Two-stage multiphase separation reinjection system for subsea oil and gas wells

Abstract

The application provides a two-stage multiphase separation reinjection system for an underwater oil and gas well, which comprises a first-stage pre-separation device, a second-stage multiphase deposition separation device and a reinjection assembly. The first-stage pre-separation device comprises a gas-liquid preliminary separation module and an oil-water preliminary separation module, which can be selected and used according to the underwater space and the flow characteristics, so as to realize the preliminary separation and impact buffering of the flow. The second-stage multiphase deposition separation device is provided with an oil-water separation chamber and an oil-gas chamber, and is communicated through an overflow device, and respectively receives the oil-water phase and the oil-gas after the first-stage separation, and carries out deep separation. The reinjection assembly comprises a sand removal device and a water injection assembly, which is used for discharging the separated water to the sea or reinjecting the formation after sand removal. The system can adapt to complex working conditions, reduce the volume and wall thickness of the second-stage device, save the construction cost; at the same time, the formation pressure is maintained and the recovery rate is improved through reinjection, and the number of submarine pipelines is reduced by adopting oil-gas mixed transportation, so that the underwater production is realized in an economical and efficient manner.

Description

Technical Field

[0001] This invention relates to the field of subsea oil and gas extraction technology, and in particular to a two-stage multiphase separation reinjection system for subsea oil and gas wells. Background Technology

[0002] In the field of oil and gas development, the mixture extracted from oil and gas fields always contains various oil phases, natural gas, water, and solid particles. It is necessary to separate substances of different phases to purify petroleum products, natural gas, and other products, and to remove impurities from the water for discharge or reinjection. Separation tanks are frequently used in industrial mixture separation, utilizing the principle of gravity separation to achieve three-phase separation. Typically, one end of the tank has an inlet pipe, while other locations have outlet pipes for oil, gas, and water. An overflow plate is installed inside the tank. Taking advantage of the immiscibility of oil and water and the lower density of oil than water, the oil and water are drawn into different spaces.

[0003] The ocean contains abundant oil and gas resources, but the exploitation of these resources faces limitations due to deep-water development technologies. Subsea production and separation technologies are significantly more difficult than terrestrial resource development, both in terms of technology and cost. Separation tank technology, typically used for separating terrestrial products, is unsuitable for marine oil, gas, and water separation due to its large size, complex structure, and difficult installation. Furthermore, the high-pressure environment underwater places high demands on separation equipment; while ordinary separation tanks are large, high pressure necessitates significantly increased wall thickness, resulting in excessively large and heavy equipment that hinders installation. In addition, while typical oil, gas, and water separation tanks separate the three phases and transport them separately, for deep-water development, the more pipelines required, the higher the cost of subsea extraction. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention provides a two-stage multiphase separation and reinjection system for subsea oil and gas wells, aiming to address the problem that separation tanks used on land are unsuitable for subsea production in related technologies.

[0005] This invention provides a two-stage multiphase separation reinjection system for subsea oil and gas wells, comprising:

[0006] A primary pre-separation device includes a gas-liquid preliminary separation module and an oil-water preliminary separation module, wherein the inlet of at least one of the gas-liquid preliminary separation module and the oil-water preliminary separation module is connected to a subsea wellhead. A two-stage multiphase deposition and separation device includes an oil-water phase inlet, a light phase inlet, a liquid phase outlet, an oil-gas outlet, an oil-water separation chamber, and an oil-gas chamber. The oil-water phase inlet and the liquid phase outlet are connected to the oil-water separation chamber, and the light phase inlet and the oil-gas outlet are connected to the oil-gas chamber. The oil-water separation chamber is connected to the oil-gas chamber via an overflow device. The oil-gas outlet is used to connect to an oil-gas mixing pipeline. The oil and gas separated by the first-stage pre-separation device enter the oil-gas chamber through the light phase inlet, and the oil-water phase separated by the first-stage pre-separation device enters the oil-water separation chamber through the oil-water phase inlet. The reinjection assembly includes a desanding device and a water injection assembly. The water injection assembly is connected to the liquid phase outlet through the desanding device. The water injection assembly is used to discharge the desanded water into the sea or inject it into the mined formation.

