Presalt Carbonate Evaluation for Santos Basin, Offshore Brazil

The Presalt carbonate wells of Lula Field, Santos Basin, Offshore Brazil, are currently producing high quality, 28 to 30 degrees API oil at an average rate of 30,000 BOPD. With reservoir pressures over 8,000 psi, and a downhole oil viscosity of 1 cP, the prolific flow rates from these high permeability, lacustrine carbonates have shown no significant decline in over five years of production. These heterogeneous, layered carbonates with variable reservoir quality typically have oil columns greater than 200 m and one of the key challenges is to identify the high permeability intervals to optimize the completion strategy. Since the first discovery well in Lula field in 2007, NMR logs have been extensively used to aid in identifying the high-permeability intervals. Laboratory NMR experiments have been performed by Petrobras on Presalt core samples, oil samples and oil-based mud filtrate at downhole conditions to better understand the NMR response in these oil-wet carbonates. The laboratory measurements have been valuable for understanding the effects of varying wettability and varying surface relaxivity due to the presence of heavy minerals, and how each can affect the NMR T-2 response. Complementing the NMR analysis, acoustic rock physics and new algorithms for quantifying vuggy porosity from ultrasonic image logs are now used to aid in identifying high-permeability zones. Finally, advanced foimation-tester analysis is used to analyze both horizontal and vertical permeability over larger intervals to aid in upscaling the formation properties for reservoir simulation.