An undersea wellhead hydrate inhibitor injection system and method

Abstract

The present application relates to a kind of hydrate inhibitor injection system and method of underwater wellhead, the system includes ethylene glycol supply system, first injection line and second injection line, ethylene glycol in the ethylene glycol supply system is injected into the first injection line and second injection line respectively by ethylene glycol metering valve, the first injection line is equipped with intercommunication valve, the second injection line is equipped with ethylene glycol injection valve, ethylene glycol in the first injection line is injected into the upstream of oil nozzle by the intercommunication valve, ethylene glycol in the second injection line is injected into the downstream of oil nozzle by ethylene glycol injection valve, and the ethylene glycol is diluted to 80%~90% by mass fraction using water.The present application reduces the design flow of methanol injection by controlling reasonable medicament concentration and injection operation procedure, to reduce the umbilical cross-sectional area, reduce the project investment, while reducing the storage amount of toxic and harmful medium methanol on the platform.

Description

Technical Field

[0001] This invention relates to a subsea wellhead hydrate inhibitor injection system and method, belonging to the field of deepwater oil and gas field development flow safety assurance technology. Background Technology

[0002] Before well opening for deep-sea gas field development, the fluid pressure upstream of the subsea production tree nozzle is 30~60MPa (well shut-in pressure) and the temperature is 2~3℃ (ambient temperature). The pressure downstream of the nozzle is 1~10MPaA (subsea pipeline pressure) and the temperature is 2~3℃ (ambient temperature).

[0003] During well start-up operations, the significant pressure difference between the upstream and downstream of the nozzle causes throttling at the nozzle. This leads to a rapid temperature drop in the natural gas due to the Joule-Thomson effect, making it highly susceptible to hydrate formation between the formation water and natural gas downstream of the nozzle. During normal operation, continuous heat exchange between the fluid within the pipeline and the ambient temperature will lower the temperature to ambient levels, creating conditions for hydrate formation downstream. Once formed, the hydrate will accumulate, clogging the pipeline and causing serious production accidents.

[0004] Currently, hydrate control at subsea wellheads involves injecting methanol upstream of the nozzle during well start-up and continuously injecting ethylene glycol downstream of the nozzle during normal production, both supplied by the supporting platform. Ethylene glycol is typically supplied and transported via subsea pipelines, while methanol is integrated into the umbilical cable via multiple stainless steel pipes. This results in an excessively large umbilical cable cross-section, leading to high manufacturing and transportation costs, as well as increased installation expenses. Summary of the Invention

[0005] To address the aforementioned problems, one objective of this invention is to provide a subsea wellhead hydrate inhibitor injection system. By controlling the appropriate reagent concentration and injection procedure, the design flow rate of methanol injection during well start-up can be reduced, thereby decreasing the umbilical cable cross-sectional area, lowering engineering investment, and simultaneously reducing the amount of toxic and harmful methanol stored on the support platform. Another objective of this invention is to provide a subsea wellhead hydrate inhibitor injection method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a subsea wellhead hydrate inhibitor injection system, including an ethylene glycol supply system and a first injection pipeline and a second injection pipeline connected to the ethylene glycol injection system. Ethylene glycol in the ethylene glycol supply system is injected into the first injection pipeline and the second injection pipeline respectively via an ethylene glycol metering valve. A connecting valve is provided on the first injection pipeline, and an ethylene glycol injection valve is provided on the second injection pipeline. Ethylene glycol in the first injection pipeline is injected upstream of the nozzle via the connecting valve, and ethylene glycol in the second injection pipeline is injected downstream of the nozzle via the ethylene glycol injection valve. The ethylene glycol is diluted with water to a mass fraction of 80% to 90%.

[0007] Furthermore, it also includes a methanol supply system, which is connected to the first injection pipeline via a third injection pipeline. The third injection pipeline is equipped with a methanol injection valve. When starting the well, methanol is mixed with ethylene glycol in the first pipeline through the methanol injection valve and then injected upstream of the nozzle. During normal operation and downstream unblocking, methanol is injected downstream of the nozzle after passing through the methanol injection valve, the connecting valve and the ethylene glycol injection valve in sequence.

