WO2009006672A1

WO2009006672A1 - Fluid-fluid separator

Info

Publication number: WO2009006672A1
Application number: PCT/AU2008/000831
Authority: WO
Inventors: Mohamed Nabil Noui-Mehidi; Eugenio Spano Rosa; Gerardo Alonso Sanchez Soto; Edson Yoshihito Nakagawa; Mayela Rivero
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2007-07-12
Filing date: 2008-06-12
Publication date: 2009-01-15

Abstract

A fluid-fluid (preferably a gas-liquid) separator (10) in which an input mixture of fluids (12) flows downwardly in an outer pipe (26) along a spiral guide vane (34) such that a first fluid of the mixture (e.g. a gas 13) and a second fluid of the mixture (e.g. a liquid 15) are separated centrifugally. The first fluid or gas (13) enters an open lower end (22) of an inner pipe (18) to be vented upwardly (14) and the second fluid or liquid (15) flows downwardly to a reservoir (32) for extraction (16). The outer pipe (26) is sealed (40) above the inlet (38) such that all of the input mixture of fluids (12) is passed along the spiral guide vane (34) and the inner pipe (18) extends a distance below the spiral guide vane (34) to provide a stabilisation baffle (50) for the separated fluid phases. The separator (10) can be made very compact and used in petroleum industry well bores up to 310 mm in diameter.

Description

Fluid-Fluid Separator

Technical Field The present invention relates to fluid-fluid separators and to systems and methods using such separators. The separators may have particular use in the petroleum industry, in the separation of for example gas and liquid phases of a process fluid, and may be especially useful for separating liquid from gas in the production of gas from a gas field. They may have wider application also.

Background

An important stage in the production of oil and natural gas in the petroleum industry is multiphase separation, in which separate product streams are produced from the initial formation fluid that enters the well bore. It may for example be desirable to separate and produce an oil stream or a gas stream from a well bore, whilst appropriately disposing of other phases, e.g. by re- injection into a producing formation, or to separate and produce both streams.

Multiphase separators need to be reliable, appropriately sized for the environment, and able to cope with the various characteristics of the mixture of fluids that is to be processed. A separator's size may be particularly important when it is to be used on a production rig or in a seabed installation, and especially when it is to be used downhole in a production well.

The present invention aims to provide fluid-fluid separators that in their various forms may have a number of advantages, and that may for example be compact in size. The separators may be particularly advantageous in downhole and seabed separation processes, although they may also have broader applicability and be usable in a number of other fields such as platform applications or on-shore applications. Disclosure of the Invention

Viewed from a first aspect, the present invention provides a fluid-fluid separator for a well bore, the separator including: an outer pipe having a fluid mixture inlet at, in use of the separator, an upper portion of the outer pipe, an inner pipe concentric with the outer pipe, the inner and outer pipes defining an annular passage therebetween, a closure element that closes the annular passage above the fluid mixture inlet, wherein the inner pipe extends below the fluid mixture inlet and has an open lower end that defines an entrance for a separated first fluid, wherein the outer pipe extends below the open lower end of the inner pipe, a spiral guide vane in the annular passage extending downwardly from the fluid mixture inlet, wherein the separator is configured such that a mixture of fluids input through the fluid mixture inlet is separated centrifugally as it passes along the guide vane, a separated first fluid entering the open lower end of the inner pipe to pass upwardly within the inner pipe, and a separated second fluid flowing downwardly past the open lower end of the inner pipe along a wall of the outer pipe.

Viewed from a second aspect, the present invention provides a method of separating a fluid stream containing a gas phase and a liquid phase, including the steps of: injecting said fluid stream into an annular passage defined by an inner pipe and an outer pipe that is concentric with said inner pipe and that extends downwardly past an open lower end of said inner pipe; passing the whole of said fluid stream downwardly along a spiral guide vane in said annular passage, such that said fluid stream is separated by centrifugal force into a gas phase and a liquid phase; collecting said gas phase from said open lower end of said inner pipe; and allowing said liquid to fall under gravity through a lower portion of said outer pipe.

Preferably the closure element of the above mentioned first aspect is spaced upwardly from said fluid mixture inlet. This has the advantage of providing an annular chamber portion above the fluid mixture inlet to accommodate surges of an input mixture of fluids, e.g. slugs of gas and/or liquid.

