Forward modeling of fracture‐induced sonic anisotropy using a combination of borehole image and sonic logs

Abstract

We develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geological fractures using a forward modeling approach. We use a classical excess‐compliance fracture model that relies on the orientation of the individual fractures, the compliances of the fractures, and the compliances of the host rock. We extract the orientation of individual fractures from borehole image log analysis. We validate the model using borehole resistivity image and sonic logs in a gas‐sand reservoir over a 160‐ft vertical interval of a well. We observe significant amounts of sonic anisotropy and numerous quasi‐vertical fractures. The maximum horizontal stress direction, given by breakouts and drilling‐induced fractures, is not aligned with the strike of natural fractures. We show that using just two adjustable fracture compliance parameters, one for natural fractures and one for drilling‐induced fractures, is an excellent first‐order approximation to explain the fracture‐induced anisotropy response. Given the presence of gas within the fractures, we assumed equal normal and tangential compliances. The two inverted normal compliances are and . Predicted fast‐shear azimuth matches measured fast‐shear azimuth over 130 ft of the 160‐ft studied interval. Predicted slowness anisotropy matches the overall variation and measured values of anisotropy for two of the three strong anisotropy zones. The medium is mostly a horizontal transverse isotropic medium (HTI) with small azimuthal variations of the symmetry axis. We also show that the measured sonic anisotropy is caused by the combination of stress and fracture effects where the predominance of one mechanism over the other is depth‐dependent.