Desander Cyclones-Oil & Gas 1.1 Aug 2001

Apex Process Systems, 2001. Page: 1 of 11

SOLID-LIQUID SEPARATION Using

DESANDER CYCLONES In the

OIL & GAS INDUSTRY

Version 1.1 August, 2001.

Prepared by:

Apex Process Systems

2/14 Christensen Road, Stapylton P.O. Box 931, Beenleigh, (Brisbane), QLD. 4207,

Australia, Phone: +61 7 3807 5499

Fax: +61 7 3807 5799 E-Mail: [email protected] Web: www.apexprocess.com.au

Desander Cyclones: Oil & Gas Industry Applications Version 1.1, August, 2001

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Table of Contents 1.0 INTRODUCTION ................................................................................................ 3 2.0 DESANDER TECHNICAL DESCRIPTION......................................................... 4

2.1 BACKGROUND ................................................................................................4 2.2 OPERATING PRINCIPLES...............................................................................5 2.3 PERFORMANCE PREDICTION .......................................................................5 2.4 COMMON DESANDER APPLICATIONS.........................................................6

2.4.1 Produced Water Desanding................................................................6 2.4.2 Well-Head Desanding ..........................................................................7 2.4.3 Well-Stream Desanding.......................................................................8

2.5 OPERATING PRINCIPLES...............................................................................9 3.0 APPLICATIONS............................................................................................... 10

3.1 PRODUCED WATER DESANDERS ..............................................................10 3.2 WELL-HEAD & WELL-STREAM DESANDERS ............................................10 3.3 EQUIPMENT SELECTION..............................................................................10

3.3.1 Desander Type ...................................................................................10 3.3.2 Solids Accumulator ...........................................................................11


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1.0 INTRODUCTION Solids are produced from all Oil & Gas wells, although the amounts vary greatly. These solids originate either from the reservoir (as sand, clay or silt), or as corrosion by-products from the down-hole tubing and other process equipment. These solids are normally up to 1 mm (1000 micron) in diameter and are found in varying amounts in different fields, with typical concentration ranges of 5- 250 ppm. Low levels of fine solids may not cause a problem, and may not require any specific action to be carried out as a consequence of solids being produced. However, higher levels of coarser solids production can cause a range of problems where levels are high enough, and may result in one or more of the following problems:

Local regulations that limit or do not allow the discharge of solids, Plugging of water re-injection wells, Erosion of process equipment (eg: Chokes, flow-lines, pumps, meters, water treatment

equipment, etc.)

Separators or flotation cells fill with sand, which requires equipment shutdowns & isolation to allow manual solids removal (resulting in production losses).

The main options used traditionally to deal with solids production have been:

To reduce oil/gas production rates to reduce the rate of sand produced, The work-over of wells to repair or install gravel packing or sand screens, Accept increased equipment shut-down for maintenance & repairs, and the consequent

lost production. Cyclonic separation equipment, in the form of Desanding Hydrocyclones have proven to be a valuable option for solids removal in many applications in the Oil & Gas industry, and many units are installed at Offshore Platforms, and a variety of Onshore Production, Refining & Processing Facilities.


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2.0 DESANDER TECHNICAL DESCRIPTION

2.1 BACKGROUND Solid-Liquid cyclones have been used for decades in various industrial applications ranging from classification of solids in the Mining industry, to removing fine solids in municipal water systems, to food processing. In the Oil & Gas industry, solid-liquid cyclones (called Desander Cyclones) have been used for many years in drilling operations, and within the last decade have been installed in a number of applications in Oil & Gas Production Operations. Desander Cyclones can operate with open or closed Underflows, although for most Oil & Gas applications, they are operated with a closed Solids Underflow for safety reasons, and because solids levels are relatively low. This low level of solids allows periodic batch dumping of the solids from the normally-closed Underflow Collection or Accumulation chamber. There are 2 generic types of Desander Cyclones used in Oil & Gas applications:

1. Large Diameter single-cyclone designs (XW Series) range from 4 30 diameter (100 mm - 750 mm.

2. Small Diameter high-efficiency multi-cluster designs (XS Series) range are typically 1- 2 diameter (10-50 mm), and consist of a vessel with several Cyclones installed inside it. Up to ~150 Liners can be installed inside a single vessel to handle relatively large flows as required.

Inlet

Inlet

Overflow Overflow

Underflow

Underflow

COMMON DESANDER CYCLONE TYPES:

SOLIDS SLURRY

CLEAN LIQUID

Single-Liner Design

Multi-Liner Design

Figure 2.1: Large & Small Desander Cyclones

These units vary in size & construction, and selection of either is dependent upon the capacity to be processed, separation cut size (of solids to be removed), and turn-down requirements, etc. The large single Cyclone units are used in applications where large flow rates & coarse separation size (typically >40 micron) is required, while the small diameter units are used in


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applications to treat any flow rate (units are sized to suit the flow) where fine separation size (>10 micron) is required.

