A Modular Wireline Sonic Tool For Measurements Of 3D (Azimuthal, Radial, And Axial Formation Acoustic Properties

Abstract

A comprehensive mechanical characterization of the wellbore rock relies on a three-dimensional (3D) characterization of the acoustic slowness in terms of radial, azimuthal, and axial variations. These acoustic rock property variations arise because of non-uniform stress distributions, mechanical or chemical nearwellbore alteration caused by the drilling process, and formation-intrinsic anisotropy. A 3D formation acoustic properties characterization is achieved through comprehensive acquisition of broadband waveforms from all borehole modes (monopole, dipole, and Stoneley) coupled with an integrated inversion of all acquired data. Compressional slowness radial variations are enabled through monopole acquisition with a wide range of transmitterreceiver spacings, from very short to very long, unique to this new technology. Shear slowness radial variations are quantified through inversions of the broadband dispersions of the dipole flexural and Stoneley modes over a wide frequency band featured by this new sonic tool. The unique design of the dipole source enables it to be fired in either pulse mode or chirp mode. Open hole wireline logs from China, Norway, Mexico, Brazil and the United States demonstrate the improved accuracy of both compressional and shear slowness measurements and their radial variations. With the increased number of axial and azimuthal receivers and an acoustically quiet and predictable structure, shear wave anisotropy is estimated more robustly with data showing reliable fast shear azimuth measurements down to 1–2% slowness anisotropy. Shear slownesses from 90 to 900 µs/ft have been measured with this new tool. Improved dispersion curves for both monopole and dipole modes lead to clear identification of formation homogeneity, inhomogeneity, isotropy and mechanisms of anisotropy. Cased hole formation evaluation data demonstrate that shear slowness through casing as high as 450 µs/ft can be measured reliably.