05 Produced Water

Upstream Process Engineering Course 5. Produced Water

Upstream Process Engineering Course

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Produced Water

1

What is produced water? • Produced water (PW) is a waste water stream produced with oil. • Amount of water increases as reservoir matures. Can be up to 98% water cut in late field life • Consists of formation water and injection water breakthrough plus some condensed water • PW stream can also include other waste water sources: ballast water, drains etc • In UKCS about 4,900 tpa of oil is discharged to sea with produced water at an average concentration of 20 mg/l (2005 figures). Upstream Process Engineering Course

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Produced Water

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Contaminants • Dispersed oil – mostly aliphatics and heavy aromatics • Dissolved oil – mostly light aromatics and more predominant in gas condensate service • Polar hydrocarbons – organic acids, glycols • Polycyclic aromatic hydrocarbons (PAHs) – naphthalene+ • Chemicals – corrosion inhibitor, scale inhibitor, demulsifier etc • Solids – scale, formation solids, corrosion and erosion products • Heavy metals: Pb, Cr, Hg, Zn etc • Salts • Low level radioactivity

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Produced Water

3

Regulation for offshore disposal • • • • • • •

• • • • •

For the NE Atlantic, OSPAR (Oslo and Paris Commission) regulates offshore discharges through national authorities (In the UK, the DTI issues oil discharge permits) Historic OSPAR limit was an average of 40 mg/l (dispersed oil) In 2006 new tighter regulation now require an average of 30 mg/l (dispersed oil) and an overall reduction of 15% in mass of oil discharged with PW compared to 2000. With rising PW volumes as UK fields mature this is a significant constraint. PW trading is coming. ‘Clean’ PW will be a commercial commodity. DTI will issue annual licences limiting the amount of oil that can be discharged by an installation. Allowances have been set aside for new developments Penalties for exceeding licensed limit set at £108 per kg. Recently reduced from £280/kg as cost of technology to reduce OIW levels is less than previously estimated. Example:100,000 Bbl/d of water at 25 mg/l discharges 145 tonnes of oil a year. If the performance were to deteriorate by say 2 mg/l, the penalty would be £1.3 million a year. Gas condensate (previously exempt) now regulated in the same way as oil 2 tpa de minimus limit Chemical use is controlled by permits based on marine environmental risk assessment Radioactive discharges controlled by permit by SEPA in Scotland and EA in England

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Produced Water

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Future trends in regulation • Regulation likely to become more onerous and closer to BP’s aspiration – zero harmful emissions – No discharge of oil to sea – No discharge of harmful chemicals to sea

• Will probably control currently unregulated components • On-line monitoring and PW flow measurement likely to be mandatory for larger installations

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Produced Water

5

Typical UKCS Produced Water System Choke

Production Separator

to flare Hydrocyclone

500 -1000 ppm Degasser

Downhole Pump

< 40 ppm

skimmed oil “clean” water < 30 ppm

• •

• •

Small oil droplets are formed as a result of shear e.g. by downhole pumps or choke valves The higher the pressure drop over the hydrocyclone, the higher the g-force and hence the better the separation (P: 2 - 30 bar) The degasser typically operates close to atmospheric pressure Residence time in degasser: approx. 2 minutes

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Deoiler Hydrocyclone

Produced Water

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Hydrocyclones • Workhorse for UKCS especially for oil removal • Removes only dispersed oil • Removes droplets down to about 8-10μm

• Need pressure differential • New designs planned to offer reduced ΔP and shearing

Source: Vortoil

• Generally can meet 30 mg/l. • Polishing step may be needed for difficult separation or to meet tighter performance standards

Source: NATCO Upstream Process Engineering Course

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Produced Water

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Coalescence • Promotes droplet coalescence • Conditions stream upstream of hydrocyclones

• Only targets dispersed oil • Some reports of clogging and excessive fouling • Better suited to clean service Upstream Process Engineering Course

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Produced Water

Source: OPUS

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Coalescence Mare’s Tail

PECT-F

Source: OPUS

Source: Cyclotech

•

PECT-F coalescing material placed inside hydrocyclone vessel

• •

Coalescer/separator • •

Coalescer/separator for CIW service Provides coalescence and separation in one unit

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Mare’s Tail placed in pipe upstream of hydrocyclone Less sensitive to fouling than other coalescing media

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Produced Water

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Adsorption • Several proprietary units CETCO, EarthCanada, RM environmental • Mostly non-regenerable media based on starch or clay • Most (not all) experience has been positive but OPEX can be high • Removes a range of contaminants: dissolved and dispersed oil, chemicals, heavy metals. • Can be useful as a “quick fix” • Produces a waste product of spent material • Consider disposal cost and method – Burning and landfill

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Produced Water

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Extraction •

• • •

•

Produced water brought into contact with a hydrocarbon fluid and oil migrates to hydrocarbon phase Acts as a solvent and removes dispersed and dissolved oil Two proprietary processes – MPPE and CTour CTour uses NGL which often needs to be conditioned before use. Used in Norway MPPE uses a bed containing the extraction fluid which is periodically regenerated using steam and the oil is recovered. Used in Netherlands for gas fields.

