Evidence of a drained to undrained frequency dependent transition from elastic and hydraulic diffusion properties on a Fontainebleau sandstone

Abstract

Direct measurement of dispersion and attenuation on fullysaturated rocks elastic properties are particularly difficult to obtain. Very few apparatus allow for measurement of frequency effects on elastic moduli. A new experimental set-up and protocol were developed to measure and understand dispersion and attenuation effects on fully-saturated rocks’ bulk modulus. Using standard samples, the apparatus was shown to allow for measurements in the frequency and pressure ranges of f ∈ [4.10;4.10]Hz and Pc ∈ [0−50]MPa respectively (Pimienta et al., 2014b). In this study, a new experimental protocol is designed to extend the frequency range of measurement and allow for measuring the possible causes for dispersion effects. Under confining pressure oscillations, the stressstrain method is extended to measurement of both volumetric strain and pore pressure (i.e. Pp) oscillations. While the strain information leads to elastic/bulk properties (i.e. K and Q K ), pore pressure leads to hydraulic diffusion properties in terms of a pseudo-Skempton coefficient (i.e. Bp) and Pp phase shift (i.e. ∆φp). Elastic and hydraulic diffusion properties of a Fontainebleau sandstone (φ ∼ 7% porosity) saturated by different fluids (i.e. air, water and glycerine) are then measured for varying frequencies f . Both elastic and hydraulic properties show large dependence to both fluids and frequencies in the frequency range of the apparatus. At Pe f f ∼ 1MPa, a large frequency-dependent increase in K is observed under glycerine saturation that directly correlates to a large Q K peak. This effect directly correlates to large variations of both Bp and ∆φp, clearly indicating a dispersion/attenuation phenomenon related to the transition from drained to undrained regime.