Seismic attenuation and dispersion in fractured porous media: The effect of heterogeneities in the background

Fluid pressure diffusion (FPD) induced by seismic waves is an important mechanism for energy dissipation in fractured media. Although most research has focused on homogeneous backgrounds embedding the fractures, in practice, the rock's background exhibits heterogeneities and their effect in terms of seismic dispersion and attenuation due to FPD is yet to be assessed. We perform numerical simulations based on quasi-static Biot's equations of poroelasticity to explore these effects. In the numerical models, a highly compressible, porous, and permeable fracture is embedded in a heterogeneous background, and the shape and distribution of these heterogeneities range from idealized to realistic. For fractured porous media with idealized heterogeneous backgrounds, the heterogeneities are considered in either hydraulic, mechanical, or hydro-mechanical properties to draft a basic understanding of the effects of these different sets of properties. The hydraulic heterogeneities in the background cause FPD between fracture and background to occur at different diffusion rates, resulting in two corresponding attenuation peaks at low frequencies. Meanwhile, mechanical heterogeneities in the background can allow for FPD inside the fracture, resulting in an additional attenuation peak at higher frequencies, compared to the attenuation peaks associated to FPD between fracture and background. For the fractured porous media with more realistic heterogeneous backgrounds, which are based on stochastic distributions of the rock physical properties, the effect of mechanical heterogeneities on attenuation and dispersion can be very small if the contrasts in these properties are smooth, but they become larger if the contrasts are strong. Our results give an understanding about the magnitude and frequency dependence of attenuation/dispersion effects that can be expected in association with heterogeneities in the background embedding a fracture. • The effect of heterogeneities in the background on attenuation and dispersion caused by fluid pressure diffusion was studied considering smooth and sharp spatial changes in properties. • For a fractured porous medium with heterogeneous background, the whole fluid pressure diffusion process is largely controlled by hydro-mechanical heterogeneities. • Background heterogeneities in compressibility cause fluid pressure diffusion not only between fracture and background but also inside the fractures, the latter with an attenuation peak at much higher frequencies.