GB2362589A

GB2362589A - Cyclone with intermediate water inlet

Info

Publication number: GB2362589A
Application number: GB0112477A
Authority: GB
Inventors: Simon Roger Henderson Davies
Original assignee: Kvaerner Process Systems AS
Current assignee: Fjords Processing AS
Priority date: 2000-05-24
Filing date: 2001-05-23
Publication date: 2001-11-28
Also published as: GB0012428D0; GB0112477D0

Abstract

A cyclonic inlet device for a well production stream separation system, the device comprising a cyclone separator (15) having an elongated body (17) into which production stream is introduced by way of a main inlet (16), and within which the production stream is caused to rotate such that the liquid mixture of the production stream migrates to the wall of the body, and the gas phase forms a gas core within the body, the body having a gas outlet (18) adjacent one axial end thereof, a liquid mixture outlet (19) adjacent the opposite axial end thereof and being characterised in that intermediate said main inlet (16) and said liquid mixture outlet (19), the body (17) is provided with secondary inlet means (24, 25) for introducing additional liquid into the body (17). Also disclosed is a gravity separator and a separation process utilising such an inlet device. The liquid is pumped or introduced under the influence of the pressure distribution within the cyclone.

Description

2362589 CYCLONIC INLET DEVICE

Background of the Invention

This invention relates to a cyclonic inlet device for use in a separation system, primarily, but not exclusively, a three-phase gravity separation system for separating the oil, water and gas phases of the production stream of an oil well.

It is to be recognised that throughout this specification, the terms "oil", 'Jwater" and gas' are used for convenience to identify the predominant constituents of the multi-phase mixture which is the well production stream. Furthermore, although for convenience the invention is described in relation to a three-phase gravity separator, it is to be recognised that the cyclonic inlet device forming the subject matter of the invention can be used as an inlet device for other forms of separation system.

In general terms, a cyclonic inlet device for a well stream separation system is intended to separate the bulk of the gas phase from the liquid phases before the well stream is discharged into the downstream region of the separating system. Thus, in a three-phase gravity separator, it is usual for the cyclonic inlet device to be housed within the vessel of the gravity separator and to separate the bulk of the gas of the well stream from the liquid phase of the well stream, permitting the gas to be discharged into the head space of the vessel, and permitting the liquid phases to be discharged in a controlled manner into the bulk of the liquid within the vessel. It is recognised that cyclonic inlet devices suppress, and in some applications, eliminate, foaming which would otherwise arise upon discharge of the well stream into the gravity separator vessel. However, it is also recognised that the liquid mixture discharged from the inlet device may well be subject, in the inlet device, to relatively high shear forces which can hinder the subsequent separation of the oil and water phases from the liquid mixture. It is an object of the present invention to provide a cyclonic inlet device wherein the aforementioned disadvantage is minimised.

Summary of the Invention

In accordance with the present invention, there is provided a cyclonic inlet device for a well stream separation system, the device comprising a cyclone separator having an elongate body into which production stream is introduced by way of a main inlet, and within which the production stream is caused to rotate such that the liquid mixture of the production stream migrates to the wall of the body, and the gas phase forms a gas core within the body, the body having a gas outlet adjacent one axial end thereof, a liquid mixture outlet adjacent the opposite axial end thereof and, intermediate said main inlet and said liquid mixture outlet, the body is provided with secondary inlet means for introducing additional liquid into the body.

Preferably said cyclone separator is provided, adjacent the liquid mixture outlet. with a gas blockage device for balancing pressure conditions within the cyclone separator, and said secondary inlet means opens into said body upstream of said gas blockage device.

Preferably said secondary inlet means opens into a region of said body which. in use, will be occupied by a low pressure region of the gas core whereby liquid can be drawn through said secondary inlet means into said body by virtue of said low pressure region.

Alternatively said secondary inlet means opens into a region of the body adjacent the wall of the body and which will be occupied in use by rotating liquid mixture whereby liquid from said secondary inlet means is drawn into said body, at least in part, by eduction.

Conveniently said secondary inlet means includes a control valve for effecting control over the flow of liquid through the secondary inlet means.

Conveniently the secondary inlet means is supplied with liquid by way of a pump.

Preferably the position of the secondary inlet is adjustable axially within said body.

The invention further resides in a gravity separator having an inlet device as defined above and wherein said secondary inlet means is supplied with liquid from a liquid layer of the gravity separator.

