Despite continued improvements in acoustic-logging technology, sonic logs
processed with industry-standard methods often remain affected by formation
damage and mud-filtrate invasion. Quantitative understanding of the process of
mud-filtrate invasion is necessary to identity and assess biases in the
standard estimates of in-situ compressional- and shear-wave (P- and S-wave)
velocities. We describe a systematic approach to quantify the effects of
mud-filtrate invasion on borehole acoustic logs and introduce a new algorithm
to estimate radial distributions of elastic properties away from the borehole
wall. Radial saturation distributions of mud filtrate and connate formation
fluids are obtained by simulating the process of mud-filtrate invasion.
Subsequently, we calculate radial distributions of the elastic properties using
the Biot-Gassmann fluid-substitution model. The calculated radial distributions
of formation elastic properties are used to simulate array sonic waveforms.
Finally, estimated P- and S-wave velocities for homogeneous,
stepwise, and multilayered formation models are compared to quantify
mud-filtrate-invasion effects on sonic measurements.
We use a nonlinear Gauss-Newton inversion algorithm to estimate radial
distributions of formation elastic parameters in the presence of invaded zones
using normalized spectral ratios of array waveform data. Inversion examples
using synthetic and field data indicate that physically consistent
distributions of formation elastic properties can be reconstructed from array
waveform data. In turn, radial distributions of formation elastic properties
can be used to construct more-realistic near-wellbore petrophysical models for
applications in reservoir simulation and production.
During and after drilling, the near-wellbore formation is often altered by
stress buildup and release, mud-filtrate invasion, chemical reactions, and many
other factors. These alterations cause the physical properties in the
near-wellbore region to be different from those of the virgin rock formation.
Stress concentration around a wellbore may cause near-wellbore damage and
induce formation anisotropy on P- and S-wave velocities. The
stress-induced anisotropy can be identified by dispersion analysis (Plona et
al. 2002). Positive radial velocity gradients focus the elastic waves
propagating away from the wellbore back toward the borehole wall. Such a
phenomenon can be identified easily from high-amplitude acoustic arrivals. In
this paper, we focus our attention on mud-filtrate-invasion effects only.
It is well known that formation properties inferred from wireline logging
measurements may not reflect true properties of virgin formations. A realistic
description of the invaded zone is important for the processing and
interpretation of sonic logs. A common model used in the open literature
assumes that a sharp interface exists between the altered zone and the
undisturbed formation (Baker 1984). The term “stepwise” is used to describe
this type of mud-filtrate-invasion model. Linear-gradient models have been
described for syntheses of acoustic waveforms as well (Stephen et al. 1985).
Actual radial distributions of elastic wave properties resulting from invasion
can be complex and are dependent on the specific petrophysical properties of
the rock as well as on the static and dynamic properties of the fluids
involved. We divide the invaded zone into a set of concentric radial layers to
represent an actual invasion profile and call it a “multilayered” model.
Subsequently, we describe a procedure for calculating the radial distributions
of formation elastic properties with the Biot-Gassmann fluid-substitution model
starting from numerically simulated radial distributions of flow
© 2006. Society of Petroleum Engineers
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- Original manuscript received:
4 June 2004
- Revised manuscript received:
29 May 2006
- Manuscript approved:
25 July 2006
- Version of record:
20 October 2006