provides segmented shut-off functionality during planned shutdowns of single wells (including single-well acquisition systems) or multiple wells (including multi-well acquisition systems). This supports the zonal release of the liquid phase pipeline system, overcomes the pressure impact of hydrostatic pressure on the acquisition system, significantly reduces the pressure of the liquid phase acquisition pipeline system with lower fluctuations (relatively horizontal), and releases free gas that may accumulate at the end of the acquisition pipeline with higher elevation after shutdown, ensuring safety in preventing the secondary formation of hydrates in the liquid phase acquisition pipeline after shutdown.

[0049] Specifically, in the system shown, the gas phase acquisition pipeline wellhead shut-off valve 21, the liquid phase acquisition pipeline wellhead shut-off valve 22, the liquid phase acquisition pipeline separation shut-off valve 23, the gas phase acquisition pipeline end shut-off valve 24, the liquid phase acquisition pipeline end shut-off valve 25, the gas phase riser initiation shut-off valve 26, and the liquid phase riser initiation shut-off valve 27 are all electrically operated shut-off ball valves, which remain open during normal operation; the initiation liquid phase acquisition pipeline pressure relief shut-off valve 28, the initiation liquid phase acquisition pipeline pressure relief regulating valve 29, the end liquid phase acquisition pipeline pressure relief shut-off valve 31, and the end liquid phase acquisition pipeline pressure relief regulating valve 32 are all electrically controlled and remain closed during normal operation. The gas phase acquisition pipeline wellhead shut-off valve 21 and the gas phase acquisition pipeline end shut-off valve 24 isolate the gas phase acquisition pipeline 1 into a wellhead section and a subsea pipeline section when it is not in operation. Since the pressure and temperature of the gas phase pipeline are higher than the conditions for secondary hydrate formation, it can be used as a pressure relief medium receiving system for the liquid phase system when it is not in operation. The liquid phase acquisition pipeline wellhead shut-off valve 22, the liquid phase acquisition pipeline separation shut-off valve 23, and the liquid phase acquisition pipeline end shut-off valve 25 isolate the liquid phase acquisition pipeline 2 and the liquid phase pump post-pipeline 4 into a wellhead section, a separation section, and a subsea pipeline section when it is not in operation. Since the pressure and temperature of the liquid phase pipeline are higher than the conditions for secondary hydrate formation, it can be used as a pressure relief medium receiving system for the liquid phase system when it is not in operation. The pressure may exceed the conditions for secondary hydrate formation, therefore pressure relief is required (through a release medium). First, open the pressure relief shut-off valve 28 and the pressure relief regulating valve 29 of the starting liquid phase acquisition pipeline to release pressure on the lower-altitude section of the starting liquid phase acquisition pipeline (the release medium is water), and the pressure should be reduced to 5 MPa. At the same time, open the pressure relief shut-off valve 31 and the pressure relief regulating valve 32 of the end liquid phase acquisition pipeline to release any free gas that may exist in the higher-altitude section of the starting liquid phase acquisition pipeline into the gas phase pipeline, and the pressure should be reduced to 3 MPa.

[0050] The prevention and control system also includes a subsea hydrate inhibitor injection system. This system comprises a main inhibitor injection pipeline 41, a subsea central manifold injection regulating valve 42, a wellhead liquid phase pipeline injection regulating valve 43, and a wellhead gas phase pipeline injection regulating valve 44. It also includes branch pipelines at the subsea center supplying hydrate inhibitors to other wells and injection pipelines supplying hydrate inhibitors to the downhole interfaces of individual wells. This subsea hydrate inhibitor injection system provides necessary hydrate inhibitor injection channels for both subsea and downhole production systems. For the subsea production system, relying on the aforementioned liquid phase pipeline hydrate control system, only intermittent, small-dose inhibitor injections are provided, significantly reducing hydrate inhibitor consumption.

