Effect of
Spherical Pore Shapes on Acoustic Properties in Carbonates
Baechle, Gregor
T.1, Layaan Al-Kharusi1, Gregor P. Eberli1, Austin Boyd2, Alan
Byrnes3 (1) University of Miami, Miami, FL (2) Schlumberger,
Ridgefield, CT (3) Kansas Geological Survey, Lawrence, KS
Twenty-eight mono-mineral oomoldic carbonate samples with near-spherical pores show a
large scatter in a velocities – porosity plot that is
caused by inter-crystalline porosity in the re-crystallized rock frame. This
finding questions the assumption that spherical pores have a dominant effect on
the p-wave velocity. Vp and
Vs is simultaneously measured at a frequency of 1MHz and under increasing
effective stress from 3 MPa to 30 MPa.
We observe large variations in velocities between 3200 m/s and 6500 m/s and a
large scatter in the p-wave velocity – porosity relationship. The p-wave
velocity shows up to 2500m/s difference at a given porosity. The velocity
increases between 250 and 750m/s with pressures from 3 to 30MPa. The bulk of
the samples show increasing Vp/Vs ratios with
pressurization, up to values between 1.7 and 1.84. The ratio of normalized bulk
versus shear modulus is ranging from 0.7 to 0.9. Several samples have been
chosen for fluid substitution and saturated “in-situ” with 7 different pore
fluids. Significant effect of fluid changes on velocity is observed. A linear
correlation exists between bulk modulus and fluid modulus (r2 > 0.97). In
contrast, shear modulus changes correlated with the viscosity of the fluids:
the lower the fluid viscosity, the lower the shear modulus. Our results
question common hypothesizes for modeling pore structure effects on acoustic
properties in carbonates; (a) P-wave velocity is controlled by the amount of
spherical pores, and (b) the velocity in oomoldic rocks
is insensitive to fluid and pressure changes because of high aspect ratio
pores.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California