Waveform Signatures in Micro-fractured Media - The Role of Effective Orthotropy

Fracture characterization from full-waveform elastic seismic data is a key technology for detecting sweet spots and monitoring production in reservoirs such as carbonates, shale oil, tight and shale gas. We analyze the effects of small-scale fractures on elastic waveform signatures using i) the Kachanov effective medium theory to describe effective stiffness tensors and ii) an efficient Lebedev-grid finite-difference modeling scheme for monoclinic and orthorhombic symmetries. To address the waveform sensitivity to crack infill together with the accuracy of the effective orthotropy approximation, we analyze elastic waveform signatures with multiple, obliquely-oriented crack sets with varying crack densities. The waveforms show that the effective orthotropy approximation based on the second-rank crack compliance tensor is not sufficiently accurate to describe elastic waveforms. This approximation can be acceptable for dry cracks and small crack densities, but waveform errors associated with neglecting the fourth-rank crack compliance tensors increase rapidly as crack density and crack infill stiffness increase. Also, waveforms are substantially different for dry versus fluid-filled cracks. Our findings suggest that waveforms may provide sufficient sensitivity to invert for components of both second- and fourth-rank crack compliance tensors. This could aid in seismic-based discrimination of multiple fractures and fracture infill parameters.