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GEOREF RECORD

Simulation of borehole sonic waveforms in dipping, anisotropic, and invaded formations

Robert K. Mallan, Carlos Torres-Verdin and Jun Ma
Simulation of borehole sonic waveforms in dipping, anisotropic, and invaded formations
Geophysics (July 2011) 76 (4): E127-E139

Abstract

A numerical simulation study has been made of borehole sonic measurements that examined shoulder-bed, anisotropy, and mud-filtrate invasion effects on frequency-dispersion curves of flexural and Stoneley waves. Numerical simulations were considered for a range of models for fast and slow formations. Computations are performed with a Cartesian 3D finite-difference time-domain code. Simulations show that presence of transverse isotropy (TI) alters the dispersion of flexural and Stoneley waves. In slow formations, the flexural wave becomes less dispersive when the shear modulus (c44) governing wave propagation parallel to the TI symmetry axis is lower than the shear modulus (c66) governing wave propagation normal to the TI symmetry axis; conversely, the flexural wave becomes more dispersive when c44>c66. Dispersion decreases by as much as 30% at higher frequencies for the considered case where c44<c66. Dispersion of Stoneley waves, on the other hand, increases in TI formations when c44>c66 and decreases when c44<c66. Dispersion increases by more than a factor of 2.5 at higher frequencies for the considered case where c44<c66. Simulations also indicate that the impact of invasion on flexural and Stoneley dispersions can be altered by the presence of TI. For the case of a slow formation and TI, where c44 decreases from the isotropic value, separation between dispersion curves for cases with and without the presence of a fast invasion zone increases by as much as 33% for the flexural wave and by as much as a factor of 1.4 for the Stoneley wave. Lastly, presence of a shoulder bed intersecting the sonic tool at high dip angles can alter flexural dispersion significantly at low frequencies. For the considered case of a shoulder bed dipping at 80 degrees , ambiguity in the flexural cutoff frequency might lead to shear-wave velocity errors of 8%-10%.


ISSN: 0016-8033
EISSN: 1942-2156
Coden: GPYSA7
Serial Title: Geophysics
Serial Volume: 76
Serial Issue: 4
Title: Simulation of borehole sonic waveforms in dipping, anisotropic, and invaded formations
Affiliation: Chevron Energy Technology, Houston, TX, United States
Pages: E127-E139
Published: 201107
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 23
Accession Number: 2012-015338
Categories: Applied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Texas at Austin, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 201209
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