The importance of fracture toughness in the estimation of seismic anisotropy and stress orientation in shale formations
Shale resource plays are associated with low permeability, and hence hydraulic fracturing is required for their stimulation and production. The effectiveness of hydraulic fracturing depends on how accurately a horizontal well is placed in the formation of interest. The direction of maximum stress and the magnitude of seismic anisotropy play an important role in the placement of a horizontal well, and effective hydraulic fracture stimulations along its length. Therefore, their estimation from seismic data can provide valuable information. While the source of seismic anisotropy is non-unique, fracture induced anisotropy as well as stress induced anisotropy are considered for an estimation of maximum stress direction and magnitude of seismic anisotropy. In this paper, we first introduce the concept of fracture toughness, which refers to the the ability of a rock to resist fracturing and propagation of pre-existing fractures. We then propose a workflow that uses its azimuthal variation for estimating these two parameters on seismic datasets from the Anadarko Basin. After estimating the maximum stress direction and magnitude of seismic anisotropy the available borehole breakout data as well as microseismic data for area of study are brought into consideration for authenticating the maximum stress direction analysis. The dipole shear logs measured at one well are used to validate the estimation for magnitude of seismic anisotropy.
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