In this paper we summarize and compare numerical simulation results of injection-induced fault reactivation and seismicity associated with underground CO2 injection and hydraulic fracturing of shale-gas reservoirs. Overall, the analysis shows that while the CO2 geologic sequestration in deep sedimentary formations are capable of producing notable events (e.g. magnitude 3 or 4); the likelihood for such felt events is much smaller in the case of shale-gas fracturing. The reason is that CO2 geological sequestration involves injection and pressure disturbances at much larger scale and duration than in the case of shale-gas fracturing. In the case of shale-gas fracturing, the expected low permeability of faults intersecting gas-saturated shales is clearly a limiting factor for the possible rupture length and seismic magnitude. For a fault that is initially impermeable, the only possibility of larger fault slip events would be opening by hydraulic fracturing allowing pressure to permeate along the fault causing a reduction in the frictional strength over a sufficiently large fault surface patch and very brittle fault properties that would allow sudden (seismic) shear slip to develop over a sufficient large rupture area. In both CO2 sequestration and shale-gas fracturing, the brittleness of the rock is an important factor, which is a site-specific factor and it should be considered when assessing the likelihood of felt seismicity.
The potential for fault reactivation and induced seismicity are issues of concern related to both geologic CO2 sequestration and shale-gas fracturing [1-7]. It is well known that underground injection may cause induced seismicity depending on site-specific conditions, such as stress and rock properties and injection parameters. In the case of shale-gas fracturing, only three cases of felt seismicity have been documented out of hundreds of thousands of hydraulic fracturing stimulation stages performed to-date . So far no sizeable seismic event that could be felt by humans on the ground surface has been documented associated with CO2 sequestration activities [2, 7]. However, at future industrial scale projects for CO2 sequestration, a much larger rates and volumes than current pilot scale projects would be required to mitigate the global emission of greenhouse gases.