Improved depth conversion with FWI – a case study

In the study region, notoriously heterogeneous shallow carbonates, here between depths of approximately 1.5- 2.5 km, give rise to short-wavelength velocity variations in the overburden, which can cause severe depth undulations at the reservoir level (~3 km depth subsea). As the reservoirs are relatively thin (~30 m), stacked, fluvial-deltaic channel sands with sharp meanders and lateral truncations, even a localised 1% velocity error can produce a 30 m depth error at 3 km, which is not acceptable for development well planning. The resolution of traveltime-tomography is limited and may not resolve overburden variability in a manner suitable for depth conversion in a development setting. Geostatistical scaling using the available well control is one option to further improve depth conversion reliability. This workflow utilizes time-depth data at well locations along with geophysically constrained statistics to derive a stable, geologically plausible background trend that accounts for the majority of depth error. The result is a regionally consistent model that ties the available well control and provides more reliable depth conversion away from wells. The background trend provided by the well-based velocity calibration is however a long-wavelength solution and is unlikely to be able to correct for the short-wavelength depth errors which can arise at depth. FWI on the other hand provides a resolution of less than half a seismic wavelength to capture thin and localized subsurface features. With short-wavelength features accurately resolved, well control can then be used to correct for any residual long-wavelength errors and ensure more reliable depth conversion away from the wells. In this study, reflection tomography and 3D TTI (tilted transverse isotropy) FWI velocity models were geostatistically calibrated to wells and compared for depth conversion accuracy. Using the calibrated reflection tomography model, early appraisal wells were still more than 50 m off prognosis at the top reservoir level. These appraisal wells were just a few hundred metres away from the exploration wells used for control. 3D TTI FWI was then run using the reflection tomography model as input. The same geostatistical calibration to wells was applied to the FWI output and ultimately provided a model with depth conversion accuracy to within 15 m at the reservoir level.