Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

Carbonate formations are characterized by multiscale heterogeneities that control their acoustic response and flow properties. At the laboratory scale, carbonate rocks do not indicate a strong correlation between P- and S-wave velocities and porosity. The velocity disparities between carbonates of similar mineralogy and porosity result from different microstructures derived from their sedimentary facies and subsequent diagenetic transformations. The still-discussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocks remains another key issue. We have developed an integrated experimental workflow that allows a consistent checking of the applicability of Biot-Gassmann’s equation and provides key geologic and microstructural information to understand the petroacoustic signature of carbonate rocks. The defined approach is based on the phase-velocity measurements performed in liquid-saturated conditions using polar and nonpolar fluids. It allows the identification of the whole set of parameters required by Biot-Gassmann’s equation including the bulk modulus of the solid matrix. This approach is implemented on samples representative of two different carbonate formations deposited in lacustrine and marine environments, respectively. The obtained results demonstrate the applicability of Biot-Gassmann’s equation for the two studied carbonate families and indicate the link between their petroacoustic signature and diagenetic history.

Extended excluded volume: Its origin and consequences

In contrast to the common intuitive/speculative approach based on an analysis of thermodynamic or structural data of (nonpolar) fluids, the statistical mechanical approach is used to extend the excluded volume concept to all other types of fluids. The (extended) excluded volume incorporates, in addition to common nonelectrostatic interactions defining the shape and size of the molecules, also the short-range part of the repulsive interactions between the embedded Coulombic sites. In this study we show that the extended excluded volume concept correctly predicts the behavior of the partial molar volume (PMV) at infinite dilution in different solvents and, particularly, differences between nonpolar and associating solvents. The concept is then applied to estimate the PMV of methanol in water.