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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 velocity of carbonates.
·
Macroporosity causes data scatter in velocity-porosity space.
·
Dual porosity DEM
Workflow for Dual DEM
(a)
Fraction of macroporosity and microporosity.
(b)
Average aspect ratio of macroporosity.
(2) Determine average aspect ratio of microporosity by best fit
multiple
(3) Use fraction of macroporosity and microporosity to
·
Laboratory data shows that compliant micropores have a strong
softening effect on the sonic velocity. · Digital image analysis of thin sections provides pore structure descriptions (fraction of micro- and macroporosity).
·
Dual porosity DEM
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