[0051] Specifically, in the system shown, the inhibitor injection main pipeline 41 connects the hydrate inhibitor injection unit of the surface treatment platform and the subsea system, providing a channel for hydrate inhibitor injection, which is delivered to the wellhead and various download points. The inhibitor injection main pipeline 41 is connected to the liquid phase pipeline central manifold 7 through the subsea central manifold injection regulating valve 42, wherein the subsea central manifold injection regulating valve 42 is an electric regulating valve, which works in conjunction with the liquid phase manifold exhaust regulating valve 14 to open. The inhibitor injection main pipeline 41 is connected to the liquid phase outlet of the gas-liquid separator 11 through the wellhead liquid phase pipeline injection regulating valve 43, and the inhibitor injection main pipeline 41 is connected to the gas phase outlet of the gas-liquid separator 11 through the wellhead gas phase pipeline injection regulating valve 44, wherein the wellhead liquid phase pipeline injection regulating valve 43 and the wellhead gas phase pipeline injection regulating valve 44 are both electric regulating valves, which are operated and opened in conjunction with the separator gas phase regulating valve 13 and the bypass shut-off valve 12 respectively when hydrate inhibitor injection is required.

[0052] The working principle of this invention is as follows:

[0053] (1) After the test production of combustible ice in the sea area adopts the depressurization method, the pressure is generally reduced to 3-4 MPa, and it is decomposed into natural gas (methane) and water. It is transported to the wellhead through two channels, and then transported to the central area through the subsea gas phase pipeline and liquid phase pipeline. After the media from multiple wells are combined, they are transported to the surface platform through risers. Since only a small separation facility is used for gas-water initial separation after decomposition in the well, the produced medium contains a part of free water in the gas phase pipeline and a part of free gas in the liquid phase pipeline. In the subsea system, natural gas can be lifted to the platform using downhole pressure energy. The maximum operating pressure is about 3 MPa. The marine ambient temperature is around 3℃ to 8℃, which basically avoids the hydrate formation range. Therefore, the risk of secondary hydrate formation in the gas phase pipeline (containing a small amount of free water) is low. However, for the liquid phase pipeline, since it needs to be pressurized to about 13-14 MPa downhole (or underwater) to provide lift pressure energy, its operating pressure is much higher than the conditions for secondary hydrate formation. Therefore, the liquid phase pipeline has reached the conditions for secondary hydrate formation, and the scale of hydrate formation depends on the content of free gas.

[0054] (2) In order to minimize the risk of secondary hydrate formation in the liquid phase pipeline, gas-liquid separation is performed at the wellhead. The separated free gas enters the gas phase pipeline by pressure difference, and the separated liquid phase (produced water) enters the liquid phase pipeline. This significantly reduces the gas content in the liquid phase pipeline and slows down the secondary hydrate formation from the perspective of component control.

[0055] (3) Since the underwater production area has a confluence function, some areas with relatively high altitudes are likely to form "dead gas" areas with relatively poor flow. Therefore, it is necessary to periodically carry out media displacement. To this end, a liquid phase manifold venting function is set up in the central manifold to periodically discharge the natural gas that still exists in the liquid phase pipeline and accumulates in the "dead gas" area, and introduce it into the gas phase pipeline to avoid freezing blockage caused by secondary hydrate formation in the liquid phase system.

[0056] (4) As the elevation of the subsea pipeline gradually increases from the wellhead, through the central collection system, and across the water surface, the higher pressures of the subsea production system all originate from hydrostatic pressure. After the pipeline is shut down, the gas phase entrained in the liquid phase will move and accumulate at higher elevations due to density differences, which will also cause a certain risk of secondary hydrate formation and blockage. To address this, a shutdown and pressure control system is installed for the single-well pipeline. After shutdown, a shut-off valve is used to isolate the pressure transmission, and the shut-down liquid phase pipeline is appropriately depressurized to avoid localized hydrate freezing and blockage caused by the accumulation of free gas. Due to the natural pressure difference, the liquid phase release medium directly enters the gas phase shut-down pipeline, without causing additional risk of secondary hydrate formation.

[0057] (5) To coordinate downhole and subsea production, a hydrate inhibitor injection system was established, which supplies hydrate inhibitors from the surface production system to the subsea system and provides hydrate inhibitor supply to the wellhead and some other sites for injection during continuous and periodic production. Because this subsea production system has conducted thorough identification and assessment of secondary hydrate formation and implemented essential control measures such as separation and depressurization for secondary hydrate formation, the need for continuous injection of hydrate inhibitors is minimal.