[0007] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, a demulsifier injection device is also connected to the pipeline connecting the first-stage pre-separation device and the subsea wellhead.

[0008] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, a hydrate inhibitor injection device is also connected to the pipeline connecting the first-stage pre-separation device and the subsea wellhead.

[0009] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, both the gas-liquid preliminary separation module and the oil-water preliminary separation module are pipeline-type separation devices, and the length-to-diameter ratio of the pipeline-type separation device is 10~30.

[0010] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, the gas-liquid preliminary separation module includes at least a T-type pipeline separator. The first mixture inlet of the T-type pipeline separator is used to connect to the subsea wellhead, the gas phase outlet of the T-type pipeline separator is connected to the light phase inlet, and the oil-water mixture outlet of the T-type pipeline separator is connected to the oil-containing water phase inlet.

[0011] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, the oil-water preliminary separation module includes at least a pipeline cyclone separator, the second mixture inlet of the pipeline cyclone separator is used to connect to the subsea wellhead, the oil and gas phase outlet of the pipeline cyclone separator is connected to the light phase inlet, and the oil-water phase outlet of the pipeline cyclone separator is connected to the oil-water phase inlet.

[0012] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, the secondary multiphase sedimentation separation device is a horizontal settling separator with a length-to-diameter ratio of 4 to 8.

[0013] According to the two-stage multiphase separation reinjection system for subsea oil and gas wells provided by the present invention, the water injection component includes a flow control component and a water injection tree, the flow control component is connected between the desanding device and the water injection tree, and the water injection tree is used to communicate with the extracted formation.

[0014] According to the two-stage multiphase separation reinjection system for subsea oil and gas wells provided by the present invention, the flow control component includes a level sensor and a first and a second water injection pipeline connected in parallel. The first water injection pipeline is equipped with a first electric valve, and the second water injection pipeline is connected in series with a second electric valve and a booster pump. The level sensor is disposed in the oil-water separation chamber. The second electric valve and the booster pump are both communicatively connected to the level sensor. When the level value of the level sensor is greater than a preset value, the level sensor controls the second electric valve to open and simultaneously controls the booster pump to start.

[0015] According to the two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by the present invention, the interior of the secondary multiphase sedimentation separation device is provided with a sand flushing pipe, one end of which is connected to the oil-bearing water phase inlet, and the other end extends to the bottom of the secondary multiphase sedimentation separation device.

[0016] The present invention has the following advantages due to the adoption of the above technical solutions: This invention provides a two-stage multiphase separation and reinjection system for subsea oil and gas wells. It features a selectable first-stage pre-separation device, allowing for dynamic customization of the first-stage separation scheme based on subsea space and inflow characteristics. This significantly improves adaptability to complex operating conditions, tunes the inflow pattern, and creates ideal hydrodynamic conditions for the subsequent pre-separation phase, thus accelerating pre-separation. The first-stage pre-separation device promotes phase accumulation, improves separation efficiency, and maintains system stability. Furthermore, the efficient pre-separation separates some media in advance, significantly reducing the processing capacity and load of subsequent processing equipment. This reduces the complexity and size of the second-stage multiphase sedimentation and separation device, allowing for relatively smaller wall thickness under high-pressure subsea conditions, thereby lowering construction costs. By incorporating reinjection components, water transportation costs are reduced, and reinjection replenishes formation energy, improving oil and gas recovery and maximizing efficiency. The mixed oil and gas transportation scheme, compared to separate oil and gas transportation schemes, reduces the construction of one transportation pipeline, thus reducing production costs. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of a two-stage multiphase separation and reinjection system for subsea oil and gas wells provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of a T-type pipe separator provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of a pipe cyclone separator provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the internal structure of a horizontal oil-water separator provided in an embodiment of the present invention.