C-Tour

Source: Akzo Nobel Upstream Process Engineering Course

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Produced Water

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Compact gas flotation • Compact design combines gas flotation with axial flow • Residence time ~ 30 seconds • Low pressure drop • Sometimes flocculent is required • Sometimes additional flotation gas is required (N2 or fuel gas) • Units offered by EPCON and CETCO and others • Have been used in UKCS and Norway and shown good results • Mostly targets dispersed oil but evidence that units can remove some aromatics through a stripping effect. Upstream Process Engineering Course

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Produced Water

12

UV/Ozone • Adaptation of onshore WWT technology • Proprietary process “AquaPurge” • Uses UV and ozone to create hydroxyl radicals OH- which react with hydrocarbons to convert to water and CO2 • Removes dissolved and dispersed hydrocarbons

• Encouraging trials onshore and on Montrose • Other trials planned Upstream Process Engineering Course

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Produced Water

13

Membranes • Filtration type process • Historically membranes have fouled in PW service

Source: GE Osmonics

• New designs use a hydrophilic surface to prevent oil coating the membrane Upstream Process Engineering Course

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Produced Water

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Gas condensate in water • • •

Less than 1% of hydrocarbons discharged to sea with produced water is gas condensate but predominance of dissolved species raises profile Gas fields are over-represented in OSPAR’s ‘name and shame’ list for installations discharging > 40 mg/l to sea Condensate is more difficult to remove from water than oil. This is due to: – – – –

• • •

Nature of water is different as much of it is condensed water not formation water Lower surface tension makes small droplets more stable Gas corrosion inhibitor tends to stabilise emulsions Often intermittent flow

Where flowrates are low and a simple minimum cost solution is required, a long residence time (several hours) may offer adequate separation especially if CI is absent Otherwise a coalescer filter/separator unit is a typical choice For larger flows hydrocyclones are often used but performance is frequently unsatisfactory and a secondary polishing treatment is required.

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Produced Water

15

Produced Water Re-injection (PWRI) • BP preferred disposal method for new projects • Encouraged by DTI (zero emissions) • Doesn’t necessarily avoid the need to have good OIW as disposal to sea is required when PWRI is not available • PWRI routing can supplement SW injection for pressure support or use aquifer for disposal only • When PW is mixed with SW care must be taken with chemical compatibility to prevent scaling and precipitation • Loss of injectivity can typically range from 0 to 40% • Temperature effect can be significant: easier to inject cold high OIW than cleaner warm water. • Filtration and management of solids can be a key issue. BP example of this is Schiehallion. • Potential to offset high CAPEX with PW trading. • Pumping power demand can increase combustion emissions

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Produced Water

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On-line OIW monitoring • • • • •

Reliable on-line OIW monitoring in UKCS is elusive Encouraged by DTI and may become mandatory for larger installations (discharging more than 100 t/y of oil to sea) Analysis method determines what is measured and may not match DTI sample reports Provides opportunity to respond more quickly to excursions Offers better understanding of factors affecting performance

•

Several units are available. BP has had good experience with the Jorin ViPA which also provides particle size measurement. It can distinguish between oil and solid particles

Oil particles Upstream Process Engineering Course

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Produced Water

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Droplet size measurement

Upstream Process Engineering Course

1000

Cumulative OIW mg/l

Platform A First Stage Separator hydrocyclones

Inlet

100

Outlet

SMALL DROPLETS Almost 30 mg/l is present as

10

droplets smaller than 10 microns

1 1

10 Oil droplet size, microns

100

100 Platform B Hydrocyclones ( First Stage Separator)

Cumulative OIW mg/l

• Much PW equipment (especially hydrocyclones) depends on gravity separation and droplet size affects performance • Size data provides useful information to explain OIW performance • Platform B’s PW is easier to treat than platform A’s as much less of the oil is present as small droplets • However, treat droplet size data with caution. Some methods can report unfeasibly small distributions.

