Fractured Reservoir Characterisation from Seismic and Well Analysis – A Case Study

Abstract

The performance of naturally fractured reservoirs is controlled by the orientation, intensity and spatial distribution of open fractures. In this paper we present the results of a recent fracture characterization study based on P-wave azimuthal anisotropy using interval velocities and RMS amplitude maps. A consistent processing sequence was applied to all azimuth sectors in order to preserve the quality of the azimuthal anisotropy information. A proprietary geostatistical decomposition technique was applied to ensure more stable and accurate estimates of the fracture intensity and direction. These results provide detailed maps of the fracture system and show good correlation with sub-seismic scale fracture analysis from FMI/FMS logs. Introduction An accurate knowledge of fracture networks is critical for the optimization of the development of naturally fractured reservoirs, since intensity and orientation of fractures significantly affect fluid flow in the reservoir rocks. Although numerous methods based on shear-wave splitting analysis are available for fracture detection, a growing interest in P-wave azimuthal amplitude variation has been observed in the last few years. Ruger and Tsvanskin (1997) demonstrated that reliable estimates of the anisotropy parameters could be obtained from P-wave amplitudes. The approximations of the reflection coefficients of Ruger (1998) were then extended into a linearised form by Jenner (2002). Angerer et al. (2003) then went on to propose an integrated approach for fracture characterisation, which yields quantitative estimates of fracture intensity and direction from wide-azimuth, large offset P-wave data. In this paper we apply this workflow to a 3D seismic dataset of approximately 20 km 2 and investigate two distinct reservoirs. To confirm the validity of the estimated anisotropy attributes this study included an analysis of the fracture network on a sub-seismic scale using image log data from 6 wells. Azimuth-friendly processing In the applied workflow, a detailed review of the acquisition geometry and the processing sequence is performed to ensure preservation of the azimuthal anisotropy information. This is followed by optimization of the binning grid to generate a homogeneous offset and azimuth distribution. The number of azimuth classes and the offset range is selected on the basis of the signal-to-noise ratio and average fold with an iterative process that takes into account the various macro-binning options. There is a trade off between the macro bin size and the number of azimuth sectors, which affect either the lateral resolution or the robustness of the anisotropy attributes computation. The best combination was obtained for a 5x5 macro bin size, 4 azimuth sectors and an offset range of 500-1400 m. This corresponds to a maximum incidence angle of 35-40 degrees for the shallower target and 20-25 degrees for the deeper target. For each of the azimuth sectors some RMS amplitude maps are extracted from time windows of 100 ms thickness centred on each of the two reservoirs. In addition, automatic high density velocity picking was performed in order to produce accurate azimuthal NMO velocities from which we were able to derive an interval velocity map.