No 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.

Recently proposed peak-frequency method is used to estimate the P-and S-wave quality factors from microseismic events. We use a downhole monitoring dataset of 10 high signal-to-noise ratio microseismic events to calculate P-and S-wave effective attenuation of a carbonate reservoir. We benchmark these results with the spectral ratio method and obtain mutually consistent results. Additionally we develop and test two techniques of peak frequency determination. We show that the peak frequency method can be successfully used in the estimation of the quality factor and it provides precise measurements of attenuation.K e y w o r d s : attenuation, microseismicity, inversion, ray tracing