A classification of phase envelopes for reservoir fluids with asphaltene onset lines: Exploring topology transitions based on compositional changes

Abstract

Reservoir fluids may present complex phase diagrams involving three-phase behavior, due to the presence of different compounds that -when exceeding certain quantities- can induce the separation of a second liquid phase. Typical and important cases are carbon dioxide, water, and asphaltenes, while the mere asymmetry in the mixture of hydrocarbons can also be responsible for a three-phase region in the fluid phase envelope. Although the behavior and practical implications are quite different in each of these cases, their phase diagrams may share some qualitative characteristics and therefore require similar methods and strategies for their calculation. A general calculation strategy, which includes details about the calculation method for certain types of three-phase envelopes and emphasizes the importance of double saturation points, was presented in a previous publication. The main objective of this study is to explore the different possible topologies that the phase envelope of a reservoir fluid may adopt as a result of compositional changes, when using an equation of state (EOS). Reference fluids (compositional data and parameters) were taken from the literature. Based on these fluids, different alterations in the amounts of defined or pseudo-components of the system were used to analyze their effect on the predicted phase diagram. A new type of behavior was identified, which is included in a classification that currently consists of four different types. Additionally, a complete and updated algorithm is presented, which enables the computation of the different topologies presented in this study. Finally, it is shown that transitions between different types of phase diagram can be observed not only as a consequence of compositional changes but also based on modeling or parametric changes in the description of a given fluid, for example by changing the lumping strategy for the heavy fractions.