[0008] Furthermore, the nozzle is located on the main pipeline, which is also equipped with a main valve and a wing valve. The main valve and the wing valve are located upstream of the nozzle, and are respectively located on both sides of the first injection pipeline.

[0009] Furthermore, a downhole safety valve is provided at one end of the main pipeline near the reservoir. A first pressure sensor and a second pressure sensor are respectively provided upstream and downstream of the downhole safety valve. The first pressure sensor and the second pressure sensor are used to detect the pressure upstream and downstream of the downhole safety valve, respectively.

[0010] Furthermore, the third injection pipeline is also equipped with a methanol metering valve, which is located upstream of the methanol injection valve.

[0011] Furthermore, the first injection line and the second injection line are respectively provided with a first check valve and a second check valve. The ethylene glycol is injected from the first injection line through the first check valve to the upstream of the nozzle, and the ethylene glycol is injected from the second injection line through the second check valve to the downstream of the nozzle.

[0012] On the other hand, the present invention also provides a method for injecting hydrate inhibitors at the subsea wellhead, based on the aforementioned subsea wellhead hydrate inhibitor injection system, characterized by comprising the following steps: When starting the well, open the connecting valve and close the ethylene glycol injection valve. Ethylene glycol is injected upstream of the nozzle through the connecting valve. After the temperature gradually rises to the hydrate control temperature, open the ethylene glycol injection valve and close the connecting valve. Ethylene glycol is then injected downstream of the nozzle through the second check valve.

[0013] Furthermore, during well start-up, the connecting valve and methanol injection valve are opened, and the ethylene glycol injection valve is closed. Ethylene glycol is mixed with methanol through the connecting valve and injected upstream of the nozzle.

[0014] Furthermore, this also includes unblocking downstream pipelines during normal operation. Specifically, when a blockage occurs in the pipeline downstream of the wellhead after well start-up, the methanol injection valve and the connecting valve are opened, and the methanol and ethylene glycol are mixed and flowed to the hydrate blockage.

[0015] Furthermore, the well opening process also includes the following steps: before injecting ethylene glycol, open the methanol injection valve and inject it into the upper part of the downhole safety valve through the first check valve. When the upstream and downstream pressures of the downhole safety valve are the same, open the downhole safety valve.

[0016] The present invention has the following advantages due to the adoption of the above technical solutions: During well start-up, ethylene glycol will be the primary agent for preventing hydrates downstream of the nozzle, supplemented by methanol. Once the temperature downstream of the nozzle gradually rises to the hydrate control temperature, the injection of ethylene glycol between the main valve and the wing valve can be stopped. Ethylene glycol will then be switched to be injected downstream of the nozzle during normal production to reduce the injection pressure during normal production and reduce energy consumption.

[0017] Because methanol has a low vapor pressure and high volatility, it is easy to vaporize and enter the moisture after being injected into pipelines and equipment, and then dissolve hydrated lumps, thus having a good unblocking effect. Methanol will be the main medium for unblocking. However, hydrate blockage is prone to occur downstream of the nozzle. Methanol can be injected downstream of the nozzle during unblocking through the connecting valve of this invention.

[0018] Reduce the design flow rate of methanol injection during well opening to decrease the cross-sectional area of ​​the umbilical cable, reduce engineering investment, and at the same time reduce the storage of toxic and harmful methanol on the platform. Attached Figure Description

[0019] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings: Figure 1 This is a schematic diagram of the underwater wellhead hydrate inhibitor injection system; Figure 2 This is a diagram of the hydrate inhibitor injection process system at the underwater wellhead. Figure 3 This is a diagram showing the pour point of ethylene glycol at different mass fractions. Detailed Implementation

