Shear Wave Logging In Semi-Infinite Saturated Porous Formations

Abstract

Recent theoretical and experimental studies have demonstrated the interest of using non-axisymmetric sources, such as dipole and quadrupole, to record shear wave events in any kind of formation (fast or slow). Non-axisymmetric 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 mode, respectively, for the dipole and the quadrupole source. We study their dispersion and attenuation in a fluidfilled borehole embedded in a homogeneous saturated porous formation. The two phase medium is modeled using Biot's theory modified in accordance with homogenization theory. Calculation of synthetic full waveform logs is also performed using the discrete wavenumber method. Whatever the formation, the most reliable information which 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 determination of in situ permeability. In the presence of fast formations the flexural and screw mode are 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.