How much carbon can we store?

One of the biggest challenges facing firms developing carbon capture and storage (CCS) systems is working out what to do with the carbon dioxide once it has been caught.

Now researchers at the Massachusetts Institute of Technology (MIT) have unveiled a new system to help tackle the problem by accurately assessing how much CO2 can be sequestered in a given geological formation.

Critics of CCS have warned that there is a risk that carbon dioxide pumped underground into semi-permeable rocks and aquifers could leak back out into the atmosphere over time, negating the benefits of capturing the gas in the first placed.

But the new software model from MIT, which was debuted this week at the International Conference on Greenhouse Gas Control Technologies in Washington, aims to mitigate that risk by determining how much CO2 can be stored safely in given geological formations.

Ruben Juanes, assistant professor of civil and environmental engineering (CEE) and one of the researchers behind the model, said that it offered CCS project developers a simple means of assessing the capacity of geological basins. "It is the first to look at large scales and take into account the effects of flow dynamics on the stored CO2," he said.

Importantly, it takes into account how CO2 will migrate from the original injection well and also incorporates a phenomenon known as capillary trapping, whereby liquefied CO2 eventually dissolves to form harmless carbonate minerals.

Report co-author Michael L. Szulczewski said that despite these complexities, the model was easy to use. "Using only pen and paper, you take geological parameters such as porosity, temperature and pressure to calculate storage capacity," he said. "Other methods suffer from major shortcomings of accuracy, complexity or scale."

The model has already been applied to the Fox Hills Sandstone in the Powder River basin straddling Montana and Wyoming and found that the formation could hold about five gigatonnes of CO2 – equivalent to more than half of all CO2 emitted by the US each year.

The research was unveiled on the same day as a separate team at MIT released a new study claiming that power companies should press ahead with less ambitious partial capture systems capable of capturing 60 per cent of plant emissions while they wait for the cost of full capture technologies to fall.

"Our approach – partial capture – can get CO2 emissions from coal-burning plants down to emissions levels of natural gas power plants," said Ashleigh Hildebrand, a graduate student in chemical engineering and the technology and policy programme and one of the authors of the report. "Policies such as California's Emissions Performance Standards could be met by coal plants using partial capture rather than having to rely solely on natural gas, which is increasingly imported and subject to high and volatile prices."

The study found that the cost per tonne of CO2 captured is roughly the same when 60 per cent of emissions are captured, as it is when 90 per cent are captured. As such, there are no economies of scale to be gained by going to 90 per cent, and companies can reduce their initial capital investment buy initially operating for a partial capture system.