A new method for characterization of fractured reservoirs has been developed from wide experience in acoustic logging and from results of theoretical research. The method is based on the use of shear (S)-wave and Stoneley-wave attenuation, two parameters obtained from the processing of full-waveform acoustic data. The combination of both parameters allows location of individual fractures and fractured zones. The propagation of S-waves is based on two different physical mechanisms. For this reason the combined use of both parameters can help discriminate between individual fractures of large aperture and conductive fractured zones (Stoneley-wave attenuation), and individual fractures of lesser aperture or mostly sealed (S-wave attenuation). Fracture localization is possible in cased holes because of our ability to process full-waveform data acquired through casing with the EVA@ (Evaluation of Velocities and Attenuations) tool and because S-waves are the same with or without casing. The method was successfully tested on the Gaviota gas field, offshore Spain. Full-waveform data acquisition and processing with EVA for three wells, open or cased, helped locate or was the only way to locate fractured zones, which, after perforation and acid treatments, yielded a very satisfactory gas production.

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