Could a Recent CCS Study Really Be Right?
by Eric Larson
Substantially reducing emissions of earth-warming greenhouse gases is a huge task. One technology under development, CO2 capture and storage (CCS), may enable the main greenhouse gas, CO2, to be captured at coal-fired power plants and pumped underground. But a recent research paper by a pair of petroleum engineers has stirred up controversy by claiming that large-scale underground storage of CO2 will be downright impossible1.
If the paper, by Christine Ehlig-Economides and Michael Economides (hereinafter referred to as E&E) of Texas A&M University and the University of Houston, respectively, is right, we can probably say farewell to the possibility of limiting global warming to two degrees Celsius or less (see these studies in Nature or the Proceedings of the National Academy of Sciences). This is the threshold agreed to by leaders of most nations at the UN climate summit last December, and CCS is believed to be a key component of plans to limit climate change to below that level.
Fortunately, however, the E&E analysis is based on a model that over-simplifies reality to the point that their overall conclusion is just plain wrong.
Without CCS as an option, a couple of decades (or less) of fossil fuel use at current rates will throw enough long-lived CO2 into the atmosphere that even if society quits fossil fuels altogether after that, the warming power of the accumulated CO2 would likely be enough to heat the world beyond the two degree threshold.
The “capture” part of CCS involves well-established technology. Not so with the storage part, but the basic idea with storage is to inject the CO2 into an underground geologic formation where it would stay, one hopes, indefinitely. The E&E paper claims that there are not nearly enough suitable formations to hold a significant amount of the current CO2 emissions from fossil fuels.
In contrast to E&E’s conclusions, the larger CCS expert community actually has high confidence that large-scale CCS will be possible.
More demonstration projects needed
What I and many other scientists think is the main hurdle to having CCS become a routine activity is that there have not yet been enough large-scale projects undertaken to demonstrate that CCS can work as expected in a wide variety of geologies. More demonstration projects are also needed to give society a comfort level with the technology. There are already four large successful CCS projects ongoing (the longest one since 1996, injecting one million tons of CO2 per year under the sea floor off the Norwegian coast). But for CCS to become a routine commercial option by the end of this decade requires the success of another ten or twenty such projects.
The sweeping claims of E&E have evoked a flurry of rebuttals from the technical CCS community, including researchers at the US Department of Energy, British and Canadian scientists, the American Petroleum Institute, and a European team developing CCS.
Stefan Bachu, whom I got to know when he was visiting Princeton University, is one of the most respected scientists currently working on CCS. He is the principal scientist for CO2 Storage at Alberta Innovates – Technology Futures (formerly called the Alberta Research Council) and has written some 90 technical papers on underground CO2 storage.
“There are many reviewed papers in the literature that demonstrate … that E&E are wrong, but this paper by the Economides pair has already created lots of damage because it was picked up by the media and by people who do not have a good understanding of CO2 storage and who do not support it,” Bachu told me.
The Romanian-born Bachu characterized the stir caused by the E&E paper this way, “In my mother tongue there is a saying: something about a crazy man throwing a stone and ten wise men trying to undo the damage, and this is what’s going on now in the CCS community.”
Paper oversimplifies underground geology
To understand E&E’s over-simplifications requires knowing something about the underground formations in which CO2 storage is being considered. These would be located at least 2,500 feet below the surface and typically consist of intermingled layers of sand, gravel, and rock saturated with salty water (“brine”). “Saline aquifer” is a term used to loosely describe such formations.
CO2 injected into such formations will find its way into any available space, sometimes by pushing aside the brine. The CO2 stays in the formation primarily by “structural trapping” by “caprock,” which are rock layers that effectively prevent easy movement of the CO2 toward the surface.
The E&E analysis assumes that any CO2 storage formation must be completely enclosed by impermeable rock. Anyone who has had the unfortunate experience of pumping air into a bicycle tire until it explodes can imagine what problems closed rock walls might create. The E&E paper says that to keep from cracking the walls and causing big leaks (“fracturing the caprock”), there are very low limits on how much and at what rate CO2 can be injected. They also say there aren’t very many closed formations in the first place. Based on these assumptions, they conclude that very little CO2 storage would be possible.
Bachu says that modeling an underground space as having impermeable boundaries, “is a common and reasonably acceptable assumption in petroleum engineering.” However, “This is an absolutely incorrect assumption and wrongly applied when we are talking about aquifers and timeframes of centuries to millennia, as any hydrogeologist and any handbook of hydrogeology will attest.”
Moreover, most CCS experts believe (and testing so far confirms) that CO2 storage is possible in formations that are not closed on all sides, as long as they have good caprocks. In such formations CO2 can push brine aside without excessive pressure buildup. There are lots of these semi-open formations worldwide.
In fact, geologists believe there are suitable formations under many regions of the US, and a special report on CCS from the U.N. Intergovernmental Panel on Climate Change (for which Bachu was one of the Lead Authors) concluded that there was a high probability that there are enough suitable formations worldwide to store about 30 billion metric tons per year of CO2 (the current total emissions from global fossil fuel combustion) for at least 65 years. That’s a lot of CO2 storage potential – hopefully enough to give us a chance to wean ourselves from fossil fuels before we run out of storage space.
Despite the E&E analysis, the world has been moving steadily toward doing the needed large-scale projects to demonstrate unequivocally the feasibility of CCS. The leaders of the G-8 countries are committed to sponsoring 20 commercial-scale CCS projects around the world by 2016. In February, President Obama issued a Presidential Memorandum on CCS that called for a comprehensive federal report on CCS within six months and for committing the US to have five to ten commercial-scale demonstrations going by 2016. This would represent a major step toward understanding the real viability of CCS systems.
1C. Ehlig-Economides and M.J. Economides, “Sequestering carbon dioxide in a closed underground volume,” Journal of Petroleum Science and Engineering, 70(1-2), January 2010, Pages 123-130.