CO2 storage in under-sea caverns closely resembles natural process

Mar 06, 2002 01:00 AM

Melting ice caps, rising sea levels, floods and droughts are all a result of climate change and the increasing presence of carbon dioxide in the atmosphere is cited as a leading cause of this change. Since the industrial revolution the level of CO2 in the atmosphere has risen by 33 % -- and it is still rising.
Forestation is one way of reducing the gas because during photosynthesis trees absorb CO2 and give out oxygen. But trees are vulnerable to droughts, forest fires, floods and storms, as well as to the demands of industry. However, a complementary strategy to forestation is being tested, which involves extracting the CO2 that is found with natural gas in gas fields -- and storing it underground.
After burning, natural gas treated in this way releases less CO2 than it would have done before treatment. The CO2 can be removed before the gas is sent ashore or, at a coal-fired power station, it can be extracted from flue gases.

For the past five years Statoil, the Norwegian oil company, has been extracting CO2 from natural gas at the Sleipner field in the North Sea before the gas is sent ashore and injecting it into a large aquifer about 1 km below the seabed. Monitoring suggests that the leakage from the aquifer is negligible. Natural gas has been held in under-sea caverns for thousands of years and presumably would have stayed there for many years more, except that the caverns have been deliberately breached to extract the gas.
Scientists believe that because the CO2 storage technique closely resembles a natural process, it is inherently stable. In fact, there are some deep caverns where CO2 itself has been held naturally for millions of years as a natural result of geological processes.
Under the oceans there are also natural gas caverns from which the gas has already been extracted and which are now full of water and sand. Filling those with CO2 simply replaces one gas with another, using a storage space that has been proven to be gas-tight for millions of years.

The cavern under the Sleipner field has barely started to fill. Researchers from the British Geological Survey have calculated that just 1 % of its capacity could accept the annual output of CO2 from about 925 coal-fired or about 2,340 gas-fired 500 MW power stations. Enough capacity exists in the North Sea and in continental Europe to absorb 800 years' worth of CO2 emission from all Europe's fossil fuel power stations, while the available capacity is probably more than 800 bn tons.
The cost of the process -- stripping the CO2 from the natural gas, putting it under pressure and then injecting it -- is about EUR 45 (£ 28) a ton, close to Norway's carbon tax of EUR 40 a ton. However, Tor A. Torpe of the Sleipner Field project points out: "Operating anything in the North Sea is three times more expensive than it is ashore." A fossil fuel power station on land could strip the CO2 for EUR 30 a ton, a figure that includes the cost of pipelines to convey the gas to the nearest suitable storage cavity.

Source: The Financial Times
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