WO2015036041A1

WO2015036041A1 - Hydrocarbon separation apparatus with recirculation loop

Info

Publication number: WO2015036041A1
Application number: PCT/EP2013/069053
Authority: WO
Inventors: Knut Bech
Original assignee: Statoil Petroleum As
Priority date: 2013-09-13
Filing date: 2013-09-13
Publication date: 2015-03-19

Abstract

Apparatus for subsea processing of a multiphase fluid from a well for cold flow, the apparatus comprising a first pipe section comprising a separator arranged to receive said fluid and to separate at least two phase components of said fluid, and a second pipe section arranged to re-circulate at least part of said fluid through the separator.

Description

HYDROCARBON SEPARATION APPARATUS WITH RECIRCULATION LOOP

Technical field The present invention relates to the separation of phases from a mixture of phases in a fluid from a well and to the separation of hydrocarbon fluids and water.

Background Pipe separators may be used for the separation of gas, oil and water phases, for example in subsea processing of hydrocarbon fluids from a well. An example of a pipe separator is described in the patent document US 7278543. This apparatus can be applied to separate water from a mixture of gas, oil and water in a water-continuous flow of the mixture. The separated water can be disposed in the rock formation or further processed by some other device for re-injection into the hydrocarbon reservoir rock. The separated oil will normally still contain a significant amount of water.

For some applications, the quality of separation of the different liquid phases and gas provided by such pipe separators is insufficient. For example, in order to provide sufficiently low water-in-oil content for long distance "cold" subsea transport of waxy oil, improved separation is sought. A low water content reduces the chance of hydrate formation and also reduces the amount of hydrate inhibitor needed to provide cold flow transport. The cost involved in adding a large amount of hydrate inhibitors is significant in the context of other costs involved in the production process. The quality of oil for cold flow therefore needs to be high.

Separated water can be re-injected into the reservoir and a low oil and particle content is desirable if separated water is used for re-injection into the reservoir. A low oil content is required because the presence of oil in water which is re-injected into a reservoir may cause blockages in the rock formation.

Existing pipe separators are typically provided on a seabed frame. This arrangement puts a practical limit to the size due to the need of a heavy lifting vessel, and can be cost intensive compared to installation methods applied for piping, such as towing or reeling methods. A limit to the length and size of a pipe separator causes in practice also a limit to the amount of time fluids can reside in the pipe separator for a given flow rate of the fluids, and thus to the quality of the separation process.

SUMMARY

According to a first aspect of the invention, there is provided an apparatus for subsea processing of a multiphase fluid from a well for cold flow, the apparatus comprising: a first pipe section comprising a separator arranged to receive said fluid and to separate at least two phase components of said fluid; and a second pipe section arranged to re- circulate at least part of said fluid through the separator.

Said first pipe section and said second pipe section together may form a loop for recirculating said fluid. Said separator may comprises a pipe separator and/or an annular phase splitter. Said pipe separator and said annular phase splitter may be arranged such that the fluid from a well flows through said pipe separator before the fluid flows through the annular phase splitter. The at least two phase components may comprise at least two of water, oil and gas.

The annular phase splitter may comprise an inner pipe and an outer pipe, said outer pipe being located around said inner pipe, wherein said inner pipe has a plurality of perforations to provide fluid communication between an inside of the inner pipe and a region between an outer surface of said inner pipe and an inner surface of said outer pipe. The annular phase splitter may comprise first and second apertures extending through the wall of the outer pipe providing respective outlets for said at least two phase components.

The inner pipe of said annular phase splitter may provide a flow path downstream of said plurality of perforations to provide an outlet for remaining fluid towards said second pipe section. The pipe separator may be arranged to produce a stratified flow of said multiphase fluid from a well. The separator may be arranged to extend horizontally in use. The first pipe section may comprise an insulated portion arranged to connect the apparatus to a well manifold. At least part of the pipe separator may have an internal diameter which is substantially 1.75 times larger than the internal diameter of at least part of the insulated portion. The second pipe section may be coupled to a pig launcher for pigging the first pipe section. The second pipe section may be coupled to a pump for circulating the fluid through the first pipe section and the second pipe section and the pump may be capable of re-circulating fluid for flushing the first pipe section and the second pipe section, for breaking emulsion in the fluid and for mixing the constituents in the fluid.

An injection line may be connected to the second pipe section for injecting production chemicals such as emulsion breaker. According to a second aspect of the invention, there is provided a separator as used in the apparatus of the first aspect of the invention.

According to a third aspect of the invention, there is provided a method of processing fluid from a well using the apparatus of the first aspect of the invention.

