Seismic attenuation and velocity dispersion in fractured rocks: The role played by fracture contact areas

摘要

The presence of fractures in fluid-saturated porous rocks is usually associated with strong seismic P-wave attenuation and velocity dispersion. This energy dissipation can be caused by oscillatory wave-induced fluid pressure diffusion between the fractures and the host rock, an intrinsic attenuation mechanism generally referred to as wave-induced fluid flow. Geological observations suggest that fracture surfaces are highly irregular at the millimetre and sub-millimetre scale, which finds its expression in geometrical and mechanical complexities of the contact area between the fracture faces. It is well known that contact areas strongly affect the overall mechanical fracture properties. However, existing models for seismic attenuation and velocity dispersion in fractured rocks neglect this complexity. In this work, we explore the effects of fracture contact areas on seismic P-wave attenuation and velocity dispersion using oscillatory relaxation simulations based on quasi-static poroelastic equations. We verify that the geometrical and mechanical details of fracture contact areas have a strong impact on seismic signatures. In addition, our numerical approach allows us to quantify the vertical solid displacement jump across fractures, the key quantity in the linear slip theory. We find that the displacement jump is strongly affected by the geometrical details of the fracture contact area and, due to the oscillatory fluid pressure diffusion process, is complex-valued and frequency-dependent. By using laboratory measurements of stress-induced changes in the fracture contact area, we relate seismic attenuation and dispersion to the effective stress. The corresponding results do indeed indicate that seismic attenuation and phase velocity may constitute useful attributes to constrain the effective stress. Alternatively, knowledge of the effective stress may help to identify the regions in which wave induced fluid flow is expected to be the dominant attenuation mechanism.