Yibing Zheng

Imaging near-borehole structure using acoustic logging data with pre-stack F-K migration

Summary We apply a pre-stack F-K migration technique to the fullwave acoustic logging data to obtain an image of geological structures in the near-borehole region. The data acquisition configuration of acoustic logging, i.e., the span of receivers along the receiver array and a large number of tool positions along the borehole, well suits the requirement for the chosen migration method. The pre-stack F-K migration has no dip angle limitation and therefore can handle large angle dips (with respect to the borehole axis) quite well. It can also efficiently handle a large volume of data in one processing. Because it solves the migration problem in the frequency-wavenumber domain, the F-K migration can distinguish between up-dips and down-dips for a bed boundary intersecting the borehole. The technique has been tested with synthetic and field data sets. A deviated well acoustic logging data set will be used to demonstrate the ability of the technique to obtain clear structural images away from the borehole.

Borehole Acoustic Dipole Response With Drilling-induced Vertical Fractures

Understanding the effect of drilling-induced fractures on borehole dipole-flexural waves is important for interpreting the result of cross-dipole anisotropy measurement. This effect was analyzed using a discrete-wavenumber finite element method. We studied the dispersion characteristics of the acoustic dipole modes in an isotropic formation with various drilling-induced fractures along the borehole. Within the frequency range of dipole acoustic logging, these fractures induce two major dipole modes with different phase velocities and distinctive polarization directions along and perpendicular to the fracture orientation. The result indicates that shallow and deep fractures influence the dipole modes differently. Shallow fractures split the dipole velocities at high frequencies while deep fractures cause the splitting at low frequencies. As a result, deep fractures can induce significant shearwave azimuthal anisotropy measured by cross-dipole logging. Other aspects of fractures were also analyzed. Wider fractures cause larger velocity differences than narrower ones. Multiple fractures have a similar effect compared with a single fracture. The result of this work provides a theoretical foundation for interpreting the crossdipole shear-wave anisotropy measurement in the presence of drilling-induced fractures.

Borehole flexural-wave response to vertical solid-filled fractures

Summary The drilling process, hydraulic fracturing and perforation can create fractures along the borehole, which are often filled with yielded rocks. The yielded rocks have much reduced mechanical rigidity than the original undamaged formation. Understanding the effect of this type of fractures on the acoustic dipole responses not only is important for correctly interpreting the acoustic anisotropy measurement but also will help characterizing the fractures using acoustic logging. We use a finite element method with discrete wavenumbers to model the acoustic modes in a borehole with these fractures. Dispersion properties of the dipole waves are calculated with various fracture parameters. The dipole mode polarized perpendicular to the fracture travels at a lower speed than the dipole mode polarized along the fracture at low frequencies. The velocity difference of the two dipole modes increases with the increasing fracture extension into the surrounding formation. The width of the fracture affects the dipole modes mostly at high frequencies. The effects of the fractures can still be felt in a cased borehole. The result of this study can be used to provide a guideline for understanding and interpreting the cross-dipole measurement in fractured formations.