Can small events (M < 0) observed during hydraulic fracture stimulations initiate large events (M > 0)?

Abstract

It is becoming evident that under different circumstances, such as injection rates and geologic conditions, it might be possible to generate larger magnitude (M > 0) events during hydraulic fracture stimulations. By utilizing fully integrated passive seismic monitoring programs, we can examine the relationship between the occurrence of small magnitude events (M < 0) and the fracture and stress conditions that may lead to larger events. In our investigations, we identify that over all scales of observation the events follow a self-similar behavior; however, the small magnitude events are generally lower in stress release than observed for the larger events. These differences can be explained by the observed failure mechanisms where smaller events tend to be driven by shear-tensile failures of pre-existing discrete fractures (joints) whereas the larger events appear to be dominated by shear-driven failure processes associated with pre-existing faults. These observations suggest that there can be sufficient stress transfer and stress buildup resulting from the smaller events associated with hydraulic stimulation to allow for nearby pre-existing faults to slip in shear.