In reservoirs producing primarily from secondary porosity like fractures, the flow is largely controlled by the fracture system and characterizing these fractures is key to effectively managing these reservoirs. Aligned fractures will affect the average stiffness of the rocks even when fractures are smaller than seismic wavelengths. Small scale aligned fractures lead to variations in the seismic velocities that are directionally dependent, an effect known as anisotropy. Measuring this seismic anisotropy provides information on the fracture systems properties, such as fracture density and orientation. Such information can be used to identify sweet spots and in general be used for better reservoir management, like guiding directional and infill drilling.
Since fractures are often nearly vertical to bedding the dominant seismic effect is azimuthal anisotropy, which is observed both on P- and S-waves. AVOA (azimuthal AVO-effect) and variations in the NMO velocities have been used to estimate P-wave anisotropy. S-waves will split into one fast and one slow mode and this splitting is very sensitive to small changes in the subsurface. The fast shear-wave direction aligns with the fracture strike and the amount of time splitting between the fast and slow mode depends on fracture density.
Apart from the difference in sensitivity between the P- and S-wave measurements there are attributes of the fracture system that can only be observed on the S-wave data. The asymmetry in the converted waves (PS-data) can say something about the tilt of the fractures. Multicomponent seismic provides access to both the P- and S-wave azimuthal information. Combining these two can enable the extraction of information about the fluid in the fractures. In this paper we will give an overview of the theory and several cases where multi-component data has been used to characterize fractures and fracture attributes.
Whilst many fractured reservoirs have been profitably produced, few of them have been depleted in an efficient manner. Nelson points out the importance of classifying fractured reservoirs based on the amount of heterogeneity and anisotropy observed in the porosity and permeability. Quantifying this anisotropy with surface seismic data should provide an optimal strategy for fractured reservoir management by integrating the geophysical data from all scales with the engineering data.