[0020] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0021] like Figure 1 and Figure 2As shown, Embodiment 1 of the present invention provides a subsea wellhead hydrate inhibitor injection system, including an ethylene glycol supply system and a first injection pipeline 12 and a second injection pipeline 13 connected to the ethylene glycol injection system. Ethylene glycol in the ethylene glycol supply system is injected into the first injection pipeline 12 and the second injection pipeline 13 respectively via an ethylene glycol metering valve 1. A connecting valve 2 is provided on the first injection pipeline 12, and an ethylene glycol injection valve 5 is provided on the second injection pipeline 13. Ethylene glycol in the first injection pipeline 12 is injected upstream of the nozzle 6 via the connecting valve 2, and ethylene glycol in the second injection pipeline 13 is injected downstream of the nozzle 6 via the ethylene glycol injection valve 5. The ethylene glycol is diluted with water to a mass fraction of 80%~90%.

[0022] It should be noted that the ethylene glycol referred to in this invention refers to the ethylene glycol used in the initial filling and injection, diluted with water.

[0023] like Figure 3 As shown, the minimum design temperature of the downstream pipeline of the nozzle is -29℃. Usually, before well start-up, in order to balance the pressure upstream and downstream of the nozzle, the pipeline system needs to be pressurized to control the temperature of natural gas after throttling through the nozzle to be higher than -24℃. The commercially available ethylene glycol that is usually purchased is 99.9% concentration with a pour point of -10℃. If it is injected upstream of the nozzle during well start-up, the ethylene glycol will solidify upstream, reducing the upstream pressure and causing freezing blockage downstream of the nozzle.

[0024] The present invention pre-dilutes commercially available ethylene glycol with fresh water to a mass fraction of 80%~90%, lowering the pour point to -29.8°C to meet the pour point requirements of the inhibitor during well start-up, and then fills the ethylene glycol pipeline as the initial pre-filling medium.

[0025] Ethylene glycol in the ethylene glycol supply system is supplied to the wellhead via two pipelines through ethylene glycol metering valve 1. One pipeline injects ethylene glycol downstream of the nozzle 6 through ethylene glycol injection valve 5, and the other pipeline injects ethylene glycol upstream of the nozzle 6 through the connecting valve 2. During well start-up, the connecting valve 2 is opened, and ethylene glycol is injected upstream of the nozzle 6 through the connecting valve of the first injection pipeline 12. Once the temperature downstream of the nozzle 6 gradually rises to the hydrate control temperature, the injection of ethylene glycol upstream of the nozzle 6 is stopped, ethylene glycol injection valve 5 is opened, and connecting valve 2 is closed. This switches the ethylene glycol aqueous solution supplied by the ethylene glycol supply system to the normal production process, where ethylene glycol is injected downstream of the nozzle 6 to reduce the injection pressure during normal production and reduce energy consumption.

[0026] To reduce methanol usage, the required injection volume of hydrate inhibitor ethylene glycol will be calculated based on the hydrate control needs during well start-up, and this volume will be used as the design flow rate of ethylene glycol for well start-up.

[0027] To increase safety, the aqueous wellhead hydrate inhibitor injection system also includes a methanol supply system. The methanol supply system is connected to the first injection pipeline 12 via a third injection pipeline 14. The third injection pipeline 14 is equipped with a methanol injection valve 5. The third injection pipeline 14 is connected to the second injection pipeline 13 via a connecting valve 2.

[0028] When starting the well, methanol is mixed with ethylene glycol in the first injection pipeline 12 through the methanol injection valve 5 and then injected upstream of the nozzle 6. During normal operation, when the downstream pipeline is unblocked, methanol is injected downstream of the nozzle 6 after passing through the methanol injection valve 4, the connecting valve 2 and the ethylene glycol injection valve 5 in sequence.

[0029] While the diluted ethylene glycol meets the required pour point and viscosity, methanol has a lower viscosity and freezing point, posing a greater risk of hydrate blockage during well opening. To increase safety and fully utilize the methanol system's capacity, the calculation of ethylene glycol volume is primary during well opening, with the methanol system operating at maximum capacity as an auxiliary measure. Once the temperature rises to 14°C, the hydrate control process is complete, and ethylene glycol can be injected downstream of nozzle 6. This invention allows for complete elimination of methanol during well opening; however, for safety reasons, it is recommended to use methanol as a supplement.

