Full Wave Synthetic Acoustic Logs In Saturated Porous Media Part II: Simple Configuration
Name
1986.6 Schmitt.pdf
Size
2.55 MB
Format
Adobe PDF
Checksum (MD5)
399f527b787d1df549ac4d7225c385bd
Author(s)
Schmitt, D. P.
Date Issued
1986
Publisher
Massachusetts Institute of Technology. Earth Resources Laboratory
Series/Report no.
Earth Resources Laboratory Industry Consortia Annual Report;1986-06
Abstract
The discrete wavenumber method is used to compute synthetic full waveform acoustic
logs in an axisymmetric fluid-filled borehole surrounded by a porous medium. The two
phase formation is modeled following Biot's theory. It is modified by the introduction
of an unified definition of the coupling coefficients related to the pore shape in accordance
with homogenization theory. Both permeable and impermeable borehole walls are
considered. The study focuses on the permeabilty, source center frequency and radius
effects on guided modes and especially the Stoneley wave for various types of formations
and saturant fluids. Comparison is made with homogeneous elastic formations.
Plots of the spectral energy density in the frequency axial wavenumber domain allow
the recognition and identification of the different wave types.
When the pores are occluded at the borehole wall, the wavefield behavior is nearly
equivalent to that of an elastic formation (with body wave attenuation added). The
Stoneley wave is only very slightly affected.
When the fluid flow is free, the distortions are more important because of the relative
motion between the two phases at the borehole wall. With increasing permeability,
Stoneley wave absolute and relative energy decreases and is relegated to a narrower low
frequency band. It is then associated with a phase velocity lower than the" tube" wave
velocity in an elastic formation. Its dispersion increases, especially in the low frequency
range. Its attenuation correspondingly increases, reinforced by an increase of the shear
wave attenuation. Due to an apparent increase of the borehole radius, the low cut-off
frequencies of the pseudo-Rayleigh modes are shifted toward lower frequencies. The P
to guided wave amplitude ratio is enhanced. For a given permeability, porosity and pore
geometry, these characteristics are enhanced when the saturant fluid mobility increases
and/or when the borehole radius decreases.
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