Abstract

A horizontal well is most productive in tight reservoirs when it intersects a large number of vertical fractures, yet strata near the borehole remain mechanically stable. Azimuthal velocity analysis and P-wave amplitude versus offset (AVO) using 3D wide-azimuth prestack surface seismic data provide a remote yet detailed way to map a fracture pattern away from well control. We estimate fracture direction and relative fracture intensity from such data at two gas fields in Saudi Arabia. Our results show a small azimuthal variation in P-wave velocity (maximum 5%) and a larger variation in azimuthal AVO at the reservoir (larger than 100%). Computed fracture attributes for field 1 show a consistent east-west fracture direction. However, in field 2, fracture azimuth is variable but generally east-west and north-south, with the strongly anisotropic north-south orientation correlating with faults and areas of large structural curvature in the reservoir. In both fields, azimuthal AVO analysis shows a more consistent estimate of fracture orientation than velocity analysis. These estimates have been instrumental in planning prolific and safe horizontal wells

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