Conveniently the liquid from said layer is drawn through the secondary inlet means by the pressure difference between said layer and the gas core.

Alternatively the liquid from said layer is drawn through the secondary inlet means by eduction.

Alternatively, or in addition, a pump is utilised to supply liquid from said liquid layer of the gravity separator to said secondary inlet means. Preferably said liquid layer is a water layer.

The invention still further resides in a process for separating the phases of an oil well production stream comprising supplying production stream to a separation system by way of a cyclonic inlet device and supplying additional liquid to the cyclonic inlet device between a main inlet of the cyclonic inlet device and an underflow outlet of the device.

Preferably said additional liquid is derived from the subsequently separated water phase of the production stream.

Alternatively additional is supplied from a supply point or reservoir separate from the separation system.

Preferably said additional liquid is water.

Brief Description of the Drawings

In the accompanying drawings:Figure 1 is a diagrammatic representation of the cyclonic inlet device of a three-phase gravity separator in accordance with a first example of the present invention, and Figures 2, 3, 4, 5 and 6 are views similar to Figure 1 of five further examples of the present invention.

Detailed Description of Preferred Embodiments of the Invention

Turning first to Figure 1 of the accompanying drawings, there is illustrated a gravity separator including a conventional separator vessel 11 within which the oil well production stream separates, in known manner, into water, oil and gas layers 12, 13, 14 respectively. The vessel 11 is provided with appropriate outlets (not shown) for the separated oil, gas and water and it will be recognised by those skilled in the art that this is a very simplistic view of a gravity separator. It is however sufficient for an understanding of the present invention which relates to the cyclonic inlet device of a separation system, rather than to the structure of the gravity separator vessel and its outlets.

The oil well production stream is supplied to the vessel 11 through a cyclonic inlet device 15 which incorporates an inlet pipe 16 extending through the wall of the vessel 11 to a cyclone separator body 17 housed within the vessel 11. The body of the cyclone separator has a gas (overflow) outlet 18 at one axial end and a liquid mixture (underflow) outlet 19 at its opposite axial end. In a manner well understood by those ski 1 led in the art, the production stream entering the body 17 by way of the inlet pipe 16 is caused to rotate within the body 17 so that the denser phases, that is to say the oil and water phases as a mixture, migrate to the wall of the body 17 and an axially extending gas core is formed down the centre of the body.

Adjacent the underflow outlet 19 the body is provided internally with a gas blockage device 21 which produces a controlling effect balancing the pressure conditions existing at the underflow outlet 19 with the pressure conditions existing at the overflow outlet 18 over a range of inlet pressures at the inlet 16 of the cyclone separator, whereby the operation of the cyclone separator is tolerant of variations of inlet flow conditions. Furthermore, the back-pressure created by the gas blockage arrangement 21 ensures that undesired passage of gas through the underflow outlet 18 is substantially eliminated. Although the gas blockage device 21 is extremely desirable in many applications there may be occasions where it is unnecessary. Moreover the use of dollar plates and the like to impede the lower end of the gas core is not excluded.

Surrounding the underflow outlet end of the body 17 is an outlet duct arrangement 22 having outlet apertures 23 ensuring that the liquid mixture issuing from the underflow outlet 19 of the cyclone separator is discharged into the vessel 11 above the water layer 12. It will be recognised that there may be an emulsion layer formed at the interface of the oil and water layers 12, 13 and desirably the outlet apertures 23 open into the oil layer 13. It is to be recognised however that it is within the ambit of this invention to provide an adjustable outlet ducting 22 so that control over the position within the vessel 11 at which the liquid mixture is discharged into the vessel can be effected. For example, there may be circumstances in which it is desired to discharge the liquid mixture issuing from the underflow outlet 19 into the water layer 12. An adjustable outlet duct arrangement would facilitate setting up the cyclonic inlet device to produce such an effect.

It will be recognised that in addition to directing the liquid mixture to the desired region of the vessel 11, the ducting 22 also damps any swirl wh ich there may be in the 1 iquid mixture issu i ng from the outlet 19 so that there is generally a quiescent flow from the apertures 23.

The gas core formed within the mass of the liquid mixture rotating in the body 17 exits into the head space (the gas layer 14) of the vessel 11 by way of the overflow outlet 18.

