Albacora Leste Field Development: Reservoir Aspects and Development Strategy

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18056, "Albacora Leste Field Development: Reservoir Aspects and Development Strategy," by W.P. Lemos, M.R. Baiao de Castro, C.M. Soares, J.F. Rosalba, and A.A.G. Meira, Petrobras, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. Albacora Leste, one of the largest Campos basin deepwater oil fields, comprises Miocene sandstones with high porosity and permeability. Net thickness ranges from 5 to 35 m, suggesting horizontal wells. After the deposition stage, erosive channels introduced flow barriers that generated different reservoir compartments. The full-length paper presents the key aspects of the reservoirs as well as the strategy adopted during project implementation. Introduction The giant Albacora Leste deepwater oil field in the northern part of the Campos basin has reservoir depths ranging from 2300 to 2600 m. The reservoirs are high-quality siliciclastic Miocene sandstones with 30% average porosity, 3000-md average absolute permeability, and 16.5 to 21.5°API oil. Total recovery for Albacora Leste is estimated to be 90×106 m3 of oil and 10×109 m3 of gas. Discovered in 1986, the challenging field environment slowed field development because it could be supported only by technologies emerging at that time such as long horizontal gravel-packed wells and thermally insulated pipelines. Development-well drilling began in 2002. A 28 600-m3-oil-capacity floating production, storage, and offloading vessel (FPSO), the P-50, will be used for field production. Peak oil production will be reached at the end of 2006 when all production wells will be producing. Seismic The first 3D-seismic data were acquired in 1987, but their acquisition parameters and processing routine were not sufficiently good for reservoir characterization. During 2001, a new 3D seismic was purchased that supported reservoir characterization for the field development. Fig. 1 shows the difference between the 1987 and 2001 surveys for the main reservoir zone. The new seismic data brought significant improvements to interpretation of the Albacora Leste geological model. Because of improved seismic resolution, reservoir boundaries, sandstone channels, and erosive channels were defined better. Usually, reservoirs show low impedance in the seismic cubes and are clearly distinct from the surrounding hemipelagic shales that show higher impedance. In Albacora Leste, presence of low-impedance shale and interlaminated rocks (very thin shale/sandstone stacked sequences) causes negative-amplitude seismic responses, similar to the best reservoir responses. Usually the low-impedance shale and the interlaminated rocks can be identified by amplitude-variation-with-offset (AVO) analysis and elastic inversion, but in the case of Albacora Leste, this does not solve the problem because of the low seismic resolution. For the same reason, identification of small gas caps is difficult. Models indicate that the small gas thicknesses observed in the drilled wells would not have been detected previously even using prestacking techniques because of seismic pulse interference effects. In 2005, a new 3D seismic with a larger number of traces on a smaller bin was purchased to obtain larger frequency content. This seismic will provide better support for future reservoir management and possibly for 4D studies to identify poorly drained reservoir volumes.