Analysis and Interpretation of CLustered Microseismicity at Geothermal and Petroleum Reservoirs

Analysis of events induced during ARCO's Deep Well Treatment and Injection project demonstrated "real-time" hypocenter modeling for hydraulic fracture diagnostics. The injection produced a hydraulic fracture that exhibited strong spatial asymmetry. Analysis of the results suggest that the phenomenon of "screenout" may be one mechanism contributing to the observed seismicity's spatial and temporal characteristics and the direction of the asymmetric failure may be related to heterogeneities in formation depositional structures.

A swarm of events in 1993 near the northern boundary of the Larderello Geothermal field occurred just below an important regional seismic reflector known as the "K" horizon. The distribution of events suggests that seismic slip on steeply dipping, northwest-southeast striking Tortonian age normal faults could account for the observations and that high pressure fluids circulating in the fracture zone of the "K" horizon cause stress changes and slip on localized patches of the faults. Due to the relative depth of this swarm, compared to reservoir depths, this cluster was attributed to the natural geodynamics of this complex area.

Analysis of a set of events from the LAGO seismogenic zone in the Larderello Geothermal field again showed a spatial association with the seismic "K" horizon and a lateral NW-SE linear distribution that correlated well with the direction of regional extension.

Finally, this thesis reports the results of the relative event relocation for a 1997 swarm of events from the Monte Amiata geothermal field. The hypocenters are located very near a northwest-southeast striking fault at depths correlated with the deeper of two actively producing fracture zones of the Piancastagnaio field.