Changes in dynamic shear moduli of carbonate rocks with fluid substitution

Abstract

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 [Formula: see text] higher than the velocities of the dry samples. Some show no change, and a few even show a decrease in velocity. Shear-wave velocity [Formula: see text] generally decreases, but nine samples show an increase of up to [Formula: see text]. Water saturation decreases the shear modulus by up to [Formula: see text] in some samples and increases it by up to [Formula: see text] in others. The average increase in the shear modulus with water saturation is [Formula: see text]; the average decrease is [Formula: see text]. The [Formula: see text] ratio shows an overall increase with water saturation. In particular, rocks displaying shear weakening have distinctly higher [Formula: see text] 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 [Formula: see text] for samples exhibiting shear weakening. In samples showing shear strengthening, Gassmann-predicted velocity values are as much as [Formula: see text] 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.

Rock physics template from laboratory data for carbonates

“…They also mentioned that the pore shape is not easy to quantify but is one important rock property for the elastic properties. Other researchers like for example Khazanehdari and Sothcott (2003), Baechle et al (2009), Adam et al (2006 or Pimienta et al (2014) add this. They all summarized that the change of dynamic shear modulus with saturation, especially for carbonates is a result of rock-fluid interaction and the underlying rock microstructure.…”

Laboratory measurements of the effect of fluid saturation on elastic properties of carbonates at seismic frequencies

“…; Wang ; Marion and Jizba ; Assefa, McCann, and Sothcott ; Baechle et al . , ; Røgen et al . ; Sharma et al .…”

Application of Gassmann’s equation for laboratory data from carbonates from Austria

“…Eberli et al (2003) note "different pore types cluster in the porosity-velocity field, indicating that scattering at equal porosity is caused by the specific pore type and their resultant elastic property". Baechle et al (2005Baechle et al ( , 2009) present an interpretation of their derived data concerning various pore types. They also observed the shear weakening effect and compared the measured velocities to ones calculated with Gassmann's equation.…”