"Cleaning up Produced Water During Offshore Drilling

Environmental Impact of the Offshore Oil and Gas Industry by Stanislav Patin - a unique summary of world-wide studies on the environmental issues associated with offshore oil and gas exploration and development

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Muddied Waters

A Survey of Offshore Oilfield Drilling Wastes and Disposal Techniques to Reduce the Ecological Impact of Sea Dumping

by Jonathan Wills, M.A., Ph.D., M.Inst.Pet., for Ekologicheskaya Vahkta Sakhalina (Sakhalin Environment Watch); 25th May 2000

Minimising Waste Discharges and Their Effects (continued)

Cleaning up Produced Water Streams

As more information has become available on the possible long-term effects of pouring such very large quantities of even mildly polluted water into the sea every day, governments, the industry and environmental campaigners have continued the search for ways to reduce the volume of produced water, to re-inject it into the rocks below the seabed, and to make it as clean as possible when there is no alternative to overboard discharge ( For an early discussion of the problems, see: Read, A.D. 1978. Treatment of oily water at North Sea installations - a progress report. In Johnstone, C.S. and Morris, R.J. (eds). Oily Water Discharges. Regulatory, Technical and Scientific Considerations. pp.127-136 Applied Science Publishers, Barking, England. ).

In Norway, for example, the Jotun and Balder fields do not normally discharge any produced water. It is all re-injected from floating production, storage and offloading vessels (FPSOs) (SFT spokesman, Oslo, May 2000. pers. comm.)

In 1995 the American Petroleum Institute called a special two-day meeting, chaired by Joseph Smith of Exxon Production Research, to look at ways of cleaning up the wastewater stream from offshore installations. (American Petroleum Institute. 1995. op. cit. See also: OSPAR. 1992b. PARCOM Recommendation 92/6 on Best Available Technology for Produced Water Management on Offshore Gas and Oil Installations. Brussels.) Smith's work group responsible for producing the report of the meeting included representatives of Chevron, Conoco, Marathon, Phillips Petroleum, Shell and Texaco.

The report identified the following factors as contributing to the toxicity of produced water: very small particles, salinity (9% or greater), volatile compounds, extractable organics (acidic, basic, neutral), ammonia and hydrogen sulphide. Six water treatment technologies already proven onshore, were evaluated and costed - for offshore use. Although each method presented technical problems, none of them was insuperable. The report made it clear that, by using combinations of different technologies, it is possible to reduce the pollutants in produced waster to almost undetectable levels. Further research was needed, however, before accurate cost estimates could be made. The accompanying table shows the technologies assessed by the API group.

In short, it is usually not necessary to discharge production water to the sea. It would not be done on land. It has been almost eliminated in some European countries' offshore oil and gas fields. It continues elsewhere for economic rather than technical reasons.

Table 14: Technology to clean up Production Water

(After API, 1995)

Technology	Processes	Advantages	Disadvantages	Costs
Carbon adsorption	Modular granular activated carbon systems.	Removes hydrocarbons and acid, base and neutral compounds; low energy requirements; higher throughput than other treatments (except biological); treats a broad range of contaminants; very efficient at removing high Mwt. Organics.	Fouling of carbon granules is a problem; produces waste stream of carbon and backwash; requires some pre-treatment of produced water stream.	"Middle range" of costs.
Air stripping	Packed tower with air bubbling through the produced water stream.	Can remove 95% of volatile compounds as well as benzene, toluene, naphthalene, phenanthrene, anthracene, pyrene and phenols; H2S and ammonia can be stripped but pH must be adjusted; higher temperature improves removal of semi-volatiles; small size, low weight and low energy requirements; simple to operate; well-known technology.	Can be fouled by oil; risk of iron and calcium scales forming; generates an off-gas waste stream that may require treatment; requires some pre-treatment of produced water stream.	Low capital and operating costs; overall treatment cost US$0.02 to $0.10/1,000 gallons, plus $0.50 to $1.50/1,000 gal if off-gas control by activated carbon is required.
Filtration	Very fine membranes.	Effective removal of particles and dispersed and emulsified oil; small size, low weight and low energy requirements; high throughput rates.	Does not remove volatiles or dissolved compounds. Does not affect salinity; oil, sulfides or bacteria may foul membrane, which requires daily cleaning; waste streams may contain radioactive material; requires some pre-treatment of produced water stream.	Low capital and operating costs (similar to air stripping).
Ultra-violet light	Irradiation by UV lamps	Destroys dissolved organics and both volatile and non-volatile organic compounds, including organic biocides; does not generate additional waste stream; handles upset or high-loading conditions.	Will not treat ammonia, dispersed oil droplets, heavy metals or salinity; relatively high energy requirements; UV lamps may become fouled; residues may be toxic if peroxide used; requires some pre-treatment of produced water stream.	Similar capital costs to chemical oxidation with ozone but operating costs lower because no waste stream.
Chemical oxidation	Ozone and/or hydrogen peroxide oxidation	Removes H2S and particulates; treats hydrocarbons, acid, base and neutral organics, volatiles and non-volatiles; low energy requirements if peroxide system used; straightforward to operate.	High energy inputs for ozone system; oil may foul catalyst; may produce sludge and toxic residues; requires some pre-treatment of produced water stream.	"Middle range" of costs.
Biological treatment	Aerobic system with fixed film biotower or suspended growth (e.g. deep shaft)	Treats biodegradable hydrocarbons and organic compounds, H2S, some metals and, in some conditions, ammonia; "fairly low" energy requirements; handles variable loadings, if acclimated.	Large, heavy plant required for long residence times; build-up of oil and/or iron may hinder biological activity; aeration may cause calcium scale to form; may produce gas and sludge requiring treatment; requires some pre-treatment of produced water stream.	"No costs estimated"