Abstract
Typically, seismic networks are deployed downhole to monitor injection-related activity and are designed to detect and analyze microseismic events at very short distances. During hydraulic fracture treatments, thousands of events can be generated with magnitudes in the range of -M4 to –M0. These networks record relatively high frequency signals (> 15Hz) and use time records with a short duration; however, they are limited in low-frequency response. This lack of bandwidth results in underestimated magnitudes for events with larger magnitudes (from about –M0 to +M4) that may occur during stimulations. To correct this issue, recording with high -sensitivity, near-surface sensors with low-frequency capabilities (>0.5Hz) extends the frequency range thereby allowing for the correct assessment of magnitude. In this paper, we investigate the spatial and temporal variations in seismicity associated with hydraulic fracture treatments of a shale play in North America using two different monitoring configurations, with multiple, fibre-optic based wireline arrays, each with twenty to forty-eight three-component levels of omni 15Hz geophones deployed vertically in close proximity to the treatment well and using a near-surface network of five stations consisting of three component Force Balance Accelerometers and 4.5Hz geophones. The near-surface monitoring network detected hundreds of events ranging in magnitude from M0 to M3, the largest of which were also recorded on a nearby national seismic network station. The downhole events did not see such large events, but recoreded thousands of events up to M0. Here, we compare parameters from the downhole data to the near-surface data to try to answer what role the larger events are playing in the reservoir and how they can be put into proper context with the smaller, downhole data. The failure mechanisms and scaling relationships for these large events suggest that they are the response of larger features in the reservoir, slipping in response to the stress perturbations induced by the treatment. Based on these observations, we suggest that a hybrid solution is appropriate for reservoirs with structural controls in order to appropriately assess the reservoir response to stimulations.
