Micro-seismicity associated with CO2 geological storage activities in Hontomín, Spain

Abstract

We present the first local micro-seismicity catalogue including focal mechanisms for the Hontomín plant, the only site of its kind in Spain and one of the very scarce CO2 storage facilities in Europe. The catalogue reveals a correlation between micro-seismicity occurrence and bottom hole pressure (BHP) at the injection well. A burst of micro-seismicity ranging from -1 to 0.4 ML, thus imperceptible to human senses, took place coeval with the longest duration test of the period studied when the BHP was the highest. Events clustered at ∼ 0.4 km below the well bottom hole. Focal mechanisms could be well resolved for 3 events with similar results, showing a strong strike-slip component and a minor reverse component, with similarly oriented nodal planes. The local stress fields inverted from the focal mechanism solutions exhibit an orientation of the maximum horizontal stress axis (SHmax) that ranges from NE to ESE and differs from the principal stress orientations obtained in previous works for the same area. The micro-seismicity locations and tensors obtained are useful inputs to models aiming to better understand the deformation effects of fluids’ injection underground.This research was funded by the SENSE (Assuring integrity of CO2 storage sites through ground surface monitoring) project no. 299664 through ACT (EC project no. 691712). In this context, the Hontomín case study was subsidized by Agencia Estatal de Investigación (project no. PCI2019-111951-2). Also, the analysis of strain and stress tensor data was partially funded by the project FAMRAD PID2020-113407RB-I00