Frequency-dependent attenuation as a potential indicator of oil saturation
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.Journal of Applied Geophysics 82, pp. 119-128, 2012.