How the compositional balance between key-components can induce three-phase behavior in reservoir fluids

PISONI, GERARDO O.; CISMONDI, MARTÌN

CAMPINAS

Conferencia; XII IBEROAMERICAN CONFERENCE ON PHASE EQUILIBRIA AND FLUID PROPERTIES FOR PROCESS DESIGN; 2022

UNICAMP

From a thermodynamic or phase-equilibrium perspective, a reservoir fluid can be considered a multicomponent system with constant composition, which goes through different pressure and temperature conditions throughout different –real or hypothetic- production processes, from the reservoir up to the first separation stages, generating in some cases the appearance and disappearance of different types of phases. Consequently, the isoplethic diagram or constant-composition phase envelope is generally the most used phase diagram in the oil and gas industry. The information obtained from this type of diagrams is extremely important for planning the extraction, transport, storage and processing of the reservoir fluid. In turn, the topology and the different types of equilibrium or regions that a phase envelope can present depend on the fluid composition. It is known that components like carbon dioxide, water or asphaltenes, as well as the mere asymmetry of the hydrocarbon mixture, can lead to the appearance of three phase equilibrium regions within the phase envelope (heterogeneous region). In view of the vast diversity of possible reservoir fluid compositions and processes based on compositional changes like miscible gas EOR, knowing the pressure and temperature range in which the three-phase equilibrium region exists, and the effect that the concentration of the different components that form the reservoir fluid has on such equilibria is very important, both from scientific and applied points of view. Cismondi [1] developed an algorithm that allows identifying and computing double saturation points (DSP) on a phase envelope, as predicted from an Equation of State (EoS) model. The presence of DSP’s indicates the existence of three-phase equilibrium regions for the studied composition. Therefore, this algorithm can help in the study of compositional effects on the existence of three-phase regions. Nevertheless, mapping the compositional possibilities becomes tedious and cumbersome, not to say impossible, when the algorithm has to be applied to single compositions, one by one, even for simplified compositions based on few key-components. On the other hand, Pisoni et al [2] proposed calculation strategies to compute the complete Characteristic Map of a Ternary System, determining the pressure and temperature range in which the three phase equilibrium exists. Although quite limited in terms of number of components, this approach provides a more straightforward strategy to qualitatively study the main effects of certain components, or pseudo-components representing fractions, on the three-phase behavior of reservoir fluids.The objective of this work is to carry out a deep exploration of some compositional effects on the three-phase behavior of reservoir fluids, using both approaches, complementing the strategies and comparing results. To that end, ternary systems will be defined, which in a simplified way, allow studying the effects of CO2, asphaltenes or asymmetry on a reservoir fluid phase behavior.