Bookmark and ShareDr Victoria Johnson is lead researcher on climate and energy at nef.

The capture and the long-term storage of CO2 is now central to plans for reducing CO2 emissions from large-scale fossil fuel uses. But new and controversial research argues the storage potential of CO2 may have been overestimated.

Carbon Capture and Storage (CCS) involves the capture of the greenhouse gas CO2 produced from the combustion of fossil fuels. The captured and compressed CO2 is then transported to a location for long-term storage. While several proposals for the storage phase exist, geological storage has received the most attention. This is partly because it is believed to have the least logistical constraints.

While discrete components of a geological CCS system are mature, there is a broad consensus [subscription required] that significant technological and cost improvements are necessary for commercial CCS deployment. But in the absence of large-scale CCS demonstration plants, the technology is still surrounded by a haze of uncertainty such as cost and speed of deployment.

Adding to these uncertainties, a new study published in The Journal of Petroleum Science and Engineering argues the potential for geological storage has been significantly overestimated. The results have prompted a very public and highly technical spat. A large body of experts from industry and academia have now contested the paper’s claims.

According to Ehlig-Economides et al, geological storage of CO2 needs to occur in a closed-system to avoid leakage. But the pressure in the system cannot exceed the formation fracture pressure. Above this threshold, the injection of fluid into the formation will cause the rock to fracture. This means that in a storage reservoir nearly saturated by formation fluids – oil, gas or brine – like deep saline aquifers or mature oil fields, the storage potential for CO2 is limited by the reservoir’s current pressure and the overall fracture pressure of the formation. Should the rock fracture, CO2 could migrate, escape to the surface, or trigger seismic events. The failure of past analyses to consider this limit, they argue, means that potential geological storage capacity has been significantly overestimated.

Their results suggest that CCS will require from 5 to 20 times more underground reservoir volume than has been envisioned by many. This not only means there may not be enough space for carbon, but the infrastructure will be vastly more expensive. In an email exchange with Michael Economides, a co-author of the paper, he told me:

You realize that we probably come from different ends of the spectrum on what to do if any with CO2 emissions but sequestration is not the answer no matter what.

The main criticism of the Ehlig-Economides et al study is the oversimplification of underground geology in their model. According to one critique, their analysis is built on two key assumptions: effective geological storage requires the presence of hydrologically isolated, completely closed geological structure; and any other storage system is guaranteed to leak. It is argued that both these assumptions are flawed, and empirical (albeit limited) evidence suggests otherwise.

Ehlig-Economides et al have responded by saying that whilst they only considered two types of geological storage – deep saline aquifers and mature oil and gas fields, they stand by their analyses, dismissing the criticisms as ‘nonsense’. They contend that critics are merely protecting vested interests.

All CO2 storage potential estimates are based on generalisations of sub-surface geology. Many critics of the Ehlig-Economides et al study conclude the only way to reduce some of the uncertainties such as storage potential is through increased demonstrations and commercial deployment.

CCS features heavily in the portfolio of solutions to decarbonise the global economy, and there are now a growing number of government-sponsored pilot projects worldwide. However, widespread deployment of CCS unlikely before 2020. With research [subscription required] suggesting that if emissions are still 25% above 2000 levels in 2020, the risk of exceeding 2 °C shifts to more likely than not, whether Ehlig-Economides et al are right or not, a bigger question should be whether the promise of CCS is crowding out proven or less uncertain technologies such as wind, wave, tidal and solar that could be deployed now. These would all have immediate impact on emissions that are still currently growing at around 3% each year.

Of course, there are many other problems related to CCS. These include: further locking ourselves into a centralised (and inherently inefficient) fossil fuel based energy system, the parasitic effect CCS has on the efficiency of power stations, and the environmental impacts of coal, gas and oil mining to name a few. We reviewed CCS in more detail in our recent report Growth isn’t Possible: Why we need a new economic direction.

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