Monitoring of Fluid Substitution in Rocks Using Passive Ultrasonic Interferometry

An alternative laboratory technique for measuring the elastic constants of solid samples, based on the analysis of the cross-correlation spectra of the vibratory response of randomly excited short solid cylinders, has been recently proposed. The aim of this paper is to demonstrate the ability of the technique, called passive ultrasonic interferometry (UI), to monitor fluid substitution (here water and ethylene glycol) in different rock samples. Velocity variation due to fluid substitution is easily measured if the wave attenuation in fluid-saturated rock is not too large. The experimental results are in agreement with the predictions of Biot-Gassmann poroelastic theory. The effect of substituting water with a stiffer saturating fluid, such as ethylene glycol, is to increase the overall bulk modulus of the rock, without any substantial effect on shear modulus. Furthermore the experimental results compare well with those independently obtained on the same rock samples with conventional pulse transmission (PT) technique using ultrasonic transducers. However, in detail, the bulk moduli measured with PT are systematically slightly larger than the corresponding moduli measured with UI. The difference is more marked with the more viscous saturating fluid, here ethylene glycol, than with water. This can be explained by dispersion due to viscosity effect.