Experimental determination and thermodynamic modeling of phase equilibrium in systems with CO 2 + decane + hexadecane

Abstract

The phase behavior of reservoir fluids is crucial for evaluating the feasibility of oilfield development. Variations in pressure, temperature, and composition during production can trigger complex phase transitions that affect flow. One of the inherent challenges in studying the phase behavior of reservoir fluids is related to their opacity and the evident complexity of determining phase equilibrium data due to their intrinsic characteristics. For this reason,

model systems are used to represent specific behaviors through key components. This study presents experimental phase equilibrium data and thermodynamic modeling for the carbon dioxide + decane + hexadecane ternary system at temperatures of T = (283.15, 298.15, and 323.15) K and pressures up to 20 MPa. These temperatures were chosen to cover both subcritical and supercritical conditions for carbon dioxide (CO2). Specifically, CO2 is subcritical at 283.15 K and 298.15 K, and supercritical at 323.15 K, given its critical temperature of 304.21 K. The experiments were carried out using different proportions of hexadecane in the binary mixture (hexadecane/decane), i.e., 20%, 40%, 60%, and 80% molar fraction of the liquid mixture, to its impact evaluate on phase behavior. Vapor-liquid equilibrium was observed for the ternary systems with up to 40% hexadecane in the hydrocarbon liquid mixture at 298.15 K. For subcritical temperature conditions, liquid-liquid equilibrium was observed starting from this composition. At 323,15 K, dew points were observed at CO2 mole fractions greater than 90%. However, complexity arises when the molar composition of hexadecane reaches 60% or higher in the binary liquid hydrocarbon mixture. Under these conditions, phase transitions from liquid to liquid-liquid and from liquid-liquid to vapor-liquid-liquid were observed, along with the barotropic inversion phenomenon, at a temperature of 298.15 K. The Peng-Robinson equation of state was used to model the experimental points, with binary interaction parameters for the component pairs being obtained: the CO2/decane pair was obtained by regression of experimental data of this work, while the CO2/hexadecane pair was determined through regression of literature data. The results reveal complex behaviors as the hexadecane composition in the mixture increases. This study provides valuable data on model systems representing crude oil, highlighting complex behaviors in ternary systems with high carbon dioxide content.