Preferably the fluid mixture inlet is configured such that it directs fluid tangentially into the annular passage. This has the advantage that it promotes the swirling motion of the input mixture of fluids.

Viewed from a third aspect, the invention provides a gas-liquid separator including: an input stage in which process fluid is input; a first or primary separation stage below said input stage, including a spiral guide vane for separating gas and liquid phases of said process fluid by a centrifugal action as said process fluid flows downwardly through said primary separation stage; and a second separation stage below said primary separation stage, in which said liquid phase flows downwardly under gravity to a liquid reservoir stage, and in which said gas phase is vented upwardly centrally of said secondary separation stage, wherein the input stage is sealed such that all of the process fluid is passed through the primary separation stage.

It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below.

For a better understanding of the invention and to show how it may be performed, embodiments of the invention are described below, by way of example only, with reference to the accompanying drawings. It is to be understood that the particularity of the following description and drawings does not limit the generality of the inventive concepts that are taught in the present application and that are embodied in the described implementations.

Brief Description of the Drawings

Figure 1 is a schematic cross-sectional view from the side of a fluid-fluid separator according to an embodiment of the invention;

Figure 2 is a schematic cross-sectional view from above through the fluid mixture inlet of the separator of Fig. 1 ; Figure 3 is a schematic cross-sectional view through a cross baffle for stopping swirl of a separated liquid phase of the separator of Fig. 1 ; and

Figure 4 is a schematic cross-sectional view from the side of a gas- liquid separator located downhole in a subsea well bore.

Figure 5 is a graph giving an operational envelope for a separator according to an embodiment of the invention.

Detailed Description of Embodiments

Referring to Fig. 1 , a fluid-fluid separator 10 separates a multiphase fluid 12 into a gas phase 14 and a separate liquid phase 16. Thus the separator 10 is a gas-liquid separator.

The separator 10 includes an inner pipe 18 that has, in use of the separator 10, an open lower end 20 defining an entrance 22 for a separated first fluid, i.e. in this embodiment a gas, and an upper end 24 that communicates with appropriate gas processing equipment (not shown). An outer pipe 26 is concentric with the inner pipe 18 and extends downwardly past the open lower end 22 of the inner pipe 18. The outer pipe 26 includes an upper portion 28 that receives the mixture of fluids 12 and a lower portion 30 that defines a collection area 32 for a separated second fluid, i.e. in this embodiment a liquid and communicates with appropriate liquid processing equipment (not shown).

A spiral guide vane 34 is located in the annular passage 36 defined between the inner and outer pipes 18 and 26 to form a swirl tube. The mixture of fluids 12, i.e. the multiphase fluid 12 of gas and liquid, is input to the separator 10 via a horizontal fluid mixture inlet 38 in the outer pipe 26 that is tangential to the annular passage 36, as shown in Fig. 2, so that the fluid stream 12 enters the annular passage 36 tangentially and produces a swirl motion.

A closure element, e.g. a top plate 40 between a pair of flanges 42, seals off the annular passage 36 above the fluid mixture inlet 38, so that all of the fluid 12 is forced along the spiral guide vane 34. The top plate 40 is spaced from the fluid mixture inlet 38 to form an annular chamber portion 44 to accommodate surges of the fluid 12, e.g. slugs of gas and/or liquid.

As the swirled and pressurised multiphase fluid stream 12 flows through along the guide vane 34, it gains tangential momentum, and a high centrifugal field is established that separates gas from liquid. The liquid, which is the heavier phase, is pushed towards the inner surface 46 of the outer pipe 26 (see reference 15 which illustrates the liquid film), whilst the gas, which is the lighter phase, is extracted, and forms a swirling film (see reference 13) near the outer surface 48 of the inner pipe 18.

The inner pipe 18 extends downwardly past the lower end of the spiral vane 34 to form a baffle portion 50. This length 50 of straight piping helps to minimize atomization (i.e. droplets shearing off) of the separated liquid phase 15, and acts to stabilize the separated liquid and gas films 15 and 13, and reduce carry-over of liquid into the gas stream 14. Thus the inner pipe 18 provides a stabilization baffle 50 that extends downwardly from the spiral guide vane 34 to the open lower end 22 of the inner pipe 18.

