Fast and slow flexural waves in a deviated borehole in homogeneous and layered anisotropic formations

Details Fast and slow flexural waves in a deviated borehole in homogeneous and layered anisotropic formations Fast and slow flexural waves in a deviated borehole in homogeneous and layered anisotropic formations

Apr 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010geoji.181..417h&link_type=abstract

Geophysical Journal International, Volume 181, Issue 1, pp. 417-426.

Astronomy and Astrophysics

Astronomy

Numerical Solutions, Guided Waves, Seismic Anisotropy, Wave Propagation

Scientific paper

Dipole acoustic fields in an arbitrarily deviated well penetrating a homogeneous as well as a stratified transversely isotropic formation are simulated using a 3-D finite-difference time-domain algorithm in cylindrical coordinates. The modelling results show that a dipole source can excite a fast- and a slow-flexural mode due to the shear wave anisotropy when the borehole is inclined with respect to the symmetry axis of transverse isotropy. Both flexural slownesses change with the wellbore deviation angle. The splitting of flexural modes is prominent in full wave arrays when the shear anisotropy is strong enough. It is revealed that the dipole orientation influences the relative amplitudes of the fast- and slow-flexural waves but it has no effect on their slownesses or phases. In a vertical well parallel to the symmetry axis, the two flexural waves degenerate and propagate at the same speed. The degenerated flexural wave travels approximately at the shear speed along the borehole wall except in a few formations. Our study shows, for example, that it is about 10 per cent slower than the shear wave in Mesaverde clayshale 5501. Even in that kind of formations, however, extraction of the fast- and slow-shear velocities from the flexural modes is still possible if the borehole deviation is large enough. To examine the effect of layering, we modelled the full waves in a formation with a sandwich. When the well is perpendicular to the layer interfaces, reflection is obvious and can be recognized. It becomes weaker or even invisible as the deviation angle increases, so it is difficult to detect a thin layer embedded in a formation directly from reflected waves. The sandwich can, instead, be recognized from the irregularity in the spectra of the full waveforms displayed versus depth.
[Correction added after online publication 25th February 2009; the original spelling of `homogenous' in the title has been corrected to `homogeneous'.[

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