WO2015093935A1

WO2015093935A1 - Multiphase sand separation system

Info

Publication number: WO2015093935A1
Application number: PCT/MY2014/050016
Authority: WO
Inventors: Arul JOTHY; Rashid SERAKAWI; Shahrul Azman B Zainal Abidin; Chin Kiat SEE
Original assignee: Ngltech Sdn. Bhd.; Petroliam Nasional Berhad (Petronas)
Priority date: 2013-12-18
Filing date: 2014-12-16
Publication date: 2015-06-25
Also published as: AU2014367387A1; AU2014367387A2; AU2014101586A4

Abstract

A multiphase sand separation system comprising an expanded pipe for flow stabilisation, with an inlet for receiving multiphase material, a first outlet for gas and a second outlet for liquid and/or solids; and a cyclonic desander including an inlet for receiving material from the second outlet of the expanded pipe, a first outlet for liquid and a second outlet for solids; characterised in that a pressure controller valve is provided to maintain a pressure difference between the first outlet of the expanded pipe and the first outlet of the cyclonic desander, thereby ensuring that the differential pressure across the cyclonic desander is maintained at a fixed preset pressure drop, irrespective of the flow regime, flow and pressure surges and the flow rates, ensuring that it continuously and consistently operates at its highest sand removal efficiency value.

Description

MULTIPHASE SAND SEPARATION SYSTEM Field of Invention

The invention relates to a system for separating sand from a multiphase fluid, in particular, but not necessarily limited to, oil and gas.

Background

Solids are undesirable in oil and gas production systems as solids in the production stream can lead to wear, poor performance and blockages. It is challenging to remove solids from the production stream with high efficiency, at a low pressure drop, while ensuring small particle removal.

Solids are not always expected but can occur for a variety of reasons, such as an increase in the gas-oil ratio (e.g. due to gas injection), low-pressure production, increased water production, well clean up or well/sand collapse. Production of sand from oil and gas wells causes a number of problems for the operators of the wells, including:

• wear and erosion at the fittings, choke valves and flowlines at the wellhead area;

· sand accumulation and erosion at the pipelines resulting in flow restriction, corrosion and damage of pipelines, requiring other maintenance work such as pigging to clean the lines; • wear and tear of pumps, compressors, piping and hydrocyclones at the processing facilities;

• build-up of sand in gravity separators, affecting their performance and demanding regular maintenance;

· blockages of instrument nozzles and a negative impact on separation equipment such as the clogging of filters, blocking of hydrocyclones and filling up of separators;

• high cost to upgrade materials exposed to sand in order to minimize erosion; and/or

· restricted production from wells that produce excessive sand, thus causing a loss of revenue;

Hence the operators of oil and gas fields require reliable systems that can remove sand effectively upstream of the process and production system to eliminate the above listed problems.

There are a number of sand separation systems available in the market to remove sand. Most of these are cyclonic-separation systems and include reverse flow hydrocyclones, which generate high g-forces to separate sand from the produced gas and liquid phases. Some of the features of the known cyclonic sand separation systems affect the efficiency of sand separation significantly.

Reverse flow hydrocyclones are well known in the industry and are used for performing a variety of phase separation duties. They have a steep inverted cone shape with a tangential entry vent. The tangential entry vent causes spinning of the fluid mixture and generates high g-forces. The design of the hydrocyclones is such that the low and high density phases are separated as a result of the spinning action and the generation of high g-forces. The low density phase is forced upwards and exits the hydrocyclone through a vortex finder located in the top section of the unit, and the high density phase exits from the bottom of the unit. The two phases (low and high density) therefore move in opposite directions and it is for this reason that hydrocyclones are referred to as reverse flow cyclones.

It is known to use hydrocyclones (or termed henceforth as cyclonic desanders) as an effective way of removing solids from a multiphase flow. Whilst cyclonic desanders are very efficient for separation of sand from a multiphase fluid, they have a very limited operating envelope and will significantly lose their sand removal efficiency when operating outside this envelope.

