Abstract

The 1D interlayer-flow (or White's) model is based on Biot's theory of poroelasticity and explains low-frequency seismic wave attenuation in partially saturated rocks by wave-induced fluid flow between two alternating poroelastic layers, each saturated with a different fluid. We have developed approximate equations for both the minimum possible value of the quality factor, Q, and the corresponding fluid saturation for which Q is minimal. The simple approximate equations provide a better insight into the dependence of Q on basic petrophysical parameters and allow for a fast assessment of the minimal value of Q. The approximation is valid for a wide range of realistic petrophysical parameter values for sandstones partially saturated with gas and water, and shows that values of Q can be as small as two. We ap-plied the interlayer-flow model to study the reflection coefficient of a thin (i.e., between 6 and 11 times smaller than the incident wavelength) layer that is partially saturated with gas and water. The reflection coefficient of the layer, caused only by a contrast in attenuation between the layer and the nonattenuating background medium, can be larger than 10% for Q<4 within the layer. The reflection coefficient was calculated with finite difference simulations of wave propagation in heterogeneous, poroelastic solids and in equivalent viscoelastic solids. The reflection coefficient of the layer is also estimated with an analytical solution using a complex velocity for the layer. The numerical and analytical results agree well. Our results indicate that reflection coefficients of gas reservoirs can be significantly increased and frequency dependent in the low-frequency range because of attenuation within the reservoir caused by wave-induced flow.

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