Identifying principle shear directions in anisotropic formations with acoustic logging-while-drilling

摘要

An LWD tool 102 is provided on a drill string in a borehole 104 to determine the principle axes of the formation 100. The LWD tool is provided with an acoustic transmitter capable of exciting dipole waves and optionally monopole and quadrupole waves. An array of acoustic sensors provided on the LWD tool are capable of receiving wave data in at least four orthogonal directions. The transmitter may be fired in dipole mode in two orthogonal directions. An Alford rotation is applied to mathematically transform received composite signals to determine the anisotropy of the formation including the orientation of fast and slow shear slowness. When the transmitter excites dipole waves the sensors receive in-line and cross-line wave data resulting from dipole propagation through both the formation and the LWD tool. When the transmitter excites monopole waves the receivers receive monopole wave data and monopole-induced quadrupole data resulting from monopole propagation through both the formation and the LWD tool. A negative correlation between monopole and induced quadrupole received data can indicate the fast shear direction whilst a positive correlation can indicate the slow shear direction.