Inversion of 4-C borehole flexural waves to determine anisotropy in a fractured carbonate reservoir

Abstract

Observations of anisotropy in fractured carbonate reservoirs can yield important information such as the orientation of aligned crack systems and crack density. A new generation of borehole acoustic tools provides the data necessary to observe anisotropy around the borehole. The flexural waves, excited by two dipole sources operating perpendicularly to each other in these tools, are recorded in two perpendicular directions, resulting in four‐component flexural wave recordings, Superposition of sources and rotation of receivers are the basic elements for the inversion of the waveforms to determine fast or slow mode orientations. Since the two sources are not necessarily of the same strength, a reciprocity‐based source equalization is carried out as part of the inversion. We have inverted four‐component borehole flexural waves, recorded in a vertical well, penetrating three lithologic units of a fractured carbonate reservoir in northern Oman. For the lower unit, a northeast‐southwest azimuth of flexural wave anisotropy is well resolved, which is parallel to the strike of open fractures observed in the borehole, preferential flow directions, and to the regional present‐day in‐situ maximum horizontal stress. Furthermore, the northeast‐southwest direction corresponds with polarization directions of the fast shear wave observed in a multicomponent surface seismic survey over the reservoir. The middle unit is most likely isotropic, except for a depth interval of low gamma readings, where a northeast‐southwest orientation is found for the fast mode. For the upper unit, a north‐south fast mode orientation is found, which does not correspond to the expected northeast‐southwest direction. Borehole geometry data indicate that the shape of the hole is circular for the middle unit but elongated for the upper and lower unit. Caliper orientations track the flexural wave anisotropy azimuths in these two units. Because of the elongation of the borehole, the observed flexural mode orientations, as well as the magnitude of flexural wave anisotropy, cannot be related to formation anisotropy directly, which obscures the determination of fracture orientation and fracture density from four‐component borehole flexural waves.