At seismic frequencies, wave-induced fluid flow is a major cause of
P-wave attenuation in partially saturated porous rocks. Attenuation
is of great importance for the oil industry in the interpretation
of seismic field data. Here, the effects on P-wave attenuation resulting
from changes in oil saturation are studied for media with coexisting
water, oil, and gas. For that, creep experiments are numerically
simulated by solving Biot's equations for consolidation of poroelastic
media with the finite-element method. The experiments yield time-dependent
stress?strain relations that are used to calculate the complex P-wave
modulus from which frequency-dependent P-wave attenuation is determined.
The models are layered media with periodically alternating triplets
of layers. Models consisting of triplets of layers having randomly
varying layer thicknesses are also considered. The layers in each
triplet are fully saturated with water, oil, and gas. The layer saturated
with water has lower porosity and permeability than the layers saturated
with oil and gas. These models represent hydrocarbon reservoirs in
which water is the wetting fluid preferentially saturating regions
of lower porosity. The results from the numerical experiments showed
that increasing oil saturation, connected to a decrease in gas saturation,
resulted in a significant increase of attenuation at low frequencies
(lower than 2 Hz). Furthermore, replacing the oil with water resulted
in a distinguishable behavior of the frequency-dependent attenuation.
These results imply that, according to the physical mechanism of
wave-induced fluid flow, frequency-dependent attenuation in media
saturated with water, oil, and gas is a potential indicator of oil
saturation.