QUANTIFYING BYPASSED PAY THROUGH 4-D POST-STACK INVERSION

In this study, we performed a 4D inversion study to identify remaining potential within three sands of the Amberjack field, Gulf of Mexico. Petrophysical analysis was critical in determining the changes in reservoir properties due to pressure and fluid changes in each of the sands. This analysis showed that changes in acoustic impedance could be tied to changes in pressure as well as changes in the fluid saturation from oil to brine. Post-stack seismic volumes from a base and monitor survey were processed and calibrated to minimize differences between the two surveys. Post-stack acoustic impedance (P-Impedance) inversion was performed on both surveys and a difference volume was calculated between the post-stack inversion results. Maps were generated of acoustic impedance, acoustic impedance difference, and top reservoir depths for each of the sands. Cutoffs were applied to these maps to match hydrocarbons that had been produced from the reservoir. Once the cutoffs were established, remaining potential maps could be generated. The goal of this study was to define fluid changes within the Amberjack field, offshore Louisiana. The required inputs included pressure and fluid property data, well locations, well logs, and post-stack seismic data from both a base and monitor survey. Fluid substitution of the well logs showed an increase in acoustic impedance indicates that the fluid saturation has changed. For all three sands, a decrease in acoustic impedance indicates that the pressure has changed . By examining the changes in acoustic impedance between the base and monitor survey we were able to identify areas that had been swept. We applied cutoffs to the minimum P-impedance map, the top reservoir depth map, and the average P-impedance difference map to match the reservoir volumes that had been produced. Adjusting the cutoffs we can match the volumes for both the initial volume and what had been produced. Once we have our volume matches we can calculate maps showing the remaining potential based upon the minimum acoustic impedance and the average acoustic impedance difference maps. We can then quantify the remaining potential.