Two articles on the April 2nd issue of Nature look into some of the physics, chemistry, and geology associated with carbon capture and storage (CCS) as a possible form of greenhouse gas mitigation. The first largely summarizes the results of the second. Each stresses how significant amounts of carbon dioxide (CO2) are already trapped in groundwater in the subsurface environment, suggesting that the artificial addition of more may be safe and effective. Leaks are avoided due to the “presence of sealing, low-permeability rock formations above the targeted layer,” such as those found above natural gas fields. The article considers CO2-rich natural gas fields in North America, China and Europe as natural analogs for future CCS sites. It concludes that relatively little (about 10%) of the CO2 gets incorporated into rocks, from which it is unlikely to escape. Most remains in water, from which future emissions are more possible. It concludes that the hydrogeological characteristics of future CCS sites will need to be carefully considered, bearing in mind that most of the CO2 will apparently end up saturated in water.
None of this provides definitive support for CCS as a mitigation option. Rather, it provides some guidance into the further research necessary to determine if it can be safe and environmentally effective. Notably, this research also gives no consideration to the economics of CCS deployment, nor to the timelines across which it can be achieved. Indeed, these articles could be taken as evidence of the relative infancy of the scientific consideration of subsurface disposal of carbon dioxide, something that governments assuming its near-term commercial viability should note.
One factor that differentiates CCS sites from natural deposits is that the former will need artificial wells drilled into them.
Over thousands of years, that seems likely to increase the probability of a leak.
Carbon capture and storage
SIR – You were correct in saying that the enormous cost of the geological sequestration of carbon dioxide is a major impediment to its implementation (“Going underground”, August 18th). However, your article contains some important factual and conceptual misrepresentations. For example, it concludes: “Ultimately…the safety risks are a secondary issue. The technique will be successful only if the cost of hiding a tonne of carbon underground falls lower than the cost of emitting a tonne into the air.” We strongly disagree.
Safety risks are paramount, especially if geological sequestration of CO2 was cheap enough to be widely adopted. A recent report from the National Research Council on seismic risks associated with energy technologies pointed to the potential for large earthquakes to be associated with carbon sequestration. In our recent article in the Proceedings of the National Academy of Sciences we argued that even relatively small quakes could damage the caprock seal enough to release the CO2 back into the atmosphere.
Although we argued that such leakage could negate the desired emission reductions, we never said that if leakage were sudden it might kill people, as implied in your article.
Mark Zoback
Steven Gorelick
Stanford University
Stanford, California
“Global CO2 emissions from fossil fuels were ~33 bn metric tons in 2019. To store just 15% of this amount (5 bn metric tons), a CCS compression, transportation and storage industry would have to handle 6 bn cubic meters of CO2 every year by volume. How much is that? For context, that’s more than the 5 bn cubic meters of oil that’s produced, transported and refined each year around the globe. In other words, CCS infrastructure would have to be even greater than the one used for the world’s annual oil consumption just to sequester 15% of global emissions. There are applications where CCS makes sense (enhanced oil recovery and small amounts of commercial CO2 demand). But as a big picture solution to CO2 emissions, CCS buildout requirements are daunting.”
https://privatebank.jpmorgan.com/content/dam/jpm-wm-aem/global/pb/en/insights/eye-on-the-market/tenth-annual-energy-paper-full.pdf
“We already know the emissions number; it’s 51 billion tons each year. As for the cost of removing a ton of carbon from the air, that figure hasn’t been firmly established, but it’s at least $200 per ton. With some innovation, I think we can realistically expect it to get down to $100 per ton, so that’s the number I’ll use.
That gives us the following equation:
51 billion tons per year x $100 per ton = $5.1 trillion per year
In other words, using the DAC approach to solve the climate problem would cost at least $5.1 trillion per year, every year, as long as we produce emissions. That’s around 6 percent of the world’s economy.”
Gates, Bill. How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need. New York: Random House, 2021. p. 63
“Taking all these factors into account, the math suggests you’d need somewhere around 50 acres’ worth of trees, planted in tropical areas, to absorb the emissions produced by an average American in her lifetime. Multiply that by the population of the United States, and you get more than 16 billion acres, or 25 million square miles, roughly half the landmass of the world. Those trees would have to be maintained forever. And that’s just for the United States—we haven’t accounted for any other country’s emissions.”
Gates, Bill. How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need. New York: Random House, 2021. p. 129
Capturing carbon dioxide is just part of the process. Next it has to be safely locked away. The rift’s geology is particularly good for this, too. It has bands of porous basalt (a volcanic rock) that stretch across thousands of square kilometres. This makes the region “ideal” for carbon capture and storage, according to a paper published in 2021 by George Otieno Okoko and Lydia Olaka, both of the University of Nairobi. After carbon dioxide has been sucked from the air it is dissolved in water (in the same way one would make sparkling water). This slightly acidic and bubbly liquid is then injected into the rock. There it reacts with the basalt to form carbon-rich minerals—in essence, rocks—which means the gas will not leak back into the atmosphere.
https://www.economist.com/middle-east-and-africa/2023/06/15/why-kenya-could-take-the-lead-in-carbon-removal