The importance of natural fractures for development and production of hydrocarbon reservoirs requires little justification. While in clastic reservoirs fractures can cause permeability anisotropy and thus affect field development, in carbonates and tight sands they are often critical for reservoir production. If open fractures have a preferential direction (which is almost always the case), they cause azimuthal seismic anisotropy, making seismic a powerful tool for the characterization of fractured reservoirs.

In inhomogeneous porous media, the mechanism of wave-induced fluid flow causes significant attenuation and dispersion of seismic waves. In connection with this phenomenon, we study the impact of spatial permeability fluctuations on the dynamic behavior of porous materials. This heterogeneous permeability distribution further complicates the ongoing efforts to extract flow permeability from seismic data. Based on the method of statistical smoothing applied to Biot's equations of poroelasticity, we derive models for the dynamic-equivalent permeability in 1D and 3D randomly inhomogeneous media. The low-frequency limit of this permeability corresponds to the flow permeability governing fluid flow in porous media. We incorporate the dynamic-equivalent permeability model into the expressions for attenuation and dispersion of P-waves, also obtained by the method of smoothing. The resulting attenuation and dispersion model is confirmed by numerical computations in randomly layered poroelastic structures. The results suggest that the effect of wave-induced fluid flow can be observed in a broader frequency range than previously thought. The peak attenuation shifts along the frequency axis depending on the strength of the permeability fluctuations. We conclude that estimation of flow permeability from seismic attenuation is only possible if permeability fluctuations are properly accounted for.

To explore the validity and limitations of the theoretical model of wave propagation in porous rocks with periodic distribution of planar fractures, we perform numerical simulation using a poroelastic reflectivity algorithm. The numerical results are found to be in good agreement with the analytical model. not only for periodic fractures, but also for random distribution of constant thickness fractures.

P162 Z-99 Frequency Dependent Anisotropy of Fractured Summary 1 Porous Rocks G. LAMBERT B. GUREVICH AND M. BRAJANOVSKI Curtin University of Technology Department of Exploration Geophysics GPO Box U1987 Perth Western Australia 6845. Email: Gracjan.Lambert@geophy.curtin.edu.au We propose a method for computing the anisotropic stiffness tensor c as a function of frequency for a porous fluid-saturated rock permeated by a system of aligned planar fractures. This method utilises the previously obtained expression for the stiffness corresponding to the unidirectional deformation perpendicular to the fracture plane (denoted ) along with the known high- and low-frequency limits for the stiffnesses and c

Presence of open fractures in a fluid‐saturated porous rock causes significant attenuation and dispersion of seismic waves due to wave induced fluid flow between pores and fractures. Recently Brajanovski et al. (2003) developed a model for attenuation and dispersion of compressional waves in a porous rock with a periodic system of aligned fractures using the theory of wave propagation in layered porous media. In order to verify the theoretical model of Brajanovski et al. (2003) numerical experiments were performed using OASES, a general purpose computer code for modeling seismo‐acoustic propagation in stratified media extended to layers described by the Biot theory of poroelasticity. The numerical simulations were found to be in very good agreement with theoretical results for the periodic system of fractures. More importantly, there was good agreement even for random fracture spacing, with divergence between theoretical and numerical results only appearing when fracture thickness was also random.

Fractures in a porous rock can be modelled as very thin and highly porous layers in a porous background. Elastic moduli of such a fractured medium can be obtained using the result of Norris (1993) for wave propagation in periodically layered poroelastic media. When this porous fractured system is dry, it is equivalent to a transversely isotropic dry elastic porous material with linear-slip interfaces. When saturated with a liquid this system exhibits significant attenuation and velocity dispersion due to wave-induced fluid flow between pores and fractures. The characteristic frequency of such attenuation and dispersion depends on the background permeability, fluid viscosity, as well as fracture density and spacing. The theoretical results are in good agreement with numerical simulations using the reflectivity algorithm generalized to poroelasticity. For randomly distributed microfractures the frequency dependent anisotropy can be modelled using a combination of low frequency predictions based on anisotropic Gassmann equations and a frequency correction based on the dispersion relationship of Hudson et al. (1996). Comparison with laboratory experiments confirms that this combined model gives an accurate prediction of saturated elastic properties and angular dependencies of elastic wave velocities versus frequency.