WO2005094962A1

WO2005094962A1 - Separation system

Info

Publication number: WO2005094962A1
Application number: PCT/IB2005/001166
Authority: WO
Inventors: Davoud Tayebi
Original assignee: Kvaerner Process Systems A.S.
Priority date: 2004-04-02
Filing date: 2005-04-04
Publication date: 2005-10-13
Also published as: GB0407565D0

Abstract

The invention relates to a system (10) for separating solids from a multiphase fluid stream (11). The system comprises first separator means (12) for separating gas from the fluid stream to provide a degassed fluid stream (24). Second separator means (28) separates solids from the degassed fluid stream. The first separator means includes a compact cyclonic degasser (12).

Description

Separation System

The present invention relates to a system for separating solids from a multiphase fluid stream.

In oil and gas production, fluids are extracted from a well. These fluids, termed a wellstream, may contain gases, liquids and solids in varying proportions. The solids typically comprise sand, but may also include other components such as salt or scale particles. However, in order to reduce wear, erosion or even blockage of valves or other process equipment separation of the solids is usually required as a first step. Also, the presence of solids such as sand can adversely affect the performance of the gas-liquid separation process. Solids can simply settle down in the separators reducing the available separator volume. In some cases, such as re-injection of produced water back into a reservoir, solids must be separated before any injection can take place. Otherwise, the solids may block pores and destroy the permeability of the reservoir formation.

It is known to use cyclone desanders as an effective way of removing solids from a multiphase flow. Examples include vessel style cyclones and liner-style cyclones.

One problem affecting all known cyclonic types of equipment for separating solids is that the wellstream flow may vary and fluctuate between different regimes such as stratified flow, annular flow, plug flow or slug flow. Slug flow is especially problematic because the liquid and gas volume fractions change dramatically. At one moment the flow may be almost fully gas and the next moment almost fully liquid. When using a cyclone desander, for example, gas flow creates a much higher spin in the cyclone resulting in a given separation performance. With a slug flow, at one moment the cyclone experiences a high gas velocity causing a high degree of spin, and the next moment when a liquid slug enters the cyclone the liquid flow causes spin to be lost/dramatically reduced with a resulting loss of separation efficiency.

This means that it is very difficult to design equipment which can operate effectively at all times, unless the equipment includes large diameter vessels, or a complicated process is used involving many process stages. Large diameter vessels not only occupy more space (which may be at a premium, for example on off-shore production platforms), but because these processes operate at high pressure, they need to be large diameter pressure vessels, which are bulky and very expensive pieces of equipment.

It is an aim of the present invention to provide a separation system that substantially alleviates the aforementioned problems.

According to the present invention there is provided a system for separating solids from a multiphase fluid stream, the system comprising: first separator means for separating gas from the fluid stream to provide a degassed fluid stream; and second separator means for separating solids from the degassed fluid stream, wherein the first separator means includes a compact cyclonic degasser.

The term compact, as used herein, may be understood in terms of the internal volume of the first separator means. A time parameter can be defined as the total internal volume of a separator vessel divided by the total combined (gas, liquid and solids) flow rate through the separator. The smaller the time parameter is, the more compact is the unit. For a known gravity separator this time parameter may be several minutes, typically between 3 and 5 minutes. For the compact cyclonic degasser of first separator means of the present invention, the time parameter should be less than 2 minutes and is preferably 30 seconds or less.

Preferably, the first separator means comprises slug-catching means. More preferably, the slug-catching means comprises a lower region of a vessel of the compact cyclonic degasser.

It is an advantage that, by separating the gas phase from the fluid stream, the solids remain in the fluid entering the second separator means. This fluid is predominantly a liquid phase containing solid particles, and so the second separator means can be designed for optimal removal of solids from the liquid. It is a further advantage that by providing a slug catcher in the first separator means, efficient separation of the gas phase from the liquid phase can be achieved before the liquid slug enters the flow to the second separator means. The slug catcher also ensures that a continuous stable liquid flow is provided to the second separator means, thereby ensuring stable operation for the removal of solids.

