Produced Water Treatment (PWT) versus Produced Water Re-Injection (PWRI)
Produced water is an unwanted by-product of oil and gas production, and in many oilfields more water is produced than oil. Globally the ratio is about 3:1 water to oil, therefore with some 250 million barrels per day of water to be disposed of or re-used.
For many decades, produced water injection to disposal wells, into aquifers or ‘lost circulation zones’ has been common place in onshore fields, and so has re-injection (PWRI) into the originating oil reservoir, but offshore it has been common to discharge produced water to sea after removal of the free oil down to around 40 ppm oil-in-water. In some sea areas a somewhat easier specification of 100 ppm has applied, but some ‘tighter’ specifications have been applied in relatively enclosed seas or lakes e.g. 5 ppm in the Arabian Gulf. Recently North Sea limits have been reduced from 40 to 30 ppm, with limits also put on the total amounts of oil which may be discharged per oilfield, backed up by heavy fines for exceeding the agreed amounts, at over £100,000 per tonne of oil in the produced water.
Also, for several decades there has been considerable concern about the harmful, or potentially harmful, effects on the environment of the dissolved hydrocarbons and treatment chemicals in produced water, and in many areas toxicity testing is required and limits set so that discharge concentrations are well below the apparently harmful values. The actual quantities of treatment chemicals used in an oilfield or platform may also be limited.
This gives the impression that the environment is being adequately protected, and indeed some oil companies have now backed away from their clearly-stated promises made around year 2000, that all their produced water would be re-injected by, say, 2005, and so their produced water would be ‘zero discharge’. Gradually this was changed to ‘zero harmful discharge’. But now some are claiming that it would be more environmentally harmful to actually re-inject the water than to continue discharging to the sea. This is on the basis that PWRI typically requires higher pump pressures and so more energy and so more CO2 discharge to the atmosphere, than simply discharging water to the sea (which is clearly true). Also, PWRI is said to require more power to inject than seawater or aquifer water, on the basis that produced water is much dirtier and so needs higher pressures to propagate fractures in injection wells and so continue to inject water.
So, PWRI is a topical issue, despite its long history [1 to 7], and these aspects are expanded upon in this short course along with discussion of produced water treatment options (PWT). Oil-in-water monitoring is not discussed in this workshop/training course.