Overboard discharge is not the only destination for treated produced water on an FPSO. Where discharge is restricted by environmental permit, where the treated water cannot meet the discharge limit, or where reservoir management makes water injection attractive, produced water is reinjected into the reservoir or a disposal formation rather than discharged to sea. For a plain-English explanation of the full produced water journey on an FPSO, see What Happens to Produced Water on an FPSO?.
This article explains what produced water reinjection (PWRI) is, why operators choose it over overboard discharge, what water quality is required for injection and how the monitoring requirements differ from those for discharge.
| Key thing to remember Produced water reinjection requires a higher water quality standard than overboard discharge. Oil concentration, suspended solids content, particle size, bacterial count and chemical compatibility must all be controlled to protect the injection well and the reservoir formation. The monitoring programme for a PWRI system is therefore more extensive than for a discharge-only system. |
What Is Produced Water Reinjection?
Produced water reinjection (PWRI) is the process of treating produced water from oil and gas production and injecting it back into the subsurface either into the producing reservoir itself or into a separate disposal formation rather than discharging it overboard.
On an FPSO, the produced water treatment package removes oil, solids and other contaminants from the water stream to a level suitable for injection. The treated water is then pumped via a water injection system into injection wells, where it enters the subsurface formation.
PWRI serves two distinct purposes depending on the operating context:
- Reservoir pressure support injecting water into the reservoir maintains reservoir pressure as hydrocarbons are produced, improving oil recovery and extending field life. This is known as water flooding and is a primary enhanced oil recovery technique.
- Environmental disposal where overboard discharge is not permitted or where the treated water cannot meet the discharge limit, PWRI provides a disposal route that avoids environmental impact at surface.
When Do Operators Choose Reinjection Over Overboard Discharge?
The decision between overboard discharge and produced water reinjection depends on a combination of regulatory, technical and commercial factors. The most common drivers for choosing reinjection are:
Regulatory restriction on discharge
In some operating jurisdictions, overboard discharge of produced water is prohibited or severely restricted either by zero-discharge requirements in the field permit or by particularly stringent discharge limits that the treatment system cannot reliably achieve. Where the treated water cannot consistently meet the discharge limit, reinjection is the only compliant disposal route. For an overview of how discharge limits vary by jurisdiction, see Why Do Produced Water Discharge Limits Vary by Country?.
Reservoir pressure support
In fields where reservoir pressure declines as hydrocarbons are produced, water injection maintains pressure and improves sweep efficiency increasing oil recovery. Where the produced water volume is sufficient and the water quality is suitable for injection, PWRI combines environmental disposal with a production engineering benefit.
Treatment system limitations
Where the produced water treatment package cannot consistently reduce oil concentration and solids content to within the discharge limit particularly in fields with high water cut, variable crude composition or challenging produced water chemistry reinjection removes the discharge compliance risk by eliminating the overboard discharge stream entirely.
Offshore location restrictions
In ecologically sensitive areas, marine protected areas or locations where the receiving environment is particularly vulnerable, regulators may impose restrictions on produced water discharge that make reinjection the preferred or mandatory disposal route regardless of the treated water quality.
How Does Produced Water Reinjection Differ From Overboard Discharge?
The table below summarises the key differences between overboard discharge and produced water reinjection across the dimensions that matter most for EPC project teams and offshore operators.
