Saline Aquifers: Storage for Greenhouse Gases
A new study by researchers at MIT shows that there is enough capacity in deep saline aquifers in the United States to store at least a century's worth of carbon dioxide emissions from the nation's coal-fired power plants. Most analyses have simply estimated the overall volume of the formations, without considering the dynamics of how the carbon dioxide would infiltrate them. The MIT team's analysis is led by Ruben Juanes, the ARCO Associate Professor in Energy Studies in the Department of Civil and Environmental Engineering, and is part of the doctoral thesis work of graduate students Christopher MacMinn and Michael Szulczewski. Using tiny glass beads, the researchers simulated the way liquified carbon dioxide would spread through salty water in the pores of deep rock formations.
Transcript
00:00:00 My name is Ruben Juanes, and I'm a professor in the Department of Civil & Environmental Engineering at MIT. In my research group, we study (among other things) carbon capture and storage. Carbon capture and storage is a promising technology to reduce CO2 emissions in the atmosphere from large, stationary sources, like coal-fired & gas-fired power plants. The idea is to capture CO2 from the flue gas of these plants and then inject it into deep geologic reservoirs for long-term storage. These reservoirs are typically
00:00:31 one to three kilometers underground, well below the freshwater aquifers used for drinking water. They are bounded above by one or several caprocks, that prevent the upward flow of CO2, back to the surface. A key question for the future of carbon capture & storage is: for how long can it stabilize CO2 emissions? To answer this question, it is essential to understand how CO2 behaves in the subsurface. We model the subsurface fluid dynamics of CO2 storage. To begin with, we model the injection of the CO2, to ensure
00:01:06 that the injection pressure does not become too high, and fracture the caprock. We also model what happens to the CO2 after injection, to ensure that it does not travel to a potential leakage pathway, like a large fracture shown here in red. After injection, the CO2 will migrate upslope beneath the caprock, because it's buoyant. As it migrates, the plume of CO2 will become arrested by two trapping mechanisms. One mechanism is capillary trapping. During migration, the CO2 will become immobilized into blobs by capillary forces in the wake of the plume, show as light grey in the cartoon.
00:01:42 The photograph shows residual CO2 in a tank packed with glass beads that simulates the storage reservoir. The second trapping mechanism is solubility trapping. Since CO2 is soluble in water, the plume of CO2 will dissolve into the groundwater, shrinking as it migrates. It turns out that the water with dissolved CO2 is more dense than groundwater, and therefore dissolution will lead to an unstable configuration: a layer of dense, CO2-rich fluid, sitting over the lighter ambient groundwater. As a result, the dissolved CO2 will sink away from the plume, to the
00:02:18 bottom of the reservoir, in a process known as convective dissolution, which we illustrate with a hi-res computer simulation. This process will greatly accelerate the dissolution rate, and together with capillary trapping, will eventually cause all of the CO2 to become completely trapped. To determine how much CO2 could be stored in the United States, we studied 20 of the largest, most promising deep saline aquifers over the country. For each saline aquifer, we calculated the maximum amount of CO2
00:02:48 that could be stored subject to two constraints: the ejection pressures must be low enough to avoid damaging the caprock, and the CO2 must be completely trapped before migrating to a major leakage pathway. The footprints of trapped CO2 in these saline aquifers is shown here in blue. We found that the United States can store enough CO2 to stabilize emissions at their current rate for over 100 yrs. This result suggests that with a favorable political and economic framework, carbon capture and storage can be
00:03:20 a viable climate change mitigation option in this country for the next century.