This article was first published in Platform’s Carbon Web newsletter, issue 9.

Is carbon capture and storage a safe climate mitigation option?

As climate change increasingly becomes a defining political theme for the 21st Century, coal, oil and gas companies have not suffered the existential crisis that might have been expected. Instead, they are betting on a technological solution to the problem, in the form of carbon capture and storage. But, ask Gabriele von Goerne and David Santillo, how safe is the technology?

To avoid dangerous anthropogenic climate change, which would place millions of people and the natural systems on which they depend at risk, global greenhouse gas emissions need to be reduced by at least 80% by the middle of this century. This, and more, can be achieved by a combination of greater energy efficiencies, phasing out the use of coal and switching from fossil fuels to renewable energies. But this vision of the future is not one that fossil fuel companies can accept. Led by the coal industry, those companies are insisting that carbon capture and storage (CCS) can square the circle between everincreasing sales of their products and a major decrease in greenhouse gas emissions.

The stakes could not be higher. If at any point in the future the technology failed, resulting in either gradual or sudden leakage of stored carbon dioxide (CO2), the world could be faced with substantial, unexpected greenhouse gas emissions about which little or nothing could be done, as well as the potential for severe direct impacts on ecosystems in the vicinity of such leaks. Given that the storage would have to remain intact for many centuries, an extremely high level of confidence in the system’s integrity would be necessary before proceeding with CCS.

Storage science shortfall

CCS technology is still very much in development. Its principle sounds simple – CO2 that would usually be emitted to the atmosphere is captured at the power plant, transported and injected into deep geological formations where, according to theory, it is stored safely for a long period of time. But in reality, the process turns out to be highly complex, not least because the scale of both sources and storage formations are so vast, and knowledge and experience so limited.

Scenarios indicate that a single 1000MW coal-fired power plant, producing 8.6 million tons of CO2 per year for 30 years, could generate an underground CO2 plume which, within a further 20-50 years, could extend over an area of between 200 and 360 km2, depending on the type and thickness of the storage formation1. Continuous injection of CO2 will also cause formation pressures to rise over large areas, not only in the plume area but well beyond.

Simulations indicate that after 30 years of injection, a pressure increase of 1 bar could extend over an area of about 2500 km2,2 which will modify the local mechanical stress field and could cause deformation of the surrounding geological formation itself. This would make it far more likely that the cap rock could be compromised, particularly where there are any existing weaknesses, such as faults or fracture zones, providing pathways for CO2, and, in the case of saline aquifers, metalladen brines to escape to the biosphere.

Cap rock integrity is therefore an essential key for storing CO2 safely in geological formations. However, largescale injection of CO2 will induce a range of strongly coupled physical and chemical processes, including multiphase fluid flow, changes in effective stress and solute transport, and even chemical reactions between fluids and minerals in the geological formation. The more impurities present in the CO2 stream, the more complex and unpredictable the system inevitably becomes.

To date, most risk assessments and models assume that only pure CO2 will be stored. In reality, this is very unlikely to be the case. Less-pure CO2 waste streams, also containing other substances like SOx, NOx, hydrogen sulphide or even mercury, are significantly cheaper to generate (albeit with the possibility of higher transport costs), requiring less technological investment and energy to separate from a flue gas, coal gasification process, etc3. This economic incentive makes it likely that some companies will choose to store mixed gases.

Keeping carbon captured

Yet these mixtures and impurities could have a major impact on storage integrity. Mineral trapping of CO2 in a storage formation is hampered by hydrogen sulphide (H2S), for example. Although it has been suggested by some that large amounts of co-injected H2S should not prove problematic, interaction with the rock formation cannot be ruled out. Moreover, if conditions in a geological formation allow sulfur to be oxidized, or if CO2 was to be costored with SO2, very different patterns of pH distribution and mineral alteration would be expected compared to those arising from CO2 injection alone. Mineral alteration can lead to significant changes in porosity, and hence permeability, which could modify the fluid flow4. SO2 is much more corrosive in the presence of water than CO2 , such that the mobilization of metals in groundwater and overlying soils or sediments may be higher, leading to a greater risk of trace metal contamination in the surrounding environment5.

Even if only pure CO2 was injected, it could still induce dissolution of minerals, especially iron-bearing oxides, that could mobilize toxic trace metals6 and ultimately create pathways through the sealing rock for CO2, displaced brines and other associated substances7. Although current geophysical techniques allow broad identification and characterisation of fractures in a rock formation, relatively fine (open or sealed) fractures may remain undetected at the time of injection, representing possible pathways for CO2 sometime in the future.

A much more obvious and, perhaps, immediate pathway for leakage are the wells themselves, whether those used for injection or others in the vicinity which have, at some point, connected with the formation. The potential for leakage of CO2 through existing and abandoned wells is particularly relevant in regions that have been intensively explored and exploited for hydrocarbon reserves, such as in the North Sea. Although well completion and abandonment practices have evolved considerably over time, even wells drilled and abandoned by today’s standards are unlikely to be entirely resistant to the corrosive effects of CO2 that comes in contact with water.

In short, the risks and uncertainties surrounding CCS are significant, manifold and complex. Despite assurances from industry and government, leakage of CO2 from storage reservoirs cannot be ruled out. Although the IPCC regards the risks to be low, it is vital to remember that problems may occur long after injection has ended, well beyond the timeframes over which the efficacy and safety of CCS has so far been demonstrated. The big question decisionmakers need to ask themselves is not just whether they want to take the risk, but whether it is responsible and sustainable for them to pass the burden of a continued reliance on fossil fuels to future generations.

The choice is real – CCS is not unavoidable – if only they put their efforts and money into renewable energies and energy efficiencies, the real solutions to climate change.

Dr Gabriela von Goerne is a geologist working with the Climate & Energy Unit of Greenpeace’s office in Hamburg, Germany. Dr David Santillo is a marine biologist and environmental chemist working with the Greenpeace Research Laboratories, based at the University of Exeter in the UK

1 Benson S., Hoversten M., Gasperikova E., Haines M. (2004): Monitoring protocols and life-cycle costs for geologic storage of carbon dioxide. Proceedings of the 7th International Conference on Greenhouse Gas Control technologies, Vancouver, Canada

2 Pruess K., Xu T., Apps J., Garcia J. (2003): Numerical modeling of aquifer disposal of CO2. SPE Journal, 49-60

3 Andersson K., Johnsson F., Strömberg L. (2003): An 865 Mwe lignite-fired power plant with CO2 capture – a technical feasibility study. VGB Conference “Power Plants in Competition – Technology, Operation and Environment”, Cologne.

4 Xu T., Apps J., Pruess K., Yamamoto H. (2007): Numerical modeling of injection and mineral trapping of CO2 with H2S and SO2 in a sandstone formation. Chemical Geology xx (2007) xxx-xxx

5 IPCC (2005): Special report on Carbon dioxide capture and storage. p250

6 Schütt T., Wigand M., Spangenberg E. (2005): Geophysical and geochemical effects of supercritical CO2 on sandstones. In: Carbon dioxide capture for storage in deep geologic formations (Eds.: D.C.Thomas, S.M. Benson) Vol.2, Chapter 7, 767-786

7 Kharaka Y., Cole D., Hovorka S., Gunter W., Knauss K., Freifeld B. (2006): Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins. Geology, Vol.34, 577-580