At the end 20 of the baffle portion 50, the separated gas phase 13-14 of the fluid stream 12 naturally vents via the open lower end 22 of the inner pipe 18 upwardly via the inner pipe 18 to gas processing equipment. The separated liquid phase 15 of the fluid stream 12 flows downwardly under gravity along the inner wall 46 of the outer pipe 26 to the collection area 32. As it falls, gas entrained in the liquid may be separated by gravity, i.e. may bubble out and also vent via gas opening 22.

A cross baffle 52, as shown in cross-section in Fig. 3, is provided in the lower portion 30 of the outer pipe 26 in the liquid collection area 32 in the centre of the pipe 26, so as to break up or stop the liquid swirling motion in the liquid collection area 32. A gas vortex 17 will penetrate into the liquid collection area 32 centrally of the pipe 26, and both gas and liquid vortices will be present in the central region of the collection area 32. The cross baffle 52 comprises diametral or radial plates 54 arranged at 90° to each other that extend a distance along the pipe 26 to facilitate suppression of the vortex motion of the liquid 15-16 and allow gas bubbles in the liquid to rise due to buoyancy and escape into the gas entrance 22. The cross baffle 52 may therefore help to increase gas separation and to reduce gas entrainment in the liquid stream 16 (that is, it reduces gas carry-under).

In experiments using test embodiments of the separator, gas carry-under of less than 7% has been achieved in a separated liquid stream which includes salts such that the liquid mimics sea water.

Either or both of the gas phase 14 and liquid phase 16 may be produced from the separator 10 for use, e.g. distribution and sale. If one of the streams is not of use, it may be appropriately treated and disposed of.

The separator 10 can be made to be compact and robust, and is simple to manufacture and reliable in use. It may for example be made from suitable metal piping and guide vanes welded together. It may be especially useful in harsh environments and in environments where space is critical.

The design promotes high separation performance under a large operational range, e.g. of superficial velocities (i.e. of flow rates through a specific area) and of gas/liquid ratios. The same design may therefore be used in many different environments without change, and may be used in situations that must accommodate fluctuations in the characteristics of the multiphase fluid.

The passing of the mixture of fluids 12 downwardly along a spiral guide vane 34 to provide a primary gas and liquid separation stage enables the separator 10 to use both centrifugal force and gravity to provide a high degree of separation, as well as to be compact in design.

The input stage with the tangential fluid mixture inlet 38 and sealing of the annular passage 36 via the plate 40 prevents liquid spill over into the gas stream 14, whilst also assisting in the separation by keeping the pressure of the mixture of fluids 12 within the chamber 44 high and providing a swirling motion into the spiral guide vane 34.

The use of a single spiral vane guide 34 has been found to work well, as it allows for high g-forces to be applied to the mixture of fluids 12. The spiral guide vane 34 may have a constant pitch, and may be angled up to 20 degrees (from the horizontal). It may for example have a pitch in the range of about 10 to about 15 degrees. It may have up to 10 turns, with 5 to 7 being found to be particularly useful and to handle a wide range of multiphase flow rates. The design should be such as to give a "swirl number" (i.e. the ratio of the tangential velocity relative to the axial velocity - which depends of the gas/liquid ratio) at the end of the spiral vane 34 that is as high as possible. Embodiments of the invention have been designed which deliver a g-force equivalent up to 4000g in the gas stream at the discharge end of the spiral guide vane 34.

The annular chamber 44 may for example be sized to have a length of say 3 to 5 times the internal diameter of the outer pipe 26.

The stabilizing baffle 50 portion of the inner pipe 18 may be of suitable length so as to provide appropriate stability to the gas and liquid films 13 and 15. It has been found that a baffle 50 length of about 1 to 4 times the inner diameter of the inner pipe 18 provides good results over a wide liquid content range. An increase in liquid content in the fluid flow may be accommodated by an increase in length of the stabilizing baffle 50.

Good results have been found with a straight length of baffle 50 and a square edged gas entrance 22. The diameter of the entrance 22 should be adequate to accept all of the gas, with the entrance 22 diameter helping to define the gas vortex 17 produced around the inner pipe 18.

The inner and outer pipes 18 and 26 may be sized for desired gas flow rates, superficial velocities and gas-liquid ratios of the fluid 12. In use, a desired gas flow rate may dictate a suitable outer pipe 26 size for stability. It has been found that an inner pipe 18 inner diameter of about 75 to 80% of the outer pipe 26 inner diameter provides a useful design.