[0019] Figure label: 100: Primary pre-separation unit; 110: T-type pipeline separator; 111: First mixture inlet; 112: Gas phase outlet; 113: Oil-water mixture outlet; 120: Pipeline cyclone separator; 121: Second mixture inlet; 122: Oil-gas phase outlet; 123: Oil-containing water phase outlet; 200: Secondary multiphase deposition separation unit; 210: Oil-containing water phase inlet; 220: Light phase inlet; 230: Liquid phase outlet; 240: Oil-gas outlet; 250: Oil-water separation chamber; 260: Oil-gas chamber; 270: Overflow device; 300: Oil-gas mixed transport pipeline; 410: Demulsifier injection device; 420: Hydrate inhibitor injection device; 510: First electric valve; 521: Second electric valve; 522: Booster pump; 530: Water tree; 540: Sand removal device. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are 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, they should not be construed as limitations on this invention.

[0022] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0023] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0024] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0025] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0026] This invention provides a two-stage multiphase separation and reinjection system for subsea oil and gas wells, comprising a primary pre-separation device, a secondary multiphase sedimentation and separation device, and reinjection components. The primary pre-separation device includes a gas-liquid preliminary separation module and an oil-water preliminary separation module, with the inlet of at least one of these modules connected to the subsea wellhead. The secondary multiphase sedimentation and separation device includes an oil-water phase inlet, a light phase inlet, a liquid phase outlet, an oil and gas outlet, an oil-water separation chamber, and an oil and gas chamber. The oil-water phase inlet and liquid phase outlet are connected to the oil-water separation chamber, and the light phase inlet and oil and gas outlet are connected to the oil and gas chamber. The oil-water separation chamber is connected to the oil and gas chamber via an overflow device, and the oil and gas outlet is used to connect to an oil and gas mixed transport pipeline. The oil and gas separated by the primary pre-separation device enters the oil and gas chamber through the light phase inlet, and the oil-water phase separated by the primary pre-separation device enters the oil-water separation chamber through the oil-water phase inlet. The oil and gas outlet is used to connect to the oil and gas mixed transport pipeline. The reinjection assembly includes a desandering device and a water injection assembly. The water injection assembly is connected to the liquid phase outlet through the desandering device and is used to discharge the desandered water into the sea or inject it into the extracted formation. The two-stage multiphase separation reinjection system for subsea oil and gas wells provided by this invention features a selectable first-stage pre-separation device. This device allows for dynamic customization of the first-stage separation scheme based on the subsea space and inflow characteristics, significantly improving adaptability to complex operating conditions and effectively buffering inflow impacts to ensure the stability of the downstream system. Furthermore, connecting the first-stage pre-separation device to the front end of the second-stage multiphase sedimentation separation device reduces the complexity and volume of the second-stage device, allowing for a relatively smaller wall thickness under high-pressure underwater conditions, thus reducing construction costs. By incorporating the reinjection assembly, water transportation costs are reduced, and by replenishing formation energy through reinjection, oil and gas recovery is improved, maximizing efficiency. The oil and gas co-transport scheme, compared to separate oil and gas transportation schemes, reduces the construction of one transportation pipeline, thus reducing production costs.

[0027] The following is combined with Figures 1-4 The present invention describes a two-stage multiphase separation reinjection system for subsea oil and gas wells.

[0028] An embodiment of the present invention provides a two-stage multiphase separation and reinjection system for subsea oil and gas wells, including a primary pre-separation device 100, a secondary multiphase deposition and separation device 200, and a reinjection assembly.

[0029] The primary pre-separation device 100 includes a gas-liquid preliminary separation module and an oil-water preliminary separation module, and the inlet of at least one of the gas-liquid preliminary separation module and the oil-water preliminary separation module is connected to the subsea wellhead.

[0030] For the gas-liquid preliminary separation module and the oil-water preliminary separation module, one can be selected to connect to the underwater wellhead according to the underwater operating space, the characteristics of the incoming medium and the processing index requirements, so as to perform customized pre-separation of the incoming flow.

[0031] For example, when the incoming flow is high in gas content, a gas-liquid preliminary separation module can be connected to the subsea wellhead for preliminary gas-liquid separation. When the incoming flow is low in gas content, an oil-water preliminary separation module can be connected to the subsea wellhead for preliminary oil-water separation.

