According to a study funded in part by the U.S. Department of Energy (DOE) and conducted by Massachusetts Institute of Technology (MIT) researchers, a smaller portion of carbon dioxide (CO2) that is injected into the ground may be converted into rock than was previously presumed.
The team, working in the Department of Earth, Atmospheric and Planetary Sciences at MIT, said a complete physical picture able to predict the structure developing within the porous medium was lacking, which prompted the study. The findings were published in the journal Proceedings of the Royal Society A on Jan. 21.
It is widely believed that the sequestration of CO2 in subsurface reservoirs could play an important role in reducing greenhouse gas emissions, while enabling continued use of fossil fuels for electricity generation. In 2013, the U.S. power plant sector emitted 2.1 billion metric tons of CO2, which was over 66% of all CO2 emissions reported in the Environmental Protection Agency’s Greenhouse Gas Reporting Program. Some estimates suggest that 80% to 90% of power plant CO2 emissions could be reduced using carbon capture and sequestration (CCS) technology.
CCS is a three-step process. First, the CO2 is captured from power plant flue gases. Next the CO2 must be compressed and transported to a storage location, usually through pipelines. Lastly, supercritical CO2 is injected into deep underground rock formations, often more than a mile beneath the surface.
Once injected into a brine-rock environment, a carbonate-rich region is created amid brine. Within the carbonate-rich region, minerals dissolve and migrate from regions of high-to-low concentration along with other dissolved carbonate species. This causes mineral precipitation at the interface between the two regions.
The problem the MIT researchers identified is that precipitation in a small layer reduces diffusivity and eventually causes mechanical trapping of the CO2. Consequently, only a small fraction of the CO2 is converted to solid mineral. The remainder either dissolves in water or is trapped in its original form.
If CO2 stays in its gaseous or liquid phase, it remains mobile and could potentially migrate back to the atmosphere. Yet, while the findings suggest a potential drawback, the authors feel more research is needed because many details factor into individual outcomes. Things such as concentration gradients, porosity of rocks, and connectivity between pores have an effect on how much CO2 ultimately mineralizes.
Previous DOE estimates suggested that from 1.8 trillion to 20 trillion metric tons of CO2 could be stored underground in the U.S. The MIT study reveals some new features that may need to be considered when identifying optimal geologic formations for long-term sequestration and when estimating total quantities of CO2 that those sites can accommodate.
—Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)