Effects of Permeability Barriers and Pore Fluids on S-wave Attenuation

Abstract

We numerically perform stress relaxation experiments using Biot’s equations for consolidation of poroelastic media to study seismic attenuation of S-waves caused by wave-induced fluid flow. Our model consists of periodically distributed mesoscopic-scale circular heterogeneities with lower porosity and permeability than the background, which contains 80% of the total pore space of the medium. This model represents a hydrocarbon reservoir, where the background is fully saturated with oil or gas (or water, for comparison), and the low porosity heterogeneities are always saturated with water. Varying the dry bulk and shear moduli in the medium, a tendency is observed in the relative behavior of the S-wave attenuation among the different saturation scenarios. First, in the gas-saturated media the S-wave attenuation is very low and much lower than in the oil-saturated or in the fully water-saturated media. Second, at low frequencies the S-wave attenuation is significantly higher in the oil-saturated media than in the fully water-saturated media. Additionally, we observed that impermeable barriers in the background can cause a significant increase in S-wave attenuation. Based on the theory of wave-induced fluid flow, our results suggest that S-wave attenuation could be used as an indicator of fluid content and permeability changes in a reservoir.