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. The main characteristics of these flow regimes are that the instantaneous flow rate of each phase flowing through any part of the pipeline varies significantly. These flow fluctuations seriously affect the performance of cyclonic desanders and reduce their efficiency. 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 cyclonic 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.

A further problem associated with most cyclonic sand separation systems is that as the separation of sand from the mixture of liquid, gas and sand is carried out in a single step, the liquids and gases are forced to flow upwards and exit from the top outlet of the hydrocyclone, while only the sand flows downwards into a collection vessel located below the hydrocyclone. The reverse flow of the liquid phase against the downwards flowing sand results in a significant quantity of sand being carried up with the liquid phase instead of being deposited in the collection vessel. Changes in flow regime can add to this effect. The carryover of sand prevents complete separation of the sand and thus the problems associated with sand production are neither eliminated nor even significantly reduced.

Coupled with the above is the relative high pressure drop associated with desanding of multiphase production fluid. The pressure drop is relative high across the cyclonic desander, typically ranges from 2 to 5 bar for multiphase fluids. In addition, due to the fluctuating flow regimes experienced in pipes handling multiphase fluids, the pressure drop across the cyclonic desander also fluctuates resulting in erratic flow and fluctuating pressures at the downstream system. This can potentially result in downstream system process upsets. The limited operating envelope of cyclonic desanders also limits the turndown capability of these units (i.e. operational capability when the flow rate is slower). Typically cyclonic desander have a turndown limit ranging from 30% to 50% of the design flowrate. Apart from limiting the normal turndown flow of the system, this will also result in loss of performance due to transient flow fluctuations that are inherent in multiphase flow conditions.

Slugging is a challenging problem in most conventional crude oil production receiving facilities particularly those receiving production from multi-phase subsea pipelines as they are susceptible to slugs. This is particularly the case for production facilities receiving crude and associated gas from remote wells via pipelines and risers. Slugs generated in the pipelines and risers not only require processing facilities to be increased in size to accommodate the slugs, but also results in production upsets associated with the high speed at which the slugs arrive and the transient pressure fluctuations due to the surge of gas following the arrival of the slug. Under these scenarios the production facilities, like inlet heat exchangers, production separators and downstream gas compressors, will generally not be able to cope with this transient slugging phenomenon resulting in production upsets and possibly shutdown. For cyclonic desanders these large transient flow surges will render the system ineffective, will introduce huge pressure drops and may even impact the integrity of the equipment when large slugs with associated pressure surges arrive at the unit. For this reason, cyclonic desanders are seldom installed upstream of the production unit that receives multiphase fluids from subsea pipelines. 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.

An aim of the invention is to address at least some of the above issues and provide a sand separation system which can separate sand more effectively than known systems under diverse conditions.

Summary of Invention

In an aspect of the invention, there is provided a multiphase sand separation system comprising:

a flow stabilisation section including an expanded pipe with an inlet for receiving multiphase material, a first outlet for gas and a second outlet for liquid and/or solids; and

a sand separation section including an inlet for receiving material from the second outlet of the flow stabilisation section, a first outlet for liquid and a second outlet for solids;

characterised in that a pressure controller section is provided to maintain a pressure difference between the first outlet of the flow stabilisation section and the first outlet of the sand separation section. In one embodiment the multiphase material comprises oil and/or gas from a wellstream, typically comprising gas, liquid and solids.

In one embodiment the flow stabilisation section comprises an expanded pipe, which is typically oriented in an inclined or vertical position. Typically the first outlet thereof is located at the upper end of the pipe, and the second outlet is located at the lower end of the pipe.

In one embodiment the expanded pipe has a minimum diameter such that the flow regime therein is stratified. Typically the pipe is sized so as to accommodate the highest anticipated incoming slug volume.

