Seismic and aseismic slips induced by large-scale fluid injections

A 3600 m deep well has been used to conduct large water injection tests in the Rhine Graben. The total volume injected during the fall 1993 reconnaissance program reached 44000 m(3). Induced seismicity was monitored with both a downhole and a surface seismic network. About 20000 events have been recorded by the downhole tools and 165 events with the surface network. The largest observed magnitude reached 1.9, as determined from signal duration observed on the surface network. Borehole televiewer observations show that some slip events were larger than 4 cm at the borehole wail, a value much larger than the slip motion associated with microseismic events, as evaluated From events' magnitude. It is concluded that these observed slip events were aseismic. This implies that induced seismicity is not a good marker for the efficiency of this hydraulic stimulation. It only helps to identify zones of high pore pressure during injection.