According to a fourth aspect of the invention, there is provided a method of processing a multiphase fluid from a well, the method comprising: receiving said fluid in a pipe; separating at least part of the phases from said multiphase fluid in a separator; recirculating at least part of said multiphase fluid into said pipe. The separated phases may comprise oil and water. The method may further comprise re-injecting said separated water into a reservoir.

BRIEF DESCRIPTION OF DRAWINGS Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 illustrates schematically an apparatus for processing fluids. Figure 2 illustrates schematically a further apparatus for processing fluids.

Figure 3 illustrates schematically an annular separator as may be used in the apparatus of Figures 1 or 2. Figure 4 is a flow diagram of a separation method. DETAILED DESCRIPTION

Herein disclosed is a subsea apparatus for processing of a multiphase well fluid for cold flow. The apparatus comprises a separator arranged to receive a multiphase fluid from a well. When the multiphase fluid flows through the separator, phase components of the fluid such as oil, gas and water are separated from each other and from the fluid and are diverted away from the separator through respective outlets. Some remainder fluid will still contain multiple phases. The apparatus is arranged such that the residence time of the multiphase fluid in the separator is increased by re-circulating the remainder fluid through the separator such that the quality of the separation is increased.

The apparatus comprises a first pipe section comprising a separator arranged to receive said fluid and to separate at least two phase components of said fluid and a second pipe section which is arranged to re-circulate at least part of said fluid through the separator. The first pipe section and said second pipe section together form a loop for re-circulating the multiphase fluid. The separator may comprise two parts: a pipe separator and an annular phase splitter, whereby the pipe separator is upstream with respect to the annular phase splitter. The flow of the fluid is maintained by the combination of the first and second pipe sections.

The annular phase splitter may comprise an inner pipe and an outer pipe, said outer pipe being located around said inner pipe, wherein said inner pipe has a plurality of perforations to provide fluid communication between an inside of the inner pipe and a region between an outer surface of said inner pipe and an inner surface of said outer pipe. The annular phase splitter may comprise first and second apertures extending through the wall of the outer pipe providing outlets for the separated phase components. The inner pipe of the annular phase splitter may provide a flow path downstream of said plurality of perforations to provide an outlet for remaining fluid towards the second pipe section. The pipe separator may be arranged to produce a stratified flow into the annular phase splitter, whereby the different phases flow in separate adjacent horizontal layers. The separator may be arranged to extend horizontally in use and the apparatus including the first and second pipe sections may be arranged in the horizontal plane.

The first and second pipe sections may be partly or fully covered by thermal insulation to connect the apparatus to a well manifold. At least part of the pipe separator has an internal diameter which is substantially 1 .75 times larger than the internal diameter of at least part of the insulated portion. The second pipe section may be coupled to a pig launcher and a receiver for round pigging the first pipe section. The second pipe section may be coupled to a pump for circulating the fluid through the first pipe section and the second pipe section and said pump may be capable of recirculating fluid for flushing the first pipe section and the second pipe section, for breaking emulsion in the fluid and for mixing the constituents in the fluid. An injection line may be connected to the second pipe section for injecting production chemicals such as emulsion breaker.

With reference to Figure 1 , a plurality of wells 1 is connected to a common manifold 2. The output of the manifold will be a mixture of fluids and solids including oil, gas and water. The output is connected to an insulated pipe 3. The length of the pipe is chosen such that a first stage separation of gas, oil and water is achieved by an at least partially stratified flow. This pipe is thereby a pipe separator. The length may be around 500m to achieve the required residence time for separation of the phases. The pipe is insulated to avoid a drop in temperature which may give rise to wax formation. The separator is close to the well relative to the total distance over which the fluids are transported to reduce the temperature drop and any risk of wax and hydrate formation upstream from the separator. An electrical heating method, such as direct electrical heating, can be applied to heat the fluids and to reduce the chance of wax formation. The separator is also close to the well to avoid reduced separation properties due to a pressure drop and to avoid the need for a pump between the well and the separator. A pump may mix up phases during pumping and is therefore preferably avoided between the well and pipe 3. The insulated pipe is connected to a pipe 4 which is larger in diameter than pipe 3, for example 1 .75 times larger, and this larger pipe creates a slow stratified flow of the fluid from the well. In a stratified flow, the different phases generally flow in adjacent horizontal layers. Pipe separator 4 is connected to an annular phase splitter 5 which has a first output 6 for water and a second output 7 for oil and gas. A phase splitter is illustrated in more detail in Figure 3 and described below. The annular phase splitter may not achieve a complete separation of the multiphase fluid from the well and a remainder fluid is received in a further pipe 8 which is coupled back to the insulated pipe 3, thereby forming a loop. The further pipe 8 may be insulated or heated such that the temperature of the fluids does not significantly drop during re-circulation. The further pipe may have an internal diameter which is similar to that of pipe 4, or, alternatively, similar to the internal diameter of pipe 3. The remainder fluid is mixed in the insulated pipe 3 with fluid from the output of the manifold 1 . The fluid flow slows down in the annular phase splitter due to the loss of the separated oil through first output 6 and the loss of separated water second output 7. By way of example, the fluid speed may be 0.8 m/s on entry into the annual phase splitter and may slow down to 0.1 m/s towards the end portion of the annual phase splitter. A pump 9 is provided in the further pipe for driving the remainder fluid back to the insulated pipe. The pump breaks up stable water droplets in the oil, which improves the separation of oil and water on return in the pipe separator. In addition to the provision of a pump, a chemical emulsion breaker may be added to the fluid. The pump can also be used for creating a strong fluid flow in the loop to flush the loop and wash out any contaminants. In a strong fluid flow, the separation of phases will not take place to the same extent as in a weak flow and therefore a strong flow will generally only be used for cleaning purposes. The further pipe is also connected to a pig launcher 10 and a receiver for round pigging the pipes. A valve 1 1 is provided in the further pipe. The system illustrated in Figure 1 may be used for water-continuous flow emerging from the well. A system used for oil-continuous flow would require longer pipe sections and a larger diameter of the pipes when compared to a system used for water-continuous flow.

