f Use of frequency dependent anisotropy

Conventional methods for seismic fracture detection are based on observation of seismic anisotropy. The anisotropy is assumed to be independent of frequency, and consequently we are able to produce maps of fracture orientation and intensity. Nevertheless, recent work has challenged the assumption of the frequency independence of anisotropy, and suggests that information on the orientation and dominant scale lengths of open fractures can in principle be inferred from observation of frequency dependent anisotropy. Here we present several examples on the use of frequency-dependent anisotropy in tight gas reservoirs. The first example is a walkaround VSP from an on-shore fractured gas reservoir in North America that allows us to demonstrate the existence of frequency dependent P-wave anisotropy and its links to preferred fluid flow direction. The second example is a multiazimuth walkaway VSP data from a fractured reservoir in the UK continental shelf where we believe that we are able to determine the orientations of the open fractures using P-wave attenuation anisotropy. The third example is a multicomponent VSP data from a tight gas reservoir in North America, in which fractured scale length is inferred from the frequency dependent shear-wave splitting. Our results suggests that the concept of frequency dependent anisotropy provides considerable scope for improving seismic fracture characterisation based on both P- and S- wave data by providing information on the scale lengths of the open fractures.