[0032] The two-stage multiphase deposition separation device 200 includes an oil-water phase inlet 210, a light phase inlet 220, a liquid phase outlet 230, an oil-gas outlet 240, an oil-water separation chamber 250, and an oil-gas chamber 260. The oil-water separation chamber 250 and the oil-gas chamber 260 are distributed along the axial direction of the two-stage multiphase separation device, and the oil-water separation chamber 250 is connected to the oil-gas chamber 260 via an overflow device 270. The oil-water phase inlet 210 is located at one end of the two-stage multiphase deposition separation device 200 where the oil-water separation chamber 250 is located and is connected to the oil-water separation chamber 250. The liquid phase outlet 230 is located at the bottom of the oil-water separation chamber 250 and is connected to the interior of the oil-water separation chamber 250. The light phase inlet 220 is located at the top of the oil-gas chamber 260 and is connected to the interior of the oil-gas chamber 260. The oil-gas outlet 240 is connected to the oil-gas chamber 260.

[0033] During production, the oil phase and gas phase separated by the primary pre-separation device 100 enter the oil and gas chamber 260 of the secondary multiphase deposition separation device 200 through the light phase inlet 220, and the oil-water phase separated by the primary pre-separation device 100 enters the oil-water separation chamber 250 through the oil-water phase inlet 210.

[0034] Because water is denser than oil, the water in the oil-water separation chamber 250 is located in the lower layer, and the oil is located in the upper layer. During the flow to the oil-gas chamber 260, the overflow device 270 blocks the water in the oil-water separation chamber 250, and the oil passes through the overflow device 270 and enters the oil-gas chamber 260.

[0035] Since the oil-water phase also contains some solid particles such as sand, the reinjection assembly includes a desanding device 540 and a water injection assembly. The water separated in the oil-water separation chamber 250 is discharged through the liquid phase outlet 230, and then first desanded by the desanding device 540, and then reinjected into the mined formation or seawater by the water injection assembly.

[0036] The oil and natural gas located in the oil and gas chamber 260 are connected to the oil and gas mixed transport pipeline 300 through the oil and gas outlet 240.

[0037] The two-stage multiphase separation reinjection system for subsea oil and gas wells provided by this invention allows for flexible switching and combination of modular pre-separation structures. It can dynamically customize the first-stage separation scheme according to the underwater space and incoming flow characteristics, significantly improving the adaptability to complex working conditions and effectively buffering the impact of incoming flow to ensure the stability of the downstream system.

[0038] The two-stage multiphase separation and reinjection system for subsea oil and gas wells provided by this invention uses pipeline back pressure as the core of its control system, eliminating the need for traditional complex control components. By automatically adjusting the valves at the front end of the oil and gas mixed-transport pipeline through the pipeline pressure downstream, the pipeline outlet pressure is stabilized, allowing for a stable and continuous output of the gas phase under stable back pressure. Through back pressure adaptive control and a combined level-pressure control, the system can withstand flow fluctuations, ensuring stable operation while reducing control costs and design complexity.

[0039] The two-stage multiphase separation reinjection system for subsea oil and gas wells provided by this invention reduces the cost of water transportation and, at the same time, improves oil and gas recovery by reinjecting formation energy, thereby maximizing benefits.

[0040] The two-stage multiphase separation reinjection system for subsea oil and gas wells provided by this invention adopts a mixed oil and gas transportation scheme, which reduces the construction of one transportation pipeline and reduces production costs compared to the scheme of separate oil and gas transportation.

[0041] In some embodiments, a demulsifier injection device 410 and a hydrate inhibitor injection device 420 are also connected to the pipeline connecting the primary pre-separation device 100 to the subsea wellhead. The integrated process of pre-demulsification and hydrate inhibition broadens the system's processing range for complex fluids such as highly emulsified fluids and fluids prone to hydrate formation, effectively ensuring the continuity of the separation process under extreme composition conditions.

[0042] In some embodiments, both the gas-liquid preliminary separation module and the oil-water preliminary separation module are pipeline-type separation devices, and the length-to-diameter ratio of the pipeline-type separation device can be 10 to 30.

[0043] A pipe structure with an aspect ratio of 10 to 30 helps to allow sufficient settling distance at the front of the pipe-type separation device. The larger the aspect ratio, the better the fluid settling and separation effect, or the more space the cyclone structure has to develop a swirling flow field for sufficient separation. A larger aspect ratio also results in a smaller diameter for the same volume; under the same depth and high pressure, a smaller diameter corresponds to a smaller wall thickness, which helps to reduce the relative weight of the device. At the same time, sufficient flow area must be maintained, so the aspect ratio cannot be increased indefinitely. In pipe-type separation devices, an aspect ratio of 10 to 30 typically provides sufficient space for flow development, promoting separation and improving separation efficiency.

