The Birth of an Underground Gas Storage in a Depleted Gas Reservoir—Results From Integrated Seismic and Ground Deformation Monitoring

In the Po Valley (Italy), near Lodi, a depleted methane reservoir was recently converted into an underground gas storage (UGS) facility. We describe the new monitoring infrastructure that integrates seismic and ground deformation capabilities. We also present results obtained before and after the start of UGS operation, namely: (a) the so-called “background,” for seismicity and deformation estimated over several decades before the UGS; (b) the “seismic baseline,” assessed using the new monitoring network over nearly 2 years before the gas injection began; and (c) the seismicity and deformation measured over the first 3 years of UGS operation. In practice, we observe the phenomena associated with the “birth” of a UGS with remarkable instrumental capabilities. Following three cycles of injection/extraction, about 30 events consistent with natural, tectonically related seismicity were located within 30 km of the UGS. Moreover, the observed uplift of about 2 cm is consistent with theoretical expectations of ground deformation. Our study confirms that UGS in depleted gas reservoirs, if well managed, can result in negligible, if any, human-induced seismicity and limited ground surface deformation. It also shows the importance of measuring the undisturbed conditions prior to industrial activities over a sufficiently long period of time, to correctly interpret the phenomena observed later.