Good results for preventing swirl and allowing gravity separation of entrained gas have been found for a cross baffle 52 length of about one to 1 ^λh times the inner diameter of the outer pipe 26. All that is required of this baffle 52 is that it stop the swirl, and lengths beyond that which does this are wasted and could perhaps promote undesirable consequences, e.g. such as blockages.

The cross baffle 52 is placed diametrically within the outer pipe 26 ideally just below where the gas and liquid vortices occur in the liquid collection area 32. The distance between the cross baffle 52 and the gas entrance 22 may be for example about 10 times the diameter of the outer pipe 26.

The parameters of most importance in the design of a separator 10 to suit a given application are the pitch and number of turns of the spiral guide vane 34, the inner diameter of the gas or inner pipe 18 and the length of the baffle portion 50 of the gas or inner pipe 18. The graph of Figure 5 is a plot of the superficial velocity of a separated liquid phase (V_L) against the superficial velocity of a separated gas phase (V_G) from an input fluid mixture into an experimental test embodiment of the invention. The dimensions of this experimental test embodiment were a diameter of 50 mm (2 inches) for the outer pipe and a diameter of 38 mm (1.5 inches) for the production pipe (inner pipe) with a helical pitch of 38 mm (1.5 inches). The space length above the fluid mixture inlet was equivalent to five (5) times the inner pipe diameter and the inlet diameter was 25.4 mm (1 inch). The baffle length below the end of the spiral guide vane was equivalent to one (1 ) diameter of the inner pipe. The total length of the separator was 40 times the diameter of the outer pipe.

The graph of Figure 5 gives an operational envelope for liquid carry-over (i.e. liquid entrained in the gas stream) and delimits the range of gas and liquid superficial velocities (i.e. flow rates through a specific area) below which the separator efficiency is 100%, i.e. gas and liquid are perfectly separated. The envelope is equivalent to a stability boundary in terms of composition of fluids in the inlet which indicates whether liquid carry-over happens or not depending on the operating location in the graph. Such an envelope is also used to scale up separator systems from an experimental scale to a full scale when superficial velocities are taken into consideration as one of the scaling parameters.

Fig. 4 (in which the same reference numerals for the features of the separator 10 as used in Fig. 1 have been used) shows a separator 10 formed in the manner of Fig. 1 downhole in a production well bore 60 sunk on the seabed 62. The well bore 60 is lined by a casing 64, which is perforated with holes 66 in a production zone 68 so as to allow process fluid 70 to flow into the well bore 60 from the surrounding productive formation 72.

The separator 10 is supported by the inner pipe 18, and has a packer 74 provided about the pipe 18 above the separator inlet 38 and associated with the closure top plate 40, and a packer 76 provided on the outer pipe 26 above a liquid outlet end 30. A pump 78 is provided on the end of the separator 10 to pump the liquid 16 separated from the process fluid 70 back into the formation 72 via further perforations 80 in the casing 64.

Gas 14 produced from the fluid stream 70 can be vented via piping 18 to a subsea processing station or to a rig or onshore processing plant.

It is expected that embodiments of the invention could be designed for implementation in gas fields with high gas-liquid ratio (GLR) with a liquid content possibly up to 45% v/v at operating conditions, and a preferable maximum liquid content of about 15% for a no liquid carry over condition. It is also expected that such embodiments could operate with high gas flow rates of at least 150 MMSCFD and be used in confined spaces such as down-hole diameters up to 12¹Λ inches (310mm). The ratio of the length of separator 10 to the production pipe (inner pipe 18) diameter can be at least 60 and the length of the primary separation stage (defined by the spiral guide vane 34) need be only 15 times the production pipe 18 diameter, thus allowing a relatively short and compact primary separation stage.

The invention described herein is susceptible to alterations, additions and/or modifications other than those specifically described as would be understood by a person skilled in the art and it is to be understood that the invention encompasses all such alterations, additions and/or modifications which fall within the scope of the following claims.