Figure 2 illustrates a further apparatus wherein features which correspond to features in Figure 1 have been assigned corresponding reference signs. With reference to Figure 2, a subsea pipe loop is illustrated, wherein the fluid flow emerges from a well manifold 2, then flows through pipe segment 3, which constitutes a pipe separator, through pipe segment 4, which constitutes a pipe separator with increased inner diameter, trough full-bore valve 12, trough pipe segment 5, which constitutes an annular phase splitter, through pipe segment 8, which constitutes the first section of the re-circulation loop, through valve 1 1 , through pump 9, and finally though pipe segment 13, which constitutes the second part of the re-circulation loop as well as a second stage separator for the remainder fluid. The re-circulating flow from pipe segment 13 is re-combined with the production flow in pipe 3. The pump 9 can be used to re-circulate a small flow during normal operation or a large flow during flushing of the system for removing sand. An outlet pipe 6 is connected to annular phase splitter 5 for taking separated oil away from the annular phase splitter. A valve 14 is provided in outlet pipe 6. An outlet pipe 7 is connected to the annular phase splitter for taking separated water away from the annular phase splitter. A booster pump 15 is provided in outlet pipe 7.

The output from the annular phase splitter is connected to a water injection well 20 via outlet pipe 7. The separated water can thereby be re-injected into the formation. A booster pump 15 is provided for generating sufficient pressure to re-inject the water into the formation.

Pipe segment 4 may have a diameter of at least 1 .75 times the diameter of pipe segment 3. Sand and other particular deposits may be allowed to settle in pipe segment 4. The remainder fluid in the re-circulating flow entering pipe segment 8 consists of a mixture of oil and water, and possible some reminiscent gas, and constitutes a small fraction of the total production from the wells. Valve 1 1 and pump 9 control the amount of re-circulation, together with pump 15 and control valve 14. Pump 9 works on the fluid mixture to de-stabilize emulsions. De-emulsifying chemical can be added from a storage or line 13 between valve 1 1 and pump 9. Pipe segment 13 works as a pipe separator with low flow velocity, and correspondingly as a second-stage separator for the re-circulating flow. The subsea pipe loop can be cleaned by circulating a by-pass pig from device 10. Valves and controls for a pigging operation are not shown in Figures 1 and 2.

Sand jetting and/or hydrate inhibition is facilitated by a combined local subsea storage and pump station 16. A mixture of water and hydrate inhibitor such as mono-ethylene glycol (MEG) can be injected into pipe segment 4 via line 17 connecting pump station 16 to pipe segment 4. A further line 18 is provided to form a connection from pipe segment 4 and line 17 to outlet pipe 6, and a valve 19 is provided in line 18. Valve 12 can be closed while valve 19 is opened for the hydrate inhibitor and/or sand mixture to flush into outlet pipe 6 for further transport to other equipment. Hydrate inhibition of the wellheads and piping associated with wells 2 and 20 is possible via tubes 21 extending from the storage/pump station 16 to wells 2 and 20.

Pipe sections 3, 4, 5, 8 and 13 may all be insulated to reduce heat loss of the fluids into the environment. Avoiding heat loss can be used to reduce wax deposition. The pipe sections 3, 4, 5, 8 and 13 preferably have a limited slope, whereby the angle of the pipe sections with respect to the horizontal is preferably small enough to avoid flow of water due to gravity in a direction opposite the flow from the well.

