To assess saturation effects on acoustic properties in carbonates, we measure ultrasonic velocity on 38 limestone samples whose porosity ranges from 5% to 30% under dry and water-saturated conditions. Complete saturation of the pore space with water causes an increase and decrease in compressional- and shear-wave velocity as well as significant changes in the shear moduli. Compressional velocities of most water-saturated samples are up to 500 m/s higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity (VS) generally decreases, but nine samples show an increase of up to 230 m/s. Water saturation decreases the shear modulus by up to 2 GPa in some samples and increases it by up to 3 GPa in others. The average increase in the shear modulus with water saturation is 1.23 GPa; the average decrease is 0.75 GPa. The VP/VS ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher VP/VS ratios. Grainstone samples with high amounts of microporosity and interparticle macro-pores preferentially show shear weakening, whereas recrystallized limestones are prone to increase shear strengths with water saturation. The observed shear weakening indicates that a rock-fluid interaction occurs with water saturation, which violates one of the assumptions in Gassmann's theory. We find a positive correlation between changes in shear modulus and the inability of Gassmann's theory to predict velocities of water-saturated samples at high frequencies. Velocities of water-saturated samples predicted by Gassmann's equation often exceed measured values by as much as 400 m/s for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as 600 m/s lower than measured values. In 66% of samples, Gassmann-predicted velocities show a misfit to measured water-saturated P-wave velocities. This discrepancy between measured and Gassmann-predicted velocity is not caused solely by velocity dispersion but also by rock-fluid interaction related to the pore structure of carbonates. Thus, a pore analysis should be conducted to assess shear-moduli changes and the resultant uncertainty for amplitude variation with offset analyses and velocity prediction using Gassmann's theory.
ASJC Scopus subject areas
- Geochemistry and Petrology