Summary
We present a study of shear-wave splitting in stimulated microseismic events recorded in a highly anisotropic shale sequence. We see up to 50% anisotropy, which can be primarily attributed to the alignment of phyllosilicate minerals and kerogen in petroleum bearing shale. The polarisation of the leading shear-wave is predominantly confined to the horizontal plane (i.e., SH-waves in a VTI medium), although a small but significant population of the measurements show intermediate fast polarisation angles that are neither horizontal nor vertical. Azimuthal variations in the percentage anisotropy suggest a potential fracture set close to the direction of maximum horizontal compressive stress. These results also exhibit a complicated pattern of multiple shear-wave arrivals that seem to be due to fractured shale layers. Further modelling will be used to help better interpret the observed rapid vertical variations in shale anisotropy due to both intrinsic VTI and fractureinduced HTI mechanisms.
Introduction
The alignment of phyllosilicate minerals strongly influences the intrinsic seismic anisotropy of shale rocks (e.g., Valcke et al., 2006), but it is also influenced by the kerogen content in petroleum bearing shales (Sayers et al., 2013). Whilst these rocks are porous, they are often not permeable enough to be economically valuable without the hydraulic stimulation of fractures. Applied stresses can reopen pre-existing fractures or stimulate new fractures parallel to the direction of maximum horizontal compressive stress (SH) (e.g., Baird et al., 2013). In either case this normally leads to fracture alignment, which is also an effective mechanism for seismic anisotropy. Thus observations of seismic anisotropy hold the potential to better diagnose the economic potential of petroleum bearing shale formations, both before and after stimulation. Shear-wave splitting, or the propagation of two independent shear-waves, is probably the most unambiguous indicator of seismic anisotropy. Here we use observations of shear-wave splitting in microseismic events induced in a shale gas formation to better discriminate between intrinsic and fracture-induced anisotropy.
