Azimuthal pre-stack depth migration for in-situ stress evaluation in a fractured carbonate oil reservoir: Predrill prediction of instantaneous shut-in pressure gradients

Abstract

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 shale, which will flow oil with a sufficient fracture network. The on-going drilling and completion program is horizontal wells plus hydraulic stimulation. The azimuthal seismic attributes were compared to ISIP (instantaneous shut-in pressure) gradients, taken as indicating minimum horizontal stress, and other calibration data as discussed in the companion papers presenting the azimuthal amplitude results. Azimuthal interval velocities derived from azimuthal prestack depth migrated data provide insight into the in-situ horizontal stress field. The success of the hydraulic stimulations depended upon having a low minimum horizontal stress and the presence of natural fractures that flow oil. The results of this study are: 1) The magnitude of the slow interval velocity (VINTslow) in the reservoir showed a local proportional sensitivity to the local minimum horizontal stress, as estimated from the ISIP gradients. 2) the interval velocity anisotropy (VINTfast-slow) was seen to be locally inversely proportional to ISIP gradients throughout much of the survey. 3) Based on the observations in the first two results, pre-drill qualitative predictions for ISIP gradients were corroborated in a horizontal well drilled after we had finished processing and interpretation