Geomechanical Modeling of Fault Responses and the Potential for Notable Seismic Events during Underground CO2 Injection

The importance of geomechanics associated with large-scale geologic carbon storage (GCS) operations is now widely recognized. There are concerns related to the potential for triggering notable (felt) seismic events and how such events could impact the long-term integrity of a CO2 repository (as well as how it could impact the public perception of GCS). In this context, we review a number of modeling studies and field observations related to the potential for injection-induced fault reactivations and seismic events. We present recent model simulations of CO2 injection and fault reactivation, including both aseismic and seismic fault responses. The model simulations were conducted using a slip weakening fault model enabling sudden (seismic) fault rupture, and some of the numerical analyses were extended to fully dynamic modeling of seismic source, wave propagation, and ground motion. The model simulations illustrated what it will take to create a magnitude 3 or 4 earthquake that would not result in any significant damage at the groundsurface, but could raise concerns in the local community and could also affect the deep containment of the stored CO2. The analyses show that the local in situ stress field, fault orientation, fault strength, and injection induced overpressure are critical factors in determining the likelihood and magnitude of such an event. We like to clarify though that in our modeling we had to apply very high injection pressure to be able to intentionally induce any fault reactivation. Consequently, our model simulations represent extreme cases, which in a real GCS operation could be avoided by estimating maximum sustainable injection pressure and carefully controlling the injection pressure. In fact, no notable seismic event has been reported from any of the current CO2 storage projects, although some unfelt microseismic activities have been detected by geophones. On the other hand, potential future commercial GCS operations from large power plants will require injection at a much larger scale. The large-scale pressure buildup associated with such an injection operation, associated crustal straining, and potential undetected faults might be of greatest concern. We analyzed cases with undetectable faults and argue that such faults would likely be less than 1 km long and therefore seismic magnitudes could be estimated to be less than about 3.6, even if the entire fault would to be reactivated. However, not withstanding the potential for triggering notable (felt) seismic events, the potential for buoyancy-driven CO2 to reach potable ground water and the ground surface is more important from safety and storage-efficiency perspectives. We also know from natural and industrial analogues that reshear of fractures and faults in the caprock is important in determining the storage potential. Thus, fault reactivation, even associated with relatively small seismic or aseismic events, could potentially increase CO2 seepage out of the intended storage complex and therefore reduce the effectiveness of a CO2 storage operation. Under these circumstances, we recommend a staged, learn-as-you-go approach, involving a gradual increase of injection rates combined with continuous monitoring of geomechanical changes, as well as siting beneath a multiple layered overburden for multiple flow barrier protection, should an unexpected deep fault reactivation occur.