Azimuthal anisotropy analysis of P-wave seismic data and estimation of the orientation of the in-situ stress fields---An example from the Rock-Springs uplift, Wyoming

Estimating the orientations and magnitudes of the maximum and minimum horizontal in-situ stress fields is important for characterizing naturally fractured, unconventional, and carbon-sequestrated reservoirs. For naturally fractured reservoirs they are needed to guide directional drilling, for unconventional reservoirs they are used for optimal placements of hydraulic fractures, and for carbon sequestrated reservoirs they are used to avoid fracturing of the overlying seal rocks during and after carbon dioxide (CO2) sequestration. The orientations and magnitudes of the in-situ stress fields can be calculated at the well locations and to obtain them over a three-dimensional (3D) area, the well data are usually interpolated over the interpreted horizons of stacked 3D seismic data volumes. Such an approach may be adequate in mature exploration areas with sufficiently good well control, but could be misleading when well control is sparse. Because the in-situ stress fields induce anisotropy in observed seismic data, here we combine prestack waveform inversion with azimuthal analysis to develop a robust workflow for interpreting the azimuthal anisotropy of P-wave seismic data. Applying this workflow to real seismic data from the Rock springs uplift, Wyoming and relating the results with the well data, we find that the principal anisotropy direction extracted from seismic data corresponds exactly with the orientation of the maximum in-situ horizontal stress field. Therefore, the azimuthally sorted P-wave seismic data once calibrated with the well data could potentially be used for predicting the in-situ stress fields over a 3D area of interest using this method. The methodology outlined here is purely qualitative in nature, and can only provide the orientations of the in-situ stress fields but not their magnitudes. However, we also outline the steps for developing an anisotropic prestack waveform inversion that can potentially obtain their magnitudes in addition to their orientations.