Comparison between conventional and NMR approaches for formation evaluation of presalt interval in the Buzios Field, Santos Basin, Brazil

Brazilian presalt account for more than 70% of petroleum production in the country. More than ten years after the announcement of its discovery, presalt carbonates remain presenting many challenges regarding their reservoir's characterization. Well log analysis and formation evaluation of carbonate rocks are difficult tasks due to the complex and heterogeneous nature of these rocks and its associated mineral phases such as magnesian clays (Mg-clays). This study proposes a new workflow for presalt reservoirs formation evaluation that incorporates nuclear magnetic resonance (NMR) logs in the estimation of petrophysical properties such as clay volume, porosity, water saturation and net pay. This workflow aims to be useful for initial assessments when a limited amount of data is available. We compare this approach with conventional methods widely applied in formation evaluation to verify the impact that use of different methodologies can have in the final assessments. These methods are applied in Barra Velha and Itapema formations of the Buzios Field, Santos Basin. Our results show that a hybrid method which combines NMR and conventional logs for clay content and water saturation is more robust for estimation of those properties in Barra Velha and Itapema formations. Clay content based on gammaray logs is more assertive to represent the shales observed in the Itapema Formation. However, clay content based on NMR logs is essential to properly identify Mg-clays in the Barra Velha Formation. Regarding the water saturation, the use of only Archie's equation can result in net pay regions below the oil-water contact if their parameters were not properly determined. An accurate determination of these parameters requires a huge amount of data and measurements that are not always available. So, the hybrid approach for water saturation shows that the use of NMR logs above the transition zone is an optimistic alternative to overcome the limitations of the Archie's equation in carbonate rocks. In general, effective porosity from NMR and neutron-density logs presented similar results. Nevertheless, NMR effective porosity was more accurate to regions with dolomitized and silicified carbonates and Mg-clays. In magnesian-clays interval, NMR effective porosity fitted two times better to the laboratory data than neutron-density effective porosity. So, in our analysis, the incorporation of NMR logs and its integration with conventional logs resulting in a hybrid approach provided a more assertive formation evaluation in the studied area, being essential its use in complex environments as presalt reservoirs.