ABSTRACT

New borehole geophysical techniques to characterize conductive fractures were tested in Rhourde El Baguel. Fractures crossing the borehole were investigated using Ultrasonic Borehole Image (UBI) scans and full-waveform sonic data generated by a Dipole Shear Sonic Imager (DSI) tool. The full-waveform sonic data included acquisition of compressional waves, Stoneley waves and 4-component dipole shear waves. The UBI provided high-resolution information regarding the location and orientation of fractures penetrated by the borehole. Analysis of crossed-dipole shear wave data revealed strong shear anisotropy consistent with locations of fractured intervals identified in core and UBI analysis. Also, the direction of the fast shear arrival closely matched the principal fracture direction identified from UBI analysis, suggesting that fracture direction may be determined using crossed-dipole sonic analysis. Reflected Stoneley-wave analysis was used to determine if individual fracture sets are conductive. Direct Stoneley waves excite fluid movement into conductive fractures crossing the borehole, generating reflected Stoneley waves. The amplitude of the reflected Stoneley wave depends on the conductivity of the fractures; processing yielded depth-continuous curves of Stoneley wave reflectivity. The processing was done over different frequency bands, with the low frequency band giving a significantly larger response, consistent with theoretical results for conductive fractures. Peaks in the low frequency curve coincident with fractures identified by UBI and core analysis confirmed locations of conductive fractures. In addition, interpreted locations of conductive fractures are consistent with locations of producing intervals determined by production log analysis.

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