Evaluating Micro-Seismic Events Triggered by Reservoir Operations at the Geothermal Site of Gross Schonebeck (Germany)

This study aims at evaluating the spatial and temporal distribution of 26 micro-seismic events which were triggered by hydraulic stimulation at the geothermal site of Gross Schonebeck (Germany). For this purpose, the alteration of the in-situ stress state and the related change of slip tendency for existing fault zones due to stimulation treatments and reservoir operations is numerical simulated. Changes in slip tendency can potentially lead to reactivation of fault zones, the related movement can lead to the occurrence of seismic events. In the current numerical study, results obtained based on the thermal-hydraulic-mechanical coupled simulation are compared to field observations. In particular, the study focuses on describing the fault reactivation potential: (1) under in-situ stress conditions; (2) during a waterfrac stimulation treatment; and (3) during a projected 30 years production and injection period at the in-situ geothermal test-site Gross Schonebeck. The in-situ stress state indicates no potential for fault reactivation. During a waterfrac stimulation treatment, micro-seismic events were recorded. Our current evaluation shows an increase of slip tendency during the treatment above the failure level in the direct vicinity of the micro-seismic events. During the projected production and injection period, despite increased thermal stress, the values for slip tendency are below the threshold for fault reactivation. Based on these results, and to prove the applied method to evaluate the observed micro-seismic events, a final discussion is opened. This includes the in-situ stress state, the role of pre-existing fault zones, the adopted criterion for fault reactivation, and a 3D rock failure criterion based on true triaxial measurements.