Hydraulic fracture induced changes in borehole modal dispersions

Three dimensional finite-difference codes with a perfectly matched layer (PML) at the computational boundary enable us to study the influence of planar fractures on refracted headwaves as well as dispersive borehole modes. This paper describes borehole dispersions in the presence of different fracture geometries intersecting a fluid-filled borehole. Sonic signatures consist of compressional, fast-shear, and slow-shear slownesses obtained from the processing of an array of waveforms by the slowness-time coherence (STC) algorithm together with the axisymmetric Stoneley and dipole flexural dispersions obtained from the processing of monopole and dipole waveforms recorded at an array of hydrophone receivers placed in a fluid-filled borehole. Characteristic differences in the dispersion signatures and evidence of dipole shear-slowness anisotropy are indicators of the orientation and radial extent of fractures introduced by the hydraulic fracturing treatment. Long axial fractures of width 5 mm exhibits slower dispersisons in the intermediate frequency band with the flexural polarization parallel to the fracture plane and slower dispersions at low frequencies with the flexural polarization perpendicular to the fracture plane. Cross-dipole shear-slowness differences are signatures associated with axial fractures. In contrast, fractures parallel to the borehole cross-sectional plane produce a characteristic drop in the Stoneley slowness dispersion and an increase in the dipole flexural dispersion at low frequencies. Reflections from cross-sectional fractures are observed on both the Stoneley and flexural waveforms. Interestingly, higher order Stoneley and flexural modes are not observed in the presence of such reflections. In conclusion, we have identified clear differences between the Stoneley and dipole flexural dispersions before and after the fracturing operation. These characteristic differences can help in identifying the presence of open fractures that contribute to - he flow of hydrocarbons into the producing well.