Hydromechanical properties of Fontainebleau sandstone: Experimental determination and micromechanical modeling

摘要

We measured ultrasonic velocity, hydraulic permeability, and specific storage capacity of eight blocks of Fontainebleau sandstone, a well-sorted, medium-grained, almost pure quartz-sandstone of Oligocene age, covering a range in connected porosity from 3% to 10% and varying significantly in pore geometry for a given porosity. Ultrasonic P-wave velocity measured on water-saturated samples covers the full range predicted by variational Hashin-Shtrikman bounds. Permeability and specific storage were determined by the linear pressurization method at different effective pressures up to 180 MPa and room temperature. The permeability of tested samples varies from 10?13 m2 down to 10?20 m2, depending primarily on porosity and pore geometry, and subordinately on effective pressure. Specific storage capacity always exceeds the contribution of pore fluid compression. The excess corresponds to the contribution of pore deformability controlled by pore geometry and matrix material properties. Since the sandstone samples are composed of a single solid component, quartz, we were able to calculate various poroelastic parameters (Biot-Willis and Skempton coefficients, drained and undrained bulk moduli) crucial for estimating hydromechanical coupling in fluid-saturated aggregates. The micromechanical modeling of the effective pressure sensitivity of the hydraulic transport and poroelastic parameters by the contiguity model of Takei (1998) yields good agreement between static and dynamic elastic parameters for saturated samples, partial agreement between the pressure dependence of effective elastic parameters and Hertzian contact mechanics, and an explanation for pressure-insensitive permeability of samples with the highest porosity.