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)

Wcisło M., Stabile T. A., Telesca L., Eisner L.

Geophysics, 83(2) pp. B25-B31, 2018 | doi: 10.1190/geo2017-0123.1

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 Qp and Qs 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 Qp ∼ 48 and Qp/Qs ratio of less than 1 in the vicinity of the injection well was found, in comparison with Qp ∼ 60 for the rest of the region. Furthermore, the Vp/Vs 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 Vp/Vs 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.