[0044] For example, the gas-liquid preliminary separation module can be a T-type pipeline separator 110, which includes a first mixture inlet 111, a gas phase outlet 112, and an oil-water mixture outlet 113. The first mixture inlet 111 is used to connect to the subsea wellhead, the gas phase outlet 112 is connected to the light phase inlet 220 so that the separated gas enters the oil-gas chamber 260 through the light phase inlet 220, and the oil-water mixture outlet 113 is connected to the oil-water phase inlet 210 to inject the separated gas oil-water mixture into the oil-water separation chamber 250 for oil-water separation.

[0045] The gas-liquid preliminary separation module also includes a pipeline cyclone separator 120, which includes a second mixture inlet 121, an oil-gas phase outlet 122, and an oil-water phase outlet 123. The second mixture inlet 121 is used to connect to the subsea wellhead, the oil-gas phase outlet 122 transports the separated oil phase containing a small amount of gas to the oil-gas chamber 260, and the oil-water phase outlet 123 transports the separated oil-water phase to the oil-water separation chamber 250 for separation.

[0046] Furthermore, the aforementioned two-stage multiphase sedimentation separation device 200 can be a horizontal settling tank with a length-to-diameter ratio of 4 to 8. Because of the inclusion of a primary pre-separation device 100, the volume of the horizontal settling tank can be appropriately reduced. This reduction in volume decreases the pressure on the tank, thereby allowing for thinner walls. Reducing the volume is equivalent to reducing the product of the length and the square of the diameter of the horizontal settling tank. At the same length-to-diameter ratio, the diameter needs to be reduced. Under the same operating pressure, a smaller diameter results in a thinner wall, thus significantly reducing the equipment weight.

[0047] For conventional land-based separators, considering the relationship between throughput and equipment weight, an aspect ratio (L / D ratio) of 2 to 6 is generally more reasonable. However, due to the influence of external high pressure underwater, the container walls are relatively thicker, thus requiring a certain increase in L / D ratio to maintain a new balance between material usage and throughput for the high-pressure equipment. Based on empirical calculations, while ensuring separation throughput, a L / D ratio of 4 to 8 is more reasonable for horizontal settling separators.

[0048] In some embodiments, the water injection assembly includes a flow control component and a water injection tree 530. The flow control component is connected between the desanding device 540 and the water injection tree 530. The water injection tree 530 is used to reinject the desanded water back into the mined formation. The flow control component is mainly used to control the liquid level balance in the horizontal settling tank, control the oil-water separation boundary, and ensure the normal operation of the horizontal settling tank.

[0049] Specifically, the flow control device may include a liquid level sensor, a first water injection pipeline and a second water injection pipeline connected in parallel, a first electric valve 510 installed on the first water injection pipeline, and a second electric valve 521 and a booster pump 522 connected in series on the second water injection pipeline. The liquid level sensor is installed inside the oil-water separation chamber 250 to detect the liquid level inside the oil-water separation chamber 250 and is linked with the second electric valve 521 and the booster pump 522.

[0050] When the level sensor detects that the liquid level in the oil-water separation chamber 250 is higher than the preset value, the level sensor controls the second electric valve 521 to open and simultaneously controls the booster pump to start, increasing the discharge capacity and causing the liquid level in the oil-water separation chamber 250 to drop. When the liquid level drops to within the normal operating range of the horizontal oil-water separator, the booster pump stops working.

