A Case Study of Seismic Anisotropy Analysis on Multi-Azimuth OBC Seismic Data in an Offshore Carbonate Oil Field, U.A.E.

Abstract

Fractures can be observed in micro or macro scale at outcrops and many oil/gas reservoirs can be fractured as well. A good understanding of the fracture network is hence critical for reservoir development since it could have significant impact on fluid flow. Full azimuth 3D seismic data provides a means of detecting and characterizing fractures before drilling. Seismic anisotropy investigation for fracture detection has been carried out in a carbonate oil field located in offshore United Arab Emirates (U.A.E). Target reservoirs are lower cretaceous carbonates mainly consisting of limestone with alternating porous and dense intervals which become dolomitic with depth. A large 3D-2C OBC seismic data sets was acquired over this giant oil field with orthogonal patch geometry covering all azimuths in 2001. The seismic data was reprocessed in 2006 to improve mainly pre-stack data quality and maintain azimuthal integrity. Velocity and amplitude analysis by ellipse fitting for the azimuthal anisotropy intensity and the direction have been conducted on seismic super CMP gathers, PreStack Migrated (PrSTM) partial stacked data and azimuthally picked velocities. A multi-line walkaway and walk-around VSP survey from a single point of well control was also used to analyze and to verify 3D seismic results. The VSP amplitude and AVOAz results around the well show harmonic amplitude variation with offset and azimuth. However, amplitude analysis on partial stack data shows relatively high seismic anisotropy intensity especially in noisy data areas and the direction is not consistent with the other results. The azimuth direction of partial stacked amplitude is close to the receiver line orientation and thus appears susceptible to acquisition geometry and processing applications. On the other hand, all velocity analysis results show a NE-SW high velocity anisotropy trend. This is consistent with the regional ZAGROS stress field which also has a NE-SW as SHmax, consistent with FMI and core interpreted open fracture data. Six core and FMI data sets were used in this study to verify the results. The velocity analysis result is consistent with the core & FMI data at 5 of the 6 well locations in terms of the azimuthal direction. These results of this study highlight the importance of reviewing multi-sources of information when available, evaluating pros and cons of the datasets with specific attention paid to data volume, noise sensitivity and resolution.

Fracture Characterization by Integrating Seismic-Derived Attributes including Anisotropy and Diffraction Imaging with Borehole Fracture Data in an Offshore Carbonate Field, United Arab Emirates

“…This may explain the stacked anisotropy anomalies that are observed in Figure 6. Overburden effects have been noted by Liu et al (2011); Ikawa and Mercado (2010); and Luo et al (2007). Note that in a recent publication, Liu et al (2015) have performed 3D synthetic study of estimating the fracture density of small-scale vertical fractures when large-scale vertical fractures are present.…”

“…We can either use azimuth velocity-based attributes (low resolution) or azimuth amplitudebased attributes (high resolution) to infer relative anisotropy magnitude (a proxy for fracture intensity) and anisotropy orientation (a proxy for fracture orientation). In our previous study (Liu et al, 2011), azimuthal amplitude variation was used, and we found that the overburden effect was identified as one of the major influencing factors affecting the reliability of fracture interpretation in the targets (Ikawa and Mercado, 2010;Liu et al, 2011). The seismic anisotropy magnitude was computed over intervals for the major reservoirs defined by the seismic interpretation surfaces.…”