Pore Fluid Effects on Seismic P- and S-wave Attenuation in Rocks with Double Porosity and Patchy Saturation

Biot’s equations of consolidation are numerically solved to yield the stress-strain relations, used to calculate the complex moduli of 2D poroelastic media with mesoscopic-scale heterogeneities. With these moduli, attenuation caused by the mechanism of wave-induced fluid flow is determined. In our models, rocks are represented by media containing circular heterogeneities of much lower porosity and permeability than the background. The background contains 80% of the total porous space in the medium and is fully saturated with oil, gas, or water, while the heterogeneities are always fully saturated with water. We observe that, at low seismic frequencies (1-20 Hz), the P-wave attenuation can be very high in the medium saturated with 80% of oil or gas. The S-wave attenuation in the medium with 80% of oil is much higher than in the one with 80% of gas. At very low frequencies (1 Hz), the S-wave attenuation in the medium with 80% of oil is also much higher than in the one with 100% of water. This occurs because the maximum value of the S-wave attenuation is shifted to lower frequencies with increasing fluid viscosity. Numerical and laboratory experiments can be used to establish patterns for these relationships.