Laboratory insights on acoustics of granular porous media with wettability changes

Elastic wave propagation in partially saturated porous rocks has been extensively addressed; however, the presence of fluids distributed over the pore surface at different degrees of wettability introduces additional complexities, which remain largely unexplored. We establish an ultrasonic experimental basis for observing changes between hydrophilic and weakly hydrophilic conditions in partially saturated granular media. For this purpose, glass beads are silane-treated to alternate the wettability condition. We validate our glass-bead experimental system by measuring P-wave velocities under uniaxial pressure in dry condition and comparison with published data in almost-identical experimental conditions. For partially saturated bead packings, particular attention is paid to ensure uniform distribution of water throughout the pore space across different saturation levels. We measure the P- and S-wave velocities and attenuations with increasing saturation before and after chemical treatment, respectively. The results show that the silane treatment improves the water–bead coupling, leading to higher velocity and lower attenuation of coherent waves. We use a combination of the theories of Gassmann, Wood and -Walton to predict the velocity-saturation relation of coherent waves under different wettability conditions. Notably, once reaching a critical saturation, incoherent high-frequency waves are observed propagating with the speed predicted by Gassmann’s theory. The silane treatment reduces the amplitude of these incoherent waves to some degree, probably due to the improvement of water–bead coupling. We interpret the observed coherent and incoherent waves in terms of the wettability-dependent ability for water to bridge neighboring beads. These liquid bridges create distinct force chain patterns thereby altering the wave transmissivity. Even though the glass beads make up an idealized granular medium, our experimental findings lend support to a wave-based characterization of wettability changes in granular media.