Background to the Study
 

Induced seismicity refers to seismicity caused by human/external activity above natural background levels in a given tectonic setting and is distinguished from triggered seismicity, where human activity affects earthquake recurrence intervals, magnitude or other attributes. The physics of triggered and induced seismicity is thought to be identical and both need to be considered during geological storage of CO2. Induced seismicity may be observed during impounding of dams, mining and tunnelling operations, quarrying, underground solids/cuttings disposal, waste fluids disposal, oil and gas production, geothermal energy production and geological storage of gases and occasionally by rainfall. In some cases induced seismicity has led to projects being suspended, for example enhanced geothermal activities in Basel, Switzerland.

Injection and consequent geological storage of CO2 may affect subsurface stress and alter in-situ fluid pressure and hence potentially induce seismicity. It is necessary to evaluate potential for and effects of induced seismicity during risk assessment of storage projects. A best practice approach has already been proposed by the US WESTCARB Partnership based on protocols related to geothermal activities.

Induced seismicity may be caused by mechanical loads which can cause changes to the stress regime. Fluid pressures also play a key role in seismicity as pore pressures act against gravitational and tectonic forces and, if increased sufficiently, may cause rock failure. Pre-existing fractures may be stable in the stress regime before fluid injection, but fluid injection increases the pore pressure, which acts in opposition to the normal stress. If pore pressure is great enough to overcome the normal stress, then shear failure will occur.

Other factors that may affect seismicity are thermal and chemical stresses, which can have a weakening effect on the rock. This is likely in geothermal reservoirs, though usually occurs in conjunction with seismicity caused by changes in fluid pressure.

Induced seismicity can also be associated with hydraulic fracturing; this is when a rock is purposefully fractured by injecting water at high pressure with an aim to increase permeability. This has been observed during enhanced geothermal activities and in shale gas production.

Learnings from induced seismicity in other areas, e.g. geothermal activities and hydrocarbon exploration may be applied, but differences with CCS need to be taken into account; such as depth differences, type of sediment into which injection will take place, tectonic activity in the area, injection pressure, volume injected and the length of injection. These values will, for example be very different from those for enhanced geothermal activity which will likely be deeper, into basement rock, possibly in a tectonic area with higher injection pressures for short bursts.

Induced seismicity will also depend on several other factors, which may include the stress regime, fault orientation and locations, and rock friction. It is necessary that site characterisation takes into account any potential for induced seismicity; however, as more information becomes available during the lifetime of the project, through the monitoring programme, the risk in regards to induced seismicity can be reassessed. This may be in the form of real-time monitoring of any ongoing induced seismicity.

CO2CRC, a consortium based in Australia and New Zealand, was commissioned by IEAGHG to undertake this study.

Key Messages

• The risks associated with induced seismicity at CCS sites can be reduced and mitigated using a systematic and structured risk management programme.

• Statistical models presently show the most promise for forecasting seismicity, but improved physical models are under development and may be key in the future. Both types will need to be tailored to the injection site.

• Site performance and management guidelines should be established prior to injection to facilitate: 1) definition of the acceptable levels and impacts of induced seismicity; 2) optimisation of the monitoring and mitigation programmes; and 3) the establishing of key control measures. Such guidelines have been developed for Enhanced Geothermal Systems and should provide the starting point for a management strategy of induced seismicity at CCS sites.