Nibrara Surface Seismic Data - Resolution Enhancement and Reservoir Characterization Through Wavefield Re-Datuming and Inversion
Dr. Jyoti Behura
Knowledge and utilization of in-situ stress and fracture distributions in the subsurface determine the operational success in most unconventional plays. Besides stress and fracture distributions, seismic attributes derived from amplitude-variation-with-offset (AVO), velocity, and attenuation analyses play key roles in the characterization and development of these plays.
In this case study of the Niobrara, we use only the surface seismic data to decipher the stress distribution from azimuthal velocity analysis, fracture orientation and rock properties from azimuthal AVO inversion, and fault properties through attenuation inversion. Because the robustness and accuracy of the above attributes are extremely sensitive to the overburden effects, we utilize a novel re-datuming technique to eliminate all effects of overburden. The re-datumed common-offset common-azimuth (COCA) gathers clearly show azimuthal moveout (related to in-situ stress distribution). The re-datuming also generates high-resolution gathers and enhances the signal-to-noise ratio which should help in geo-steering and in possibly distinguishing Niobrara and Codell formations.
The azimuthal velocity analysis yields the direction of Vfast that corresponds to the direction of maximum stress. The study shows that this direction for the Niobrara-Codell formations in the Vogl area is predominantly W-E and WSW-ENE. The attenuation attribute shows that the regional fault oriented N-S within the Vogl area is of significantly lower attenuation compared to the surroundings. The most likely explanation for such low attenuation is that the fault has undergone calcite mineralization and is therefore sealing. Such an inference has major implications for not only field development but also for environmental impact of hydraulic fracturing because this regional fault would not conduct hydraulic-fracturing fluids to the surface. The AVO attributes show a distinct change across the regional fault which again points to the possibility of the fault being sealing. Such a contrast might arise from differences in pore-pressure or in lithology. Moreover, the AVAZ attribute is similar to the Vfast direction which indicates that the dominant fracture direction coincides with the in-situ maximum horizontal stress direction.