Unknown Properties of Scattered Waves from Fractured Reservoir Beds Unraveled by Numerical Modeling

Grid-characteristic method was used to unravel following properties of compressional and converted diffractions from mesorfractured bed (zone).

Relatively high strength of scattered waves from fluid-saturated mesofractures is due to: formation of response owing to vertical deviation of mesofractures; much overlap of interfering diffractions when scattered wavefront gets formed; no phase change on converted diffraction from each single mesofracture. Scattered compressional waves show important specificity in amplitude behavior away from source (AVO-curve). For reservoir beds with zero or insignificant abrupt impedance change, these waves have zero reflection coefficient when they are normal-incident onto fluid-saturated fractured bed; this coefficient rises sharply away from the source location. The situation is similar for positive abrupt impedance change. For reflections from boundaries and nonfractured beds, AVO-curve is substantially different. It is at maximum for normal-incident waves.

Scattered converted wave from fractured features exceeds all waves in strength in near field on X-component data and exhibits following specificities: appreciable asymmetry of wavefields (different wave structure) on either side of receiver spread; phase change at source location (on X-component data); shift of zero on AVO-curve even if fracture tilt is minor; response with one- or two-cycle signal from bed top only due to high absorption of shear waves propagating from bed base.