In one embodiment the sand separation section comprises at least one cyclonic desander. Typically the first outlet thereof is located at the upper end of the cyclonic desander, and the second outlet is located at the lower end of the cyclonic desander.

In one embodiment the cyclonic desander is provided with a sand boot for receiving sand. Typically the sand boot is connected to an accumulator which receives sand and can be isolated from the cyclonic desander and sand boot for sand disposal.

In one embodiment the cyclonic desander and sand boot are integral with the lower section of the pipe in the flow stabilisation section. In one embodiment the pressure controller section comprises a control valve and a differential pressure controller for maintaining a constant differential pressure thereacross. In one embodiment the differential pressure controller allows the differential pressure to be selectable as a preset pressure drop.

Gas is routed from the first outlet of the expanded pipe in the flow stabilisation section, via the control valve, to the first outlet of the sand separation section, where it comingles with liquid exiting the first outlet of the cyclonic desander. This configuration ensures that the differential pressure between the inlet and the first outlet of the cyclonic desander is maintained at a fixed preset pressure drop, irrespective of the flow regime, flow and pressure surges and the flow rates, ensuring that it continuously and consistently operates at its highest sand removal efficiency value. In one embodiment the preset pressure drop is selectable in the range of about 50kPa to about 300kPa.

In one embodiment the expanded pipe is provided with a level transmitter for determining the level of liquid in the expanded pipe. Typically the level transmitter is connected to a level controller which determines the preset value for the differential pressure controller. This provides a control loop for controlling the liquid level in the expanded pipe, especially during slugging whereby incoming slugs could potentially fill the entire volume of the expanded pipe. Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figure 1 is a block diagram of a multiphase sand separation system according to an embodiment of the invention

Figure 2 is a schematic view of a multiphase sand separation system according to an embodiment of the invention (a) with an inclined expanded pipe; (b) with a vertical expanded pipe.

Figure 3 is a schematic view of a multiphase sand separation system according to a further embodiment of the invention

Figure 4 is a schematic view of a multiphase sand separation system according to a yet further embodiment of the invention

Figure 5 is a schematic view of a multiphase sand separation system according to a still further embodiment of the invention Detailed Description

With reference to Figure 1, there is illustrated a multiphase sand separation system 2 comprising a flow stabilisation section 4, a sand separation section 12, and a differential pressure controller section 20.

The flow stabilisation section 4 includes an inlet 6 for receiving the flow of full wellstream material 26, a first outlet 8 for gas, and a second outlet 10 for liquid and/or solids. Liquid slugs are accommodated in this section, and transient pressure surges are suppressed. Gas is at least partially separated from the multiphase fluid to leave a second mixture of sand and liquid with possibly some entrained gas.

The sand separation section 12 includes an inlet 14 for receiving material from the second outlet of the flow stabilisation section 4, a first outlet 16 for liquid and a second outlet 18 for solids. The second outlet is connected to a sand accumulator 22 for receiving the separated sand. Typically the sand separation section comprises a cyclonic desander where sand from its second outlet 18 is routed to the sand accumulator 22 and sand is periodically withdrawn therefrom using a valving arrangement. Liquid from the first outlet of the cyclonic desander comingles with the gas stream from the first outlet 8 of the flow stabilisation section 4, resulting in steady sand-free FWS fluid 24.

The pressure controller section 20 is connected between the first outlet 8 of the flow stabilisation section 4 and the first outlet 16 of the sand separation section 12. Separated gas (which may contain some entrained liquids) from the first outlet 8 of the flow stabilisation section 4 is routed via the differential pressure controller and comingles with the fluid from the first outlet from the sand separation section. The controller section maintains a selected pressure drop between the first outlet of the flow stabilisation section and the first outlet of the sand separation section, irrespective of the inlet fluid flow rate, conditions or flow regime.

