Finite Difference Modeling of Seismic Responses to Intersecting Fracture Sets

Abstract

Fractured reservoir characterization is becoming increasingly important for the petroleum industry. Currentmethods for this task are developed based on effectivemedia theory, which assumes the cracks or fractures in a reservoir are much smaller than the seismic wavelength. A discrete fracturemodel has to be used for large-scale fractures. We describe an approach of using a finite difference method for modeling seismic wave propagation in rock formations with intersecting fracture sets. We then use the code to study the behavior of seismic waves, particularly scattering due to such fracture sets with various spacing and compliances. The scattering pattern due to fractures varies azimuthally. We find that converted PS and PSP waves from the bottom of the fractured layers show strong interference by the scattered waves. We observe coherent scattered waves in shot gathers parallel to the fracture orientation and significant backscattering at near offsets and forward scattering at far offsets in the gathers normal to the fracture orientation. When two sets of fractures are present, scattering becomes stronger and more complex scattered waves appear in the gathers. The scattering becomes stronger with increasing the fracture compliances and decreasing spacing (still on the order of seismic wave length). When the fracture sets are not orthogonal to each other, the gathers still show coherent scattering in the fracture orientations. Azimuthal characteristics of the scattered waves may be used to analyze fracture orientations, spacing, and relative compliance of intersecting fracture sets.