Modeling C7+ and Asphaltene Compositional Grading in Oil Reservoirs

A previously proposed framework to model compositional grading in continuous or semicontinuous mixtures is here extended to model segregation profiles of reservoir-fluid columns containing heavy fractions and asphaltenes. The gravity-induced variation in plus-fraction molecular weights is described with a a two-moment formulation of gravitational equilibria, to predict composition, distribution functions and reservoir pressure gradients in multi-ensemble fluid columns displaying gas-cap, oil leg and tar-mat regions in the reservoir. By coupling a saturation pressure calculation to the segregated profiles, the oil-gas and oil-tar contacts in the reservoir are easily calculated. We first present a description of the method for computation of these gradients. Comparison of this method with measured data of two reservoir fluid columns shows the adequacy of the proposed approach.