Shear wave logging in semi-infinite saturated porous formations

Abstract

Recent theoretical and experimental studies have demonstrated the interest of using nonaxisymmetric sources, such as dipole and quadrupole, to record shear wave events in any kind of formation (fast or slow). Nonaxisymmetric sources excite surface waves whose Airy phase is predominant in the records at intermediate frequencies (≂3 to 6 kHz). They are referred to as the flexural and screw modes, respectively, for the dipole and the quadrupole sources. Their dispersion and attenuation are studied in a fluid-filled borehole embedded in a homogeneous saturated porous formation. The two-phase medium is modeled using Biot’s theory modified in accordance with the homogenization theory. Calculation of synthetic full waveform logs is also performed using the discrete wavenumber method. Whatever the formation, the most reliable information that can be extracted from low-frequency parts of the wavetrains is the formation shear wave velocity and attenuation. It is of great interest in the inversion of Stoneley wave velocity and attenuation for the evaluation of in situ permeability. In the presence of fast formations, the attenuation of the Airy phase of both the flexural and screw modes is sensitive to the in situ permeability. The same conclusion holds for the slow formations only when they are saturated by a fluid of low viscosity and high compressibility.