Dynamic Permeability of Thin Layered Media - Theory vs Numerical Simulations

An increasing interest in gas hydrates as a potential energy source gave reason for numerous field studies, laboratory and numerical experiments, that have revealed some interesting aspects of sediments containing gas hydrates. While there exist several models explaining observed increased seismic velocities, the mechanism of formation of gas hydrates and the reasons for observed strong attenuation are not fully understood. Two rock physical models are controversly debated: one attributes occurrence of hydrates to the properties of the rock's matrix, the other relates presence of hydrates to the properties of the pore fluid.<br>In our approach we assume, that an occurrence of hydrates affects the properties of the fluid, frame and grain of the host sediment. A poroelastic generalization of the O'Doherty-Anstey theory indicates that this would result in increased values for attenuation. To work with realistic models of multilayered, poroelastic media and to account for observed strong fluctuations in hydrate-bearing sedimentary layers we investigate exponentially correlated, random media. Numerical and analytical results confirm, that correlated fluctuations in properties of the frame, grain and fluid cause significant attenuation values. Especially in the lower seismic frequency range they are comparable to those observed in field measurements.<br>