Behavior of induced microseismic events with large magnitude

Hydraulic stimulation of geothermal and oil/gas reservoir is one of the conventional techniques used for enhancing the productivity from reservoirs. In most cases, the stimulation process induces microseismic events. Based on the activity, location, magnitude and source mechanism of such events, the 3D localization and characterization of the reservoir can be carried out with practical resolution. Typically, microseismic events from a reservoir have moment magnitudes of less than zero, and most of them are detectable only by downhole sensors with high sensitivity. However, it is known that some of these events have higher magnitudes and can be felt at the surface. These large events can be hazardous from an environmental point of view, while resulting in enhanced permeability in the reservoir at the same time. The authors have analyzed the spatio-temporal distribution, and source mechanism of such microseismic events having large magnitudes (big events) observed during the hydraulic stimulations at Australian hot fractured rock (HFR) site in the Cooper Basin (Asanuma et al., SEG Exp. Abst., 2004) and also at the European hot dry rock (HDR) site in Soultz, France (Asanuma et al., Trans. GRC, 2004). A comparison between the origin time of these big events and the hydraulic records showed that many of the big events occurred after the shut-ins at both the sites. Besides, during pumping, most of these events did not show a clear correlation to the wellhead pressure and the rate of pumping. In most cases, the source mechanism of the big events were consistent with the shear slip of a preexisting fracture. We have also found that some of the big events at the Australian site brought very clear extension of the seismic cloud into zones that were seismically silent before, suggesting that some kind of hydraulic barrier was overcome by these big events. The observational data also showed that the microseismic events at those sites originated mainly from a slip of asperities in existing fractures under a nearly critical stress state of the shear slip. The largest source radius estimated from the moment magnitude at the European site was around 300m. It is an accepted fact that the typical length of the existing joints in granite at the stimulated zone is around ten meters. Hence, we conclude that the source mechanism of these induced microseismic events at the European site cannot simply be interpreted as due to a slip of asperities alone, and an investigation into the correlation of the stress concentration at the joints with the magnitudes is also warranted.