Acoustic anisotropy analysis is used in a wide variety of applications, such as fracture characterisation, wellbore stability, production enhancement, and geosteering. However, the methods by which acoustic anisotropy are determined are not always well understood, both by the end user and the data analyst. Azimuthal variations in velocities may be due to stress variations, intrinsic anisotropy, bed boundaries, or some combination thereof. Environmental effects such as hole inclination, centralization, wellbore condition, dispersion and source/receiver matching affect the viability of the data and must be considered in the interpretation. Untangling the various acoustic anisotropy factors is essential to effectively interpreting the results.This paper begins with a discussion of the types of acoustic anisotropy, followed by a review of common industry methods for extracting anisotropy from wireline and LWD azimuthal sonic data. Environmental factors such as tool centralization, irregular borehole shape, poor tool calibration, and dispersion are considered, paying particular attention to the practical limitations of acquiring data suitable for high quality anisotropy analysis in adverse conditions.Quality control techniques are discussed in some detail, as there are various causes of “false anisotropy” that should be recognized so as not to incorrectly interpret processing artefacts as formation features. Quality control plots are suggested to aid the non-specialist in determining whether the anisotropy results are viable.Intrinsic, induced, and geometric anisotropy are discussed in detail, along with consideration of the depth if sensitivity of acoustic measurements. Finally, a case study is presented to illustrate the art of untangling overlapping acoustic anisotropy responses.
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