2.2 OPERATING PRINCIPLES Desander Cyclones are pressure-driven devices and require a pressure drop to operate. A feed stream mixture of liquids & solids enters each Cyclone (whether large or small) through a tangential inlet, which forces the fluids to spin in a spiral motion. As the internal diameter is reduced down the length of the cone, the vortex flow (spiral motion) is accelerated. This creates centrifugal forces which cause the heavier solid particles to move outwards to the internal wall of the cone. These solids continue to spin in a spiral pattern around the Cyclone internal wall, and travel down the length of the cone, to discharge through the Apex Nozzle or Underflow spigot. The tapering cone shape forces the de-sanded liquid to flow towards the core where its direction is reversed upward through the Vortex Finder as the overflow stream. The solids that exit through the bottom Apex collect in an Accumulation chamber, where they are periodically purged, while the desanded overflow (>99% of throughput) discharges continually.

2.3 PERFORMANCE PREDICTION Solid-Liquid Cyclones are particle-size classification devices. Each type, size, & geometry of cyclone will separate particles of a specific size, although it can be easily summarized as: Smaller cyclones capture smaller particles. (See Figure 2.2) All Cyclones have a Reduced Graded Efficiency Curve, which indicates the particle sizes in the inlet stream to the Cyclone, compared to the % Probability of that particle to be recovered (captured) by that device. The separation size is defined as the particle size that has a 98% probability of capture, and is termed as a Cyclones D98 size. Figure 2.2 provides a general guide for Cyclone performance relative to its diameter, based on solids of ~2.5 S.G. in a water stream.

CYCLONE SIZING GUIDE

0

10

20

30

40

50

60

1 2 4 6 10 16 20Cyclone Diameter (Inches)

98%

Sol

ids

Rem

oval

(mic

ron)

Figure 2.2: Cyclone Sizing Guide


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Where the typical size distribution of solids in the stream are known, the removal performance of a Desander Cyclone can be predicted with some accuracy. However, in most applications, the solids sizes are largely unknown, so estimates need to be made based on experience, or by the target size of solids to be removed. APS can provide a performance prediction based on the process data available, or can recommend a Desander Cyclone size based on a target solids removal size.

2.4 COMMON DESANDER APPLICATIONS Desander Cyclones are very well suited for several specific solid-liquid separation applications in the Oil & Gas industry. The 3 most common applications are as follows:

1. Produced Water Desanding (Downstream of Production Separator) 2. Well-Head Desanding (Upstream of Production Separator) 3. Well-Stream Desanding (Upstream of Production Separator)

Being either Upstream or Downstream of the Production Separator/FWKO, has a large impact on the selected Desander Cyclones design & performance characteristics.

Figure 2.3: Common Desander Options

Further details of these 3 applications:

2.4.1 Produced Water Desanding This application involves locating the Desander Cyclone unit on the water outlet line of a Production Separator or FWKO. A small diameter (XS Series) Desander is most commonly used, as it is often necessary to recover particles down to 10-15 micron for reservoir reinjection, equipment protection, or discharge regulation. The Desander Cyclone is fed by the operating pressure of the production separator, and so is typically designed to the same pressure rating. Typical installations are required to treat a solids concentration of 20-500 PPM, with an average solids size distribution of 20-100 microns. The XS Desander is typically able to remove >98% of all solid particles above ~15 micron, and reduce the remaining solids remaining to


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Normal operating pressure drop (from Desander Cyclone Inlet to Water Outlet) is typically 25-40 psig (1.7-2.75 Bar). The Desander Cyclone is typically installed upstream of the Separator level control valve (LCV), as shown in the Desander/Deoiler combination system (see Figure 2.4), although it may be located downstream of the Deoiler Cyclone if reinjection is an issue.

Figure 2.4: Produced Water Desander

The Overflow (clean) discharge of the Desander is continuous. This water is usually sent to further treatment for oil removal, which is commonly done by a Deoiler Cyclone or flotation cell. Solids are dumped from the Desander as a slurry which is typically sent to a collection vessel or sand skip.

2.4.2 Well-Head Desanding Well-Head Desander Cyclones are typically located at or near the Well Head, and typically upstream of the Flow-line choke, where they are usually required to be designed to the same rating as the Christmas Tree assembly (eg: 5,000/10,000 psi). They can provide a compact option that allows higher sand-free production rates while eliminating the need for down-hole sand control. Well-Head Desanders can be installed on most oil or gas wells (see Figure 2.5).