In a preferred embodiment, the first separator means further comprises a liquid level control system for the slug catcher ensuring a continuous stable liquid flow to the second separator means, thereby ensuring stable operation for the removal of solids.

The first separator means may further comprises a scrubber element and/or an axial flow cyclone for further separation of liquid from gas separated from the fluid stream.

In a preferred embodiment, the second separator means is a liner-style desander.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a solids separation system according to the invention;

Figure 2 is a typical example of a process line diagram of a wellstream separation process incorporating the solids separation system of Figure 1 and an optional sand washing or cleaning system; and

Figure 3 is a graph comparing the operational envelope of a conventional wellstream desanding system, and the system of Figure 2.

Referring first to Figure 2, a wellstream separation process includes a separator 50 for separating gas, oil and water from wellstream fluids using known separation methods. The separator is provided with an inlet 52 for receiving the incoming wellstream fluids, and separate outlets for gas 54, oil 56 and water 58. The equipment used for such separation methods is susceptible to the presence of solids, in particular sand particles, carried by the wellstream fluids. These solid particles may adversely affect the efficiency of the separation process, or damage the equipment. Solids can simply settle down in the separators reducing the available separator volume. It is therefore a requirement to remove the solids particles from the wellstream fluids.

In the process shown in Figure 2, sand particles (and other solids) are removed using a separation system as described in more detail below in relation to Figure 1. An unprocessed wellstream 11 enters a compact cyclonic degasser 12, which forms part of a solids separation system 10. The liquid phase leaves the system 10 by way of a liquid outlet 34 into a feed line 60 to the separator 50.

Referring to Figure 1, the separation system 10 for removing solids from the wellstream 11 includes a first separator in the form of a compact cyclonic degasser 12. The compact cyclonic degasser 12 has a vessel 16 with an inlet 14 for receiving the fluid flow of the wellstream 11. Gas is separated from the wellstream fluids in a manner that will be explained in more detail below. The compact cyclonic degasser 12 has an upper gas outlet 22 and a lower outlet 23 for the degassed fluid.

The lower outlet 23 feeds into a connecting pipe 24, which leads to an inlet 26 of a second separator in the form of a desander 28. The desander 28 comprises a vessel 30, which is preferably filled with a plurality of small diameter liner style cyclones 32. This style of desander is preferred because the small diameter cyclones provide a high fluid acceleration, resulting in efficient separation of solid particles. The desander 28 has a liquid outlet 34 and a solids outlet 36. If required, as illustrated in Figures 1 and 2 the solids outlet can lead, via a first valve 38, to a solids accumulation tank 40. In other cases the solids may be deposited directly into another vessel, bags, containers or other arrangements for solids discharge. The disposal or discharge system may be a continuously operated system. In some other cases such as subsea processing systems for produced water cleaning before re-injection of the water into the reservoir, the separated solids may be re-introduced into the oil flow line again (for example in the oil flow from the oil outlet 56 of the separator 50 shown in Figure 2) if there is no other discharge alternative available. The solids accumulation tank 40 in Figure 2 has an outlet 42, which is provided with a second valve 44 to enable accumulated solids to be discharged at intervals. The wellstream fluids entering the inlet 14 of the compact cyclonic degasser 12 pass into an inlet chamber 15. Below the inlet chamber 15 is a swirl chamber 17 in which a swirl velocity is imparted to the fluids as they enter a cyclone section 20. The fluids are thus spun in a cyclonic flow. This causes the liquid and gas phases to be separated such that the cyclonic and gravitational forces acting on the liquid cause it to move radially outwardly and then down the wall of the cyclone 20 towards a lower region 20a. The solids are therefore either carried in suspension in the liquid down to the lower region 20a, or are spun out of the gas by the cyclonic motion to merge into the liquid phase near the wall of the cyclone section 20 from where they are carried down to the lower region 20a. The main bulk of the separated liquid flow, including solids, leaves the cyclone section 20 through an outlet in the lower region 20a. The liquid (and solids therein) leaves the vessel 16 through the outlet 23. Therefore substantially all of the solid phase is carried through the degasser 12 by the liquid phase.

