Time-lapse scenario modeling and VSP analysis for an EOR reservoir in Oman

Extensive reservoir modeling analysis and VSP processing are undertaken for a pilot steam injection project in Oman to understand the relationship between reservoir properties and seismic properties and how that changes with steam injection. Effects of different permeability distributions in steam-flooded reservoir models are analyzed in terms of their seismic response. A full-wave walkway time-lapse VSP is designed and processed to investigate possible 4D anomalies. Fluid substitution modeling (Gassmann’s substitution) is used to calculate modified petrophysical properties after initial steam injection. Prediction results in a 5 percent lower VP and 2 percent lower density. Predicted modifications in petrophysical parameters are sufficient to produce observable 4D effects in PP VSP. Time-lapse anomalies get enhanced when the near-surface VSP data are excluded. In this case, multiple and shear-wave suppression may actually compromise the observed 4D effects. Use of the muted upgoing wave–field for migration enhances the 4D amplitudes, but compromises the lateral resolution. The predicted PP time-lapse results and time-lapse attribute maps are similar to real field VSP results. The PS reflections in the present case are restricted to produce any measurable changes (for smaller angle gathers). I predict that a 6 percent decrease in density (due to steam saturation) may be sufficient for the Oman oil field to produce measurable changes in both PP and PS amplitudes for walkway VSP.