Frequency-dependent seismic anistropy and its implication for estimating fracture size in low porosity reservoirs

Frequency-dependent anisotropy has been observed and can be explained by two mechanisms, i.e. seismic scattering by heterogeneities, such as open aligned fractures, and fluid flow in fractured porous rock. If the proper mechanism is understood, it may provide a mean of going beyond the concept of the conventional static equivalent medium theories to potentially estimate size of meso-scale fractures. In this paper, we present some numerical results and synthetic seismograms using the far-field Green’s function in fractured porous media. The model that we use takes account of fluid interactions at two scales: meso-scale fractures inserted into a background porous rock with micro-cracks. The fracture lengths can be much larger than grain-scale pores or micro-cracks, but are less than the seismic wavelength, as a result velocity dispersion occurs at the low or seismic frequency, and variation of shear-wave anisotropy with frequency is expected. Our results show that time-delays between split shear-waves vary with fracture sizes: as fracture length increases, shear-wave anisotropy decreases. Our study has indicated the great potential of using seismic anisotropic measurements to estimate fracture sizes, which are critical for fluid flow in fractured rock.