Gisela Viegas

Rupture Behaviour of Induced Seismicity

With a hybrid monitoring array of 15 Hz downhole sensors and lower frequency sensors on the surface, we can look in detail at the differences in rupture processes between lower magnitude seismicity usually associated with hydraulic fractures (M 0). We find that in the dataset we examine from the Horn River Basin in NE BC, that the lower magnitude events are associated with more lubricated, slower slip, and simple rupture processes whereas the larger magnitude events are more consistent with a more complicated rupture process with less of a fluid component. These findings are important to develop an understanding of the generating processes behind larger magnitude events observed during hydraulic fracture completions.

Felt Seismicity Related to Hydraulic Fracturing

Summary Hydraulic fracturing is well known to generate seismic events, most of these events are very small magnitude and are of great use when recorded by a properly calibrated array of geophones to delineate the geometry or the fractures and in other microseismic monitoring applications. However, there have been increasing numbers of reports of larger magnitude seismicity. In this presentation, we discuss the instrumentation aspects of properly recording and these larger magnitude events. We discuss a case study where seismicity was recorded by a near-surface network of 4.5 Hz geophones and forcebalanced accelerometers that corresponded to events up to moment magnitudes of 3, large enough to both be felt on surface and to be recorded by distance regional seismic stations up to 100 km away. These events are also accompanied by hundreds of events seen on the near-surface network with magnitudes between 1 and 3. The presence of these events has implications for previous microseismic studies where generally highfrequency (15Hz) geophones are employed to derive the locations. In these cases, the magnitudes of large events together with parameters like the radius of the rupture, will be systematically underestimated. Therefore, this saturation effect will cause a general mis-estimation of the discrete fracture network activated during the fracture. Events that may appear isolated below zone, if they are large enough, can have size dimensions in the range of hundreds of meters to kilometers. Features of such scales can have dramatic effects on the observed seismicity and so their accurate identification using instruments in the appropriate bandwidth is critical to obtaining an accurate picture of the DFN and the potential for seismic hazard associated with hydraulic fracturing.