Sonic logging in deviated boreholes penetrating an anisotropic formation: Laboratory study

Abstract

Development of deepwater fields requires drilling deviated or horizontal wells. Many formations are highly anisotropic, that is, the P- and S-wave velocities vary with propagation direction. Sonic logs acquired in these wells need to be corrected for anisotropy effects before the logs can be used in formation evaluation and seismic applications. In this study, we use a laboratory model made of an orthorhombic Phenolite block to study acoustic logging in deviated wells. We first measure the qP-, qSV-, and SH-wave group velocities by using body waves at angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90° relative to the slowest P-wave principal axis of the Phenolite block. We then drill holes at the same angles in the block. We record monopole and dipole sonic waveforms in these holes and extract the qP-, qSV-, SH-, and Stoneley-wave velocities by using the slowness-time semblance method. The velocities measured through the use of monopole logging and dipole logging vary with borehole deviations. We find that an equivalent transversely isotropic (TI) model can fit the measured qP-, qSV-, and Stoneley-wave velocities very well. The S-wave velocities at low to medium borehole deviations can be used to differentiate an orthorhombic material from a TI one.