Detection of multiple fracture sets using observations of shear-wave splitting in microseismic data

As industry moves towards more unconventional reservoirs, such as tight gas, the ability to characterise natural and induced fracture sets in a reservoir becomes ever more important. Seismic anisotropy, which splits shear waves, can be used to image fracture sets, but when used with surface seismics, the need to remove overburden effects, and the lack of inclinational coverage, makes it difficult to characterise fractures using SWS alone. Shear waves produced by microseismic events and recorded on downhole arrays travel with a range of azimuths and inclinations, and only travel through rocks in and around the reservoir. As such, they present an excellent S-wave source for measuring seismic anisotropy in reservoirs. In this paper we develop a method to invert SWS measurements for multiple sets of aligned fractures. We demonstrate the procedure using data from Weyburn, where at least two sets of fractures are present, and, to show that it is possible to discriminate between one and multiple fracture sets, data from a hydro-frac where only one set is present. We also use synthetic data to highlight some pitfalls that can be encountered when multiple fracture sets are not accounted for.