| Overboard discharge | Produced water reinjection (PWRI) | |
| Destination of treated water | Discharged overboard within permit limits | Injected into the reservoir or a disposal formation |
| Regulatory basis | Field permit and national environmental law | Reservoir management plan and formation injection permit |
| Oil concentration limit | Permit specific typically 15 to 40ppm depending on jurisdiction | No fixed global standard solids, bacteria and compatibility must also be controlled |
| Water quality requirements | Oil concentration within the discharge limit | Tighter than discharge solids content, bacterial count and chemical compatibility must also meet injection well requirements |
| Monitoring focus | Continuous oil-in-water monitoring at treatment package outlet | Oil, solids and bacterial monitoring to protect injection well and reservoir integrity |
| Primary driver | Environmental compliance staying within discharge limit | Reservoir pressure support, zero discharge requirement, or permit restriction on overboard discharge |
| Fate of recovered oil | Skimmed from treatment system and returned to production | Skimmed from treatment system and returned to production |
The most significant difference from a monitoring and treatment perspective is the water quality requirement. Overboard discharge requires the water to meet an oil concentration limit set by the field permit. Reinjection requires the water to meet a broader set of quality parameters oil, solids, particle size, bacteria and chemistry all of which must be controlled to protect the injection well and the reservoir formation.
What Water Quality Is Required for Produced Water Reinjection?
The water quality required for produced water reinjection is typically more demanding than for overboard discharge. The specific requirements depend on the reservoir characteristics, the injection well design and the field operator’s injection well management plan but the general quality parameters that must be controlled are consistent across most PWRI applications.
| Quality parameter | Typical requirement | Why it matters for injection |
| Oil in water concentration | Typically lower than the overboard discharge limit often below 10ppm | Residual oil causes formation plugging and degrades injectivity over time |
| Total suspended solids (TSS) | Typically below 2 to 5mg/l depending on reservoir and well characteristics | Solids plug pore spaces in the reservoir formation, reducing injectivity |
| Particle size distribution | Typically below 2 to 5 microns median particle size | Larger particles block pore throats more rapidly than fine particles |
| Bacterial count | Controlled below a threshold specified in the injection well management plan | Bacteria cause reservoir souring (hydrogen sulphide generation) and well damage through microbial induced corrosion |
| Chemical compatibility | Injected water must be compatible with formation water and reservoir mineralogy | Incompatible water can cause scale precipitation in the formation and injection well |
| Oxygen content | Low typically below 20ppb dissolved oxygen | Oxygen promotes bacterial growth and corrosion in injection wells |
Meeting these quality requirements demands a more comprehensive treatment system than overboard discharge alone. Where overboard discharge may be achieved with hydrocyclones and flotation, PWRI typically requires additional filtration stages media filters, cartridge filters or membrane systems to achieve the solids and particle size specifications required for injection.
| Injection well injectivity decline Poor water quality is the primary cause of injection well injectivity decline the gradual reduction in the rate at which water can be injected into the formation. Injectivity decline caused by solids plugging or bacterial growth in the near-wellbore zone is expensive to remediate and can require well workover or abandonment. Maintaining water quality within the injection well management plan specifications is essential for protecting the long-term productivity of the injection well. |
How Does Monitoring Differ Between Discharge and Reinjection Systems?
For overboard discharge, the primary monitoring requirement is continuous oil-in-water monitoring at the treatment package outlet confirming that oil concentration is within the field permit discharge limit before water reaches the sea.
For produced water reinjection, the monitoring programme is more comprehensive because multiple water quality parameters must be controlled simultaneously:
- Oil-in-water concentration monitored at the treatment package outlet to confirm oil has been reduced to within the injection specification
- Total suspended solids monitored to confirm the filtration system is maintaining solids below the injection threshold
- Particle size distribution monitored to confirm that particles are below the size that causes formation plugging
- Bacterial count periodically sampled and analysed to confirm that the biocide programme is controlling bacterial growth
- Dissolved oxygen monitored where oxygen ingress into the injection water system is a risk
The oil-in-water monitor in a PWRI system measures oil concentration in the treated water before injection, confirming that residual oil is within the injection specification. The OCD Xtra is suitable for produced water applications where calibration against specific crude oil types is required and where measurement accuracy across a broader concentration range is needed making it well suited to monitoring the lower oil concentrations required for injection water compared to overboard discharge.
What Is the Treatment Process for Produced Water Reinjection?
