Summary
Besides the typical negative magnitude fractures induced in the treated formations by hydraulic fracture stimulation programs, small positive magnitude events associated to small faults located underneath or cross-cutting the reservoir also frequently occur. In this study we investigate discriminant source and rupture characteristics to distinguish between the two event types and also between reservoir fracture types. To achieve this goal we estimate source and failure properties of –M3 to M1 seismic events recorded during a hydraulic fracturing stimulation of a shale reservoir in Horn River Basin, Canada, for which the >M0 events are associated with slip on a pre-existing fault underneath the reservoir. Comparison between static and dynamic source parameters suggests distinct signatures of the two event types associated to two distinct failure processes. Positive magnitude events occurring beneath the reservoir have slightly higher static and dynamic stress drops, higher apparent stress and energy release, and rupture faster than shallower reservoir events. These differences reflect fracturing of harder rocks at higher confining stresses for the deep events, but also a possible release of a larger quantity of strain energy stored within the fault zone. The lower stress and energy release and slower rupturing fractures observed in the reservoir fractures, as well as overshoot type failure (slip weakening failure) indicates fluid lubrication by pore pressure increase and frictional resistance reduction. Some trends are also observed when looking only at reservoir fractures, where variations in average rupture velocities correlate with variations in formation depth and fault azimuth reflecting a dominance of the local stress field over other factors. Average rupture velocities also correlates with elapsed time showing an observable imprint of the changed local conditions during treatment over the regional conditions. Identification of more spatial and temporal families of events with similar rupture behaviors and source characteristics can be used as a proxy for specific fracture network development and hydrocarbon production and included in geo-mechanical models and fracture treatment designs. Reservoir and fault related induced events release less stress and radiates less energy than natural occurring tectonic earthquakes of comparable size at similar depths indicating a potential fluid influence in these failures. Considering the ongoing debate regarding the cause-effect relationship between fluid injection programs and nearby deeper earthquakes this study suggests that source parameters can be used as a discriminant factor between the two types of earthquakes.