The separated gas phase enters a vortex finder 18 which passes vertically upwards through the inlet chamber 15. A blockage device (not shown) may be included to prevent the gas from entering back into the liquid in the lower region 20a of the cyclone section 20. This ensures an efficient separation of gas from the fluid. The gas travels up to the top of the compact cyclonic degasser 12 where a device 19 in the form of a scrubber and/or an axial flow cyclone may be provided to remove liquid droplets which may be carried by the gas. This liquid drops back down through an annular space 19a between the inlet chamber and the vessel 16 to a bottom region 21 of the vessel 16.

The compact cyclonic degasser 12 is so described because the volume of the vessel 16 is much smaller than other known separators. The high acceleration due to the swirl in the cyclone improves separation efficiency and makes the volume compact. The volume of a separator may be characterised by a time parameter defined as the total internal volume of the separator vessel 16 divided by the actual combined gas, liquid, solids (and other) flow rate through the separation process. The smaller the time parameter, the more compact the unit. For the compact cyclonic degasser 12 the time parameter may be 30 seconds or less. By comparison, for a gravity separator this time constant may be several minutes, typically between 3 and 5 minutes. The wellstream 11 is made up of a combination of gas, solid and liquid phases. The proportions of each phase, and the nature of the flow regime in the wellstream 11 are variable and may include slug flow in which there are slugs of predominantly liquid phase flow interspersed with regions of a predominantly gas phase flow.

When a liquid slug enters the inlet 14, the cyclonic motion may not be sufficient to create a significant spin velocity in the cyclone section 20. In this case the un- separated fraction of the liquid slug will simply fall down the vessel into the lower region 20a. Normally, the spin in the cyclone 20 separates gas and liquid (with solids therein).

The lower region 20a of the cyclone section 20, and the bottom region 21 of the vessel 16 are inter-connected and together act as an accumulator volume for accumulated liquid (with solids therein). Provided that sufficient liquid has accumulated in this volume, a steady flow can be maintained in the connecting pipe 24 to the desander 28, such that the effect of the slug flow in the wellstream 11 has been substantially removed. The compact cyclonic degasser 12 therefore acts as a slug catcher. The liquid in the accumulator volume has a level 21a, which may be controlled in a manner that will be described in more detail hereafter.

However, a compact cyclonic degasser where no accumulator volume is present may be used. In that case there may be no internal cyclone section 20 inside the vessel 16. Instead the swirl chamber 17 sets the fluid in rotation inside vessel 16: This causes liquid and gas phases to be separated such that the cyclonic and gravitational forces acting on the liquid (with solids therein) cause it to move radially outwardly and then down the wall of the vessel 16 towards the bottom region 21. The liquid (with solids therein) leaves the cyclonic degasser 12 through the outlet 23. The separated gas phase returns back up through the vortex finder 18. The gas leaves the cyclonic degasser through outlet 22.

The fluid in the connecting pipe 24 enters the desander inlet 26, and the solids particles are separated from the liquid by the flow through the cyclones 32, in a known manner. The liquid phase leaves the desander by way of the liquid outlet 34, while the solids particles leave through the solids outlet 36 (in this example) to accumulate in the solids accumulation tank 40. The accumulated solids can be removed from the solids accumulation tank 40, through the outlet 42, for example by closing the first valve 38 and opening the second valve 44.

A liquid level control system may be included in the compact cyclonic degasser 12 to ensure a continuous and stable liquid flow to the second separator (desander 28), thereby ensuring stable operation for the removal of solids. The liquid level control system employs a device that manipulates pressure and flow, for example a control valve, a pump, an eductor, an orifice, choke valve or other means.

