P- and PS-Wave Vector Wavefields for Anisotropic Petrophysics

Abstract

Predicting petrophysical properties of lithology, density and fractures using seismic data is an essential part of reservoir evaluation. In addition to lithology characterization, “frackability” has become a very important area of investigation for high-grading locations for drilling and hydraulic fracture stimulation. Most seismic studies that estimate P- and S-wave impedance, density and brittleness or formation strength use conventional P-wave data and isotropic elastic inversion methods. However, converted-wave (PS-wave) joint inversion and S-wave splitting methods have successfully been used to improve determination of seismic properties for shale plays as well as other unconventional resource plays.

Anisotropic behaviour related to layered media (VTI), fracture properties, stress direction and the geomechanics of shales are increasingly more important for seismic analysis, imaging and reservoir characterization. Vector wavefields are sensitive to these properties and can help identify optimal drilling and stimulation locations. Also, it has been shown that use of conventional elastic parameters for characterizing “brittleness” should include anisotropic corrections to obtain a more accurate response. Including PS-wave seismic data is beneficial for isotropic elastic inversion and should improve anisotropy estimates for identification of potential fracture locations.

Elastic inversion of azimuthally anisotropic amplitude variations (AVAz) is also becoming more important. When layered media are fractured, orthorhombic symmetry of P-wave amplitude depends on S-wave birefringence. PS-waves are ideal for determining this S-wave splitting information from layerstripping and their reflectivity provides additional equations for joint inversion with P-waves. Two coefficients, a radial RPSV and transverse RPSH reinforce anisotropic signatures similar to P-wave reflectivity RP. Vector wavefields contain all the necessary information for S-wave anisotropy from short wavelength AVAz as well as from long wavelength moveout behaviour.