Variations of attenuation and VP/VS ratio in the vicinity of wastewater injection: A case study of Costa Molina 2 well (High Agri Valley, Italy)

Abstract

By analyzing seismicity induced by the injection of wastewater into the Costa Molina 2 well, we characterized attenuation in the area. Attenuation quality factors [Formula: see text] and [Formula: see text] were calculated using the peak-frequency method, which is suitable for small-magnitude earthquakes registered in the region. Anomalously high attenuation with values of effective [Formula: see text] and [Formula: see text] ratio of less than 1 in the vicinity of the injection well was found, in comparison with [Formula: see text] for the rest of the region. Furthermore, the [Formula: see text] ratio was also found to be high in the close vicinity of the injection well. We determine how sedimentary rocks possibly change their geomechanical properties due to salt-water injection and find that observed changes of attenuation (increase) and [Formula: see text] ratio (increase) are consistent with the geomechanical models and experiments. Our findings indicate that the observed anomaly in the vicinity of the Costa Molina 2 well is due to the (1) changes of the properties of the rock mass affected by injection and (2) 3D structure in the overburden rocks.

Using ground motion prediction equations to monitor variations in quality factor due to induced seismicity: a feasibility study

“…For example, Toksoz et al (1979) and Johnston et al (1979) conclude that, for sandstone samples, Q can change from 100 to 10 based on the saturation and differential pressure. Similar results have been obtained by Hutching et al (2019) by analyzing earthquakes recorded during field operations at The Geysers geothermal field and by Wcisło et al (2017) who found high attenuation values (Q P ~ 48 and Q P /Q S < 1) analyzing seismicity induced by the injection of wastewater in the Costa Molina 2 well in southern Italy. Wandycz et al (2019) measured the variation of the anelastic attenuation based on the microseismicity recordings in Northern Poland during hydraulic stimulation of the two unconventional gas reservoirs.…”

How do Local Earthquake Tomography and inverted dataset affect earthquake locations? The case study of High Agri Valley (Southern Italy)

“…Concerning this, our images are in partial agreement with their study: indeed, the seismic rays, travelling from the cluster of fluid-induced events to stations close to the CM2 well [ Figure 15(b)] have ray-paths partially in very high Vp/Vs materials and partially in low Vp/Vs layers. Since high Vp/Vs values retrieved by Wcisło et al (2018) mainly describe an average representation of the whole properties of the subsurface materials, we think that images retrieved by this study may partially explain their results.…”

“…Also, Wcisło et al (2018), in their analyses on seismic data of injection induced cluster collected by stations very close to the CM2 injection well, retrieved high Vp/Vs values. Concerning this, our images are in partial agreement with their study: indeed, the seismic rays, travelling from the cluster of fluid-induced events to stations close to the CM2 well [ Figure 15(b)] have ray-paths partially in very high Vp/Vs materials and partially in low Vp/Vs layers.…”

“…The main aims were to characterize the crustal structure of the area and the different kinds of seismicity involving the HAV. These goals were achieved by means of local earthquake tomographies (Valoroso et al 2011;Improta et al 2017), earthquake relocations, focal mechanisms and statistical analyses (Valoroso et al 2009;Stabile et al 2014aStabile et al , 2014bImprota et al 2015;Stabile et al 2015;Telesca et al 2015;Buttinelli et al 2016;Improta et al 2017) and joint Vp/Vs and attenuation studies (Wcisło et al 2018).…”

Estimation of the quality factor based on the microseismicity recordings from Northern Poland

“…Eisner et al (2013) proposed derived theory to predict the peak frequency of observed microseismic event waveforms and showed it is consistent with surface and downhole monitoring datasets. Wcisło and Eisner (2016) developed the peak frequency methodology further and developed a new methodology to obtain more accurate results both on downhole monitoring dataset and for the surface monitoring (Wcisło et al 2017).…”