Modeling Sonic Velocity in Carbonates Using Thin Sections*
Gregor Baechle1, Arnout Colpaert2, Gregor P. Eberli1, and Ralf J. Weger1
Search and Discovery Article #40313 (2008)
Posted July 25, 2008
from oral presentation at AAPG Annual Convention,
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.
· 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 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 using different aspect ratios of microporosity fraction.
(3) Use fraction of macroporosity and microporosity to model both, shear and bulk moduli (and velocity) from thin sections.
· 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 model incorporates micro- and macroporosity and enables Vs and Vp predictions.