Reservoir Implications of Measured Thermodynamic Equilibrium of Crude Oil Components: Gases, Liquids, the Solid Asphaltenes, and Biomarkers

Abstract

Abstract The process of compositional equilibration of reservoir crude oil requires excellent reservoir connectivity. For example, the measured of asphaltene gradients for indications of connectivity is now commonplace. In addition, equilibrated fluids imply various other important fluid and reservoir properties. However, some measurements of distinguishing equilibration from disequilibrium have been ambiguous. Here, we provide measurement protocols which provide robust determination of equilibrium within a framework of reservoir fluid geodynamics. Thermodynamic modeling of measured asphaltene gradients vertically and laterally with considerations of solution gas provides a robust determination of equilibrium. In addition, two-dimensional gas chromatography with its high-resolution compositional evaluation especially within a geochemical context can corroborate conclusions from asphaltene gradient analysis. Outlier locations in the reservoir can often be found and aid considerably in delineating the important reservoir fluid geodynamic processes operative in the reservoir and validating the assessments regarding equilibrium. The evolution of lateral equilibration is shown in a reservoir with known initial (at time of charge) and present-day lateral gradients. Modeling clarifies the convection which has occurred over geologic time to minimize these lateral gradients. Different processes of mass transport in reservoirs are compared. Equilibration processes and corresponding asphaltene gradient measurements are analyzed for differing geologic times including a very young (Pleistocene and Pliocene) and very old (Cretaceous) reservoir charges in different reservoirs. The important role of convection is shown, with its critical requirement of a density inversion.