To cope with large variations in the flow (as during slugging conditions), a control means 70 downstream of the outlet 22 of the compact cyclonic degasser 12 (see Figure 2) is used to balance the pressure drops in the gas flow against the pressure drop through the connecting pipe 24, desander 28 and feed line 60. Dependent on the amplitudes of fluctuations the control means 70 may be either a) an orifice if the fluctuations are very small and the correct balance pressure drop is known, or b) a control valve when active adjustment of the level is needed during operation.

The control routine monitors the level in the compact cyclonic degasser 12, which will vary dependent on the pressure balance in the downstream lines (that is again dependent on the incoming flow rate and composition). The level control routine opens or chokes the control valve 70 in order to keep the level constant.

When the compact cyclonic degasser 12 is acting as a slug catcher a soft sensing level control routine may be used that allows the level to fluctuate within high and low limits. The high and low limits have a range smaller than the minimum/maximum operational level range. If the level is within the high and low limits, it is allowed to fluctuate. This may be achieved by decreasing the gain of the level control loop. If the level is outside the high and low limits the level controller routine increases the gain and the level is quickly returned to the set point. Other means of tuning the gain include use of integration and derivative control parameters to further optimise the control. Feed forward is another option in order to enhance the control where a monitoring device is used upstream in the wellstream line 11.

Because the pressure drops are dependent of the incoming « flow, a control valve in the wellstream 11 upstream of the compact cyclonic degasser 12 may also be used to control the flow rate in order to manipulate the level.

The level control system may be different depending on where the gas outlet 22 is directed. If this flow is recombined with the liquid flow from the desander outlet 34, or with the liquid flow at some other point downstream, the control system that balances the pressure drops as described above is preferred.

However, if the flows from the gas outlet 22 and liquid outlet 34 are not recombined, but are directed to two independent locations, other control options may be better suited. For example, the gas flow may be directed to downstream scrubbers and compressors (not shown in the Figures) and the liquid flow to the separator 50 in Figure 2. In this case a control valve downstream of the outlet 22 may be used to maintain the pressure in the compact cyclonic degasser 12. The level may be controlled by any valve, pump or other means to influence pressure and flow either in the connector pipe 24 or the feed line 60.

Alternatively, the control valve downstream of the outlet 22 may be used to control the level.

The flow rate will normally be controlled by the controller routines already acting on the downstream separator 50 or other components located downstream in the feed line 60. Another option is upstream flow control of the flow through the system by a control valve located upstream of the compact cyclonic degasser 12 in the wellstream 11.

The system may also be configured to balance the flows between two or more parallel separation processes. The flow split between the parallel units may be adjusted by control valves, either upstream or downstream of the solids separation. For slug flow conditions a dimensionless parameter may be defined as the volume of the gas/liquid separator divided by the volume of the liquid, gas, solids(or other) during one slug period. If this dimensionless parameter is 1, the gas/liquid separator vessel size is equal to the volume of gas, liquids, solids (or other) during one slug period. If this dimensionless parameter is less than 1 the slug volume is larger than the vessel.

For a traditional gravity separator this dimensionless parameter is typically at least 5 to 10 (or more). This is because the slug should be contained within certain levels that represent a fraction of the separator vessel volume, for example 10%-20 , to ensure that effective separation is maintained.

For the compact cyclonic degasser 12 the dimensionless parameter does not need to be larger than 1. The slug volume may be larger than the vessel volume, without significantly affecting separation performance.

Referring again to the process shown in Figure 2, as an optional feature, the accumulation tank 40 forms part of a sand washing system. This is used to recover any wellstream liquid phase fluids that have not been separated from the solids phase in the desander 28. A source of fluidisation water 62 is supplied to the solids accumulation tank 40 to wash the solids. A stream of a liquid phase (mainly comprised of the fluidisation water) is taken from an outlet 64 of the accumulation tank and fed to a further desander 66, for example using an eductor or a slurry pump. The liquid phase stream will carry solids particles with it, and these are separated in the further desander 66. Separated liquid from the further desander 66 is fed back into the feed line 60, while the eparated solids are fed back into the solids accumulation tank 40.

