Abstract
Understanding wave propagation in granular sediments is important for
subsurface characterization. The presence of fluid and wettability
condition result in additional complexities. While it is known that wave
propagation in dry granular porous media is dominated by the presence of
force chains, their influence in (partially) saturated granular porous
media with different wettability conditions remains largely unexplored.
To make progress in this direction, we design laboratory experiments by
combining core flooding and ultrasonic measurement in glassbead packings
that are chemically treated to alternate the wettability. The P- and
S-wave velocity-saturation relation and attenuation-saturation relation
are obtained from the waveforms for both water- and gas-wetting samples.
The results show that there is a transition from an attenuating but
stable P-wave pulse at low and moderate saturation to a set of
incoherently scattered waves at high saturation. The incoherent
scattering in the gas-wetting case is negligibly small, whereas it is
more pronounced in the water-wetting case. We conclude that only if
water wets the grains, can the liquid enter the grain contacts. These
liquid bridges are thought to locally reinforce the force chains and to
increase their characteristic length scale. This leads to an increase in
P-wave velocity and promotes incoherent scattering since the ratio of
dominant wavelength to characteristic length scale decreases. In the gas
wetting case, however, the presence of gas prevents the water from
direct contact with the glass beads and therefore stops the formation
and growth of the liquid bridges within the force chain network.