Investigation of borehole cross-dipole flexural dispersion crossover through numerical modeling

Crossover of the dispersion of flexural waves recorded in borehole cross-dipole measurements is interpreted as an indicator of stress-induced anisotropy around a circular borehole in formations that are isotropic in the absence of stresses. We have investigated different factors that influence flexural wave dispersion. Through numerical modeling, we determined that for a circular borehole surrounded by an isotropic formation that is subjected to an anisotropic stress field, the dipole flexural dispersion crossover is detectable only when the formation is very compliant. This might happen only in the shallow subsurface or in zones having high pore pressure. However, we found that dipole dispersion crossover can also result from the combined effect of formation intrinsic anisotropy and borehole elongation. We found that a small elongation on the wellbore and very weak intrinsic anisotropy can result in a resolvable crossover in flexural dispersion that might be erroneously interpreted as borehole stress-induced anisotropy. A thorough and correct interpretation of flexural dispersion crossover thus has to take into account the effects of stress-induced and intrinsic anisotropy and borehole cross-sectional geometry.