Fracture densities and fracture azimuths evident in the azimuthal amplitudes from the top of a fractured carbonate oil reservoir

Abstract

Summary A high-fold full-azimuth full-offset 3D P-P reflection survey was acquired and processed for the purpose of characterizing a naturally fractured carbonate reservoir. The reservoir is a thick carbonate, overlain by a shale, which will flow oil with a sufficient fracture network. The calibration data for the natural fractures are: 1) a loggingwhile-drilling (LWD) Star Trak™ resistivity image log in a horizontal borehole, and 2) micro-seismic (m-s) data. The 3D P-P reflection data were processed through azimuthal prestack depth migration (PSDM), and the azimuthal traveltimes were quantified in a non-sectored analysis and then removed into attribute volumes, leaving flat common image gathers (CIGs). A companion paper presents the minimum horizontal stress (taken as the Instantaneous Shut In Pressure gradients) correlation with the azimuthal interval velocity slow. After the offset vector tile (OVT) PSDM, four azimuth sectors (0-180 degrees), each with four angle stacks, enabled the AVOaz computation for Top/Reservoir, as well as the computations for the azimuthal variation of each angle of incidence. Inversions, both azimuthally-isotropic as well as azimuthally (only VS was allowed to change azimuthally), gave estimates of rock properties. The azimuthal amplitudes themselves gave insight into the natural fractures network. The results presented in this paper are: 1) The azimuthal variation of the near-offset amplitudes (7 o ) best matched the calibration data of the Star Trak image log and microseismic (m-s) data; the AVOaz results were acceptable, but did not match as well as the 7° angle of incidence azimuthal amplitudes; 2) the m-s events for most of the frac stages studied showed the NE-SW spatial distribution indicating a NE-SW maximum horizontal stress orientation, consistent with the NE-SW azimuth of VINTfast; 3) at several frac stage centers, the m-s events showed detailed spatial patterns that track the azimuth and density of the natural fractures as interpreted from azimuthal amplitudes, specifically, the 7° angle of incidence azimuthal amplitudes. 4) To explain the m-s data, knowledge of the in-situ stress (azimuthal VINT) and the natural fractures (azimuthal amplitudes) must be used.