Produced Water Treatment: How to Choose the Right Train from Separator to Injection Well

8 hours ago

3 min read

At a glance. Build a produced water train from the end point backwards. Fix the target first: the injection specification set by the reservoir, or the discharge limit set by the regulator. Then add stages until the oil and solids left after each one suit the next: primary separation, secondary deoiling by hydrocyclone or induced gas flotation, and tertiary filtration by walnut shell filter and cartridge polishing.

What sets the target?

For injection, the reservoir sets it: tight formations plug quickly, so they need very low oil and solids and fine filtration. For discharge, the regulator sets it. Offshore in the North-East Atlantic, for example, the OSPAR performance standard limits dispersed oil to 30 mg/L as a monthly average. Agree the target, including particle size for injection water, before choosing equipment.

What does each stage do?

Stage	Equipment	Removes
Primary	Separators, skim tanks	Free oil and large droplets
Secondary	Deoiling hydrocyclones, induced gas flotation (IGF)	Dispersed oil droplets and some fine solids
Tertiary	Walnut shell filters	Residual oil and fine solids
Polishing	Cartridge filters	Fine suspended solids before injection pumps

How does droplet size decide the technology?

Every separation method works down to a certain droplet size. Hydrocyclones use the pressure already in the system to spin out droplets, have no moving parts and suit the larger dispersed droplets. Induced gas flotation attaches fine gas bubbles to smaller droplets and lifts them to the surface, so it handles finer oil and copes with variable feed. Walnut shell filters trap the fine residue in a deep media bed that is scrubbed and backwashed in place. Cartridge polishing then removes the fine solids that would plug injection wells. Droplet size analysis of the actual water is the best basis for choosing.

Hydrocyclone or flotation first?

Hydrocyclones need driving pressure, from the upstream separator or a booster pump, and take little space, which suits offshore platforms. IGF works at low pressure, tolerates changes in feed better and removes smaller droplets, but takes more space. Many trains use both: hydrocyclones for the bulk of the oil, then flotation or walnut shell filtration to reach the target.

Sand comes first

Sand from the wells erodes chokes, hydrocyclone liners and pumps, and fills vessels. A wellhead desander removes it at source, before it reaches the water train.

Specification checklist

Water flow now and at plateau, in barrels a day or m³/h
Oil-in-water and solids at the inlet, with droplet size data if available
Injection specification or discharge limit
Pressure available from the separators
Space and weight limits
Flotation gas available: produced gas, fuel gas or nitrogen
Materials for the water chemistry, including chlorides and H₂S
Redundancy needed to keep injection running during maintenance

Tech Inc. produced water equipment

Tech Inc. supplies deoiling hydrocyclone skids, induced gas flotation units, walnut shell filters, cartridge polishing skids, complete produced water treatment packages and wellhead desanders. Send your water analysis and target to mail@techincresearch.com.

Frequently asked questions

Do I need both a hydrocyclone and flotation?

Not always. If the droplets are large and the target is moderate, hydrocyclones may be enough. Finer droplets or a tight injection specification usually need flotation or walnut shell filtration as well.