Effect of Spherical Pore Shapes on Acoustic Properties in Carbonates*
Gregor T. Baechle1, Layaan Al-Kharusi1, Gregor P. Eberli1, Austin Boyd2,
and Alan Byrnes3
Search and Discovery Article #50047 (2007)
Posted July 30, 2007
*Adapted from oral presentation at AAPG Annual Convention, Long Beach, April 1-4, 2007
1University of Miami, Miami, FL ([email protected])
2Schlumberger, Ridgefield, CT
3Kansas 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.
Not dominant pore type, but minor inter-crystalline pore type in rock frame causes slow velocity at given porosity.