Towards a Cleaner Marine Environment
6. Oil and gas extraction at sea
Oil and gas are extracted through pipes extending seven to eight km down into the earth's crust, at depths of down to 60 metres in the Danish sector of the North Sea. Production has increased rapidly since the first oil was brought to the surface from Danfield in 1972. About 40 platforms, distributed over 16 oil and gas fields, are in operation today. Production exceeded the Danish consumption of oil and gas for the first time in 1991. 1999 was also a record year, in which production exceeded Denmark's total energy consumption by 12 percent. Although North-Sea oil gives the Danish state a generous income, oil production also has an impact on the marine environment. Drilling fluid and produced water are the primary causes of ecological problems.
Drilling waste can pollute
Drilling waste (which is discharged into the sea after partial purification) occurs during drilling operations. This drilling waste consists of the drill cuttings and the drilling fluid, which is pumped down through the drill itself, during drilling and which contains various chemical additives. The drilling waste contains residues of the drilling fluid, in the form of minerals, chemicals and traces of heavy metals. Drilling fluid is used to lubricate and cool the drill bit and, by virtue of its weight, to prevent blow-out, in which oil and gas erupt violently when the drill penetrates an oil pocket. The mud is squeezed out through holes in the drill bit, from which it is forced back up to the platform within the jacket of the drilling pipe, together with the drill cuttings.
Oil residues in produced water
The oil and gas extracted from wells contain a certain amount of water. This "produced water" is separated out on the platform and purified according to a set of rules, before it is discharged into the sea. But produced water still contains oil residues, which amount to an annual total of over 200 tonnes (1999) and which are discharged into the North Sea from Danish oil fields. This quantity of water has been increasing in recent years, in part because of increased production. Produced water also contains chemicals used in the process. Some of these chemicals can probably affect the marine environment.
Environmental requirements on the off-shore industry
The off-shore industry's impact on the marine environment is monitored continually. Investigations show that the effects on the sea-bed around platforms include e.g. the high hydrocarbon content of the sediment and the paucity of life on the sea-bed. For this reason, the environmental authorities regulate the off-shore industry to ensure that its production is environmentally acceptable. The guiding principle is that only the best available technology be used. Just now, the goal is to reduce the oil content of the produced water through improved purification. At the same time, there is an on-going transition to chemicals less hazardous to the environment.
