Felt Seismicity Related to Hydraulic Fracturing

Hydraulic fracturing is well known to generate seismic events; most are very small magnitude (M <0) and are of great use to delineate geometry or fractures and potentially outline the effective stimulation volume. However, there have been increasing numbers of reports of larger magnitude seismicity. In this presentation, we discuss the instrumentation aspects of properly recording these larger magnitude events. We discuss a case study where seismicity was recorded by a near-surface network of 4.5 Hz geophones and force-balanced accelerometers that corresponded to events up to moment magnitudes of 3, large enough to both be felt on surface and be recorded by distance regional seismic stations up to 100 km away. The presence of these events has implications for previous microseismic studies where generally high-frequency (15Hz) geophones were employed. In these cases, the magnitudes of large events together with parameters like the rupture radius will be systematically underestimated. This saturation will cause underestimation of discrete fracture network dimensions activated during the fracture. Events in the case study can have size dimensions in the range of hundreds of meters, have dramatic effects on the interpretation issues like migration of frac fluids and proper targeting of formations.