Produced water: discharge or reinjection? How the treatment differs

Produced water, the water that comes up with oil and gas, is usually the largest waste stream an oil field handles, and the volume tends to rise as a field matures. It has to go somewhere: back into the sea or a river, back into the ground, or, increasingly, into reuse.

Each route sets a different specification, and the specification decides the treatment. Designing the treatment before the destination is settled is one of the most common and expensive mistakes on produced water projects.

Route 1: discharge

Discharge to sea (offshore) or to surface water (onshore) is regulated environmentally. The main concern is what the water does to the receiving environment.

The headline parameter is usually oil in water. For example:

• In the North-East Atlantic, the OSPAR performance standard for dispersed oil is a monthly average of 30 mg/l for offshore installations, with national regulators setting permits.
• On the US Outer Continental Shelf, EPA effluent guidelines (40 CFR Part 435) limit oil and grease to a monthly average of 29 mg/l and a daily maximum of 42 mg/l.

Permits can also cover toxicity, production chemicals, heavy metals, naturally occurring radioactive material (NORM), temperature and salinity. Onshore discharge limits are often much tighter. Always work from the actual permit: the figures above are a starting point, not a design basis.

Typical treatment for discharge: a three-phase separator, then hydrocyclones, induced or dissolved gas flotation, and sometimes a polishing step such as nutshell filters or adsorbents to remove dissolved organics.

Route 2: reinjection

When produced water is reinjected, into the reservoir for pressure support or into a disposal well, the concern changes from the environment to the reservoir and the wells. The question is whether the water will plug the formation, scale the wells or sour the reservoir.

Reinjection specifications are set by the operator, from reservoir studies and injectivity tests, and usually cover:

• Suspended solids and particle size — compared with the pore throat size of the formation. Tight formations need much finer filtration.
• Oil in water — oil droplets and solids together plug the formation face.
• Compatibility and scaling — mixing produced water with seawater or another source can precipitate barium, strontium or calcium sulphate.
• Oxygen — introduced through open tanks, it causes corrosion and reacts with iron to form solids.
• Bacteria — especially sulphate-reducing bacteria, which can sour the reservoir and cause corrosion.

Typical treatment for reinjection: separation and flotation as for discharge, then filtration matched to the formation, deoxygenation, closed systems to keep air out, and scale inhibitor and biocide dosing.

Why the difference matters

Aspect	Discharge	Reinjection
Who sets the limit	Environmental regulator	Operator, from reservoir data
Main concern	Environmental impact	Injectivity, scaling, souring, corrosion
Key parameters	Oil in water, toxicity, chemicals	Solids, particle size, oil, oxygen, bacteria
Typical failure	Permit exceedance	Falling injectivity, well workovers

A plant designed for discharge can struggle with reinjection solids specifications, and a plant designed for reinjection may not meet a discharge permit on dissolved components. If both routes are possible — for example reinjection with discharge as a backup during well downtime — the design has to meet both.

Before choosing a technology

1. Fix the destination, and the specification that goes with it.
2. Characterise the water: oil droplet size, solids, dissolved components, scaling ions, bacteria and how they change over the field’s life.
3. Check compatibility if waters are mixed.
4. Design for the future, as water cut and chemistry change.
5. Review what is already installed. Existing plants often have unused capacity once the bottlenecks are understood.

Waterna provides independent produced water treatment engineering — characterisation, technology selection and design review — for discharge, reinjection and reuse.