Fracture detection using P-wave AVO measurements

Abstract

Physical modelling of seismic wave propagation can provide insights into complex structural, stratigraphic and reservoir characterisation problems. The detection and delineation of fracture zones is very important in develop- ment drilling. Determination of fracture orientation within a reservoir may play an important role in EOR programs. The fundamental principle used in AVO studies is the extraction of shear wave information present in the reflection amplitude variation with source?receiver distance. The objective of this study was to determine if the shear information in 3-D AVO surveys can be utilised for fracture detection and delineation. A 3-D physical modelling experiment was conducted over a simulated fracture system at the Allied Geophysical Laboratories at the University of Houston. The 'fracture system' consisted of three isotropic homogeneous layers with a transversely isotropic Phenolite disc embedded in the central portion of the middle layer. The three layers are: an upper layer of black resin 2741LV; a middle layer of blue Stycast resin 2850MT (containing the anisotropic disc); and a lower layer of Evercoat casting resin. Two 3-D seismic surveys were acquired over the model, one with acquisition lines oriented parallel to, and a second survey with the acquisition lines perpendicular to, the fracture direction. Multi-offset and multi-azimuth seismic experiments were also conducted over the simulated fracture system. After applying corrections for spherical divergence and source?receiver directivity the reflection amplitudes from the black/Stycast resin interface were analysed. The results show that the 'fractured' disc is characterised by low AVO gradients in both surveys. In addition, the 3-D AVO gradient estimates from the survey perpendicular to the fracture orientations are 30% to 40% lower than the AVO gradient estimates from the survey with line orientation parallel to the fracture direction. Also, AVO effects along different line orientations suggest that at near offsets the amplitude behavior is similar for all azimuths but at far offsets an anomalous amplitude is seen along an azimuth perpendicular to the fracture zone.