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Figure 2.5: Schematic of Well-Head Desander Installation

The main benefit of removing the solids at the Well-Head is that the erosion potential of the solids is primarily eliminated from the entire downstream process. Although Well-Head Desanders do not remove all solids, they remove the coarser solids that are the primary cause of erosion. An additional benefit that the sand dumped from the Solids Accumulator is very low in hydrocarbons as the sand is typically water wet in the reservoir, and will remain so, unless they become contaminated or coated with oil during processing. Removing the sand ahead of the separators prevents oil adsorption.

2.4.3 Well-Stream Desanding A Well-Stream Desander is typically installed downstream of the flow-line choke, (see Figure 2.6), and often on the main well-collection manifold, which received flows from a number of wells. The main benefits of installing a Well-Stream Desander over a Well-Head Desander is that the Unit can be designed to a lower pressure rating, and a number of wells can be processed with a single larger unit. However, it is typical for the pressure to be lower downstream of the flow-line chokes, and so the gas will occupy a larger volume. This requires a larger capacity unit to handle the larger volumetric flow-rate of Gas, Oil, Water & Solids, than if it was installed upstream of the Flow-line choke. The selection of either a Well-Head or Well-Stream Desander depends on the process conditions and the objectives in installing a Desander Unit. In most other respects, a Well-Stream Desander has similar characteristics to a Well-Head Desander.


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Figure 2.6: Schematic of Well-Stream installed location

2.5 OPERATING PRINCIPLES Well-Head & Well-Stream Desanders operate on the same principles as liquid-only Desanders, except that they are designed to treat multi-phase flows. The presence of free gas in the feed stream has the dual effect of increasing inlet fluid velocity, and decreasing the fluid mixture density & viscosity. Both effects work together to increase the solids recovery efficiency, although the gas volume must be accommodated by an overall higher processing capacity.


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3.0 APPLICATIONS

3.1 PRODUCED WATER DESANDERS Removal of solids from the water stream downstream of the Production Separator/FWKO is a common application for Desander Cyclones, where solids are removed for one or more of the following reasons:

Local regulations that limit or do not allow the discharge of solids, Plugging of water re-injection wells, Erosion of downstream Deoiler Cyclones, and/or level control valves, Downstream Flotation cells fill with sand.

3.2 WELL-HEAD & WELL-STREAM DESANDERS Used for a range of applications: 1. Removal of formation sand from wells before they enter the process equipment 2. Removal of sand flow back from Coiled-Tubing frac. jobs, well-bore sand-flush, acidizing,

etc., to prevent choke erosion and large quantities of solids from entering the downstream process equipment.

3. Removal of sand from oil & gas well streams to protect downstream equipment, and avoid choke, manifold, and flow line erosion, heat exchangers, etc.

The Well-head Desander can be installed up or downstream of the choke depending on the nature and economics of the problem. Each location has specific benefits as defined below: Upstream of Choke: Protection of choke from erosion Lower actual volume of fluids to be treated, therefore the unit will be smaller in size No requirement for PSV as unit is designed for full shut-in pressure Downstream of choke: Lower design pressure, hence a simpler, more economic design Potential to treat several wells through a single unit

3.3 EQUIPMENT SELECTION

3.3.1 Desander Type A Desander system is designed based upon the required separation size and the total system capacity. The main factor when selecting a Desander Cyclone is to decide where the solids are to be removed from the process:

1. Upstream of the Flow-line choke, 2. Upstream of the Production Separator/FWKO, 3. Upstream of the Water Treatment System.

Once this selection has been made, the next criteria to decide is what level of solids removal is required. For example:

1. Removal of solids >15 micron?


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2. Removal of solids >50 micron? 3. Removal of solids >100 micron?

3.3.2 Solids Accumulator The Solids Accumulator is often a part of a Desander Cyclone system, as it allows a higher degree of flexibility in operation. All Desanders are provided with an integral Accumulation Chamber as the bottom part of the Desander vessel, but some systems may require a separate Accumulator. A separate accumulator is used when one or more of the following occur;

Operating pressure is >100 psig (7 Bar) Solids collection must be contained at all times (i.e. for hazardous content) Air must not be allowed to come in contact with the production fluid, or conversely gases are

not allowed to escape,

Solids content is very high, and low purge frequency is desired Primarily the operating pressure or solids content will dictate the use of a separate Accumulator. The Accumulator can be of any design, shape, or orientation, as it is only used to collect solids, although it must be physically located directly beneath the Desander Vessel. As the solids fall from the Desander Vessel to the Accumulator by gravity, a vertical path is the most efficient means to allow solids collection in the Accumulator.

Desander Cyclones-Oil & Gas 1.1 Aug 2001

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