The treatment process for produced water reinjection builds on the same primary treatment stages used for overboard discharge but typically requires additional polishing stages to achieve the tighter quality specifications for injection. For a full explanation of the produced water treatment process, see the Produced Water Treatment on FPSOs: A Compliance Guide for EPC Teams.
A typical PWRI treatment train includes:
- Primary separation three-phase separator or free water knockout removes bulk oil and gas from the produced water stream
- Hydrocyclone de-oiling removes dispersed oil droplets above approximately 10 to 15 microns
- Gas flotation induced or dissolved gas flotation removes finer oil droplets in the 5 to 15 micron range
- Filtration media filters (walnut shell, sand), cartridge filters or membrane systems remove solids and fine particles to the injection well specification
- Biocide dosing controls bacterial growth in the water system
- Oxygen scavenging removes dissolved oxygen where ingress into the system is a risk
- Oil-in-water monitoring confirms oil concentration in the treated water before injection
What Should EPC Teams Consider When Designing a PWRI System?
For EPC process and instrumentation engineering teams, the key decisions in PWRI system design are:
- Injection well management plan requirements the water quality specifications that must be met are defined in the injection well management plan and should be confirmed before treatment system design begins
- Treatment system capacity PWRI treatment trains must be designed for the expected produced water volume and composition across the full field life, including increasing water cut in mature production phases
- Filtration system selection the filtration technology must be capable of achieving the particle size and solids specifications required for injection, not just the oil concentration limit for discharge
- Monitoring and sampling programme the monitoring points, parameters, frequency and analytical methods must be defined in the injection well management plan and designed into the system
- Injection pump design the injection pump must be capable of delivering the required injection rate and pressure across the range of reservoir conditions expected during field life
- Compatibility testing the treated produced water must be tested for compatibility with formation water and reservoir mineralogy before the injection programme begins
Rivertrace provides oil-in-water monitoring solutions for PWRI applications as part of a broader portfolio of produced water monitoring products. Full details of the advisory support available for FPSO produced water projects including PWRI monitoring are on the Produced Water Discharge Monitoring for FPSOs page.
Frequently Asked Questions
What is produced water reinjection?
Produced water reinjection (PWRI) is the process of treating produced water from oil and gas production and injecting it back into the subsurface either into the producing reservoir for pressure support or into a separate disposal formation rather than discharging it overboard. It serves both an environmental disposal function and, in some cases, a reservoir management function by maintaining reservoir pressure.
Why do some operators choose reinjection over overboard discharge?
Operators choose reinjection where overboard discharge is restricted by environmental permit or zero-discharge requirements, where the treated water cannot consistently meet the discharge limit, where reservoir pressure support is beneficial for oil recovery, or where the receiving environment is particularly sensitive. In some jurisdictions, reinjection is the only permitted disposal route for produced water.
What water quality is required for produced water reinjection?
Produced water for reinjection must meet tighter quality standards than for overboard discharge. As well as oil concentration, total suspended solids content, particle size distribution, bacterial count and chemical compatibility with the formation water and reservoir mineralogy must all be controlled. The specific requirements are defined in the injection well management plan and depend on the reservoir characteristics and injection well design.
How does monitoring for produced water reinjection differ from monitoring for discharge?
Overboard discharge monitoring focuses primarily on oil-in-water concentration at the treatment package outlet. PWRI monitoring is more comprehensive, covering oil concentration, total suspended solids, particle size distribution, bacterial count and dissolved oxygen. The oil-in-water monitor in a PWRI system measures against the injection water specification rather than an overboard discharge permit limit, which is typically a lower oil concentration threshold.
What causes injection well injectivity decline?
Injection well injectivity decline is primarily caused by solids plugging of pore spaces in the near-wellbore zone, bacterial growth and biofilm formation in the injection well, scale precipitation from incompatible water chemistry, and corrosion damage from oxygen ingress. Maintaining water quality within the injection well management plan specifications particularly for solids content and bacterial count is the primary defence against injectivity decline.