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Modeling Oil Composition in Reservoirs Submitted to Biodegradation in Offshore Oilfields in the Brazilian Southeastern Margin


Volumetric and compositional petroleum prediction constitutes a crucial requirement to the exploratory process in areas subjected to biodegradation. This work was based on a detailed study of oil and gas samples from offshore oilfields in the Brazilian southeastern margin, and on numerical simulations to quantify the alterations in geochemical and physical properties in the petroleum accumulations due to biodegradation. Among the laboratory procedures, gas chromatography was utilized to subdivide the oils in compositional classes (C6-C15, C15-C25, up to C55+), PVT analyses to quantify the fluid properties (API gravity, density, GOR, and others) and distillation to estimate the oil mass losses with the advancement of biodegradation. The integration of these techniques allowed to estimate around 50% of mass losses in a degraded oil with 15° API. Furthermore, petroleum systems modeling using PetroMod (Schlumberger) was applied to simulate petroleum generation, migration and accumulation as well as the compositional evolution of biodegraded oils. These quantitative approaches are based on the assumption that different compound classes in oil are biodegraded at different rates at the oil-water contact. First, the volumes and the original properties of non-biodegraded accumulations were reproduced accurately by using a PVT-based compositional kinetic scheme. Then, taking samples of non-biodegraded oils as reference for the original oils in biodegraded intervals, rates were adjusted for each compound class until a good match was achieved between measured and calculated compositions. The simulated biodegraded accumulations reproduced the mass losses, API gravities and compositions of oils consistent with those assessed by the integration of laboratory techniques. These new achievements enable, using the set of biodegradation parameters and reservoir filling history, to assess the biodegradation risk, as well as the volume and oil composition in new prospects located in areas subject to biodegradation.