A pipe 24 extends axially within the cyclone separator body 17 passing through the gas blockage device 21, the underflow outlet 19, and the duct 22 to open at its lower end in the water layer region of the vessel 11. The opposite end of the pipe 24 is closed, but has openings 25 in its side wall which communicate, in use, with a low pressure region of the gas core which forms along the axis of the cyclone separator. It will be recognised therefore that as a result of the pressure difference between the openings 25 and the opposite end of the pipe 24, liquid from the water layer 12 will be drawn upwardly through the pipe 24 and will be discharged into the body 17 by way of the apertures 25. Thus the pipe 24 and apertures 25 provide a secondary liquid inlet by way of which liquid enters the body 17.

It will be recognised that during operation of the cyclone separator of the cyclonic inlet device 15, the rotating liquid mixture migrating downwardly towards the underflow outlet 19 is subjected to significant shear forces. In many, but not all applications the liquid mixture will be a water-continuous mixture containing oil droplets and the shear forces imposed upon this mixture can break up the oil droplets thus hindering subsequent gravity separation of the liquid mixture into the oil and water layers 12, in the vessel 11. Introducing additional water by way of the pipe 24 increases the mass of liquid which is rotating, and thus damps rotation. This has the effect of dissipating the energy of rotation and thus reducing the shear forces imposed upon the liquid mixture, particularly at 8_ the point at which the liquid mixture passes between the tapering wall of the body 11 and the correspondingly tapering gas blockage device 21. It will be recognised of course that at this point in the process the bulk of gas wi.11 have been separated from the liquid mixture by virtue of the high rotational forces adjacent the inlet 16 of the cyclone separator 17 and thus the addition of water by way of the pipe 24 will not have any disadvantageous effect on gas separation. In other words, initially the high rotational forces are maintained to promote efficient degassing and foam elimination, and only thereafter is the rotation damped to minimise the effect of shear forces on the liquid mixture.

In addition, the provision of additional water, for example by way of the pipe 24, provides the ability to adjust the oillwater dispersion characteristics of the liquid mixture. It is recognised that a watercontinuous mixture is easier to separate and the addition of water minimises the risk of the liquid mixture becoming oil continuous for example should there be an oil slug in the production flow.

Figure 2 illustrates a modification of the arrangement shown in Figure 1 in which the pipe 24 extends, for most of its length, externally of the inlet device 15. In some applications, the use of an external pipe 24 may be preferable, particularly where there is insufficient space beneath the duct 24 and the bottom wall of the vessel 11 for the pipe 24 to protrude. Moreover, Figure 2 shows that it is not essential for the liquid to be extracted from the water layer immediately adjacent the inlet device 15. There may be advantages in extracting liquid from the water layer downstream of the device 15 where further gravity separation has occurred.

-g- The arrangements illustrated in Figures 1 and 2 rely upon the reduced pressure in the gas core to draw liquid into the body 15 through the pipe 24. In Figure 3, the pipe 24 is shown extending axially in the body 17 (as.in Figure 1) but the pipe 24 communicates with the interior of the body 17 by way of two or more radially extending pipes 26, the radially outermost ends of which terminate in the High rotational speed region of the liquid mixture layer between the axis of the body 17 and the inner wall of the body 17. Moreover, desirably the open ends of the pipes 26 are curved to follow the direction of rotation of liquid within the body 17 and liquid is drawn through the pipe 24 and pipes 26 by eduction, that it to say by the effect of the liquid flow over the open ends of the pipes 26. Whiletwoor more pipes 26 are suggested there may be applications in which a single pipe 26 will suffice.

Figure 4 illustrates a modification of the arrangement illustrated in Figure 3. In Figure 4, the pipe 24 is external to the inlet device 15 (as in Figure 2) but rather than passing through the wall of the body 15 and terminating in the gas core region, the pipe 24 opens into a circumferential gallery 27 encircling the body 17. Two or more inlet pipes 28 are equiangularly spaced around the inner wall of the body 17 and are curved to follow the direction of the rotation of the liquid adjacent the inner wall of the body 17. Each of the pipes opens into the body 17 in the high rotational speed region of the liquid mixture layer and, at its opposite end, communicates with the gallery 27. Thus as with the arrangement illustrated in Figure 3, liquid can be drawn through the pipes 28, the gallery 27 and the pipe 24 from the water layer 12 into the cyclone separator 17, by eduction. Again, while two or more pipes 28 are suggested there may be applications in which a single pipe 28 will suffice.

