Abstract
This paper reviews the use of shear-wave splitting in exploration seismology, how it began and how it has been used to characterize fractured hydrocarbon reservoirs in the past forty years since it was first confirmed in earthquake seismology and subsequently introduced to the hydrocarbon industry.Shear-wave splitting (or birefringence) refers to the phenomenon that when a shear-wave enters a fractured medium, it necessarily splits into a fast and slow wave.The fast wave is usually polarized along the fracture strike, the slow-wave perpendicular to the strike, and the time-delay between the fast and slow wave is proportional to the fracture density.Therefore, recording and analysis of the resultant shear-wave splitting in fractured hydrocarbon reservoirs can give us better definition of fracture orientation and density.Furthermore, frequency-related changes in shear-wave splitting derived from multicomponent seismic data can provide us with vital information about fracture scale-length, fluid type and distribution.Current surveys and various research works have revealed that shear-wave splitting is widely present in the Earth's crust, and more profound splitting occurs in the near-surface (the first 1200m of the Crust) than the deeper subsurface.This and some other shear-wave data quality issues restricted the application of shear-wave splitting in seismic exploration to certain areas with favourable conditions such as a relatively simple near surface and relatively thick and heavily fractured reservoirs, together with detailed calibrations of shear-wave splitting from shear-wave VSPs.Overall: data quality, near surface and reservoir conditions are the three key factors which affect the applications of shear-wave splitting.Only when shear-wave data quality and the cost-effectiveness in shear-wave data acquisition become comparable with that of the P-wave, can the full potential of shear-wave splitting be realized.
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