Claims

1. A fluid-fluid separator for a well bore, the separator including: an outer pipe having a fluid mixture inlet at, in use of the separator, an upper portion of the outer pipe, an inner pipe concentric with the outer pipe, the inner and outer pipes defining an annular passage therebetween, a closure element that closes the annular passage above the fluid mixture inlet, wherein the inner pipe extends below the fluid mixture inlet and has an open lower end that defines an entrance for a separated first fluid, wherein the outer pipe extents below the open lower end of the inner pipe, a spiral guide vane in the annular passage extending downwardly from the fluid mixture inlet, wherein the separator is configured such that a mixture of fluids input through the fluid mixture inlet is separated centrifugally as it passes along the guide vane, a separated first fluid entering the open lower end of the inner pipe to pass upwardly within the inner pipe, and a separated second fluid flowing downwardly past the open lower end of the inner pipe along a wall of the outer pipe.

2. The separator of claim 1 , wherein said fluid mixture inlet is configured such that it directs fluid tangentially into said annular passage.

3. The separator of claim 1 or 2, wherein said closure element is spaced from said fluid mixture inlet.

4. The separator of any preceding claim, wherein said spiral guide vane seals against said inner and outer pipes.

5. The separator of any preceding claim, wherein said spiral guide vane is a single guide vane.

6. The separator of any preceding claim, wherein said spiral guide vane has a constant pitch of up to substantially 20 degrees.

7. The separator of any preceding claim, wherein said spiral guide vane has a constant pitch of between substantially 10 and substantially 15 degrees.

8. The separator of any preceding claim, wherein said spiral guide vane has up to 10 turns.

9. The separator of any preceding claim, wherein said spiral guide vane has between 5 and 7 turns.

10. The separator of any preceding claim, wherein said inner pipe provides a stabilization baffle that extends downwardly from said spiral guide vane to the open lower end of the inner pipe to reduce liquid atomization and stabilize the separated first and second fluids at the walls of, respectively, the inner and outer pipes.

1 1. The separator of claim 10, wherein said stabilization baffle is at least one inner diameter of said inner pipe long.

12. The separator of claim 10 or 1 1 , wherein said stabilization baffle is less than or substantially four inner diameters of said inner pipe long.

13. The separator of claim 10, 1 1 or 12, wherein said stabilization baffle includes perforations therein.

14. The separator of any preceding claim, including a swirl stopping baffle positioned diametrically within said outer pipe at a location within a fluid reservoir formed by said separated second fluid for reducing swirl flow.

15. A separator as claimed in any preceding claim which is configured as a gas-liquid separator, wherein said separated first fluid is a gas and said separated second fluid is a liquid.

16. A method of separating a fluid stream containing a gas phase and a liquid phase, including the steps of: injecting said fluid stream into an annular passage defined by an inner pipe and an outer pipe that is concentric with said inner pipe and that extends downwardly past an open lower end of said inner pipe; passing the whole of said fluid stream downwardly along a spiral guide vane in said annular passage, such that said fluid stream is separated by centrifugal force into a gas phase and a liquid phase; collecting said gas phase from said open lower end of said inner pipe; and allowing said liquid to fall under gravity through a lower portion of said outer pipe.

17. The method of claim 16 wherein the fluid stream is injected tangentially into the annular passage.

18. The method of claim 16 or 17, including the step of reducing atomization of said liquid phase by extending said inner pipe past an end of said guide vane to provide a stabilization baffle.

19. The method of claim 16, 17 or 18 including the step of providing a cross baffle in said lower portion of said outer pipe to reduce swirl in said liquid phase.

20. A gas-liquid separator including: an input stage in which process fluid is input; a first or primary separation stage below said input stage, including a spiral guide vane for separating gas and liquid phases of said process fluid by a centrifugal action as said process fluid flows downwardly through said primary separation stage; and a second separation stage below said primary separation stage, in which said liquid phase flows downwardly under gravity to a liquid reservoir stage, and in which said gas phase is vented upwardly centrally of said secondary separation stage, wherein the input stage is sealed such that all of the process fluid is passed through the primary separation stage.

21. The separator of claim 20, wherein said spiral guide vane extends around a central inner pipe, said inner pipe defining a gas inlet at its lower end for venting said gas phase upwardly, said inner pipe extending downwardly from an end of said spiral guide vane, so as to stabilize gas and liquid films formed by said separated gas and liquid phases.

22. The separator of claim 20 or 21 , wherein said separator includes a baffle located diametrically in said liquid reservoir stage for disrupting liquid swirl.

23. The separator of any of claims 20 to 22, wherein the input stage includes an inlet for inputting process fluid tangentially into said input stage.