Single-well S-wave imaging has several attractive features because of its directional sensitivity and usefulness for fracture characterization. To provide a method for single-well acoustic imaging, we analyzed the effects of wave radiation, reflection, and borehole acoustic response on S-wave reflection measurements from a multicomponent dipole acoustic tool. A study of S-wave radiation from a dipole source and the wave’s reflection from a formation boundary shows that the S-waves generated by a dipole source in a borehole have a wide radiation pattern that allows imaging of reflectors at various dip angles crossing the borehole. More importantly, the azimuthal variation of the S-waves, in connection with the multicomponent nature of a cross-dipole tool, can determine the strike of the reflector. We used our theoretical foundation for borehole S-wave imaging to formulate an inversion procedure for field data processing. Application to field data validates the theoretical results and demonstrates the advantages of S-wave imaging. Application to near-borehole fracture imaging clearly demonstrates S-wave azimuthal sensitivity to fracture orientation.

Imaging near-borehole structures using acoustic-logging data depends on the data-processing method used. We demonstrate that extracting the up- and downgoing reflections from the data and using those separately for the imaging can significantly improve the image quality. A parametric wave separation is first applied to array data to separate direct and reflection waves. In particular, we use a transmitter array gathered from successive source positions to extract the upgoing reflection. Reflection data can also be enhanced by stacking data along the reflection moveout in array using approximate structural dip information. Implementation of this method to acoustic-logging data processing improves imaging quality, making it possible to image near-borehole geologic structures using conventional array acoustic-logging. A potentially important application of drilling steering is using logging-while-drilling acoustic measurements.

We have developed a novel constrained inversion method for estimating a radial shear-wave velocity profile away from the wellbore using dipole acoustic logging data and have analyzed the effect of the radial velocity changes on dipole-flexural-wave dispersion characteristics. The inversion of the dispersion data to estimate the radial changes is inherently a nonunique problem because changing the degree of variation or the radial size of the variation zone can produce similar wave-dispersion characteristics. Nonuniqueness can be solved by developing a constrained inversion method. This is done by constraining the high-frequency portion of the model dispersion curve with another curve calculated using the near-borehole velocity. The constraint condition is based on the physical principle that a high-frequency dipole wave has a shallow penetration depth and is therefore sensitive to the near-borehole shear-wave velocity. We have validated the result of the constrained inversion with synthetic data testing. Combining the new inversion method with four-component crossed-dipole anisotropy processing obtains shear radial profiles in fast and slow shear polarization directions. In a sandstone formation, the fast and slow shear-wave profiles show substantial differences caused by the near-borehole stress field, demonstrating the ability of the technique to obtain radial and azimuthal geomechanical property changes near the wellbore.

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