Inferring Hydraulic Pressure Changes from Induced Seismicity Observations: Three Showcases from Geothermal Reservoirs

Abstract

We apply a recently developed approach for inferring in situ fluid pressure changes from induced seismicity observations to datasets from geothermal reservoirs at St. Gallen (Switzerland), Paralana (Australia), and Cooper Basin (Australia), respectively. The approach, referred to as seismohydraulic pressure mapping (SHPM), is based on mapping the seismic moment of induced earthquakes. Relative fluid pressure changes are inferred from the stress deficit of fracture patches slipping repeatedly. The SHPM approach was developed for the specific scenario, where induced earthquakes occur on a single, larger-scale plane with slip being driven by the regional stress field. We demonstrate that this scenario applies to the three datasets under investigation, indicating that geothermal systems in crystalline rock could typically be fault-dominated. For all datasets, individual earthquake source geometry could not be determined from source spectra due to the attenuation of the high signal frequencies. Instead, SHPM was applied assuming a constant stress drop in a circular crack model. Absolute values of inferred pressure change scale with the assumed stress drop while the spatiotemporal pattern of pressure changes remains similar even when varying stress drop by one order of magnitude. We demonstrate how the associated mismapping of seismic moment tends to average out when hypocentres are densely spaced. Our results indicate that SHPM could provide important information for calibrating numerical reservoir models.