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smaller than those being proposed for large power plant and industrial plant operations,17 these projects provide data for assessment of the potential for induced seismic activity associated with large-scale CCS.

Induced Seismicity Risks

The risk of induced seismicity from CCS is currently difficult to assess accurately. The NETL reported that no harmful induced seismicity had been associated with any of the global CCS storage demonstration projects as of February 2011.18 However, the volumes of CO2 injected at these sites so far are small in comparison to the volumes being considered for future proposed large CCS projects. Unlike most water disposal wells, CCS involves continuous CO2 injection at high rates under high pressures for very long periods of time. The potential therefore exists to increase pore pressures throughout a volume with the storage reservoir that is much larger than those affected by other energy technologies. Given that the potential magnitude of an induced seismic event correlates strongly with the fault rupture area, which in turn relates to the magnitude of pore pressure increase and the volume in which it exists, it would appear that CCS may have the potential for significant seismic risk. The combination of hydro-chemical-mechanical effects such as mineral dissolution may also exacerbate the problem (Espinoza et al., 2011). Some factors could also serve to mitigate risk such as low viscosity and lower injection pressure and limits of permanent pressure change in the reservoir depending upon variables such as reservoir thickness.

DISCUSSION

Geothermal, enhanced geothermal, oil and gas, unconventional oil and gas, and CCS technologies all involve fluid withdrawal and/or injection, thereby providing the potential to induce seismic events. The rates, volumes, pressure, and duration of the injection vary with the technology as do the potential sizes of the earthquakes, the mechanisms to which the earthquakes are attributed (Table 3.4), and the possible risk and hazards of the induced events.

Induced seismicity is commonly characterized by large numbers of small earthquakes that persist during, and in some cases significantly after, fluid injection or removal. At several sites of seismicity caused by or likely related to energy technologies, calculations based on the measured injection pressure and the measured or the inferred state of stress in

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17 Approximately 3,000 million metric tonnes (~3,300 million tons) of CO2 are reported to have been emitted by the United States in 2009 from the combined activities of electricity and heat production, manufacturing and construction, and other industrial processes including petroleum refining, hydrocarbon extraction, coal mining, and other energy-producing industries. Data available at www.iea.org/co2highlights/co2highlights.pdf.

18 See www.netl.doe.gov/technologies/carbon_seq/FAQs/permanence4.html.



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