Produced Water Treatment: What Is Actually Used in the Field

"Treatment" covers a wide range here, from a tank that lets oil float to the top to membrane processes that would not look out of place in a municipal plant. What gets installed depends entirely on what the water is going to do next, and for most Alberta produced water the answer is "go down a disposal well", which sets a much lower bar than reuse would.

This guide runs through what is actually deployed, roughly in order of how common it is.

Start With the Requirement

Before any equipment decision, the question is what the water has to be fit for.

• Disposal down a Class II well: the requirement is that it will not damage the injection formation or the equipment on the way there. Free oil and solids need to come out; dissolved salt does not need to be touched. This is the least demanding case and covers the bulk of Alberta produced water.
• Reinjection for enhanced recovery: more demanding, because the water is going into a producing reservoir you care about. Oil in water, suspended solids, bacteria and oxygen all matter, because plugging the injection formation damages the scheme.
• Reuse in completions: depends on the frac design and chemistry compatibility. Often less demanding than people assume.
• Discharge or agricultural use: would require desalination. This is very rarely done in Alberta on cost grounds.

Specifying treatment for a higher standard than the destination requires is one of the more common and expensive mistakes.

Gravity Separation

This is the foundation and it is on essentially every battery. A three-phase separator takes the well stream and splits it into gas, oil and water using nothing more exotic than residence time and density difference. It is cheap, reliable and has no moving parts to speak of.

What it will not do is remove finely dispersed oil or emulsions. A separator gets the bulk of the free oil out and leaves a water stream still carrying some.

Skim Tanks and Free Water Knockouts

The next stage down the train, giving the water more residence time so additional oil can rise and be skimmed. Simple, effective for what it does, and the performance is almost entirely a function of how much residence time you give it and how well the inlet is designed to avoid short-circuiting.

Many small properties stop here, because the water is going to disposal and this is enough.

Chemical Treatment

Chemistry does a lot of the work that equipment would otherwise have to do:

• Demulsifiers break oil-in-water and water-in-oil emulsions so that gravity separation can work at all.
• Coagulants and flocculants make fine solids and oil droplets aggregate into something large enough to settle or float.
• Scale inhibitors prevent carbonate and sulphate scale from forming in flowlines, vessels and, critically, in the injection formation.
• Corrosion inhibitors protect steel that is in constant contact with brine.
• Biocides control sulphate-reducing bacteria, which sour systems and produce iron sulphide that plugs injection wells.

Chemical programmes are frequently either over- or under-dosed because nobody has tested in a while. It is worth reviewing periodically; both directions cost money.

Induced Gas Flotation

Where skimming is insufficient, flotation introduces fine gas bubbles that attach to oil droplets and carry them to the surface far faster than buoyancy alone would. It handles dispersed oil that gravity separation leaves behind, and it is common where water is destined for reinjection into a producing formation.

It costs more to run than a skim tank and it needs attention, so it appears where the water quality requirement justifies it.

Filtration

Media filters, typically sand or walnut shell, remove suspended solids and some residual oil. Walnut shell filters in particular are widely used in produced water service because they tolerate oil that would blind a sand bed. Cartridge filters sit further downstream as polishing.

Filtration is where reinjection water quality is usually finally made, and filter performance is one of the more useful things to trend, because a rising differential pressure tells you something upstream has changed.

What Gets Discussed More Than Installed

Reverse osmosis, thermal distillation, mechanical vapour recompression, electrodialysis and various emerging desalination technologies all appear regularly in conference papers and vendor material. They work. The issue is that removing dissolved salt is energy-intensive, and the resulting concentrated brine still has to be disposed of, so you have spent a lot of money to reduce a volume rather than eliminate a problem.

In a jurisdiction with deep, well-characterised disposal formations and an established disposal industry, the economics rarely favour desalination. They can change where disposal capacity is constrained, where water is scarce enough to have real value, or where regulation restricts disposal. None of those currently describes most of southern Alberta.

Common Questions