Design of Passive Seismic Monitoring for Underground Gas Storage in the Vienna Basin, Austria

We present a case study for designing a passive seismic monitoring network in a large underground gas storage site. The study included measurements of background seismic noise at selected locations across the site and background seismic noise along a string of receivers cemented in a shallow monitoring borehole. We measured the noise levels over a period of three months and studied the background seismic noise reduction in the shallow monitoring borehole. Specifically this study reveals a significant decrease with depth of the temporal variations in background seismic noise levels at the shallow monitoring borehole relative to the surface. We compare location accuracy from the downhole and surface monitoring arrays, and combinations of these to determine the optimal monitoring array. We also evaluate the detection threshold for microseismic events in the proposed shallow borehole monitoring array based on measured noise levels and empirical observations of microseismic events in similar reservoirs.