[0030] In this invention, during the well start-up process, the prevention and control of hydrates downstream of nozzle 6 will be mainly based on ethylene glycol and supplemented by methanol. After the temperature downstream of nozzle 6 gradually rises to the hydrate control temperature, the injection of ethylene glycol upstream of nozzle 6 can be stopped. Ethylene glycol will be switched to be injected downstream of nozzle 6 during normal production to reduce the injection pressure during normal production and reduce energy consumption.

[0031] The subsea wellhead hydrate inhibitor injection system serves as a subsea distribution unit. After methanol and ethylene glycol undergo hydrate inhibition during well start-up and normal operation via this subsea distribution unit, they flow out from different wellheads. The methanol supply system includes a first support platform located on the sea surface and an umbilical cable connecting to the first support platform. The ethylene glycol supply system includes a second support platform located on a sponge and an ethylene glycol pipeline connecting to the second support platform.

[0032] The nozzle 6 is located on the main pipeline, and the main pipeline is also equipped with a main valve 7 and a wing valve 8. The main valve 7 and the wing valve 8 are located upstream of the nozzle 6, and the main valve 7 and the wing valve 8 are respectively located on both sides of the first injection pipeline 12.

[0033] A downhole safety valve 11 is also provided at one end of the main pipeline near the reservoir. A first pressure sensor P1 and a second pressure sensor P2 are respectively provided upstream and downstream of the downhole safety valve 11. The first pressure sensor P1 and the second pressure sensor P2 are used to detect the pressure upstream and downstream of the downhole safety valve 11, respectively.

[0034] The third injection pipeline 14 is also equipped with a methanol metering valve 3, which is located upstream of the methanol injection valve 4.

[0035] The first injection line 12 and the second injection line 13 are respectively provided with a first one-way valve 9 and a second one-way valve 10. The ethylene glycol is injected from the first injection line 12 through the first one-way valve 9 to the upstream of the nozzle 6, and the ethylene glycol is injected from the second injection line 13 through the second one-way valve 5 to the downstream of the nozzle 6.

[0036] The ethylene glycol injection system of this invention has two pipelines leading to the wellhead via ethylene glycol metering valve 3. One pipeline injects glycol downstream of nozzle 6 through ethylene glycol injection valve 5 and second check valve 10. The other pipeline injects glycol downstream of nozzle 6 through connecting valve 2 and then through first check valve 9, reaching the space between main valve 7 and wing valve 8. Methanol is introduced from the umbilical cable via methanol metering valve 3 and methanol injection valve 4, then connected between connecting valve 2 and first check valve 9. Because methanol has low vapor pressure and high volatility, it easily vaporizes after injection into the pipeline and equipment, dissolving hydrated lumps and effectively unblocking them. Methanol will serve as the primary unblocking medium. Since hydrated blockages easily occur downstream of nozzle 6, methanol can be injected downstream of nozzle 6 via connecting valve 2 of this invention for unblocking purposes.

[0037] The injection method based on the above-mentioned subsea wellhead hydrate inhibitor injection system includes the following steps: including well start-up, specifically including the following steps: When starting the well, open the connecting valve 2 and close the ethylene glycol injection valve 5. Ethylene glycol is injected upstream of the nozzle 6 through the connecting valve 2. After the temperature gradually rises to the hydrate control temperature, open the ethylene glycol injection valve 5 and close the connecting valve 2. Ethylene glycol is then injected downstream of the nozzle through the second check valve 10.

[0038] To improve safety, a methanol supply system is introduced. The present invention further includes the following steps: when starting the well, the connecting valve 2 is opened, the ethylene glycol injection valve 5 is closed, and the methanol injection valve 4 is opened at the same time. Ethylene glycol is mixed with methanol through the connecting valve 2 and injected upstream of the nozzle 6.

