Shear wave splitting and fluid flow in highly anisotropic shale gas reservoirs

We present a study of shear-wave splitting using microseismic sources in a highly anisotropic shale gas reservoir. We observe very strong anisotropy (up to 30% in some cases) with a predominantly VTI symmetry (vertical transverse isotropy). Splitting is strongest for near-horizontal rays where the SH leads SV, however, for steeper inclinations SV is faster and we observe a triplication in the SV wavefront. Although much of this anisotropy can be explained due to the intrinsic anisotropy of the phyllosilicate material, we suggest that an additional contribution from horizontal microcracks with low hydraulic connectivity may be required to produce the observed patterns of anisotropy. This has important implications for fracture treatments, as the reactivation and interaction of these horizontal cracks with induced hydraulic fractures may be an important mechanism to facilitate gas flow.