Also, in the process shown in Figure 2, the source of fluidisation water 62 provides water to assist in flushing the accumulated liquid and solids from the lower region 21 of the compact cyclonic degasser 12 through to the desander 28, and to assist in flushing the solids from the desander 28 through to the solids accumulation tank 40. In the example shown in Figure 2 gas separated from the wellstream fluids in the compact cyclonic degasser 12 leaves via the gas outlet 22 into the feed line 60, to be recombined with the liquid phase from the desander 28 and the further desander 66. The combined gas and liquid phase fluids are then fed to the separation process 50.

Alternatively (not shown in figure 2) the gas leaving via the gas outlet 22 may be taken to separate gas phase processing, or may be fed directly into the separator 50.

Referring to Figure 3, an operational envelope for the solids separation system of Figure 1 is shown in comparison to that for a conventional separation system. The horizontal axis L of Figure 3 shows the liquid flow as a proportion of the maximum liquid flow (100%) that the system can handle. The vertical axis G shows the gas flow as a proportion of the maximum gas flow (100%) that the system can handle.

In a conventional system, solids removal is only effective at gas and liquid flows above a level of 50% or more of the maximum values, as shown by the region 100 in Figure 3. At lower gas flows there is insufficient gas velocity to generate the required separation forces. At lower liquid flows slug flow conditions prevail which cause a loss of separation performance.

These problems do not occur in the system 10 of Figure 1. Firstly, the system can operate at any proportion of the gas flow because the gas phase is not present during the solids removal stage. Secondly, the slug-catching capabilities of the compact cyclonic degasser 12 allow operation of the solids separation system even when there is slug flow. This means that the operational envelope is much greater, as shown by the region 200 in Figure 3.

The greater operational envelope means that it is possible to design a system which is capable of separating solids from wellstream fluids for a widely fluctuating gas and liquid proportions and varying flow regimes. This allows a much smaller system to be constructed with smaller pressure vessels and resulting savings on space, cost and energy usage.

Claims

1. A system for separating solids from a multiphase fluid stream, the system comprising: first separator means for separating gas from the fluid stream to provide a degassed fluid stream; and second separator means for separating solids from the degassed fluid stream, wherein the first separator means includes a compact cyclonic degasser.

2. The system of claim 1 wherein the first separator means comprises slug- catching means.

3. The system of claim 2 wherein the compact cyclonic degasser comprises a vessel having a cyclone section and the slug-catching means comprises a lower region of the vessel.

4. The system of claim 3 wherein the cyclone section and the lower region together define an accumulator volume for accumulating liquid.

5. The system of any one of claims 2 to 4 wherein the first separator means further comprises a liquid level control means for the slug catching means, ensuring a continuous stable liquid flow to the second separator means, thereby ensuring stable operation for the removal of solids.

6. The system of claim 5 wherein the control means is configured to balance pressure drops in the separated gas flow and the liquid flow from the system.

7. The system of claim 5 or claim 6 wherein the liquid level control means includes a soft sensing level control routine whereby the liquid level is allowed to fluctuate between high and low level limits.

8. The system of any preceding claim wherein the first separator means further comprises a scrubber element and/or an axial flow cyclone for further separation of liquid from gas separated from the fluid stream.

9. The system of any preceding claim wherein the second separator means is a liner-style desander.

10. The system of claim 1, wherein the compact cyclonic degasser has an internal volume characterised by a time parameter of less than 2 minutes, the time parameter being defined as the total internal volume of the compact cyclonic degasser divided by the total combined (gas, liquid and solids) flow rate through the first separator means.

11. The system of claim 2 wherein the time parameter is 30 seconds or less.