Impact of Natural Gas in the Marine Environment

by Dr. Irene Novaczek
from: “Environmental Impact of the Offshore Oil and Gas Industry,”

During drilling and extraction of gas deposits from the sea floor, releases of gas into the marine environment are inevitable. Gas is dumped into the sea mixed in with produced water, may leak from pipelines, tankers and underwater storage tanks, or may be released during catastrophic well blowouts, explosions and smaller accidental spills. Spills and blowouts occur due to drilling equipment failure, corrosion of pipelines, human error, earthquakes, ice, storms, shipping accidents etc.

Pipeline failures are most commonly due to material or welding defects, but pipelines may also be hit by anchors or trawls, or affected by earthquakes or ground erosion. The environmental consequences of releases of natural gas into the sea are especially severe when they happen near shore, in shallow waters or in areas with slow water circulation.

Among the most dangerous situations are gas tanker accidents, which may trigger explosions when rapidly evaporating gas at the sea surface stimulates formation of gas clouds, which then can combust and explode, destroying every living thing in areas of up to 400 square km.

Toxic to Fish and Shellfish

Many people think that natural gas would just bubble up to the surface and quickly evaporate off but in fact a significant portion dissolves in the water and is highly toxic to marine life. The gas can rapidly penetrate the bodies of fish, doing direct damage to gills, skin, chemoreceptors and eyes, and filling up the gas bladder, making the fish unable to control its buoyancy.

At concentrations of 0.02 – 0.05 mg/l, gas will be sensed by fish and they will move away. If however, fish are exposed to concentrations above 1 mg/l they become excited within seconds of contact, then disoriented and unable to flee. Within 15 – 20 minutes fish exposed to such concentrations show signs of acute poisoning, and they die within 1-2 days of exposure. Shellfish are also killed by exposure to gas. Zooplankton and phytoplankton can tolerate higher concentrations of gas than fish or shellfish can (i.e. they die at 2 – 5 mg/l).

Accidental gas releases on a migratory route of fish such as salmon, either in the sea or from a pipeline close to a river, can block a spawning migration. A localized release can thus have a regional impact.

In the Gulf of Mexico, high concentrations of methane were found around offshore drilling rigs during a 1975 study by Sackett and Brooks. In the North Sea and in California, bottom ecosystems are disturbed and their species compositions altered where there is gas seepage. Biomass of living things declines and shellfish disappear.

Some fish, such as flounders, are more sensitive to gas than other species. Juveniles are more sensitive than older fish. Fish also become more sensitive if repeatedly exposed to low concentrations of gas. Fish are more vulnerable when water temperatures are high or when oxygen concentrations are low (as in an eutrophic estuary in summer). Under conditions of cold temperature and high pressure, gas may react with water to form hydrates. These can be trapped and accumulate under ice in winter and be converted to methane as water temperature rises in spring, with serious environmental consequences. If the gas is “sour,” or contains sulfides, it is much more highly toxic to marine life.

Air Pollution from Flaring

Flaring or burning off of natural gas at drilling sites causes air pollution. Pollutants are commonly removed from the air by precipitation and deposited in the sea around the well. Impacts of this pollution have not been well studied.

Rigs as Fish Aggregating Devices

Petroleum developers often point to the Gulf of Mexico and say that rigs there are good for the fishery because catch rates have gone up. Indeed, rigs placed on sandy bottom where there is no other shelter act like artificial reefs. They attract fish and it is easy for sport fishers to catch them. Analysis of the situation shows that although rigs do act like fish aggregating devices, there is no increase in the abundance of the stock, it merely gets redistributed and easier to catch with hook and line. In the North Sea, where fishes are bottom species, there has been a negative relationship between the number of oil and gas rigs and fish catches.


[From WS December 2001/January 2002]

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