Seismic response of fractures by numerical simulation

Abstract

SUMMARY Fractures occur on a wide range of scales and are important in the study of hydrocarbon reservoirs. Seismic simulation by finite-difference modelling using an equivalent medium method can devise the elastic parameters of a cell intersected by a fracture as those of a medium with an equivalent seismic response. Numerical experiments confirm that diffractions from the fracture tips are a strong component of the total wavefield. However, a comparison of boxcar, linear, angular and elliptic tapering suggests that there is little dependence on shape because the energy involved in a single diffraction is much lower than the incident energy. An open, fluid filled fracture has stronger effect on the wavefield than the wet and dry, multiple crack models, because an open fracture would have a stronger dissimilarity to the background rock. The density of microcracks within a fracture also has strong effect on the seismic response, however the properties of those cracks are not significant to the overall seismic response. Considering a distribution of a large number of fractures, even when the overall density of fractures is held constant, longer fractures attenuate seismic energy more than smaller ones. For the orientation effect, fractures oriented in the direction of propagation seem to affect the wavefield more than those perpendicular because of the incident wave striking the fracture at an angle greater than the critical angle. Experiments on clustering of the fractures indicate that although clusters which are large compared to the wavelength may attenuate and ‘shield’ more than for a uniform distribution, smaller ones in fact attenuate less, because of the ‘healing’ effect. These are important results when trying to characterize the fracture properties including density, clustering, size and orientation of a fractured reservoir from field seismic data.