Borehole sonic data has long been a key component of geophysical logging programs. Compressional, shear and Stoneley slowness logs have been applied in the surface and borehole seismic, petrophysical, geomechanical and other domains.
Recent advances in borehole sonic acquisition and interpretation has altered the landscape of borehole sonic applications. The magnitude and scope of these advancements is substantial and is changing industry practices and workflows.
These advancements include significantly improved slowness measurements, advanced slowness frequency dispersion analysis, characterization of acoustic anisotropy (even in environments of extremely low acoustic anisotropy), slownesses measured as a function of radial depth from the wellbore, advanced imaging techniques, increased information on fracture systems and enhanced estimations of permeability from Stoneley waves.
The advanced slowness frequency dispersion analysis yields new information about the type of acoustic anisotropy present in the reservoir and the near wellbore environment, whether the rock has been altered by the drilling process and to what extent. In extremely slow formations and other difficult acquisition environments dispersion analysis is used as an excellent quality control indicator to guarantee that the computed slowness accurately represents formation properties.
The newly developed sonic interpretations mark a significant leap forward in the understanding of natural fractures. Enhanced evaluations can be achieved via a combination of broadband frequency monopole and dipole and lower frequency, deeper reading Stoneley measurements. In hard-rock environments, natural fractures play an important role in production. A good understanding of fracture attributes such as aperture and permeability are necessary for optimal well completion and the achievement of maximum production.
With reference to field examples, this paper presents a summary of the recently developed and developing sonic acquisition and interpretation techniques and their application to reservoir characterization.