National Ground Water Association (NGWA)

Modeling underground sequestration of carbon dioxide

When it comes to predicting the fate of captured carbon dioxide (CO2) stored underground, 2008 Henry Darcy Distinguished Lecturer Michael Celia, Ph.D., and his colleagues have invented a better mousetrap for decision-makers trying to mitigate the effects of CO2 emissions into the atmosphere.

Celia wrapped up his year as the National Ground Water Research and Educational's Foundation Darcy Distinguished Lecturer at the 2008 National Ground Water Association Expo and Annual Meeting with his presentation entitled 'Geological Storage as a Carbon Mitigation Option.'

Manmade emissions of CO2 have increased the atmospheric concentration of CO2 by about 35 percent during the past 200 years. The current concentration, at about 385 parts per million, represents the highest CO2 concentration in the last 500,000 years. Projected future emissions will lead to a doubling of the preindustrial CO2 concentration within the next 50 years. Manmade CO2 emissions into the atmosphere are a major contributor to global warming.

Carbon capture and storage currently offers the best hope of allowing continued use of coal for power generation while addressing the problem CO2 emissions into the atmosphere, said Celia.
'The idea is that the CO2 will remain deep in the subsurface long enough that we can essentially solve the climate problem,' he said. But modeling what happens to CO2 once it is stored underground is so complex as to be impractical for regulators and others grappling with the issue.

'Our interest is in trying to think through what it means to put very large amounts of CO2 underground. If you don't put away large amounts, you don't have much impact. Anyway you look at it, it's a big problem,' Celia said.

Both short- and long-term concerns are how the CO2 will displace other fluids that are underground and how that displacement may contaminate ground water. For instance, large saline aquifers offer some of the largest capacity for CO2 storage, but displacement of brine water can be a problem. Also, deep midcontinent sedimentary basins offer one of the best environments for CO2 injection, but millions of old oil and gas wells can serve as conduits for leakage of the CO2. Furthermore, CO2, when injected deep underground, is less dense than water and tends to rise upward, thus, creating the potential for fluid displacement or leakage.

With these variables and more, trying to predict what will happen to the CO2 when it is stored underground can result in a 'very interesting but very complicated' computer model.

What Celia and his colleagues have done is taken a philosophical approach to modeling that says, 'Let's take a step back and figure out the most important things that are going on. If we can figure those out, we will throw away everything else, and that ultimately gives us mathematical models that are relatively simple.'

That is precisely what Celia has done to provide a very practical tool for those who want to capture and store CO2 underground.

'We think about the use of modeling in a regulatory framework, in terms of the utility of the models,' he said. With his model, for instance, 'We can make a very quick back-of-the-envelope estimate of how big a CO2 plume is going to be. In my view, these are the kind of very practical tools regulators and others can use,' Celia said.

The model's simplicity allows users to run thousands of scenarios in a short period of time to inform their decision-making.

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