With reference to Figure 2a, a particular embodiment of the invention is illustrated wherein full wellstream material 26 is first routed to the flow stabilisation section 4, which comprises an Expanded Inclined Pipe (termed EIP henceforth) with a gas outlet 8 at the high point and liquid outlet 10 at the low point. ,

The EIP has a minimum pipe diameter such that the flow regime in the EIP is stratified, and is sized so as to accommodate the highest anticipated incoming slug volume.

The incoming fluid which comprises of gas and liquids with sand (termed multiphase fluid) undergoes coarse gas/liquid separation at the EIP by ensuring that the flow regime is stratified with the denser phase (i.e. liquids with solids) flowing down to the bottom of the EIP and the lighter phase (i.e. gas) flowing up through the top of the EIP.

Therefore as sand is denser than the liquid phase, the sand particles will predominantly be entrained with the liquid phase at the bottom of the EIP. In addition, as the liquids within the EIP may not have sufficient residence time for complete degassing, it is expected that some gas will also be entrained with the liquid stream. Conversely, as the EIP is only sized for coarse gas liquid separation, some liquid droplets are expected to be entrained with the gas stream. The liquid stream with sand and entrained gas from the outlet 10 at the bottom of the EIP is then routed to the sand separation system 12.

The sand separation system 12 comprises a Cyclonic Desander Device 30 (henceforth termed CDD). The CDD may be a conventional single or multiple cyclonic device(s) used for separation of sand from a gas and/or liquid stream. Associated with the CDD is the sand boot 32 and sand accumulator 22 for collection of sand separated from the CDD. A valving arrangement 34 is provided to allow isolation of the sand accumulator 22 from the sand boot and CDD for a sand disposal operation .

The gas stream exits the high point of the EIP via outlet 8 and is routed to a control valve 36 in the pressure controller section 20. A differential pressure controller 38 throttles the control valve 36 to maintain a constant differential pressure across the valve 36. Downstream of the control valve 36, the gas co-mingles with liquid that exits the first outlet of the CDD 30.

This configuration ensures that the differential pressure across the CDD 30 is maintained fixed, irrespective of the flow regime, flow and pressure surges and the flow rates of the inlet 26 of the flow stabiliser section 4. The set point of the differential pressure controller is selected to provide the pressure drop required across the selected CDD to achieve the desired sand separation at the design fluid flowrates. Typically the pressure drop will be in the range of 50kPa to 300kPa depending on the design specifications of the CDD.

The following is the mechanism on how the system operates:

1. Under design flow conditions, gas and liquid will be separated at the flow stabiliser section and the liquids with sand will flow through the CDD. The differential pressure (and consequently the fluid mass flow) across the CDD is maintained at a specific set point (via the pressure controller section 20) such that the liquid level in the EIP is maintained at a fixed level near the bottom of the EIP.

2. When a design slug of liquid volume arrives, the liquids will fill the EIP (with a small gas cap). Liquid draw-off rate through the CDD will only increase marginally

(due to the incremental static head of the slug volume in the EIP). This ensures that the flow through the CDD remains approximately constant at its optimal design rate even when slugging occurs. 3. Under liquid turndown rates, as the differential pressure across the CDD is maintained constant, the volumetric flow of liquids through the CDD will be more than the incoming flow. This will result in the liquid level in the EIP dropping until more gas may start to breakthrough. This again ensures that the volumetric flow of fluids through the CDD is approximately constant at close to its optimal value, thus in turn ensuring that the sand removal efficiency is maintained even during turndown conditions. During this scenario, the differential pressure control valve will be throttled closed to maintain the differential pressure, thus diverting more gas through the CDD.

