Written evidence submitted by James Verdon et al . , University of Bristol ( CCS 02 )

 Large-scale subsurface fluid injection has the potential to trigger earthquakes. In addition to seismic hazard, reactivation of faults and fractures during CO2 injection has the potential to compromise storage integrity. These risks need to be taken into account when selecting potential carbon capture and storage (CCS) sites.  Deployment of seismometers at current large-scale CCS projects (more than 1 million tonnes of CO2 per year) is not routine. However, two such projects, Weyburn and In Salah, have used seismic monitoring arrays, and low-magnitude induced seismic activity has been detected at both (Verdon et al., 2013). Characterisation of such seismic events, even if they are too small to be noticed by humans at the surface, is crucial for generating a complete understanding of the geomechanical response to CO2 injection, and therefore the risk to secure storage posed.  The detection thresholds and event location accuracies needed to assess the both the seismic hazard and the risk to storage integrity are beyond the capacity of current national seismometer networks. Therefore, dedicated seismic monitoring arrays will need to be installed at proposed CCS sites. Ideally, such arrays will be installed well in advance of injection, facilitating baseline monitoring of pre-existing, naturally-occurring seismic activity.  Seismic monitoring should continue during the CO2 injection phase. Identification of anomalous seismicity must be used to guide injection programs (injection rate, pressure, and the number of injection wells) to mitigate both the seismic hazard and the risk posed by fault reactivation to the hydraulic integrity of the targeted storage formation.  We further recommend that geomechanical appraisal prior to injection should characterise rock properties, the state of stress, and the presence of any faults and fractures in and around the target reservoir,