Effects of single vertical fluid-filled fractures on full waveform dipole sonic logs

We conducted a parametric study of the effects of single, finite-width, adjacent, vertical, fluid-filled fractures on full-wave borehole dipole logs using the transformed boundary integral equation method. We found that a fracture has significant effects on dipole response if it is within half the wavelength, or two to three times the borehole diameter, from the borehole center. Dual flexural waves and a leaky fracture mode resulted from the waveguide composed of the borehole and fracture. The first flexural wave was controlled by both the borehole and fracture, whereas the second flexural wave was controlled primarily by the borehole but influenced by the fracture. The separation of these two flexural waves increased and the strength of the first one decreased when the distance between the fracture and the borehole increased. The leaky fracture mode was represented by a sharp minimum in the amplitude spectrum, and energy leakage to the fracture reached a maximum when the fracture intersected the borehole. These two unique characteristics distinguish a single fracture from anisotropy. We also estimated location, orientation, and aperture of the fracture through waveform and spectral analyses.