Results of the downhole microseismic monitoring at a pilot hydraulic fracturing site in Poland, Part II: shear wave splitting analysis

Observations of azimuthal seismic anisotropy provide useful information, notably on stress orientation and the presence of pre-existing natural fracture systems, during hydraulic fracturing operations. Seismic anisotropy can be observed through the measurement of shear-wave splitting (SWS) on waveforms generated by microseismic events and recorded on downhole geophone arrays. In this paper we present measurements of azimuthal anisotropy from a Lower Paleozoic shale play in northern Poland. The observed orthorhombic anisotropic symmetry system is dominated by a vertically-transverse isotropy (VTI) fabric, produced both by the alignment of anisotropic platy clay minerals and by thin horizontal layering, and overprinted by a weak azimuthal anisotropy. Despite the dominating VTI fabric, we successfully identify a weaker HTI fabric striking east-southeast. We do this by constraining the rock-physics model inversion with VTI background parameters incorporated from other geophysical methods: microseismic velocity model inversion, 3D reflection seismic and borehole cross-dipole sonic logs. The obtained orientation is consistent with a pre-existing natural fracture set that has been observed using XRMI image logs from a nearby vertical well. The present-day regional maximum horizontal stress direction differs from the observed fracture strike by approximately 45°. This implies that the SWS measurements recorded during hydraulic stimulation of shale gas reservoir are imaging the pre-existing natural fracture set which influences the treatment efficiency, instead of the present-day stress.