The overall system thus ensures that the pressure drop across the CDD is maintained constant at its optimal value which in turn also maintains a constant volumetric flow of fluid through the CDD. This combined effect results in a very consistent performance of the CDD by operating consistently at its highest sand removal efficiency value, irrespective of the flow conditions (slugging, plug flow, mist flow, stratified, etc.) of the incoming multiphase fluid. Hence the liquid (and/or gas at turndown conditions) that exits the CDD will consistently meet the sand removal criteria that the CDD is designed to meet. The sand free fluid from the CDD and gas from the differential pressure control valve will recombine and is routed to the downstream system. As the turbulences, pressure surges, slugs, etc. are suppressed by the above mentioned system, the multiphase flow to the downstream system will be very steady. This will significantly improve the performance of the downstream system which may be a production choke valve, heat exchanger or separator. In addition, in the case of a downstream separator, it does not need to be sized for liquid slug handling as the slugs have already been handled and suppressed by the above mentioned system. Advantageously, as sand particles are predominantly in the liquid phase, only the predominantly liquid phase is routed to the desander. This allows the desander to be compact whilst enabling it to be sized for minimal pressure drop with high sand removal efficiency.

In addition, flow fluctuations to the desander, including slug flow, are mitigated thus ensuring consistently good performance of the desander. This is achieved by the upstream flow stabilisation device that suppresses flow fluctuations to the desander. Furthermore, pressure drop across the desander is maintained at a constant value, irrespective of the incoming flowrate, thus further ensuring consistent performance of the desander.

With a constant pressure drop across the cyclonic desander, the system will enable virtually unlimited turndown. Under design liquid flow conditions, liquids laden with sand and some entrained gas will be routed to the cyclonic desander as the pressure balance of the system will ensure that a liquid static head will be maintained in the upstream gas/liquid separation section. When flow is at turndown rates, liquid level will drop and more gas will breakthrough to ensure that the pressure drop across the cyclonic desander is maintained. This configuration ensures that preferentially, liquid (laden with sand) is routed to the cyclonic desander. At turndown liquid rates, more gas will start to breakthrough to the cyclonic desander to maintain the fixed pressure drop. In the limiting case, when flowrate is very low, all the gas and liquid will be routed to the cyclonic desander and during this period, the differential pressure control valve may be fully closed. Hence, very low turndown rates are achieved without the need to readjust the number of liners and/or the specification of the liner(s) in the desander. Apart from the above, gas pressure surges that typically follow slugs are mitigated by the pressure balance line (via the differential pressure control valve).

The comingled flow to the downstream system will be steady and uniform as all the slugs and flow surges have been suppressed by the system.

The availability of the system is high even with a control valve installed at the gas balance line as in the event for example the valve fails in the closed position, all the production fluid will be routed through the cyclonic desander, albeit, depending on the production rate, at a higher pressure drop across the cyclonic desander.

In summary the benefits of the system over a conventional system includes:

• Very high and consistent sand removal efficiency, down to 98% removal of sand particles of 10 microns and larger.

• Low and fixed system pressure drop, ranging from 50kPa to 300kPa.

· High turndown limit

• Suppresses slugs, turbulences and pressure surges and prevents propagation of these flow characteristics to the downstream system. With reference to Figure 2b, an embodiment of the invention is illustrated which is similar in principle to that shown in Figure 2a, but the flow stabiliser section 28" comprises an expanded pipe which is vertical instead of inclined. This alternative flow stabiliser configuration utilises fluid swirl in addition to pipe expansion to contain and dampen transient and unsteady flow fluctuations (liquid slugs, pressure surges, turbulent flows etc), accommodate the largest anticipated slug volume, and enhance gas/liquid separation performance. This alternative flow stabiliser configuration may be provided with other internals as deemed fit to enhance the gas/liquid separation. This configuration is also used to save space and accommodate space constraints in situations where it is more viable to install a vertical vessel or pipe instead of an inclined one.

