CO2 Storage in Deep Saline Aquifers

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Brett Smith for

A group of researchers at MIT, led by Ruben Juanes, published a study this week that showed deep saline aquifers in the United States are capable of storing a century´s worth of carbon dioxide produced by the nation´s coal or gas fueled power plants.

The researchers were able to accurately model carbon dioxide storage in the aquifers, or layers of water-bearing permeable rock, that would be located well below those water sources used for human consumption or agriculture, according to the study published in the Proceedings of the National Academy of Sciences (PNAS) journal.

This development could be a huge windfall for the carbon capture and storage industry´s (CCS) effort to reduce carbon emissions. Current systems to solely capture carbon emissions have been proven to be very effective. A newly proposed clean coal plant to be built near Edinburgh, Scotland by the American company Summit Power Group would have systems that capture carbon emissions at 90 percent efficiency, according to Severin Carrell of The Guardian.

The bigger problem in reducing coal-fired power plant emissions is what to do with the carbon dioxide once it is captured. The proposed Scottish power plant has run into opposition because the current plan for these captured gases would be to use them in an effort to pump oil out of the North Sea, which is not considered an environmentally friendly use.

Deep underground locations are currently viewed as the most promising places in the U.S. to store the emissions. The process known as geo-sequestration would involve the injection of supercritical carbon emissions into geological formations. The deep saline aquifers described in the study would be bounded above by a cap-rock formation that would prevent the flow of emissions up toward the surface.

In a video presentation, researcher Mike Szulczewski described how the MIT group modeled the sub-surface flow of supercritical fluid both during and after the injection process. He said the injection stage was modeled “to ensure that the injection pressure does not become too high and fracture the cap rock.” This would cause significant CO2 leakage from the aquifer.

“We also modeled what happens to the CO2 after injection to ensure it does not travel to a potential leakage pathway like a large fracture,” said Szulczewski.

After injection, the carbon dioxide is potentially trapped in the aquifer by two different mechanisms: capillary and solubility trapping. Capillary trapping, also called residual gas trapping, occurs when the injected CO2 plume passes through porous rock, disconnecting some of the supercritical fluid into the pores though surface tension. Researches simulated this through the use of tiny multi-colored glass beads that trapped liquid as it passed through them.

Solubility trapping happens because carbon dioxide saturated water is heavier than the aquifer´s unsaturated water by 1 percent. This will cause the CO2 to sink to the bottom of the aquifer over a period of time. This type of trapping is less likely to leak since the supercritical fluid is not able to rise to the surface because of buoyancy.

Using the two criteria of proper injection pressure and aquifer trapping potential, the researchers were able to reach the conclusion that storage utilization of the nation´s deep saline aquifers would allow for the stabilization of U.S. emissions at the current rate for over 100 years.