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

Abstract

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.

Can we ever trust the shear-wave log?

“…Figure 4 shows a numerical simulation of the dispersion behavior of the fast and slow shear waves under a uniaxial stress of 10 MPa in Berea Sandstone (Fang et al, 2015). The crossover of the dispersion curves as a function of frequency is well understood and is the result of stress rotation from far to near field.…”

Dispersion of flexural waves in a borehole with a tensile fracture in an anisotropic stress environment

“…Fang et al . (2015) studied the effect of stress‐induced anisotropy on the flexural dispersion. Wang and Fehler (2018a, b) investigated the influence of cement and casing pipe on the semblance and the dispersion curves.…”

Editorial: Recent Advances in Borehole Geophysics in Unconventional Resources: Applications, Techniques and Challenges

“…Innovations in LWD NMR enable quantification of porosity, irreducible and clay bound water in real-time. The elastic parameters of the formation, the fractures outside of the borehole, and even the brittleness during the hydraulic fracturing would be determined by the acoustic logging data, especially from the acoustic logging-while drilling and the borehole remote reflection or imaging [2][3][4][5][6]. The images from borehole acoustic reflection imaging and electronic imaging provide input for geo-mechanical models that are used to plan or optimize the wellbore stabilization and stimulation treatments.…”