Improved Depth Conversion with FWI - a Case Study from the NW Shelf, Australia

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Horizontal drilling for development of thin reservoirs requires seismic depth conversion to be more accurate than the bed thickness, which may be of the order of 2% of depth or less. In highly heterogeneous overburden, traveltime tomography may not sufficiently resolve lateral velocity variations causing depth “busts” in the underlying image. Full Waveform Inversion provides model resolution to half a seismic wavelength or better, which in turn provides an accurate, data-driven velocity model with reliable depth conversion accuracy away from the wells, following geostatistical velocity model calibration. At an oil and gas development field in the NW shelf, Australia, 3D TTI FWI resolves the detailed morphology of Miocene carbonates at around 2km depth. Geostatistical scaling of the reflection tomography model did not capture this rugosity and led to development well depth errors at the Triassic reservoir level at 3 km of up to 50 m, just a few hundred metres from exploration wells. The same geostatistical scaling applied to the FWI model provides a statistically stable uncertainty of approximately 15 m, or less than 2% of depth at reservoir level. The robust depth conversion model can be confidently used for future development well planning and reservoir volume estimates.