Multi-Component Full Wave Data for Reservoir Fracture Analysis

Brian Donnelly
ION Geophysical Corporation, London, United Kingdom

For a fractured reservoir knowledge of the dominant fracture orientation is key to understanding field development and prospectivity. We describe a method for acquiring, processing and interpreting multi-component data to give a reliable picture of fracture morphology, by using the particular properties of converted shear waves. We illustrate this using a case-history approach. The first consideration is survey design. Converted wave recording and anisotropy analysis require a full azimuth distribution, which must be reflected in a survey design free of azimuthal bias. Similarly point-sensor recording removes the effects of array directionality and inter-element statics. The sensors are multi-component, recording all components of the seismic wave-field, hence Full Wave. Converted waves propagate more slowly than p-waves so a denser receiver spacing is required. As well as p-wave seismic, this technique yields converted wave data from the horizontal channels. This is rotated to the correct source-receiver azimuth, then the 3 volumes - p-wave and 2 c-wave - are processed through PSTM. Comparison of the volumes by horizon registration gives a measure of Vp/Vs ratio, which can be a direct lithology indicator. The next stage makes use of shear-wave splitting, or birefringence, whereby shear waves are polarised in anisotropic media by the presence of fractures. Analysis of the slow and fast polarisation directions gives a direct indication of fracture orientation, as the fast direction is aligned along the dominant fracture plane. Using advanced visualisation technology data is interpreted to give a highly detailed picture of reservoir fracturing, also derived attributes such as Vp/Vs and shear impedance. Application of this method has proven to be highly successful in field development and productive well-planning.