Temporal evolution of a seismic sequence induced by a gas injection in the Eastern coast of Spain

S. Ruiz-Barajas, N. Sharma, V. Convertito, A. Zollo, B. Benito
2017 Scientific Reports  
Induced seismicity associated with energy production is becoming an increasingly important issue worldwide for the hazard it poses to the exposed population and structures. We analyze one of the rare cases of induced seismicity associated with the underwater gas storage operations observed in the Castor platform, located in the Valencia gulf, east Spain, near a complex and important geological structure. In September 2013, some gas injection operations started at Castor, producing a series of
more » ... ismic events around the reservoir area. The larger magnitude events (up to 4.2) took place some days after the end of the injection, with EMS intensities in coastal towns up to degree III. In this work, the seismic sequence is analyzed with the aim of detecting changes in statistical parameters describing the earthquake occurrence before and after the injection and identifying possible proxies to be used for monitoring the sequence evolution. Moreover, we explore the potential predictability of these statistical parameters which can be used to control the field operations in injection/storage fluid reservoirs. We firstly perform a retrospective approach and next a perspective analysis. We use different techniques for estimating the value of the expected maximum magnitude that can occur due to antropogenic activities in Castor. Induced seismicity typically refers to low magnitude earthquakes (usually M w < 5.0), that are caused by human activities which alter the stress field of the Earth's crust 1 . According to Ellsworth 2 the induced seismicity may include both the earthquakes that primarily release stresses created by the fluid injection (or other industrial activity) in the near volume surrounding the downhole injection point and triggered events that release tectonic stresses as, for instance, an effect of pore pressure increase at critically stressed faults. This kind of seismicity has been observed in relation to changes in the dam water level, hydro-fracturing operations, fluid injections and extraction in geothermal areas 2-4 . Historically, some best-documented and well-known cases of induced seismicity related to injection of fluids into the crust have been provided (e.g., the Denver earthquakes as reported by Healy et al. 5 ; Simpson 6 ). Recently, energy development technologies that involve fluid injection operations are being increasingly used in different projects worldwide and different cases of injection-induced seismicity have been reported. However, it is rare to find cases of induced seismicity associated with underwater gas storage operations. In the present study we analyze one of the rare and best recorded examples of seismic sequences induced by gas storage. In particular, we focus on Castor field, which is the biggest underground reservoir of natural gas in Spain, located in the Gulf of Valencia (NE Mediterranean) (Fig. 1) . The offshore platform is located 22 km away from the nearest inland town (Vinaroz, Castellón). The target reservoir is contained within a fault-limited, horst structure with an average depth of more than 1700 m below the mean sea level. The main fault of this structure is the Amposta fault. The recorded seismicity of the zone, before the gas injection operations, is the lowest in the eastern Spain, where only two earthquakes with magnitudes larger or equal to M w 5.0 occurred (1885, M = 5.4 and 1995, M = 5.0) at distances greater than 50 km from the reservoir. After the beginning of injection in September 2013, a sequence of seismic events occurred, reaching a maximum magnitude of 4.2 and macroseismic intensities in coastal towns of degree III on the EMS-98 scale. While the first part of the sequence can be directly related to the injection process, Cesca et al. 7 argue that the second Published: xx xx xxxx OPEN www.nature.com/scientificreports/ 2 Scientific RepoRts | 7: 2901 |
doi:10.1038/s41598-017-02773-2 pmid:28588269 pmcid:PMC5460170 fatcat:3xivb4spkzcrpfourk67ybboxi