Using a numerical approach, we explore wave-induced fluid flow effects
in partially saturated porous rocks in which the gas-water saturation
patterns are governed by mesoscopic heterogeneities associated with
the dry frame properties. The link between the dry frame properties
and the gas saturation is defined by the assumption of capillary
pressure equilibrium, which in the presence of heterogeneity implies
that neighbouring regions can exhibit different levels of saturation.
To determine the equivalent attenuation and phase velocity of the
synthetic rock samples considered in this study, we apply a numerical
upscaling procedure, which permits to take into account mesoscopic
heterogeneities associated with the dry frame properties as well
as spatially continuous variations of the pore fluid properties.
The multiscale nature of the fluid saturation is taken into account
by locally computing the physical properties of an effective fluid,
which are then used for the larger-scale simulations. We consider
two sets of numerical experiments to analyse such effects in heterogeneous
partially saturated porous media, where the saturation field is determined
by variations in porosity and clay content, respectively. In both
cases we also evaluate the seismic responses of corresponding binary,
patchy-type saturation patterns. Our results indicate that significant
attenuation and modest velocity dispersion effects take place in
this kind of media for both binary patchy-type and spatially continuous
gas saturation patterns and in particular in the presence of relatively
small amounts of gas. The numerical experiments also show that the
nature of the gas distribution patterns is a critical parameter controlling
the seismic responses of these environments, since attenuation and
velocity dispersion effects are much more significant and occur over
a broader saturation range for binary patchy-type gas-water distributions.
This analysis therefore suggests that the physical mechanisms governing
partial saturation should be accounted for when analysing seismic
data in a poroelastic framework. In this context, heterogeneities
associated with the dry frame properties, which do not play important
roles in wave-induced fluid flow processes per se, should be taken
into account since they may determine the kind of gas distribution
pattern taking place in the porous rock.