Rock physics and seismic modeling ofshale reservoirs with horizontal fractures

摘要

The presence of horizontal fractures enhances seismic anisotropy of shales. Calculation based on the effective medium theory indicates that horizontal fractures have little effects on velocities along the direction parallel to fractures,but can significantly reduce velocities along the direction normal to fractures. Seismic responses of shales with horizontal fractures are calculated based on the reflector model and the anisotropic propagator matrix method,in which the reflections are a combination of the contrast in impedance due to the variations in fracture density,anisotropic propagation of waves within the shales,and the tuning and interferences associated with layer thickness. Calculated results indicate that seismic reflections are sensitive to reservoir layer thickness and fracture density. Anisotropic propagation alters amplitudes and phases of reflections. It corresponds to higher reflection amplitudes for the case of surrounding sandstone with higher velocity because the increase in fracture density increases the contrast in impedance between the shale and sandstone. In contrast,the surrounding sandstone with lower velocity corresponds to lower reflection amplitudes for the increase in fracture density.