With reference to Figure 3, a further embodiment of the invention is illustrated, which is similar in principle to that shown in Figure 2, but wherein the CDD 30' and the sand boot 32' is now integral within the lower section of the EIP 28' . In effect, the CDD is thus submerged in the liquid 40 at the lower end of the EIP 28', but operates in substantially the same way, separating the solids received at the inlet 14' which are deposited in the separate sand boot section 32' via outlet 18' . This will save space and weight of the system. The system may be provided with other internals as deemed fit to enhance the separation. In addition, another alternative is to configure the EIP as a horizontal pressure vessel operating as a separator with gas/liquid separation enhancing internals. With reference to Figure 4, a yet further embodiment of the invention is illustrated, which is like that described in figure 2b, wherein the expanded pipe 28" is configured as a vertical pipe piece or vertical vessel, and operates in a similar principle to perform coarse gas/liquid separation and to accommodate the largest anticipated slug volume. However, in this embodiment, the CDD 30' is integral within the expanded pipe/vessel with the sand boot 32' as a separate compartment. Depending on the handling requirements, the two compartments may be constant diameter or of a different diameter. This configuration is used again to save space and weight and where it is more viable to install a vessel or pipe which is substantially vertical instead of inclined. The system may or may not be provided with other internals as deemed fit to enhance the separation. With reference to Figure 5, a still further embodiment of the invention is illustrated, except that a cascade control loop is provided for differential pressure control at the gas outlet 8" of the system. A level transmitter 42 is used to give an indication of level of the liquid 40 in the coarse separation device 28",. The measured signal from the level transmitter 42 is fed to a level controller and the level controller output provides the set-point to the differential pressure controller 38 at the gas outlet line. Thus the level control signal functions as the master controller and the differential pressure is the slave for this cascade control loop. Advantageously if slugs come in, causing a fast rise in liquid volume and pressure, the level is measured and reported by the transmitter 42 to the controller 38 which can then increase the flow rate through the CDD 30' thereby reducing the level more quickly to prevent overflow from the expanded pipe 28".

It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the device which does not affect the overall functioning of the device.

Claims

1. A multiphase sand separation system comprising:

characterised in that a pressure controller section is provided to maintain a pressure difference between the first outlet of the flow stabilisation section and the first outlet of the sand separation section.

2. A multiphase sand separation system according to claim 1 wherein the pressure controller section comprises a control valve and a differential pressure controller for maintaining a constant differential pressure across the control valve.

A multiphase sand separation system according to claim 1 or 2 wherein the differential pressure controller allows the differential pressure to be selectable as a preset value.

4. A multiphase sand separation system according to claim 3 wherein the preset value is a pressure drop in the range of about 50kPa to about 300kPa.

A multiphase sand separation system according to claim 3 or 4 wherein the expanded pipe is provided with a level transmitter for detecting the level of liquid in the expanded pipe and a level controller which selects the preset value for the differential pressure controller based on the level of liquid detected.

6. A multiphase sand separation system according to any preceding claim wherein the expanded pipe is inclined or substantially vertical.

A multiphase sand separation system according to any preceding claim wherein the first outlet of the expanded pipe is located at the upper end thereof, and the second outlet of the expanded pipe is located at the lower end thereof.

A multiphase sand separation system according to any preceding claim wherein the expanded pipe has a minimum diameter such that the flow regime therein is stratified.

A multiphase sand separation system according to any preceding claim wherein the expanded pipe is sized so as to accommodate the highest anticipated incoming slug volume.

10. A multiphase sand separation system according to any preceding claim wherein the sand separation section comprises at least one cyclonic desander.

11. A multiphase sand separation system according to claim 10 wherein the first outlet of the cylonic desander is located at the upper end thereof, and the second outlet of the cylonic desander is located at the lower end thereof.

12. A multiphase sand separation system according to any preceding claim wherein the sand separation section is provided with a sand boot and/or a sand accumulator.

13. A multiphase sand separation system according to claim 12 wherein the cyclonic desander and sand boot are integral with the lower section of the expanded pipe.

14. A multiphase sand separation system according to any preceding claim wherein the multiphase material comprises oil and/or gas from a wellstream.