Analysis of Data Recorded in a Walk-Around VSP in the Arabian Gulf

Abstract

Abstract Two walk-around VSPs were recorded in an oil well in the Arabian Gulf: a deep setting with the aim of characterizing fractures within carbonate reservoirs, and a shallow setting designed to investigate the cause of a lost-circulation zone. This paper presents a detailed analysis of the walk-around VSP interpretation. Fracture characterisation was achieved by analyzing shear-wave splitting from a sequence of shear-wave generators. The paper will give a brief outline of the theory and method of assessing azimuthal anisotropy through shear-wave splitting analysis, followed by a review of the processed VSP data submitted by the VSP contractor in the form of Excel data. The walk-around VSP data interpretation is then presented for both the target carbonate oil and gas reservoirs and a shallow lost-circulation zone. Several cores were taken across the reservoir interval and fracture analysis of the cores is compared to the results of the walk-around VSP interpretation. Two fracture properties were defined: density and azimuth. The variation of fracture density with depth was interpreted from the walk-around VSP data using the ratio of transverse to radial shear wave amplitudes as a proxy for fracture density. Both seismic wave modes provided information on the fracture azimuth, obtained through measurement of the fast shear direction and the azimuth of P-wave first arrivals. Fracture analysis of cores from two discrete reservoir intervals confirmed the value of the deep setting walk-around VSP data as well as the accuracy of the interpretation. Strong azimuthal anisotropy in the shallow setting walk-around VSP data was interpreted to indicate a zone of high fracture density, in which the fractures were oriented parallel to the regional stress field and the fracture zone coincided precisely with the lost circulation layer. The shear-wave splitting data recorded in these walk-around VSPs provided robust measurements of azimuthal anisotropy over a long well-bore interval. This could be interpreted in terms of natural fracture density and azimuth, both in the main producing reservoir and in a shallow lost circulation zone. The results corroborated and extended the fracture characterization interpretation available from more restricted core data.