[0051] In some embodiments, the secondary multiphase deposition separation device 200 is internally provided with a sand flushing channel. One end of the sand flushing channel is connected to the oil-water phase inlet 210, and the other end extends to the bottom of the secondary multiphase deposition separation device 200. Utilizing the pressure difference between the oil-water phase inlet 210 and the liquid phase outlet 230, the oil-water phase passing through the sand flushing channel continuously impacts the sediment at the bottom of the secondary multiphase deposition separation device 200, suspending it and then carrying it into the sand removal device 540 for removal. This fundamentally avoids clogging problems and significantly extends the service life of the system.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A two-stage multiphase separation reinjection system for subsea oil and gas wells, characterized in that, include: The primary pre-separation device (100) includes a gas-liquid preliminary separation module and an oil-water preliminary separation module, wherein the inlet of at least one of the gas-liquid preliminary separation module and the oil-water preliminary separation module is connected to a subsea wellhead. A two-stage multiphase deposition separation device (200) includes an oil-water phase inlet (210), a light phase inlet (220), a liquid phase outlet (230), an oil and gas outlet (240), an oil-water separation chamber (250), and an oil and gas chamber (260). The oil-water phase inlet (210) and the liquid phase outlet (230) are connected to the oil-water separation chamber (250), and the light phase inlet (220) and the oil and gas outlet (240) are connected to the oil and gas chamber (260). The water separation chamber (250) is connected to the oil and gas chamber (260) through the overflow device (270), and the oil and gas outlet (240) is used to connect to the oil and gas mixing pipeline (300). The oil and gas separated by the first-stage pre-separation device (100) enters the oil and gas chamber (260) through the light phase inlet (220), and the oil-containing water phase separated by the first-stage pre-separation device (100) enters the oil and water separation chamber (250) through the oil-containing water phase inlet (210). The reinjection assembly includes a desanding device (540) and a water injection assembly. The water injection assembly is connected to the liquid phase outlet (230) through the desanding device (540). The water injection assembly is used to discharge the desanded water into the sea or inject it into the mined formation.

2. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1, characterized in that, A demulsifier injection device (410) is also connected to the pipeline connecting the primary pre-separation device (100) and the underwater wellhead.

3. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1 or 2, characterized in that, A hydrate inhibitor injection device (420) is also connected to the pipeline connecting the primary pre-separation device (100) and the underwater wellhead.

4. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1, characterized in that, Both the gas-liquid preliminary separation module and the oil-water preliminary separation module are pipeline-type separation devices, and the length-to-diameter ratio of the pipeline-type separation device is 10~30.

5. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 4, characterized in that, The gas-liquid preliminary separation module includes at least a T-type pipe separator (110). The first mixture inlet of the T-type pipe separator (110) is used to connect to the subsea wellhead. The gas phase outlet of the T-type pipe separator (110) is connected to the light phase inlet (220). The oil-water mixture outlet of the T-type pipe separator (110) is connected to the oil-containing water phase inlet (210).

6. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 4, characterized in that, The oil-water preliminary separation module includes at least a pipeline cyclone separator (120), the second mixture inlet (121) of the pipeline cyclone separator (120) is used to connect to the subsea wellhead, the oil-gas phase outlet (122) of the pipeline cyclone separator (120) is connected to the light phase inlet (220), and the oil-water phase outlet (123) of the pipeline cyclone separator (120) is connected to the oil-water phase inlet (210).

7. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1, characterized in that, The secondary multiphase sedimentation separation device (200) is a horizontal sedimentation separation tank with a length-to-diameter ratio of 4 to 8.

8. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1, characterized in that, The water injection assembly includes a flow control assembly and a water injection tree (530). The flow control assembly is connected between the desanding device (540) and the water injection tree (530). The water injection tree (530) is used to communicate with the mined strata.

9. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 8, characterized in that, The flow control component includes a level sensor and a first water injection pipeline and a second water injection pipeline connected in parallel. The first water injection pipeline is equipped with a first electric valve (510), and the second water injection pipeline is connected in series with a second electric valve (521) and a booster pump (522). The level sensor is located in the oil-water separation chamber (250). The second electric valve (521) and the booster pump (522) are both communicatively connected to the level sensor. When the level value of the level sensor is greater than a preset value, the level sensor controls the second electric valve (521) to open and simultaneously controls the booster pump (522) to start.

10. The two-stage multiphase separation and reinjection system for subsea oil and gas wells according to claim 1, characterized in that, The secondary multiphase deposition separation device (200) is equipped with a sand flushing pipe inside. One end of the sand flushing pipe is connected to the oil-containing water phase inlet (210), and the other end extends to the bottom of the secondary multiphase deposition separation device (200).

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