Feasibility study for the microseismic monitoring of the natural gas reservoir of “Sant’Alberto” (Po Plain, Italy)

Abstract

In this paper we present a feasibility study for the surface seismic monitoring of the “Sant’Alberto” gas reservoir (Po Plain, Northern Italy). The project was commissioned to INGV in the framework of the activities planned by the oil & gas E&P company PoValley Energy (PVE), aiming to exploit the underground deposit. After estimating the ambient seismic noise of the area, we hypothesized a microseismic network composed of five stations installed in a 8.0 x 8.0 km2 area centered on the surface projection of the reservoir. Moreover, due to the high noise levels observed, we also considered the possible installation of a borehole station at 200 m depth. Detection and localization thresholds were estimated by comparing the simulated power spectral density of hypothetical seismic sources located in the crustal volume surrounding the reservoir, with the mean power spectra of the ambient seismic noise observed (or hypothesized) at each station-site. Two crustal volumes for the earthquakes detection were considered: the inner domain of detection, DI (8.6 x 8.6 x 4.0 km3), within which we should ensure the highest network performance, and the extended domain of detection, DE (18.6 x 18.6 x 9.0 km3). After estimating the background level of ambient seismic noise, simulations show that: 1) five surface stations installed within the surface projection of DI ensure localization thresholds ranging between ML 0.7 and 0.8 within DI; 2) the installation of the borehole station allows to improve the detection threshold everywhere within DI, and down to ML =-0.4 nearby the reservoir; and 3) localization thresholds obtained in the crustal volume included in DE, but not in DI, show values around ML = 1.0.