Compositional Variations within a Hydrocarbon Column due to Gravity

Abstract

ABSTRACT Gravity forces may generate and stabilise variations in composition along the hydrocarbon column in thick reservoirs. From the condition of thermodynamic equilibrium in a hydrocarbon column in the field of gravity one may determine with an equation of state the variations in composition with depth. The developed calculational procedure uses the Soave and Peng-Robinson equation of state. Results are shown for various hydrocarbon systems. Aromatics content, overall pressure and the introduction of binary interaction coefficients in the equation of state are seen to have strong influence on the predictions of gravity segregation. Observed compositional variations and corresponding variations in bubble points/dew points as well as the occurrence of natural miscibility between cap gas and reservoir oil can be explained.