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Sequestration on Shaky Ground

Christine Daniloff/MIT

Study finds a natural impediment to the long-term sequestration of carbon dioxide.

Jennifer Chu | MIT News Office

Carbon sequestration promises to address greenhouse-gas emissions by capturing carbon dioxide from the atmosphere and injecting it deep below the Earth’s surface, where it would permanently solidify into rock. The U.S. Environmental Protection Agency estimates that current carbon-sequestration technologies may eliminate up to 90 percent of carbon dioxide emissions from coal-fired power plants.

While such technologies may successfully remove greenhouse gases from the atmosphere, researchers in the Department of Earth, Atmospheric and Planetary Sciences at MIT have found that once injected into the ground, less carbon dioxide is converted to rock than previously imagined.

The team studied the chemical reactions between carbon dioxide and its surroundings once the gas is injected into the Earth — finding that as carbon dioxide works its way underground, only a small fraction of the gas turns to rock. The remainder of the gas stays in a more tenuous form.

“If it turns into rock, it’s stable and will remain there permanently,” says postdoc Yossi Cohen. “However, if it stays in its gaseous or liquid phase, it remains mobile and it can possibly return back to the atmosphere.”

Cohen and Daniel Rothman, a professor of geophysics in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, detail the results this week in the journal Proceedings of the Royal Society A.

Current geologic carbon-sequestration techniques aim to inject carbon dioxide into the subsurface some 7,000 feet below the Earth’s surface, a depth equivalent to more than five Empire State Buildings stacked end-to-end. At such depths, carbon dioxide may be stored in deep-saline aquifers: large pockets of brine that can chemically react with carbon dioxide to solidify the gas.

Cohen and Rothman sought to model the chemical reactions that take place after carbon dioxide is injected into a briny, rocky environment. When carbon dioxide is pumped into the ground, it rushes into open pockets within rock, displacing any existing fluid, such as brine. What remains are bubbles of carbon dioxide, along with carbon dioxide dissolved in water. The dissolved carbon dioxide takes the form of bicarbonate and carbonic acid, which create an acidic environment. To precipitate, or solidify into rock, carbon dioxide requires a basic environment, such as brine.

The researchers modeled the chemical reactions between two main regions: an acidic, low-pH region with a high concentration of carbon dioxide, and a higher-pH region filled with brine, or salty water. As each carbonate species reacts differently when diffusing or flowing through water, the researchers characterized each reaction, then worked each reaction into a reactive diffusion model — a simulation of chemical reactions as carbon dioxide flows through a briny, rocky environment.

When the team analyzed the chemical reactions between regions rich in carbon dioxide and regions of brine, they found that the carbon dioxide solidifies — but only at the interface. The reaction essentially creates a solid wall at the point where carbon dioxide meets brine, keeping the bulk of the carbon dioxide from reacting with the brine.

“This can basically close the channel, and no more material can move farther into the brine, because as soon as it touches the brine, it will become solid,” Cohen says. “The expectation was that most of the carbon dioxide would become solid mineral. Our work suggests that significantly less will precipitate.”

Cohen and Rothman point out that their theoretical predictions require experimental study to determine the magnitude of this effect.

“Experiments would help determine the kind of rock that would minimize this clogging phenomenon,” Cohen says. “There are many factors, such as the porosity and connectivity between pores in rocks, that will determine if and when carbon dioxide mineralizes. Our study reveals new features of this problem that may help identify the optimal geologic formations for long-term sequestration”

This research was funded in part by the U.S. Department of Energy.

Reprinted with permission of MIT News

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Wilmot McCutchen's picture
Wilmot McCutchen on Jan 22, 2015 10:21 pm GMT

Shaky indeed.  Cracking the bedrock and salting the groundwater are the likely outcome of CO2 sequestration, if it’s even possible at the scale of 2 billion tonnes per year.  Trying to squeeze supercritical fluid into rock where the pore space is full of very salty water makes no sense.


Armond Cohen's picture
Armond Cohen on Jan 22, 2015 11:44 pm GMT

Readers beware. The research paper itself says nothing about the probability of CO2 being released to the atmosphere. Unfortunately, the press release ventures into that area with no basis for its catchy headline. 

We have no qualms with the research itself which is a numerical simulation looking at how minerals might form in the host rock when CO2 is injected into the brine.  The results of this simulation suggests that mineralization is not an important sequestration mechanism.  However, storage geologists don’t rely on mineral formation as a primary means for trapping CO2. This is because minerals are slow to form. In fact, the timeframe at which mineralization may occur may be well beyond the timeframe that we need to capture and store CO2. Instead,  the security of CO2 is provided by vertical separation of the injection zone from the surface through a thick section of impermeable rock. 
Unfortunately, in the press release, both one of the researchers and the MIT publicists went well beyond the scope of the paper. Interested readers should consult the paper itself.
Armond Cohen, Executive Director, Clean Air Task Force
Bruce Hill, Senior Scientist, Clean Air Task Force
Bob Meinetz's picture
Bob Meinetz on Jan 23, 2015 6:43 am GMT

Armond, do you have a link to the paper itself?


Wilmot McCutchen's picture
Wilmot McCutchen on Jan 23, 2015 8:29 pm GMT

Post-combustion CO2 capture by chemistry also looks dubious.   Amine sweetening, to remove CO2 from natural gas, is a mature technology (from the ’30s) but it can’t possibly scale from the oil field to what’s needed for a coal plant (5,000 tpd).  Water consumption would double.  Quenching the hot flue gas to 30-60C so the amine sorbents can work is possible but not economical.  Wet cooling at thermal power plants already consumes more fresh water than any other use, and doubling that waste of water during a drought is not a realistic plan.  The energy penalty of chemical capture (20-35%) is another strike against conventional CCS.  Weakening power generation during expanding demand is a sacrifice that will be hard to sell to the developing world.  Strike three is the impossibiity of secure storage underground at the scale required.  With these three strikes against it, chemical capture and sequestration is clearly not a viable option.  But the goal of mitigating global warming could be achieved by kinematic separation in open von Karman flow (for CO2 capture by stripping the nitrogen ballast) and CO2 cracking (by radial counterflow shear electrolysis).



Engineer- Poet's picture
Engineer- Poet on Jan 25, 2015 2:04 am GMT

You could always remove some of the water.  So long as it’s not saturated with salt, you could reduce the volume by evaporation and pump the remainder down with the CO2.

I doubt that any sequestration scheme is economic compared to nuclear power.

Engineer- Poet's picture
Engineer- Poet on Jan 26, 2015 3:43 am GMT

My suspicion is that the optimal scheme for sequestration of carbon is to convert it to hot liquid polyethylene and pump it into deep rock formations.  Once it cools and hardens it’s there for geologic time.

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