The dimensions of the system illustrated in Figures 1 and 2 can be chosen such that the system can be provided as a pipe bundle. In a pipe bundle, the different parts of the system shown in Figures 1 and 2 are surrounded by a steel carrier pipe. The different pumps and control systems necessary to operate the system can be located in the so-called towheads, which constitute the terminals of a pipe bundle. The pipe bundle could be constructed onshore and towed to the location in a submerged state. The construction of the system as a pipe bundle is cost effective. The annular phase splitter 5 is show in more detail in Figure 3. The phase splitter has an inner pipe 31 , which has a plurality of perforations 32 at the top and a plurality of perforations 33 at the bottom. An outer pipe 34 surrounds the inner pipe in a substantially co-axial orientation. An outlet 35 is provided at the top for connection to outlet pipe 6 for removing the separated oil and gas from the phase splitter. An outlet 36 is provided at the bottom and can be connected to outlet pipe 7 for removing the separated water from the annular phase splitter. A cross section 37 of fluid in inner pipe 31 is illustrated showing adjacent horizontal layers of fluids and gas in a stratified flow. Figure 4 illustrates a method of processing fluids in a flow diagram. The following steps are illustrated: S1 , receiving fluids in a pipe; S2, separating at least part of the phases from said multiphase fluid in a separator; S3, re-circulating at least part of said multiphase fluid into said pipe. It will be appreciated by a person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present disclosure. Different embodiments have been described above, but the skilled person will readily be able to devise other options for providing a pipe separator loop.

Claims

CLAIMS:

1 . Apparatus for subsea processing of a multiphase fluid from a well for cold flow, the apparatus comprising:

a first pipe section comprising a separator arranged to receive said fluid and to separate at least two phase components of said fluid; and

a second pipe section arranged to re-circulate at least part of said fluid through the separator.

2. The apparatus according to claim 1 , wherein said first pipe section and said second pipe section together form a loop for re-circulating said fluid.

3. The apparatus according to claim 1 or 2, wherein said separator comprises a pipe separator.

4. The apparatus according to any one of the preceding claims, wherein said separator comprises an annular phase splitter.

5. The apparatus according to claim 4, wherein said pipe separator and said annular phase splitter are arranged such that the fluid from a well flows through said pipe separator before the fluid flows through the annular phase splitter.

6. The apparatus according to any one of the preceding claims, wherein the at least two phase components comprise at least two of water, oil and gas.

7. The apparatus according to any one of claims 4 and 5, wherein said annular phase splitter comprises an inner pipe and an outer pipe, said outer pipe being located around said inner pipe, wherein said inner pipe has a plurality of perforations to provide fluid communication between an inside of the inner pipe and a region between an outer surface of said inner pipe and an inner surface of said outer pipe.

8. The apparatus according to claim 7, wherein the annular phase splitter comprises first and second apertures extending through the wall of the outer pipe providing respective outlets for said at least two phase components.

9. The apparatus according to claim 7, wherein the inner pipe of said annular phase splitter provides a flow path downstream of said plurality of perforations to provide an outlet for remaining fluid towards said second pipe section.

10. The apparatus according to claim 3, wherein said pipe separator is arranged to produce a stratified flow of said multiphase fluid from a well.

1 1 . The apparatus according to any one of the preceding claims, wherein said separator is arranged to extend horizontally in use.

12. The apparatus according to any one of the preceding claims, wherein the first pipe section comprises an insulated portion arranged to connect the apparatus to a well manifold.

13. The apparatus according to claim 12, wherein at least part of the pipe separator has an internal diameter which is substantially 1 .75 times larger than the internal diameter of at least part of the insulated portion.

14. The apparatus according to any one of the preceding claims, wherein the second pipe section is coupled to a pig launcher for pigging the first pipe section.

15. The apparatus according to any one of the preceding claims, wherein the second pipe section is coupled to a pump for circulating the fluid through the first pipe section and the second pipe section.

16. The apparatus according to claim 15, wherein said pump is capable of recirculating fluid for flushing the first pipe section and the second pipe section, for breaking emulsion in the fluid and for mixing the constituents in the fluid.

17. The apparatus according to any preceding claims, further comprising an injection line connected to the second pipe section for injecting production chemicals.

18. The apparatus according to claim 17, wherein said production chemicals comprise emulsion breaker.

19. A separator as used in the apparatus of claims 1 to 18.

20. A method of processing fluid from a well using the apparatus of claims 1 to 18.

21 . A method of processing a multiphase fluid from a well, the method comprising: receiving said fluid in a pipe;

separating at least part of the phases from said multiphase fluid in a separator; re-circulating at least part of said multiphase fluid into said pipe.

22. The method of claim 21 , wherein said separated phases comprise oil and water.

23. The method of claim 22, further comprising re-injecting said separated water into a reservoir.