Figure 5 illustrates a modification of Figure 1 which can equally be applied to the examples illustrated in Figures 2, 3 and 4. In Figure 5 it can be seen that the pipe 24 is provided with a control valve 29 in order to allow regulation of the flow rate of liquid through the pipe 24 into the body 17. Although it is not essential to do so, Figure 5 shows that the pipe 24 may be led outside the wall of the vessel 11 so that the control valve 29 is positioned externally of the vessel. Naturally the control valve 29 can be remotely operated and desirably a flow rate monitor (not shown) will monitor the rate at which liquid is flowing along the pipe 24 in order to provide control for the valve 29. In addition, control for the valve 29 may be provided by a water-cut monitor which detects the oil/ water ratio of the production stream entering the device 15 by way of the pipe 16. If a higher than desired concentration of oil is detected then the valve 29 may be opened to permit a higher rate of flow of water through the pipe 24 thereby ensuring that the oillwater ratio in the liquid mixture in the body 17 and passing through the underflow outlet 19 is within a desired range in which the mixture is water-continuous.

Figure 6 illustrates the addition, to the arrangement of Figure 5, of a pump 31 of any convenient form. The pump may, for example, be a centrifugal pump or a positive displacement pump, and its provision gives further flexibility and control in relation to the flow rate of liquid along the pipe 24 into the body 15. For example, the provision of a pump permits a flow rate along the pipe 24 in excess of that which could be achieved simply by the suction effect of reduced pressure in the gas core or by eduction. Thus the provision of a pump enables sufficient water to be supplied, as needed, to ensure that the liquid mixture in the cyclone separator is always water-continuous. Moreover, the addition of water by way of the secondary inlet 24 broadens the operating window of the inlet device 15 prior to the onset of gas "breakthrough" into the liquid mixture issuing from the outlet 19. The provision of a pump would permit the possibility of gas "breakthrough' to be eliminated and this will be helpful in operating conditions where there may be severe gas slugging in the production stream.

It will be recognised that the pump 31 could be provided in each of the examples illustrated in Figures 1 to 4, with or without the provision of the control valve 29.

In all of the examples described above, the liquid added by way of the pipe 24 is derived from the water layer of the gravity separator, and thus is producedwater. It is to be understood however that the invention is not limited to recycling of produced water, and water or other liquid from a reservoir or other form of supply could be utilised in place of produced water from the water layer of a gravity separator. For example, where the cyclonic inlet device 15 is used in conjunction with some other form of separation system, there may not be easy access to the produced water for recycling and so a reservoir of fresh or sea water, treated if necessary to avoid corrosion and scaling, may be provided.

Irrespective of the means by which the liquid is introduced into the cyclone separator, and irrespective of whether or not the liquid is produced water, there can be benefits in providing an arrangement for adjusting the axial position at which the additional liquid is introduced. It is therefore within the scope of the invention to provide an arrangement in which the axial position, along the axis of the body 17, at which the additional liquid is introduced, is adjustable. For example, where the production stream can be said to be a Oheavy oil stream'y then there may be advantage in introducing the additional water somewhat closer to the inlet 16 so that a boundary Water layer is produced at the wall of the body 17 thereby reducing viscous drag which could otherwise reduce the degassing performance of the inlet device. Such an arrangement may be particularly advantageous if the production stream is an oil-continuous heavy oil stream.

It will be recognised that the invention is not limited to the particular arrangement of cyclone separator 17 illustrated in the drawings. For example, different arrangements of the inlet 16 could be provided. The inlet 16 illustrated in the drawings is intended to be a depiction of a conventional tangential flow inlet. It is to be recognised however that cyclone separators having vane-type inlets where vanes rather than tangential positioning of the inlet, generate rotation of the mixture. Furthermore., top inlet separators could also be used.

For convenience the invention has been described in relation to a gravity separator having an internal cyclonic inlet device. It is to be understood that the same principles can be applied to a gravity separator where the inlet device is external to the vessel 11. Moreover the invention is not restricted to gravity separation systems and the inlet devices described above could be used in other forms of separation system, for example those known as compact separation systems and other forms of separation system using one or more downstream cyclone separators to separate the phases of the liquid mixture.

The invention is primarily concerned with the introduction of water either to provide a drag reducing boundary layer in an oil continuous environment or to preserve a water continuous environment where appropriate. It will be recognised that the controlled addition of water can minimise the problems in separation which are known to occur at the change from oil continuous mixtures to water continuous, in that the addition of water can drive the mixture almost instantaneously through the point of change from oil continuous to water continuous.

