Relationship of pore pressure to large microseismic events during hydraulic stimulation at Basel, Switzerland, in 2006

Previous analyses of large-magnitude microseismic events during and after hydraulic stimulation of rock formations at Basel, Switzerland, in 2006 have shown that most of the large events from the deep part of the stimulated zone originated in ruptures involving multiple asperities and that the shallow large events occurred in fractures that were sub-parallel to the main stimulated zone. The trigger mechanism of the large events remains unidentified. In this study, we investigated the relationship between pore pressure and the occurrence of large microseismic events to reveal the underlying physics. The stress state on fracture planes and the distribution of the critical pore pressure for shear slip during the stimulation were estimated using tectonic stress and fault plane solution (FPS).The spatial distribution of estimated critical pore pressures showed a local anomaly close to one of the dominant feed points, which suggests hydraulic heterogeneity in the fracture system. Temporal analysis of critical pore pressures and microseismic hypocenters showed some correlation of pore pressure to the large events. This study revealed that critical pore pressure does not correlate directly with the magnitude of microseismic events. The large events were possibly triggered by other mechanisms, such as reduction of surface friction factor or changes in the distribution or concentration of local stress.