A new modular sonic tool provides complete acoustic formation characterization

Abstract

An improved estimation of sonic slownesses and a comprehensive mechanical characterization of the wellbore rock rely on a complete characterization of the compressional and shear slowness in terms of their radial, azimuthal, and axial variations. The new modular sonic tool accomplishes this by incorporating improved monopole and cross‐dipole transmitter technology while featuring an extensive receiver array incorporating 13 axial levels of 8 azimuthal sensors each. Each receiver is individually digitized resulting in 104 waveforms per transmitter firing leading to an extremely reliable and accurate slowness estimation. This comes about through improved borehole mode extraction/rejection and enhanced wavenumber resolution at all frequencies. Formations exhibit wide, and sometimes complex, acoustical behaviors ranging from isotropic, anisotropic with its various mechanisms and significant radial slowness gradients. Radial rock property variations arise because of non‐uniform stress distributions and mechanical or chemical near‐wellbore alteration due to the drilling process. Anisotropy can be caused by intrinsic shale properties or external differential stresses. The critical data required to invert for these rock parameters underlying these acoustic behaviors are derived from the new tool through the use of broadband dispersion curves associated with propagating borehole acoustic modes. In this paper, we highlight tool features that have an important impact on seismic, borehole seismic, and sonic applications. The acquired high quality waveforms and advanced processing techniques lead to improved compressional and shear slowness estimates, radial profiling of shear and compressional slowness, enhanced anisotropy detection and mechanism identification, and reliable through casing slowness measurements. Examples are shown from several wells in Norway and Mexico.