AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Quantifying Bypassed Pay Through 4-D Post-Stack Inversion
(1) BHP, Houston, TX.
(2) Stone Energy, Lafayette, LA.
In this study, we performed a 4D inversion to identify remaining potential within three sands of the Amberjack field, Gulf of Mexico. The goal of this study was to define fluid changes within the ‘G’, ‘J’, and ‘P’ sands of the Amberjack field, offshore Louisiana. We focus on the ‘G’ sand in this talk. The upper sands were deposits from shelf edge deltas, hence are shingled sands with common base connectivity and limited upper connectivity. 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 and in 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. The base survey was from a Western Spec survey acquired in 1994, and the monitor survey was from a 2002 proprietary survey. Post-stack acoustic 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. Finally, remaining potential maps could be generated. Detailed petrophysical analysis was done on the BP A025ST1 well. We found we can measure changes in the reservoir using post-stack acoustic impedance inversion techniques. We calculated a difference model between the base and monitor surveys. Maps were generated over the zones of interest. Inputs included gross and net pay sands for the ‘G’ and ‘J’ sands for three fault blocks, a constant porosity for the sands, and production information. Given these maps and constants, we can calculate reservoir pore volumes, the minimum P-impedance map from the inversion of the base survey, and the average P-impedance difference map from the difference volume between the surveys. We applied cutoffs to match the reservoir volumes that had been produced. Then we could calculate maps showing the remaining potential. By examining the changes in acoustic impedance between the base and monitor survey, we were able to identify areas that had been swept and map remaining potential.