[0039] The method also includes unblocking the downstream pipeline during normal operation. Specifically, when a blockage occurs in the pipeline downstream of the nozzle, the methanol injection valve 4 and the connecting valve 2 are opened. Methanol and ethylene glycol are mixed and flow through the second one-way valve 10 to the hydrate blockage site. Because methanol has a low vapor pressure and high volatility, it easily vaporizes after being injected into the pipeline and equipment, entering the moisture and dissolving the hydrate clumps, thus effectively unblocking. Methanol will serve as the main medium for unblocking. Since hydrate blockage is prone to occur downstream of the nozzle, methanol can be injected downstream of the nozzle during unblocking through the connecting valve of this invention.

[0040] To dissolve hydrates on the wellbore wall during the safety valve shutdown process, the well start-up process further includes the following steps: Before injecting ethylene glycol, open methanol injection valve 4 and inject it into the upper part of downhole safety valve 11 through the first one-way valve 9. When the upstream and downstream pressures of downhole safety valve 11 are equal, open downhole safety valve 11. Methanol has a low saturated vapor pressure and low density; therefore, before opening downhole safety valve 11, the injected methanol can be distributed in the gas phase, dissolving hydrates on the wellbore wall during the shutdown process.

[0041] For specific applications, the following examples are provided: The gas field has a total of 12 production wells. The maximum shut-in pressure for a single well is 58.8 MPa. The maximum upstream flow pressure of production nozzle 6 is 50.6 MPaA, and the maximum downstream flow pressure is 14.9 MPaA. If methanol is used for well start-up, the methanol injection flow rate is 45 m³ / h. 3 / d, the umbilical cable requires the integration of three 70km, 1.25in methanol stainless steel pipelines into a self-supporting platform. During normal production, to prevent hydrate formation, the ethylene glycol injection system is designed with a capacity of 1213m³. 3 / d, requires self-supporting platform 2 to transport through 114km, 6in steel pipe.

[0042] In this invention, 90% ethylene glycol by mass is used as a hydrate inhibitor for well start-up, with a maximum ethylene glycol flow rate of 60 m³ / h during start-up. 3 / d. By adding ethylene glycol required for normal production in other wells, the design capacity of ethylene glycol is increased to 1250m³. 3 / d, the ethylene glycol pipeline size remains unchanged. Since methanol is only used as an auxiliary agent for well opening and for hydrate unblocking, the flow rate can be optimized to 12m³ / d. 3 Therefore, the methanol pipeline within the umbilical cable can be optimized to a single 1.25-inch stainless steel methanol pipeline, reducing umbilical cable investment by 89 million. Based on the storage capacity of the toxic and hazardous methanol on Platform 1, only the inhibition of hydrates within the wellbore needs to be ensured. The 300 m³ required for well start-up from 12 wells can be reduced to 40 m³. 3 .

[0043] (1) Well start-up operation procedure Open the main valve and set the flow rate of methanol metering valve 3 to 12m³ / h.3 / d, open methanol injection valve 4, injecting methanol through the first check valve 9 to the upper part of downhole safety valve 11. When pressure sensors P1 and P2 are equal, open downhole safety valve 11. Ethylene glycol metering valve, flow rate set to 60m³ / d. 3 / d, open the connecting valve 2, mix with methanol through the first check valve 9, and inject it between the main valve 7 and the wing valve 8. Open the wing valve 8, and wait for the temperature to gradually rise to the hydrate control temperature. After the pressure is 14.9 MPa and the generation temperature is 14℃, open the ethylene glycol injection valve 5 and close the connecting valve 2. Ethylene glycol is injected downstream of the nozzle 6 through the second check valve 10, which can reduce the injection energy consumption in the production operation process.

