Using Walkaround Vertical Seismic Profiling for Azimuthal Anisotropy Analysis of a Complex Fracture System
Al Dulaijan, Khaled *1; Planchart, Carlos 1; Al-Khater, Salman 1; Al-Dajani, Abdulfattah 1
(1) Saudi Aramco, Dhahran, Saudi Arabia.
Open fractures can be excellent conduits for fluid flow, therefore knowing the orientation of the fractures is important for optimal field development of fractured reservoirs. This knowledge is especially true for unconventional reservoirs (e.g., tight sand gas, and shale gas). Using Walkaround Vertical Seismic Profiling (VSP) technology presents a unique opportunity to identify azimuthal anisotropy for use in mapping potential fractures and their orientation around the borehole.
Saudi Aramco recently completed the acquisition, processing and analysis of a walkaround VSP survey in an unconventional tight gas sand to help identify azimuthal anisotropy in the reservoir and its orientation.
In this paper, we present the results of the azimuthal anisotropy analysis using traveltimes, polarization and shear-wave splitting analysis. We also compare the azimuthal anisotropy results with the inferred fracture orientations derived from the dipole sonic and image logs.
The fracture system in the study area was found to be more complicated than systems of earlier studies in the area. The image logs suggest two sets of open fractures dominated by a N-NE trending set, and two sets of cemented fractures dominated by a E-SE trending set. Our analysis shows that the azimuthal anisotropy seen in the traveltime data (P-wave and converted PS-wave) is consistent with the major orientation of the cemented fractures obtained from the image logs (i.e., E-SE). On the other hand, the azimuthal anisotropy seen in the amplitude data (P-wave polarization, P-wave Q, and S-wave splitting) is consistent with the major orientation of the open fractures obtained from the image logs (N-NE). This conclusion also confirms the maximum horizontal stress orientation obtained from geomechanical analysis.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain