ABSTRACT

The downhole hydraulic fracturing process, besides fracturing formation rocks, generates small, secondary fractures around the borehole, allowing evaluation of the result of fracturing using borehole sonic measurements. We analyzed the near-borehole fracture network from an existing laboratory hydraulic fracturing experiment to study the fracture distribution around the borehole. The result indicates that the fracture distribution exhibits fractal characteristics. The fractal dimension is high in the near-borehole region and decreases away from borehole. Because the fractal dimension increases with fracture density, this indicates that fracturing produces a high fracture-density zone in the near-borehole region. The high concentration of the hydraulic fractures in turn can causes significant attenuation in the sonic-logging waveforms acquired after fracturing. The fracturing-induced sonic attenuation, averaged over the sonic frequency band, can be estimated using a median frequency shift method. Comparison of the attenuation of the fracturing interval with that of an unfractured interval, or with that of the same interval before fracturing allows for evaluating the result of fracturing and the fracture extension along the borehole. The application of the method is demonstrated with field-data examples and validated by comparing results from existing borehole techniques, thus offering a useful technique for evaluating the result of hydraulic fracturing using the borehole sonic-wave attenuation.

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