Fracture compliance estimation using a combination of image and sonic logs

Abstract

We estimate fracture elastic compliances using a combination of image and sonic logs from a typical borehole survey in a naturally fractured reservoir. Borehole dipole sonic anisotropy related to the effect of drilling-induced (DI) and natural fractures is modeled using a classical excess-compliance fracture model. We extract the orientation of individual fractures from borehole image log analysis. The analysis is focused on the log depth interval where fracture-induced anisotropy is observed, and where the maximum horizontal stress direction is not aligned with the strike of natural fractures. Fracture compliances are estimated in two steps by first selecting the fractures that honor the fast-shear azimuth, and second by using a grid search method minimizing the sonic slowness anisotropy difference. Two fracture compliance parameters are used, one per fracture type (natural and DI). We first model the effect of natural and DI fractures including all fractures observed on the images. Predicted fast-shear azimuths are shown to be slightly biased by the presence of non-compliant fractures in each fracture type. Next, we introduce a selection criteria based on the fast-shear azimuth to reject non-compliant fractures. Finally, we estimate fracture compliances using sonic slowness difference before and after the selection process and discuss the results.