[0058] This invention also discloses a method for preventing secondary formation of hydrates in combustible ice collection pipelines, specifically including the following methods:

[0059] The production of combustible ice in the sea area adopts the downhole pressure extraction mode. The extracted gaseous medium (natural gas) is transported to the underwater central area through gas phase collection pipelines. After merging with other single-well gas phase collection pipelines, it is transported to the water surface through gas phase risers. The energy of the transport relies on the downhole pressure energy during extraction. The extracted liquid medium (decomposed water) is transported to the underwater central area through liquid phase collection pipelines, liquid phase booster pumps, and liquid phase pump post-pump pipelines. After merging with other single-well liquid phase collection pipelines, it is transported to the water surface through liquid phase risers. The energy of the transport relies on the downhole pressure energy during extraction, the pressure energy of the transport pump, etc.

[0060] The gas phase pipeline operates at a maximum pressure of approximately 3 MPa, with a transport temperature considered to be around 5°C (based on ambient temperature). This puts the gas phase pipeline within a safe zone for hydrate formation (i.e., the transport temperature at the corresponding pressure is higher than the natural gas hydrate formation temperature). The liquid phase pipeline, however, requires a higher lift pressure (approximately 14 MPa), resulting in a transport pressure significantly higher than the natural gas hydrate formation pressure at the corresponding ambient temperature. Therefore, the liquid phase acquisition pipeline connects the liquid phase (decomposed water, containing some free gas) to the gas-liquid separator. The separated free gas is then injected into the gas phase pipeline through the separator's gas phase regulating valve. The separated liquid phase contains virtually no free gas, significantly reducing the risk of secondary hydrate formation during subsequent transport. It should be noted that the liquid phase pipeline's transport pressure can be provided collaboratively by a downhole booster pump and a liquid phase booster pump 3. Preferably, the downhole booster pump provides a pressure 1 MPa higher than the pressure inside the gas phase acquisition pipeline when the liquid phase enters the gas-liquid separator, reducing the pressure difference before and after the separator's gas phase regulating valve.

[0061] During normal delivery, keep the main inhibitor injection pipeline open and periodically open the underwater central manifold injection regulating valve to inject small doses of hydrate inhibitor into the central manifold. Simultaneously, periodically open the liquid phase manifold vent regulating valve to discharge the gas accumulated in the central manifold of the liquid phase pipeline into the gas phase pipeline system. This means discharging any "dead gas" that may exist in the liquid phase manifold into the gas phase pipeline, preventing this "dead gas" from freezing and blocking with hydrates under high pressure and low temperature. The preferred cycle is 2 hours / time.

[0062] When the gas-liquid separator needs maintenance, open the bypass shut-off valve to prevent gas-liquid separation of the produced liquid phase medium in the short term. Simultaneously, open the wellhead liquid phase pipeline injection regulating valve to continuously inject hydrate inhibitors into the liquid phase pipeline. The amount of hydrate inhibitor injected should work in synergy with the amount of hydrate inhibitor injected during downhole decomposition to ensure that the free gas in the liquid phase pipeline does not freeze during transportation. At the same time, the liquid phase manifold exhaust regulating valve should still be opened periodically to prevent secondary hydrate formation in the "dead gas" area.

[0063] When a single-well system is scheduled to shut down, the wellhead shut-off valves for the gas phase acquisition pipeline, the wellhead shut-off valve for the liquid phase acquisition pipeline, the shut-off valve after the liquid phase acquisition pipeline is separated, the end shut-off valve for the gas phase acquisition pipeline, and the end shut-off valve for the liquid phase acquisition pipeline are closed, dividing the single-well acquisition system into the wellhead area and the acquisition pipeline area. At this time, the maximum pressure in the gas phase pipeline (the section upstream of the end shut-off valve of the gas phase acquisition pipeline) is approximately 3 MPa, and the maximum pressure in the liquid phase pipeline (the section upstream of the end shut-off valve of the liquid phase acquisition pipeline) is approximately 14 MPa.

[0064] Implement segmented regional pressure reduction. (1) Free gas that may exist in the liquid phase pipeline will accumulate at higher levels after the pipeline stops operating, so pressure reduction measures are taken. Open the pressure relief shut-off valve and the pressure relief regulating valve of the starting liquid phase acquisition pipeline to discharge high-pressure liquid phase pipeline medium into the gas phase pipeline to reduce the pressure of the pipeline after the liquid phase pump. Then, open the pressure relief shut-off valve and the pressure relief regulating valve of the ending liquid phase acquisition pipeline to discharge the free gas that may accumulate in the high-pressure liquid phase pipeline into the gas phase pipeline (since the pipeline elevation of the underwater production system gradually increases from the single well to the central area, the underwater central area is the area where free gas may accumulate), and gradually reduce the pressure of the pipeline after the liquid phase pump to about 3MPa. (2) Keep the gas phase regulating valve of the separator open and gradually reduce the pressure of the gas-liquid separator to about 3MPa.

