To curb greenhouse gases, put them underground

Storing carbon dioxide in underground caves that once held oil and gas is shaping up to be one of the more promising techniques for reducing the amount of greenhouse gases that get pumped into the atmosphere, he said during a presentation at the International Technology Petroleum Conference taking place here this week.

"Gas fields have the attraction of natural seals. They have held gasses for millions of years," Espie said. "Storage in geological formations is one of the cheapest options."

Trials for the concept are already under way in Algeria and Canada. Commercial development could begin within the next five years. Ideally, storage facilities would keep carbon dioxide underground for at least 1,000 years and hopefully 10,000 or more.

The carbon problem comes as a direct result of fuel consumption. Since the industrial revolution, the amount of carbon dioxide has increased dramatically, paralleling a spike in global temperatures, Espie said. In the 20th Century, average temperatures rose about 0.6 degrees Celsius. While that doesn't sound like a lot, scientists say that a rise of 2 degrees Celsius will cause significant disruptions to water supplies and agriculture.

Currently, about 25 billion tons of carbon dioxide are injected into the atmosphere each year, of which about 7 billion tons make it into the atmosphere. That's close to the maximum, Espie said.

Fossil fuel demand, however, is rising rapidly too. By 2050, the emissions figure will double if preventative action isn't taken, according to Espie.

"By 2050, storage could provide about half of what is needed," he said. The other half could be accomplished through storing it in underground deposits of saline water.

Carbon storage involves a two-step process. First, carbon should be sequestered during the combustion process, either through industrial scrubbers or technologies like oxyfuels. With oxyfuels, fuel is ignited in a pure oxygen environment, rather than in air, which contains nitrogen. The reaction leads to wet carbon dioxide.

The carbon dioxide can then be compressed and stored. In some nations, the cost could be subsidized under an emissions credit plan. Carbon dioxide storage could also be used to extract more oil out of depleted fields; by pumping in C02, the added pressure would force out remaining oil that's been otherwise difficult to extract.

Significant risks, of course, exist. A rupture in an underground cavern can lead to a bubble of carbon dioxide. If people were to walk through the bubble, they would probably die of suffocation, Espie said. That problem, however, is fairly localized. Sensors placed in the area can provide some warning.

A large earthquake could also create a fault that would let carbon dioxide escape over a broader area. "In that case, you'd probably have to take the CO2 out," he said.

If the gas escaped into a water supply, the result would be carbonated water, but health officials would be concerned about any deleterious chemical reactions.

Still, the geochemical and geophysical data show that the concept has promise. In some cases, carbon dioxide storage could prove beneficial by redistributing minerals and making the cavern stronger, Espie said.

The biggest roadblock ultimately could be public acceptance of the idea. To that end, he suggested the industry share the data of these projects.

Administrative bodies also need to be created. Companies don't last 1,000 years, he said.