Unipole Sonic Logging While Drilling With Eccentric Tools and Anisotropy Evaluations

Logging while drilling (LWD) is a rapidly developing in-situ measurement technique due to its advantages of saving time and cost, less mud invasion during measurements, and the application in deviated or horizontal wells. Unlike the measurements in a vertical well, the LWD tools are inevitably off-centered in deviated or horizontal boreholes due to gravity. In this study, we investigate the wave propagation of unipole acoustic LWD with eccentric tools. The unipole transducer in a fluid-filled borehole can be characterized as a single point source deviated from the symmetric axis. The signals of unipole transmitters with arbitrary azimuths can approximately be equivalent to a linear combination of monopole, dipole, and quadrupole sources. According to analyses of the wave separation, it is concluded that the dipole components are dominant in the full wave arrays of unipole tools. The formulation of responses at the opposite azimuth with respect to a unipole transmitter reveals that the unipole data by this gathering mode would have less sensitivity to the tool positions. Numerical simulations by 3D finite difference further confirm this. For conventional multipole LWD including dipole and quadrupole logging, it is quite hard to evaluate the true shear velocity of the formation once the tool is highly off-centered in the borehole. For the unipole data gathered at the opposite azimuth to the source, however, the signal properties are very stable, with little dependence on either the tool eccentricity or the source orientation in horizontal wells. Both the fast and the slow shear velocities of an anisotropic formation can therefore be measured with good accuracy according to inversion tests in this study. Numerical examples further show it is feasible to evaluate the formation shear anisotropy by the unipole data during the tool rotating.