[0065] For other operating conditions, the underwater hydrate inhibitor injection system connects the surface production system to the underwater system. The underwater hydrate inhibitor injection system systematically injects hydrate inhibitors into the central manifold of the liquid phase pipeline, the liquid phase outlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator, further ensuring that the risk of secondary hydrate production in the underwater production system is controllable.

[0066] This invention is not limited to the specific embodiments described above. It extends to any new features or combinations disclosed in this specification, as well as any new steps or combinations of any new methods or processes disclosed herein.

Claims

1. A system for preventing secondary hydrate formation in a combustible ice collection pipeline, comprising a gas phase collection pipeline (1) for transporting the produced gaseous medium and a liquid phase collection pipeline (2) for transporting the produced liquid medium, characterized in that: The liquid phase acquisition pipeline (2) is connected to the liquid phase pipeline hydrate control system and is used to separate the free gas of the liquid phase produced from the wellhead. The liquid phase pipeline hydrate control system includes a gas-liquid separator (11), a bypass shut-off valve (12), and a separator gas phase regulating valve (13). The liquid phase collection pipeline (2) is connected to the gas-liquid separator (11). The liquid phase outlet of the gas-liquid separator (11) is connected to the central manifold (7) of the liquid phase pipeline. The gas phase outlet of the gas-liquid separator (11) is connected to the gas phase collection pipeline (1) through a pipeline equipped with the separator gas phase regulating valve (13), and connected to the central manifold (5) of the gas phase pipeline. The gas-liquid separator (11) is equipped with a bypass, and a bypass shut-off valve (12) is installed on the bypass. The inlet end of the bypass is connected to the upstream pipeline of the gas-liquid separator (11), and its outlet end is connected to the downstream pipeline of the gas-liquid separator (11). A shut-off valve group is provided on the gas phase acquisition pipeline (1) and the liquid phase acquisition pipeline (2), respectively. The shut-off valve group includes a gas phase acquisition pipeline wellhead shut-off valve (21) and a gas phase acquisition pipeline end shut-off valve (24) provided on the gas phase acquisition pipeline (1), and a liquid phase acquisition pipeline wellhead shut-off valve (22), a liquid phase acquisition pipeline separation shut-off valve (23), and a liquid phase acquisition pipeline end shut-off valve (25) provided on the liquid phase acquisition pipeline (2). A pressure control system is provided between the gas phase acquisition pipeline (1) and the liquid phase acquisition pipeline (2). The pressure control system includes a pressure relief shut-off valve (28) for the starting liquid phase acquisition pipeline, a pressure relief regulating valve (29) for the starting liquid phase acquisition pipeline, a pressure relief shut-off valve (31) for the ending liquid phase acquisition pipeline, and a pressure relief regulating valve (32) for the ending liquid phase acquisition pipeline. A pipeline equipped with a pressure relief shut-off valve (28) for the initial liquid phase collection pipeline and a pressure relief regulating valve (29) for the initial liquid phase collection pipeline is used to connect the upstream section of the liquid phase outlet of the gas-liquid separator (11) with the gas phase collection pipeline (1); a pipeline equipped with a pressure relief shut-off valve (31) for the end liquid phase collection pipeline and a pressure relief regulating valve (32) for the end liquid phase collection pipeline is used to connect the downstream section of the liquid phase outlet of the gas-liquid separator (11) with the gas phase collection pipeline (1). In the off-line state, the gas phase collection pipeline (1) is used as a pressure relief medium receiving pipeline for the liquid phase system.

2. The system for preventing secondary hydrate formation in combustible ice collection pipelines according to claim 1, characterized in that: The liquid phase pipeline hydrate control system also includes a liquid phase manifold exhaust regulating valve (14), which connects the liquid phase pipeline central manifold (7) and the gas phase pipeline central manifold (5) to discharge the gas accumulated in the liquid phase pipeline central manifold (7) into the gas phase pipeline system.

