Problem With Burying CO2

In this weeks issue of The Economist, an article entitled 'Plumbing the depths' expresses concern for carbon sequestration in shallow aquifers. Japan may want to rethink their efforts to reduce pollution by burying emissions.

One way of slowing climate change would be to prevent greenhouse gases from entering the atmosphere in the first place. In the case of carbon dioxide, a widely discussed suggestion is to capture it when it is produced in power stations and other large industrial plants, and store it in geological formations from which it cannot easily escape. Oilfields, natural-gas fields, aquifers filled with seawater and depleted coal seams are all possibilities.

Oil companies have long pumped carbon dioxide into depleted fields—not for environmental reasons, but because it forces out the remaining oil. America has 80 such fields, some of which are 30 years old. Aquifers full of brine, however, could be better because there are so many of them, and they often lie close to offshore oil- and gas-fields. Statoil, Norway's national oil company, started pumping carbon dioxide into aquifers under the North Sea a decade ago and BP has a similar onshore project in Algeria.

But few studies have looked at what happens once the gas is in the ground. In October 2004 a group of researchers led by Yousif Kharaka of the United States' Geological Survey in Menlo Park, California, pumped 1,600 tonnes of carbon dioxide into the Frio formation, a disused brine and oil reservoir east of Houston, Texas. The results of their experiment have just been published in Geology.

The team compressed the gas into its liquid form and pumped it into a layer of sandstone 24 metres thick, lying 1.5km (about a mile) under the surface. They have been monitoring the site ever since, and so far they have found no leaks.

What they have, however, found is that the carbon dioxide has increased the acidity of the water in the aquifer. This, in turn, has dissolved the minerals that hold the sandstone together. As their report puts it, “this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for carbon dioxide and brine leakage.”

Although changes in the rock made in a year can certainly be described as rapid, the concept of “ultimately” in geological time could be thousands—or even millions—of years. So there may be no need to worry unduly. But Dr Kharaka nevertheless suggests that the sequestration of carbon dioxide should be confined to deep aquifers, where overlying layers of shale would be likely to prevent leaks.