What Matters to Flow: West Africa Field Case History

Abstract

The subject field, located in deep-water Angola Block 15, was discovered in 1999, achieved first production in 2003 and was abandoned in 2011. Over its 7-year life, the field reached a peak production of 90 thousand BOPD and produced nearly 100 million barrels of oil from a high-quality stacked Lower Miocene deep-water channel complex. This field provides a unique opportunity to perform a look back on how well the reservoir characterization, reservoir modeling and reservoir performance predictions made at project funding matched with the final actual field performance. The West Africa field was a subsea development tied back to a FPSO (Floating Production Storage Offtake vessel). The original depletion plan called for crestal gas storage and peripheral water flood. The field was ultimately developed with a combination water flood / downdip gas injection process. The factors that drove the evolution in the depletion plan are reviewed. Despite the significant changes to the depletion plan, the funding models did a good job of predicting average reservoir behavior such as most-likely recovery and production plateau. However, both gas and water breakthrough and build up was faster than expected. These factors were offset by higher well productivity and larger in place oil volumes. The reasons for the models not matching actual performance are discussed. 4D seismic acquired after 3 years of production was particularly effective in illuminating gas and water flow pathways in the reservoir that had not been modeled or predicted. Water injection was not initiated until about three months after first production. Initial field performance under pressure depletion drive provided valuable reservoir characterization data. Early pressure decline was slower than predicted; material balance indicated that a higher STOOIP was needed as stronger aquifer support alone was not sufficient to match observed pressures. Late field life data showed a more complex situation, indicating an even stronger aquifer than had been previously modeled. The impact of aquifer strength on pressure support was not fully recognized at project funding time. For most subsea developments with limited ability for well intervention, reservoir surveillance data from permanent down-hole pressure gauges and particularly 4D seismic are critical to effective reservoir management. Learnings from this field have been applied to other West African deep-water developments.