3. The system for preventing secondary hydrate formation in combustible ice collection pipelines according to claim 1, characterized in that: The prevention and control system also includes an underwater hydrate inhibitor injection system, which includes an inhibitor injection main pipeline (41), an underwater central manifold injection regulating valve (42), a wellhead liquid phase pipeline injection regulating valve (43), and a wellhead gas phase pipeline injection regulating valve (44). The inhibitor injection main pipeline (41) connects the hydrate inhibitor injection unit of the surface treatment platform and the underwater system. The inhibitor injection main pipeline (41) is connected to the liquid phase pipeline central manifold (7) through the underwater central manifold injection regulating valve (42). The inhibitor injection main pipeline (41) is connected to the liquid phase outlet of the gas-liquid separator (11) through the wellhead liquid phase pipeline injection regulating valve (43). The inhibitor injection main pipeline (41) is connected to the gas phase outlet of the gas-liquid separator (11) through the wellhead gas phase pipeline injection regulating valve (44).

4. A method for preventing secondary hydrate formation in a combustible ice collection pipeline using the combustible ice collection pipeline hydrate secondary formation prevention system according to any one of claims 1 to 3, characterized in that: Specifically, the methods include the following: Methane hydrate is produced using a downhole pressure extraction method. The extracted gaseous medium is transported to the underwater central area through a gas phase acquisition pipeline. After merging with other single-well gas phase acquisition pipelines, it is transported to the water surface through a gas phase riser. The extracted liquid medium is transported to the underwater central area through a liquid phase acquisition pipeline, a liquid phase booster pump, and a liquid phase pump post-pump pipeline. After merging with other single-well liquid phase acquisition pipelines, it is transported to the water surface through a liquid phase riser. The liquid phase acquisition pipeline connects the liquid phase to a gas-liquid separator, and the separated free gas is injected into the gas phase pipeline through the separator's gas phase regulating valve. During normal delivery, keep the main inhibitor injection pipeline open and periodically open the underwater central manifold injection regulating valve to inject small doses of hydrate inhibitor into the central manifold. Simultaneously, periodically open the liquid phase manifold exhaust regulating valve to discharge the gas accumulated in the central manifold of the liquid phase pipeline into the gas phase pipeline system. When the gas-liquid separator needs maintenance, open the bypass shut-off valve so that the liquid phase produced medium will not undergo gas-liquid separation for a short period of time. Simultaneously, open the wellhead liquid phase pipeline injection regulating valve to continuously inject hydrate inhibitors into the liquid phase pipeline. The amount of hydrate inhibitor injected should work in synergy with the amount of hydrate inhibitor injected during downhole decomposition to ensure that the free gas in the liquid phase pipeline does not freeze during transportation. At the same time, the liquid phase manifold exhaust regulating valve should still be opened periodically.

5. The method for preventing secondary hydrate formation in combustible ice collection pipelines according to claim 4, characterized in that: When the downhole booster pump supplies liquid phase into the gas-liquid separator, the pressure is 1 MPa higher than the pressure inside the gas phase collection pipeline.

6. The method for preventing secondary hydrate formation in combustible ice collection pipelines according to claim 4, characterized in that: When a single-well system is shut down according to plan, the wellhead shut-off valves of the gas phase acquisition pipeline, the wellhead shut-off valve of the liquid phase acquisition pipeline, the shut-off valve after the separation of the liquid phase acquisition pipeline, the end shut-off valve of the gas phase acquisition pipeline, and the end shut-off valve of the liquid phase acquisition pipeline are closed, dividing the single-well acquisition system into the wellhead area and the acquisition pipeline area. Implement segmented regional pressure reduction by opening the pressure relief shut-off valve and the pressure relief regulating valve of the initial liquid phase acquisition pipeline to discharge high-pressure liquid phase medium into the gas phase pipeline, thereby reducing the pressure in the pipeline after the liquid phase pump. Subsequently, open the pressure relief shut-off valve and the pressure relief regulating valve of the final liquid phase acquisition pipeline to discharge any possible accumulated free gas in the high-pressure liquid phase pipeline into the gas phase pipeline, gradually reducing the pressure in the pipeline after the liquid phase pump. Keep the separator gas phase regulating valve open to gradually reduce the pressure of the gas-liquid separator.

7. The method for preventing secondary hydrate formation in combustible ice collection pipelines according to claim 4, characterized in that: The underwater hydrate inhibitor injection system connects the surface production system to the underwater system. The underwater hydrate inhibitor injection system injects hydrate inhibitors into the central manifold of the liquid phase pipeline, the liquid phase outlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator in a planned manner.

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