Baechle, Gregor T.¹, Ralf J. Weger¹, Gregor P. Eberli¹
(1) University of Miami / RSMAS, Miami, FL

ABSTRACT: Gassmann Revisited: Variations in Dynamic Shear Modulus of Carbonate Rocks

Understanding the effects of saturation on acoustic properties of carbonates is paramount for using the amplitude versus offset (AVO) technique and 4-D seismic surveys in carbonates. Laboratory experiments on Cretaceous and Miocene limestones display variations in the shear modulus due to saturation. This variation of the shear modulus questions the basic assumption of constant shear modulus with saturation in Gassmann's theory. Ultra-sonic velocities of 30 Cretaceous and Miocene limestone samples with porosities from 5% to over 30% were measured under dry and saturated conditions at variable confining pressures. Two groups are observed based on the calculated shear moduli: group one shows a decrease in shear modulus of the rock by up to 2 GPa, group two shows an increase by up to 3 GPa. Furthermore, velocities predicted with the Gassmann equation for the group with shear weakening overestimate the measured values by as much as 400 m/s; in the group with shear strengthening the values are underestimated by as much 600 m/s. In addition, the Vp/Vs ratio shows an overall increase with saturation. In particular, rocks displaying shear weakening have distinct higher Vp/Vs ratios. Samples with shear weakening and high Vp/Vs ratio consist mostly of rocks with interparticle/intercrystalline porosity. In contrast, carbonate rocks with dominant microporosity and moldic pore types tend to show less effects of fluid saturation on velocity. These data suggest that a) shear moduli do not remain constant during saturation, b) the rock-fluid interaction causes errors in Gassmann-predicted velocities, and c) pore types influence elastic moduli.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.