Geologic Carbon Storage: A Safe Bet. : CCS UNDERGROUND
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CCS has the potential to significantly reduce global carbon emissions.

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A discussion of the issues and policies related to carbon capture and storage technology.*

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CCS UNDERGROUND

Geologic Carbon Storage: A Safe Bet.

by ENGO NETWORK GUEST AUTHOR on 06/09/15

This is a cross-post by guest author Bruce Hill with Clean Air Task Force.

Carbon capture and storage (CCS) has been identified as a critical technology to manage the transition to a low carbon economy, not just for power generation, but for industrial activitylike manufacturing cement and steel. Recent reports from the IPCC, the United Nations, and the International Agency have affirmed this role. 

The main mechanism for CCS is deep injection of CO2 and dissolution of the CO2 into the briny water in the rock pores combined with physically trapping the CO2 beneath thousands of feet of impermeable rock.  This is the way that nature has trapped oil and natural gas in geologic structures for tens to hundreds of millions of years.  Without this natural trapping and storage ability, we wouldn't have fossil fuels today. Geologic carbon storage takes advantage of this same natural capacity for storage. Numerous studies, and in-field tests, have demonstrated the feasibility, soundness and stability of this form of carbon storage.

 

 https://www.catf.us/blogs/ahead/files/2015/02/20150203-1-e1422977457871.jpg

 

Image: Storage of CO2 requires a permeable rock formation overlain by thousands of feet of rock impermeable to vertical flow. Source:http://www.co2crc.com.au/images/imagelibrary/stor_diag/storage_saline.jpg

Geologic storage formations must be at great depth and overlain by a thick sequence of impermeable rock, capable of permanently trapping the CO2 whether it is in a liquid or gas form or dissolved in the formation water.  The ability to use deep geologic formations for storage is fully proven by the nearly 3 trillion cubic feet of natural gas currently stored by U.S. gas companies, which has been injected back into rock formations, stored for future use (See:http://ir.eia.gov/ngs/ngs.html). And in fact, CO2 injection technology is well known with a four-decade long track record of safety beginning in 1972 in Texas for the purposes of stimulating oil production. 

There is an additional possible very long-term mechanism for CCS in addition to dissolution of CO2 in briny water and trapping underneath thousands of feet of impermeable rock.  That is “mineralization” – the transformation of CO2 into solid rock, bound up with other minerals. It has been speculated that this mechanism may provide an additional layer of protection over the scale of thousands to tens of thousands of years or more (see illustration below).  But the mineralization sequestration mechanism has a small role to play in geologic storage and has never been the primary focus of CCS in time scales meaningful to humans.

 

 https://www.catf.us/blogs/ahead/files/2015/02/20150203-2.jpg 

 

Image: Mineralization plays a small role in trapping CO2 in a permeable saline formation.  Source:http://www.co2crc.com.au/aboutccs/stor_trapping.html

Yet that is the unfortunate misimpression left by recent press coverage of a report from MIT. In a January 21 paper entitled Mechanisms for mechanical trapping of geologically sequestered Carbon dioxide, MIT researchers Yossi Cohen and Dan Rothman analyzed the processes of how CO2 is “mineralized” i.e., bound up in minerals, when it is injected into a rock formation as a climate mitigation strategy.  The researchers modeled the mechanics of how injected CO2 reacts with carbonate storage reservoir rock to form minerals. (see:http://rspa.royalsocietypublishing.org/content/471/2175/20140853.) The authors concluded that only “a small fraction of injected CO2 is converted to solid mineral.” 

The paper is sound science, but a misleading press release by MIT, and subsequent press coverage that followed that release, has unfortunately mischaracterized it.  The January 20 statement issued by the MIT press covering the paper was headlined, “Sequestration on Shaky Ground,” and subtitled: “Study finds a natural impediment to the long-term sequestration of carbon dioxide.” Further, the release included an animation suggesting CO2 “bubbles” migrating upwards through rock. Finally, the junior researcher on the study, post doc student, Yossi Cohen, is quoted as saying: 

 “If it [CO2] turns into rock, it’s stable and will remain there permanently. However, if it stays in its gaseous or liquid phase, it remains mobile and it can possibly return back to the atmosphere.” 

While we applaud the paper’s contribution to the understanding of the geochemical processes associated with carbon mineralization, the research did nothing to investigate the risk of injected CO2 returning to the atmosphere over any meaningful long-term time scale in the absence of this mechanism. The authors have clarified this point on their website (https://rothmangroup.mit.edu/publications). 

As noted above, carbon sequestration through the formation of minerals is notthe mechanism by which geologists will ensure CO2 is permanently trapped in the subsurface. Instead, a robust geologic CO2 storage strategy combines several trapping mechanisms, including dissolution of the CO2 into the briny water in the rock pores combined with physically trapping the CO2 beneath thousands of feet of impermeable rock. The formation of minerals is a very long-term process that would not prove useful for addressing climate change if it were the chief mechanism for geologic carbon storage. In fact over long time periods, CO2 dissolved in formation water will actually become more dense and have a tendency to sink (see image below).


https://www.catf.us/blogs/ahead/files/2015/02/20150203-3-e1422977557430.jpg

 

Image: CO2 moves upwards to the top seal. Over time the CO2 dissolves into the formation water. This makes it denser and it moves downwards. Source: http://www.co2crc.com.au/images/imagelibrary/stor_diag/CO2-simulation_media.jpg 

To be sure, CO2 cannot be injected just any old place. U.S. regulations require that repositories for CO2 must be carefully selected and rigorously evaluated by geologists, geophysicists and engineers.  (See:http://water.epa.gov/type/groundwater/uic/class6/gsregulations.cfm In order to store fluids and gases, the rock must have the right properties. For example, the rock into which the CO2 is injected must be permeable and have adequate space in the rock pores to accommodate large volumes of fluids. This injection formation must lie in geologic structures beneath an extremely thick sequence of impermeable rocks that can trap the CO2. The minimum depths of injection must be greater than a half-mile in depth, and typically equivalent to several times the height of the new One World Trade Center, the tallest building in the U.S. 

Finally, the US Geological survey has investigated the capacity of deep US geologic formations to store CO2. It estimates 500 years of capacity for CO2 at today’s emissions rates.  (See: http://pubs.usgs.gov/fs/2013/3020/

So, to set the record straight, geologic storage is a proven technology that does not require the mineralization of the CO2 to ensure permanence, as the MIT press release incorrectly states.  The key to safe and permanent storage – past, present and future – is deep injection of CO2 below thick, impermeable geologic strata.


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