There may be operating environments where it is beneficial to add oil rather than water, for example to maintain or achieve a particular water cut in an oil continuous mixture. Thus the pipe 24 can be arranged to extract liquid from the oil layer 13, and it would be possible to produce a switchable construction in which the pipe 24 can be coupled by way of remotely controllable, or externally positioned, valves, to extraction pipes in both the layer 12 and the layer 13, the valves being controlled by, for example, a water cut meter monitoring the composition of the production stream in the inlet pipe 16.

Claims

1. A cyclonic inlet device for a well production stream separation system, the device comprising a cyclone separator (15) having an elongated body (17) into which production stream is introduced by way of a main inlet (16), and within which the production stream is caused to rotate such that the liquid mixture of the production stream migrates to the wall of the body, and the gas phase forms a gas core within the body, the body having a gas outlet (18) adjacent one axial end thereof, a liquid mixture outlet (19) adjacent the opposite axial end thereof and being characterised in that intermediate said main inlet (16) and said liquid mixture outlet (19), the body (17) is provided with secondary inlet means (24? 25) for introducing additional liquid into the body (17).

2. A cyclonic inlet device as claimed in Claim 1 characterised in that said cyclone separator is provided, adjacent the liquid mixture outlet (19) with a gas blockage device (21) for balancing pressure conditions within the cyclone separator, and said secondary inlet means (24, 25) opens into said body (17) upstream of said gas blockage device (21).

3. A cyclonic inlet device as claimed in Claim 1 or Claim 2 characterised in that said secondary inlet means (24, 25) opens into a region of said body (17) which, in use, will be occupied by a low pressure region of the gas core whereby liquid can be drawn through said secondary inlet means (24, 25) into said body (17) by virtue of said low pressure region.

4. A cyclonic inlet device as claimed in Claim 1 or Claim 2 characterised in that said secondary inlet means (24, 25) opens into a region of the body (17) adjacent the wall of the body and which will be occupied in use by rotating liquid mixture whereby in use liquid from said secondary inlet means (24, 25) will be drawn into said body (17), at least in part, by eduction.

5. A cyclonic inlet device as claimed in any one of the preceding claims characterised in that said secondary inlet means includes a control valve (29) for effecting control over the flow of liquid through the secondary inlet means.

6. A cyclonic inlet device as claimed in any one of the preceding claims characterised in that the secondary inlet means (24, 25) is supplied with liquid by way of a pump (31).

7. A cyclonic inlet device as claimed in any one of Claims 1 to 6 characterised in that the position of said secondary inlet (24, 25) is adjustable annually within said body (17).

8. A gravity separator characterised by an inlet device as claimed in any one of the preceding claims, the secondary inlet means of the inlet device obtaining liquid from a liquid layer of the gravity separator.

9.. A gravity separator as claimed in Claim 8 characterised in that said liquid from the liquid layer is drawn through the secondary inlet means by the pressure difference between the layer and the gas core of the inlet device.

10. A gravity separator as claimed in Claim 8 characterised in that said liquid from the liquid layer is drawn through the secondary inlet means by eduction.

11. A gravity separator as claimed in any one of Claims 8 to 10 characterised in that a pump is utilised to pump liquid from said layer of the gravity separator to said secondary inlet means.

12. A gravity separator as claimed in any one of Claims 8 to 11 characterised in that said liquid layer is a water layer.

13. A gravity separator as claimed in any one of Claims 8 to 11 characterised by control means determining the location within the gravity separator vessel from which said liquid is obtained.

14. A gravity separator characterised by an inlet device as claimed in any one of Claims 1 to 7, said secondary inlet means obtaining liquid from a supply distinct from the separated liquid layers in the gravity separator vessel.

15. A process for separating the phases of an oil well production stream comprising supplying the production stream to a separation system by way of a cyclonic inlet device and characterised by supplying additional liquid to the cyclonic inlet device between a main inlet and an underflow outlet of the cyclonic inlet device.

16. A process as claimed in Claim 15 characterised in that said additional liquid is derived from a subsequently separated phase of the production stream.

17. A process as claimed in Claim 15 characterised in that said additional liquid is supplied from a supply point or reservoir separate from the separation system.

18. A process as claimed in any one of Claims 15 to 17 characterised in that said additional liquid is water.