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Modeling Sonic
Velocity
in Carbonates
Using
Thin Sections*
By
Gregor Baechle1, Arnout Colpaert2, Gregor P. Eberli1, and Ralf J. Weger1
Search and Discovery Article #40313 (2008)
Posted July 25, 2008
*Adapted
from oral presentation at AAPG Annual Convention,
1Comparative Sedimentology
Laboratory,
2Statoil
The differential effective medium theory (DEM)
is used to model high frequency (1MHz) laboratory velocity
measurements of
carbonates under dry and water-saturated conditions.
Velocity
-porosity data
from laboratory experiments show that micropores have a strong softening
effect on the sonic
velocity
of carbonates. Quantitative image
analysis
of
250 thin sections enables us to quantify the concentration of micropores and
macropores, which forms the base of our rock physics modeling study. We
model the effect of the varying stiffness of those two pore populations on
velocity
: (a) compliant micropores and (b) stiff macropores.
To verify the model results, we compare the elastic moduli derived from ultrasonic velocities and density information with elastic
moduli obtained by DEM modeling of the same samples. This DEM model that uses
measured input parameters from quantitative digital image analysis
of the pore
structure results in an excellent prediction of acoustic properties of
carbonates. The
velocity
predictions also show significant improvement compared
to
velocity
prediction
using
other empirical equations; e.g., the Wyllie times
average equation. In addition, we show how a low rock stiffness identifies
carbonates of low permeability, indicating the potential of improved reservoir
characterization from acoustic data.
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·
Laboratory data shows that compliant micropores have a strong
softening effect on the sonic
·
Macroporosity causes data scatter in · Dual porosity DEM model that incorporates micro- and macroporosity predicts very well elastic properties.
(a)
Fraction of macroporosity and microporosity.
(b)
Average aspect ratio of macroporosity.
(2) Determine average aspect ratio of microporosity by best fit
multiple model runs
(3) Use fraction of macroporosity and microporosity to model both,
shear and bulk moduli (and
·
Laboratory data shows that compliant micropores have a strong
softening effect on the sonic
·
Digital image · Dual porosity DEM model incorporates micro- and macroporosity and enables Vs and Vp predictions.
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