PREDICTION OF UNUSUAL RETROGRADE CONDENSATION IN MODEL RESERVOIR FLUIDS

Vapour-liquid-liquid equilibrium experiments were conducted with model reservoir fluids comprising the binary pair ethane + n-decylbenzene to which nitrogen, methane, ethylene or carbon dioxide was added. These ternary mixtures were found to exhibit retrograde condensation of the heavier liquid phase, over narrow ranges of pressure, temperature and composition. The experimental data are modelled qualitatively with the Peng-Robinson equation of state in conjunction with a three-phase flash routine (CMGPROP). Relatively large pressure ranges are predicted for this type of retrograde condensation when trace amounts of the heavy hydrocarbon and nitrogen or methane are present. The maximum pressure range predicted for the ternaries with nitrogen and methane arises during the transition of the shape of the liquid-liquid-vapour zone from an oval to a tear drop shape.