|SEISMIC RISK OF GEOLOGICAL STORAGE OF CO2 |
Stanford University professors Zoback and Gorelick have recently published an article on induced seismicity of CO2 storage (Zoback MD and Gorelick SM, Earthquake triggering and large-scale geologic storage of carbon dioxide, PNAS 2012, doi:10.1073/pnas.1202473109). The article casts doubt on the large scale deployment of CO2 storage and attracted much attention in the press. The authors claim that large scale injection of CO2 would likely trigger earthquakes which could threaten the seal integrity of the storage site. Although there is no data on earthquakes caused by CO2 injection, the authors referred to evidence from waste water injection. Clean Air Task Force (CATF), Petroleum Technology Research Centre (PTRC) and Scottish Carbon Capture and Storage (SCCS) have all commented on the paper countering the statement of Zoback and Gorelick.
According to CATF, CO2 injection projects (including EOR) worldwide have not reported any significant induced seismicity and there is not enough data to construct models to link earthquakes to CO2 injection and they believe that there is huge offshore capacity in the USA (500 billion tons to 7.5 trillion tones (NETL) of CO2 storage capacity) where these formations are not prone to seismic activity (according to Zoback and Gorelick).
PTRC gives the example of Weyburn in North America where 21 million tons of CO2 have been stored safely. In Weyburn they observed only very small magnitude seismic activities and they did not observe any fault reactivation or creation.
Stuart Hazeldine from SCCS acknowledges that the Earth's crust is critically stressed and it is difficult to predict the size and timing of earthquakes, but seismic monitoring of CO2 storage sites can detect any seismic activity. As the radius of pressure increase is much larger than the radius of CO2 plume, the monitoring should apply to the radius of pressure increase. However, the pressure radius could be managed by producing water or oil to create space for the CO2.
The important question is "how much CO2 will leak if an earthquake occurs?" According to Zoback and Gorelick, induced earthquakes will form open fracture pathways and the injected CO2 will leak to the surface. As Hazeldine points out, this is unlikely to occur because the new fault needs to intersect the caprock above the free CO2 level and therefore the fault has to occur within the radius of physical CO2 trapping.
If CO2 leaks to surface it will be a localized leak and can be easily identifiable and remediated in a process similar to oil well blowouts. Moreover, more than half of the injected CO2 is likely immobile due to residual saturation effects. CO2 retention increases further with time by dissolution of CO2 in water.
All the authors agree that seismic factors must be considered in the planning and permitting process, however the current evidence does not suggest that the risk is significant.
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Ozgur Gundogan, Reservoir Engineer
Statement references and links included with kind permission from Clean Air Task Force, PTRC and Professor Stuart Haszeldine of SCCS, Edinburgh University. To read any of the statements follow the links in the article or click here.