Hydrocyclone inlet Hydrocyclone outlet

10 SMALL DROPLETS Less than 5 mg/l is present as droplets smaller than 10 microns

1

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1

10

100

Oil droplet size, microns

Produced Water

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Chemicals • BP’s goal is no discharge of harmful chemicals to sea • Production chemicals can affect PW performance for better or worse • Some can stabilise emulsions if overdosed - demulsifier • Notorious difficulty is seen when gas phase corrosion inhibitor is used • Deoiler may improve OIW quality but is highly application specific and trials can take time to identify most suitable formulation. • Good scale control is important as presence of scale can impede oil removal • Chemical cocktail can cause incompatibilities and poor OIW Upstream Process Engineering Course

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Produced Water

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Back to basics • Deoiling equipment usually relies on gravity separation (especially hydrocyclones)

• Performance governed by Stokes’ law

gD (  w   o ) Vt  18 w 2

Vt = velocity of oil droplet moving through water

• •

g: G force: more pressure drop gives higher G forces D: Droplet size: small droplets are more difficult to remove. Sources are: emulsions, shearing (control valves, pumps, artificial lift), chemicals, condensate

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μw: Water viscosity: higher temperature gives lower viscosity, high salinity gives high viscosity

•

ρo: Oil density: heavy oil is harder to remove than light oil, solids can coat the droplet surface and make it heavier

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Produced Water

20

Design and operation points • Optimise separator performance to minimise OIW leaving separator.

• Minimise number of hydrocyclone liners in service to maximise pressure drop • Be aware of effect of changes in operating conditions. Lower separator pressure (to improve production) or lower temperature (due to a subsea tieback) will worsen OIW performance

• Minimise transients in PW flow to hydrocyclones • Provide hydrocyclone back flushing facilities and use them regularly to prevent blockages • Minimise shearing. Place control valves downstream of PW treatment. Avoid pumping PW but if unavoidable use low shear pumps. • Avoid recycling PW. ‘Stale’ oil droplets are more stable and difficult to remove

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Produced Water

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Example of poor design • Recent design – – – –

Low operating temperature Low pressure drop Artificial lift LCV upstream of hydrocyclone

• Result – poor OIW discharge quality – damaged reputation – and…….. “discussions” with DTI Upstream Process Engineering Course

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Produced Water

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Sand Management • Effects of Sand Production

• Sand Production – Sand is produced in the reservoir, mainly from the pressure drop across the interface

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– Sand will tend to settle in areas where settling is encouraged e.g. separators, pipework dead legs etc. – The sand will raise the level in the vessels causing a reduction in residence time – When flowing with liquids the larger sand particles will tend to flow with the produced water, however oil coated fine sand particles will tend to remain in the oil phase potentially stabilising emulsions.

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Produced Water

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Sand Management Legislation Recent changes in regulations. Discharge of sand to sea now covered by OPPC permit. Mass of sand and oil content must be measured and reported to DTI • Sand must be cleaned of oil • Samples of sand must be taken for analysis • Wash water to be routed via produced water treatment facilities or hazardous drains • Chemicals discharged as a result of the operation must be reported to the DTI

If exemption is not applied for then the sand must be put in a safe container and sent onshore for disposal.

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Produced Water

24

Sand Management Typical Sand Removal Operations

Wellhead removal Upstream or downstream of choke valves

Reject Oil

Sand Cyclone Separator removal

Water leg removal

Separator

Hydrocyclone Sand Cyclone

Sand Accumulators

Seawater to Sparge or Tore To Skip or Sand Cleaning (Detail A or B)

To Skip or Sand Cleaning (Detail A or B)

Sand Accumulators To Skip or Sand Cleaning (Detail A or B)

Flash Gas to LP Flare

Proprietary mechanical Separator / sieve

Detail A solids outlet Seawater for wash water (if required)

Liquid outlet

Detail B

To open hazardous drain

Sand Cleaning

Wet sand Skip To open hazardous drain

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PW Degasser

Oily water To Open Haz Drains

To Overboard Disposal

Sand discharged overboard

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Produced Water

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Sand Management Sand Cyclone

Removal Technologies Oil Phase and wellhead fluids Desanding Cyclones • Sand cyclones are simple pieces of equipment for coarse and medium grade sand separation from bulk fluids utilising centrifugal sources to effect separation. • Sand, being heavier will follow the under flow into a storage receptacle while lighter fluids exit at the top of the unit. Nutshell Filter Nutshell Filters • Remove sand down to 2μm with 95% efficiency and 5μm with 98% efficiency • Also removes small oil droplets • Unit requires backwash to remove particles thus requiring two units. Upstream Process Engineering Course

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Produced Water

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Sand Management Separator Sparge Internals • Use utility seawater supplied through spray nozzles to break up settled sand • Sand exits through drain nozzles in the vessel. • Online operation disturbs interface levels • Potential carry over of sand into produced water and oil

Separator sparging internals

Merpro Tore Vessel Internals • Induce a vortex that ‘sucks’ up the sand from the vessel. • Minimises water consumption • Can be used online without disturbing interface levels. • Sand removed is cleaned and disposed of appropriately Upstream Process Engineering Course

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Tore internals Produced Water

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