Fri, Jul 05, 2013 - Page 8 News List

Closer look at carbon storage plan

Wei Kuo-yen 魏國彥

A major story published in the Apple Daily recently said that Taiwan Power Co (Taipower) and state-run oil refiner CPC Corp, Taiwan, are secretly planning to store carbon along the west coast of Taiwan “right under people’s noses.”

Residents of Yunlin and Changhua counties have expressed concern about these plans, citing a case of mass asphyxiation in the West African country of Cameroon.

On Aug. 21, 1986, Lake Nyos, a crater lake saturated in carbon dioxide, suddenly released a large cloud of carbon dioxide gas into the atmosphere, asphyxiating everything within a 25km circumference and causing the deaths of about 1,700 people and 3,500 livestock.

This tragedy was an example of the dangers involved in storing carbon dioxide.

Yet carbon dioxide capture and storage (CCS) is nothing new: It has been used by many petroleum companies to increase underground pressure and displace oil from underground fissures — a process known as carbon dioxide enhanced oil recovery (EOR). EOR is used to increase yields in aging oil fields and to store carbon dioxide underground, killing two birds with one stone.

CPC’s current plan is to pour carbon dioxide back into exhausted underground gas-bearing structures, or reservoirs, in Miaoli, from which natural gas has already been extracted, essentially pumping waste emissions back into the reservoirs that had originally held the gas in its natural state.

The structures already have impervious overlying strata, otherwise they would have been unable to hold the natural gas in the first place. Pouring the carbon dioxide gas into the reservoirs would be like placing bonds or old newspapers into a safe after you have removed your cash: It presents no risks whatever.

Taipower, for its part, intends to store carbon in deep salt water reservoirs from 2km to 3km underground. To ascertain the conditions in the underground strata, it is already drilling exploration wells that will later be reincarnated as monitoring wells.

The upper level in the stratigraphic sequence in this area is a cap layer, followed by an impermeable layer and then a storage layer.

When the carbon dioxide seeps into the salt water reservoir 3,000 meters down, it will set off a series of chemical reactions leading to a process of mineralization.

After this, the carbon dioxide will cease to act as a gas, a process known as “geological sequestration,” at which point the process is no longer pure storage. Furthermore, the strata intended to store the gas slants up toward the west. If the gas placed underground migrates, it will do so in this direction and leak out into the ocean floor of the Taiwan Strait.

The 1986 Lake Nyos incident, however, was caused by a completely different mechanism. Nyos is a vast volcanic lake 200m deep. The depth means that the gas exchange rate varies depending on the level. There is little oxygen at the bottom of the lake, where the concentrations of carbon dioxide are relatively high.

Also, the volcano at the lower corner of the lake gently emits heat, increasing the temperature in the lake. When the water hits a certain temperature it rises, displacing the cold water above, which then sinks, and it is when the water with high concentrations of carbon dioxide comes into contact with the surface that it releases the gas into the air.

Lake Nyos is in a basin, surrounded by mountains. The carbon dioxide, on this occasion, became quite concentrated before it spilled out and spread over the local area, asphyxiating people and animals.

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