Curtin/CSIRO Geophysics Seminar, 19th May 2022

Applicability of Gassmann theory to fully and partially saturated nano-porous media

Date: Thursday, 19^th of May, 2022
Time: 11:00 AM – 12:00 PM
Location: CSIRO/ARRC Auditorium, 26 Dick Perry Avenue, Kensington
Presenter: Boris Gurevich, John Curtin Distinguished Professor, Director of Centre for Exploration Geophysics, WASM: Minerals, Energy and Chemical Engineering, Curtin University

Recent progress in extraction of unconventional hydrocarbon resources has ignited the interest in the studies of nanoporous media. Since many thermodynamic and mechanical properties of nanoscale solids and fluids differ from the analogous bulk materials, it is not obvious whether wave propagation in nanoporous media can be described using the same framework as in macroporous media. Application of poroelastic models to the data measured on fluid-saturated rocks is challenging, because the poroelastic effects in such complex media are often obscured by other phenomena, such as squirt flow. Nanoporous Vycor glass, which has a narrow pore size distribution, provides an excellent medium for testing those models.

We test the validity of Gassmann’s equation using two published sets of ultrasonic measurements for a model nanoporous medium, Vycor glass, saturated with two different fluids, argon, and n-hexane. Predictions of the Gassmann theory depend on the bulk and shear moduli of the dry samples, which are known from ultrasonic measurements and the bulk moduli of the solid and fluid constituents. The solid bulk modulus can be estimated from adsorption-induced deformation or from elastic effective medium theory. Substitution of these parameters into the Gassmann equation provides predictions consistent with measured data.

To test applicability of poroelastic patchy saturation models to nano-porous materials, we consider ultrasonic measurements during adsorption and desorption of n-Hexane vapor on nanoporous Vycor glass (Page et al., 1995). As vapor pressure is increased from zero to the saturation pressure, the vapor is adsorbed on the pore walls, resulting in gradual increase of the liquid fraction. The reverse process occurs when pressure is decreased, but the ‘drying’ of the nanopoorus glass is heterogeneous, resulting in a very different velocity-saturation relationship. These results suggest that ultrasonic measurements are a promising method for studying fluid distributions in nano-porous geomaterials such as shale or coal.