| dc.description.abstract | We present a general formulation based on the Thomson-Haskell method which allows
the study of complex models. The surrounding formation may be composed of several
coaxial porous shells or of a combination of both porous and elastic layers. At any kind of interface, welded contact is assumed. The two phase media are modeled following Biot's
theory which is modified through a unified definition of frequency dependent coupling
coefficients in accordance with homogenization theory. Calculations of dispersion and
attenuation of both the Stoneley wave and first pseudo-Rayleigh mode are performed.
Also displayed are synthetic microseismograms computed using the discrete wavenumber
method.
The configuration of a damaged (invaded or flushed) zone resulting from radial
variation of permeability and/or porosity, with or without modification of the saturant
fluid is investigated. When the borehole wall is impermeable, the effects of a porous
radial layering are analogous to those observed in the presence of only elastic media,
with body wave attenuations added, whatever the formation (fast or slow). When the
borehole is permeable, as soon as the inner layer thickness reaches a few centimeters,
Stoneley wave dispersion and attenuation are representative of the rheological properties
of that inner layer. As a result, the determination of the in situ permeability of the
virgin formation, based on Stoneley wave characteristics, is ill posed.
Also studied is the presence of a mudcake at the borehole wall. Whatever its rheological properties, this additional layer leads to a decrease of Stoneley wave phase velocity
compared to the sealed pores situation. Compared to the simple open hole model with
a permeable borehole wall, the same effect can be noted but only for frequencies greater
than≃ 2 kHz, and a significantly thick mudcake. With any elastic or low permeability
mudcake, with the absence or inefficient pressure continuity between the bore and saturant fluids, one cannot generate the variations of attenuations displayed by real data. Further investigations will need experimental measurements of the mudcake properties.
Whatever the configuration, the phase velocity and attenuation of the first pseudo-Rayleigh mode starts at those of the virgin formation. The low frequency part of the
pseudo-Rayleigh wavetrain can then be used for the indirect estimation of the formation
shear wave characteristics. | en_US |
