Induced Seismicity in Underground Gas Storage - The Case of Castor, Offshore NE Spain

Abstract

Oil and Gas operations have the potential to induce earthquakes, and this is becoming an increasing concern for operators and regulators. Here we present the case of the Castor Underground Gas Storage (UGS), offshore NE Spain, where seismicity was induced during the first phases of gas injection. The microseismicity that occurred during the injection period, and which was not felt, evolved into magnitude 4 events almost two weeks after the end of the injection. The largest earthquakes of the sequence were weakly felt by coastal populations (EMS-98 Intensity III) and, in spite of posing no risk, pressed the authorities to halt all activity in the UGS. Here we compile the results from the major studies of the Castor seismicity: earthquake relocations and source mechanisms, and use new tools to characterise the seismicity and its relation to the gas injection. Cases of induced seismicity, such as Castor, where the external forcing is known, can contribute significantly to our understanding of the response of the crust to stress perturbations caused by fluid movement and the associated rupture events.