SEAWATER INJECTION –A HOLISTIC REVIEW

Ross Macdonald

Proserv Production Chemistry Networking Event - 3rd March 20201

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SWI Presentation Outline

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Water Injection

Overview

Injection Water

Quality

Production

Impacts

• Pressure

Support

• Sweeping/EOR

• Example SWI

topside system

• Solids

• Bacteria

• Oxygen

• Downhole

Scaling

• MIC

• Reservoir

Souring

WATER INJECTION OVERVIEW

WATER INJECTION – PRESSURE SUPPORT

• To maintain fixed reservoir pressure

– Voidage of produced fluids replaced with water

• Above bubble point:

– Wi = (OP * Bo) + WP

• Water Injection provides:

– Secondary Production

– Maintains PRes > PBP

• Single HC Phase in Reservoir

• Asphaltene Stability in Reservoir

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WATERFLOOD

• Sweep

• Water front moves out from the injector

• Oil displaced towards production well

• Improved recovery factor

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• EOR

• Polymer flood

• Viscosity improves vertical sweep

• Surfactants

• Reduce residual oil saturation to improve recovery

• Microbial injection/stimulation

• Partial digestion of heavy oils

SEAWATER INJECTION

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WATER QUALITY

WATER QUALITY

• Injection water specification required to maintain:

– Injectivity to the reservoir

– Integrity of the well and injection equipment

• The main seawater quality issues

– Solids

– Marine Growth

– Bacteria

– Oxygen

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WATER QUALITY – SOLIDS

• Solids present in the injection water

– Deposit in injection equipment (efficiency/corrosion)

– Fouling of the sand face (reduced injectivity)

• Fouling tolerance dependent on well/reservoir

– Pore-throat size

• Mitigation: Filtration

– Typical Specification

• Particle Size < 50 µm

• Loading < 100 mg/l

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WATER QUALITY – SOLIDS

• Water hammer from Hard Shut-in

• Hammer induces mobilization in the well bore

– Rat hole solids

– Sand face solids

• Restart drives suspended solids into near well-bore

• Mitigation: Settling period in restart procedure

– Example: 6 hours minimum hold time

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WATER QUALITY – MARINE GROWTH

• Seawater contains diverse array of biota

– Varies by location and time of year (bloom period)

• Macro/Micro fouling

– Shell-fish build up in strainers/exchangers

– Biofilm growth in vessels/pipework

• Example: Ceratium tripos dinoflagellate

– Present in the upper water column

• Mitigation: Chlorine (Hypo/Electrochlorination)

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SEAWATER INJECTION

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WATER QUALITY - BACTERIA

• Seawater contains diverse array of microorganisms

• Issues

– MIC (vessels, pipework and tubing)

– Fouling of vessel internals

– Souring: SO42- → H2S

• Monitoring: Sessile Biofouling Monitoring Tubes (SBMTs)

– Compares with and without biocide

– ATP, qPCR, MPN serial dilution…

• Mitigation: Secondary biocide treatments (batch)

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SEAWATER INJECTION

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WATER QUALITY - OXYGEN

• Oxygen in SW induces pitting corrosion of the injector well tubing

• Dissolved O2 specification depends on required life of well

– Typically <10ppb for L80 tubing

• 13CR Steel tubing more susceptible to O2-induced pitting

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• Significant risk when converting 13CR production

wells to injectors

• 13CR tubing more susceptible to O2 pitting

• Oxygenated brines induce Cl-SCC

WATER QUALITY – ACHIEVING < 10 PPB

• O2 content of SW typically in the range 8 – 11 ppm

• Vacuum Deaeration Tower reduces to ~20 - 50 ppb

• Chemical scavenging utilised to achieve < 10 ppb

– Typically NH4HSO3 or NaHSO3

2HSO3- + O2 → 2H+ + 2SO4

-

– Scavenger Catalyst can be required

• Bisulphite can promote corrosion

– Residual range: 1.28 – 2.56 ppm

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SEAWATER INJECTION

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PRODUCTION IMPACTS OF SEAWATER INJECTION

DOWNHOLE SCALING

• FW/SW incompatibility can lead to scale precipitation

– FW provides cation (Ba2+/Sr2+/Ca2+)

– SW provides the anion (SO42-)

• Deposition can block near wellbore and tubing

• Monitoring: SWBT%, ESEMs

• Mitigation: SI Squeeze Treatment, LSSW

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Formation Water Seawater

Na 13617 10933

K 105 460

Ca 596 428

Mg 130 1368

Ba 78 0

Sr 59 8

HCO3 1916 140

SO4 26 2960

Cl 22713 19766

MICROBIALLY INFLUENCED CORROSION (MIC)

• Reservoir contains a vast array of microorganisms

– Archaea: single-celled species

– SRA and Methanogens

• Sulphate ions metobolised by SRA

– Active species promote MIC in pipework/vessels

• ‘Bowl in bowl’ pits indicating MIC

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SOURING

• SRB & SRA present in injection water/reservoir

– Microbes motabilise SO42- to H2S

• Souring in SW supported fields effected by:

– Temperature

– Water Chemistry (salinity)

– Nutrients (sulphate, VFAs, phosphorous)

– Mineralogy (scavenging effects)

• Monitoring: H2S in gas phase

• Mitigation:

– Reservoir Souring Models (understand the potential)

– Nitrate Injection (promote NRPs)

– H2S Scavenger

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SUMMARY

• Water Injection required for pressure support and reservoir sweeping to maximise recovery

• Water quality specification required to maintain integrity and injectivity:

– Solids

– Bacteria

– Oxygen

• Seawater injection can pose significant risks to production

– Downhole Scaling

– Microbial Influenced Corrosion

– Reservoir Souring

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THANKS FOR LISTENING!