[0044] (2) Procedure for unblocking hydrates in downstream pipelines during normal operation When hydrate blockage occurs downstream of the production nozzle 6, methanol will be used as the unblocking medium. The methanol injection valve 4, the connecting valve 2, and the ethylene glycol injection valve 5 will be opened. Through the second check valve 10, methanol can flow to the hydrate blockage.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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 of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for injecting a subsea wellhead hydrate inhibitor, based on a subsea wellhead hydrate inhibitor injection system, the subsea wellhead hydrate inhibitor injection system comprising an ethylene glycol supply system, a methanol supply system, and a first injection pipeline and a second injection pipeline connected to the ethylene glycol injection system, wherein ethylene glycol in the ethylene glycol supply system is injected into the first injection pipeline and the second injection pipeline respectively via an ethylene glycol metering valve, the first injection pipeline is provided with a connecting valve, and the second injection pipeline is provided with an ethylene glycol injection valve, wherein the ethylene glycol in the first injection pipeline is injected upstream of the nozzle via the connecting valve, and the ethylene glycol in the second injection pipeline is injected downstream of the nozzle via the ethylene glycol injection valve, wherein the ethylene glycol is a commercial 99.9% ethylene glycol with a pour point of -12.9℃ diluted with fresh water to an ethylene glycol aqueous solution with a mass fraction of 80% to 90%, the pour point of the ethylene glycol aqueous solution is reduced to -29.8℃, and it is used as an initial pre-filling medium to fill the ethylene glycol delivery and injection pipeline; The methanol supply system is connected to the first injection pipeline via a third injection pipeline. The third injection pipeline is equipped with a methanol injection valve. When the well is started, methanol is mixed with ethylene glycol in the first injection pipeline through the methanol injection valve and then injected upstream of the nozzle. If hydrate blocks the downstream and needs to be unblocked, methanol is injected downstream of the nozzle after passing through the methanol injection valve, the connecting valve and the ethylene glycol injection valve in sequence. Its features are, The method for injecting hydrate inhibitors at the underwater wellhead includes: When starting the well, open the connecting valve and close the ethylene glycol injection valve. Ethylene glycol is injected upstream of the nozzle through the connecting valve. After the temperature gradually rises to the hydrate control temperature, open the ethylene glycol injection valve and close the connecting valve. Ethylene glycol is then injected downstream of the nozzle through the second check valve. In this system, ethylene glycol in the ethylene glycol supply system is supplied to the wellhead via two pipelines through an ethylene glycol metering valve. One pipeline injects ethylene glycol downstream of the wellhead via an ethylene glycol injection valve, and the other pipeline injects ethylene glycol upstream of the wellhead via the connecting valve. During well start-up, the connecting valve is opened, and ethylene glycol is injected upstream of the wellhead through the connecting valve of the first injection pipeline. Once the temperature downstream of the wellhead gradually rises to the hydrate control temperature, the injection of ethylene glycol upstream of the wellhead is stopped, the ethylene glycol injection valve is opened, and the connecting valve is closed. This switches the ethylene glycol aqueous solution provided by the ethylene glycol supply system to the normal production process, where ethylene glycol is injected downstream of the wellhead to reduce the injection pressure during normal production and reduce energy consumption.

2. The method according to claim 1, characterized in that, The nozzle is located on the main pipeline, which is also equipped with a main valve and a wing valve. The main valve and the wing valve are located upstream of the nozzle and are located on both sides of the first injection pipeline.

3. The method according to claim 2, characterized in that, A downhole safety valve is also provided at the end of the main pipeline near the reservoir. A first pressure sensor and a second pressure sensor are respectively provided upstream and downstream of the downhole safety valve. The first pressure sensor and the second pressure sensor are used to detect the pressure upstream and downstream of the downhole safety valve, respectively.

4. The method according to claim 3, characterized in that, The third injection line is also equipped with a methanol metering valve, which is located upstream of the methanol injection valve.

5. The method according to claim 4, characterized in that, The first injection line and the second injection line are respectively provided with a first check valve and a second check valve. Ethylene glycol is injected from the first injection line through the first check valve to the upstream of the nozzle, and ethylene glycol is injected from the second injection line through the second check valve to the downstream of the nozzle.

6. The method according to claim 5, characterized in that, The well opening process also includes the following steps: before injecting ethylene glycol, open the methanol injection valve and inject it into the upper part of the downhole safety valve through the first check valve. When the upstream and downstream pressures of the downhole safety valve are the same, open the downhole safety valve.

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